Water-based inkjet ink and image forming method

Abstract

To provide a water-based inkjet ink capable of suppressing clogging of a filter and enhancing storage stability and discharge stability in an image formation method of discharging the water-based inkjet ink having passed through the filter a plurality of times from a nozzle of an inkjet head.SOLUTION: A water-based inkjet ink is used in an image formation method of discharging an ink having passed through a filter a plurality of times from the nozzle of the inkjet head. The inkjet ink contains a pigment, a pigment dispersant, and an organic solvent and satisfies condition (a) and condition (b). Condition (a): The pigment dispersant has a hydrogen bond component of a SP value of 8 (MPa)1 / 2 or more and 23 (MPa)1 / 2 or less. Condition (b): The organic solvent includes a solvent of which the hydrogen bond component of a SP value of 18 (MPa)1 / 2 or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an aqueous inkjet ink and an image forming method.

Background Art

[0002] The inkjet method that enables digital printing without using a plate is used in various printing fields because it can form an image simply and at low cost.

[0003] Inks used in the inkjet method include aqueous inks composed of water and a small amount of organic solvents, non-aqueous inks containing organic solvents but substantially free of water, hot melt inks that heat and melt a solid ink at room temperature for printing, and active energy ray curable inks that are cured by irradiating active energy rays after printing. There are multiple types of these inks, and they are used appropriately according to the application. Among these, aqueous inks generally have less odor and high safety.

[0004] When a pigment is used as a colorant in an aqueous ink, the pigment particles may aggregate in the ink, resulting in a decrease in the storage stability and ejection stability of the ink. In order to suppress the decrease in the above storage stability and ejection stability, it is known to include a pigment dispersant in the ink.

[0005] For example, Patent Document 1 discloses an aqueous ink composition containing an organic solvent in an amount of 1.0% by mass or more and 13.0% by mass or less based on the total amount of the ink. The organic solvent includes an organic solvent having a standard boiling point of 150.0°C or higher and 280.0°C or lower, and the content of an organic solvent having a standard boiling point exceeding 280.0°C is 3.0% by mass or less based on the total amount of the ink. In Patent Document 1, in addition to the above organic solvent, a pigment and a pigment dispersant which is a water-soluble styrene acrylic resin are included, and image formation is performed while circulating the aqueous ink composition in a circulation channel in a recording head, whereby the fastness and ejection stability of the image are said to have been improved.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-121523 [Overview of the project] [Problems that the invention aims to solve]

[0007] As shown in Patent Document 1, an ink containing a pigment and a pigment dispersant is known.

[0008] Incidentally, in image forming apparatuses used for image formation by inkjet method, filters are sometimes installed to capture air bubbles and foreign matter contained in the ink. These filters are usually installed inside the inkjet head or in the flow path through which the ink passes.

[0009] Furthermore, according to the inventors' investigations, when the aqueous ink composition described in Patent Document 1 is circulated in a circulation channel within the recording head while image formation is performed in order to suppress deterioration of ejection performance, clogging of the filter is likely to occur. Similar clogging is also observed when multiple filters are provided in the ink channel that supplies ink from the ink tank to the inkjet head. Therefore, it is considered that clogging of the filter is likely to occur when the aqueous ink composition described in Patent Document 1 is passed through the filter multiple times while image formation is performed.

[0010] Furthermore, while the water-based ink described in Patent Document 1 is said to have improved ejection stability, our own investigations have shown that even the water-based ink described in Patent Document 1 does not have sufficiently improved ejection stability. In addition, there is a desire to further improve the storage stability of water-based inks by suppressing pigment aggregation and the like.

[0011] The present invention has been made in view of the above circumstances, and aims to provide an aqueous inkjet ink and an image forming method that can suppress clogging of the filter and improve the storage stability and ejection stability of the ink in an image forming method in which aqueous inkjet ink that has passed through a filter multiple times is ejected from the nozzle of an inkjet head. [Means for solving the problem]

[0012] An aqueous inkjet ink according to one embodiment of the present invention for solving the above problems is an aqueous inkjet ink used in an image forming method in which ink that has passed through a filter multiple times is ejected from a nozzle of an inkjet head, wherein the aqueous inkjet ink comprises a pigment, a pigment dispersant, and an organic solvent, and satisfies conditions (a) and (b). Condition (a) The pigment dispersant has a hydrogen bonding component with an SP value of 8 (MPa) 1 / 2 The above is 23 MPa. 1 / 2 The following is Condition (b) The organic solvent has a hydrogen bonding component of SP value of 18 (MPa) 1 / 2 Includes the following solvents

[0013] Furthermore, an image forming method according to one embodiment of the present invention for solving the above problems is an image forming method in which an aqueous inkjet ink that has passed through a filter multiple times is ejected from the nozzle of an inkjet head, and the method comprises the step of ejecting the aqueous inkjet ink from the nozzle of the inkjet head. [Effects of the Invention]

[0014] The present invention provides an aqueous inkjet ink and an image forming method that can suppress clogging of the filter and improve the storage stability and ejection stability of the ink in an image forming method in which aqueous inkjet ink that has passed through a filter multiple times is ejected from the nozzle of an inkjet head. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is a schematic diagram showing the configuration of an image forming apparatus according to one embodiment of the present invention. [Figure 2] Figure 2 is an exploded perspective view showing an overview of the inkjet head used in the image forming apparatus described above. [Figure 3] Figure 3 is a cross-sectional view of the inkjet head described above. [Modes for carrying out the invention]

[0016] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to the following embodiments.

[0017] In this specification, "discharging ink that has passed through a filter multiple times" means that at least a portion of the discharged ink has passed through the filter multiple times.

[0018] As described above, in an image forming apparatus having a circulating channel, if a filter is installed in the circulating channel, the ink flowing through the circulating channel passes through the filter multiple times, and a portion of the ink that has passed through the filter is ejected from the nozzle of the inkjet head.

[0019] Furthermore, even without a circulation channel, if there is an ink channel through which ink flows, or if multiple filters are installed inside the inkjet head, the ink will pass through the filters multiple times before being ejected from the nozzles of the inkjet head.

[0020] As described above, in image forming methods in which ink that has passed through a filter multiple times is ejected from the nozzles of an inkjet head, it was found that when an aqueous ink containing a pigment and a pigment dispersant is used, clogging of the filter occurs. This clogging of the filter sometimes resulted in the ink not being ejected from the nozzles of the inkjet head at the desired flow rate.

[0021] The inventors considered that clogging of the filter occurred due to aggregation of the pigment in the aqueous ink. Therefore, in order to suppress the aggregation of the pigment, they considered using a pigment dispersant in which the hydrogen bonding component (δ H ) of the SP value is in a specific range. Since the above δ H is an index indicating the degree of hydrophilicity and hydrophobicity, they tried to select a pigment dispersant that easily adheres to the hydrophobic pigment according to the value of δ H .

[0022] As a result of using a pigment dispersant in which the hydrogen bonding component of the SP value is 8 (MPa) 1 / 2 or more and 23 (MPa) 1 / 2 or less, it was found that the storage stability and ejection stability were improved, and thus it was understood that the aggregation of the pigment could be suppressed by the above pigment dispersant. Regarding this, it is considered that when the above δ H is 8 (MPa) 1 / 2 or more, the dispersant easily adheres to the pigment, and the pigment can be dispersed in the ink to improve the ejection stability. Also, when the above δ H is 23 (MPa) 1 / 2 or less, since the affinity between the pigment dispersant and water in the ink is not too high, it is considered that the pigment dispersant attached to the pigment can be prevented from separating from the pigment over time. Thereby, it is considered that aggregation of the pigment during storage can be suppressed and the storage stability can be improved.

[0023] Thus, although it was found that the aggregation of the pigment could be suppressed by using the above pigment dispersant, still, when the ink was passed through the filter multiple times, the clogging of the filter was not sufficiently suppressed. Regarding the cause of this, when the inventors reexamined, the following causes were considered. <0000​It is believed that pigment dispersants present in water-based inks adhere to the surface of pigment particles at a certain rate, dispersing the pigment within the ink. However, not all of the pigment dispersant added during ink preparation adheres to the pigment; a certain percentage of the dispersant remains in the ink without adhering to the pigment. On the other hand, it is thought that pigment dispersants that do not adhere to the pigment particles tend to aggregate in water-based inks, forming micelles due to the aggregation of hydrophobic parts with low affinity for water.

[0025] When an aqueous ink containing pigment with a pigment dispersant attached and the aforementioned micelles passes through a filter, shear forces from the filter are applied to the pigment, causing some of the pigment dispersant to detach from the surface of the pigment. Furthermore, shear forces from the filter are applied to the micelles, causing them to partially disintegrate and form aggregates of the pigment dispersant with hydrophobic groups exposed on the surface. It is thought that these aggregates of pigment dispersant with exposed hydrophobic groups then adhere to the pigment from which some of the dispersant has detached, resulting in the generation of slightly larger particles. When the ink passes through the filter again in this state, it is thought that more pigment from which some of the dispersant has detached or aggregates of pigment dispersant with exposed hydrophobic groups will adhere to the aggregates formed during the previous pass through the filter, further increasing the particle size. Therefore, the aforementioned filter clogging is thought to be due to the enlarged aggregates resulting from the ink containing the pigment and pigment dispersant passing through the filter multiple times.

[0026] Therefore, the inventors diligently investigated and found that the hydrogen bonding component of the SP value (δ H ) is 18 (MPa) 1 / 2 We discovered that incorporating the following organic solvents into the ink reduces the likelihood of filter clogging.

[0027] The reason for this is unclear, but it can be thought to be as follows.

[0028] The above organic solvent is δ H The δ of the pigment dispersant mentioned above HSince it takes a value close to the above, the pigment dispersant has a high affinity for the above organic solvent. Therefore, it is thought that the above organic solvent suppresses the aggregation of hydrophobic parts of the pigment dispersant and improves the dispersibility of the pigment dispersant. For this reason, it is thought that this organic solvent suppresses the formation of micelles by the pigment dispersant in the ink. By making it difficult for micelles to form, it is thought that the clogging of the filter due to aggregation of micelles and pigment as described above is suppressed.

[0029] These effects are thought to improve storage stability and dispensing stability while suppressing filter clogging.

[0030] 1. Image forming apparatus Figure 1 is a schematic diagram showing the configuration of an image forming apparatus 100 that can be used when carrying out an image forming method using inkjet ink according to one embodiment of the present invention.

[0031] The image forming apparatus 100 includes an inkjet head 101, an ink tank 102, a liquid delivery pump 103, an ink flow path 104, and a transport device 105.

[0032] In the image forming apparatus 100, ink contained in the ink tank 102 is supplied to the inkjet head 101 via the ink channel 104 by the liquid transfer pump 103. The ink supplied to the inkjet head 101 is then ejected and deposited onto the recording medium M, which is transported by the transport device 105, thereby forming an image on the recording medium M.

[0033] The inkjet head 101 has multiple nozzles for ejecting ink droplets onto a recording medium M, such as paper, which is the material to be printed.

[0034] Examples of inkjet heads used in inkjet methods include electromechanical conversion methods such as single-cavity, double-cavity, bender, piston, shear-mode, and shear-wall types, as well as electro-thermal conversion methods such as thermal inkjet and bubble jet ("bubble jet" is a registered trademark of Canon Inc.). In this embodiment, a shear-mode inkjet head is used.

[0035] The ink tank 102 is a tank for storing ink to be supplied to the inkjet head 101. The ink tank 102 may have an agitator (not shown). The ink tank 102 can be appropriately configured according to the image forming performance and size of the image forming apparatus 100.

[0036] The liquid transfer pump 103 is a pump that transfers the ink stored in the ink tank 102 to the inkjet head 101 via the ink flow path 104.

[0037] The transport device 105 is a device for transporting the recording medium M. The transport device 105 includes, for example, a belt conveyor 105a and a rotatable feed roller 105b.

[0038] In this embodiment, the image forming apparatus 100 may also have a dryer 106. The dryer 106 is a device for drying the ink applied to the recording medium M. The dryer 106 can use a known heater and an irradiator that emits an infrared lamp.

[0039] 1-1. Inkjet head Figure 2 is an exploded perspective view showing an overview of the inkjet head 101 used in the image forming apparatus 100. Figure 3 is a cross-sectional view of the inkjet head 101. As shown in Figure 2, the inkjet head 101 includes a head chip 1, a common ink chamber 2, a holding section 3, and a flexible wiring board 4.

[0040] The head chip 1 has multiple substrates. For example, the head chip 1 has a nozzle substrate 11 on which an ink ejection nozzle (not shown) is formed, a spacer substrate 12 having a communication hole (not shown) that connects the pressure chamber and the nozzle, and a pressure chamber substrate 13 having a pressure chamber (not shown) and a discharge channel (not shown) that discharges a portion of the ink supplied from the pressure chamber to the nozzle into a circulation channel 2d.

[0041] The common ink chamber 2 is formed in a hollow, roughly rectangular parallelepiped shape, with one side facing the holding portion 3 being open. An ink supply port 2a for supplying ink is provided on the side of the common ink chamber 2 facing the opening.

[0042] The common ink chamber 2 contains the ink supplied to the nozzle. As shown in Figure 3, the common ink chamber 2 has a filter F inside. This filter F divides the common ink chamber 2 into a first ink chamber 2b and a second ink chamber 2c. As the ink heading towards the nozzle passes through the filter F, any foreign matter contained in the ink is collected by the filter.

[0043] The material of filter F is not particularly limited, but examples include resins such as nylon, metals such as SUS, etc. These may be manufactured by laser processing or etching. Of these materials, SUS is preferred from the viewpoint of durability.

[0044] The aperture diameter of filter F is preferably 1 μm or more and 20 μm or less, preferably 3 μm or more and 10 μm or less, and more preferably 5 μm or more and 10 μm or less. When the aperture diameter is 20 μm or less, the aggregates of micelles and pigments mentioned above tend to clog the filter. In this embodiment, by using the ink described later, clogging can be suppressed even for filters with aperture diameters within the above range. Furthermore, when the aperture diameter is 1 μm or more, it is possible to allow pigments and resins contained in the ink to pass through more easily.

[0045] As shown in Figure 3, the common ink chamber 2 has a circulation channel 2d. The circulation channel 2d is in communication with the discharge channel formed in the pressure chamber substrate described above, and is a channel for supplying the ink discharged from the discharge channel back to the common ink chamber 2 (first ink chamber 2b).

[0046] The ink supplied again to the common ink chamber 2 (first ink chamber 2b) passes through filter F again and heads towards the nozzle. In this way, the ink that has passed through filter F multiple times is ejected from the nozzle of the inkjet head 101 after passing through filter F. In this embodiment, as shown in Figure 3, the ink that has passed through the same filter multiple times is ejected from the nozzle.

[0047] The holding portion 3 is formed in a substantially flat shape with an opening 3a in the approximate center, and is positioned to cover the opening of the common ink chamber 2. As a result, the common ink chamber 2 is connected to one side of the holding portion 3 so as to cover the opening 2a. The head chip 1 is connected to the other side of the holding portion 3 so as to cover the opening 3a. The holding portion 3 connects the common ink chamber 2 and the head chip 1 through the opening 3a.

[0048] An insertion hole 3b is provided on the outer periphery of the holding part 3. The flexible wiring board 4 is inserted through the insertion hole 3b. One end of the flexible wiring board 3 is connected to a wiring board (not shown) on the head chip 1, which will be described later. The other end of the flexible wiring board 4 is inserted through the insertion hole 3b provided in the holding part 3 from the other side of the holding part 3 and pulled out towards the common ink chamber 2.

[0049] In the inkjet head 101 configured in this way, the discharge channel of the head chip 1 described above allows a portion of the ink supplied from the common ink chamber 2 to the nozzle outlet via the pressure chamber to be extracted (branched), and the extracted ink can be discharged (transmitted) to the circulation channel 2d within the common ink chamber 2. The ink discharged (transmitted) to the circulation channel 2d can then be returned to the common ink chamber 2 (first ink chamber 2b) and circulated again. By circulating the ink in this way, it is possible to suppress the increase in ink viscosity near the nozzle caused by the sedimentation or aggregation of pigment particles in the ink, thereby improving discharge stability.

[0050] In this embodiment, the image forming apparatus 100 circulates ink inside the inkjet head 101, but the configuration is not limited to this. The image forming apparatus 100 may, for example, be configured to discharge a portion of the ink supplied to the inkjet head 101 to the outside of the inkjet head 101 and then supply the discharged ink back to the inkjet head.

[0051] Furthermore, the image forming apparatus 100 does not necessarily have a configuration for circulating ink as described above. In this case, by providing multiple filters in the ink flow path 104, ink that has passed through the filters multiple times is ejected, and an image is formed on the recording medium M.

[0052] 2. Water-based inkjet ink The aqueous inkjet ink used in the image forming method in this embodiment comprises a pigment, a pigment dispersant, and an organic solvent, and satisfies conditions (a) and (b). Condition (a) The pigment dispersant has a hydrogen bonding component with an SP value of 8 (MPa) 1 / 2 The above is 23 MPa. 1 / 2 The following is Condition (b) The organic solvent has a hydrogen bonding component of SP value of 18 (MPa) 1 / 2 Includes the following solvents

[0053] 2-1.Water In this embodiment, the water-based inkjet ink contains water. The type of water contained in the ink is not particularly limited. Preferably, the water content in the water-based inkjet ink is 45% by mass or more of the total mass of the water-based inkjet ink, and more preferably 65% ​​by mass or more of the total mass of the liquid components in the water-based inkjet ink.

[0054] 2-2. Pigments In this embodiment, the water-based inkjet ink contains pigments. Examples of pigments include yellow pigment, magenta pigment, cyan pigment, and black pigment.

[0055] Examples of yellow pigments include CI Pigment Yellow 12, CI Pigment Yellow 13, 14, 15, 15:3, 17, 74, 93, 128, 94, 138, and 155 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0056] Examples of magenta pigments include CI Pigment Red 2, CI Pigment Red 3, 5, 6, 7, 15, 16, 48;1, 53;1, 57;1, 122, 123, 139, 144, 149, 150, 166, 177, 178, 184, 222, and 238.

[0057] Examples of cyan pigments include CI Pigment Blue 15, 15;2, 15;3, 15;4, 16, 60, 62, and 66 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0058] Examples of black pigments include carbon black, CIPigment Black 7, 26, and 28.

[0059] The particle size of the pigment particles is not particularly limited, but from the viewpoint of dispersing the pigment in the ink for a long period of time, it is preferable that the volume-based median diameter is 60 nm or more and 200 nm or less. Here, the particle size of the pigment particles refers to the particle size of the pigment particles with the pigment dispersant attached. The particle size can be determined by known particle size measuring instruments using dynamic light scattering or electrophoresis. From the viewpoint of simple and highly accurate measurement, measurement by dynamic light scattering is preferred.

[0060] The pigment content in the water-based inkjet ink is preferably 5% by mass or more and 20% by mass or less, and more preferably 7% by mass or more and 15% by mass or less, relative to the total mass of the ink. When the content is 5% by mass or more, the pigment concentration can be increased, making it easier to form images with high color reproducibility with a small amount of ink. As a result, the amount of ink used can be reduced, and because the solid content concentration in the ink increases, the proportion of water decreases, which reduces the load on the recording medium during drying (such as curling and cockling) and the energy required for drying. On the other hand, when the content is 5% by mass or more, the amount of pigment increases, making aggregation between the pigment and the micelles of the pigment dispersant mentioned above, as well as aggregation between the pigments themselves, more likely to occur, which can lead to filter clogging, reduced storage stability of the ink, and decreased ejection stability. In contrast, in this embodiment, the above problems can be solved by using a water-based inkjet ink that satisfies the above conditions (a) and (b). Furthermore, when the content is 20% by mass or less, the aggregation of pigment in the water-based ink can be further suppressed.

[0061] 2-3. Pigment Dispersants In this embodiment, the water-based inkjet ink contains a pigment dispersant.

[0062] As mentioned above, the hydrogen bonding component (δ) of the SP value of the pigment dispersant H ) is 8 (MPa) 1 / 2 The above is 23 MPa. 1 / 2 The following is the case. From the perspective that pigment dispersants that do not adsorb to pigments make it more difficult for micelles to form, the δ of the pigment dispersantH This is the δ of the solvent contained in the organic solvents mentioned above. H It is preferable that the δ of the two be close. Specifically, the δ of the two H The difference in value is 1 (MPa). 1 / 2 Above 10 MPa 1 / 2 Preferably, the following is true: 3 (MPa) 1 / 2 The above is 7 MPa. 1 / 2 The following is more preferable: δ of the pigment dispersant H However, by satisfying this relationship, the pigment dispersant, which does not adsorb to the pigment, has a suitable affinity for the organic solvent contained in the ink, making it easier to disperse in the ink and less likely to form micelles.

[0063] δ of pigment dispersant H and the δ of the above solvent H From the perspective that the difference between these two factors makes it easier for each to satisfy the above range, the δ of the pigment dispersant H is 8 (MPa) 1 / 2 The above is 18 MPa. 1 / 2 It is more preferable that the following conditions be met: 11 (MPa) 1 / 2 The above is 18 MPa. 1 / 2 The following is even more preferable:

[0064] The hydrogen bonding component (δ) of the SP value of the pigment dispersant H This can be adjusted by changing the number of hydrophilic groups (described later) in the molecules of the pigment dispersant.

[0065] δ of pigment dispersant H This can be measured using the computer software Hansen Solubility Parameters in Practice 5th Edition 5.0.13 (HSPiP, manufactured by Tegara Co., Ltd.).

[0066] Specifically, first, prepare 15 or more types of organic solvents, mix 9.95 g of each organic solvent with 0.05 g of pigment dispersant A, and shake for 30 minutes to prepare a number of compositions with a concentration of 0.5% by mass corresponding to the number of types of organic solvents.

[0067] By inputting the compound name of the above organic solvent and its corresponding classification (good solvent or poor solvent) into the computer software Hansen Solubility Parameters in Practice 5th Edition 5.0.13 (HSPiP, manufactured by Tegara Co., Ltd.), the dispersion term (δ) of the SP value of pigment dispersant A can be calculated. D ) and polar term (δ P ) and hydrogen bond term (hydrogen bond component δ H The following can be calculated: When the pigment dispersant is dispersed in a dispersion medium, the pigment dispersant is dried to 130°C using a dryer to obtain a solid content of 100%, and the same measurement is performed on the pigment dispersant.

[0068] In this embodiment, the type of pigment dispersant is not particularly limited as long as condition (a) is met, but it is preferable that it has both hydrophilic and hydrophobic groups. Having hydrophobic groups in the pigment dispersant makes it easier for the pigment dispersant to adsorb to the pigment particles, and having hydrophilic groups makes it easier to disperse the pigment in the water-based ink. On the other hand, pigment dispersant that does not adsorb to the pigment tends to aggregate and form micelles with the hydrophobic groups on the inside and the hydrophilic groups on the outside, which can easily cause aggregates of these micelles and pigments to clog the filter. In contrast, the water-based inkjet ink in this embodiment satisfies conditions (a) and (b), and therefore can solve the above problem.

[0069] Examples of hydrophilic groups mentioned above include hydroxyl groups, carboxyl groups, amino groups, ketone groups, and sulfonic acid groups.

[0070] Examples of the hydrophobic groups mentioned above include alkyl groups, phenyl groups, and naphthyl groups.

[0071] The pigment dispersant may be a low-molecular-weight dispersant or a high-molecular-weight dispersant. From the viewpoint of more stably dispersing the pigment due to the steric repulsion effect, it is preferable to use a high-molecular-weight dispersant. In this specification, "high-molecular-weight dispersant" refers to a pigment dispersant having a weight-average molecular weight (Mw) of 2000 or more and 30000 or less. The weight-average molecular weight (Mw) can be measured, for example, by gel permeation chromatography (GPC) using polymethyl methacrylate as a comparative standard.

[0072] The resin constituting the polymer dispersant is, for example, a copolymer of a hydrophilic monomer and a hydrophobic monomer. In this specification, "hydrophilic monomer" means a monomer whose solubility in 100 mL of water at 25°C is 10 g or more, and "hydrophobic monomer" means a monomer whose solubility in 100 mL of water at 25°C is 10 g or less.

[0073] Examples of hydrophilic monomers include monomers containing a carboxyl group or an acid anhydride group (unsaturated polycarboxylic acids such as (meth)acrylic acid, methyl (meth)acrylate, and maleic acid, as well as maleic anhydride), and ethylene oxide-modified (meth)acrylic acid ester monomers (such as ethylene oxide-modified (meth)acrylate alkyl esters).

[0074] Examples of hydrophobic monomers include (meth)acrylic acid ester monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, and benzyl (meth)acrylate; propylene oxide-modified (meth)acrylic acid ester monomers; (meth)acrylic acid ester monomers having an alkyl group with 3 to 6 carbon atoms in the side chain; styrene monomers such as styrene, α-methylstyrene, and vinyltoluene; α-olefin monomers such as ethylene, propylene, and 1-butene; vinyl monomers having an aromatic ring group; and vinyl carboxylate ester monomers such as vinyl acetate and vinyl butyrate.

[0075] Examples of the copolymers mentioned above include styrene-acrylic acid copolymer, styrene-acrylic acid-alkyl acrylate copolymer, styrene-maleic acid copolymer, styrene-maleic acid-alkyl acrylate copolymer, styrene-methacrylic acid copolymer, styrene-methacrylic acid-alkyl acrylate copolymer, and styrene-maleic acid half-ester copolymer.

[0076] The above copolymers include random copolymers, alternating copolymers, and block copolymers. Of these, from the viewpoint of further improving the dispersibility of the pigment, the above copolymer is preferably a block copolymer having hydrophilic blocks and hydrophobic blocks.

[0077] A block copolymer is a polymer of two or more monomers, and may be a polymer chain formed by the homopolymerization of the same type of monomer, or a polymer chain formed by the copolymerization of two or more monomers. The block copolymer has hydrophilic blocks and hydrophobic blocks. In this specification, "hydrophilic block" refers to a block among the blocks constituting the copolymer that has a greater affinity for water. Also, in this specification, "hydrophobic block" refers to a block that has a lesser affinity for water. For example, a hydrophilic block is a block composed only of hydrophilic monomers, or a block in which the proportion of constituent units derived from hydrophilic monomers (the ratio of moles of hydrophilic functional groups to the molecular weight of the block) is greater than that of a hydrophobic block. Also, a hydrophobic block is a block that does not contain hydrophilic monomers and is composed only of hydrophobic monomers, or a block in which the proportion of constituent units derived from hydrophilic monomers (the ratio of moles of hydrophilic functional groups to the molecular weight of the block) is less than that of a hydrophilic block. Typically, hydrophobic blocks are sites that adsorb to pigments, and hydrophilic blocks are sites that increase the affinity of the ink to water.

[0078] The inventors have found that when the pigment dispersant included in the ink is the block copolymer described above, filter clogging is more likely to occur. The block copolymer has hydrophobic blocks and hydrophilic blocks, each with a certain chain length, so the polymer chains tend to aggregate into blocks, and it is thought that micelles are more likely to form in aqueous ink with the hydrophobic blocks facing inward and the hydrophilic blocks facing outward. Therefore, aggregation between micelles and pigment is more likely to occur, and filter clogging is more likely to occur. In contrast, in this embodiment, the above problem can be solved by using an inkjet ink that satisfies the above conditions (a) and (b).

[0079] The hydrophilic monomers constituting the hydrophilic block described above are preferably (meth)acrylic acid and methyl (meth)acrylate, from the viewpoint of imparting appropriate water solubility to the hydrophilic block.

[0080] The hydrophobic monomers constituting the above-mentioned hydrophobic block are preferably benzyl (meth)acrylate and vinyl monomers having an aromatic ring group, from the viewpoint of improving adsorption to pigments.

[0081] If the hydrophilic block described above is A and the hydrophobic block described above is B, then examples of the block copolymer structure include AB type, ABA type, BAB type, ABAB type, ABABA type, and BABAB type. In structures having multiple blocks A and B, such as ABA type and BAB type, the monomer species and monomer composition ratios constituting each block A may be the same, or different effects (for example, an effect that further enhances affinity with pigments) may be imparted to each block A by changing the monomer species and monomer composition ratios. Similarly, in structures with multiple blocks B, such as ABA type and BAB type, the monomer species and monomer composition ratios constituting each block B may be the same, or different effects may be imparted to each block B by changing the monomer species and monomer composition ratios. Of these, AB type is preferred from the viewpoint of improving adsorption to pigments and storage stability.

[0082] The method for synthesizing block copolymers is not particularly limited, but for example, AB-type block copolymers can be synthesized by the following method.

[0083] (1) Diethylene glycol dimethyl ether, 2-iod-2-cyanopropane, hydrophilic monomer, azobisisobutyronitrile (AIBN), and iodosuccinimide are added to a separable flask (reaction apparatus) equipped with a stirrer, backflow condenser, thermometer, and nitrogen inlet tube, and the mixture is stirred while flowing nitrogen. (2) A mixture of hydrophobic monomer and azobisisobutyronitrile is added to the above polymerization solution, and polymerization is carried out at 75°C for 3 hours.

[0084] Examples of commercially available pigment dispersants that are block copolymers include DISPERBYK2010 (both manufactured by Bic Chemie) and EFKA4585 (manufactured by BASF).

[0085] The fact that a pigment dispersant is a block copolymer can be determined, for example, by confirming that the molecular weight distribution (PDI) (weight-average molecular weight (Mw) of the block copolymer) / (number-average molecular weight (Mn) of the block copolymer) is 2.0 or less. The above weight-average molecular weight (Mw) and number-average molecular weight (Mn) can be determined by gel permeation chromatography (GPC).

[0086] The pigment dispersant content is preferably 20% to 50% by mass, and more preferably 25% to 40% by mass, relative to the total mass of the pigment. A content of 20% by mass or more allows for a more sufficient improvement in the dispersibility of the pigment in the ink. A content of 25% by mass or more increases the amount of pigment dispersant that forms micelles in the ink without adsorbing to the pigment, making filter clogging more likely. In contrast, in this embodiment, the above problem can be solved by using a water-based inkjet ink that satisfies either condition (a) or condition (b) described above.

[0087] Furthermore, by having the pigment dispersant content be 50% by mass or less, the amount of micelles can be reduced, making it less likely for the filter to clog.

[0088] 2-4. Organic Solvents In this embodiment, the inkjet ink contains an organic solvent.

[0089] As described above, the organic solvent has a hydrogen bonding component (δ) of the SP value. H ) is 18 (MPa) 1 / 2 It contains the following solvents.

[0090] δ of pigment dispersant H and the δ of the above solvent H From the viewpoint of making the difference between the above and the above range more likely to fall within the range described above, the δ of the solvent H is 10 (MPa) 1 / 2 The above is 18 MPa. 1 / 2 Preferably, it is 12 (MPa). 1 / 2The above is 18 MPa. 1 / 2 The following is more preferable:

[0091] δ of organic solvents H This can be obtained by referring to the values ​​included in the aforementioned computer software, Hansen Solubility Parameters in Practice 5th Edition 5.0.13 (HSPiP, manufactured by Tegara Co., Ltd.), or by obtaining it from publicly known literature values.

[0092] Examples of organic solvents include alcohols, polyhydric alcohols, amines, amides, and glycol ethers. These organic solvents may be present individually or in combination of two or more types.

[0093] Examples of the alcohols mentioned above include methanol, ethanol, propanol, butanol, and isobutanol.

[0094] Examples of the polyhydric alcohols mentioned above include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol with five or more ethylene oxide groups, propylene glycol, dipropylene glycol, and tripropylene glycol.

[0095] Examples of the above amines include ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, and tetramethylpropylenediamine.

[0096] Examples of the above amides include formamide, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0097] The hydrogen bonding component of the above SP value (δH ) is 18 (MPa) 1 / 2 Examples of solvents include methyl ethyl ketone (δ H =5.1), acetonitrile (δ H =6.1), N-methylpyrrolidone (δ H =7.2), tetrahydrofuran (δ H =8.0), 2-pyrrolidone (δ H =9.0), triethylene glycol monobutyl ether (δ H =9.1), Dimethylformamide (δ H =11.3), diethylene glycol monoethyl ether (δ H =12.2), triethylene glycol monomethyl ether (δ H =12.5), isopropanol (δ H =16.4), 1,2-hexanediol (δ H =17.3), dipropylene glycol (δ H This includes (=17.7), etc.

[0098] The content of the organic solvent is preferably 5% by mass or more and 25% by mass or less, and more preferably 10% by mass or more and 23% by mass or less, relative to the total mass of the inkjet ink. When the content is 5% by mass or more, the wettability of the ink to the nozzle surface of the inkjet head can be improved, and the ejection stability can be further improved. When the content is 25% by mass or less, the amount of organic solvent used can be reduced, and VOC emissions can be reduced.

[0099] The hydrogen bonding component of the above SP value (δ H ) is 18 (MPa) 1 / 2 The solvent content is preferably 5% to 25% by mass, and more preferably 5% to 20% by mass, relative to the total mass of the inkjet ink. A content of 5% by mass or more further enhances the dispersibility of the pigment dispersant in the aqueous ink, which does not adhere to the pigment, thereby suppressing micelle formation of the dispersant and further suppressing filter clogging. A content of 25% by mass or less enhances the δ of the aqueous ink. H is the δ of organic solventsH This prevents the pigment from getting too close to the pigment. As a result, the pigment dispersant adhering to the pigment can be prevented from separating from the pigment over time and dispersing in the water-based ink, thereby improving the dispersibility of the pigment. This improves storage stability.

[0100] In this embodiment, the total mass ratio of water and organic solvent to the liquid component of the ink is preferably 80% by mass or more, and more preferably 85% by mass or more.

[0101] 2-5. Others In this embodiment, the ink may contain, depending on the purpose, a fixing resin, a surfactant, a pH adjuster, an ultraviolet absorber, oil droplet particles, a fluorescent whitening agent, a polysaccharide, a viscosity modifier, a resistivity modifier, a film-forming agent, an antioxidant, a mildew inhibitor, a rust inhibitor, and the like. These components may be present individually or in combination of two or more types.

[0102] The fixing resin is a resin distinct from the pigment dispersant, and may be either a water-soluble or water-insoluble resin. It refers to the resin that forms a coating film when the ink deposited on the substrate is dried. Examples of fixing resins include acrylic resins, urethane resins, urethane-acrylic resins, polyester resins, polyamine resins, polyvinyl alcohol, and water-dispersible latex resins.

[0103] These resins may have crosslinkable groups. Examples of crosslinkable groups include amine groups, urethane bonds, urea bonds, polyols in which adjacent carbon molecules have hydroxyl groups, and carboxyl groups.

[0104] When the fixing resin has crosslinkable groups, the ink may contain a crosslinking agent. Examples of crosslinking agents include carbodiimide compounds, isocyanate compounds, epoxy compounds, silyl compounds, hydrazine compounds, and oxazoline compounds.

[0105] Examples of the surfactants mentioned above include anionic surfactants such as dialkyl sulfosuccinates, alkylnaphthalene sulfonates, and fatty acid salts; nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene-polyoxypropylene block copolymers; cationic surfactants such as alkylamine salts and quaternary ammonium salts; and silicone-based and fluorine-based surfactants.

[0106] Examples of pH adjusters include known acids, bases, and buffers. Of these, ammonia, monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, methylethylamine, monoethanolamine, diethanolamine, triethanolamine, ethylenediamine, methylaminoethanol, and dimethylaminoethanol, as well as their salts, are preferred because they do not easily inhibit the reaction between the crosslinking resin and the crosslinking agent.

[0107] Examples of the above-mentioned UV absorbers include benzophenone-based UV absorbers, benzotriazole-based UV absorbers, benzoate-based UV absorbers, and triazine-based UV absorbers.

[0108] 2-6. Physical Properties From the viewpoint of improving the ejection stability of ink from the nozzles of the inkjet head, the viscosity of the ink is preferably 1 cP or more and less than 100 cP. From the viewpoint of further improving the ejection stability, the viscosity of the ink is preferably 1 cP or more and 50 cP or less, and more preferably 1 cP or more and 15 cP or less.

[0109] From the viewpoint of improving the ejection stability of ink from the nozzles of the inkjet head, the surface tension of the ink is preferably 20 mN / m to 50 mN / m. From the viewpoint of improving wettability to the substrate and making the formed image more high-resolution, the surface tension of the ink is more preferably 20 mN / m to 35 mN / m. The surface tension of the ink can be adjusted to the above range by changing the type or amount of the surfactant and organic solvent.

[0110] 3. Image forming method The image forming method according to this embodiment is an image forming method in which inkjet ink that has passed through a filter multiple times is ejected from the nozzles of an inkjet head, and comprises the step of ejecting the inkjet ink from the nozzles of the inkjet head. In this embodiment, the image forming method can be performed using an image forming apparatus 100.

[0111] In this embodiment, as described above, the ink is circulated within the inkjet head 101 of the image forming apparatus 100. Therefore, the ink supplied to the inkjet head 101 passes through the same filter F multiple times.

[0112] In this embodiment, the circulation flow rate is preferably 1 g / min or more and 10 g / min or less, and more preferably 2 g / min or more and 7 g / min or less. If it is 1 g / min or more, the increase in ink viscosity near the nozzle due to the sedimentation or aggregation of pigment particles in the ink can be further suppressed, thereby improving ejection stability. Also, if it is 10 g / min or less, an excessive increase in the amount of ink ejected can be suppressed. Here, the inkjet head is treated as one unit, which is the sum of all nozzles that eject ink supplied from one supply port. In this specification, "circulation flow rate" refers to the flow rate of water-based ink flowing through the circulation channel 2d in the inkjet head 101. The above circulation flow rate can be determined, for example, by using a modified head that has a channel for discharging the ink flowing through the circulation channel 2d to the outside of the jet head, setting the ink ejection amount and the driving conditions of the liquid supply pump in the same way as when the above image formation was performed, and measuring the amount of ink discharged from the head per unit time.

[0113] In this embodiment, image formation is performed using the inkjet ink described above. This makes it possible to suppress filter clogging that occurs when the ink passes through filter F multiple times, and further improves ejection stability.

[0114] (Process for ejecting inkjet ink (Process S10)) In this process, the aforementioned inkjet ink is ejected from the inkjet head 101.

[0115] The amount of ink ejected from the inkjet head 101 is set appropriately depending on the type of inkjet head 101, but for example, it is 5 pL.

[0116] In this process, ink may be ejected from the nozzles of the inkjet head 101 and applied to the recording medium, or to an intermediate transfer medium.

[0117] The type of recording medium is not particularly limited. The recording medium may be, for example, a highly absorbent paper substrate, or a non-absorbent substrate such as film, plastic board (flexible polyvinyl chloride, rigid polyvinyl chloride, acrylic sheet, polyolefin, etc.).

[0118] (Drying process (Process S20)) The image forming method according to this embodiment may include a step (step S20) of drying the ink applied to the recording medium.

[0119] The method for drying the ink is not particularly limited and can be done using, for example, a known heater or infrared lamp.

[0120] The temperature at which the ink is dried is preferably 30°C or higher from the viewpoint of further improving fixation, and preferably 95°C or lower from the viewpoint of energy-saving fixation.

[0121] The image forming method in this embodiment can suppress filter clogging that occurs when ink passes through the filter F in the inkjet head 101 multiple times, thereby improving ejection stability. [Examples]

[0122] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0123] 1. Preparation of pigment dispersant (1) In a separable flask (reaction apparatus) fitted with a stirrer, a backflow condenser, a thermometer, and a nitrogen inlet tube, 100 parts by mass of diethylene glycol dimethyl ether, 3 parts by mass of 2-iod-2-cyanopropane, 35 parts by mass of methyl methacrylate, 20 parts by mass of acrylic acid, 2 parts by mass of azobisisobutyronitrile, and 0.1 parts by mass of iodosuccinimide were added, and the mixture was stirred while flowing nitrogen. Polymerization was carried out at a reaction temperature of 75°C for 3 hours.

[0124] (2) A mixture of 40 parts by mass of benzyl methacrylate and 0.4 parts by mass of azobisisobutyronitrile was added to the polymerization solution described above, and polymerization was carried out at 75°C for 3 hours.

[0125] (3) To the polymerization solution, an equivalent amount of aqueous potassium hydroxide solution was added to neutralize the carboxyl groups in the block polymer. Then, the block polymer-containing solution was heated to remove volatile components and obtain pigment dispersant A.

[0126] Pigment dispersant B was obtained in the same manner as above, except that the amount of methyl methacrylate added in (1) was changed to 65 parts by mass, the amount of acrylic acid added was changed to 35 parts by mass, and the amount of benzyl methacrylate added in (2) was changed to 19 parts by mass.

[0127] Pigment dispersant E was obtained in the same manner as above, except that the amount of methyl methacrylate added in (1) was changed to 30 parts by mass, the amount of acrylic acid added was changed to 8 parts by mass, and the amount of benzyl methacrylate added in (2) was changed to 50 parts by mass.

[0128] 2. Ink preparation 2-1. Preparation of Pigment Dispersion 18.0 parts by mass of cyanide pigment (pigment blue 15:3, manufactured by Tokyo Chemical Industry Co., Ltd.) was mixed with 5.4 parts by mass of pigment dispersant A, 2.4 parts by mass of propylene glycol (PG), 17.6 parts by mass of 1,2-hexanediol (1,2-HD), and 56.6 parts by mass of deionized water. The mixture was then dispersed using a sand grinder packed with 50% by volume of zirconia beads with an average particle size of 0.5 mm to prepare a cyanide pigment dispersion with a pigment content of 18.0% by mass. The average particle size of the pigment particles in the prepared pigment dispersion was 109 nm. The volume-based average particle size of the pigment particles was measured using a particle size distribution analyzer (Zeta Nanosizer 1000HS, manufactured by Malvern).

[0129] The organic solvent used in preparing the pigment dispersion was appropriately changed to match the type and ratio of the organic solvent used in preparing the water-based ink, as described later.

[0130] 2-2. Preparation of water-based inkjet inks While stirring a 55.6 parts by mass cyanide pigment dispersion, 1.1 parts by mass propylene glycol, 7.8 parts by mass 1,2-hexanediol, 0.5 parts by mass silicone-based surfactant (KF-351A, manufactured by Shin-Etsu Chemical Co., Ltd.), and 5.0 parts by mass (solids) resin particles (MD-2000, Toyobo) were added. Deionized water was then added to bring the total volume to 100 parts by mass to prepare an ink composition. The above ink composition was filtered through a 0.8 μm filter to obtain ink 1.

[0131] Inks 2 to 18 were obtained in the same manner as ink 1, except that the formulation of the pigment dispersion and the water-based ink were appropriately modified so that the type and content of pigment dispersant, the type and content of organic solvent, and the pigment concentration in the ink were as shown in Tables 1 and 2. Note that the mass % in Table 1 is the value relative to the total mass of the ink.

[0132] The pigment dispersants A to E in Tables 1 and 2 are those listed below. Note that the pigment dispersant content in Tables 1 and 2 indicates the solid content. Pigment dispersants A to E all contain both hydrophilic groups (hydrophilic blocks when they have a block structure) and hydrophobic groups (hydrophobic blocks when they have a block structure). Furthermore, the weight-average molecular weights of the pigment dispersants listed below were measured by gel permeation chromatography (GPC). Pigment dispersant A (SP value (δ H ()=10, weight-average molecular weight (Mw): 5000, block structure) Pigment dispersant B (SP value (δ H )=15, weight-average molecular weight (Mw): 6500, block structure) Pigment dispersant C (BYKJET-9151, manufactured by Bic Chemie, SP value (δ) H )=20, weight-average molecular weight (Mw): 3, 900, random structure) Pigment dispersant D (EFKA4585, manufactured by BASF, SP value (δ) H )=25, weight-average molecular weight (Mw): 4,500, block structure) Pigment dispersant E (SP value (δ H )=9, weight-average molecular weight (Mw): 5500, block structure)

[0133] Whether or not the above pigment dispersants A to E are block copolymers was confirmed by checking that their molecular weight distribution PDI was 2.0 or less. The above molecular weight distribution PDI was determined by measuring the weight-average molecular weight (Mw) and number-average molecular weight (Mn) from a calibration curve prepared using polystyrene standard samples with a gel permeation chromatography instrument (RID-6A, manufactured by Shimadzu Corporation (column: TSK-GEL, manufactured by Tosoh Corporation)), and then dividing Mw by Mn.

[0134] SP values ​​(δ) of the above pigment dispersants A to E H The answer was calculated as follows:

[0135] Twenty-five types of organic solvents, as shown in Table 3, were prepared. 9.95 g of each organic solvent was mixed with 0.05 g of pigment dispersant A and shaken for 30 minutes to prepare 25 compositions with a concentration of 0.5% by mass. Next, these compositions were left to stand for 24 hours at 25°C. After 24 hours, the compositions were visually inspected. Organic solvents in compositions in which no precipitate was observed were classified as "good solvents," while those in which precipitate was observed were classified as "poor solvents."

[0136] By inputting the compound names of the 25 types of organic solvents and their corresponding classifications (good solvent or poor solvent) into the computer software Hansen Solubility Parameters in Practice 5th Edition 5.0.13 (HSPiP, manufactured by Tegara Co., Ltd.), the dispersion term (δ) of the SP value of pigment dispersant A can be calculated. D ) and polar term (δ P ) and hydrogen bond term (δ H ) was calculated.

[0137] For pigment dispersants B to E, the SP value (δ) was determined in the same manner as for pigment dispersant A. H The following was determined. When the pigment dispersant was dispersed in the dispersion medium, the pigment dispersant was dried to 130°C using a dryer, and the same measurement was performed on the pigment dispersant with a solid content of 100%.

[0138] SP value of organic solvent A (δ H The SP value (δ) of organic solvent B was calculated using the values ​​of each organic solvent recorded in the above software. H The values ​​for each organic solvent recorded in the above software were used.

[0139] [Table 1]

[0140] [Table 2]

[0141] [Table 3]

[0142] 2. Evaluation Experiments 1-19 were conducted using inks 1-18 to perform the evaluations shown below.

[0143] 2-1. Storage stability (Experiments 1-19) Each of the obtained inks 1-18 was stored for 7 days in a constant temperature chamber at 50°C. The ratio of the ink viscosity after storage to the ink viscosity before storage (thickening ratio) was measured, and the storage stability of the inks was evaluated according to the following criteria, with A and B being considered acceptable. A. The viscosity ratio is 1.1 or less. B. The viscosity ratio is greater than 1.1 and less than or equal to 1.3. C. Thickness ratio greater than 1.3 and less than or equal to 1.5 D. Thickness ratio greater than 1.5

[0144] 2-2. Discharge stability (Experiments 1-18) A 720dpi x 720dpi head module was fabricated by arranging two independent drive heads of a piezo-type inkjet printer (360dpi, ejection volume 6pL, modified Konica Minolta head) with a circulation mechanism as shown in Figure 3, so that the nozzles were staggered. Next, it was installed on a stage transporter with the nozzle rows perpendicular to the transport direction. The inkjet head of the head module was filled with the obtained ink 1, and an inkjet recording device was configured to record a solid image in a single pass method on coated paper (OK Topcoat + 73.3gsm, manufactured by Oji Paper Co., Ltd.) transported by the stage transporter.

[0145] The above inkjet recording device was left for 1 hour at a temperature of 25°C and a relative humidity of 50% with a circulation flow rate of 2g / min. Then, 100 solid images were continuously formed on the coated paper under the conditions of a print width of 100nm x 100nm and a resolution of 720dpi x 720dpi. The same procedure was performed for inks 2 to 15.

[0146] After forming the solid image described above, the inkjet head was continuously driven under conditions of droplet volume of 3.5 pL, droplet velocity of 7.0 m / s, ejection frequency of 40 kHz, and print coverage of 100%. The number of missing nozzles was measured, and the ejection stability was evaluated according to the following criteria, with A and B being considered acceptable. A: No cases of missing nozzles were found. B: The number of missing nozzles was 1 to 3. C: The number of missing nozzles was 4-6. D: The number of missing nozzles was 7 or more.

[0147] (Experiment 19) The ejection stability was evaluated in the same manner as in Experiment 9, except that the inkjet head used was changed to an inkjet head without a circulation channel (a modified head manufactured by Konica Minolta, Inc., with only one filter pass).

[0148] 2-3. Circulation Sharing Aggregation (Experiments 1-18) Using a piezo-type inkjet recording device with the circulation mechanism shown in Figure 3, image formation was performed for 8 hours a day using ink 1 and multiple sheets of the coated paper described above. During periods when no image was being formed, the device was left idle with the ink circulating. The circulation flow rate during idle was set to 2 g / min. The circulation flow rate was determined by measuring the amount of ink discharged from the head per unit time, using a modified head equipped with a channel to discharge the ink flowing through the circulation channel 2d to the outside of the jet head, and setting the ink discharge amount and the driving conditions of the liquid transfer pump in the same way as when image formation was performed. This was repeated for one month, and after one month, the filter (material: SUS303, aperture diameter 10 μm) installed inside the head was observed. The same procedure was performed for inks 2 to 18. Based on the observation results, evaluation was performed according to the following criteria, with A and B being considered acceptable. No solid foreign matter is visible in the filter. A small amount of solid-like foreign matter is visible in filter B. A considerable amount of solid-like foreign matter is visible in the C filter.

[0149] (Experiment 19) Except for changing the inkjet head used to an inkjet head without a circulation channel (manufactured by Konica Minolta, Inc., with one filter pass), the circulation sharing evaluation was performed in the same manner as in Experiment 9.

[0150] (Reduces ink usage) In the ejection stability evaluations of Experiments 1-19, the images formed were evaluated for ink usage reduction according to the following criteria. ○ Observation confirmed that images with sufficient standard image density could be formed using less than 5 μL of ink. △ Observation confirmed that images with sufficient standard image density could be formed using 5 μL to less than 10 μL of ink. × Observation confirmed that the standard image density could be achieved, and images were formed using 10 μL or more of ink.

[0151] (VOC reduction) Inks 1-18 were each collected in 30 ml beakers. The beakers were then placed in the container of an outgassing collection device. The temperature of the collection device was set to 50°C, and high-purity N2 gas was passed through the container. The expelled gas was collected by an adsorption tube. After collection, the adsorption tube was purged and trapped, and the VOCs were measured using GC / MS. The evaluation was conducted according to the following criteria. ◎ Measurement result is less than 30 ppm ○ Measurement result is between 30 ppm and less than 50 ppm △ Measurement result is between 50 ppm and less than 80 ppm × Measurement result is 80 ppm or higher

[0152] (comprehensive evaluation) The overall evaluation was conducted according to the following criteria. ◎ Storage stability, ejection stability, and shearing evaluation are all A, and the ink usage reduction evaluation is ○, and the VOC reduction evaluation is ◎ or ○. ○ One or more of the following are rated B: storage stability, ejection stability, and shearing evaluation, OR the ink usage reduction evaluation is △, OR the VOC evaluation is △. × One or more of the following evaluations—storage stability, dispensing stability, and shearing—fails to meet the acceptance criteria.

[0153] These evaluation results are summarized in Table 4.

[0154] [Table 4]

[0155] In experiments 1-8, which used water-based inks satisfying the above-mentioned conditions (a) and (b), the evaluation of storage stability, dispensing stability, and shearing aggregation was all better than in experiments 9-18. It is thought that the pigment dispersant satisfying condition (a) improved the dispersibility of the pigment, thereby improving storage stability and dispensing stability, while the solvent satisfying condition (b) improved the dispersibility of the pigment dispersant that did not adhere to the pigment, making it difficult for micelles to form, and thus suppressing filter clogging.

[0156] In particular, the δ of the pigment dispersant H And, the δ of organic solvents H In experiments 2-4, where the difference was smaller, filter clogging was suppressed more than in experiment 1. H This is thought to be because the smaller difference in these factors allows for improved dispersibility of the pigment dispersant that did not adhere to the pigment, making micelle formation more difficult.

[0157] Furthermore, in Experiment 2, filter clogging was suppressed more effectively than in Experiment 5, and storage stability was improved compared to Experiment 6. This is thought to be because Experiment 2 contained a higher amount of solvent satisfying condition (b) than Experiment 5, resulting in a more pronounced effect of the solvent in suppressing micelle formation. Additionally, in Experiment 2, the amount of solvent was moderately lower than in Experiment 6, which is thought to have allowed the pigment dispersant adhering to the pigment to separate from the pigment over time, further suppressing pigment aggregation during storage.

[0158] In experiments 9 and 16, when the dispersant had a block structure, filter clogging occurred more frequently. This is thought to be because the block copolymer dispersant has a block-like, cohesive polymer chain, making it easier to form micelles. In contrast, in experiments 2 and 7, the effect of suppressing filter clogging was maintained even when the dispersant was a block copolymer. This is because δ H For dispersants that satisfy condition (a), δ H This is thought to be because micelle formation was suppressed by using a solvent that closely matches (satisfies condition (b)).

[0159] In experiments 10 and 18, increasing the pigment dispersant content to 25% by mass or more relative to the pigment mass improved the dispersibility of the pigment, thereby enhancing storage stability and dispensing stability. However, this also led to increased filter clogging. This is thought to be because the increased amount of pigment dispersant made micelle formation more likely. In contrast, in experiments 2 and 8, when the pigment dispersant content to 25% by mass or more was similarly increased, the effect of suppressing filter clogging was maintained while improving storage stability and dispensing stability. For a pigment dispersant satisfying condition (a), the δ of the dispersant was... H It is believed that using a solvent that more closely matches the target solvent (a solvent that satisfies condition (b)) allowed for the suppression of micelle formation even when the amount of pigment dispersant increased.

[0160] In experiments 16 and 17, increasing the pigment concentration led to greater filter clogging. This is thought to be because the amount of pigment that could aggregate with micelles due to the pigment dispersant increased. In contrast, in experiments 2-4, the effect of suppressing filter clogging was maintained even when the pigment concentration was increased from 5% by mass to 10% by mass. This is thought to be because conditions (a) and (b) were met, making it difficult for micelles that could aggregate with the pigment to form.

[0161] In experiments 9-18, which did not meet conditions (a) and (b), at least one of the following evaluations—storage stability, dispensing stability, and shearing aggregation—decreased. In experiments 10, 12, and 14, filter clogging was suppressed to some extent, but storage stability could not be improved. δ H Although the high affinity of the pigment dispersant for water in the ink made micelle formation difficult, it is thought that the pigment dispersant attached to the pigment separated during storage, causing the pigment to aggregate.

[0162] In Experiment 19, the ink was passed through the filter only once, so no clogging of the filter occurred. [Industrial applicability]

[0163] By using the aqueous inkjet ink of the present invention, in an image forming method in which aqueous inkjet ink that has passed through a filter multiple times is ejected from the nozzle of an inkjet head, clogging of the filter can be suppressed, and the storage stability and ejection stability of the ink can be improved. Therefore, the present invention is useful, for example, in an image forming method using a circulating channel. [Explanation of Symbols]

[0164] 100 Image forming apparatus 101 Inkjet Head 102 Ink Tanks 103 Liquid transfer pump 104 Ink channel 105 Conveying device 106 Dryer M recording medium

Claims

1. A water-based inkjet ink for use in an image forming method in which ink that has passed through a filter multiple times is ejected from the nozzle of an inkjet head, The aforementioned aqueous inkjet ink comprises a pigment, a pigment dispersant, and an organic solvent. The conditions (a) and (b) are met, Water-based inkjet ink. Condition (a) The pigment dispersant has a hydrogen bonding component with an SP value of 8 (MPa) １／２ The above is 23 MPa. １／２ The following is Condition (b) The organic solvent has an SP value of 18 (MPa) １／２ Includes the following solvents

2. The aqueous inkjet ink according to claim 1, wherein the aqueous inkjet ink passes through the same filter multiple times in the image forming method.

3. The aqueous inkjet ink according to claim 1 or 2, wherein the pigment dispersant is a block copolymer having a hydrophilic block and a hydrophobic block.

4. The aqueous inkjet ink according to any one of claims 1 to 3, wherein the pigment content is 5% by mass or more relative to the total mass of the aqueous inkjet ink.

5. The aqueous inkjet ink according to any one of claims 1 to 4, wherein the content of the pigment dispersant is 25% by mass or more with respect to the total mass of the pigment.

6. The aqueous inkjet ink according to any one of claims 1 to 5, wherein the content of the organic solvent is 5% by mass or more and 25% by mass or less based on the total mass of the aqueous inkjet ink.

7. The hydrogen bonding component of the SP value is 18 (MPa). １／２ The aqueous inkjet ink according to any one of claims 1 to 6, wherein the content of the solvent is 5% by mass or more and 25% by mass or less with respect to the total mass of the aqueous inkjet ink.

8. An image forming method comprising ejecting water-based inkjet ink that has passed through a filter multiple times from the nozzles of an inkjet head, The process comprises the step of ejecting the aqueous inkjet ink described in any one of claims 1 to 7 from the nozzle of the inkjet head, Image forming method.

Images