Inkjet inks, ink sets, and inkjet recording devices

Abstract

We provide inkjet inks with excellent image density, storage stability, and ejection stability. [Solution] The inkjet ink contains a pigment, a binder resin made of styrene-(meth)acrylic acid copolymer, and a first aqueous medium containing a hydrophobic solvent and water. The inkjet ink has a 30% dry viscosity of 120 mPa·s or more and 3800 mPa·s or less. The inkjet ink has a binder resin content of 0.50% by mass or more and 3.00% by mass or less. The binder resin has a styrene ratio of 20% by mass or more and 40% by mass or less, and a (meth)acrylic acid ratio of 60% by mass or more and 80% by mass or less. The inkjet ink has an HSP distance of 8.0 or more and 12.0 or less between the binder resin and the hydrophobic solvent.

Description

[Technical Field]

[0001] The present invention relates to inkjet ink, ink set, and inkjet recording device. [Background technology]

[0002] Patent documents 1 and 2 disclose techniques for improving the image density of an image formed on a recording medium using inkjet inks. The inkjet inks described in patent documents 1 and 2 are configured to dry and thicken rapidly after landing on the recording medium. As a result, the pigments in the inkjet inks described in patent documents 1 and 2 tend to remain on the surface of the recording medium, making it easier to obtain images with high image density on the recording medium. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2016-175951 [Patent Document 2] Patent No. 4794830 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, with inkjet inks, if the ink is designed to easily dry out and thicken, storage stability and ejection stability tend to decrease. In other words, with inkjet inks, there is often a trade-off relationship between the image density in the image formed on the recording medium and storage stability and ejection stability. Patent documents 1 and 2 do not address the suppression of the decrease in storage stability and ejection stability in inkjet inks.

[0005] In view of the above circumstances, the object of the present invention is to provide an inkjet ink that is excellent in image density, storage stability and ejection stability, as well as an ink set and an inkjet recording device using the same. [Means for solving the problem]

[0006] To achieve the above objective, an inkjet ink according to one embodiment of the present invention contains a pigment, a binder resin made of styrene-(meth)acrylic acid copolymer, and a first aqueous medium containing a hydrophobic solvent and water. The above inkjet ink has a viscosity of 120 mPa·s to 3800 mPa·s at 25°C after a drying treatment in which the mass is heated at 40°C until it decreases by 30%. In the above-mentioned inkjet ink, the binder resin content is 0.50% by mass or more and 3.00% by mass or less. In the aforementioned binder resin, the proportion of styrene is 20% by mass or more and 40% by mass or less, and the proportion of (meth)acrylic acid is 60% by mass or more and 80% by mass or less. In the above-mentioned inkjet ink, the HSP distance between the binder resin and the hydrophobic solvent is 8.0 to 12.0.

[0007] The hydrophobic solvent may contain at least one of 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether. The total content of 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether may be 3.00% by mass or more and 25.00% by mass or less.

[0008] The LogP of the hydrophobic solvent may be greater than 0.00 and less than or equal to 1.50.

[0009] The viscosity at 25°C before the drying treatment may be 7.0 mPa·s or more and 10.0 mPa·s or less.

[0010] The first aqueous medium may further contain a first water-soluble organic solvent having a LogP of -2.00 or more and 0.00 or less.

[0011] In the inkjet ink, the total content of the hydrophobic solvent and the first water-soluble organic solvent may be more than 3.00% by mass and 50.00% by mass or less.

[0012] An ink set according to an embodiment of the present invention includes the inkjet ink and a cleaning liquid. The cleaning liquid contains 0.15% by mass or more and 10.50% by mass or less of poly(meth)acrylic acid, a surfactant, and a second aqueous medium containing water.

[0013] The content of the surfactant in the cleaning liquid may be 0.10% by mass or more and 1.00% by mass or less.

[0014] The second aqueous medium may further contain a second water-soluble organic solvent having a LogP of -2.00 or more and 0.00 or less.

[0015] The content of the second water-soluble organic solvent in the cleaning liquid may be 5.00% by mass or more and 30.00% by mass or less.

[0016] The first aqueous medium and the second aqueous medium may contain a common water-soluble organic solvent having a LogP of -2.00 or more and 0.00 or less.

[0017] An inkjet recording apparatus according to an embodiment of the present invention uses the above ink set. The inkjet recording apparatus includes a recording head having a recording unit that discharges the inkjet ink onto a recording medium and a cleaning unit that cleans the ink discharge surface of the recording unit with the cleaning liquid.

Advantages of the Invention

[0018] The present invention can provide an inkjet ink with excellent image density, storage stability, and ejection stability, as well as an ink set and an inkjet recording device using the same. [Brief explanation of the drawing]

[0019] [Figure 1] This figure shows an example of an inkjet recording device according to one embodiment of the present invention. [Figure 2] This figure shows the recording head and the underside of the head housing. [Figure 3] This is a side view of the first recording head in Figure 1. [Figure 4] This figure shows one step of the cleaning operation performed by the inkjet recording device shown in Figure 1. [Figure 5] This diagram shows the next step after Figure 4. [Figure 6] This diagram shows the next step after Figure 5. [Figure 7] This diagram shows the next step after Figure 6. [Modes for carrying out the invention]

[0020] Embodiments of the present invention will now be described. In this invention, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic," meaning that "(meth)acrylic" includes both "acrylic" and "methacrylic."

[0021] [Inkjet ink] (Schematic configuration) An inkjet ink according to one embodiment of the present invention (hereinafter also simply referred to as "ink") contains a pigment, a binder resin, and a first aqueous medium. The ink according to this embodiment is typically an aqueous ink that is ejected from the recording head of an inkjet recording device onto a recording medium to form an image on the recording medium. The recording medium on which an image is formed with the ink according to this embodiment is an ink-permeable recording medium composed of fibers such as cellulose fibers, and examples include plain paper, copy paper, recycled paper, thin paper, and thick paper.

[0022] In the ink according to this embodiment, viscosity is controlled to achieve both high image density in the image formed on the recording medium and high storage stability and ejection stability. Specifically, in the ink according to this embodiment, a drying process is performed in which the ink is heated at 40°C until the total mass of the ink decreases by 30%, and the viscosity obtained by measuring at 25°C after the drying process (hereinafter also referred to as "30% dry viscosity") is controlled.

[0023] In this embodiment, the ink has a 30% dry viscosity of 120 mPa·s or higher, from the viewpoint of improving the image density in the image formed on the recording medium. Furthermore, in this embodiment, the ink has a 30% dry viscosity of 3800 mPa·s or lower, from the viewpoint of improving storage stability and ejection stability, as well as making it easier to remove from the ink ejection surface of the recording head with a cleaning solution.

[0024] Furthermore, in the ink according to this embodiment, the viscosity measured at 25°C before the drying process (hereinafter also referred to as "initial viscosity") is preferably 7.0 mPa·s or higher from the viewpoint of improving the image density in the image formed on the recording medium. Also, in the ink according to this embodiment, the initial viscosity is preferably 10.0 mPa·s or lower from the viewpoint of improving ejection stability.

[0025] In the ink according to this embodiment, the binder resin is composed of styrene-(meth)acrylic resin (styrene-(meth)acrylic acid copolymer), and the first aqueous medium contains a hydrophobic solvent. In the ink according to this embodiment, the above-mentioned 30% dry viscosity can be achieved mainly by the composition of the binder resin and the compatibility of the combination of the binder resin and the hydrophobic solvent, and a configuration can be achieved that combines high image density in the image formed on the recording medium with high storage stability and ejection stability.

[0026] (Pigment) Examples of pigments to be incorporated into the ink according to this embodiment include yellow pigment, orange pigment, red pigment, blue pigment, purple pigment, and black pigment. Examples of yellow pigments include CI Pigment Yellow (74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, and 193). Examples of orange pigments include CI Pigment Orange (34, 36, 43, 61, 63, and 71). Examples of red pigments include CI Pigment Red (122 and 202). Examples of blue pigments include CI Pigment Blue (15, more specifically 15:3). Examples of purple pigments include CI Pigment Violet (19, 23, and 33). Examples of black pigments include CI Pigment Black (7).

[0027] In the ink according to this embodiment, the pigment content is preferably 2.00% by mass or more and 15.00% by mass or less, and more preferably 5.00% by mass or more and 10.00% by mass or less. In the ink according to this embodiment, by setting the pigment content to 2.00% by mass or more, it is easier to obtain a high image density in the image formed on the recording medium. In addition, in the ink according to this embodiment, by setting the pigment content to 15.00% by mass or less, it is easier to remove from the ink ejection surface of the recording head by the cleaning fluid.

[0028] Furthermore, the ink according to this embodiment may also contain pigment particles together with a pigment-coated resin blended as a dispersant. The pigment particles are composed of, for example, a core containing pigment and a pigment-coated resin coating the core. The pigment-coated resin is dispersed in, for example, a first aqueous medium. In the ink according to this embodiment, from the viewpoint of improving color density, hue, or stability, the median volume diameter (D50) of the pigment particles is preferably 30 nm or more and 300 nm or less, and more preferably 90 nm or more and 130 nm or less.

[0029] (Binder resin) In the ink according to this embodiment, by incorporating a styrene-(meth)acrylic resin of a specific configuration as the binder resin, it becomes easier to achieve both high image density in the image formed on the recording medium and high storage stability and ejection stability. In the binder resin incorporated into the ink according to this embodiment, the ratio of styrene to (meth)acrylic acid with respect to the total repeating units is in the range of 20:80 to 40:60. That is, in the binder resin incorporated into the ink according to this embodiment, the ratio of styrene to the total repeating units is 20% by mass or more and 40% by mass or less, and the ratio of (meth)acrylic acid to the total repeating units is 60% by mass or more and 80% by mass or less. In the ink according to this embodiment, the average particle size of the binder resin is preferably about 100 nm, and more specifically, it is preferably 50 nm or more and 150 nm or less from the viewpoint of stability and ejection performance.

[0030] In the ink according to this embodiment, the binder resin content is 0.50% by mass or more in order to effectively obtain the effects of the binder resin described above. Furthermore, in the ink according to this embodiment, the binder resin content is 3.00% by mass or less in order to prevent an excessive increase in viscosity.

[0031] (1st aqueous medium) The first aqueous medium incorporated into the ink according to this embodiment is a medium containing a hydrophobic solvent and water. The first aqueous medium may function as a solvent or as a dispersion medium. In addition to the hydrophobic solvent and water, the first aqueous medium may further contain a first water-soluble organic solvent as an organic solvent other than the hydrophobic solvent. In the ink according to this embodiment, it is preferable that the total content of the hydrophobic solvent and the first water-soluble organic solvent is greater than 3.00% by mass and 50.00% by mass or less.

[0032] In the ink according to this embodiment, the octanol / water partition coefficient LogP (hereinafter sometimes simply referred to as LogP) of the hydrophobic solvent is preferably greater than 0.00 and less than or equal to 1.50, and more preferably between 0.20 and 0.40. In the ink according to this embodiment, by setting the LogP of the hydrophobic solvent to greater than 0.00 and less than or equal to 1.50, the ink exhibits high hydrophobicity when it dries and the moisture content decreases. Therefore, with the ink according to this embodiment, it is easier to obtain high image density in the image formed on the recording medium. In this embodiment, for organic solvents for which literature values ​​exist, the literature value can be used as LogP, and for organic solvents for which there are no literature values, a value calculated by calculation software (for example, "ChemDraw" from PerkinElmer) can be used.

[0033] The hydrophobic solvent incorporated into the ink according to this embodiment preferably contains at least one of 3-methyl-1,5-pentanediol (LogP: 0.35) and triethylene glycol monobutyl ether (LogP: 0.44). Furthermore, in the ink according to this embodiment, the total content of 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether is preferably 3.00% by mass or more and 25.00% by mass or less, more preferably 3.00% by mass or more and 25.00% by mass or less, and more preferably 8.00% by mass or more and 20.00% by mass or less. This makes it easier to adjust the 30% dry viscosity of the ink according to this embodiment to the above range.

[0034] The first water-soluble organic solvent incorporated into the ink according to this embodiment is an organic solvent with a LogP of 0.00 or less. The LogP of the first water-soluble organic solvent is preferably between -2.00 and 0.00, and more preferably between -1.50 and -0.50. The first water-soluble organic solvent incorporated into the ink according to this embodiment preferably includes at least one of 1,3-propanediol (LogP: -1.04) and 1,2-propanediol (LogP: -0.92).

[0035] In the ink according to this embodiment, the content of the first water-soluble organic solvent is preferably 3.00% by mass or more and 30.00% by mass or less, and more preferably 15.00% by mass or more and 25.00% by mass or less. By including the first water-soluble organic solvent in a proportion of 3.00% by mass or more and 30.00% by mass or less, the 30% dry viscosity of the ink according to this embodiment can be easily adjusted within the above range.

[0036] In the ink according to this embodiment, the total content of the hydrophobic solvent and the first water-soluble organic solvent is preferably more than 3.00% by mass and 50.00% by mass or less, and more preferably 25.00% by mass or more and 35.00% by mass or less.

[0037] In the ink according to this embodiment, the water content is preferably 40.00% by mass or more and 80.00% by mass or less, and more preferably 55.00% by mass or more and 65.00% by mass or less. In the ink according to this embodiment, by setting the water content to 40.00% by mass or more and 80.00% by mass or less, the ejection stability by the inkjet recording device can be improved.

[0038] (Combination of binder resin and hydrophobic solvent) In this embodiment, the combination of binder resin and hydrophobic solvent that is compatible is determined by the similarity of the physical properties of the binder resin and the hydrophobic solvent. In this embodiment, the similarity of the physical properties of the binder resin and the hydrophobic solvent is determined using the Hansen solubility parameter (HSP).

[0039] The Hansen solubility parameter is a value used to predict the solubility of substances and is composed of the following three parameters. ·Dispersion term δ D : Energy due to intermolecular dispersion forces ·Polarization term δ P : Energy due to dipole-dipole interactions between molecules ·Hydrogen bonding term δ H : Energy due to hydrogen bonding between molecules The three parameters that make up the Hansen solubility parameter can be regarded as coordinates in a three-dimensional space (Hansen space). When two specific substances are placed in the Hansen space, the closer the distance Ra between the two substances (hereinafter also referred to as "HSP distance R" a ), the more similar the properties of the two substances tend to be.

[0040] In this embodiment, the proximity of the physical properties between the binder resin and the hydrophobic solvent is determined by the length of the HSP distance R between the binder resin and the hydrophobic solvent. The HSP distance R between the binder resin and the hydrophobic solvent a can be obtained as the distance between the coordinates of the binder resin and the coordinates of the hydrophobic solvent plotted in the Hansen space. That is, the HSP distance R between the binder resin and the hydrophobic solvent a can be calculated by the following formula using the dispersion term δ a in the binder resin, the polarization term δ D1 , and the hydrogen bonding term δ P1 , as well as the dispersion term δ H1 , the polarization term δ D2 , and the hydrogen bonding term δ P2 in the hydrophobic solvent. H2

[0041]

Equation

[0042] In the ink according to this embodiment, the HSP distance R between the binder resin and the hydrophobic solvent a ​The HSP distance R between the binder resin and the hydrophobic solvent is 8.0 or greater and 12.0 or less. In the ink according to this embodiment, the HSP distance R between the binder resin and the hydrophobic solvent is 8.0 or greater and 12.0 or less. a By setting the HSP distance R to 12.0 or less, the viscosity can be increased as the binder resin dissolves into the first aqueous medium during drying (water evaporation). Furthermore, in the ink according to this embodiment, the HSP distance R between the binder resin and the hydrophobic solvent is a Setting the value to 8.0 or higher makes it easier to ensure storage stability and dispensing stability.

[0043] If the Hansen solubility parameter of at least one of the binder resin and hydrophobic solvent is unknown, the Hansen solubility parameter can be measured by the following method. First, place 1 part by mass of the substance whose Hansen solubility parameter is to be measured (hereinafter sometimes referred to as the target substance) and 49 parts by mass of a solvent for which the Hansen solubility parameter is known (for example, a solvent for which literature values ​​exist) into a sealable container. Next, thoroughly mix the target substance and solvent by handshake the container. Next, leave the container to stand for 12 hours at room temperature (23°C). Next, invert the container and observe the bottom surface. If there are no precipitates or aggregates on the bottom surface of the container, it is determined that the solvent has dissolved the target substance. Repeat this test while appropriately changing the type of solvent. This will determine 10 combinations of solvents, consisting of solvents that dissolve the target substance and solvents that do not dissolve the target substance. Ideally, the combination of 10 solvents should consist of approximately half (for example, 4-6) solvents that dissolve the target substance, with the remaining solvents not dissolving it. Based on the test results for the 10 solvents, a sphere called a Hansen sphere is drawn in the Hansen space.

[0044] This section explains how to draw a Hansen sphere. A sphere (Hansen sphere) is drawn in Hansen space that includes the coordinates of the solvent in which the target substance was dissolved, but does not include the coordinates of the solvent in which the target substance was not dissolved. The coordinates of the center of the drawn Hansen sphere represent the Hansen solubility parameter of the target substance. The size of the Hansen sphere varies depending on the type of target substance. Specifically, target substances that dissolve in various solvents with different properties have a large radius R0 of the Hansen sphere. Conversely, target substances that dissolve only in solvents with limited, specific properties have a small radius R0 of the Hansen sphere.

[0045] (Other ingredients) The ink according to this embodiment may contain components other than those listed above, as needed. For example, the ink according to this embodiment may contain a surfactant. By incorporating a surfactant into the ink according to this embodiment, the compatibility and dispersion stability of each component can be improved. Furthermore, the surfactant in the ink according to this embodiment can impart wettability to the recording medium. Nonionic surfactants are preferred as the surfactant to be incorporated into the ink according to this embodiment.

[0046] Examples of nonionic surfactants to be incorporated into the ink according to this embodiment include acetylene glycol surfactants (surfactants containing acetylene glycol compounds), silicone surfactants (surfactants containing silicone compounds), and fluorine surfactants (surfactants containing fluororesins or fluorine-containing compounds). Examples of acetylene glycol surfactants include ethylene oxide adducts of acetylene glycol and propylene oxide adducts of acetylene glycol. It is preferable that the ink contains an acetylene glycol surfactant.

[0047] In the ink according to this embodiment, the surfactant content is preferably 0.10% by mass or more and 1.00% by mass or less, and more preferably 0.30% by mass or more and 0.60% by mass or less.

[0048] Furthermore, in addition to surfactants, the ink according to this embodiment may contain various additives as needed, such as dissolution stabilizers, drying inhibitors, antioxidants, viscosity modifiers, pH adjusters, neutralizing agents, and antifungal agents.

[0049] [Ink Set] An ink set according to one embodiment of the present invention comprises the ink according to the above embodiment and a cleaning fluid. In the ink set according to this embodiment, the ink and the cleaning fluid are stored separately and used separately without being mixed. More specifically, the ink and cleaning fluid constituting the ink set according to this embodiment are configured such that the ink adhering to the ink ejection surface of the recording head can be effectively removed by the cleaning fluid.

[0050] [Cleaning solution] (Brief explanation) The cleaning fluid for the ink set according to this embodiment contains poly(meth)acrylic acid, a surfactant, and a second aqueous medium. The cleaning fluid according to this embodiment is configured to effectively remove the ink according to the above embodiment, which tends to thicken upon drying, from the ink ejection surface of the recording head.

[0051] (Poly(meth)acrylic acid) In the cleaning solution according to this embodiment, poly(meth)acrylic acid has the function of enhancing the cleaning effect of the cleaning solution on the ink according to the above embodiment, which is prone to thickening upon drying and has a high dry viscosity of 30%, and acts effectively as a builder agent. Furthermore, poly(meth)acrylic acid has a stronger effect of enhancing the cleaning effect of the cleaning solution on the ink according to the above embodiment compared to general builders. For this reason, in the ink set according to this embodiment, by combining the ink according to the above embodiment with the cleaning solution according to this embodiment, it is possible to effectively remove ink adhering to the ink ejection surface of the recording head with the cleaning solution.

[0052] In the cleaning solution according to this embodiment, the poly(meth)acrylic acid content is preferably 0.15% by mass or more and 10.50% by mass or less, and preferably 0.15% by mass or more and 5.00% by mass or less. In the cleaning solution according to this embodiment, by setting the poly(meth)acrylic acid content to 0.15% by mass or more and 10.50% by mass or less, it becomes easier to remove ink from the ink ejection surface of the recording head. This makes it possible to improve the ejection stability of the inkjet recording device.

[0053] In the cleaning solution according to this embodiment, the mass-average molecular weight of poly(meth)acrylic acid is preferably 3,000 to 27,000, and more preferably 3,000 to 7,000. By setting the mass-average molecular weight of poly(meth)acrylic acid to 3,000 to 27,000 in the cleaning solution according to this embodiment, the ink can be more reliably removed from the ink ejection surface of the recording head by the cleaning solution. This improves the ink ejection stability of the inkjet recording device.

[0054] In the cleaning solution according to this embodiment, the mass-average molecular weight of poly(meth)acrylic acid can be adjusted by changing the polymerization conditions of poly(meth)acrylic acid (more specifically, the amount of polymerization initiator used, polymerization temperature, polymerization time, etc.). In the cleaning solution according to this embodiment, the poly(meth)acrylic acid may be partially or completely neutralized by a basic compound. In this embodiment, the mass-average molecular weight is the value measured using gel permeation chromatography.

[0055] (Surfactants) The surfactant incorporated into the cleaning solution according to this embodiment has the ability to adhere to the dirt (pigment, etc.) fixed to the object to be cleaned (ink ejection surface), to release the dirt from the object to be cleaned, and to disperse the released dirt in the cleaning solution.

[0056] Examples of surfactants to be incorporated into the cleaning solution according to this embodiment include those similar to the surfactants exemplified in the description of the ink. Among the surfactants mentioned above, silicone surfactants (surfactants containing silicone compounds) are preferred as the surfactant in the cleaning solution.

[0057] In the cleaning solution according to this embodiment, the surfactant content is preferably 0.10% by mass or more and 1.00% by mass or less, and more preferably 0.30% by mass or more and 0.60% by mass or less.

[0058] (Second aqueous medium) The second aqueous medium incorporated into the cleaning solution according to this embodiment is a water-containing medium, and preferably a water-containing medium containing a second water-soluble organic solvent and water. The second aqueous medium may function as a solvent or as a dispersion medium.

[0059] In the cleaning fluid according to this embodiment, the water content is preferably 55.00% by mass or more and 95.00% by mass or less, and more preferably 75.00% by mass or more and 85.00% by mass or less. In the cleaning fluid according to this embodiment, by setting the water content to 55.00% by mass or more and 95.00% by mass or less, the ink can be more easily removed from the ink ejection surface of the recording head. This can improve the ink ejection stability of the inkjet recording device.

[0060] The second water-soluble organic solvent blended into the cleaning solution according to this embodiment is an organic solvent with a LogP of 0.00 or less, similar to the first water-soluble organic solvent. The LogP of the second water-soluble organic solvent is preferably between -2.00 and 0.00, and more preferably between -1.50 and -0.50.

[0061] Examples of the second water-soluble organic solvent to be incorporated into the cleaning solution according to this embodiment include water-soluble organic solvents similar to those exemplified in the description of the first water-soluble organic solvent above. In the ink set according to this embodiment, the first water-soluble organic solvent of the ink and the second water-soluble organic solvent of the cleaning solution may be the same solvent or different solvents, but it is preferable that they be the same solvent. 1,3-propanediol is preferred as the second water-soluble organic solvent to be incorporated into the cleaning solution according to this embodiment. Therefore, in the ink set according to this embodiment, it is more preferable that both the first water-soluble organic solvent of the ink and the second water-soluble organic solvent of the cleaning solution are 1,3-propanediol.

[0062] In the cleaning solution according to this embodiment, the content of the second water-soluble organic solvent is preferably 5.00% by mass or more and 30.00% by mass or less, and more preferably 15.00% by mass or more and 25.00% by mass or less. By containing the second water-soluble organic solvent in a proportion of 5.00% by mass or more and 35.00% by mass or less, the cleaning solution according to this embodiment makes it easier to remove ink from the ink ejection surface of the recording head. This improves the ink ejection stability of the inkjet recording device.

[0063] (Other ingredients) The cleaning solution according to this embodiment may further contain a basic compound as an additive (pH adjuster) as needed. Sodium hydroxide is preferred as the basic compound to be incorporated into the cleaning solution according to this embodiment. Furthermore, the cleaning solution according to this embodiment may further contain other known additives as needed (more specifically, for example, dissolution stabilizers, drying inhibitors, antioxidants, viscosity modifiers, and antifungal agents).

[0064] [Inkjet recording device] In the inkjet recording apparatus 1 according to one embodiment of the present invention, the ink set according to the above embodiment is used. That is, in the inkjet recording apparatus 1, the ink according to the above embodiment is used as at least one of the inks for forming an image on the recording medium, and the cleaning fluid of the ink set according to the above embodiment is used for cleaning the ink ejection surface of the recording head. The inkjet recording apparatus 1 will be described below with reference to the drawings. Note that the drawings shown are schematic representations mainly of the components for ease of understanding, and the size, number, etc. of each component shown may differ from the actual dimensions due to the convenience of drawing creation. Also, the size, number, etc. of each component shown may be changed as appropriate.

[0065] Figure 1 shows the main components of an inkjet recording device 1. The inkjet recording device 1 shown in Figure 1 comprises a recording head 2, a head housing 3 that holds the recording head 2, and a transport unit 4 that transports a recording medium (not shown). The recording head 2 has a first recording head 2a, a second recording head 2b, a third recording head 2c, and a fourth recording head 2d. The transport unit 4 has a pair of transport rollers, a first roller 4a and a second roller 4b, and a transport belt 4c that is stretched between the first roller 4a and the second roller 4b. The transport unit 4 transports the recording medium placed on the transport belt 4c in a certain direction (to the right, indicated by the arrow in Figure 1). Hereinafter, the direction in which the recording medium is transported may be referred to as the transport direction X. The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d eject ink to form an image when the recording medium is transported directly below them.

[0066] The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d are each supported at a height such that the distance from the upper surface of the conveyor belt 4c is a predetermined length.

[0067] The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d each record (form) an image on a recording medium being transported on a transport belt 4c. The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d each have an ink tank (not shown). The ink tanks of the first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d each contain four different colors of ink (black, cyan, magenta, and yellow) (first ink, second ink, third ink, and fourth ink). The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d each inkjet-dispense the ink of each color stored in the ink tank (not shown) onto the recording medium through nozzles. The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d each eject inkjet material onto the recording medium in a predetermined order, with a fixed ejection interval between them. This forms a color image on the recording medium.

[0068] Figure 2 shows the lower surface of the recording head 2 and head housing 3 in Figure 1. The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d each extend in a direction perpendicular to the transport direction X (hereinafter sometimes referred to as the width direction A).

[0069] The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d have almost identical structures. Therefore, the details of the recording head 2 will be explained below using the first recording head 2a as an example. However, the same explanation applies to the second recording head 2b, the third recording head 2c, and the fourth recording head 2d.

[0070] Figure 3 is a side view of the first recording head 2a shown in Figure 1. As shown in Figure 3, the first recording head 2a includes a recording unit 5 that ejects ink onto the image forming area of ​​the recording medium, and a cleaning unit 6 that cleans the ink ejection surface F1 of the recording unit 5 with a cleaning solution.

[0071] Multiple nozzles (not shown) for ejecting ink are arranged on the ink ejection surface F1 on the lower surface of the recording unit 5.

[0072] As shown in Figure 3, the cleaning unit 6 includes a cleaning fluid supply unit 6a and a wiper 6b, which are located near the ink ejection surface F1 of the recording unit 5. The cleaning fluid supply surface F2 on the lower surface of the cleaning fluid supply unit 6a is provided with a plurality of cleaning fluid discharge holes (not shown) for discharging cleaning fluid. The cleaning fluid supply unit 6a supplies cleaning fluid stored in a cleaning fluid tank (not shown) to the cleaning fluid supply surface F2 through each cleaning fluid discharge hole. The wiper 6b has the function of wiping the ink ejection surface F1. The wiper 6b is, for example, a rubber wiper. The linear pressure of the wiper 6b is, for example, 5.0 N / m or more and 13.0 N / m or less.

[0073] Figures 4 to 7 show a series of steps in the cleaning operation of the inkjet recording device 1. As shown in Figure 4, in the cleaning operation, first a small amount of ink I is purged from the recording unit 5 (purging operation). This clears any nozzle clogging in the recording unit 5. The purged ink I adheres to the ink ejection surface F1 of the recording unit 5. Simultaneously with the purging operation, cleaning fluid C is supplied from the cleaning fluid supply unit 6a during the cleaning operation (cleaning fluid supply operation). The supplied cleaning fluid C adheres to the area near the ink ejection surface F1 of the recording unit 5 (the cleaning fluid supply surface F2 on the underside of the cleaning fluid supply unit 6a).

[0074] Next, as shown in Figure 5, the wiper 6b is pressed against the cleaning fluid supply surface F2 on the lower surface of the cleaning fluid supply unit 6a. Then, as shown in Figure 6, the wiper 6b moves horizontally (to the left in Figure 6). This causes the wiper 6b to wipe away the cleaning fluid C adhering to the cleaning fluid supply surface F2 and the ink I adhering to the ink ejection surface F1 (wiping action). At this time, the cleaning fluid C mixes with the ink I. As a result, as shown in Figure 7, the cleaning fluid C adhering to the cleaning fluid supply surface F2 and the ink I adhering to the ink ejection surface F1 are removed. Thus, the recording unit 5 is cleaned by the cleaning fluid C.

[0075] The above describes the series of steps involved in the cleaning operation of the inkjet recording device 1. The second recording head 2b, third recording head 2c, and fourth recording head 2d also have a recording unit 5 that ejects ink and a cleaning unit 6 that cleans the ink ejection surface F1 of the recording unit 5 with cleaning fluid, similar to the first recording head 2a. The cleaning unit 6 for the second recording head 2b, third recording head 2c, and fourth recording head 2d also includes a cleaning fluid supply unit 6a located near the ink ejection surface F1 of the recording unit 5 and a wiper 6b.

[0076] The inkjet recording apparatus 1 according to this embodiment is not limited to those shown in Figures 1 to 7.

[0077] The inkjet recording device 1 shown in Figures 1 to 7 was equipped with four line-type recording heads 2 corresponding to four colors of ink. However, the number of recording heads 2 equipped in the inkjet recording device 1 is not particularly limited and can be, for example, one to ten, and preferably three to five.

[0078] Furthermore, the type, combination, and ejection sequence of inks in the inkjet recording device 1 are not particularly limited.

[0079] Furthermore, in the inkjet recording device 1 shown in Figures 1 to 7, the cleaning unit 6 had a cleaning liquid supply unit 6a located near the ink ejection surface F1 of the recording unit 5, and a wiper 6b. However, the inkjet recording device 1 may also perform a wiping operation by directly applying the cleaning liquid to the wiper 6b. The cleaning unit 6 of the inkjet recording device 1 only needs to be able to clean the ink ejection surface F1 of the recording unit 5 with the cleaning liquid.

[0080] Furthermore, the inkjet recording device 1 may be a multifunction device that also has the functions of a scanner, copier, printer, or facsimile.

[0081] As the recording medium for image formation in the inkjet recording device 1, a permeable recording medium (for example, plain paper) is preferred. When image formation is performed on plain paper with a known inkjet recording device (in particular, a known inkjet recording device equipped with a line-type recording head), the image density tends to be insufficient. In contrast, the inkjet recording device 1 according to this embodiment can form an image with the desired image density even when image formation is performed on plain paper.

[0082] [Examples and Comparative Examples] Ink preparation and evaluation were carried out as examples and comparative examples of the present invention. Note that the following examples merely illustrate one example of the present invention, and the present invention is not limited to the configurations of the following examples.

[0083] (Ink preparation) Preparation of pigment dispersion 18.00 parts by mass of carbon black (Cabot's "Black Pearls® 800") as a black pigment, 4.00 parts by mass of pigment coating resin (Bic Chemie Japan Co., Ltd.'s "DISPERBYK®-190"), and 78.00 parts by mass of ion-exchanged water were mixed using a disperser. The resulting mixture was dispersed using a bead mill until the D50 of the pigment particles reached 110 nm. The D50 of the pigment particles was measured using a dynamic light scattering particle size distribution analyzer (Malvern's "Zetasizer® Nano ZS"). This yielded a black pigment dispersion.

[0084] Synthesis of binder resins b1-b5 First, the binder resin b1 was synthesized. A four-necked flask was equipped with a stirrer, a nitrogen inlet tube, a condenser, a stirrer, and a dropping funnel. This four-necked flask served as the reaction vessel. Next, 100.0 g of isopropyl alcohol and 300.0 g of methyl ethyl ketone were added to the reaction vessel. Then, the contents of the reaction vessel were heated to 70°C (reflux) while bubbling with nitrogen gas. Separately, 20.0 g of styrene, 40.0 g of acrylic acid, 20.0 g of methyl methacrylate, and 0.400 g of azobisisobutyronitrile (AIBN, polymerization initiator) were mixed to obtain a monomer solution. Next, while maintaining the temperature of the contents of the reaction vessel at 70°C (reflux), the above monomer solution was added dropwise to the reaction vessel over approximately 2 hours. After dropwise addition, the temperature of the contents of the reaction vessel was maintained at 70°C (under reflux) for 6 hours.

[0085] Next, while maintaining the temperature of the reaction vessel contents at 70°C (reflux), 150 g of a methyl ethyl ketone solution containing 0.200 g of AIBN was added dropwise to the reaction vessel over 15 minutes. After the addition, the temperature of the reaction vessel contents was maintained at 70°C (reflux) for 5 hours. Then, the contents of the reaction vessel were subjected to reduced pressure while stirring to remove MEK. This yielded binder resin b1.

[0086] Binder resins b2 to b5 were synthesized in the same manner as binder resin b1. For binder resins b1 to b5, the amounts of styrene and (meth)acrylic acid were varied. Table 1 shows the amounts of styrene and (meth)acrylic acid in binder resins b1 to b5. Table 1 also shows the HSP distance R between 3-methyl-1,5-pentanediol (MPD) and binder resins b1 to b5, calculated for each. a This is also shown. As shown in Table 1, the HSP distance R of binder resins b1 to b3 a While the HSP distance R of the binder resin b4 was between 8.0 and 12.0, a The HSP distance R of the binder resin b5 is less than 8.0. a It was over 12.0.

[0087] [Table 1]

[0088] • Ink preparation The pigment dispersion, binder resin, hydrophobic solvent, water-soluble organic solvent, surfactant, and water were mixed to obtain a mixture, which was then stirred at 400 rpm using a stirrer ("Three One Motor BL-600" manufactured by Shinto Kagaku Co., Ltd.). Foreign matter and coarse particles were removed using a filter with a pore size of 5 μm to obtain the inks according to the examples and comparative examples. In both the examples and comparative examples, an acetylene surfactant ("Surfinol® 420" manufactured by Nisshin Chemical Industry Co., Ltd.) was used as the surfactant.

[0089] (Viscosity measurement) • Method for measuring initial viscosity The viscosity (initial viscosity) of the ink at 25°C before drying was measured using a torque-balanced servo-type rotational viscometer (TV-100EL, manufactured by Toki Sangyo Co., Ltd.).

[0090] • Method for measuring 30% dry viscosity 100.00 parts by mass of the ink to be measured were placed in a petri dish. The petri dish was placed in a 40°C oven for drying. During the drying process, the mass of the ink to be measured in the petri dish was measured over time. The drying process was terminated when the mass of the ink to be measured in the petri dish decreased to 70.00 parts by mass (a decrease of 30.00 mass%). The viscosity of the ink at 25°C after drying (30% dry viscosity) was measured using a torque-balanced servo-type rotational viscometer (TV-100EL, manufactured by Toki Sangyo Co., Ltd.).

[0091] (Evaluation method) The inks used in the examples and comparative examples were evaluated for storage stability, ejection stability, and image density.

[0092] ·Storage stability The viscosity (initial viscosity V1) of the ink to be evaluated was measured using a vibrating viscometer (VM-200T, manufactured by Nittetsu Hokkaido Control Systems Co., Ltd.). Next, approximately 30g of the ink to be evaluated was placed in a 50mL container and sealed. The container was placed in a constant temperature incubator set to an internal temperature of 60°C and kept warm for one month. After that, the container was removed from the incubator and left to stand at room temperature for 3 hours. Then, the ink to be measured was removed from the container and its viscosity (post-treatment viscosity V2) was measured using the vibrating viscometer. Based on the measured initial viscosity V1 and post-treatment viscosity V2, the viscosity change rate (%) was calculated using the following formula. Viscosity change rate (%)=100×(V2-V1) / V1 For each ink, the absolute value of the viscosity change rate was used as the evaluation value for storage stability. Each ink's evaluation value was assessed according to the following A and B criteria. For storage stability, inks with an evaluation of A are considered acceptable, and inks with an evaluation of B are considered unacceptable. A (good): 5.0% or less B (Bad): More than 5.0%

[0093] ·Discharge stability A4-sized inkjet matte paper (Seiko Epson Corporation's "Super Fine Paper") was used as the evaluation paper. A 150mm x 200mm solid image (100% print density) was continuously printed using the evaluation machine. Next, the recording head of the evaluation machine was cleaned. Then, a stripe image formed by multiple parallel fine lines was formed on the recording medium using the evaluation machine. In forming the stripe image, the line width of the fine lines was set to 1 pixel, and the spacing between adjacent fine lines (line pitch) was set to 3 pixels. Next, the stripe image formed on the evaluation paper was read using a microscope. Specifically, the spacing A between a specific fine line a and a fine line b located 16 pixels away from fine line a was measured at 204 locations. Note that between fine line a and fine line b, Three other thin lines were present. The variability (3σ) of the measured interval A was calculated using image processing software (manufactured by Kyocera Document Solutions Inc.). For each ink, the calculated variability (3σ) of interval A was used as the evaluation value for ejection stability. Each ink's evaluation value was evaluated according to the following A and B criteria. For ejection stability, inks with an evaluation of A were considered acceptable, and inks with an evaluation of B were considered unacceptable. A (Good): Less than 15 B (bad): 15 or more

[0094] Image density Image density was evaluated under conditions of 25°C and 50% RH. For image density evaluation, a print test machine (manufactured by Kyocera Document Solutions Inc.) equipped with a line-type recording head that ejects ink using a piezo method was used. This image density evaluation machine was used with a recording medium (MONDI "Color Copy" 90g / m²). 2 A solid image was formed on the resulting solid image. The image density (ID value) of the formed solid image was measured using a fluorescence spectrophotometer (Konica Minolta, Inc. "FD-5"). For each ink, the ID value was used as the evaluation value for image density. Each ink's evaluation value was evaluated according to the following A and B criteria. For image density, an A rating for the ink was considered acceptable, and a B rating for the ink was considered unacceptable. A (good): 1.20 or more B (Poor): Less than 1.20

[0095] (Examples 1-10) In the inks of Examples 1 to 10, a pigment dispersion, binder resin, hydrophobic solvent, water-soluble organic solvent, surfactant, and water were blended to have the composition shown in Table 2. In the inks of Examples 1 to 5 and 7 to 10, 3-methyl-1,5-pentanediol (MPD) was used as the hydrophobic solvent. In the ink of Example 6, triethylene glycol monobutyl ether (BTG) was used as the hydrophobic solvent. In the ink of Example 6, the HSP distance R between the binder resin b2 and BTG was... a However, the value was 8.05, and was between 8.0 and 12.0. In addition, 1,3-propanediol (1,3PD) was used as the water-soluble organic solvent in all of the inks related to Examples 1 to 10.

[0096] [Table 2]

[0097] Table 3 shows the initial viscosity and 30% dry viscosity for the inks used in Examples 1 to 10. For all inks used in Examples 1 to 10, the 30% dry viscosity was between 120 mPa·s and 3800 mPa·s.

[0098] [Table 3]

[0099] Table 4 shows the evaluation results for storage stability, ejection stability, and image density for the inks of Examples 1 to 10. All of the inks of Examples 1 to 10 passed the evaluations for storage stability, ejection stability, and image density.

[0100] [Table 4]

[0101] (Comparative Examples 1-10) In Comparative Examples 1 to 10, the pigment dispersion, binder resin, hydrophobic solvent, water-soluble organic solvent, surfactant, and water were blended to have the compositions shown in Table 5. In all of the inks related to Comparative Examples 2 to 10, 3-methyl-1,5-pentanediol (MPD) was used as the hydrophobic solvent. In the ink related to Comparative Example 1, triethylene glycol monobutyl ether (BTG) was used as the hydrophobic solvent. In the ink related to Comparative Example 1, the HSP distance R between the binder resin b1 and BTG was... a The value was 7.45. In addition, in the inks related to Comparative Examples 1-4 and 6-10, 1,3-propanediol (1,3PD) was used as the water-soluble organic solvent.

[0102] [Table 5]

[0103] The inks in Comparative Examples 1 and 3 differ from the inks in the above examples in that the binder resin content is greater than 3.00% by mass. The ink in Comparative Example 2 differs from the ink in the above examples in that it does not contain binder resin. The inks in Comparative Examples 7 and 8 have an HSP distance R between the binder resin and the hydrophobic solvent. a The inks in Comparative Examples 9 and 10 differ from the inks in that the HSP distance R between the binder resin and the hydrophobic solvent is less than 8.0. a This differs from the ink according to the above embodiment in that the value is greater than 12.0.

[0104] Table 6 shows the initial viscosity and 30% dry viscosity for the inks related to Comparative Examples 1 to 10. The inks related to Comparative Examples 1, 3 to 7, and 9 all differ from the inks related to the above-mentioned examples in that their 30% dry viscosity is greater than 3800 mPa·s. In addition, the inks related to Comparative Examples 8 and 10 all differ from the inks related to the above-mentioned examples in that their 30% dry viscosity is less than 120 mPa·s.

[0105] [Table 6]

[0106] Table 7 shows the evaluation results for storage stability, ejection stability, and image density for the inks related to Comparative Examples 1 to 10. All of the inks related to Comparative Examples 1 to 10 failed in at least one of the following areas: storage stability, ejection stability, and image density. Furthermore, for the inks related to Comparative Examples 5, 7, and 10, normal ejection was not possible, and therefore, image density evaluation could not be performed.

[0107] [Table 7] [Explanation of Symbols]

[0108] 1… Inkjet recording device 2…Recording head 2a...First recording head 2b...Second recording head 2c...3rd record head 2d...4th recording head 3…Head housing 4…Conveyor unit 5…Records Department 6…Cleaning Department 6a...Cleaning fluid supply unit 6b... Wiper I... Ink C... Cleaning liquid F1...Ink ejection surface F2...Cleaning fluid supply surface

Claims

1. It contains a pigment, a binder resin made of styrene-(meth)acrylic acid copolymer, and a first aqueous medium containing a hydrophobic solvent and water. The viscosity at 25°C after a drying treatment in which the mass is reduced by 30% by heating at 40°C is between 120 mPa·s and 3800 mPa·s. The content of the binder resin is 0.50% by mass or more and 3.00% by mass or less. In the aforementioned binder resin, the ratio of styrene is 20% by mass or more and 40% by mass or less, and the ratio of (meth)acrylic acid is 60% by mass or more and 80% by mass or less. The HSP distance between the binder resin and the hydrophobic solvent is 8.0 or more and 12.0 or less. Inkjet ink.

2. An inkjet ink according to claim 1, The hydrophobic solvent comprises at least one of 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether. The total content of 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether is 3.00% by mass or more and 25.00% by mass or less. Inkjet ink.

3. An inkjet ink according to claim 1 or 2, The LogP value of the hydrophobic solvent is greater than 0.00 and less than or equal to 1.

50. Inkjet ink.

4. An inkjet ink according to claim 1 or 2, The viscosity at 25°C before the aforementioned drying treatment is 7.0 mPa·s or more and 10.0 mPa·s or less. Inkjet ink.

5. An inkjet ink according to claim 1 or 2, The first aqueous medium further comprises a first water-soluble organic solvent having a LogP of -2.00 or higher and 0.00 or lower. Inkjet ink.

6. The inkjet ink according to claim 5, The total content of the hydrophobic solvent and the first water-soluble organic solvent is greater than 3.00% by mass and less than or equal to 50.00% by mass. Inkjet ink.

7. The device comprises the inkjet ink described in claim 1 or 2, and a cleaning solution. The cleaning solution contains 0.15% to 10.50% by mass of poly(meth)acrylic acid, a surfactant, and a second aqueous medium containing water. Ink set.

8. The ink set according to claim 7, The amount of the surfactant in the cleaning solution is 0.10% by mass or more and 1.00% by mass or less. Ink set.

9. The ink set according to claim 7, The second aqueous medium further comprises a second water-soluble organic solvent having a LogP of -2.00 or higher and 0.00 or lower. Ink set.

10. The ink set according to claim 9, The content of the second water-soluble organic solvent in the cleaning solution is 5.00% by mass or more and 30.00% by mass or less. Ink set.

11. The ink set according to claim 7, The first aqueous medium and the second aqueous medium contain a common water-soluble organic solvent having a LogP value of -2.00 or higher and 0.00 or lower. Ink set.

12. An inkjet recording apparatus using the ink set described in claim 7, The recording head comprises a recording unit that ejects the inkjet ink onto a recording medium, and a cleaning unit that cleans the ink ejection surface of the recording unit with the cleaning fluid. Inkjet recording device.

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