Ink set and inkjet recording device

Abstract

To provide an ink set which can form an image having a desired image density and can easily remove an inkjet ink by cleaning.SOLUTION: There is provided an ink set which comprises an inkjet ink and a cleaning solution. The inkjet ink contains a pigment and a first aqueous medium. The first aqueous medium contains a hydrophobic solvent and water. The viscosity of the inkjet ink after drying treatment at 25°C is 120 mPa S or more and 3800 mPa S or less. The drying treatment is to heat the inkjet ink at 40°C until the mass of the inkjet ink decreases to 30.00 mass%. The cleaning solution contains a poly(meth)acrylic acid, a surfactant and a second aqueous medium. In the cleaning solution, the content ratio of the poly(meth)acrylic acid is 0.15 mass% or more and 10.50 mass% or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Inkjet recording apparatuses are required to be able to form images having a desired image density. As a method for forming an image having a desired image density with an inkjet recording apparatus, for example, a method using inkjet ink that dries quickly and thickens after landing on a recording medium has been proposed (for example, Patent Document 1).

[0003] However, if the viscosity of the inkjet ink when dried is too high, for example, the inkjet ink attached to the ink ejection surface of a recording head dries and thickens, which may result in insufficient ejection stability of the inkjet ink. In addition, if the inkjet ink dries and thickens quickly and adheres to the ejection surface of a recording head, it will quickly dry and the pigment will become fixed, and it cannot be sufficiently removed even by cleaning with a normal cleaning liquid.

[0004] For this reason, an ink set comprising an inkjet ink and a cleaning liquid, and an inkjet recording apparatus using the ink set, is required to be able to form an image having a desired image density, and to be able to easily remove the inkjet ink by cleaning. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2016-175951 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, Patent Document 1 cannot meet the above-mentioned requirements.

[0007] The present invention has been made in view of the above problems, and an object of the present invention is to provide an ink set and an inkjet recording apparatus that can form an image having a desired image density and that allows the inkjet ink to be easily removed by cleaning. [Means for solving the problem]

[0008] The ink set according to the present invention comprises an inkjet ink and a cleaning liquid. The inkjet ink contains a pigment and a first aqueous medium. The first aqueous medium contains a hydrophobic solvent and water. The viscosity of the inkjet ink after drying treatment is 120 mPa·S or more and 3800 mPa·S or less at 25° C. The drying treatment is a treatment in which the inkjet ink is heated at 40° C. until the mass of the inkjet ink is reduced by 30.00% by mass. The cleaning liquid contains poly(meth)acrylic acid, a surfactant, and a second aqueous medium. The content of the poly(meth)acrylic acid in the cleaning liquid is 0.15% by mass or more and 10.50% by mass or less.

[0009] An inkjet recording apparatus according to the present invention includes a recording head using the ink set described above, the recording head having a recording section that ejects the inkjet ink onto an image forming area of ​​a recording medium, and a cleaning section that cleans the ink ejection surface of the recording section with the cleaning liquid. Effect of the Invention

[0010] According to the ink set and inkjet recording apparatus of the present invention, it is possible to form an image having a desired image density, and the inkjet ink can be easily removed by cleaning. [Brief description of the drawings]

[0011] [Figure 1]FIG. 11 is a diagram illustrating an example of an inkjet recording apparatus according to a second embodiment. [Diagram 2] 2 is a diagram showing the bottom surface of the recording head and head housing of FIG. 1. [Diagram 3] FIG. 2 is a diagram showing a side surface of the first recording head in FIG. [Figure 4] 2A to 2C are diagrams illustrating a process of a cleaning operation performed by the inkjet recording apparatus of FIG. [Diagram 5] FIG. 5 is a diagram showing the next step in FIG. 4. [Figure 6] FIG. 6 is a diagram showing the next step in FIG. 5. [Figure 7] FIG. 7 is a diagram showing the next step in FIG. 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described. In the following, the volume median diameter (D 50 ) is a value measured using a dynamic light scattering particle size distribution analyzer (for example, Zetasizer (registered trademark) Nano ZS manufactured by Malvern Instruments) unless otherwise specified.

[0013] For organic solvents with literature values, the literature value can be used as the octanol / water partition coefficient LogP. For organic solvents with no literature values, a value calculated using calculation software (for example, "ChemDraw" manufactured by PerkinElmer) can be used.

[0014] The viscosity of the ink before drying (initial viscosity) and the viscosity of the ink after drying (dry viscosity) are values ​​measured at 25° C. using a rotational viscometer (for example, “TV-100EL” manufactured by Toki Sangyo Co., Ltd.).

[0015] The weight average molecular weight (Mw) is a value measured by gel permeation chromatography, unless otherwise specified.

[0016] In this specification, acrylic and methacrylic may be collectively referred to as "(meth)acrylic". Each component described in this specification may be used alone or in combination of two or more. "At least one of A and B" means "A and / or B".

[0017] [First embodiment: ink set] An ink set according to a first embodiment of the present invention will be described below. The ink set according to this embodiment includes an inkjet ink (hereinafter, sometimes simply referred to as ink) and a cleaning liquid. The ink contains a pigment and a first aqueous medium. The first aqueous medium contains a hydrophobic solvent and water. The viscosity of the ink after drying treatment at 25°C is 120 mPa·S or more and 3800 mPa·S or less. The drying treatment is a treatment in which the ink is heated at 40°C until the mass of the ink is reduced by 30.00 mass%. The cleaning liquid contains poly(meth)acrylic acid, a surfactant, and a second aqueous medium. The content ratio of poly(meth)acrylic acid in the cleaning liquid is 0.15 mass% or more and 10.50 mass% or less.

[0018] The ink set according to this embodiment, having the above-mentioned configuration, can form an image having a desired image density, and the ink can be easily removed by cleaning. The reason why the ink set according to this embodiment exhibits the above-mentioned effects is presumed to be as follows.

[0019] As described above, the ink of the ink set according to the present embodiment contains a pigment and a first aqueous medium, and the first aqueous medium contains a hydrophobic solvent and water. Therefore, the ink of the ink set according to the present embodiment exhibits hydrophobicity when it dries and the moisture content is reduced. When the hydrophobicity of the ink increases, the pigment contained in the ink aggregates and the ink thickens. Therefore, the ink of the ink set according to the present embodiment is prone to pigment aggregation and thickening when it dries and loses moisture (viscosity (dry viscosity) at 25°C after drying process is 120 mPa·s or more and 3800 mPa·s or less). Ink that is prone to thickening due to drying in this way thickens quickly after landing on a recording medium, and the pigment is retained near the surface of the recording medium to form an image with a high image density. As a result, the ink of the ink set according to the present embodiment can form an image with a desired image density.

[0020] On the other hand, when the ink that is easily thickened by drying adheres to the ejection surface of the recording head, it dries immediately and the pigment is fixed, and it cannot be sufficiently removed by cleaning with a normal cleaning liquid. As a result of intensive research by the present inventors, poly(meth)acrylic acid has a function of enhancing the cleaning effect of the cleaning liquid on the ink that is easily thickened by drying and has a high dry viscosity as described above, and effectively acts as a builder agent. Furthermore, poly(meth)acrylic acid is more effective at enhancing the cleaning effect of the cleaning liquid on the above-mentioned ink than general builder agents. And, a cleaning liquid containing poly(meth)acrylic acid at a ratio of 0.15% by mass or more and 10.50% by mass or less has excellent performance in removing the above-mentioned ink. Therefore, by combining the above-mentioned ink and the above-mentioned cleaning liquid, it is possible to provide an ink set that can form an image with a desired image density and can easily remove the ink by cleaning.

[0021] The ink set according to this embodiment will be described in more detail below. Each of the components described below may be used alone or in combination of two or more.

[0022] [ink] The ink contains a pigment and a first aqueous medium. The ink preferably further contains at least one of a pigment-coating resin and a surfactant.

[0023] The viscosity of the ink after drying at 25°C is 120 mPa·s or more and 3800 mPa·s or less, preferably 120 mPa·s or more and 1750 mPa·s or less, and more preferably 120 mPa·s or more and 200 mPa·s or less. By making the viscosity of the ink after drying at 25°C 120 mPa·s or more, an image having a desired image density can be formed by the ink. Furthermore, by making the viscosity of the ink after drying at 25°C 3800 mPa·s or less, the ink can be easily removed from the ejection surface of the recording head by the cleaning liquid. This allows the ejection stability of the ink by the inkjet recording device to be optimized.

[0024] The drying process is a process in which the ink according to the present embodiment is heated at 40° C. until the mass of the ink is reduced by 30.00% by mass. For details of the heating process, the method described in the examples or a method based thereon can be used.

[0025] The viscosity of the ink at 25°C (viscosity before drying treatment) is preferably from 7.0 mPa·s to 10.0 mPa·s, and more preferably from 7.0 mPa·s to 8.0 mPa·s. By making the viscosity of the ink at 25°C from 7.0 mPa·s to 10.0 mPa·s, an image having a desired image density can be reliably formed.

[0026] (Pigments) The pigment contained in the ink constitutes pigment particles together with, for example, a pigment-coating resin. The pigment particles are constituted, for example, by a core containing the pigment and a pigment-coating resin that coats the core. The pigment-coating resin exists, for example, in a state of being dispersed in an aqueous medium. From the viewpoint of optimizing the color density, hue, or stability of the ink according to this embodiment, the volume median diameter (D 50 ) is preferably 30 nm or more and 300 nm or less, and more preferably 90 nm or more and 130 nm or less.

[0027] Examples of pigments include yellow pigments, orange pigments, red pigments, blue pigments, purple pigments, and black pigments. 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).

[0028] In the ink, 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. By making the pigment content 2.00% by mass or more, it becomes easier to form an image having a desired image density by the ink. Furthermore, by making the pigment content 15.00% by mass or less, the ink can be reliably removed from the ejection surface of the recording head by the cleaning liquid. This allows the ejection stability of the ink by the inkjet recording device to be optimized.

[0029] (pigment coated resin) The pigment coating resin adheres to the surface of the pigment to optimize the dispersion stability of the pigment in the aqueous medium. A part of the pigment coating resin may be free in the aqueous medium without adhering to the surface of the pigment. Examples of the pigment coating resin include polyether resin, (meth)acrylic resin, styrene-(meth)acrylic resin, and styrene-maleic acid resin. Examples of commercially available pigment coating resins include "DISPERBYK (registered trademark)-190" (manufactured by BYK Japan Co., Ltd.).

[0030] In the ink, the content of the pigment coating resin is preferably 0.30% by mass or more and 3.50% by mass or less, and more preferably 1.00% by mass or more and 2.00% by mass or less. By making the content of the pigment coating resin 0.30% by mass or more, it becomes easier to form an image having a desired image density by the ink. By making the content of the pigment coating resin 3.50% by mass or less, it becomes possible to more reliably remove the ink from the ejection surface of the recording head by the cleaning liquid. This makes it possible to optimize the ejection stability of the ink by the inkjet recording device.

[0031] (first aqueous medium) The first aqueous medium is a medium containing water and a hydrophobic solvent. The first aqueous medium may function as a solvent or as a dispersion medium. In addition to water and a hydrophobic solvent, the first aqueous medium may further contain a water-soluble organic solvent (first water-soluble organic solvent) as an organic solvent other than the hydrophobic solvent.

[0032] (water) The water content in the ink is preferably from 40.00% to 80.00% by mass, and more preferably from 55.00% to 65.00% by mass. By setting the water content to from 40.00% to 80.00% by mass, the ejection stability of the ink by an inkjet recording device can be optimized.

[0033] (Hydrophobic Solvent) The hydrophobic solvent is an organic solvent having an octanol / water partition coefficient LogP (hereinafter, sometimes simply referred to as LogP) of more than 0.00. The LogP of the hydrophobic solvent is preferably more than 0.00 and not more than 1.50, more preferably 0.20 or more and not more than 0.40. By making the LogP of the hydrophobic solvent more than 0.00 and not more than 1.50, the ink exhibits high hydrophobicity when it dries and the water content decreases. This makes it easier to form an image having a desired image density using the ink.

[0034] The hydrophobic solvent may be any organic solvent having a LogP of more than 0.00, and preferably contains at least one of 3-methyl-1,5-pentanediol (LogP: 0.35) and triethylene glycol monobutyl ether (LogP: 0.44).

[0035] The content of the hydrophobic solvent in the ink is preferably from 3.00% to 30.00% by mass, and more preferably from 9.00% to 15.00% by mass. By setting the content of the hydrophobic solvent to be from 3.00% to 30.00% by mass, it becomes easier to adjust the viscosity of the ink at 25°C after drying treatment to within the above range.

[0036] (First Water-Soluble Organic Solvent) The first water-soluble organic solvent is an organic solvent having a LogP of not more than 0.00. The LogP of the first water-soluble organic solvent is preferably from −2.00 to 0.00, and more preferably from −1.50 to −0.50.

[0037] The first water-soluble organic solvent may be any organic solvent having a LogP of 0.00 or less, and preferably contains at least one of 1,3-propanediol (LogP: -1.04) and 1,2-propanediol (LogP: -0.92).

[0038] In the ink, the content of the first water-soluble organic solvent is preferably from 3.00% to 30.00% by mass, more preferably from 15.00% to 25.00% by mass. By containing the first water-soluble organic solvent in an amount of from 3.00% to 30.00% by mass, it becomes easier to adjust the viscosity of the ink at 25°C after drying treatment to within the above range.

[0039] The total content of the hydrophobic solvent and the first water-soluble organic solvent in the ink is preferably more than 3.00% by mass and not more than 50.00% by mass, and more preferably from 25.00% by mass to 35.00% by mass.

[0040] (Surfactant) The surfactant optimizes the compatibility and dispersion stability of each component contained in the ink. The surfactant also imparts wettability to the recording medium to the ink. The surfactant in the ink is preferably a nonionic surfactant.

[0041] Examples of the nonionic surfactant in the ink include acetylene glycol surfactants (surfactants containing acetylene glycol compounds), silicone surfactants (surfactants containing silicone compounds), and fluorine surfactants (surfactants containing fluorine resins or fluorine-containing compounds). Examples of the acetylene glycol surfactant include ethylene oxide adducts of acetylene glycol and propylene oxide adducts of acetylene glycol. The ink preferably contains an acetylene glycol surfactant.

[0042] The content of the surfactant in the ink is preferably from 0.10% by mass to 1.00% by mass, and more preferably from 0.30% by mass to 0.60% by mass.

[0043] (Other Ingredients) The ink may further contain known additives (more specifically, for example, a dissolution stabilizer, a drying inhibitor, an antioxidant, a viscosity adjuster, a pH adjuster, and an anti-mold agent) as necessary.

[0044] (Preferred composition) The ink preferably has any one of compositions 1 to 4 shown in Table 1 below. The numerical values ​​in Table 1 below indicate mass %. For example, the notation "5.70-6.90" for the pigment of composition 1 indicates that the pigment is contained at 5.70 mass % or more and 6.90 mass % or less. The notations in the other columns are similar. "Resin" indicates a pigment-coated resin. "MPD", "1,3PD", "BTG", and "1,2PD" respectively indicate 3-methyl-1,5-pentanediol, 1,3-propanediol, triethylene glycol monobutyl ether, and 1,2-propanediol.

[0045] [Table 1]

[0046] (Ink manufacturing method) The ink can be produced, for example, by mixing a pigment dispersion liquid containing a pigment, an aqueous medium, and other components (e.g., surfactants) that are added as necessary. In producing the ink, after the components are mixed uniformly, foreign matter and coarse particles may be removed using a filter (e.g., a filter with a pore size of 5 μm or less).

[0047] [Cleaning fluid] The cleaning liquid contains poly(meth)acrylic acid, a surfactant, and a second aqueous medium.

[0048] (Poly(meth)acrylic acid) In the cleaning liquid, the content of poly(meth)acrylic acid is 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. By setting the content of poly(meth)acrylic acid to 0.15% by mass or more and 10.50% by mass or less, the ink can be easily removed from the ejection surface of the recording head by the cleaning liquid. This makes it possible to optimize the ejection stability of the ink by the inkjet recording device.

[0049] The Mw of the poly(meth)acrylic acid is preferably from 3000 to 27000, more preferably from 3000 to 7000. By setting the Mw of the poly(meth)acrylic acid to be from 3000 to 27000, the ink can be more reliably removed from the ejection surface of the recording head by the cleaning liquid. This allows the ejection stability of the ink by the inkjet recording device to be optimized.

[0050] The Mw of the poly(meth)acrylic acid can be adjusted by changing the polymerization conditions of the poly(meth)acrylic acid (more specifically, the amount of the polymerization initiator used, the polymerization temperature, the polymerization time, etc.). The poly(meth)acrylic acid may be partially or completely neutralized with a basic compound.

[0051] (Surfactant) Surfactants adhere to the periphery of dirt (pigments, etc.) that has adhered to the object to be cleaned (the ink ejection surface) and have the ability to release the dirt from the object to be cleaned and the ability to disperse the released dirt into the cleaning liquid.

[0052] Examples of the surfactant in the cleaning liquid include the same surfactants as those exemplified in the description of the ink. Among the above-mentioned surfactants, the surfactant in the cleaning liquid is preferably a silicone surfactant (a surfactant containing a silicone compound).

[0053] The content of the surfactant in the cleaning liquid is preferably from 0.10% by mass to 1.00% by mass, and more preferably from 0.30% by mass to 0.60% by mass.

[0054] (Second aqueous medium) The second aqueous medium is a medium containing water, and is preferably a medium containing water and a water-soluble organic solvent (second water-soluble organic solvent). The second aqueous medium may function as a solvent or as a dispersion medium.

[0055] (water) In the cleaning liquid, the content of water is preferably 55.00% by mass to 95.00% by mass, more preferably 75.00% by mass to 85.00% by mass. By setting the content of water to 55.00% by mass to 95.00% by mass, the ink can be more easily removed from the ejection surface of the recording head by the cleaning liquid. This allows the ejection stability of the ink by the inkjet recording device to be optimized.

[0056] (Second Water-Soluble Organic Solvent) The second water-soluble organic solvent, like the first water-soluble organic solvent, is an organic solvent having a LogP of 0.00 or less. The LogP of the second water-soluble organic solvent is also preferably from -2.00 to 0.00, more preferably from -1.50 to -0.50.

[0057] The second water-soluble organic solvent may be, for example, the same water-soluble organic solvent as the water-soluble organic solvent exemplified in the description of the first water-soluble organic solvent. The first water-soluble organic solvent and the second water-soluble organic solvent may be the same solvent or different solvents, but are preferably the same solvent. The second water-soluble organic solvent is preferably 1,3-propanediol. Therefore, it is more preferable that the first water-soluble organic solvent and the second water-soluble organic solvent are each 1,3-propanediol.

[0058] In the cleaning liquid, the content of the second water-soluble organic solvent is preferably 5.00% by mass or more and 30.00% by mass or less, more preferably 15.00% by mass or more and 25.00% by mass or less. When the cleaning liquid contains the second water-soluble organic solvent at a content of 5.00% by mass or more and 35.00% by mass or less, the ink can be more easily removed from the ejection surface of the recording head by the cleaning liquid. This allows the ejection stability of the ink by the inkjet recording device to be optimized.

[0059] (Other Ingredients) The cleaning liquid may further contain a basic compound as an additive (pH adjuster) if necessary. The basic compound that the cleaning liquid may contain is preferably sodium hydroxide. In addition, the cleaning liquid may further contain other known additives (more specifically, for example, a dissolution stabilizer, a drying inhibitor, an antioxidant, a viscosity adjuster, and an antifungal agent) if necessary.

[0060] (Preferred composition) The cleaning liquid preferably has any one of compositions 1 to 4 shown in Table 2 below. The numerical values ​​in Table 2 below indicate mass %. For example, the notation "18.00-22.00" for 1,3PD in composition 2 indicates that the 1,3PD content is 18.00% or more and 22.00% or less by mass. The notations in the other columns are similar. "1,3PD" indicates 1,3-propanediol. "PA1" indicates polyacrylic acid with Mw=5000. "PA2" indicates polyacrylic acid with Mw=25000.

[0061] [Table 2]

[0062] (Cleaning liquid manufacturing method) The cleaning liquid can be produced, for example, by mixing poly(meth)acrylic acid, a surfactant, a second aqueous medium, and a component added as needed (for example, an aqueous solution of a basic compound). In producing the cleaning liquid, after the components are uniformly mixed, foreign matter and coarse particles may be removed using a filter (for example, a filter with a pore size of 5 μm or less).

[0063] [Second embodiment: Inkjet recording apparatus] An inkjet recording apparatus according to a second embodiment of the present invention will be described below. The inkjet recording apparatus according to this embodiment is an inkjet recording apparatus equipped with a recording head that uses the ink set according to the first embodiment. The recording head has a recording section that ejects ink onto an image forming area of ​​the recording medium, and a cleaning section that cleans the ink ejection surface of the recording section with a cleaning liquid. Details regarding the ink set have been explained in the first embodiment, and will therefore be omitted. Since the inkjet recording apparatus according to this embodiment uses the ink set according to the first embodiment, it is possible to form an image having a desired image density, and the ink can be easily removed by cleaning.

[0064] [Structure of inkjet recording device] The inkjet recording apparatus according to the present embodiment will be described below with reference to the drawings. Note that the drawings are mainly schematic illustrations of the respective components for ease of understanding, and the size, number, etc. of each illustrated component may differ from the actual size, number, etc. of each illustrated component may be changed as appropriate.

[0065] FIG. 1 is a diagram showing a main part of an inkjet recording device 1 which is an example of an inkjet recording device according to the present embodiment. The inkjet recording device 1 shown in FIG. 1 includes a recording head 2, a head housing 3 which holds the recording head 2, and a transport unit 4 which transports a recording medium (not shown). The recording head 2 includes 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 includes a pair of transport rollers, a first roller 4a and a second roller 4b, and a transport belt 4c which 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 (the right direction indicated by the arrow in FIG. 1). Hereinafter, the direction in which the recording medium is transported may be referred to as a transport direction X. When the recording medium is transported directly below the first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d, ink is ejected to form an image.

[0066] The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d, and the transport unit 4 are each supported at a height such that the distance from the upper surface of the transport 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 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 ink (first ink, second ink, third ink, and fourth ink) of four different colors (black, cyan, magenta, and yellow). The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d each eject ink of each color stored in an ink tank (not shown) onto a recording medium through a nozzle. The first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d each eject ink onto a recording medium in a predetermined order at a fixed ejection interval, thereby forming a color image on the recording medium.

[0068] Fig. 2 is a diagram showing the bottom surface of the recording head 2 and the head housing 3 in Fig. 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 the same structure. Therefore, the following will describe the details of the recording head 2 using the first recording head 2a as an example. However, the same description also applies to the second recording head 2b, the third recording head 2c, and the fourth recording head 2d.

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

[0071] On an ink ejection surface F1 on the lower surface of the recording unit 5, a plurality of nozzles (not shown) that eject ink are arranged.

[0072] As shown in FIG. 3, the cleaning unit 6 has a cleaning liquid supply unit 6a disposed near the ink ejection surface F1 of the recording unit 5, and a wiper 6b. A plurality of cleaning liquid ejection holes (not shown) for ejecting cleaning liquid are provided on a cleaning liquid supply surface F2 below the cleaning liquid supply unit 6a. The cleaning liquid supply unit 6a supplies cleaning liquid stored in a cleaning liquid tank (not shown) to the cleaning liquid supply surface F2 through the cleaning liquid ejection holes. The wiper 6b has a 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] 4 to 7 show a series of steps in the cleaning operation by the inkjet recording apparatus 1. As shown in FIG. 4, in the cleaning operation, first, a small amount of ink I is purged from the recording unit 5 (purging operation). This clears nozzle clogging and the like in the recording unit 5. The purged ink I adheres to the ink ejection surface F1 of the recording unit 5. In the cleaning operation, simultaneously with the purging operation, cleaning liquid C is supplied from the cleaning liquid supply unit 6a (cleaning liquid supplying operation). The supplied cleaning liquid C adheres to the vicinity of the ink ejection surface F1 of the recording unit 5 (cleaning liquid supplying surface F2 on the lower surface of the cleaning liquid supply unit 6a).

[0074] Next, as shown in FIG. 5, the wiper 6b is pressed against the cleaning liquid supplying surface F2 on the lower surface of the cleaning liquid supply unit 6a. Next, as shown in FIG. 6, the wiper 6b moves horizontally (to the left in FIG. 6). As a result, the wiper 6b wipes the cleaning liquid C adhering to the cleaning liquid supplying surface F2 and the ink I adhering to the ink ejection surface F1 together (wiping operation). At this time, the cleaning liquid C mixes with the ink I. As a result, as shown in FIG. 7, the cleaning liquid C adhering to the cleaning liquid supplying surface F2 and the ink I adhering to the ink ejection surface F1 are removed. As a result, the recording unit 5 is cleaned by the cleaning liquid C.

[0075] The above describes a series of steps in the cleaning operation by the inkjet recording device 1. Similarly to the first recording head 2a, the second recording head 2b, the third recording head 2c, and the fourth recording head 2d each 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 liquid. The cleaning unit 6 for the second recording head 2b, the third recording head 2c, and the fourth recording head 2d each has a cleaning liquid supply unit 6a disposed near the ink ejection surface F1 of the recording unit 5, and a wiper 6b.

[0076] Although an example of the inkjet recording apparatus according to this embodiment has been described above, the inkjet recording apparatus according to this embodiment is not limited to the one shown in FIGS.

[0077] 1 to 7, an inkjet recording device 1 equipped with four line-type recording heads corresponding to four colors of ink has been described as an example of the recording heads. However, the number of recording heads equipped in the inkjet recording device according to this embodiment is not particularly limited, and can be, for example, 1 to 10, and preferably 3 to 5.

[0078] Furthermore, in the inkjet recording apparatus according to this embodiment, the types, combinations, and ejection order of inks are not particularly limited.

[0079] 1 to 7, the inkjet recording device 1 has been described as an example in which the cleaning unit 6 includes a cleaning liquid supply unit 6a disposed near the ink ejection surface F1 of the recording unit 5, and a wiper 6b. However, the inkjet recording device according to this embodiment may also be an inkjet recording device in which the wiping operation is performed by applying cleaning liquid directly to the wiper. The cleaning unit of the inkjet recording device according to this embodiment may be any device capable of cleaning the ink ejection surface F1 of the recording unit 5 with cleaning liquid.

[0080] Furthermore, the inkjet recording apparatus according to this embodiment may be a multifunction machine that also has the functions of a scanner, a copier, a printer, or a facsimile.

[0081] The recording medium on which the inkjet recording apparatus according to the present embodiment forms an image is preferably a permeable recording medium (e.g., plain paper). When a known inkjet recording apparatus (especially a known inkjet recording apparatus equipped with a line-type recording head) forms an image on plain paper, the image density tends to be insufficient. In contrast, the inkjet recording apparatus according to the present embodiment can form an image having a desired image density even when forming an image on plain paper. EXAMPLES

[0082] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0083] [Ink set] Ink sets of examples and comparative examples were prepared by the following method. First, the raw materials used will be described. Table 3 shows details of the organic solvents used in the examples and comparative examples. Note that 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether were hydrophobic solvents (specific organic solvents).

[0084] [Table 3]

[0085] [Ink preparation] Inks (I-1) to (I-5) used in the examples and comparative examples were prepared by the following method. The composition of each ink is shown in Table 4 below.

[0086] (Preparation of Pigment Dispersion) 18.00 parts by mass of carbon black ("Black Pearls (registered trademark) 800" manufactured by Cabot Corporation) as a black pigment, 4.00 parts by mass of a pigment coating resin ("DISPERBYK (registered trademark)-190" manufactured by BYK Japan K.K.), and 78.00 parts by mass of ion-exchanged water were mixed using a disper. The resulting mixture was milled using a bead mill to remove the D of the pigment particles. 50 The pigment particles were dispersed until the diameter was 110 nm. 50 was measured using a dynamic light scattering particle size distribution measuring device ("Zetasizer (registered trademark) Nano ZS" manufactured by Malvern Instruments). As a result, a black pigment dispersion was obtained as the pigment dispersion.

[0087] (Preparation of Ink (I-1)) A mixed liquid was obtained by mixing 35.00 parts by mass of a black pigment dispersion (6.30 parts by mass of black pigment, 1.40 parts by mass of pigment coating resin, and 27.30 parts by mass of ion-exchanged water), 20.00 parts by mass of 3-methyl-1,5-pentanediol, 10.00 parts by mass of 1,3-propanediol, 0.50 parts by mass of a surfactant (Surfynol (registered trademark) 420, manufactured by Nissin Chemical Industry Co., Ltd., an acetylene surfactant), and 34.50 parts by mass of ion-exchanged water.

[0088] The above mixture was stirred at 400 rpm using a stirrer ("Three-One Motor BL-600" manufactured by Shinto Scientific Co., Ltd.). Next, the stirred mixture was filtered using a filter with a pore size of 5 μm to remove foreign matter and coarse particles. As a result, ink (I-1) was obtained.

[0089] (Preparation of Inks (I-2) to (I-5)) Inks (I-2) to (I-5) were prepared in the same manner as ink (A), except that the types and amounts of raw materials used were changed as shown in Table 4 below. The numbers in Table 4 below indicate the parts by mass of each component. "Resin" indicates a pigment-coated resin. "MPD", "1,3PD", "BTG", and "1,2PD" indicate 3-methyl-1,5-pentanediol, 1,3-propanediol, triethylene glycol monobutyl ether, and 1,2-propanediol, respectively.

[0090] [Table 4]

[0091] [Preparation of cleaning solution] Cleaning liquids (C-1) to (C-9) used in the examples and comparative examples were prepared by the following method. The composition of each cleaning liquid is shown in Table 5 below.

[0092] (Preparation of cleaning liquid (C-1)) 78.40 parts by mass of ion-exchanged water, 20.00 parts by mass of 1,3-propanediol, 0.50 parts by mass of a surfactant ("Silface (registered trademark) SAG503A" manufactured by Nissin Chemical Industry Co., Ltd., silicone surfactant), 0.10 parts by mass of 1N sodium hydroxide aqueous solution, and 1.00 parts by mass of polyacrylic acid ("Polyacrylic acid 5000" manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Mw=5000) were mixed to obtain a mixed liquid. This mixed liquid was used as cleaning liquid (C-1).

[0093] (Preparation of cleaning solutions (C-2) to (C-9)) Cleaning liquids (C-2) to (C-9) were prepared in the same manner as the cleaning liquid (C-1), except that the types and amounts of the raw materials used were changed as shown in Table 5 below. The numbers in Table 5 below indicate the parts by mass of each component. "CL" indicates the cleaning liquid. "1,3PD" indicates 1,3-propanediol. "SA" indicates a surfactant. "PA1" indicates polyacrylic acid with Mw=5000. In the following, as the polyacrylic acid with Mw=5000, "Polyacrylic acid 5000" manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. was used as described above. "PA2" indicates polyacrylic acid with Mw=25000. In the following, as the polyacrylic acid with Mw=25000, "Polyacrylic acid 25000" manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. was used. "STPP" and "CMC" indicate sodium tripolyphosphate and sodium carboxymethylcellulose, respectively. In the following, sodium tripolyphosphate and sodium carboxymethylcellulose were used as sodium tripolyphosphate and sodium carboxymethylcellulose manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., respectively.

[0094] [Table 5]

[0095] [Ink set preparation] As shown in the following Table 6, any one of the inks (I-1) to (I-5) was combined with any one of the cleaning liquids (C-1) to (C-9), thereby preparing the ink sets of Examples 1 to 7 and Comparative Examples 1 to 6.

[0096] [Viscosity measurement] In the examples, the viscosity of the ink before and after drying was measured at 25° C. using a torque balance servo type rotational viscometer (TV-100EL, manufactured by Toki Sangyo Co., Ltd.).

[0097] (Ink viscosity measurement before drying process) Using the method described above, the viscosities (initial viscosities) of the inks (I-1) to (I-5) before drying treatment, which were the subject of the measurement, were measured at 25° C. The measurement results are shown in Table 6 below.

[0098] (Ink viscosity measurement after drying process) 100.00 parts by mass of the ink to be measured (specifically, any of inks (I-1) to (I-5)) was placed in a petri dish. The petri dish was placed in an oven at 40°C and dried. During the drying process, the mass of the measurement object in the petri dish was measured over time. Then, when the mass of the measurement object in the petri dish had decreased to 70.00 parts by mass (the mass of the measurement object had decreased by 30.00% by mass), the drying process was terminated. Using the method described above, the viscosity (dry viscosity) at 25°C of the inks (I-1) to (I-5) to be measured after the drying process was measured. The measurement results are shown in Table 6 below.

[0099] [evaluation] The image density of the formed image and the cleanability were evaluated for the ink sets of Examples 1 to 7 and Comparative Examples 1 to 6 using the following evaluation machine. Note that the cleanability refers to the ease with which ink can be removed from the ejection surface of the recording head during cleaning using an inkjet recording device. Furthermore, unless otherwise specified, the recording medium (evaluation paper) was A4-sized plain paper (Mondi's "Color Copy (registered trademark)", basis weight 90 g / m 2 The evaluation results are shown in Table 6 below.

[0100] [Image Density] The image density was evaluated under an environment of 25°C temperature and 50% RH humidity. A print tester (manufactured by Kyocera Document Solutions Inc.) equipped with a line-type recording head that ejects ink using a piezoelectric method was used as an evaluation machine (inkjet recording device) for evaluating the image density. A solid image was formed on a recording medium using this evaluation machine for evaluating the image density. The image density (ID value) of the formed solid image was measured with a fluorescent spectrodensitometer ("FD-5" manufactured by Konica Minolta Inc.). The image density was judged according to the following criteria.

[0101] (Image density standard) A (Good): ID value is 1.20 or higher. B (bad): ID value is less than 1.20.

[0102] [Cleaning ability] The cleaning performance of the ejection surface of the recording head was evaluated under a normal temperature and humidity environment (an environment with a temperature of 25°C and a humidity of 60% RH). An inkjet recording device (a modified version of the inkjet color production printer "TASKalfa Pro15000c" manufactured by Kyocera Document Solutions Inc.) equipped with a recording head was used as an evaluation machine for the cleaning performance evaluation. The recording head had a recording section that ejects ink and a cleaning section that cleans the ink ejection surface of the recording section with a cleaning liquid. The cleaning section had a cleaning liquid supply section that supplies cleaning liquid to the cleaning liquid supply surface near the ink ejection surface of the recording section, and a wiper (linear pressure: 8.0 N / m) that wipes the ink ejection surface. The ink tank of the evaluation machine was filled with the ink (specifically, any of the inks (I-1) to (I-5)) included in the ink set to be evaluated. In addition, the cleaning liquid tank of the evaluation machine was filled with the cleaning liquid (specifically, any of the cleaning liquids (C-1) to (C-9)) included in the ink set to be evaluated. Using this evaluation machine for evaluating cleaning performance, an image with a printing rate of 4% was continuously printed on 1000 sheets of recording medium using the ink included in the ink set (specifically, any of inks (I-1) to (I-5)) (printing operation). After this printing operation, a purging operation, a cleaning liquid supplying operation, and a wiping operation were performed as cleaning operations. This printing operation and cleaning operation (purging operation, cleaning liquid supplying operation, and wiping operation) constituted one set, and 10 sets were repeated. Thereafter, the ejection surface of the recording head was visually observed to confirm the presence or absence of ink that had not been cleaned and remained. The cleaning performance was judged according to the following criteria.

[0103] (Criteria for evaluating cleaning ability) A (Good): No ink adheres to the ejection surface. B (Fail): Ink is attached to the ejection surface.

[0104] [Table 6]

[0105] As shown in Tables 3 to 6, the ink sets of Examples 1 to 7 included any one of the inks (I-1), (I-2), (I-4), and (I-5) and any one of the cleaning liquids (C-1) to (C-3), and (C-7). The inks of the ink sets of Examples 1 to 7 (specifically, the inks (I-1), (I-2), (I-4), and (I-5)) contained a pigment and an aqueous medium (first aqueous medium). The first aqueous medium contained a hydrophobic solvent. The viscosity (dry viscosity) of the ink at 25°C after a drying process (a process in which the ink is heated at 40°C until the mass of the ink is reduced by 30.00% by mass) was 120 mPa·S or more and 3800 mPa·S or less. The cleaning liquids of the ink sets of Examples 1 to 7 (specifically, cleaning liquids (C-1) to (C-3) and (C-7)) contained poly(meth)acrylic acid, a surfactant and an aqueous medium (second aqueous medium). In the cleaning liquids (C-1) to (C-3) and (C-7), the content of poly(meth)acrylic acid was 0.15% by mass or more and 10.50% by mass or less. The ink sets of Examples 1 to 7 were able to form images with high image density and desired image density even on plain paper. Furthermore, the ink sets of Examples 1 to 7 were easy to remove ink from the ejection surface of the recording head of the inkjet recording device by cleaning.

[0106] On the other hand, the ink (I-3) of the ink set of Comparative Example 1 had a low dry viscosity of less than 120 mPa·S, and the image density of the formed image was poor.

[0107] In addition, the ink set of Comparative Example 2 was able to form an image with the desired image density even on plain paper by using ink (I-2) as the ink. However, by using cleaning liquid (C-4) that does not contain poly(meth)acrylic acid as the cleaning liquid, the ink (I-2) remained on the ejection surface of the recording head of the inkjet recording device after cleaning. Thus, the cleaning liquid (C-4) that does not contain poly(meth)acrylic acid was able to completely remove the ink (I-3) with a low dry viscosity of less than 120 mPa·S as in Comparative Example 1 from the ejection surface by cleaning, but was unable to completely remove the ink with a high dry viscosity such as ink (I-2) from the ejection surface.

[0108] Furthermore, as in Comparative Example 3 and Comparative Example 4, the cleaning solutions (C-5) and (C-6) containing poly(meth)acrylic acid at a content of less than 0.15 mass % or more than 10.50 mass % were unable to completely remove inks with high dry viscosity, such as ink (I-1) or (I-2), from the ejection surface.

[0109] Furthermore, even with the cleaning solutions (C-8) and (C-9) in which a known builder agent was used instead of poly(meth)acrylic acid as in Comparative Examples 5 and 6, it was not possible to completely remove inks with high dry viscosities such as ink (I-2) from the ejection surface. [Industrial Applicability]

[0110] The ink set and inkjet recording apparatus of the present invention can be used to form an image on a recording medium. [Explanation of symbols]

[0111] 1: Inkjet recording device 2: Recording head 2a: First recording head 2b: second recording head 2c: 3rd recording head 2d: 4th recording head 3: Head housing 4: Transport unit 5: Recording section 6: Cleaning section 6a: Cleaning liquid supply unit 6b: Wiper I: Ink C: Cleaning fluid F1: Ink ejection surface F2: Cleaning liquid supply surface

Claims

1. An ink set comprising an inkjet ink and a cleaning liquid, The ink-jet ink contains a pigment and a first aqueous medium, The first aqueous medium contains a hydrophobic solvent and water, the viscosity of the ink-jet ink at 25° C. after drying is 120 mPa·S or more and 3800 mPa·S or less; the drying treatment is a treatment of heating the ink-jet ink at 40° C. until the mass of the ink-jet ink is reduced by 30.00% by mass; The cleaning liquid contains poly(meth)acrylic acid, a surfactant, and a second aqueous medium, The ink set, wherein the cleaning liquid contains the poly(meth)acrylic acid in an amount of 0.15% by mass or more and 10.50% by mass or less.

2. The ink set according to claim 1 , wherein a content of the hydrophobic solvent in the inkjet ink is 3.00% by mass or more and 30.00% by mass or less.

3. The ink set according to claim 1 or 2, wherein the hydrophobic solvent has an octanol / water partition coefficient Log P of more than 0.00 and not more than 1.

50.

4. 3. The ink set according to claim 1, wherein the hydrophobic solvent comprises at least one of 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether.

5. 3. The ink set according to claim 1, wherein the viscosity of the ink-jet ink at 25° C. before the drying treatment is 7.0 mPa·s or more and 10.0 mPa·s or less.

6. 3. The ink set according to claim 1, wherein the content of the surfactant in the cleaning liquid is 0.10% by mass or more and 1.00% by mass or less.

7. The first aqueous medium further contains a first water-soluble organic solvent, 3. The ink set according to claim 1, wherein the first water-soluble organic solvent has an octanol / water partition coefficient Log P of -2.00 or more and 0.00 or less.

8. The ink set according to claim 7 , wherein a total content of the hydrophobic solvent and the first water-soluble organic solvent in the inkjet ink is more than 3.00% by mass and not more than 50.00% by mass.

9. the second aqueous medium contains a second water-soluble organic solvent and water, 3. The ink set according to claim 1, wherein the second water-soluble organic solvent has an octanol / water partition coefficient Log P of -2.00 or more and 0.00 or less.

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

11. The first aqueous medium further contains a first water-soluble organic solvent, the second aqueous medium contains a second water-soluble organic solvent and water, the first water-soluble organic solvent and the second aqueous medium each have an octanol / water partition coefficient Log P of -2.00 or more and 0.00 or less; The ink set according to claim 1 , wherein the first water-soluble organic solvent and the second water-soluble organic solvent are the same solvent.

12. 3. An inkjet recording apparatus comprising a recording head using the ink set according to claim 1, The recording head includes: a recording unit that ejects the inkjet ink onto an image forming area of ​​a recording medium; a cleaning section that cleans the ink ejection surface of the recording section with the cleaning liquid.

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