Inkjet ink

Abstract

To provide an inkjet ink having excellent flying property and wettability to a recording medium.SOLUTION: There is provided an inkjet ink comprising a pigment, a specific surfactant and an aqueous medium. The specific surfactant is a compound represented by the following general formula (I). The aqueous medium includes water and a specific organic solvent. The octanol / water partition coefficient LogKow of the specific organic solvent is -1.80 or more and -1.00 or less. In the general formula (I), m and n each independently represent a number of 0.0 or more and 11.0 or less. m+n is 9.0 or more and 11.0 or less. x represents a number of 1.0 or more and 20.0 or less. y represents a number of 0.0 or more and 10.0 or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to ink-jet inks. [Background technology]

[0002] Some inkjet recording devices use aqueous inkjet inks containing a pigment and an aqueous medium. In order to form clear images, inkjet inks are required to have excellent flight properties. Here, the flight properties of inkjet inks refer to the ability of ink droplets to remain together as one droplet without splitting until they land on a recording medium when ejected from a recording head of an inkjet recording device. Inkjet inks with low flight properties split ink droplets in the air before landing on a recording medium, and split into a main ink droplet (main droplet) and minute ink droplets (satellite droplets). Then, while the main droplet lands on the desired position on the recording medium, the satellite droplets scatter around the main droplet and land. Therefore, images formed with inkjet inks with low flight properties have poor clarity.

[0003] In addition, from the viewpoint of forming a clear solid image, the inkjet ink is required to have excellent wettability to the recording medium. Here, the wettability of the inkjet ink to the recording medium refers to the property of the ink droplets of the inkjet ink that have landed on the recording medium to adequately wet and spread on the recording medium from the landing position as the center. Images formed with ink that has low wettability to the recording medium have large gaps between adjacent pixels, making it difficult to form a clear solid image.

[0004] In response to such demands, for example, an inkjet ink containing a coloring material, a first surfactant having a predetermined structure, a second surfactant having a predetermined structure, and water has been proposed (Patent Document 1). This inkjet ink is said to have good wettability to recording media. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2022 / 224789 Summary of the Invention [Problem to be solved by the invention]

[0006] However, even the inkjet ink described in Patent Document 1 does not fully satisfy the flying properties and wettability to a recording medium.

[0007] The present invention has been made in view of the above problems, and an object of the present invention is to provide an inkjet ink that is excellent in flying properties and wettability to a recording medium. [Means for solving the problem]

[0008] The inkjet ink according to the present invention contains a pigment, a specific surfactant, and an aqueous medium. The specific surfactant is a compound represented by the following general formula (I). The aqueous medium contains water and a specific organic solvent. The octanol / water partition coefficient LogK of the specific organic solvent is ow is greater than or equal to -1.80 and less than or equal to -1.00.

[0009] [ka]

[0010] In the general formula (I), m and n each independently represent a number between 0.0 and 11.0. m+n is between 9.0 and 11.0. x represents a number between 1.0 and 20.0. y represents a number between 0.0 and 10.0. Effect of the Invention

[0011] The inkjet ink according to the present invention is excellent in flying properties and wettability to recording media. 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 ("Zetasizer Nano ZS" manufactured by Malvern Instruments) unless otherwise specified. 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.

[0013] The measured weight average molecular weight (Mw), unless otherwise specified, is a value measured using gel permeation chromatography.

[0014] Unless otherwise specified, the dynamic surface tension is a value measured using a bubble pressure type dynamic surface tensiometer (KRUSS BP-100 manufactured by KRUSS) at 25° C. and a frequency of 1 Hz.

[0015] Octanol / water partition coefficient LogK ow If no specification is given, the literature value can be used as a reference. ow may be calculated using various calculation software (for example, "Hansen Solubility Parameter in Practice (HSPiP) Ver. 5.2.06" (developed by Prof. Steven Abbott et al.)).

[0016] Unless otherwise specified, the measured acid value is a value measured in accordance with "JIS (Japanese Industrial Standards) K0070-1992".

[0017] <Ink> An inkjet ink (hereinafter, sometimes simply referred to as ink) according to an embodiment of the present invention will be described below. The ink of this embodiment contains a pigment, a specific surfactant, and an aqueous medium. The specific surfactant is a compound represented by the following general formula (I). The aqueous medium contains water and a specific organic solvent. The octanol / water partition coefficient LogK of the specific organic solvent is ow (Hereinafter, simply "LogK ow ") is greater than or equal to -1.80 and less than or equal to -1.00.

[0018] [ka]

[0019] In the general formula (I), m and n each independently represent a number between 0.0 and 11.0. m+n is between 9.0 and 11.0. x represents a number between 1.0 and 20.0. y represents a number between 0.0 and 10.0.

[0020] The ink of this embodiment may contain multiple types of compounds represented by general formula (I). In this case, in general formula (1), m, n, x, and y respectively represent the number average values ​​of m, n, x, and y in the multiple types of compounds represented by general formula (I).

[0021] The ink of the present embodiment, having the above-mentioned configuration, is excellent in flying properties and wettability to a recording medium. The reason for this is presumed to be as follows. With known inks, attempts are sometimes made to improve flying properties by increasing the surface tension of the ink. On the other hand, with known inks, attempts are sometimes made to improve wettability to a recording medium by decreasing the surface tension of the ink. However, with known inks, there is a trade-off between flying properties and wettability to a recording medium, and it is difficult to optimize both simultaneously.

[0022] In contrast, the ink of the present embodiment is made of an organic solvent with moderately high hydrophilicity (LogK owSince the ink contains a specific organic solvent having a surface tension of -1.80 or more and -1.00 or less), it has a relatively high surface tension. In addition, the specific surfactant does not reduce the surface tension of the ink droplets very much immediately after the ink droplets are ejected from the recording head, as a result of various tests. On the other hand, the specific surfactant can effectively reduce the surface tension of the ink droplets after the ink droplets land on the recording medium. The ink of this embodiment can continue to hold together as one droplet due to the relatively high surface tension derived from the specific organic solvent immediately after the ink droplets are ejected from the recording head (excellent flight properties). On the other hand, the ink of this embodiment can moderately wet and spread on the recording medium due to the surface tension reduction effect derived from the specific surfactant after the ink droplets land on the recording medium (excellent wet and spread properties on the recording medium). As a result, the ink of this embodiment is excellent in both flight properties and wettability on the recording medium. The ink of this embodiment will be described in more detail below.

[0023] [Dynamic surface tension] The dynamic surface tension of the ink of this embodiment with a surface life of 10 milliseconds is preferably 39 mN / m or more and 50 mN / m or less, and more preferably 40 mN / m or more and 44 mN / m or less. The dynamic surface tension of the ink of this embodiment with a surface life of 10 milliseconds corresponds to the surface tension immediately after it is ejected as an ink droplet from a recording head. By setting the dynamic surface tension at 39 mN / m or more with a surface life of 10 milliseconds, the flying properties of the ink of this embodiment can be optimized. By setting the dynamic surface tension at 50 mN / m or less with a surface life of 10 milliseconds, the wettability of the ink of this embodiment to a recording medium can be optimized.

[0024] The dynamic surface tension of the ink of this embodiment with a surface life of 1000 milliseconds is preferably 38 mN / m or less, and more preferably 32 mN / m to 36 mN / m. The dynamic surface tension of the ink of this embodiment with a surface life of 1000 milliseconds corresponds to the surface tension at the stage after the ink droplet lands on the recording medium. By setting the dynamic surface tension to 38 mN / m or less with a surface life of 1000 milliseconds, the wettability of the ink of this embodiment to the recording medium can be optimized.

[0025] [Pigment] In the ink of this embodiment, the pigment constitutes a pigment particle together with, for example, a pigment coating resin. The pigment particle is constituted, for example, by a core containing a pigment and a pigment coating resin coating the core. The pigment coating resin exists, for example, in a state of being dispersed in a solvent. From the viewpoint of optimizing the color density, hue, or stability of the ink of this embodiment, the volume median diameter of the pigment particles is preferably 30 nm or more and 200 nm or less, and more preferably 70 nm or more and 130 nm or less.

[0026] 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).

[0027] In the ink of this embodiment, the pigment content is preferably 1.0% by mass or more and 10.0% by mass or less, and more preferably 4.0% by mass or more and 8.0% by mass or less. By making the pigment content 1.0% by mass or more, the ink of this embodiment can easily form an image having a desired image density. In addition, by making the pigment content 10.0% by mass or less, the fluidity of the ink of this embodiment can be ensured.

[0028] [Pigment-coated resin] The pigment coating resin is a resin that is soluble in the aqueous medium of the ink of this embodiment. A part of the pigment coating resin is present, for example, on the surface of the pigment particle, optimizing the dispersibility of the pigment particle. A part of the pigment coating resin is present, for example, in a dissolved state in the aqueous medium of the ink of this embodiment.

[0029] As the pigment coating resin, styrene-(meth)acrylic resin is preferred. The styrene-(meth)acrylic resin has a repeating unit derived from at least one monomer selected from among (meth)acrylic acid alkyl esters and (meth)acrylic acid, and a styrene unit. As the (meth)acrylic acid alkyl esters, for example, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, and butyl (meth)acrylate can be mentioned. As the styrene-(meth)acrylic resin, a copolymer (X) of styrene, methyl methacrylate, methacrylic acid, and butyl acrylate is preferred. The copolymer (X) is preferably neutralized with an equal amount of a base (e.g., potassium hydroxide and sodium hydroxide).

[0030] In all repeating units contained in the copolymer (X), the content of repeating units derived from styrene is preferably 10% by mass or more and 20% by mass or less. In all repeating units contained in the copolymer (X), the content of repeating units derived from methyl methacrylate is preferably 10% by mass or more and 20% by mass or less. In all repeating units contained in the copolymer (X), the content of repeating units derived from methacrylic acid is preferably 35% by mass or more and 45% by mass or less. In all repeating units contained in the copolymer (X), the content of repeating units derived from butyl acrylate is preferably 25% by mass or more and 35% by mass or less.

[0031] In the ink of this embodiment, the content of the pigment coating resin is preferably 0.5% by mass to 6.0% by mass, and more preferably 1.5% by mass to 4.0% by mass. By setting the content of the pigment coating resin to 0.5% by mass to 6.0% by mass, the ejection stability of the ink of this embodiment can be optimized.

[0032] The acid value of the pigment coating resin is preferably from 30 mgKOH / g to 200 mgKOH / g, more preferably from 70 mgKOH / g to 130 mgKOH / g. By setting the acid value of the pigment coating resin to be from 30 mgKOH / g to 200 mgKOH / g, the dispersibility of the pigment can be further improved while optimizing the storage stability of the ink of this embodiment.

[0033] The acid value of the pigment coating resin can be adjusted by changing the amount of monomer used in synthesizing the pigment coating resin. For example, the acid value of the pigment coating resin can be increased by using a monomer (more specifically, acrylic acid, methacrylic acid, etc.) having an acidic functional group (e.g., a carboxyl group) in synthesizing the pigment coating resin.

[0034] The Mw of the pigment coating resin is preferably from 10,000 to 50,000, and more preferably from 15,000 to 25,000. By setting the Mw of the pigment coating resin to be from 10,000 to 50,000, the image density of the formed image can be optimized while suppressing an increase in the viscosity of the ink of this embodiment.

[0035] The Mw of the pigment coating resin can be adjusted by changing the polymerization conditions for the pigment coating resin (more specifically, the amount of polymerization initiator used, polymerization temperature, polymerization time, etc.).

[0036] In the polymerization of the pigment-coated resin, the amount of the polymerization initiator used is preferably 0.001 to 5 mol, more preferably 0.01 to 2 mol, per mol of the monomer mixture. In the polymerization of the pigment-coated resin, for example, the polymerization temperature can be 50° C. to 70° C., and the polymerization time can be 10 to 24 hours. The polymerized pigment-coated resin is preferably neutralized with an equal amount of a basic compound before being used as a raw material for the ink of this embodiment. The basic compound is preferably a hydroxide of an alkali metal ion (e.g., NaOH or KOH).

[0037] [Specific surfactant] The specific surfactant is a compound represented by the above-mentioned general formula (I). In the general formula (I), x preferably represents a number of 6.0 or more and 13.0 or less, more preferably represents a number of 6.0 or more and 8.0 or less, or more preferably represents a number of 11.0 or more and 13.0 or less. y preferably represents a number of 0.0 or more and 3.0 or less, more preferably represents 0.0 or represents a number of 2.0 or more and 3.0 or less. In addition, in the general formula (I), x preferably represents a number of 6.0 or more and 13.0 or less, and y preferably represents a number of 0.0 or more and 3.0 or less, more preferably represents a number of 6.0 or more and 8.0 or less, and y represents a number of 2.0 or more and 3.0 or less, or more preferably represents a number of 11.0 or more and 13.0 or less, and y represents 0.0.

[0038] Examples of commercially available specific surfactants include the "Softanol (registered trademark) M series" manufactured by Nippon Shokubai Co., Ltd. (specifically, Softanol (registered trademark) 30, 50, 70, 90, and 120) and the "Softanol (registered trademark) EP series" manufactured by the same company (specifically, Softanol (registered trademark) EP5035, EP7025, EP7045, EP7085, EP9050, EP90150, and EP12030). In the "Softanol (registered trademark) M series," y in general formula (1) is 0.0 in all cases. In the "Softanol (registered trademark) EP series," y in general formula (1) is 1.0 or more in all cases.

[0039] In the ink of this embodiment, the content of the specific surfactant is preferably 0.1% by mass or more and 1.2% by mass or less, and more preferably 0.3% by mass or more and 0.7% by mass or less. By making the content of the specific surfactant 0.1% by mass or more, the wettability of the ink of this embodiment to the recording medium can be optimized. By making the content of the specific surfactant 1.2% by mass or less, the flying performance of the ink of this embodiment can be optimized.

[0040] [Other surfactants] The ink of the present embodiment may contain only the specific surfactant as a surfactant, or may further contain a surfactant other than the specific surfactant (hereinafter, referred to as other surfactant). The content of the specific surfactant in the surfactant is preferably 60% by mass or more, and more preferably 80% by mass or more.

[0041] Other surfactants include, for example, 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).Acetylene glycol surfactants include, for example, ethylene oxide adducts of acetylene glycol and propylene oxide adducts of acetylene glycol.

[0042] In the ink of this embodiment, the content of the other surfactant is preferably from 0.01% by mass to 1.0% by mass, and more preferably from 0.05% by mass to 0.2% by mass.

[0043] [Aqueous medium] The aqueous medium includes water and the specific organic solvent. The aqueous medium may function as a solvent or as a dispersion medium. The aqueous medium may include water-soluble organic solvents (Log K ow The water-soluble organic solvent may further include a water-soluble organic solvent having a viscosity index less than -1.80 or more than -1.00 (hereinafter, sometimes referred to as other water-soluble organic solvent).

[0044] In the ink of this embodiment, the content of the aqueous medium is preferably from 75.0% by mass to 97.0% by mass, and more preferably from 87.0% by mass to 95.0% by mass.

[0045] (water) In the ink of this embodiment, the water content is preferably 25.0% by mass to 80.0% by mass, and more preferably 45.0% by mass to 55.0% by mass. By setting the water content to 25.0% by mass to 80.0% by mass, the surface tension of the ink of this embodiment (particularly the dynamic surface tension at a surface life of 10 milliseconds and the dynamic surface tension at a surface life of 1000 milliseconds) can be optimized.

[0046] (Specific organic solvent) LogK of specific organic solvent ow is -1.80 or more and -1.00 or less. Examples of the specific organic solvent include the organic solvents shown in the following Table 1. As the specific organic solvent, glycerin or 1,3-propanediol is preferable.

[0047] [Table 1]

[0048] In the ink of this embodiment, the content of the specific organic solvent is preferably 12.0% by mass or more and 40.0% by mass or less, and more preferably 25.0% by mass or more and 35.0% by mass or less. By making the content of the specific organic solvent 12.0% by mass or more, the flying properties of the ink of this embodiment can be optimized. By making the content of the specific organic solvent 40.0% by mass or less, the wettability of the ink of this embodiment to the recording medium can be optimized.

[0049] (Other water-soluble organic solvents) Examples of the other water-soluble organic solvent include glycol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, thiodiglycol, and dimethylsulfoxide. In the aqueous medium, the content of the other water-soluble organic solvent is preferably 10% by mass or less, more preferably 1% by mass or less, and even more preferably 0% by mass.

[0050] [Other ingredients] The ink of this embodiment may further contain known additives (for example, a dissolution stabilizer, a drying inhibitor, an antioxidant, a viscosity adjuster, a pH adjuster, and an anti-mold agent) as necessary.

[0051] [Preferred composition] The ink of this embodiment preferably has any one of compositions 1 to 8 shown in Table 2 below. In Table 2 below, "ratio" indicates a numerical range of a preferred content ratio [mass %]. For example, "5.0-7.0", which is the pigment ratio of composition 1, indicates that the pigment is contained in an amount of 5.0 mass % or more and 7.0 mass % or less. "Resin" indicates a pigment-coated resin. "S-1" and "S-2" for the types of specific surfactants respectively indicate the surfactants (S-1) and (S-2) used in the examples.

[0052] [Table 2]

[0053] [Ink manufacturing method] The ink of this embodiment can be produced, for example, by uniformly mixing a pigment dispersion liquid containing a pigment, a specific surfactant, and water with a stirrer. In producing the ink of this embodiment, after the components are uniformly mixed, foreign matter and coarse particles may be removed with a filter (for example, a filter with a pore size of 5 μm or less).

[0054] (Pigment dispersion) The pigment dispersion is a dispersion containing a pigment. The pigment dispersion preferably further contains a pigment-coating resin. The dispersion medium of the pigment dispersion is preferably water.

[0055] The content of the pigment in the pigment dispersion is preferably from 5.0 to 30.0% by mass, more preferably from 15.0 to 25.0% by mass, and the content of the pigment coating resin in the pigment dispersion is preferably from 2.0 to 12.0% by mass, more preferably from 5.0 to 7.0% by mass.

[0056] The pigment dispersion can be prepared by wet-dispersing the pigment, the pigment-coated resin, a dispersion medium (e.g., water), and optional components (e.g., other surfactants) using a media-type wet disperser. In the wet dispersion using a media-type wet disperser, small-particle-size beads (e.g., D 50 The material of the beads is not particularly limited, but hard materials (for example, glass and zirconia) are preferable.

[0057] When the pigment dispersion is added in the production of the ink of this embodiment, the ratio of the pigment dispersion to all the raw materials of the ink is, for example, 10.0% by mass or more and 40.0% by mass or less. EXAMPLES

[0058] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0059] [Preparation of surfactant] Surfactants (S-1) to (S-3) shown in Table 3 below were prepared as surfactants. Among the surfactants (S-1) to (S-3), surfactants (S-1) to (S-2) were specific surfactants. Surfactant (S-3) was an acetylene glycol surfactant. Table 3 below shows the values ​​of m+n, x, and y in the general formula (I) for surfactants (S-1) to (S-2).

[0060] [Table 3]

[0061] [Preparation of pigment dispersion] A pigment dispersion for use in preparing an ink was prepared. In preparing the pigment dispersion, first, a pigment coating resin solution containing the pigment coating resin (R) and water was prepared.

[0062] (Preparation of pigment coated resin solution) An alkali-soluble resin having a repeating unit derived from methacrylic acid (MAA unit), a repeating unit derived from methyl methacrylate (MMA unit), a repeating unit derived from butyl acrylate (BA unit), and a repeating unit derived from styrene (ST unit) was prepared. This alkali-soluble resin had a mass average molecular weight (Mw) of 20,000 and an acid value of 100 mgKOH / g. The mass ratio of each repeating unit in this alkali-soluble resin was "MAA unit: MMA unit: BA unit: ST unit = 40: 15: 30: 15". This alkali-soluble resin was mixed with an aqueous sodium hydroxide solution (neutralization treatment). The alkali-soluble resin was neutralized with NaOH by the neutralization treatment. In the neutralization treatment, the amount of the aqueous sodium hydroxide solution used was 1.05 times the theoretical amount of the aqueous sodium hydroxide solution required to neutralize the alkali-soluble resin. As a result, a pigment-coated resin solution containing a pigment-coated resin (R) (neutralized alkali-soluble resin) and water was obtained.

[0063] The Mw of the above-mentioned alkali-soluble resin was measured under the following conditions using gel permeation chromatography ("HLC-8020GPC" manufactured by Tosoh Corporation). A calibration curve was prepared using TSKgel standard polystyrenes F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000 manufactured by Tosoh Corporation and n-propylbenzene.

[0064] (Conditions for measuring mass average molecular weight) Column: Tosoh Corporation's "TSKgel SuperMultiporeHZ-H" (4.6mm I.D. x 15cm semi-microcolumn) Number of columns: 3 Eluent: Tetrahydrofuran ·Flow rate: 0.35mL / min Sample injection volume: 10μL ·Measurement temperature: 40℃ Detector: IR detector

[0065] (Distributed Processing) A magenta pigment (CI Pigment Red 122), the above-mentioned pigment-coated resin solution containing pigment-coated resin (R), an acetylene glycol surfactant "Olfine (registered trademark) E1010" (an ethylene oxide adduct of acetylene glycol) manufactured by Nissin Chemical Industry Co., Ltd., and ion-exchanged water were added to the vessel of a media-type wet disperser (DYNO (registered trademark)-MILL manufactured by Willy & Bachofen (WAB)) so as to obtain the composition shown in Table 4 below.

[0066] The "water" content in Table 4 below indicates the total content of the ion-exchanged water charged into the vessel described above and the water contained in the pigment-coated resin solution (more specifically, the water contained in the aqueous sodium hydroxide solution used to neutralize the alkali-soluble resin, and the water generated by the neutralization reaction of the alkali-soluble resin and sodium hydroxide).

[0067] [Table 4]

[0068] Next, the contents of the vessel were wet-dispersed. Zirconia beads (particle diameter 1.0 mm) were used as media. The amount of media put in was 70 volume % of the capacity of the vessel. The dispersion conditions were a temperature of 10°C and a peripheral speed of 8 m / sec. As a result, a pigment dispersion liquid was obtained.

[0069] The volume median diameter (D 50 Specifically, the obtained pigment dispersion was diluted 300 times with ion-exchanged water to prepare a measurement sample. The D of the pigment particles in the measurement sample was measured using a dynamic light scattering particle size distribution measurement device (Malvern Zetasizer Nano ZS). 50 The D of the pigment particles in the measurement sample was 50 of pigment particles contained in the pigment dispersion liquid 50 The measurement was carried out 10 times, and the average of the measurement results was taken as the D of the pigment particles. 50 The D of the pigment particles contained in the pigment dispersion liquid was adopted. 50 was 100 nm.

[0070] <Ink Preparation> Inks of Examples 1 to 8 and Comparative Examples 1 to 6 having the compositions shown in Table 5 below were prepared by the following method.

[0071] [Example 1] 38.1 parts by mass of the pigment dispersion (5.7 parts by mass of pigment, 2.3 parts by mass of pigment coating resin (R)), a surfactant shown in Table 5 below (0.5 parts by mass of surfactant (S-1) in Example 1), an organic solvent X shown in Table 5 below (30.0 parts by mass of glycerin in Example 1), and ion-exchanged water were charged into a beaker. The amount of ion-exchanged water used was an amount that would make the amount of the contents of the beaker 100.0 parts by mass (31.4 parts by mass in Example 1). The contents of the beaker were mixed at a rotation speed of 400 rpm using a stirrer ("Three-One Motor BL-600" manufactured by Shinto Scientific Co., Ltd.) to obtain a mixed liquid. The mixed liquid was filtered using a filter (pore size 5 μm) to remove foreign matter and coarse particles contained in the mixed liquid. In this manner, the ink of Example 1 was obtained.

[0072] [Examples 2 to 8 and Comparative Examples 1 to 6] Inks of Examples 2 to 8 and Comparative Examples 1 to 6 were prepared in the same manner as in Example 1, except that the types and amounts of each component were changed as shown in Table 5.

[0073] In Table 5, "EBEE" stands for ethylene glycol butyl ethyl ether.

[0074] [Table 5]

[0075] [Dynamic surface tension measurement] At 25° C., the dynamic surface tension of the ink of each of the examples and comparative examples at a surface life of 10 milliseconds and at a surface life of 1000 milliseconds was measured by the maximum bubble pressure method using a bubble pressure type dynamic surface tensiometer (KRUSS "BP100", capillary diameter 0.4 mm). The measurement results are shown in Table 6 below.

[0076] <Evaluation> The inks of each of the examples and comparative examples were evaluated for their ink flying properties (ease of ink droplets to gather when ejected) and their wetting and spreading properties on a recording medium by the following methods. The evaluation results are shown in Table 6 below. The evaluations were performed at a temperature of 25°C and a humidity of 15% RH unless otherwise specified.

[0077] [Evaluation machine] A line head type inkjet recording device (a tester manufactured by Kyocera Document Solutions Inc.) was used as the evaluation machine. The evaluation machine was equipped with a piezoelectric type recording head. The recording head was set to a head temperature of 32° C. and an ink ejection speed of 8 m / sec. The recording head was filled with the ink to be evaluated (specifically, any of the inks of the examples and comparative examples).

[0078] [Flightability] In the evaluation of flight performance, the ink to be evaluated was ejected from the recording head of the evaluation machine. A high-speed camera (Shimadzu Corporation's "Hyper Vision HPV-X2") was used to photograph the ink droplets as they were ejected. The length of the tail was calculated from the number of pixels from the rear end of the main droplet to the rear end of the tail of the photographed ink droplet. Ink droplets with long tails are prone to generating satellite droplets when the tail breaks off during flight. Therefore, the length of the tail serves as a guide to the flight performance of the ink. The flight performance of the ink was judged according to the following criteria based on the length of the tail.

[0079] (Evaluation criteria for flight ability) A (Good): The tail length is less than 0.31 mm. B (bad): The tail length is 0.31 mm or more.

[0080] [Wetting and spreading] In the evaluation of the wetting and spreading properties, copy paper ("CC90" manufactured by Mondi) was used as the recording medium. Using the evaluation machine, a stripe image consisting of one dot line (a vertical line with a line width of one pixel) was formed on the recording medium. The line width of the formed one dot line was measured at 10 points, and the average value (average line width) was calculated. The wetting and spreading properties of the ink were judged according to the following criteria.

[0081] (Standard for wettability) A (good): The average line width was 75 μm or more. B (poor): The average line width was less than 75 μm.

[0082] [Table 6]

[0083] As shown in Tables 1 to 6, the inks of the examples contained a pigment, a specific surfactant, and an aqueous medium. The specific surfactant was a compound represented by the general formula (I). The aqueous medium contained water and a specific organic solvent. The LogK owwas −1.80 or more and −1.00 or less. The inks of the examples were excellent in flying properties and wetting and spreading properties.

[0084] On the other hand, the inks of Comparative Examples 1 and 3 did not contain a specific organic solvent, but instead contained propylene glycol, which has relatively high hydrophobicity. The inks of Comparative Examples 1 and 3 had excessively low dynamic surface tensions at a surface life of 10 milliseconds due to the influence of propylene glycol. As a result, the inks of Comparative Examples 1 and 3 had poor flying properties.

[0085] The inks of Comparative Examples 2 and 4 did not contain a specific organic solvent, but instead contained ethylene glycol butyl ethyl ether, which has relatively high hydrophilicity. The inks of Comparative Examples 2 and 4 had excessively high dynamic surface tensions at a surface life of 1000 milliseconds due to the influence of ethylene glycol butyl ethyl ether. As a result, the inks of Comparative Examples 2 and 4 had poor wetting and spreading properties on the recording medium.

[0086] The inks of Comparative Examples 5 and 6 did not contain a specific surfactant, but instead contained an acetylene glycol surfactant. Although the inks of Comparative Examples 5 and 6 contained a surfactant, the dynamic surface tension at a surface life of 1000 milliseconds could not be sufficiently reduced. As a result, the inks of Comparative Examples 5 and 6 had poor wetting and spreading properties on the recording medium. [Industrial Applicability]

[0087] The inks of the present embodiments can be used to form images.

Claims

1. The ink contains a pigment, a specific surfactant, and an aqueous medium, The specific surfactant is a compound represented by the following general formula (I): The aqueous medium contains water and a specific organic solvent, Octanol / water partition coefficient LogK of the specific organic solvent ow is -1.80 or more and -1.00 or less. [Case 1] (In the general formula (I), m and n each independently represent a number of 0.0 or more and 11.0 or less, m+n is 9.0 or more and 11.0 or less, x represents a number of 1.0 or more and 20.0 or less, and y represents a number of 0.0 or more and 10.0 or less.)

2. The dynamic surface tension at a surface life of 10 milliseconds is 39 mN / m or more and 50 mN / m or less, 2. The ink-jet ink of claim 1, having a dynamic surface tension of 38 mN / m or less at a surface life of 1000 milliseconds.

3. The ink-jet ink according to claim 1 or 2, wherein the content of the specific surfactant is from 0.1% by mass to 1.2% by mass.

4. The ink-jet ink according to claim 1 or 2, wherein a content of the specific organic solvent is from 12.0% by mass to 40.0% by mass.

5. In the general formula (I), x represents a number of 6.0 or more and 13.0 or less; The ink-jet ink according to claim 1 or 2, wherein y represents a number of 0.0 or more and 3.0 or less.

6. 3. The ink-jet ink according to claim 1, wherein the specific organic solvent includes at least one of glycerin and 1,3-propanediol.

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