Inkjet ink

Abstract

To provide an inkjet ink having excellent ink ejection stability.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 -1.50 or more and 0.30 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 1.0 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] In order to obtain images having a desired quality, inkjet inks are required to have excellent ejection stability.

[0003] As an example, an inkjet ink containing a first surfactant having a specific structure, a second surfactant having a specific structure, and water has been proposed (Patent Document 1). It is said that the inkjet ink described above tends to have good ejection properties by containing a specific amount of at least one fixing resin selected from polymers and waxes. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2022 / 224789 Summary of the Invention [Problem to be solved by the invention]

[0005] However, even the above-mentioned ink-jet inks cannot fully satisfy the ejection stability of the ink-jet ink.

[0006] 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 having excellent ejection stability. [Means for solving the problem]

[0007] 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 -1.50 and less than or equal to 0.30.

[0008] [ka]

[0009] 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 1.0 and 10.0. Effect of the Invention

[0010] The inkjet ink according to the present invention has excellent ejection stability. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] 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 (e.g., Malvern's Zetasizer (registered trademark) Nano ZS). 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".

[0012] The measured weight average molecular weight (Mw), unless otherwise specified, is a value measured using gel permeation chromatography.

[0013] Octanol / water partition coefficient LogK owIf 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.)).

[0014] Unless otherwise specified, the measured acid value is a value measured in accordance with "JIS (Japanese Industrial Standards) K0070-1992".

[0015] <Inkjet 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 -1.50 and less than or equal to 0.30.

[0016] [ka]

[0017] 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 1.0 and 10.0.

[0018] 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).

[0019] The present inventors have conducted extensive research to improve the ejection stability of ink. As a result, although the reason is unclear, it was found that the specific surfactant represented by the general formula (I) and a specific range of LogK ow It has been found that by combining an aqueous medium containing a specific organic solvent having the above structure, an ink having excellent ejection stability can be obtained. Specifically, even when an image is formed after the ink has not been ejected from the recording head for a while, the ink droplets ejected from the recording head do not become light in color or transparent when observed by the naked eye, and a desired image can be obtained. The ink of the present embodiment will be described in more detail below.

[0020] [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.

[0021] 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).

[0022] In the ink of this embodiment, the pigment content is preferably 1.00% by mass or more and 10.00% by mass or less, and more preferably 4.00% by mass or more and 8.00% by mass or less. By making the pigment content 1.00% 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 10.00% by mass or less, the fluidity of the ink of this embodiment can be ensured.

[0023] [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.

[0024] 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 basic compound (e.g., potassium hydroxide and sodium hydroxide).

[0025] In the total repeating units contained in the copolymer (X), the content of repeating units derived from styrene is preferably 10.00% by mass or more and 20.00% by mass or less. In the total repeating units contained in the copolymer (X), the content of repeating units derived from methyl methacrylate is preferably 10.00% by mass or more and 20.00% by mass or less. In the total repeating units contained in the copolymer (X), the content of repeating units derived from methacrylic acid is preferably 35.00% by mass or more and 45.00% by mass or less. In the total repeating units contained in the copolymer (X), the content of repeating units derived from butyl acrylate is preferably 25.00% by mass or more and 35.00% by mass or less.

[0026] In the ink of this embodiment, the content of the pigment coating resin is preferably from 0.50% to 6.00% by mass, and more preferably from 1.50% to 4.00% by mass. By setting the content of the pigment coating resin to be from 0.50% to 6.00% by mass, the ejection stability of the ink of this embodiment can be optimized.

[0027] The acid value of the pigment coating resin is preferably from 30 mgKOH / g to 200 mgKOH / g, and 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 pigment dispersibility can be further optimized while optimizing the storage stability of the ink of this embodiment.

[0028] 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.

[0029] 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 from 10,000 to 50,000, it is possible to optimize the image density of the formed image and the ejection stability of the ink while suppressing an increase in the viscosity of the ink of this embodiment.

[0030] 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.).

[0031] 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).

[0032] [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 3.0 or more and 15.0 or less, more preferably a number of 6.0 or more and 8.0 or less. y preferably represents a number of 2.0 or more and 5.0 or less, more preferably 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 3.0 or more and 15.0 or less, and y preferably represents a number of 2.0 or more and 5.0 or less, more preferably a number of 3.0 or more and 15.0 or less, and y preferably represents a number of 2.0 or more and 3.0 or less.

[0033] The specific surfactant is a nonionic surfactant whose main structure is polyoxyalkylene alkyl ether, and has an ether bond (-O-) in the molecule that does not dissociate into ions even when dissolved in water. The specific surfactant is somewhere between hydrophobic and hydrophilic.

[0034] Commercially available specific surfactants include, for example, the "SOFTANOL (registered trademark) EP series" manufactured by Nippon Shokubai Co., Ltd. (specifically, EP5035, EP7025, EP7045, EP7085, EP9050, EP90150, and EP12030). All of the "SOFTANOL (registered trademark) EP series" have y of 1.0 or more in general formula (1) and have a -[CH2CH(CH3)O]- group. All of the "SOFTANOL (registered trademark) EP series" have the same basic skeleton and have similar effects even if the sizes of the side chains differ to some extent.

[0035] In the ink of this embodiment, the content of the specific surfactant is preferably from 0.10% to 1.50% by mass, and more preferably from 0.30% to 0.70% by mass. By setting the content of the specific surfactant to be from 0.10% to 1.50% by mass, the ejection stability of the ink of this embodiment can be optimized.

[0036] [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 60.00% by mass or more, and more preferably 80.00% by mass or more.

[0037] Other surfactants include, for example, surfactants in which y is less than 1.0 in the general formula (I), 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.

[0038] In the ink of this embodiment, the content of the other surfactant is preferably from 0.01% by mass to 1.00% by mass, and more preferably from 0.05% by mass to 0.20% by mass.

[0039] [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 It may further contain a water-soluble organic solvent having a water-soluble organic solvent having a water-soluble organic solvent content of less than -1.50 or more than 0.30 (hereinafter, sometimes referred to as other water-soluble organic solvent).

[0040] In the ink of this embodiment, the content of the aqueous medium is preferably from 75.00% by mass to 97.00% by mass, and more preferably from 87.00% by mass to 95.00% by mass.

[0041] (water) In the ink of this embodiment, the water content is preferably 25.00% by mass or more and 80.00% by mass or less, and more preferably 45.00% by mass or more and 55.00% by mass or less. By setting the water content to 25.00% by mass or more and 80.00% by mass or less, the ejection stability of the ink of this embodiment can be optimized.

[0042] (Specific organic solvent) LogK of specific organic solvent owis -1.50 or more and 0.30 or less. The specific organic solvent is an organic solvent that is close to the middle between hydrophobicity and hydrophilicity. Examples of the specific organic solvent include the organic solvents shown in Table 1 below. As the specific organic solvent, 1-propanol, 2-propanol, propylene glycol, or triethylene glycol monomethyl ether is preferred.

[0043] [Table 1]

[0044] In the ink of this embodiment, the content of the specific organic solvent is preferably 10.00% by mass or more and 50.00% by mass or less, and more preferably 15.00% by mass or more and 45.00% by mass or less. By setting the content of the specific organic solvent to 10.00% by mass or more and 50.00% by mass or less, the ejection stability of the ink of this embodiment can be optimized.

[0045] (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 10.00% by mass or less, more preferably 1.00% by mass or less, and even more preferably 0.00% by mass.

[0046] [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.

[0047] [Preferred composition] The ink of the present 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 preferable content ratio [mass %]. For example, "5.00-7.00", which is the pigment ratio of composition 1, indicates that the pigment is contained in an amount of 5.00 mass % or more and 7.00 mass % or less. "Resin" indicates a pigment-coated resin. "S-1", "S-4", and "S-5" of the types of specific surfactants respectively indicate the surfactants (S-1), (S-4), and (S-5) used in the examples. "TGME" of the types of specific organic solvents indicates triethylene glycol monomethyl ether. "PG" indicates propylene glycol. "IPA" indicates 2-propanol. "NPA" indicates 1-propanol.

[0048] [Table 2]

[0049] [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).

[0050] (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.

[0051] The content of the pigment in the pigment dispersion is preferably from 5.00% to 30.00% by mass, more preferably from 10.00% to 20.00% by mass, and the content of the pigment coating resin in the pigment dispersion is preferably from 2.00% to 12.00% by mass, more preferably from 5.00% to 7.00% by mass.

[0052] 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.

[0053] When a 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 of this embodiment is, for example, 20.00% by mass or more and 50.00% by mass or less. EXAMPLES

[0054] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0055] [Preparation of surfactant] Surfactants (S-1) to (S-5) shown in Table 3 below were prepared as surfactants. In the product names in Table 3 below, "(R)" indicates a registered trademark. Among the surfactants (S-1) to (S-5), surfactants (S-1), (S-4) and (S-5) were specific surfactants. Surfactant (S-2) was a surfactant in which y = 0 in general formula (I). Surfactant (S-3) was an acetylene glycol surfactant. Table 3 below lists the values ​​of m + n, x and y in general formula (I) for surfactants (S-1) to (S-2) and (S-4) to (S-5).

[0056] [Table 3]

[0057] [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.

[0058] (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.

[0059] 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.

[0060] (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

[0061] (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.

[0062] 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).

[0063] [Table 4]

[0064] 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.

[0065] 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 (Zetasizer (registered trademark) Nano ZS manufactured by Malvern Instruments). 50 The D of the pigment particles in the measurement sample was 50of 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.

[0066] <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.

[0067] [Example 1] 38.10 parts by mass of the pigment dispersion (5.70 parts by mass of pigment, 2.30 parts by mass of pigment coating resin (R)), a surfactant shown in Table 5 below (0.50 parts by mass of surfactant (S-1) in Example 1), an organic solvent X shown in Table 5 below (30.00 parts by mass of propylene glycol 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 100.00 parts by mass (31.40 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 way, the ink of Example 1 was obtained.

[0068] [Examples 2 to 10 and Comparative Examples 1 to 8] Inks of Examples 2 to 10 and Comparative Examples 1 to 8 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.

[0069] In Table 5, "TGME" represents triethylene glycol monomethyl ether, "DGME" represents dipropylene glycol monopropyl ether, and "DGMEA" represents diethylene glycol monoethyl ether acetate.

[0070] [Table 5]

[0071] <Evaluation> The ink ejection stability of the inks of the examples and comparative examples was evaluated by the following method. The evaluation results are shown in Table 6. The evaluation was performed at a temperature of 10° C. and a humidity of 15% RH unless otherwise specified.

[0072] A prototype ejection evaluation machine equipped with a recording head ("KJ4B-QA") manufactured by Kyocera Corporation was used as the evaluation machine. The recording head was set to have 2656 nozzles per head, a resolution of 600 dpi, and a drive frequency of 20 kHz. The recording head was filled with the ink to be evaluated (more specifically, any of the inks of the examples and comparative examples). The paper transport speed of the recording medium was 840 mm / sec.

[0073] [Discharge stability] First, the following first to third tests were carried out.

[0074] [First test] Using the evaluation machine, a 300 x 300 dot solid image was formed on a recording medium (glossy paper "KA4100PGP" manufactured by Seiko Epson Corporation). Immediately after the formation of the solid image, a line image of one dot line serving as a reference was formed in the conveying direction of the recording medium (hereinafter, sometimes referred to as the first line image). Five seconds after the formation of this first line image, a line image of one dot line (hereinafter, sometimes referred to as the second line image) was formed in the conveying direction of the recording medium. The density and color tone of the first dot dropped in this first line image and the density and color tone of the first dot dropped in the second line image were visually observed using an optical microscope.

[0075] [2nd test ~ 3rd test] The second and third tests were carried out using the same procedure as the first test, except that the time from the formation of the first line image to the formation of the second line image was changed to 10 seconds or 30 seconds.

[0076] The ink ejection stability was evaluated by comparing the density and color tone of the first dot dropped in the first line image formed in each test with the density and color tone of the first dot dropped in the second line image. If the evaluations of the first to third tests were all "B" or higher, the ejection stability was determined to be good.

[0077] (Evaluation criteria for ejection stability) A (particularly good): In the second line image, dots were formed that had no difference in density or color tone compared to the dots in the first line image. B (Good): In the second line image, there were differences in density and color tone compared to the dots in the first line image, but dots of good density and color tone were formed. C (slightly poor): The second line image contains dots that are lighter in color and have unclear contours compared to the dots in the first line image. D (Poor): The dots become transparent and cannot be recognized.

[0078] [Table 6]

[0079] 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 ow was greater than or equal to -1.50 and less than or equal to 0.30. The inks of the examples were excellent in the evaluation of ejection stability.

[0080] On the other hand, the inks of Comparative Examples 1 and 2 did not contain the specific organic solvent or specific surfactant, but instead contained a surfactant (S-2) in which y=0 in general formula (I) as a surfactant, and contained a LogK ow DGME or LogK less than -1.50 ow As a result, the inks of Comparative Examples 1 and 2 were poor in the evaluation of ejection stability.

[0081] Moreover, the inks of Comparative Examples 3 and 4 did not contain the specific surfactant, but instead contained a surfactant (S-2) in general formula (I) where y = 0. As a result, the inks of Comparative Examples 3 and 4 had poor evaluation results for ejection stability.

[0082] The inks of Comparative Examples 5 and 6 did not contain a specific organic solvent, but instead contained an organic solvent having a LogK ow DGME or LogK less than -1.50 ow As a result, the inks of Comparative Examples 5 and 6 were poor in the evaluation of ejection stability.

[0083] The inks of Comparative Examples 7 and 8 did not contain the specific surfactant, but instead contained an acetylene glycol surfactant. As a result, the inks of Comparative Examples 7 and 8 had poor evaluation results for ejection stability. [Industrial Applicability]

[0084] 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 -1.50 or more and 0.30 or less. [Formula 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 1.0 or more and 10.0 or less.)

2. The ink-jet ink according to claim 1 , wherein the content of the specific surfactant is from 0.10% by mass to 1.50% by mass.

3. The ink-jet ink according to claim 1 or 2, wherein a content of the specific organic solvent is from 15.00% by mass to 45.00% by mass.

4. In the general formula (I), x represents a number between 6.0 and 8.0, The ink-jet ink according to claim 1 or 2, wherein y represents a number of 2.0 or more and 3.0 or less.

5. Octanol / water partition coefficient LogK of the specific organic solvent ow The ink-jet ink according to claim 1 or 2, wherein is -1.00 or more and 0.20 or less.

6. 3. The ink-jet ink according to claim 1, wherein the specific organic solvent includes at least one of propylene glycol, triethylene glycol monomethyl ether, 1-propanol, and 2-propanol.

Images