Red Ink for Inkjet

Abstract

To provide a red ink for inkjet that is excellent in color developability, heat resistance and storage stability.SOLUTION: The red ink for inkjet contains a pigment and a specific organic solvent, and has an absorption maximum wavelength in the range of 545 nm or more and 555 nm or less wavelength. The specific organic solvent includes 1,5-pentanediol or 3-methyl-1,5-pentanediol. The pigment may include C.I. Pigment Red 112, C.I. Pigment Red 122, C.I. Pigment Red 168, C.I. Pigment Red 48:1, C.I. Pigment Red 53:1, or C.I. Pigment Red 188.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a red ink for inkjet.

Background Art

[0002] In recent years, inkjet recording apparatuses have been rapidly advancing. For example, when using photographic paper as a recording medium, an inkjet recording apparatus can form a high-quality image comparable to a silver halide photograph.

[0003] Along with this, various performances are required for the inkjet ink used in an inkjet recording apparatus. Specifically, in addition to the color developability and storage stability that have been conventionally required, the inkjet ink is required to have a performance (heat resistance) such that the formed image does not change color even when exposed to high temperatures. Discoloration of the image when exposed to high temperatures occurs, for example, when the resin component contained in the inkjet ink is carbonized. An image in which the resin component is carbonized has yellow added to the original hue.

[0004] Discoloration of the image due to carbonization of the resin component is more likely to affect inkjet inks having a hue farther from yellow in the color wheel. Specifically, carbonization of the resin component has little effect on yellow ink for inkjet, but has a relatively strong effect on red ink for inkjet and an extremely strong effect on blue ink for inkjet. Currently, the development of a red ink for inkjet that is excellent in heat resistance and less likely to cause discoloration of the image due to carbonization of the resin component is underway.

[0005] As a red ink for inkjet that is excellent in color developability and storage stability, for example, a red ink for inkjet containing a pigment containing C.I. Pigment Red 150, a water-soluble organic solvent, and a radical polymer has been proposed (Patent Document 1).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, even with the red ink for inkjet described in Patent Document 1, it is difficult to sufficiently suppress the discoloration of the image due to the carbonization of the resin component.

[0008] The present invention has been made in view of the above problems, and an object thereof is to provide a red ink for inkjet that is excellent in color developability, heat resistance, and storage stability.

Means for Solving the Problems

[0009] The red ink for inkjet according to the present invention contains a pigment and a specific organic solvent, and has an absorption maximum wavelength in the range of 545 nm or more and 555 nm or less. The specific organic solvent includes 1,5-pentanediol or 3-methyl-1,5-pentanediol.

Effects of the Invention

[0010] The red ink for inkjet according to the present invention is excellent in color developability, heat resistance, and storage stability.

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention will be described. In the following, unless otherwise specified, the measured value of the volume median diameter (D 50 ) is a value measured using a dynamic light scattering particle size distribution measuring device ("Zetasizer Nano ZS" manufactured by Malvern) if not otherwise specified.

[0012] Hereinafter, unless otherwise specified, the measured value of the acid value is a value measured in accordance with "JIS (Japanese Industrial Standard) K0070: 1992". The measured value of the mass average molecular weight (Mw) is a value measured using gel permeation chromatography if not otherwise specified.

[0013] In this specification, acrylic and methacrylic may be collectively referred to as "(meth)acrylic".

[0014] <Red Ink> Hereinafter, an inkjet red ink according to an embodiment of the present invention (hereinafter, may be simply referred to as red ink) will be described. The red ink of the present invention contains a pigment and a specific organic solvent, and has an absorption maximum wavelength in the range of 545 nm or more and 555 nm or less. The specific organic solvent includes 1,5-pentanediol or 3-methyl-1,5-pentanediol.

[0015] Note that the absorption maximum wavelength of the red ink of the present invention can be obtained by measuring the absorption spectrum in the wavelength range of 300 nm to 800 nm using a spectrophotometer. The wavelength at which the absorbance is maximum in the absorption spectrum is the absorption maximum wavelength.

[0016] The use of the red ink of the present invention is not particularly limited, but it is suitable as a red ink used in an inkjet recording apparatus equipped with a line head.

[0017] By having the above-described configuration, the red ink of the present invention is excellent in color development property, heat resistance, and storage stability. The reason is presumed as follows. Known red inks usually have an absorption maximum wavelength near 530 nm. An image formed by such a known red ink is likely to visually change when the resin component is carbonized and yellow is added to the hue. On the other hand, the red ink of the present invention has an absorption maximum wavelength in the range of 545 nm or more and 555 nm or less. By having the above-described absorption maximum wavelength, the red ink of the present invention is less likely to visually change even when yellow is added to the hue of the formed image. Therefore, the red ink of the present invention is excellent in heat resistance.

[0018] On the one hand, if only the absorption maximum wavelength of the red ink with a general composition is adjusted within the above range, the lightness of the formed image (L value in Lab values) tends to increase, and the visual image density tends to decrease (that is, there is a tendency that sufficient color developability cannot be obtained). In order to improve the color developability, it is effective to add a highly hydrophobic humectant to the red ink. After the red ink containing a highly hydrophobic humectant is ejected onto the surface of the recording medium, the pigment tends to remain on the surface of the recording medium without penetrating into the interior of the recording medium, so that an image with high lightness can be formed. On the other hand, the pigment necessary to achieve the above absorption maximum wavelength tends to reduce the storage stability of the red ink. When a highly hydrophobic humectant is added to such a red ink, the storage stability of the red ink is further reduced. Thus, it is difficult to achieve both the color developability and storage stability of the red ink by simply adjusting the absorption maximum wavelength of the red ink with a general composition within the above range. In contrast, the red ink of the present invention contains a specific organic solvent containing 1,5-pentanediol or 3-methyl-1,5-pentanediol as a humectant. Since the specific organic solvent has appropriate hydrophobicity, it improves the color developability without reducing the storage stability of the red ink of the present invention. Thus, the red ink of the present invention has excellent heat resistance, color developability, and storage stability by having the above absorption maximum wavelength and containing a specific organic solvent with appropriate hydrophobicity.

[0019] Hereinafter, the red ink of the present invention will be described in more detail. Each component described below may be used alone or in combination of two or more.

[0020] [Pigment] In the red ink of the present invention, the pigment constitutes pigment particles together with, for example, a pigment dispersion resin. The pigment particles are composed of, for example, a core containing the pigment and a pigment dispersion resin coating the core. The pigment dispersion resin is present, for example, dispersed in a solvent. From the viewpoint of improving the color density, hue, or stability of the red ink of the present invention, the D of the pigment particles 50 is preferably 30 nm or more and 200 nm or less, and more preferably 70 nm or more and 130 nm or less.

[0021] The red ink of the present invention preferably contains a red pigment. Examples of the pigment contained in the red ink of the present invention include C.I. Pigment Red 112, C.I. Pigment Red 122, C.I. Pigment Red 168, C.I. Pigment Red 48:1, C.I. Pigment Red 53:1, or C.I. Pigment Red 188. By using these pigments, it becomes easier to adjust the absorption maximum wavelength of the red ink of the present invention to a range of 545 nm or more and 555 nm or less.

[0022] In the red ink of the present invention, the content ratio of the pigment is preferably 1.0% by mass or more and 12.0% by mass or less, more preferably 4.0% by mass or more and 8.0% by mass or less. By setting the content ratio of the pigment to 1.0% by mass or more, an image having a desired image density can be obtained with the red ink of the present invention. Further, by setting the content ratio of the pigment to 12.0% by mass or less, the fluidity of the red ink of the present invention can be improved.

[0023] [Pigment dispersion resin] The pigment dispersion resin has water solubility and suppresses the aggregation of the pigment by adhering to the surface of the pigment.

[0024] Examples of the pigment dispersion resin include copolymers of at least one monomer selected from (meth)acrylic acid alkyl esters, styrene, and vinyl naphthalene and at least one monomer selected from (meth)acrylic acid and maleic acid.

[0025] As the pigment-dispersing resin, a resin having repeating units derived from (meth)acrylic acid ((meth)acrylic acid units), repeating units derived from (meth)acrylic acid alkyl esters ((meth)acrylic acid alkyl ester units), and styrene units is preferable. In this case, among all the repeating units of the pigment-dispersing resin, the proportion of (meth)acrylic acid units is preferably 4.5% by mass or more and 8.0% by mass or less. Among all the repeating units of the pigment-dispersing resin, the proportion of (meth)acrylic acid alkyl ester units is preferably 35% by mass or more and 70% by mass or less. Among all the repeating units of the pigment-dispersing resin, the proportion of styrene units is preferably 27% by mass or more and 60% by mass or less. As the pigment-dispersing resin, a resin having repeating units derived from methacrylic acid, repeating units derived from methyl methacrylate, repeating units derived from butyl acrylate, and styrene units is more preferable.

[0026] In the red ink of the present invention, the content ratio of the pigment-dispersing resin is preferably 0.5% by mass or more and 8.0% by mass or less, and more preferably 1.5% by mass or more and 4.0% by mass or less. By setting the content ratio of the pigment-dispersing resin to 0.5% by mass or more, aggregation of the pigment can be more effectively suppressed. By setting the content ratio of the pigment-dispersing resin to 8.0% by mass or less, occurrence of nozzle clogging can be suppressed.

[0027] The acid value of the pigment coating resin is preferably 50 mgKOH / g or more and 150 mgKOH / g or less. By setting the acid value of the pigment coating resin to 50 mgKOH / g or more and 150 mgKOH / g or less, the dispersibility of the pigment can be improved and the storage stability of the red ink of the present invention can be further improved.

[0028] The acid value of the pigment-dispersing resin can be adjusted by changing the monomers used when synthesizing the pigment-dispersing resin. For example, when synthesizing the pigment-dispersing resin, by using monomers having acidic functional groups (for example, carboxy groups) (for example, acrylic acid and methacrylic acid), the acid value of the pigment-dispersing resin can be increased.

[0029] The mass average molecular weight of the pigment-dispersing resin is preferably 10,000 or more and 50,000 or less, more preferably 15,000 or more and 30,000 or less. By setting the mass average molecular weight of the pigment-dispersing resin to be 10,000 or more and 50,000 or less, an image having a desired image density can be obtained while suppressing an increase in the viscosity of the red ink of the present invention.

[0030] The mass average molecular weight of the pigment-dispersing resin can be adjusted by changing the polymerization conditions of the pigment-dispersing resin (for example, the amount of polymerization initiator used, the polymerization temperature, and the polymerization time). The pigment-dispersing resin is preferably neutralized in an equivalent amount with a base (for example, KOH or NaOH).

[0031] [Specific organic solvent] The specific organic solvent contains 1,5-pentanediol or 3-methyl-1,5-pentanediol. In the red ink of the present invention, the specific organic solvent functions as a humectant.

[0032] In the red ink of the present invention, the content ratio of the specific organic solvent is preferably 5.0% by mass or more and 40.0% by mass or less, more preferably 15.0% by mass or more and 25.0% by mass or less. By setting the content ratio of the specific organic solvent to be 5.0% by mass or more, the color developability of the red ink of the present invention can be further improved. By setting the content ratio of the specific organic solvent to be 40.0% by mass or less, the storage stability of the red ink of the present invention can be further improved.

[0033] [Water] The red ink of the present invention preferably contains water as a solvent. When the red ink of the present invention contains water, the content ratio of water in the red ink of the present invention is, for example, 40.0% by mass or more and 65.0% by mass or less.

[0034] [Other water-soluble organic solvents] The red ink of the present invention preferably further contains a water-soluble organic solvent other than a specific organic solvent. Examples of the other water-soluble organic solvents include glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, thiodiglycol, glycerin, and dimethyl sulfoxide.

[0035] Examples of the glycol ether compounds include diethylene glycol diethyl ether, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, and propylene glycol monomethyl ether. The glycol ether compound preferably has a butyl ether structure. As a specific glycol ether compound, triethylene glycol monobutyl ether is preferable.

[0036] Examples of the lactam compounds include 2-pyrrolidone and N-methyl-2-pyrrolidone.

[0037] Examples of the nitrogen-containing compounds include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

[0038] Examples of the acetate compounds include diethylene glycol monoethyl ether acetate.

[0039] The red ink of the present invention preferably further contains a glycol ether compound. By further containing a glycol ether compound in the red ink of the present invention, after being discharged onto the surface of the recording medium, penetration of the pigment into the interior of the recording medium can be more effectively suppressed. As a result, the color developability of the red ink of the present invention can be further improved.

[0040] As for the content ratio of the glycol ether compound in the red ink of the present invention, it is preferably 0.5% by mass or more and 10.0% by mass or less, and more preferably 2.0% by mass or more and 6.0% by mass or less. By setting the content ratio of the glycol ether compound to 0.5% by mass or more and 10.0% by mass or less, the color development property of the red ink of the present invention can be further improved.

[0041] Further, the red ink of the present invention preferably further contains glycerin. By the red ink of the present invention further containing glycerin, the ejection property of the red ink can be improved. As for the content ratio of glycerin in the red ink of the present invention, it is preferably 0.5% by mass or more and 15.0% by mass or less, and more preferably 3.0% by mass or more and 8.0% by mass or less. By setting the content ratio of glycerin to 0.5% by mass or more and 15.0% by mass or less, the ejection property of the red ink of the present invention can be further improved.

[0042] [Surfactant] The red ink of the present invention preferably further contains a surfactant. The surfactant improves the permeability (wettability) of the red ink of the present invention with respect to the recording medium. Examples of the surfactant include anionic surfactants, cationic surfactants, and nonionic surfactants. As the surfactant, a nonionic surfactant is preferable.

[0043] Examples of the nonionic surfactant include polyoxyethylene dodecyl ether, polyoxyethylene hexadecyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan monooleate ether, monodecanoyl sucrose, and ethylene oxide adducts of acetylene glycol. As the nonionic surfactant, an ethylene oxide adduct of acetylene glycol is preferable.

[0044] When the red ink of the present invention contains a surfactant, as for the content ratio of the surfactant in the red ink of the present invention, it is preferably 0.05% by mass or more and 3.0% by mass or less, and more preferably 0.2% by mass or more and 1.0% by mass or less.

[0045] [Other components] The red ink of the present invention may further contain known additives (more specifically, for example, dissolution stabilizers, anti-drying agents, antioxidants, viscosity modifiers, pH adjusters, and fungicides), if necessary.

[0046] [Method for producing red ink] The red ink of the present invention can be produced, for example, by uniformly mixing a pigment dispersion containing a pigment and a pigment dispersion resin, a specific organic solvent, and other components (for example, water, glycol ether compounds, glycerin, and surfactants) that are blended as necessary, using a stirrer. In the production of the red ink of the present invention, after uniformly mixing each component, foreign substances and coarse particles may be removed by a filter (for example, a filter having a pore size of 5 μm or less).

[0047] (Pigment dispersion) The pigment dispersion is a dispersion containing a pigment and a pigment dispersion resin. Water is preferably used as the dispersion medium of the pigment dispersion. The pigment dispersion preferably further contains a surfactant in order to improve the dispersibility of the pigment particles.

[0048] In the pigment dispersion, the D of the pigment particles composed of the pigment and the pigment dispersion resin 50 is preferably 50 nm or more and 200 nm or less, and more preferably 70 nm or more and 130 nm or less.

[0049] The D of the pigment particles 50 can be measured, for example, using a dynamic light scattering particle size distribution measuring device (for example, "Zetasizer Nano ZS" manufactured by Malvern) with a solution obtained by diluting the pigment dispersion 300 times with ion-exchanged water as a sample.

[0050] As the pigment content ratio in the pigment dispersion liquid, 5.0 mass% or more and 25.0 mass% or less is preferable, and 10.0 mass% or more and 20.0 mass% or less is more preferable. As the content ratio of the pigment dispersion resin in the pigment dispersion liquid, 2.0 mass% or more and 10.0 mass% or less is preferable, and 4.0 mass% or more and 8.0 mass% or less is more preferable. When the pigment dispersion liquid contains a surfactant, as the content ratio of the surfactant in the pigment dispersion liquid, 0.1 mass% or more and 2.0 mass% or less is preferable, and 0.3 mass% or more and 1.0 mass% or less is more preferable.

[0051] The pigment dispersion liquid can be prepared by wet-dispersing a pigment, a pigment dispersion resin, a dispersion medium (for example, water), and components (for example, a surfactant) added as necessary using a media type wet disperser. In the wet dispersion by the media type wet disperser, as the media, for example, small particle size beads (for example, beads with D 50 of 0.5 mm or more and 1.0 mm or less) can be used. The material of the beads is not particularly limited, but a hard material (for example, glass and zirconia) is preferable.

[0052] When adding the pigment dispersion liquid in the production of the red ink of the present invention, the ratio of the pigment dispersion liquid to all the raw materials of the red ink is, for example, 25.0 mass% or more and 60.0 mass% or less.

Examples

[0053] Hereinafter, examples of the present invention will be described. However, the present invention is not limited to the following examples.

[0054] In the examples, the Mw of the pigment dispersion resin was measured under the following conditions using gel permeation chromatography ("HLC-8020GPC" manufactured by Tosoh Corporation). The calibration curve was created using TSKgel standard polystyrene manufactured by Tosoh Corporation, namely F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000, and n-propylbenzene.

[0055] (Measurement conditions for mass average molecular weight) · Column: "TSKgel SuperMultiporeHZ-H" manufactured by Tosoh Corporation (semi-micro column with 4.6 mm I.D. × 15 cm) · Number of columns: 3 · Eluent: Tetrahydrofuran · Flow rate: 0.35 mL / min · Sample injection volume: 10 μL · Measurement temperature: 40 °C · Detector: IR detector

[0056] <Study 1: Types of pigments> First, the types of pigments were studied. The preparation methods of each raw material used in the production of red ink are shown below.

[0057] (Preparation of pigment dispersion resin) A pigment dispersion resin (R-1) having repeating units derived from methacrylic acid (MAA units), repeating units derived from methyl methacrylate (MMA units), repeating units derived from butyl acrylate (BA units), and repeating units derived from styrene (ST units) was prepared. This pigment dispersion resin (R-1) 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 pigment dispersion resin (R-1) was "MAA units: MMA units: BA units: ST units = 6.5: 30.0: 30.0: 33.5". This pigment dispersion resin (R-1) was mixed with an aqueous sodium hydroxide solution containing sodium hydroxide. The aqueous sodium hydroxide solution contained 1.05 times the amount of sodium hydroxide required for equivalent neutralization of the pigment dispersion resin (R-1). As a result, the pigment dispersion resin (R-1) was neutralized with an equivalent amount (strictly, 105% amount) of KOH. As a result, a pigment dispersion resin solution containing the pigment dispersion resin (R-1) and water was obtained.

[0058] (Preparation of pigment dispersions (D-1) to (D-7)) As shown in Table 1 below, a pigment (refer to Table 2 below for the type of pigment), the above-mentioned pigment dispersion resin solution, "Orfin (registered trademark) E1010" manufactured by Nissin Chemical Industry Co., Ltd. as a nonionic surfactant (ethylene oxide adduct of acetylene diol), and ion-exchanged water were put into a vessel.

[0059] Note that the content ratio of "water" in Table 1 below indicates the total content ratio of the ion-exchanged water put into the above-mentioned vessel and the water contained in the pigment dispersion resin solution (specifically, the water contained in the sodium hydroxide aqueous solution used for neutralizing the pigment dispersion resin, and the water generated by the neutralization reaction of the pigment dispersion resin and sodium hydroxide).

[0060]

Table 1

[0061]

Table 2

[0062] Subsequently, using zirconia beads (particle size 0.5 mm) as a medium and a wet disperser ("Nano Glen Mill" manufactured by Asada Iron Works Co., Ltd.), the content of the above-mentioned vessel was dispersed. The dispersion conditions were a temperature of 10°C, a peripheral speed of 8 m / sec, and a discharge rate of 300 g / min. As a result, pigment dispersions (D-1) to (D-7) were obtained.

[0063] [Examples 1 to 5 and Comparative Examples 1 to 2] Ion-exchanged water was charged into a flask equipped with a stirrer (the "Three-in-One Motor (registered trademark) BL-600" manufactured by Shin-Tong Science Co., Ltd.). While stirring the contents with the above-described stirrer (stirring speed: 400 rpm), the above-described pigment dispersion (refer to Table 4 below for the type of pigment dispersion), a nonionic surfactant ("Surfynol (registered trademark) 420" manufactured by Nissin Chemical Industry Co., Ltd., an acetylene glycol surfactant), triethylene glycol monobutyl ether which is a glycol compound, 3-methyl-1,5-pentanediol which is a specific organic solvent, and glycerin were charged in this order. The types and proportions of the charged amounts of each raw material were as shown in Table 3 below.

[0064] [Table 3]

[0065] In order to remove foreign substances and coarse particles from the obtained mixed solution, the mixed solution was filtered using a filter with a pore size of 5 μm. Thereby, red inks of Examples 1 to 5 and Comparative Examples 1 to 2 were obtained.

[0066] (Measurement of the absorption maximum wavelength) Using ion-exchanged water, the ink to be measured (specifically, any one of the inks of Examples 1 to 5 and Comparative Examples 1 to 2) was diluted 1000 times. The obtained diluted solution was filled into a measurement cell, and the measurement cell was set in the sample chamber of a spectrophotometer ("U-3010" manufactured by Hitachi High-Tech Science Corporation). Using the spectrophotometer, the absorption spectrum of the diluted solution in the wavelength range of 300 nm to 800 nm was measured. Based on the absorption spectrum, the absorption maximum wavelength of each ink was determined. The obtained absorption maximum wavelengths are shown in Table 4 below.

[0067] [Evaluation] For the red inks of Examples 1 to 5 and Comparative Examples 1 to 2, the color development property, heat resistance, and storage stability were evaluated by the following methods, respectively. The evaluation results are shown in Table 4 below. The evaluation was carried out in an environment at a temperature of 25°C and a humidity of 60% RH unless otherwise specified.

[0068] (Color development property) As an evaluation device, an inkjet recording apparatus (a prototype manufactured by Kyocera Document Solutions Inc.) was used. The evaluation device was equipped with a piezo type line head having nozzles (aperture diameter of the opening: 10 μm). The evaluation target (specifically, any of the red inks of Examples 1 to 5 and Comparative Examples 1 to 2) was set in the line head of the evaluation device. Subsequently, a solid image of 1 cm × 1 cm was formed on A4 plain paper ("Paper One (registered trademark)" manufactured by APRIL) using the evaluation device (ink loading amount 0.55 mg / cm 2 ). At this time, the volume per dot (volume per drop) of the red ink ejected from each nozzle of the line head was set to 9.5 pL. Next, the plain paper (evaluation paper) on which the solid image was formed was dried for 12 hours. Next, using a fluorescence spectrophotometer ("FD-5" manufactured by Konica Minolta Inc.), the Lab values (L value, a value, and b value) of the solid image formed on the above-mentioned evaluation paper were measured. Regarding the color development property of the red ink, when the L value was 60.0 or less, it was determined to be good (A), and when the L value exceeded 60.0, it was determined to be poor (B).

[0069] (Heat resistance) The evaluation paper used for the evaluation of color development property was heat-treated at 105 °C for 72 hours. Next, regarding the evaluation paper after the heat treatment, using the above-mentioned fluorescence spectrophotometer, the Lab values (L value, a value, and b value) of the solid image formed on the above-mentioned evaluation paper were measured. The change amount (ΔL) of the L value, the change amount (Δa) of the a value, and the change amount (Δb) of the b value before and after the heat treatment were respectively obtained. By applying ΔL, Δa, and Δb to the following formula, the change amount (ΔE) of the Lab value due to the heat treatment was calculated. Regarding the heat resistance of the red ink, when ΔE was less than 5.0, it was determined to be good (A), and when Δ3 was 5.0 or more, it was determined to be poor (B). ΔE = 〔(ΔL) 2 +(Δa) 2 +(Δb) 2 〕 1 / 2

[0070] (Storage stability) The viscosity of the evaluation target (specifically, any one of the red inks of Examples 1 to 5 and Comparative Examples 1 and 2) was measured using a falling ball type automatic microviscometer ("AMVn" manufactured by Anton Paar). In the measurement of viscosity, a capillary with a diameter of 1.6 mm and a steel ball with a diameter of 1.5 mm and a specific gravity of 7.63 were used, the falling ball angle was 70 degrees, and the temperature was 25°C. After the measurement, the viscosity (initial viscosity) of each evaluation target was calculated using the dedicated software of the above-mentioned "AMVn" manufactured by Anton Paar.

[0071] Next, after putting the evaluation target into a plastic wide-mouth bottle ("I Boy" sold by AS ONE Corporation), the lid of the plastic wide-mouth bottle was closed, and heat treatment was performed at 60°C for 2 weeks. The viscosity of the evaluation target after the heat treatment (viscosity after heating) was measured by the same method as the measurement of the initial viscosity. By applying the initial viscosity and the viscosity after heating to the following formula, the change rate of viscosity due to the heat treatment was calculated. The storage stability of the evaluation target was determined to be good (A) when the absolute value of the change rate of viscosity was within 5%, and determined to be bad (B) when the absolute value of the change rate of viscosity exceeded 5%. Change rate of viscosity = 100×(viscosity after heating - initial viscosity) / initial viscosity

[0072]

Table 4

[0073] As shown in Tables 1 to 4, the red inks of Examples 1 to 5 had an absorption maximum wavelength in the range of 545 nm or more and 555 nm or less. The red inks of Examples 1 to 5 were excellent in color development property, heat resistance, and storage stability.

[0074] On the other hand, the red inks of Comparative Examples 1 and 2 had an absorption maximum wavelength in the range exceeding 555 nm. The red inks of Comparative Examples 1 and 2 were insufficient in heat resistance. This is considered to be because the red inks of Comparative Examples 1 and 2 form an image of a hue that is easily affected by the carbonization of the resin.

[0075] <Examination 2: Type of organic solvent> Next, the types of organic solvents used as humectants were examined. As the organic solvents used in the examination, the compounds shown in Table 5 below were used. The hydrophobicity levels of the compounds shown in Table 5 below are shown as follows. 1,4BD < 1,5PD < MPD < 1,6HD

[0076] [Table 5]

[0077] [Examples 6 - 9 and Comparative Examples 3 - 4] Red inks of Examples 6 - 9 and Comparative Examples 3 - 4 were prepared in the same manner as the preparation of the inks of Examples 1 - 5 and Comparative Examples 1 - 2, except that the types and proportions of the input amounts of the respective raw materials were as shown in Table 6 below.

[0078] [Table 6]

[0079] To remove foreign substances and coarse particles from the obtained mixed solution, the mixed solution was filtered using a filter with a pore size of 5 μm. Thereby, red inks of Examples 6 - 9 and Comparative Examples 3 - 4 were obtained.

[0080] The absorption maximum wavelengths of the red inks of Examples 6 - 9 and Comparative Examples 3 - 4 were measured in the same manner as the measurement of the absorption maximum wavelengths of the red inks of Examples 1 - 5 and Comparative Examples 1 - 2. The obtained absorption maximum wavelengths are shown in Table 7 below.

[0081] [Evaluation] The color development property, heat resistance, and storage stability of the red inks of Examples 6 - 9 and Comparative Examples 3 - 4 were evaluated in the same manner as the evaluation of the red inks of Examples 1 - 5 and Comparative Examples 1 - 2. The evaluation results are shown in Table 7 below.

[0082] [Table 7]

[0083] As shown in Table 7, the red inks of Examples 6 to 9 contained 1,5-pentanediol or 3-methyl-1,5-pentanediol, which are specific organic solvents with moderately high hydrophobicity, as humectants. The red inks of Examples 6 to 9 were excellent in color development, heat resistance, and storage stability.

[0084] On the other hand, the red ink of Comparative Example 3 contained 1,4-butanediol, which is a relatively hydrophilic organic solvent, as a humectant. It is judged that the red ink of Comparative Example 3 could not retain the pigment on the surface of the recording medium after being ejected onto the surface of the recording medium. Therefore, it is judged that the red ink of Comparative Example 3 has poor color development and forms an image with a relatively large L value. Further, the red ink of Comparative Example 3 also had poor heat resistance.

[0085] The red ink of Comparative Example 4 contained 1,6-hexanediol, which is an overly hydrophobic organic solvent, as a humectant. Since the red ink of Comparative Example 4 contained an overly hydrophobic organic solvent, it was judged to have poor storage stability.

[0086] In summary, a red ink containing a pigment and a specific organic solvent, having an absorption maximum wavelength in the range of 545 nm or more and 555 nm or less, and the specific organic solvent containing 1,5-pentanediol or 3-methyl-1,5-pentanediol is judged to be excellent in color development, heat resistance, and storage stability.

Industrial Applicability

[0087] The red ink of the present invention can be used for forming an image.

Claims

1. containing a pigment and a specific organic solvent, having an absorption maximum wavelength in the range of 545 nm or more and 555 nm or less, wherein the specific organic solvent contains 1,5-pentanediol or 3-methyl-1,5-pentanediol, a red ink for inkjet.

2. The pigment is C.I. Pigment Red 112, C.I. Pigment Red 122, C.I. Pigment Red 168, C.I. Pigment Red 48:1, C.I. Pigment Red 53:1, or C.I. Pigment Red 188, the red ink for inkjet according to Claim 1.

3. The content ratio of the specific organic solvent is 5.0% by mass or more and 40.0% by mass or less, the red ink for inkjet according to Claim 1 or 2.

4. further containing a glycol ether compound, the content ratio of the glycol ether compound is 0.5% by mass or more and 10.0% by mass or less, the red ink for inkjet according to any one of Claims 1 to 3.

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