JP7690784B2 - Red ink for inkjet printers - Google Patents

Description

The present invention relates to a red ink for inkjet printing.

In recent years, inkjet recording devices have made rapid advances. For example, when using photo paper as the recording medium, inkjet recording devices can produce high-quality images that rival those of silver halide photographs.

As a result, various performance requirements are now being placed on the inkjet inks used in inkjet recording devices. Specifically, in addition to the traditional color development and storage stability required of inkjet inks, they are now required to have the ability to withstand high temperatures without discoloring the images they form. Discoloration when an image is exposed to high temperatures occurs, for example, when the resin components contained in the inkjet ink are carbonized. When the resin components are carbonized, the image takes on a yellow hue in addition to its original hue.

Image discoloration due to carbonization of resin components is more likely to affect inkjet inks whose hues are farther from yellow on the color wheel. Specifically, carbonization of resin components does not have much effect on inkjet yellow inks, but has a relatively strong effect on inkjet red inks and an extremely strong effect on inkjet blue inks. For the time being, efforts are underway to develop inkjet red inks that have excellent heat resistance and are less susceptible to image discoloration due to carbonization of resin components.

As an example of a red inkjet ink with excellent color development and storage stability, a red inkjet ink containing a pigment including C.I. Pigment Red 150, a water-soluble organic solvent, and a radical polymer has been proposed (Patent Document 1).

International Publication No. 2018/088222

However, even with the red inkjet ink described in Patent Document 1, it is difficult to sufficiently prevent discoloration of the image caused by carbonization of the resin component.

The present invention was made in consideration of the above problems, and its purpose is to provide a red inkjet ink that has excellent color development, heat resistance, and storage stability.

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

The red inkjet ink of the present invention has excellent color development, heat resistance, and storage stability.

Hereinafter, embodiments of the present invention will be described. In the following, unless otherwise specified, the measured value of the volume median diameter ( _D50 ) is a value measured using a dynamic light scattering particle size distribution analyzer ("Zetasizer Nano ZS" manufactured by Malvern Instruments).

In the following, unless otherwise specified, the measured acid value is a value measured according to "JIS (Japanese Industrial Standards) K0070:1992." Furthermore, unless otherwise specified, the measured mass average molecular weight (Mw) is a value measured using gel permeation chromatography.

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

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

The maximum absorption wavelength of the red ink of the present invention can be determined 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 maximum absorption wavelength.

The uses of the red ink of the present invention are not particularly limited, but it is suitable as a red ink for use in an inkjet recording device equipped with a line head.

The red ink of the present invention has excellent color development, heat resistance, and storage stability due to the above-mentioned configuration. The reason for this is presumed to be as follows. Publicly known red inks usually have a maximum absorption wavelength near 530 nm. Images formed with such publicly known red inks are prone to visually change when the resin component is carbonized and yellow is added to the hue. In contrast, the red ink of the present invention has a maximum absorption wavelength in the wavelength range of 545 nm to 555 nm. Since the red ink of the present invention has the above-mentioned maximum absorption wavelength, it is difficult to visually change even if yellow is added to the hue of the image formed. For this reason, the red ink of the present invention has excellent heat resistance.

On the other hand, if the absorption maximum wavelength of a red ink of a general composition is adjusted to the above range, the brightness of the formed image (L value in Lab values) tends to be high and the visual image density tends to be low (i.e., sufficient color development tends to be not obtained). In order to improve color development, it is effective to add a highly hydrophobic humectant to the red ink. After being ejected onto the surface of the recording medium, the red ink to which a highly hydrophobic humectant has been added 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 brightness can be formed. On the other hand, the pigment required to realize the above absorption maximum wavelength tends to reduce the storage stability of the red ink. If a highly hydrophobic humectant is added to such a red ink, the storage stability of the red ink is further reduced. In this way, it is difficult to achieve both color development and storage stability of the red ink by simply adjusting the absorption maximum wavelength of a red ink of a general composition to 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. The specific organic solvent has a suitable degree of hydrophobicity, and therefore improves the color development without reducing the storage stability of the red ink of the present invention. In this way, the red ink of the present invention has excellent heat resistance, color development, and storage stability by containing a specific organic solvent having the above-mentioned maximum absorption wavelength and suitable hydrophobicity.

The red ink of the present invention will be described in more detail below. Each of the components described below may be used alone or in combination of two or more.

[Pigment]
In the red ink of the present invention, the pigment constitutes a pigment particle together with, for example, a pigment dispersion resin. The pigment particle is constituted, for example, by a core containing a pigment and a pigment dispersion resin that coats the core. The pigment dispersion resin exists, for example, in a state of being dispersed in a solvent. From the viewpoint of improving the color density, hue, or stability of the red ink of the present invention, the _D50 of the pigment particle is preferably 30 nm or more and 200 nm or less, and more preferably 70 nm or more and 130 nm or less.

The red ink of the present invention preferably contains a red pigment. The pigment contained in the red ink of the present invention is preferably 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 wavelength range of 545 nm or more and 555 nm or less.

In the red ink of the present invention, the pigment content is preferably 1.0% by mass or more and 12.0% 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, it is possible to obtain an image having a desired image density using the red ink of the present invention. Furthermore, by making the pigment content 12.0% by mass or less, it is possible to improve the fluidity of the red ink of the present invention.

[Pigment dispersion resin]
The pigment dispersing resin is water-soluble and adheres to the surface of the pigment to suppress the aggregation of the pigment.

Examples of pigment dispersion resins include copolymers of at least one monomer selected from the group consisting of (meth)acrylic acid alkyl esters, styrene, and vinyl naphthalene, and at least one monomer selected from the group consisting of (meth)acrylic acid and maleic acid.

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

In the red ink of the present invention, the content of the pigment dispersion 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 making the content of the pigment dispersion resin 0.5% by mass or more, it is possible to more effectively suppress pigment aggregation. By making the content of the pigment dispersion resin 8.0% by mass or less, it is possible to suppress the occurrence of nozzle clogging.

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 while further improving the storage stability of the red ink of the present invention.

The acid value of the pigment dispersion resin can be adjusted by changing the monomer used when synthesizing the pigment dispersion resin. For example, the acid value of the pigment dispersion resin can be increased by using a monomer (e.g., acrylic acid and methacrylic acid) having an acidic functional group (e.g., a carboxy group) when synthesizing the pigment dispersion resin.

The mass average molecular weight of the pigment dispersion resin is preferably 10,000 or more and 50,000 or less, and more preferably 15,000 or more and 30,000 or less. By setting the mass average molecular weight of the pigment dispersion resin to 10,000 or more and 50,000 or less, it is possible to obtain an image having the desired image density while suppressing an increase in the viscosity of the red ink of the present invention.

The mass average molecular weight of the pigment dispersion resin can be adjusted by changing the polymerization conditions of the pigment dispersion resin (e.g., the amount of polymerization initiator used, the polymerization temperature, and the polymerization time). It is preferable that the pigment dispersion resin is neutralized with an equal amount of a base (e.g., KOH or NaOH).

[Specific organic solvent]
The specific organic solvent includes 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.

In the red ink of the present invention, the content of the specific organic solvent is preferably 5.0% by mass or more and 40.0% by mass or less, and more preferably 15.0% by mass or more and 25.0% by mass or less. By making the content of the specific organic solvent 5.0% by mass or more, the color development of the red ink of the present invention can be further improved. By making the content of the specific organic solvent 40.0% by mass or less, the storage stability of the red ink of the present invention can be further improved.

[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 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.

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

Examples of 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. A specific glycol ether compound is preferably triethylene glycol monobutyl ether.

Examples of lactam compounds include 2-pyrrolidone and N-methyl-2-pyrrolidone.

Examples of nitrogen-containing compounds include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

An example of an acetate compound is diethylene glycol monoethyl ether acetate.

It is preferable that the red ink of the present invention further contains a glycol ether compound. By further containing a glycol ether compound, the red ink of the present invention can more effectively suppress the pigment from penetrating into the interior of the recording medium after being ejected onto the surface of the recording medium. As a result, the color development of the red ink of the present invention can be further improved.

The content of the glycol ether compound in the red ink of the present invention 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 of the glycol ether compound to 0.5% by mass or more and 10.0% by mass or less, the color development of the red ink of the present invention can be further improved.

The red ink of the present invention preferably further contains glycerin. By further containing glycerin in the red ink of the present invention, the ejection properties of the red ink can be improved. The content of glycerin in the red ink of the present invention 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 making the content of glycerin 0.5% by mass or more and 15.0% by mass or less, the ejection properties of the red ink of the present invention can be further improved.

[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 to a recording medium. Examples of the surfactant include anionic surfactants, cationic surfactants, and nonionic surfactants. The surfactant is preferably a nonionic surfactant.

Examples of nonionic surfactants 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, ethylene oxide adducts of acetylene glycol are preferred.

When the red ink of the present invention contains a surfactant, the content of the surfactant in the red ink of the present invention 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.

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

[Method of manufacturing red ink]
The red ink of the present invention can be produced, for example, by uniformly mixing a pigment dispersion liquid containing a pigment and a pigment dispersion resin, a specific organic solvent, and other components (e.g., water, a glycol ether compound, glycerin, and a surfactant) that are blended as necessary, using a stirrer. In producing the red ink of the present invention, after the components are uniformly mixed, foreign matter and coarse particles may be removed using a filter (e.g., a filter with a pore size of 5 μm or less).

(Pigment Dispersion)
The pigment dispersion is a dispersion containing a pigment and a pigment dispersion resin. The dispersion medium of the pigment dispersion is preferably water. The pigment dispersion preferably further contains a surfactant to improve the dispersibility of the pigment particles.

In the pigment dispersion, the _D50 of the pigment particles constituted by the pigment and the pigment dispersion resin is preferably from 50 nm to 200 nm, and more preferably from 70 nm to 130 nm.

The _D50 of the pigment particles can be measured, for example, by using a solution obtained by diluting the pigment dispersion 300 times with ion-exchanged water as a sample, using a dynamic light scattering particle size distribution measuring device (for example, Zetasizer Nano ZS manufactured by Malvern Instruments).

The pigment content in the pigment dispersion is preferably 5.0% by mass or more and 25.0% by mass or less, and more preferably 10.0% by mass or more and 20.0% by mass or less. The pigment dispersion resin content in the pigment dispersion is preferably 2.0% 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. When the pigment dispersion contains a surfactant, the surfactant content in the pigment dispersion is preferably 0.1% by mass or more and 2.0% by mass or less, and more preferably 0.3% by mass or more and 1.0% by mass or less.

The pigment dispersion can be prepared by wet-dispersing a pigment, a pigment dispersion resin, a dispersion medium (e.g., water), and a component (e.g., a surfactant) added as required, using a media-type wet disperser. In wet dispersion using a media-type wet disperser, for example, small particle size beads (e.g., beads with a _D50 of 0.5 mm or more and 1.0 mm or less) can be used as the media. The material of the beads is not particularly limited, but hard materials (e.g., glass and zirconia) are preferred.

When adding a pigment dispersion in the production of the red ink of the present invention, the ratio of the pigment dispersion to the total raw materials of the red ink is, for example, 25.0% by mass or more and 60.0% by mass or less.

The following describes examples of the present invention. However, the present invention is not limited to the following examples.

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 polystyrenes F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000 manufactured by Tosoh Corporation, and n-propylbenzene.

(Conditions for measuring mass average molecular weight)
Column: "TSKgel SuperMultiporeHZ-H" (4.6 mm ID x 15 cm semi-microcolumn) manufactured by Tosoh Corporation
Number of columns: 3 Eluent: tetrahydrofuran Flow rate: 0.35 mL/min Sample injection volume: 10 μL
・Measurement temperature: 40℃
Detector: IR detector

<Study 1: Types of pigments>
First, we investigated the type of pigment. Below, we show the preparation methods of each raw material used in the production of red ink.

(Preparation of pigment dispersion resin)
A pigment dispersion resin (R-1) 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 pigment dispersion resin (R-1) had a mass average molecular weight (Mw) of 20,000 and an acid value of 100 mg KOH/g. The mass ratio of each repeating unit in this pigment dispersion resin (R-1) was "MAA unit: MMA unit: BA unit: ST unit = 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 equal neutralization of the pigment dispersion resin (R-1). As a result, the pigment dispersion resin (R-1) was neutralized with an equal amount (strictly speaking, 105% amount) of KOH. As a result, a pigment dispersion resin solution containing the pigment dispersion resin (R-1) and water was obtained.

(Preparation of Pigment Dispersions (D-1) to (D-7))
A pigment (see Table 2 below for the type of pigment), the above-mentioned pigment dispersion resin solution, "Olfine (registered trademark) E1010" (an ethylene oxide adduct of acetylene diol) manufactured by Nissin Chemical Industry Co., Ltd. as a nonionic surfactant, and ion-exchanged water were charged into a vessel so as to obtain the composition shown in Table 1 below.

The "water" content in Table 1 below indicates the total content of the ion-exchanged water added to the vessel and the water contained in the pigment dispersion resin solution (more specifically, the water contained in the aqueous sodium hydroxide solution used to neutralize the pigment dispersion resin, and the water generated by the neutralization reaction of the pigment dispersion resin and sodium hydroxide).

The contents of the vessel were then dispersed using zirconia beads (particle diameter 0.5 mm) as media and a wet disperser (Nanograin Mill, manufactured by Asada Iron Works Co., Ltd.). 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.

[Examples 1 to 5 and Comparative Examples 1 to 2]
Ion-exchanged water was charged into a flask equipped with a stirrer (Three-One Motor (registered trademark) BL-600, manufactured by Shinto Scientific Co., Ltd.). While the contents were stirred with the above-mentioned stirrer (stirring speed: 400 rpm), the above-mentioned pigment dispersion (see 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 type and amount of each raw material were as shown in Table 3 below.

The resulting mixture was filtered using a filter with a pore size of 5 μm to remove foreign matter and coarse particles. This resulted in the red inks of Examples 1 to 5 and Comparative Examples 1 and 2.

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

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

(Color development)
An inkjet recording device (a prototype manufactured by Kyocera Document Solutions Inc.) was used as the evaluation machine. The evaluation machine was equipped with a piezo-type line head having a nozzle (opening diameter: 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 machine. Next, a 1 cm×1 cm solid image was formed on A4 plain paper (April's "Paper One (registered trademark)") using the evaluation machine (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, the Lab values (L value, a value, and b value) of the solid image formed on the above-mentioned evaluation paper were measured using a fluorescence spectrodensitometer (Konica Minolta's "FD-5"). The color development of the red ink was judged as good (A) when the L value was 60.0 or less, and as poor (B) when the L value was more than 60.0.

(heat resistance)
The evaluation paper used for evaluating color development was heat-treated at 105° C. for 72 hours. Next, the Lab values (L value, a value, and b value) of the solid image formed on the evaluation paper after heat treatment were measured using the above-mentioned fluorescence spectrodensitometer. The change in L value (ΔL), the change in a value (Δa), and the change in b value (Δb) before and after the heat treatment were obtained. The change in Lab value (ΔE) due to the heat treatment was calculated by applying ΔL, Δa, and Δb to the following formula. The heat resistance of the red ink was judged as good (A) when ΔE was less than 5.0, and as poor (B) when Δ3 was 5.0 or more.
ΔE = [(ΔL) ² + (Δa) ² + (Δb) ² ] ^1/2

(Storage stability)
The viscosity of the evaluation objects (specifically, any of the red inks of Examples 1 to 5 and Comparative Examples 1 and 2) was measured using a falling-ball automatic micro viscometer ("AMVn" manufactured by Anton Paar). In measuring the 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 object was calculated using the dedicated software of the above-mentioned "AMVn" manufactured by Anton Paar.

Next, the evaluation object was placed in a plastic wide-mouthed bottle ("Eye Boy" sold by AS ONE Corporation), the lid of the plastic wide-mouthed bottle was closed, and the evaluation object was heat-treated at 60°C for 2 weeks. The viscosity of the evaluation object after heat treatment (post-heated viscosity) was measured in the same manner as for measuring the initial viscosity. The rate of change in viscosity due to heat treatment was calculated by applying the initial viscosity and the post-heated viscosity to the following formula. The storage stability of the evaluation object was judged as good (A) when the absolute value of the rate of change in viscosity was within 5%, and as poor (B) when the absolute value of the rate of change in viscosity was more than 5%.
Viscosity change rate = 100 x (viscosity after heating - initial viscosity) / initial viscosity

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

On the other hand, the red inks of Comparative Examples 1 and 2 had a maximum absorption wavelength in the range of more than 555 nm. The red inks of Comparative Examples 1 and 2 had insufficient heat resistance. This is believed to be because the red inks of Comparative Examples 1 and 2 formed images with hues that were easily affected by carbonization of the resin.

<Study 2: Types of organic solvents>
Next, the type of organic solvent used as a moisturizing agent was examined. The compounds shown in Table 5 below were used as the organic solvents used in the examination. The hydrophobicity of the compounds shown in Table 5 below is shown below.
1,4BD<1,5PD<MPD<1,6HD

[Examples 6 to 9 and Comparative Examples 3 to 4]
The red inks of Examples 6 to 9 and Comparative Examples 3 and 4 were prepared in the same manner as in the preparation of the inks of Examples 1 to 5 and Comparative Examples 1 and 2, except that the types and amounts of each raw material were as shown in Table 6 below.

The resulting mixture was filtered using a filter with a pore size of 5 μm to remove foreign matter and coarse particles. This resulted in the red inks of Examples 6 to 9 and Comparative Examples 3 to 4.

The maximum absorption wavelengths of the red inks of Examples 6 to 9 and Comparative Examples 3 and 4 were measured using the same method as used to measure the maximum absorption wavelengths of the red inks of Examples 1 to 5 and Comparative Examples 1 and 2. The maximum absorption wavelengths obtained are shown in Table 7 below.

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

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 moderate hydrophobicity, as a moisturizing agent. The red inks of Examples 6 to 9 had excellent color development, heat resistance, and storage stability.

On the other hand, the red ink of Comparative Example 3 contained 1,4-butanediol, a relatively hydrophilic organic solvent, as a moisturizing agent. It is determined that the red ink of Comparative Example 3 was unable to retain the pigment on the surface of the recording medium after being ejected onto the surface of the recording medium. Therefore, it is determined that the red ink of Comparative Example 3 had poor color development and formed an image with a relatively large L value. The red ink of Comparative Example 3 also had poor heat resistance.

The red ink of Comparative Example 4 contained 1,6-hexanediol, an excessively hydrophobic organic solvent, as a moisturizing agent. Because the red ink of Comparative Example 4 contained an excessively hydrophobic organic solvent, it was determined that the storage stability of the ink was poor.

In summary, red inks that contain a pigment and a specific organic solvent, have a maximum absorption wavelength in the range of 545 nm to 555 nm, and in which the specific organic solvent contains 1,5-pentanediol or 3-methyl-1,5-pentanediol, are judged to have excellent color development, heat resistance, and storage stability.

The red ink of the present invention can be used to form images.

Claims (4)

The ink contains a pigment and a specific organic solvent,
It has a maximum absorption wavelength in the wavelength range of 545 nm or more and 555 nm or less,
The specific organic solvent comprises 1,5-pentanediol or 3-methyl-1,5-pentanediol. The inkjet red ink of claim 1, wherein the pigment comprises 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 or 2, wherein the content of the specific organic solvent is 5.0% by mass or more and 40.0% by mass or less. Further containing a glycol ether compound,
4. The ink-jet red ink according to claim 1, wherein the content of the glycol ether compound is from 0.5% by mass to 10.0% by mass.