Water-based ink for inkjet recording.

Abstract

This invention provides an aqueous inkjet recording ink that offers high image density on plain paper, excellent curl resistance, good redispersibility, and scratch resistance. [Solution] The water-based inkjet recording ink according to this embodiment contains a pigment, a pigment-coated resin, a water-soluble organic solvent, polyethylene glycol with a molecular weight of 400 in an amount of 10% to 20% by mass, 3% to 7% by mass of 3-methyl-1,5-pentanediol, 0.5% to 5% by mass of 2-ethyl-1,3-hexanediol, and water.

Description

Technical Field

[0001] The present invention relates to an aqueous ink for inkjet recording.

Background Art

[0002] In inkjet recording using aqueous ink, improvement of image density on plain paper, excellent redispersibility, and suppression of curling are required. For example, Patent Document 1 describes an inkjet ink containing a surfactant in addition to water and a pigment. Further, Patent Document 2 describes an inkjet ink containing betaines which are N-trialkyl-substituted products of amino acids in addition to a pigment and water. Furthermore, Patent Document 3 describes an aqueous recording ink having a compound with a chemical structure having 7 or more carbon atoms and having two OH groups, and an inkjet recording ink containing a colorless aqueous liquid composition.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the inkjet ink described in Patent Document 1, redispersibility is improved by adding a surfactant, but if the amount of hydrophobic solvent increases, the cohesive force of the pigment increases, so the addition of a surfactant alone may not satisfy the redispersibility requirement. Also, in the inkjet ink described in Patent Document 2, betaines are added to suppress curling, but if the moisture content of the ink is high, curling tends to worsen, and the addition of betaines alone may not satisfy the suppression of curling. Furthermore, in the inkjet ink described in Patent Document 3, a substance with 7 or more carbon atoms and 2 OH groups (polyethylene glycol) is added to suppress curling. However, the addition of polyethylene glycol alone may not satisfy the required image density.

[0005] In view of the above circumstances, the object of the present invention is to provide an aqueous inkjet recording ink that can improve image density on plain paper and has excellent curl resistance and good redispersibility. [Means for solving the problem]

[0006] An aqueous inkjet recording ink according to one embodiment of the present invention contains a pigment, a pigment-coated resin, polyethylene glycol with a molecular weight of 400 in an amount of 10% to 20% by mass, 3% to 7% by mass of 3-methyl-1,5-pentanediol, 0.5% to 5% by mass of 2-ethyl-1,3-hexanediol, and water.

[0007] This water-based inkjet recording ink can improve image density and provide good redispersibility and excellent curl resistance by incorporating 10% to 20% by mass of polyethylene glycol with a molecular weight of 400. Furthermore, this water-based inkjet recording ink can improve image density and provide good redispersibility by incorporating 3% to 7% by mass of 3-methyl-1,5-pentanediol. In addition, this water-based inkjet recording ink can improve image density and provide good scratch resistance by incorporating 0.5% to 5% by mass of 2-ethyl-1,3-hexanediol. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an aqueous inkjet recording ink that can improve image density on plain paper and has excellent curl resistance and good redispersibility. [Modes for carrying out the invention]

[0009] Embodiments of the present invention will be described below.

[0010] The present invention provides an aqueous inkjet recording ink (hereinafter sometimes simply referred to as "ink") which comprises a pigment, a pigment-coated resin, polyethylene glycol, 3-methyl-1,5-pentanediol, 2-ethyl-1,3-hexanediol, and water.

[0011] In the ink according to this embodiment, by incorporating 10% to 20% by mass of polyethylene glycol with a molecular weight of 400, image density can be improved, and good redispersibility and excellent curl resistance can be achieved. Furthermore, in this water-based inkjet recording ink, by incorporating 3% to 7% by mass of 3-methyl-1,5-pentanediol, image density can be improved, and good redispersibility can be achieved. In addition, in this water-based inkjet recording ink, by incorporating 0.5% to 5% by mass of 2-ethyl-1,3-hexanediol, image density can be improved, and good abrasion resistance can be achieved. Thus, the ink according to this embodiment can improve image density on plain paper and has excellent curl resistance and good redispersibility. The details of each component of the ink according to this embodiment will be described below.

[0012] (Pigment) In the ink of the present invention, the pigment, for example, constitutes pigment particles together with a pigment-coating resin. The pigment particles are composed of, for example, a core containing the pigment and a pigment-coating resin coating the core. The pigment-coating resin exists, for example, dispersed in a solvent. From the viewpoint of optimizing the color density, hue, or stability of the ink of the present invention, the volume median diameter of the pigment particles is preferably 30 nm to 200 nm, and more preferably 70 nm to 130 nm.

[0013] 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).

[0014] In the ink of the present invention, the pigment content is preferably 0.50% by mass or more and 10.00% by mass or less, and more preferably 1.50% by mass or more and 5.00% by mass or less. By setting the pigment content to 0.50% by mass or more, the ink of the present invention becomes even easier to form images having the desired image density. Furthermore, by setting the pigment content to 10.00% by mass or less, the fluidity of the ink of the present invention can be ensured.

[0015] (Pigment-coated resin) The pigment-coated resin is a resin soluble in the aqueous medium of the ink of the present invention. A portion of the pigment-coated resin is present, for example, on the surface of the pigment particles, optimizing the dispersibility of the pigment particles. A portion of the pigment-coated resin is present, for example, dissolved in the aqueous medium of the ink of the present invention.

[0016] As the pigment-coating resin, styrene-acrylic resin is preferred. Styrene-acrylic resin is a copolymer of styrene and at least one monomer selected from (meth)acrylate alkyl ester and (meth)acrylic acid. Styrene-acrylic resin preferably has repeating units derived from (meth)acrylic acid ((meth)acrylic acid units), repeating units derived from (meth)acrylate alkyl ester ((meth)acrylate alkyl ester units), and styrene units.

[0017] Examples of alkyl (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and octyl (meth)acrylate. Methyl methacrylate or butyl acrylate are preferred as the alkyl (meth)acrylate.

[0018] Of the total repeating units in the pigment-coated resin, the proportion of (meth)acrylic acid units is preferably 30% to 50% by mass. Of the total repeating units in the pigment-coated resin, the proportion of (meth)acrylate alkyl ester units is preferably 35% to 55% by mass. Of the total repeating units in the pigment-coated resin, the proportion of styrene units is preferably 5% to 25% by mass. The pigment-coated resin is more preferably a resin having repeating units derived from methacrylic acid, repeating units derived from methyl methacrylate, repeating units derived from butyl acrylate, and styrene units.

[0019] In the ink, the pigment-coating resin content is preferably 0.10% by mass or more and 4.00% by mass or less, and more preferably 0.50% by mass or more and 2.00% by mass or less. By setting the pigment-coating resin content to 0.10% by mass or more, the dispersibility of the pigment components can be further improved. By setting the pigment-coating resin content to 4.00% by mass or less, the ejection stability of the ink of the present invention can be further optimized.

[0020] The acid value of the pigment-coated resin is preferably 30 mgKOH / g or more and 200 mgKOH / g or less, more preferably 70 mgKOH / g or more and 130 mgKOH / g or less. By setting the acid value of the pigment-coated resin to 30 mgKOH / g or more and 200 mgKOH / g or less, the dispersibility of the pigment can be further improved while optimizing the storage stability of the ink of the present invention.

[0021] The acid value of the pigment-coated resin can be adjusted by changing the amount of the monomer used when synthesizing the pigment-coated resin. For example, when synthesizing the pigment-coated resin, by using a monomer having an acidic functional group (for example, a carboxy group) (more specifically, acrylic acid, methacrylic acid, etc.), the acid value of the pigment-coated resin can be increased.

[0022] The mass average molecular weight Mw of the pigment-coated resin is preferably 10,000 or more and 50,000 or less, more preferably 15,000 or more and 25,000 or less. By setting the mass average molecular weight Mw of the pigment-coated resin to 10,000 or more and 50,000 or less, while suppressing an increase in the viscosity of the ink of the present invention, the image density of the formed image can be further optimized.

[0023] The mass average molecular weight Mw of the pigment-coated resin can be adjusted by changing the polymerization conditions of the pigment-coated resin (more specifically, the amount of the polymerization initiator used, the polymerization temperature, the polymerization time, etc.).

[0024] In the polymerization of the pigment-coated resin, the amount of the polymerization initiator used is preferably 0.001 mol or more and 5 mol or less, more preferably 0.01 mol or more and 2 mol or less, per 1 mol of the monomer mixture. In the polymerization of the pigment-coated resin, for example, the polymerization temperature can be 50°C or more and 70°C or less, and the polymerization time can be 10 hours or more and 24 hours or less. The polymerized pigment-coated resin is preferably neutralized with an equivalent amount of a basic compound and then used as a raw material for the ink of the present invention. As the basic compound, a hydroxide of an alkali metal ion (for example, NaOH or KOH) is preferable.

[0025] (Polyethylene glycol) The polyethylene glycol incorporated into the ink according to this embodiment has a molecular weight of 400. Furthermore, the polyethylene glycol content in the ink according to this embodiment is 10% by mass or more and 20% by mass or less. By incorporating 10% by mass or more of polyethylene glycol with a molecular weight of 400 into the ink according to this embodiment, moisture retention is ensured, and good redispersibility and excellent curl resistance can be achieved. Additionally, by incorporating 20% ​​by mass or less of polyethylene glycol into the ink according to this embodiment, the cohesive force becomes greater than the dispersive force, resulting in high image density.

[0026] (3-methyl-1,5-pentanediol) In the ink according to this embodiment, the content of 3-methyl-1,5-pentanediol is 3% by mass or more and 7% by mass or less. In the ink according to this embodiment, by setting the content of 3-methyl-1,5-pentanediol to 3% by mass or more, the cohesive force of the pigment is increased, and a high image density can be achieved. Furthermore, in the ink according to this embodiment, by setting the content of 3-methyl-1,5-pentanediol to 7% by mass or less, the cohesive force of the pigment is not increased too much, and good redispersibility can be achieved.

[0027] (2-ethyl-1,3-hexanediol) In the ink according to this embodiment, the content of 2-ethyl-1,3-hexanediol is 0.5% by mass or more and 5% by mass or less. In the ink according to this embodiment, by setting the content of 2-ethyl-1,3-hexanediol to 0.5% by mass or more, sufficient permeability is achieved, and excellent scratch resistance can be obtained. Furthermore, in the ink according to this embodiment, by setting the content of 2-ethyl-1,3-hexanediol to 5% by mass or less, the permeability does not become too strong, and high image density can be obtained.

[0028] (water) In the ink according to this embodiment, for example, ion-exchanged water, purified water, or distilled water can be used as water. In the ink according to this embodiment, in order to obtain relatively low viscosity, it is preferable that the water content is 60.0% by mass or more and 80.0% by mass or less.

[0029] (Other ingredients) The ink according to this embodiment may contain other components as needed. For example, the ink according to this embodiment may contain a surfactant. Surfactants have the effect of improving the wettability of the ink to the storage medium and improving the compatibility and dispersion stability of each component contained in the ink. Examples of surfactants to be incorporated into the ink according to this embodiment include anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants. Nonionic surfactants are preferred as surfactants to be incorporated into the ink according to this embodiment, and among them, acetylene glycol-based surfactants are particularly preferred.

[0030] Furthermore, the ink according to this embodiment may also contain various additives as needed, such as dissolution stabilizers, drying inhibitors, antioxidants, viscosity modifiers, pH adjusters, neutralizing agents, and antifungal agents.

[0031] [Examples and Comparative Examples] Ink preparation and evaluation were carried out as examples and comparative examples of the present invention. Note that the following examples are merely examples of the present invention, and the configuration of the present invention is not limited to the configurations of these examples.

[0032] [Preparation of pigment-coated resin solution] An alkali-soluble resin was prepared containing repeating units derived from methacrylic acid (MAA units), methyl methacrylate (MMA units), butyl acrylate (BA units), and styrene (ST units). 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 units:MMA units:BA units:ST units = 40:15:30:15". This alkali-soluble resin was mixed with an aqueous potassium hydroxide solution (KOH concentration: 10% by mass) (neutralization treatment). The alkali-soluble resin was neutralized with an equal amount of KOH by the neutralization treatment. This yielded a pigment-coated resin solution containing pigment-coated resin (R) and water.

[0033] The mass-average molecular weight Mw of the alkali-soluble resin described above was measured using gel permeation chromatography (HLC-8020GPC, manufactured by Tosoh Corporation) under the following conditions. Calibration curves were 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.

[0034] (Measurement conditions for mass-average molecular weight) • Column: TSKgel SuperMultiporeHZ-H manufactured by Tosoh Corporation (semi-micro column with 4.6mm I.D. x 15cm) • Number of columns: 3 • Eluent: Tetrahydrofuran ·Flow rate: 0.35mL / min • Sample injection volume: 10 μL ·Measurement temperature: 40℃ • Detector: IR detector

[0035] [Preparation of black pigment dispersion] The following mixtures were prepared as shown in Table 1 below: carbon black (Mitsubishi Chemical Corporation's "#900"), the aforementioned pigment-coated resin solution containing pigment-coated resin (R), acetylene glycol surfactant "Orphine® EXP4200" manufactured by Nisshin Chemical Industry Co., Ltd., and ion-exchanged water, and then placed into the vessel of a media-type wet disperser (Willie E. Bakkofen (WAB)'s "DYNO®-MILL").

[0036] [Table 1]

[0037] The percentage of "water" in Table 1 below represents the total percentage of water contained in the ion-exchanged water added to the vessel described above, and the water contained in the pigment-coated resin solution (specifically, the water contained in the sodium hydroxide aqueous solution used to neutralize the alkali-soluble resin, and the water produced by the neutralization reaction between the alkali-soluble resin and sodium hydroxide).

[0038] Next, the contents of the vessel were wet-dispersed. Zirconia beads (particle size 1.0 mm) were used as the media. The amount of media added was 70% by volume relative to the volume of the vessel. The dispersion conditions were a temperature of 10°C and a peripheral speed of 8 m / sec. This yielded a cyanide pigment dispersion.

[0039] The median diameter (D50) of the pigment particles contained in the obtained cyanide pigment dispersion was measured. Specifically, the obtained cyanide pigment dispersion was diluted 300 times with deionized water and used as the measurement sample. The D50 of the pigment particles in the measurement sample was measured using a dynamic light scattering particle size distribution analyzer (Malvern "Zetasizer Nano ZS"). The D50 of the pigment particles in the measurement sample was defined as the D50 of the pigment particles contained in the cyanide pigment dispersion. Ten measurements were performed, and the average value of each measurement result was adopted as the D50 of the pigment particles. The D50 of the pigment particles contained in the cyanide pigment dispersion was 100 nm.

[0040] [Ink preparation] The ink was prepared using the following method. [Example 1]

[0041] 50 parts by mass of carbon black dispersion, 10 parts by mass of PEG400, 3 parts by mass of 3-methyl-1,5-pentanediol, 0.5 parts by mass of 2-ethyl-1,3-hexanediol, and 39.5 parts by mass of deionized water were placed in a beaker. Using a stirrer (Shinto Kagaku Co., Ltd. "Three One Motor BL-600"), the contents of the beaker were mixed at a rotation speed of 400 rpm for 30 minutes to obtain a mixture. The mixture was filtered using a filter (pore size 5 μm) to remove foreign matter and coarse particles contained in the mixture. In this way, the ink of Example 1 was obtained.

[0042] [Examples 2-6 and Comparative Examples 1-6] The inks for Examples 2-6 and Comparative Examples 1-6 were prepared in the same manner as the ink preparation for Example 1, except that the types and amounts of each component were changed as shown in Table 2.

[0043] <Preparing the evaluation unit> Water-based ink (the subject of evaluation) was filled into each of the four recording heads (each a line head) of an inkjet recording device (a prototype evaluation unit manufactured by Kyocera Document Solutions Inc.). The water-based ink ejection conditions were set so that the ink ejection amount per drop was 11 pL. The negative pressure generated by the suction unit of the transport belt was set to 0.6 kPa. The evaluation unit was thus prepared. Each recording head was an inkjet head manufactured by Kyocera Document Solutions Inc. (model number: KJ4B-QA, ink ejection amount per drop: 11 pL). The recording heads were arranged with a spacing of 20 mm so that their longitudinal direction was perpendicular to the paper transport direction.

[0044] <Ink Evaluation> The inks used in the examples and comparative examples were evaluated for image density, redispersibility, curl resistance, and scratch resistance.

[0045] <Image density> A 10cm x 10cm solid image was printed using Fuji Xerox Co., Ltd.'s "C2" A4 size paper as the recording medium, with the amount of ink ejected from one recording head being 11 pL. After storing the plain paper with the formed image in a normal temperature and humidity environment for 24 hours, the print density of the image area was measured using a portable reflectance densitometer RD-19 (Gretag Macbeth Corporation), and the average of the print density of 10 locations within the solid image was defined as the print density. A print density of 1.2 or higher was judged as A (pass), and a print density of less than 1.2 was judged as B (fail).

[0046] <Redispersibility> A 3cm x 3cm piece of SUS304 was coated with ink using a spin coater and then left to dry in a 40°C constant temperature bath for 72 hours. The dried SUS piece was immersed in a petri dish containing deionized water for 5 minutes. If the ink coating dissolved cleanly in the water, the ink's redispersibility was rated "A (pass)". If it did not dissolve at all or peeled off in a film, it was rated "B (fail)".

[0047] <Curl> A solid image was formed in the center of a sheet of A4 plain paper using the same method as described above for evaluating print density. Immediately after image formation, the A4 plain paper was placed on a horizontal surface. At that time, the surface of the plain paper with the solid image formed on it was placed facing the surface of the surface of the surface. Using the surface of the surface of the surface as a reference, the heights of the four corners of the plain paper were measured and the average value (average of the heights of the four points) was obtained. The obtained average value was evaluated according to the following criteria. A (Pass): The average height of the four corners of a standard sheet of paper is 20 mm or less. B (Fail): The average height of the four corners of the plain paper exceeds 20 mm.

[0048] Furthermore, if the average height of the four corners of a standard sheet of paper is 10 mm or less, it is considered possible to hold the second (back) recording medium in double-sided printing by using a 0.8 kPa suction fan during printing, through suction and adsorption by the transport belt and paper straightening using the decal mechanism.

[0049] <Abrasion resistance> The abrasion resistance was evaluated under conditions of 25°C and 60% RH. The ink ejection rate per pixel was set to 11 pL. Using the evaluation machine, a solid image (4 cm x 5 cm) was printed on one sheet of paper (hereinafter sometimes referred to as Paper A). Next, the abrasion test described below was performed. In the abrasion test, an unused sheet of paper (hereinafter sometimes referred to as Paper B) was placed on top of the solid image printed on Paper A. Next, a 1 kg weight with a base of 4 cm x 5 cm was placed on Paper B. Then, by moving Paper B and the weight together so that only the weight's own weight was applied, Paper B was rubbed back and forth five times against the solid image on Paper A. After the abrasion test, the image density of areas on Paper A where no solid image was formed was measured at 224 locations using a reflectance densitometer (X-Rite "RD-19"). The highest value among the measured image densities was taken as the evaluation value. A lower evaluation score indicates less color transfer due to rubbing and superior scratch resistance of the image. The scratch resistance of images formed using ink was determined according to the following criteria. A (Pass): Evaluation score is less than 0.03 B (Fail): Evaluation score of 0.03 or higher

[0050] The results are shown in Table 2. Examples 1 to 6 can satisfy image density, redispersibility, curl resistance, and abrasion resistance by adjusting the amounts of PEG400, MPD (3-methyl-1,5-pentanediol), and 2E13HD (2-ethyl-1,3-hexanediol) within the claimed range.

[0051] In Comparative Example 1, if the MPD is less than 3% by mass, the image density cannot be satisfied due to insufficient pigment cohesiveness. In Comparative Example 2, if the MPD is greater than 7% by mass, the pigment cohesiveness is too high and the redispersibility cannot be satisfied. In Comparative Example 3, if the PEG400 is less than 10% by mass, the redispersibility and curl resistance cannot be satisfied due to insufficient moisture retention. In Comparative Example 4, if the PEG400 is greater than 20% by mass, the image density cannot be satisfied because the dispersive force is greater than the cohesive force. In Comparative Example 5, if the 2-ethyl-1,3-hexanediol is less than 0.5% by mass, the abrasion resistance cannot be satisfied due to insufficient penetration. In Comparative Example 6, if the 2-ethyl-1,3-hexanediol is greater than 5% by mass, the penetration is too strong and the image density cannot be satisfied.

[0052] [Table 2]

Claims

[Claim 1] Pigments and Pigment-coated resin and Polyethylene glycol having a molecular weight of 400, comprising 10% by mass or more and 20% by mass or less, 3% by mass to 7% by mass of 3-methyl-1,5-pentanediol, 0.5% by mass or more and 5% by mass or less of 2-ethyl-1,3-hexanediol, Water and A water-based inkjet recording ink containing [specific ingredient].

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