inkjet ink

Abstract

This invention provides an inkjet ink that can suppress pigment aggregation in a solvent, even when using an ionic preservative. [Solution] The inkjet ink contains a pigment, a preservative, a zwitterionic compound, and water. The zwitterionic compound is a zwitterionic compound that takes on a zwitterionic structure in an environment with a pH of 7.5 to 9. The preservative contains at least one of 2-(2-hydroxy-5-methylphenyl)benzotriazole, orthophenylphenol, sodium dihydroxydimethoxybenzophenone disulfonate, para-aminobenzoic acid, ferulic acid, and benzalkonium chloride. In the inkjet ink, the orthophenylphenol content is 4% by mass or less, and the ferulic acid content is 7% by mass or less.

Description

Technical Field

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

Background Art

[0002] Some inkjet recording systems involve operators such as users and service technicians refilling ink into ink tanks. For the ink used in such systems, it is required that it is harmless even if contacted by an operator and that it is not easily deteriorated due to the growth of mold or the like. Further, such ink has a problem that since it is exposed to the atmosphere during work by an operator, evaporation of the solvent easily occurs.

[0003] In ink containing pigments, as the solvent evaporates, the pigments tend to aggregate, making it difficult to maintain the dispersion state of the pigments in the solvent. On the other hand, a technique of making the ink less likely to dry by blending a humectant typified by glycerin or polyethylene glycol is widely known. Further, Patent Document 1 discloses a technique capable of more effectively suppressing the drying of ink by blending a hygroscopic material to give the ink hygroscopicity.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in inks containing hygroscopic materials, the ionic hygroscopic materials ionize, increasing the amount of ions and thus reducing the dispersion stability of the pigment in the solvent. Furthermore, many highly safe preservatives that inhibit the growth of mold and other contaminants are ionic compounds. Therefore, inks containing such preservatives are also prone to reduced dispersion stability of the pigment in the solvent.

[0006] In view of the above circumstances, the object of the present invention is to provide an inkjet ink that can suppress the aggregation of pigments in a solvent, even when using an ionic preservative. [Means for solving the problem]

[0007] To achieve the above objective, an inkjet ink according to one embodiment of the present invention contains a pigment, a preservative, a zwitterionic compound, and water. The above-mentioned zwitterionic compound is a zwitterionic compound that adopts a zwitterionic structure in an environment with a pH of 7.5 to 9. The above preservatives are at least one of 2-(2-hydroxy-5-methylphenyl), benzotriazole, orthophenylphenol, chlorcresol, sodium salicylate, sodium dihydroxydimethoxybenzophenone disulfonate, sodium sorbate, thymol, trichlorohydroxydiphenyl ether, para-aminobenzoic acid, methyl para-hydroxybenzoate, para-chlorophenol, zinc pyrithione, piroctone olamine, phenylbenzimidazole sulfonic acid, phenol, ferulic acid, resorcinol, cetylpyridinium chloride, benzalkonium chloride, and benzethonium chloride. The above inkjet ink contains 4% by mass or less of orthophenylphenol and 7% by mass or less of ferulic acid.

[0008] In this inkjet ink, drying is suppressed by the action of zwitterionic compounds. Furthermore, by incorporating zwitterions that are neutralized themselves, the dielectric constant can be increased without affecting the electrical double layer. As a result, the electrostatic repulsion between pigments in this inkjet ink is strengthened, suppressing pigment aggregation even in compositions using ionic preservatives, and the dispersion state of the pigments in the solvent is less likely to be impaired. Therefore, the dispersion state of the pigments in this inkjet ink is easily maintained over long periods of time.

[0009] The above zwitterionic compound is at least one of a betaine-type compound, a sulfobetaine-type compound, and an N,N,N-trialkylN-oxy compound, and does not necessarily contain any of the linear alkane skeleton, triethylene glycol skeleton, or tripropylene glycol skeleton having 4 or more carbon atoms.

[0010] The above preservative may include at least one of 2-(2-hydroxy-5-methylphenyl)benzotriazole, orthophenylphenol, sodium dihydroxydimethoxybenzophenone disulfonate, para-aminobenzoic acid, ferulic acid, and benzalkonium chloride. [Effects of the Invention]

[0011] As described above, the present invention can provide an inkjet ink that can suppress pigment aggregation in a solvent, even when using an ionic preservative. [Modes for carrying out the invention]

[0012] Embodiments of the present invention will be described below.

[0013] [Ink composition] (Schematic configuration) An inkjet ink according to one embodiment of the present invention (hereinafter also simply referred to as "ink") contains a pigment a, a preservative b, a drying inhibitor c, and water. The ink according to this embodiment is a water-based ink that is ejected from the recording head of an inkjet recording device onto a recording medium to record an image on the recording medium. Examples of recording media on which images are recorded with the ink according to this embodiment include plain paper, copy paper, recycled paper, thin paper, thick paper, glossy paper, and OHP.

[0014] In the ink according to this embodiment, even with a configuration using an ionic preservative b, the aggregation of pigments can be effectively suppressed by the action of the drying inhibitor c. The details of each component of the ink according to this embodiment will be described below.

[0015] (Pigment a) The ink according to this embodiment contains pigment a as a coloring agent, from the viewpoint of improving the ability to prevent color mixing and the water resistance of images recorded on a recording medium. Pigment a may be either an inorganic pigment or an organic pigment. In addition, if necessary, these may be used in combination with an extender pigment.

[0016] Specific examples of inorganic pigments usable in the ink according to this embodiment include, for example, carbon black and metal oxides, with carbon black being particularly preferred for black inks. Examples of carbon black include furnace black, thermal lamp black, acetylene black, and channel black.

[0017] Specific examples of organic pigments that can be used in the ink according to this embodiment include azo pigments, diazo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxazine pigments, perylene pigments, perinone pigments, thioindigo pigments, anthraquinone pigments, and quinophthalone pigments.

[0018] In the ink according to this embodiment, the hue is not particularly limited, and any chromatic pigment such as yellow, magenta, cyan, blue, red, orange, and green can be used. Specific examples of preferred chromatic pigments include CI pigment yellow, CI pigment red, CI pigment orange, CI pigment violet, CI pigment blue, and CI pigment green. In the ink according to this embodiment, one or more selected from these chromatic pigments can be used as pigment a.

[0019] (Preservative b) The preservative b incorporated into the ink according to this embodiment is a highly safe ionic compound that has the effect of suppressing the growth of mold and other microorganisms and does not cause harm even if workers come into contact with it. Preservative b is typically selected from among the preservatives listed in "Ministry of Health and Welfare Notification No. 331 of September 29, 2000, Cosmetic Standards" that ionize when dissolved in water.

[0020] Specifically, preservative b is at least one of 2-(2-hydroxy-5-methylphenyl), benzotriazole, orthophenylphenol, chlorcresol, sodium salicylate, sodium dihydroxydimethoxybenzophenone disulfonate, sodium sorbate, thymol, trichlorohydroxydiphenyl ether, para-aminobenzoic acid, methyl para-hydroxybenzoate, para-chlorophenol, zinc pyrithione, piroctone olamine, phenylbenzimidazole sulfonic acid, phenol, ferulic acid, resorcinol, cetylpyridinium chloride, benzalkonium chloride, and benzethonium chloride.

[0021] In the ink according to this embodiment, in the configuration where at least one of 2-(2-hydroxy-5-methylphenyl) benzotriazole, ortho-phenylphenol, sodium dihydroxydimethoxybenzophenone disulfonic acid, para-aminobenzoic acid, ferulic acid, and benzalkonium chloride is used as the preservative b, the maximum allowable addition amount is relatively large, and a greater merit can be obtained by the action of suppressing the aggregation of the pigment a in the solvent.

[0022] In the ink according to this embodiment, it is preferable that the content of the preservative b is 1% by mass or more in order to sufficiently obtain the action of the preservative b. On the other hand, in the ink according to this embodiment, it is preferable not to blend the preservative b in an amount exceeding the maximum blending amount shown in the above cosmetic standards. Further, in the ink according to this embodiment, when ortho-phenylphenol is used, the content of the preservative b is 4% by mass or less, and when ferulic acid is used, the content of the preservative b is 7% by mass or less.

[0023] (Anti-drying agent c) The anti-drying agent c blended in the ink according to this embodiment has high moisture retention, and thereby exhibits the action of preventing the evaporation of the solvent. In the ink according to this embodiment, as the anti-drying agent c, an amphoteric ion compound having an amphoteric ion structure in an environment where the pH is 7.5 or more and 9 or less is used. An amphoteric ion compound is a compound having a cation center and an anion center in the molecule and being neutral as a whole.

[0024] In addition to the action of suppressing the evaporation of the solvent, the anti-drying agent c exhibits the action of suppressing the aggregation of the pigment a in the solvent. That is, in the ink according to this embodiment, by blending an amphoteric ion that is neutral as a whole, the dielectric constant can be increased without affecting the electric double layer. As a result, in the ink according to this embodiment, the electrostatic repulsive force between the pigments a becomes stronger, so that the aggregation of the pigment a is suppressed even in the configuration using the ionic preservative b, and the dispersion state of the pigment in the solvent is hardly impaired. For this reason, in the ink according to this embodiment, high dispersion stability of the pigment a in the solvent can be obtained, and the dispersion state of the pigment a is likely to be maintained over a long period of time.

[0025] The zwitterionic compound constituting the drying inhibitor c incorporated into the ink according to this embodiment is preferably at least one of a betaine-type compound, a sulfobetaine-type compound, and an N,N,N-trialkylN-oxy compound in order to more effectively suppress the aggregation of pigment a in the solvent. Furthermore, from a similar viewpoint, it is preferable that the drying inhibitor c does not contain any of the following: a linear alkane skeleton having 4 or more carbon atoms, a triethylene glycol skeleton having 4 or more carbon atoms, and a tripropylene glycol skeleton having 4 or more carbon atoms.

[0026] In the ink according to this embodiment, it is preferable that the content of the drying inhibitor c is 3% by mass or more in order to sufficiently suppress the aggregation of the pigment a. Furthermore, in the ink according to this embodiment, it is preferable that the content of the drying inhibitor c is 10% by mass or less in order to prevent precipitation during drying.

[0027] (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, from the viewpoint of drying properties and ejection reliability, it is preferable that the water content is 60% by mass or more and 70% by mass or less.

[0028] (Other ingredients) The ink according to this embodiment may contain components other than those mentioned above, as needed. For example, the ink according to this embodiment may contain a dispersant that enhances the dispersibility of pigment a in a solvent. Examples of dispersants include pigment dispersing resins and surfactants.

[0029] The pigment dispersion resin consists of fine particles of resin that adsorb to the surface of pigment a, thereby improving the dispersibility of pigment a in the solvent. The molecular weight of the pigment dispersion resin is preferably around tens of thousands. Examples of pigment dispersion resins include acrylic resins, styrene-acrylic resins, styrene-maleic acid resins, and urethane resins, with styrene-acrylic resin being preferred among these. In the pigment dispersion resin, it is preferable that the acid value be 150 mgKOH / g or higher from the viewpoint of improving the dispersibility of pigment a in the solvent, creating fine particles, and improving the color development and coloring power of pigment a. On the other hand, in the pigment dispersion resin, it is preferable that the acid value be 300 mgKOH / g or lower from the viewpoint of improving the storage stability of the ink.

[0030] The surfactant added as a dispersant reduces the interfacial tension between pigment a and the solvent, thereby improving the dispersibility of pigment a in the solvent. Examples of such surfactants include nonionic surfactants and anionic surfactants.

[0031] Furthermore, in addition to the dispersant, the ink according to this embodiment may contain various additives as needed, such as deliquescent agents, dissolution stabilizers, antioxidants, viscosity modifiers, pH adjusters, and neutralizing agents.

[0032] [Examples] As an embodiment of the present invention, ink preparation and evaluation were performed.

[0033] (Ink preparation) Preparation of pigment-dispersed resins An alkali-soluble resin was prepared 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). 100 parts by mass of this alkali-soluble resin was mixed with an aqueous potassium hydroxide solution containing 10.5 parts by mass of potassium hydroxide. This neutralized the alkali-soluble resin with an equal amount (strictly speaking, 105%) of KOH. This yielded a pigment dispersion resin solution containing pigment dispersion resin and water.

[0034] Preparation of pigment dispersion Pigment a ("Lionol® Blue FG-7330" manufactured by Toyo Color Co., Ltd., component: copper phthalocyanine, color index: pigment blue 15:3), the aforementioned pigment dispersion resin solution, "Orphine® E1010" (ethylene oxide adduct of acetylenediol) manufactured by Nisshin Chemical Industry Co., Ltd. as a surfactant, and deionized water were placed in a 0.6 L vessel to achieve the composition shown in Table 1 below. The contents of the vessel were then wet-dispersed using a media-type wet disperser ("DYNO®-MILL" manufactured by Willy E. Bakkofen (WAB)).

[0035] [Table 1]

[0036] In Table 1, the "water" content represents the total content of the ion-exchanged water added to the vessel described above, and the water contained in the pigment dispersion resin solution (specifically, the water contained in the potassium hydroxide aqueous solution used to neutralize the alkali-soluble resin, and the water produced by the neutralization reaction between the alkali-soluble resin and potassium hydroxide).

[0037] Next, the contents of the vessel described above were dispersed using zirconia beads (particle size 0.5 mm) as a media and a wet disperser ("Nanoglenmill" manufactured by Asada Iron Works Co., Ltd.). The dispersion conditions were a temperature of 10°C and a peripheral speed of 8 m / sec. This yielded a pigment dispersion.

[0038] • Preparation of resin emulsions A four-necked flask (capacity 1000 mL) was equipped with a stirrer, nitrogen inlet tube, condenser, agitator, and dropping funnel. This was used as the reaction vessel. Next, 100 g of isopropyl alcohol and 300 g of methyl ethyl ketone were added to the reaction vessel. Then, the contents of the reaction vessel were heated under reflux at 70°C while bubbling nitrogen. Separately, 50.0 g of butyl acrylate (BA), 10.0 g of lauryl acrylate (LA), 25.0 g of methyl methacrylate (MMA), 15.0 g of 2-ethylhexyl acrylate (2EHA), and 0.400 g of azobisisobutyronitrile (AIBN, polymerization initiator) were mixed to obtain a monomer solution. The monomer solution was added dropwise to the reaction vessel over approximately 2 hours while the contents of the reaction vessel were heated under reflux at 70°C. After the dropwise addition, the contents of the reaction vessel were heated under reflux at 70°C for a further 6 hours. Next, a solution containing 0.200 g of AIBN and 50 g of methyl ethyl ketone was added dropwise to the reaction vessel over 15 minutes. After the dropwise addition, the contents of the reaction vessel were heated under reflux at 70°C for a further 5 hours. In this way, a resin solution containing resin X was obtained. Next, the obtained resin solution was heated under reduced pressure to 70°C to evaporate the solvents (methyl ethyl ketone and isopropyl alcohol) and obtain a dry resin X.

[0039] 45g of water and 50g of resin X were placed in a flask and heated to 70°C. The mixture was then allowed to stand for 15 minutes to allow the resin X to dissolve in the water. Next, 5g of a nonionic surfactant (Kao Corporation's "Emulgen® 1153S-70," a polyoxyethylene alkyl ether) was added to the flask and slowly stirred. The resulting dispersion was then processed using a high-pressure crusher (GEA Nilosoavi's "Panda PLUS2000") (600MPa, 3 passes). This yielded a resin emulsion.

[0040] • Ink preparation Ion-exchanged water, preservative b, drying inhibitor c, and trimethylolpropane (manufactured by Tokyo Chemical Co., Ltd.) were added to a flask equipped with a stirrer ("Three One Motor (registered trademark) BL-600," manufactured by Shinto Kagaku Co., Ltd.). While stirring the contents with the aforementioned stirrer (stirring speed: 400 rpm), a 50% sodium hydroxide aqueous solution (Tokyo Chemical Co., Ltd.) was added dropwise to adjust the pH to 7.6-8.0. Further ion-exchanged water was added to adjust the ratio to the specified preparation ratio, and the pigment dispersion and resin emulsion were added in the order described above. The proportions of each raw material added were as shown in Table 2 below. To remove foreign matter and coarse particles from the resulting mixture, the mixture was filtered using a syringe filter with a pore size of 5 μm to obtain the ink. The type and content of preservative b were varied for each ink. The preservatives b1 to b20 used in the examples are shown in Table 3. Table 3 also shows the maximum amounts that can be added to preservatives b1 to b20.

[0041] [Table 2]

[0042] [Table 3]

[0043] (Ink evaluation) The dispersion stability and drying inhibition effect of the inks used in the examples were evaluated.

[0044] • Method for evaluating dispersion stability In the ink preparation process, after filtering as described above, the ink was placed in a sealed container and left at 30°C for 7 days. The ink was then filtered again using a syringe filter with a pore size of 5 μm to check for the presence or absence of filtration residue. The dispersion stability of each ink was evaluated according to the following criteria A and B. For dispersion stability, inks with an evaluation of A are considered acceptable, and inks with an evaluation of B are considered unacceptable. A (Good): No filtration residue. B (Poor): Filtration residue present.

[0045] • Method for evaluating the effect of inhibiting drying The drying inhibition effect was evaluated only for inks that passed the dispersion stability test. A modified Kyocera Document "TASKalfa Pro 15000c" was used for the drying inhibition effect evaluation. After filling the recording head with ink, it was left for 5 minutes in an environment of 25°C and 40% humidity. Then, a pattern image for checking the clogging of the recording head was recorded on a recording medium. The number of non-ejecting nozzles of the recording head N0 before the cleaning operation and the number of non-ejecting nozzles N1 after one cleaning operation were measured. The drying inhibition effect of each ink was evaluated according to the following criteria of A, B, and C. For drying inhibition effect, inks with an evaluation of A passed, and inks with evaluations of B and C failed. A (Good): Both N0 and N1 are 10 or less. B (Slightly Poor): N0 is greater than 10 and N1 is 10 or less. C (Defective): Both N0 and N1 are over 10.

[0046] (Samples 1-26) Ink samples 1 to 26 were prepared using the method described above. In all samples 1 to 26, lithium chloride (LiCl), a hygroscopic agent, was used as the drying inhibitor c. The type and content of preservative b were varied in samples 1 to 26. Table 4 shows the type and content of preservative b, as well as the evaluation results of dispersion stability for samples 1 to 26.

[0047] [Table 4]

[0048] All of samples 1 through 26 failed to meet the requirements for dispersion stability. This is thought to be because, in all of samples 1 through 26, which used an ionic desiccant as the desiccant c, the aggregation of pigment a occurred due to the presence of ions constituting the desiccant in addition to the ions constituting the preservative b.

[0049] (Samples 27-52) Ink samples 27-52 were prepared using the method described above. All samples 27-52 used glycerin, a humectant, as the drying inhibitor c. The type and content of preservative b were varied in samples 27-52. Table 5 shows the evaluation results for the type and content of preservative b, as well as the dispersion stability and drying inhibition effect, for samples 27-52.

[0050] [Table 5]

[0051] Samples 27-29, 33, 37, 44, 45, 49, and 50 all failed to meet the requirements for dispersion stability. Furthermore, even the other samples that passed the dispersion stability requirement failed to meet the drying inhibition requirement. This is thought to be because, in samples 27-52, which used glycerin as the drying inhibitor c, the drying inhibitor c's drying prevention effect was insufficient, and the aggregation of pigment a could not be suppressed due to the influence of the ions constituting preservative b.

[0052] (Samples 53-78) Ink samples 53-78 were prepared using the method described above. All samples 53-78 used DL-carnitine, a humectant, as the drying inhibitor c. The type and content of preservative b were varied in samples 53-78. Table 6 shows the evaluation results for the type and content of preservative b, as well as the dispersion stability, for samples 53-78.

[0053] [Table 6]

[0054] In all samples 53-78, the dispersion stability was unsatisfactory. This is thought to be because, in all samples 53-78, which used DL-carnitine as the drying inhibitor c, the drying inhibitor c did not effectively prevent drying, and the aggregation of pigment a could not be suppressed due to the influence of the ions constituting the preservative b.

[0055] (Samples 79-104) Ink samples 79-104 were prepared using the method described above. All samples 79-104 used betaine, a zwitterionic compound, as the drying inhibitor c. The type and content of preservative b were varied in samples 79-104. Table 7 shows the type and content of preservative b, as well as the evaluation results of dispersion stability for samples 79-104.

[0056] [Table 7]

[0057] Samples 79, 81-95, and 97-104 in the examples all passed the tests in terms of both dispersion stability and drying inhibition. On the other hand, samples 80 and 96 failed the tests in terms of dispersion stability. This is thought to be because sample 80 had an excess of orthophenylphenol, and sample 96 had an excess of ferulic acid.

[0058] (Samples 105-130) Ink samples 105-130 were prepared using the method described above. All samples 105-130 used 3-(1-pyridino)propanesulfonic acid, a sulfobetaine-type zwitterionic compound, as the drying inhibitor c. The type and content of preservative b were varied in samples 105-130. Table 8 shows the results of the evaluation of the type and content of preservative b, as well as the dispersion stability, for samples 105-130.

[0059] [Table 8]

[0060] Samples 105, 107-121, and 123-130 in the examples all passed the tests in terms of both dispersion stability and drying inhibition effect. On the other hand, samples 106 and 122 failed the tests in terms of dispersion stability. This is thought to be because sample 106 had an excess of orthophenylphenol, and sample 122 had an excess of ferulic acid.

[0061] (Samples 131-156) Ink samples 131-156 were prepared using the method described above. For all samples 131-156, trimethylamine N-oxide, an N,N,N-trialkyl N-oxy compound, was used as the drying inhibitor c. The type and content of preservative b were varied in samples 131-156. Table 9 shows the results of the evaluation of the type and content of preservative b, as well as the dispersion stability, for samples 131-156.

[0062] [Table 9]

[0063] Samples 131, 133-147, and 149-156 in the examples all passed the tests in terms of both dispersion stability and drying inhibition effect. On the other hand, samples 132 and 148 failed the tests in terms of dispersion stability. This is thought to be because sample 132 had an excess of orthophenylphenol, and sample 148 had an excess of ferulic acid.

Claims

1. Pigments and A zwitterionic compound that adopts a zwitterionic structure in an environment with a pH of 7.5 to 9, At least one preservative comprising 2-(2-hydroxy-5-methylphenyl), benzotriazole, orthophenylphenol, chlorcresol, sodium salicylate, sodium dihydroxydimethoxybenzophenone disulfonate, sodium sorbate, thymol, trichlorohydroxydiphenyl ether, para-aminobenzoic acid, methyl para-hydroxybenzoate, para-chlorophenol, zinc pyrithione, piroctone olamine, phenylbenzimidazole sulfonic acid, phenol, ferulic acid, resorcinol, cetylpyridinium chloride, benzalkonium chloride, and benzethonium chloride, An inkjet ink containing water, with an orthophenylphenol content of 4% by mass or less and a ferulic acid content of 7% by mass or less.

2. An inkjet ink according to claim 1, The zwitterionic compound is at least one of a betaine-type compound, a sulfobetaine-type compound, and an N,N,N-trialkylN-oxy compound, and does not contain any of the linear alkane skeleton, triethylene glycol skeleton, or tripropylene glycol skeleton having four or more carbon atoms. Inkjet ink.

3. An inkjet ink according to claim 1 or 2, The preservative comprises at least one of 2-(2-hydroxy-5-methylphenyl)benzotriazole, orthophenylphenol, sodium dihydroxydimethoxybenzophenone disulfonate, para-aminobenzoic acid, ferulic acid, and benzalkonium chloride. Inkjet ink.

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