inkjet ink

Abstract

We provide inkjet inks that can achieve both dispersion stability and image density. [Solution] The inkjet ink contains a pigment, a water-soluble humectant with a log Kow of 0 or more, a benzyl methacrylate / methacrylic acid copolymer having a weight-average molecular weight of 10,000 or more and 30,000 or less, and a molecular weight distribution of 2.2 or more and 5.0 or less, and water.

Description

Technical Field

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

Background Art

[0002] Patent Documents 1 to 3 disclose inks for inkjet for recording an image on a recording medium such as paper. For an ink for inkjet, high dispersion stability for maintaining the dispersed state of a pigment in a solvent is required. On the other hand, for an ink for inkjet, high image density for recording a clear image with high visibility on a recording medium is also required.

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] In an ink for inkjet, in order to improve the image density of an image recorded on a recording medium, it is preferable that the pigment aggregates on the recording medium. That is, in an ink for inkjet, in order to achieve both dispersion stability and image density, the behavior of the pigment that is contradictory between in the solvent and on the recording medium is required.

[0005] In view of the above circumstances, an object of the present invention is to provide an ink for inkjet capable of achieving both dispersion stability and image density.

Means for Solving the Problems

[0006] To achieve the above objective, an inkjet ink according to one embodiment of the present invention contains a pigment, a water-soluble humectant with a log Kow of 0 or more, a benzyl methacrylate / methacrylic acid copolymer having a weight-average molecular weight of 10,000 or more and 30,000 or less, and a molecular weight distribution of 2.2 or more and 5.0 or less, and water.

[0007] This inkjet ink uses a water-soluble humectant with relatively high hydrophobicity, which makes it easier for the pigment to aggregate on the recording medium, resulting in high image density. Furthermore, this inkjet ink uses a benzyl methacrylate / methacrylic acid copolymer as the pigment dispersion resin. In this pigment dispersion resin, the hydrophobic segment, benzyl methacrylate, improves the dispersion stability of the pigment, while the hydrophilic segment, methacrylic acid, improves image density. Furthermore, in this pigment dispersion resin, by limiting the molecular weight to a specific range and ensuring a relatively broad molecular weight distribution, it is possible to give relatively large molecules and relatively small molecules mutually different functions. Specifically, molecules with relatively large molecular weights have a strong adsorption capacity to pigments, thus maintaining the dispersibility of the pigment in the solvent. On the other hand, molecules with relatively small molecular weights have a relatively weak adsorption capacity to pigments, so by being released from the pigment on the recording medium, they enhance the aggregation of the pigment. These features enable this inkjet ink to achieve both dispersion stability and high image density. [Effects of the Invention]

[0008] As described above, the present invention can provide an inkjet ink that can achieve both dispersion stability and image density. [Modes for carrying out the invention]

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

[0010] [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 water-soluble humectant b, a pigment dispersion resin 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.

[0011] In the ink according to this embodiment, by using a combination of a water-soluble humectant b with a specific configuration and a pigment-dispersing resin c with a specific configuration, it is possible to achieve both dispersion stability and image density. The details of each component of the ink according to this embodiment will be described below.

[0012] (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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] (Water-soluble humectant b) The ink according to this embodiment is formulated with a water-soluble humectant b that has relatively high hydrophobicity. As a result, pigment a is more likely to aggregate on the recording medium in the ink according to this embodiment. Consequently, a high image density can be obtained on the recording medium with the ink according to this embodiment, that is, a clear image with high visibility can be recorded on the recording medium. From the viewpoint of obtaining a high image density on the recording medium, the log Kow of the water-soluble humectant b is 0 or greater. Furthermore, from the viewpoint of ensuring solubility in water, it is preferable that the log Kow of the water-soluble humectant b is -2.0 or less. log Kow is the water / octanol coefficient. The value of log Kow in this invention is a value calculated from the Hansen solubility parameter software "HSPiP".

[0017] In the ink according to this embodiment, it is preferable that the content of the water-soluble humectant b is 4.0% by mass or more in order to fully obtain the effect of the water-soluble humectant b. Furthermore, in the ink according to this embodiment, it is preferable that the content of the water-soluble humectant b is 40% by mass or less in order to obtain high dispersion stability.

[0018] (Pigment-dispersed resin c) In the ink according to this embodiment, a pigment dispersion resin c is blended as a dispersant for enhancing the dispersibility of pigment a in a solvent by adsorbing on the surface of pigment a. In the ink according to this embodiment, fine particles of a benzyl methacrylate / methacrylic acid copolymer are used as the pigment dispersion resin c. In the pigment dispersion resin c, benzyl methacrylate, which is a hydrophobic segment, has an effect of improving the dispersion stability of pigment a, and methacrylic acid, which is a hydrophilic segment, has an effect of improving the image density.

[0019] Further, in the pigment dispersion resin c, the weight average molecular weight is 10,000 or more and 30,000 or less, and the molecular weight distribution is 2.2 or more and 5.0 or less. Thereby, in the pigment dispersion resin c, molecules with a relatively large molecular weight and molecules with a relatively small molecular weight can have different functions from each other. That is, the molecules with a relatively large molecular weight in the pigment dispersion resin c have a strong adsorption force for pigment a, and even in the water-soluble humectant b with relatively high hydrophobicity, the adsorption state to the pigment is impaired, so it exhibits a function of maintaining the dispersibility of the pigment in the solvent. Also, the molecules with a relatively small molecular weight in the pigment dispersion resin c have a relatively weak adsorption force for the pigment, so they exhibit a function of enhancing the aggregability of the pigment by dissociating from the pigment on the recording medium. In this embodiment, the weight average molecular weight and the molecular weight distribution are measured under the following conditions using gel permeation chromatography (HLC-8020GPC manufactured by Tosoh Corporation). ·Column: "TSKgel SuperMultiporeHZ-H" manufactured by Tosoh Corporation (semi-micro column of 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 The calibration curve is prepared by selecting seven types of F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000 and n-propylbenzene from TSKgel standard polystyrene manufactured by Tosoh Corporation.

[0020] In the ink according to this embodiment, in order to sufficiently obtain the action of the above pigment dispersion resin c, the content of the pigment dispersion resin c is preferably 3.0% by mass or more. Further, in the ink according to this embodiment, in order to obtain high dispersion stability, the content of the pigment dispersion resin c is preferably 4.0% by mass or less.

[0021] (Water) In the ink according to this embodiment, as the water, for example, ion-exchanged water, purified water, distilled water, etc. can be used. In the ink according to this embodiment, from the viewpoints of drying property and ejection reliability, the water content is preferably 40% by mass or more and 60% by mass or less.

[0022] (Other components) In the ink according to this embodiment, components other than the above may be blended as necessary. For example, a surfactant can be blended in the ink according to this embodiment. The surfactant has the action of enhancing the wettability of the ink with respect to the storage medium and the action of enhancing the compatibility and dispersion stability of each component contained in the ink. Examples of the surfactant to be blended in the ink according to this embodiment include anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants. The surfactant is preferably a nonionic surfactant. The nonionic surfactant is preferably a surfactant having an acetylene bond, and more preferably a surfactant having an acetylene glycol structure or an acetylene alcohol structure.

[0023] Further, in the ink according to this embodiment, in addition to the surfactant, various additives such as a penetrant, a dissolution stabilizer, an anti-drying agent, an antioxidant, a viscosity modifier, a pH adjuster, a neutralizing agent, and a fungicide may be blended as necessary.

[0024] [Examples] As Examples 1 to 4 of the present invention, the ink was prepared and evaluated. Hereinafter, the contents common to Examples 1 to 4 will be described, and then the individual contents of each of Examples 1 to 4 will be described.

[0025] (Ink preparation) In Examples 1 to 4, a pigment dispersion was first prepared by dispersing pigment a in water. The pigment dispersion was prepared by blending pigment a, pigment dispersion resin c, sodium hydroxide, Olphine® E1010, and water in the amounts shown in Table 1.

[0026] [Table 1]

[0027] Sodium hydroxide was added as a neutralizing agent to neutralize the pigment dispersion resin c. Olfin® E1010 was added as a dispersant to improve the dispersibility of pigment a in the solvent, and is a nonionic surfactant manufactured by Nisshin Chemical Industry Co., Ltd. In all of Examples 1 to 4, pigment a was pigment blue 15:3 ("Lionol Blue FG-7351" manufactured by Toyo Color Co., Ltd.), and deionized water was used as the water.

[0028] The pigment dispersion was prepared by mixing the above components using a wet dispersion method with a media-type wet disperser. Examples of media-type wet dispersers include wet dispersers (more specifically, the "NanoGlenMill" manufactured by Asada Iron Works Co., Ltd., the "MSC Mill" manufactured by Nippon Coke Industries, Ltd., and the "DinoMill" manufactured by Shinmaru Enterprises, Ltd.).

[0029] In wet dispersion using a media-type wet disperser, media (zirconia beads with a diameter of 0.5 mm) were set in the vessel, and the discharge rate was controlled to 200-600 g / min to adjust the average particle size of the pigment dispersion, in which the dispersant adhered to pigment a dispersed in water, to 90-110 nm. The particle size distribution of the pigment dispersion was measured using a Zetasizer Nano manufactured by Sysmex Corporation, with the pigment dispersion diluted 300 times with deionized water.

[0030] Next, inks according to Examples 1 to 4 were prepared. The inks according to Examples 1 to 4 were prepared by blending the above-mentioned pigment dispersion, surfactant, water-soluble humectant b, and water in the quantities shown in Table 2. In all of Examples 1 to 4, Olphine® E1010 was used as the surfactant.

[0031] [Table 2]

[0032] In the preparation of the inks in Examples 1 to 4, the components shown in Table 2 were added in order while stirring the solvent with a stirrer. Furthermore, foreign matter, dirt, and coarse particles were removed by filtering each ink after stirring using a filter with a pore size of φ5 μm.

[0033] (Ink evaluation) The dispersion stability and image density of the inks related to Examples 1 to 4 were evaluated.

[0034] • Method for evaluating dispersion stability In evaluating dispersion stability, the particle size of the pigment dispersion, a composite in which pigment dispersion resin c is adsorbed onto pigment a, was measured before and after drying. In the drying process, 10g of ink was placed in an iBoy container, and the solvent components were evaporated until the amount of ink in the iBoy container reached 6g. The ratio of the change in particle size of the pigment dispersion before and after drying to the particle size of the pigment dispersion before drying was calculated. The calculated ratio was used as the evaluation value for the dispersion stability of each ink. The evaluation value for the dispersion stability of each ink was evaluated according to the following A and B criteria. For dispersion stability, inks with an evaluation of A were considered passable, and inks with an evaluation of B were considered failing. A (good): 10% or less B (Bad): More than 10%

[0035] • Method for evaluating image density For the evaluation of image density, an inkjet recording device (line type, manufactured by Kyocera Document Solutions Inc.) was used as the test machine. The drive voltage of the test machine was set so that the amount of ink ejected from one recording head was 12 pL. A 10 cm x 10 cm solid image was formed on a recording medium (Xerox "Vitality") using the test machine. After leaving the recording medium with the formed image for 12 hours, the image density of the solid image formed on the recording medium was measured using a fluorescence spectrophotometer (FD-5, manufactured by Konica Minolta, Inc.). Ink with an image density of 1.3 or higher was judged as passing, and ink with an image density of less than 1.3 was judged as failing.

[0036] (Example 1) In Example 1, ink samples 1 to 5 were prepared using the above method with benzyl methacrylate (BzMMA) / methacrylic acid (MAA) copolymers (BzMMA: 64% by mass, MAA: 36% by mass, acid value: 160 mg KOH / g) having various weight-average molecular weights and molecular weight distributions as pigment dispersion resin c, and the above evaluation was performed on samples 1 to 5. In all samples 1 to 5 of Example 1, 3-methyl-1,5-pentanediol (log Kow: 0.35) was used as the water-soluble humectant b.

[0037] Table 3 shows the weight-average molecular weight and molecular weight distribution of the pigment dispersion resin c used in samples 1 to 5, as well as the evaluation results for dispersion stability and image density for samples 1 to 5. Samples 2 to 4 all received good evaluation results for both dispersion stability and image density. On the other hand, sample 1, which has a narrow molecular weight distribution of pigment dispersion resin c, failed the dispersion stability test. This is thought to be because sample 1 lacked high molecular weight molecules with strong adsorption capacity to pigment a in pigment dispersion resin c. Furthermore, sample 5, which has a large molecular weight and a wide molecular weight distribution of pigment dispersion resin c, failed the image density test. This is thought to be because sample 5 lacked low molecular weight molecules with weak adsorption capacity to pigment a in pigment dispersion resin c.

[0038] [Table 3]

[0039] (Example 2) In Example 2, ink samples 6-9 were prepared using pigment dispersion resins c with various hydrophobic and hydrophilic segments by the method described above, and the above evaluation was performed on samples 6-9. In sample 6, a BzMMA / MAA copolymer was used as the pigment dispersion resin c. In sample 7, a BzMMA / acrylic acid (AA) copolymer was used as the pigment dispersion resin c. In sample 8, a styrene (St) / MAA copolymer was used as the pigment dispersion resin c. In sample 9, a St / AA copolymer was used as the pigment dispersion resin c. The acid value of the pigment dispersion resin c used in samples 6-9 was 160 mgKOH / g. In addition, 3-methyl-1,5-pentanediol (log Kow: 0.35) was used as the water-soluble humectant b in all samples 6-9 of Example 2.

[0040] Table 4 shows the hydrophobic and hydrophilic segments of the pigment dispersion resin c used in samples 6-9, as well as the evaluation results for dispersion stability and image density for samples 6-9. Sample 6 obtained good evaluation results for both dispersion stability and image density. On the other hand, samples 8 and 9, in which the hydrophobic segment of the pigment dispersion resin c was not benzyl methacrylate, both failed to meet the requirements for dispersion stability. Furthermore, samples 7 and 9, in which the hydrophilic segment of the pigment dispersion resin c was not methacrylic acid, failed to meet the requirements for image density.

[0041] [Table 4]

[0042] (Example 3) In Example 3, ink samples 10-14 were prepared using the above method with BzMMA / MAA copolymers having various weight-average molecular weights as the pigment dispersion resin c, and the above evaluation was performed on samples 10-14. In all samples 10-14 of Example 3, 3-methyl-1,5-pentanediol (log Kow: 0.35) was used as the water-soluble humectant b.

[0043] Table 5 shows the weight-average molecular weight and molecular weight distribution of pigment dispersion resin c used in samples 10-14, as well as the evaluation results for dispersion stability and image density for samples 10-14. Samples 11-13 all showed good evaluation results for both dispersion stability and image density. On the other hand, sample 10, in which pigment dispersion resin c has a small molecular weight, failed to meet the requirements for dispersion stability. This is thought to be because sample 10 lacked molecules with a large molecular weight that have strong adsorption capacity to pigment a in pigment dispersion resin c. Similarly, sample 14, in which pigment dispersion resin c has a large molecular weight, failed to meet the requirements for image density. This is thought to be because sample 14 lacked molecules with a small molecular weight that have weak adsorption capacity to pigment a in pigment dispersion resin c.

[0044] [Table 5]

[0045] (Example 4) In Example 4, samples 15-23 were prepared using water-soluble humectants b with varying log Kow values, and the above evaluation was performed on samples 15-23. In all samples 15-23 of Example 4, a BzMMA / MAA copolymer was used as the pigment dispersion resin c.

[0046] Table 6 shows the material names and log Kow values ​​of the water-soluble humectant b used in samples 15-23, as well as the evaluation results for dispersion stability and image density for samples 15-23. Samples 21-23, where the log Kow of water-soluble humectant b was 0 or greater, all showed good evaluation results for both dispersion stability and image density. On the other hand, samples 15-20, where the log Kow of water-soluble humectant b was less than 0, all failed to meet the image density requirements. This is likely because samples 15-20 did not exhibit the desired effect of water-soluble humectant b in promoting the aggregation of pigment a on the recording medium.

[0047] [Table 6]

Claims

[Claim 1] Pigments and A water-soluble humectant with log Kow of 0 or more, A benzyl methacrylate / methacrylic acid copolymer having a weight-average molecular weight of 10,000 or more and 30,000 or less, and a molecular weight distribution of 2.2 or more and 5.0 or less, Water and, Inkjet ink containing [this ingredient].

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