inkjet ink

Abstract

This invention provides an inkjet ink that achieves both high penetration into the recording medium and good cohesiveness during ejection. [Solution] The inkjet ink contains a pigment, a cyclohexyl methacrylate / methacrylic acid copolymer having a weight-average molecular weight of 7,000 to 15,000 and a molecular weight distribution of 1.0 to 1.5, and water. The surface tension of the above inkjet ink is 30 mN / m or less.

Description

Technical Field

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

Background Art

[0002] Patent Document 1 discloses ink for inkjet for recording an image on a recording medium such as paper. In order to record an image on a recording medium at high speed in an inkjet recording apparatus, it is advantageous that the ink for inkjet has high permeability to the recording medium. Generally, in ink for inkjet, in order to increase the permeability to the recording medium, it is effective to reduce the surface tension by a surfactant or the like.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in ink for inkjet, generally, the lower the surface tension, the easier it is for satellite droplets separated from the main droplet to occur when ejected from the recording head. Therefore, in ink for inkjet with low surface tension, for example, problems such as a decrease in image quality due to satellite droplets landing around the position where the main droplet lands on the recording medium are likely to occur.

[0005] In view of the above circumstances, an object of the present invention is to provide an ink for inkjet that can achieve both high permeability to a recording medium and good integrity during ejection.

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 cyclohexyl methacrylate / methacrylic acid copolymer having a weight-average molecular weight of 7,000 to 15,000 and a molecular weight distribution of 1.0 to 1.5, and water. The surface tension of the above inkjet ink is 30 mN / m or less.

[0007] This inkjet ink uses a cyclohexyl methacrylate / methacrylic acid copolymer as the pigment dispersion resin. When a pigment dispersion resin is adsorbed onto a pigment, the resin loses its inherent viscosity. However, in this inkjet ink, the adsorption of the pigment dispersion resin to the pigment is improved by the action of the cyclohexyl methacrylate that constitutes the hydrophobic segment, resulting in low viscosity. Furthermore, this inkjet ink has a relatively small weight-average molecular weight and a sharp molecular weight distribution, resulting in low viscosity of the pigment dispersion resin itself. This also means that the pigment dispersion resin that is not adsorbed to the pigment is less likely to cause an increase in viscosity. In this inkjet ink, the action of this pigment-dispersing resin makes it less likely for satellite droplets to form when ejected from the recording head, resulting in droplets that are more cohesive. Therefore, with this inkjet ink, even if the surface tension is reduced to improve penetration into the recording medium, it is possible to ensure cohesiveness during ejection. [Effects of the Invention]

[0008] As described above, the present invention can provide an inkjet ink that can achieve both penetration into the recording medium and cohesiveness during ejection. [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 pigment dispersion resin b, a surfactant 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. The recording medium on which the image is recorded with the ink according to this embodiment is made of fibers such as cellulose fibers, and examples include plain paper, copy paper, recycled paper, thin paper, and thick paper.

[0011] In this embodiment, the ink is configured to have a low surface tension in order to improve penetration into the recording medium, specifically, a surface tension of 30 mN / m or less. Furthermore, it is preferable that the surface tension of the ink in this embodiment be 25 mN / m or more. In this embodiment, the surface tension is measured using a high-precision surface tension meter "DY-700" manufactured by Kyowa Interface Science Co., Ltd. In this embodiment, by using a pigment dispersion resin b with a specific configuration, even with a low surface tension configuration, it is possible to ensure that the ink is less likely to generate satellite droplets when ejected from the recording head and tends to clump together as a single droplet (hereinafter referred to as "clumping property"). The details of each component of the ink in 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] (Pigment-dispersed resin b) The ink according to this embodiment contains a pigment dispersion resin b as a dispersant to enhance the dispersibility of pigment a in the solvent by adsorbing onto the surface of pigment a. In the ink according to this embodiment, fine particles of a cyclohexyl methacrylate / methacrylic acid copolymer are used as the pigment dispersion resin b. In the pigment dispersion resin b, cyclohexyl methacrylate constitutes a hydrophobic segment, and methacrylic acid constitutes a hydrophilic segment. When the pigment dispersion resin b is adsorbed onto pigment a, the resin's inherent viscosity disappears. In the inkjet ink according to this embodiment, the adsorption of the pigment dispersion resin b to pigment a is improved by the action of cyclohexyl methacrylate which constitutes the hydrophobic segment, resulting in low viscosity.

[0017] In addition, in the pigment-dispersing resin b, the weight-average molecular weight is 7,000 or more and 15,000 or less, and the molecular weight distribution is 1.0 or more and 1.5 or less. That is, in the pigment-dispersing resin b, since the weight-average molecular weight is relatively small and the molecular weight distribution is sharp, there are few large-molecular-weight molecules. Therefore, the viscosity of the pigment-dispersing resin b itself is low. For this reason, in the ink according to the present embodiment, the pigment-dispersing resin b that is not adsorbed to the pigment a is also unlikely to cause an increase in viscosity. In the present 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 SuperMultipore HZ-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.

[0018] Therefore, in the ink according to the present embodiment, due to the action of such a pigment-dispersing resin b, the viscosity remains low, so when it is ejected from the recording head, satellite droplets are unlikely to occur and it is likely to become droplets with high coherence.

[0019] In the ink according to the present embodiment, in order to sufficiently obtain the action of the pigment-dispersing resin b described above, the content of the pigment-dispersing resin b is preferably 4.0% by mass or more. Further, in the ink according to the present embodiment, in order to ensure high dispersion stability and prevent the adsorption amount of the pigment-dispersing resin b to the pigment a from becoming excessive, the content of the pigment-dispersing resin b is preferably 8.0% by mass or less.

[0020] (Surfactant c) In the ink according to this embodiment, in order to make the surface tension 30 mN / m or less, surfactant c can be blended. Surfactant c has the effect of enhancing the wettability of the ink with respect to the storage medium and the effect of enhancing the compatibility and dispersion stability of each component contained in the ink. Examples of surfactant c to be blended in the ink according to this embodiment include anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants. Surfactant c 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.

[0021] (Water) In the ink according to this embodiment, for example, ion-exchanged water, purified water, distilled water, etc. can be used as water. In the ink according to this embodiment, from the viewpoints of drying property and ejection reliability, the water content is preferably 5.0 mass% or more and 7.0 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, in the ink according to this embodiment, a surfactant can be used as a dispersant having the effect of enhancing the dispersibility of pigment a in the solvent. The surfactant blended as the dispersant is blended separately from surfactant c, and by reducing the interfacial tension between pigment a and the solvent, the dispersibility of pigment a in the solvent is enhanced. As such a surfactant, for example, a nonionic surfactant or an anionic surfactant can be used.

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

[0024] [Examples] As Examples 1 to 3 of the present invention, ink was prepared and evaluated.

[0025] (Ink adjustment) In Examples 1-3, a pigment dispersion was first prepared by dispersing pigment a in water. The pigment dispersion was prepared by blending pigment a, pigment dispersion resin b, 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 pigment dispersion resin b. 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 3, pigment a was pigment blue 15:3 (Toyo Color Co., Ltd. "Lionol Blue FG-7351"), 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 "NanoGlen Mill" manufactured by Asada Iron Works Co., Ltd., the "MSC Mill" manufactured by Nippon Coke Industries Co., Ltd., and the "Dino Mill" 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, the ink according to Example 1 was prepared. The ink according to Example 1 was prepared by blending the above-mentioned pigment dispersion, surfactant c, water-soluble humectant, and water in the amounts shown in Table 2. In Example 1, "Surfinol 104" manufactured by Nisshin Chemical Industry Co., Ltd. was used as surfactant c, and 3-methyl-1,5-pentanediol was used as the water-soluble humectant. The amount of surfactant c was determined for each of the 11 to 15 samples.

[0031] [Table 2]

[0032] In preparing the ink according to Example 1, 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] Furthermore, inks according to Examples 2 and 3 were prepared. The inks according to Examples 2 and 3 were prepared by blending the above-mentioned pigment dispersion, surfactant c, water-soluble humectant, and water in the amounts shown in Table 3. In Examples 2 and 3, "Surfinol 104" manufactured by Nisshin Chemical Industry Co., Ltd. was used as surfactant c, and 3-methyl-1,5-pentanediol was used as the water-soluble humectant.

[0034] [Table 3]

[0035] In preparing the inks for Examples 2 and 3, the components shown in Table 3 were added sequentially while stirring the solvent with a stirrer. Furthermore, foreign matter, debris, and coarse particles were removed by filtering each ink after stirring using a filter with a pore size of φ5 μm.

[0036] The following describes an example of a method for preparing a cyclohexyl methacrylate / methacrylic acid copolymer used as pigment dispersion resin b in each ink. First, a stirrer, nitrogen inlet tube, condenser (stirrer), and dropping funnel were set up in a four-necked flask (capacity: 1000 mL). Next, 100 g of isopropyl alcohol and 300 g of methyl ethyl ketone were placed in the flask. The contents of the flask were heated under reflux at 70°C while bubbling nitrogen into it.

[0037] Furthermore, 64.0 g of cyclohexyl acrylate, 36.0 g of methacrylic acid, and 0.4 g of azobisisobutyronitrile (AIBN, polymerization initiator) were mixed to obtain a monomer solution. The monomer solution was added dropwise to a flask over approximately 2 hours under reflux at 70°C. After addition, heating under reflux at 70°C was continued for a further 6 hours. A methyl ethyl ketone solution containing 0.2 g of AIBN was added dropwise to the flask over 15 minutes. After addition, heating under reflux at 70°C was continued for a further 5 hours. In this way, a cyclohexyl methacrylate / methacrylic acid copolymer was obtained.

[0038] (Ink evaluation) The inks from Examples 1 to 3 were evaluated for their penetration into recording media and their ability to clump together during ejection.

[0039] • Method for evaluating permeability For the evaluation of penetrating properties, a short-time abrasion resistance test was conducted. Specifically, for each ink, higher short-time abrasion resistance indicated higher penetrating properties, while lower short-time abrasion resistance indicated lower penetrating properties. For the short-time abrasion resistance test, an inkjet recording device (Kyocera Document Solutions Inc.'s "TASKalfa Pro 15000c") was used as the test machine. The test machine was set to a drive voltage such that 11 pL of ink was ejected from one recording head. The test machine was used to form a 10cm x 10cm solid image on a recording medium (Xerox Corporation's "Vitality"). Within 15 seconds of formation, another recording medium was pressed against the solid image with a load of 500g and slid back and forth five times across the solid image. The image density of the sliding surface of the other recording medium was then measured using a fluorescence spectrophotometer (FD-5, Konica Minolta, Inc.). The image density obtained from the measurement was used as the evaluation value for the penetrating properties of each ink. The penetrating properties of each ink were evaluated according to the following criteria A and B. Regarding permeability, inks with a rating of A are considered acceptable, while inks with a rating of B are considered unacceptable. A (good): 0.2% or less B (Bad): More than 0.2%

[0040] • Method for evaluating coherence In evaluating ink cohesion, the length of the droplets that appeared in the image recorded on the recording medium with each ink was measured. In other words, it was assumed that droplets were formed by satellite droplets with each ink, and the shorter the droplet length, the higher the cohesion, and the longer the droplet length, the lower the cohesion. To measure the droplet length, an inkjet recording device (TASKalfa Pro 15000c manufactured by Kyocera Document Solutions Inc.) was used, and a straight line image (dot line) was formed on the recording medium (A4 size paper) with a margin set to 5 mm. The length of the droplets extending from the dot line in the transport direction was measured, and the maximum value was used as the cohesion evaluation value. The cohesion evaluation value of each ink was evaluated according to the following A and B criteria. For cohesion, an A evaluation ink was considered acceptable, and a B evaluation ink was considered unacceptable. A (good): 0.2mm or less B (defective): More than 0.2mm

[0041] (Example 1) In Example 1, samples 1 to 5 of inks with varying surface tensions were prepared by adjusting the content of surfactant c, and the above evaluation was performed on samples 1 to 5. In all samples 1 to 5 of Example 1, cyclohexyl methacrylate (CyMMA) / methacrylic acid (MAA) copolymer (CyMMA: 64% by mass, MAA: 36% by mass, acid value: 160 mg KOH / g) was used as the pigment dispersion resin b.

[0042] Table 4 shows the surfactant c content, surface tension, and evaluation results for permeability and cohesiveness of samples 1-5. Samples 3-5 all received good evaluation results for both permeability and cohesiveness. On the other hand, samples 1 and 2, which had low surface tension, failed the permeability test. This is likely because samples 2 and 3 did not penetrate the recording medium sufficiently.

[0043] [Table 4]

[0044] (Example 2) In Example 2, ink samples 6-8 were prepared using pigment dispersion resins b with various hydrophobic segments by the method described above, and the above evaluation was performed on samples 6-8. In sample 6, a CyMMA / MAA copolymer was used as the pigment dispersion resin b. In sample 7, a benzyl methacrylate (BzMMA) / MAA copolymer was used as the pigment dispersion resin b. In sample 8, a styrene (St) / MAA copolymer was used as the pigment dispersion resin b. In all samples 6-8, the surface tension was 30 mN / m or less.

[0045] Table 5 shows the hydrophobic segment, hydrophilic segment, molecular weight distribution, and acid value of pigment dispersion resin b used in samples 6-8, as well as the evaluation results for permeability and cohesiveness for samples 6-8. Sample 6 received good evaluation results for both permeability and cohesiveness. On the other hand, samples 7 and 8, in which the hydrophobic segment of pigment dispersion resin b was not cyclohexyl methacrylate, both failed to meet the cohesiveness requirements. This is thought to be because, in samples 7 and 8, the adsorption of pigment dispersion resin b to pigment a was insufficient, and the viscosity did not decrease sufficiently.

[0046] [Table 5]

[0047] (Example 3) In Example 3, ink samples 9 to 23 were prepared using pigment dispersion resin b with various weight-average molecular weights and molecular weight distributions by the method described above, and the above evaluation was performed on samples 9 to 23. In all samples 9 to 23 of Example 3, cyclohexyl methacrylate (CyMMA) / methacrylic acid (MAA) copolymer (CyMMA: 64% by mass, MAA: 36% by mass, acid value: 160 mg KOH / g) was used as the pigment dispersion resin b. In all samples 9 to 23, the surface tension was 30 mN / m or less.

[0048] Table 6 shows the hydrophobic segment, hydrophilic segment, molecular weight distribution, and acid value of the pigment dispersion resin b used in samples 9-23, as well as the evaluation results of permeability and cohesion for samples 9-23. Samples 10-12, 15-17, and 19-21 showed good evaluation results for both permeability and cohesion. On the other hand, sample 9, which had a small molecular weight distribution in pigment dispersion resin b, failed the permeability test. This is thought to be because sample 9 had insufficient adsorption capacity of pigment dispersion resin b to pigment a, resulting in insufficient permeability to the recording medium. Sample 13, which had a large molecular weight distribution in pigment dispersion resin b, failed the cohesion test. This is thought to be because sample 13 had high viscosity due to the action of large molecular weight molecules present in pigment dispersion resin b. Sample 14, which had a small weight-average molecular weight in pigment dispersion resin b, failed the permeability test. This is thought to be because sample 14 had insufficient adsorption capacity of pigment dispersion resin b to pigment a, leaving only pigment a on the surface of the recording medium. Sample 18, which had a high weight-average molecular weight in pigment dispersion resin b, failed the cohesiveness test. This is thought to be because the viscosity of sample 18 increased due to the action of pigment dispersion resin b, which has a high weight-average molecular weight. Sample 22, which had a small molecular weight distribution in pigment dispersion resin b, also failed the cohesiveness test. This is thought to be because a localized difference in surface tension occurred in sample 22. Sample 23, which also had a small molecular weight distribution in pigment dispersion resin b, failed the permeability test. This is thought to be because in sample 23, the adsorption force of pigment dispersion resin b to pigment a was insufficient, and only pigment a remained on the surface of the recording medium.

[0049] [Table 6]

Claims

[Claim 1] Pigments and A cyclohexyl methacrylate / methacrylic acid copolymer having a weight-average molecular weight of 7,000 or more and 15,000 or less, and a molecular weight distribution of 1.0 or more and 1.5 or less, It contains water, The surface tension is 30 mN / m or less. Inkjet ink.

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