inkjet ink

Abstract

We provide an ink that can achieve both high image density and long-term storage stability. [Solution] The inkjet ink contains a pigment, a block copolymer in which the hydrophobic polymer block is composed of benzyl methacrylate and the hydrophilic polymer block is composed of methacrylic acid, and water. The acid value of the block copolymer is 120 mg KOH / g or more and 220 mg KOH / g or less. The content of the block copolymer is 0.6% by mass or more and 2.6% by mass or less. The mass ratio of the block copolymer to the pigment is 0.12 or more and 0.51 or less. In the inkjet ink, the mass ratio of solids in the supernatant liquid obtained by centrifugation is 0.5% by mass or more and 3.0% by mass or less.

Description

Technical Field

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

Background Art

[0002] Inks for recording images on recording media such as paper are required to have a high image density in the images recorded on the recording media. Patent Document 1 discloses a technique for increasing the image density. In the technique described in Patent Document 1, a block copolymer composed of a methacrylate-based hydrophobic polymer block and a hydrophilic polymer block containing methacrylic acid is blended as a pigment-dispersing resin in an ink for inkjet.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In an ink for inkjet, in order to increase the image density, it is common to weaken the adsorption force of the pigment-dispersing resin to the pigment and increase the aggregability of the pigment on the recording medium by separating the pigment-dispersing resin from the pigment on the recording medium. However, in an ink for inkjet, when the adsorption force of the pigment-dispersing resin to the pigment is weakened, the dispersibility of the pigment in the solvent becomes unstable, so that the long-term storage stability is likely to decrease.

[0005] In view of the above circumstances, an object of the present invention is to provide an ink capable of achieving both high image density and long-term storage stability.

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 block copolymer in which the hydrophobic polymer block is composed of benzyl methacrylate and the hydrophilic polymer block is composed of methacrylic acid, and water. The acid value of the above block copolymer is between 120 mg KOH / g and 220 mg KOH / g. The content of the above block copolymer is 0.6% by mass or more and 2.6% by mass or less. The mass ratio of the block copolymer to the pigment is 0.12 or more and 0.51 or less. In the above inkjet ink, the mass ratio of solids in the supernatant liquid obtained by centrifugation is 0.5% by mass or more and 3.0% by mass or less.

[0007] In this inkjet ink, the pigment dispersion resin is a block copolymer with a structure divided into hydrophobic polymer blocks and hydrophilic polymer blocks, and benzyl methacrylate is used as the hydrophobic polymer block, which provides high adsorption to the pigment. Furthermore, this inkjet ink is prepared so that the amount of pigment dispersion resin that does not adsorb to the pigment is within a predetermined range. These factors improve image density in this inkjet ink by inhibiting the penetration of pigment into the recording medium. Furthermore, by using methacrylic acid as the hydrophilic polymer block in the pigment dispersion resin and setting the acid value of the pigment dispersion resin between 120 mg KOH / g and 220 mg KOH / g, high long-term storage stability is achieved. [Effects of the Invention]

[0008] As described above, the present invention can provide an inkjet ink that can achieve both high image density and long-term storage stability. [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, 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 the ink according to this embodiment, by using a pigment dispersion resin b with a specific configuration and keeping the amount of pigment dispersion resin b that does not adsorb to pigment a within a predetermined range, it is possible to achieve both image density and long-term storage stability. 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] (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. The pigment dispersion resin b also functions as a binder to retain pigment a on the recording medium. In the ink according to this embodiment, fine particles of a benzyl methacrylate / methacrylic acid block copolymer are used as the pigment dispersion resin b. In other words, the pigment dispersion resin b is a block copolymer with a structure divided into a hydrophobic polymer block composed of benzyl methacrylate and a hydrophilic polymer block composed of methacrylic acid.

[0017] In pigment dispersion resin b, a high adsorption capacity for pigment a is obtained by constructing the hydrophobic polymer block with benzyl methacrylate. As a result, in the ink according to this embodiment, the dispersion state of pigment a in the solution is more easily maintained over a long period of time, and the penetration of pigment a into the recording medium is inhibited, thereby improving image density. Furthermore, in the ink according to this embodiment, the long-term storage stability can be further improved by constructing the hydrophilic polymer block with methacrylic acid.

[0018] Furthermore, in the ink according to this embodiment, the amount of pigment dispersion resin b that is free in the solvent without being adsorbed onto pigment a is controlled. The amount of pigment dispersion resin b free in the solvent can be determined from the supernatant obtained by centrifuging the ink. In other words, when centrifugation is performed on the ink, the pigment dispersion, which is a composite in which pigment dispersion resin b is adsorbed onto pigment a, precipitates, while the pigment dispersion resin b free in the solvent remains in the supernatant. Since the solid content remaining in the supernatant is substantially only pigment dispersion resin b, the amount of pigment dispersion resin b free in the solvent in the ink can be determined by measuring the amount of solid content remaining in the supernatant.

[0019] In this embodiment, the pigment dispersion is precipitated by centrifugal separation at a rotation speed of 140,000 rpm (1,050,000 G) for 3 hours. In this embodiment, the mass of solids present in the supernatant is measured by thermomass spectrometry. The ink according to this embodiment is prepared so that the mass ratio of solids in the supernatant obtained by centrifugal separation is within a predetermined range. Specifically, the ink according to this embodiment is prepared so that the mass ratio of solids in the supernatant is between 0.5% by mass and 3.0% by mass. In the ink according to this embodiment, by setting the mass ratio of solids in the supernatant to 3.0% by mass or less, the effect of improving image density and long-term storage stability by the pigment dispersion resin b is effectively obtained. On the other hand, in the ink according to this embodiment, by setting the mass ratio of solids in the supernatant to 0.5% by mass or more, the scratch resistance of the image recorded on the recording medium can be improved.

[0020] Furthermore, in the ink according to this embodiment, the acid value of the pigment dispersion resin b is 120 mg KOH / g or more and 220 mg KOH / g or less. In the ink according to this embodiment, by setting the acid value of the pigment dispersion resin b to 120 mg KOH / g or more, the dispersibility of pigment a in the solvent can be ensured, and further benefits such as finer particle size of the pigment dispersion and improved color development and tinting power by pigment a can also be obtained. In addition, in the ink according to this embodiment, by setting the acid value of the pigment dispersion resin b to 220 mg KOH / g or less, long-term storage stability can be ensured.

[0021] In the ink according to this embodiment, from the viewpoint of ensuring the rubbing resistance of the image recorded on the recording medium, the content of the pigment dispersion resin b in the ink is 0.6% by mass or more, and the mass ratio of the pigment dispersion resin b to the pigment a is 0.12 or more. Further, in the ink according to this embodiment, from the viewpoints of preventing a decrease in image density due to a decrease in coloring power and preventing a decrease in the dispersibility of the pigment a due to an increase in viscosity, the content of the pigment dispersion resin b in the ink is 2.6% by mass or less, and the mass ratio of the pigment dispersion resin b to the pigment a is 0.51 or less.

[0022] (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, it is preferable that the water content is 50% by mass or more and 60% by mass or less.

[0023] (Other components) In the ink according to this embodiment, components other than those described above may be blended as necessary. For example, in the ink according to this embodiment, a surfactant can be blended for the purpose of enhancing the compatibility and dispersion stability of each component. The surfactant has the effect of enhancing the wettability of the ink with respect to the memory medium and the effect 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.

[0024] Furthermore, the ink according to this embodiment may also contain a surfactant as a dispersant that enhances the dispersibility of pigment a in the solvent. The surfactant added as a dispersant is added separately from the surfactant added to enhance the compatibility and dispersion stability of each of the above components, and enhances the dispersibility of pigment a in the solvent by reducing the interfacial tension between pigment a and the solvent. For example, nonionic surfactants and anionic surfactants can be used as such surfactants.

[0025] Furthermore, in addition to the dispersant, the ink according to this embodiment may contain various additives as needed, such as water-soluble humectants, dissolution stabilizers, drying inhibitors, antioxidants, viscosity modifiers, pH adjusters, neutralizing agents, and antifungal agents.

[0026] [Examples and Comparative Examples] Ink preparation and evaluation were performed as examples and comparative examples of the present invention.

[0027] (Ink preparation) First, a pigment dispersion was prepared in which pigment a was dispersed in water. The pigment dispersion was prepared by blending pigment a, pigment dispersion resin b, methyl ethyl ketone (MEK), sodium hydroxide, 1 N aqueous sodium hydroxide solution, and water in the quantities shown in Table 1.

[0028] [Table 1]

[0029] In both the examples and comparative examples, carbon black was used as pigment a, benzyl methacrylate / methacrylic acid block copolymer was used as pigment dispersion resin b, and ion-exchanged water was used as water.

[0030] To obtain a pigment dispersion, first, pigment dispersion resin b was dissolved in a solvent (methyl ethyl ketone) using a shaker. To this solution, 1 N aqueous sodium hydroxide solution and water were added so that the neutralization rate of pigment dispersion resin b reached a predetermined value. Then, pigment a was added and dispersed. After that, the solution was subjected to a solvent removal treatment to obtain the pigment dispersion.

[0031] Next, inks for the examples and comparative examples were prepared. The inks for the examples and comparative examples were prepared by blending the above-mentioned pigment dispersion with Surfinol® 420, triethylene glycol monobutyl ether, 3-methyl-1,5-pentanediol, glycerin, and water in the amounts shown in Table 2. Surfinol® 420 was added as a surfactant. Triethylene glycol monobutyl ether, 3-methyl-1,5-pentanediol, and glycerin were all added as water-soluble humectants. Ion-exchanged water was used as the water.

[0032] [Table 2]

[0033] In preparing each ink, the components shown in Table 2 were added sequentially while stirring the solvent with a stirrer. Furthermore, foreign matter, debris, and coarse particles were removed by filtering the stirred ink using a filter with a pore size of φ5 μm.

[0034] (Ink measurement and evaluation) For the inks in the examples and comparative examples, the solid content of the supernatant liquid was measured, and the image density and storage stability were evaluated.

[0035] • Measurement of solid content in the supernatant liquid For measuring the solid content in the supernatant of each ink, each ink was first centrifuged for 3 hours at a rotation speed of 140,000 rpm (1,050,000 G) using an ultracentrifuge (Eppendorf Hi-Mac Technologies Ltd. "himac® CS150FNX", rotor: S140AT). This precipitated the pigment dispersion, which is a composite of pigment a and pigment dispersion resin b adsorbed onto each ink. Next, 30 μL of the supernatant of each ink after centrifugation was transferred to an aluminum container for thermomass analysis. Then, the mass M1 of the 30 μL supernatant was measured. Next, thermomass analysis was performed using a thermomass spectrometer (Hitachi High-Tech Science Corporation "TG / DTA7200") according to the scheme shown in Table 3 below. Finally, the mass loss M2 of the sample was measured between temperatures of 200°C and 500°C. The mass loss M2 was considered to be the mass of solid content in the supernatant. The mass ratio of solids in the supernatant liquid was calculated according to the following formula. Mass ratio of solids in the supernatant liquid [mass %] = 100 × M² / M¹

[0036] [Table 3]

[0037] • 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, and plain paper was used as the recording medium. The amount of ink ejected from one recording head in the test machine was set to 12 pL per dot. Then, a solid 10cm x 10cm image was formed on the recording medium using the test machine.

[0038] After storing the recording medium on which the image was formed for 12 hours in a normal temperature and humidity environment, 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.), and the resulting value was used as the image density evaluation value. The image density evaluation value for each ink was evaluated according to the following A and B criteria. For image density, inks with an evaluation of A were considered acceptable, and inks with an evaluation of B were considered unacceptable. A (good): 1.3 or higher B (Poor): Less than 1.3

[0039] • Method for evaluating long-term storage stability For the evaluation of long-term storage stability, an E-type viscometer (TV-100EL, manufactured by Toki Sangyo Co., Ltd.) was used to measure the viscosity V1 before storage and the viscosity V2 after storage for 30 days in a 60°C environment. Based on the measured viscosity V1 before storage and viscosity V2 after storage, the viscosity change rate [%] was calculated using the following formula, and the viscosity change rate was used as the evaluation value for long-term storage stability. Viscosity change rate (%)=100×(V1-V2) / V1 The long-term storage stability of each ink was evaluated according to the following A and B criteria. Ink with an A rating is considered acceptable for long-term storage stability, while ink with a B rating is considered unacceptable. A (good): -0.5% or more and 0.5% or less B (Poor): Less than -0.5% or greater than -0.5%

[0040] (Examples 1-11) In Examples 1 to 11, inks were prepared according to the information shown in Table 4. Table 4 shows the acid value of pigment dispersion resin b, the proportions of pigment dispersion resin b, MEK, 1 N sodium hydroxide aqueous solution, and water in the pigment dispersion, the content of pigment dispersion resin b in the ink, the mass ratio of pigment dispersion resin b to pigment a, the mass ratio of MEK to water (including water in the 1 N sodium hydroxide aqueous solution), and the neutralization rate of pigment dispersion resin b by sodium hydroxide.

[0041] [Table 4]

[0042] In all of the inks in Examples 1 to 11, a pigment dispersion resin b with an acid value of 120 mg KOH / g or more and 220 mg KOH / g or less was used. In addition, in all of Examples 1 to 11, the content of pigment dispersion resin b was set to 0.6% by mass or more and 2.6% by mass or less, and the mass ratio of pigment dispersion resin b to pigment a was set to 0.12 or more and 0.51 or less. Furthermore, in all of Examples 1 to 11, the amount of MEK added was determined so that the mass ratio of MEK to water was 0.15 or more and 0.35 or less when preparing the pigment dispersion. In addition, in all of Examples 1 to 11, the amount of 1 N aqueous sodium hydroxide solution added was determined so that the neutralization rate of pigment dispersion resin b was 40% or more and 80% or less when preparing the pigment dispersion.

[0043] The neutralization rate of pigment dispersion resin b refers to the percentage of acid groups that are neutralized, when the total number of acid groups in pigment dispersion resin b is set to 100%. In other words, the neutralization rate of pigment dispersion resin b can be calculated as the percentage of the amount of sodium hydroxide added (100 × N2 / N1) relative to the theoretical value N1, where N1 is the theoretical value of the mass of sodium hydroxide required to completely neutralize pigment dispersion resin b, and N2 is the amount of sodium hydroxide added.

[0044] Table 5 shows the measurement results of the solid content in the supernatant liquid for the inks of Examples 1 to 11, as well as the evaluation results of image density and storage stability.

[0045] [Table 5]

[0046] In all of the inks from Examples 1 to 11, the mass ratio of solids in the supernatant liquid was 0.5% by mass or more and 3.0% by mass or less, and both image density and long-term storage stability were satisfactory. This confirmed that by blending pigment a and pigment dispersion resin b in a balanced manner, setting the acid value of pigment dispersion resin b to 120 mg KOH / g or more and 220 mg KOH / g or less, setting the mass ratio of MEK to water to 0.15 or more and 0.35 or less, and setting the neutralization rate of pigment dispersion resin b to 40% or more and 80% or less, an appropriate amount of pigment dispersion resin b is released into the solvent, thereby achieving both image density and long-term storage stability.

[0047] (Comparative Examples 1-8) In Comparative Examples 1 to 8, inks were prepared according to the information shown in Table 6. Table 6 shows the acid value of pigment dispersion resin b, the proportions of pigment dispersion resin b, MEK, 1 N sodium hydroxide aqueous solution, and water in the pigment dispersion, the content of pigment dispersion resin b in the ink, the mass ratio of pigment dispersion resin b to pigment a, the mass ratio of MEK to water (including water in the 1 N sodium hydroxide aqueous solution), and the neutralization rate of pigment dispersion resin b by sodium hydroxide.

[0048] [Table 6]

[0049] Comparative Examples 1 and 3 differ from the above examples in that the mass ratio of MEK to water is greater than 0.35 and the neutralization rate of pigment dispersion resin b is less than 40%. Comparative Examples 2 and 4 differ from the above examples in that the mass ratio of MEK to water is less than 0.15 and the neutralization rate of pigment dispersion resin b is greater than 80%. Comparative Example 5 differs from the above examples in that the acid value of pigment dispersion resin b is less than 120 mgKOH / g. Comparative Example 6 differs from the above examples in that the acid value of pigment dispersion resin b is greater than 220 mgKOH / g. Comparative Example 7 differs from the above examples in that the content of pigment dispersion resin b in the ink is less than 0.6% by mass and the mass ratio of pigment dispersion resin b to pigment a is less than 0.12. Comparative Example 8 differs from the above examples in that the content of pigment dispersion resin b in the ink is greater than 2.6% by mass and the mass ratio of pigment dispersion resin b to pigment a is greater than 0.51.

[0050] Table 7 shows the measurement results of the solid content in the supernatant liquid for the inks related to Comparative Examples 1 to 8, as well as the evaluation results of image density and storage stability.

[0051] [Table 7]

[0052] In all of the inks related to Comparative Examples 1 to 4, the mass ratio of solids in the supernatant exceeded 3.0% by mass, resulting in failure of both image density and long-term storage stability. Furthermore, in Comparative Examples 5 to 8, pigment a was not sufficiently dispersed, making it impossible to measure the solids in the supernatant and to evaluate image density and storage stability.

[0053] In Comparative Examples 1 and 3, where the mass ratio of MEK to water is greater than 0.35, the increase in MEK covering the pigment dispersion resin b reduces the probability of contact between the benzyl methacrylate chain of the pigment dispersion resin b and the pigment a. This is thought to result in a larger amount of pigment dispersion resin b being released into the solvent without being adsorbed onto the pigment a. Furthermore, in Comparative Examples 1 and 3, where the neutralization rate of the pigment dispersion resin b is less than 40%, the insufficient dispersibility of the pigment dispersion resin b in water is thought to be the reason why a larger amount of pigment dispersion resin b is released into the solvent without being adsorbed onto the pigment a.

[0054] In Comparative Examples 2 and 4, where the neutralization rate of pigment dispersion resin b exceeds 80%, the COO of methacrylic acid in pigment dispersion resin b - It is thought that the transfer of MEK to the benzyl methacrylate chain reduced the probability of contact between the benzyl methacrylate chain of pigment dispersion resin b and pigment a, resulting in a larger amount of pigment dispersion resin b being released into the solvent without being adsorbed onto pigment a.

[0055] In Comparative Examples 5 and 6, where the acid value of pigment dispersion resin b was outside the range of 120 mg KOH / g to 220 mg KOH / g, the adsorption state of pigment dispersion resin b to pigment a was unstable, which is thought to have resulted in insufficient dispersion of pigment a. Furthermore, in Comparative Examples 7 and 8, where the content of pigment dispersion resin b was outside the range of 0.6% to 2.5% by mass, and the mass ratio of pigment dispersion resin b to pigment a was outside the range of 0.12 to 0.51, the dispersive effect of pigment dispersion resin b on pigment a was insufficient, which is thought to have resulted in insufficient dispersion of pigment a.

Claims

[Claim 1] Pigments and A block copolymer in which a hydrophobic polymer block is composed of benzyl methacrylate and a hydrophilic polymer block is composed of methacrylic acid, It contains water, The acid value of the block copolymer is 120 mg KOH / g or more and 220 mg KOH / g or less. The content of the block copolymer is 0.6% by mass or more and 2.6% by mass or less. The mass ratio of the block copolymer to the pigment is 0.12 or more and 0.51 or less. The mass ratio of solids in the supernatant liquid obtained by centrifugation is 0.5% by mass or more and 3.0% by mass or less. Inkjet ink.

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