Pigment dispersion for aqueous inkjet and method for producing same

Abstract

The present invention relates to a pigment dispersion for aqueous inkjet, which contains a colored organic pigment, a dispersant, an organic solvent and water, and wherein: the dispersant is at least one type selected from the group consisting of benzyl acrylate polymer dispersants and styrene-maleic anhydride polymer dispersants having an acid value of 15 mgKOH / g or less; the organic solvent is at least one type selected from the group consisting of a monovalent aliphatic alcohol and a polyvalent aliphatic alcohol, and the total amount of the organic solvent is 3-8 wt% of the entire aqueous inkjet pigment dispersion. The present invention can provide a pigment dispersion capable of producing an aqueous inkjet ink having good filter filterability, and a method for producing the pigment dispersion.

Description

Technical Field

[0001] This invention relates to pigment dispersions for water-based inkjet printing and methods for manufacturing the same, and particularly to pigment dispersions for water-based inkjet printing using colored organic pigments as pigments and methods for manufacturing the same. Background Technology

[0002] In recent years, due to improvements in image quality and printing speed, inkjet printers have been used not only for home printing but also for printing business documents and other corporate printing. On the other hand, in commercial and industrial printing applications such as the latter, there is a further demand for higher image quality and faster printing speeds. To address these demands, various studies have been conducted (e.g., patent documents 1-3).

[0003] Patent document 1 proposes an inkjet ink that can record images with excellent text quality, which are not easily smudged even when drawn with a marker, and can record images with excellent durability such as resistance to marking. In the water-based inkjet ink containing pigment and polyurethane resin, a specific polyurethane resin is used, and the dynamic surface tension of the water-based ink is set within a specified range.

[0004] In Patent Document 2, as the required properties of coloring compositions (inks) become more demanding, the dispersibility of coloring materials in existing dispersants is insufficient, and when the amount of dispersant used is increased, heat resistance issues such as color changes due to heating during or after ink spraying become problematic. Therefore, a dispersant with excellent dispersibility even in small amounts is provided, and a polymeric product containing a specific block copolymer within a specified range and a coloring composition containing the polymeric product are proposed.

[0005] Patent document 3 provides an inkjet recording water-based ink that improves the rub resistance when recording on coated paper. It proposes that the resin particles contained in the inkjet recording water-based ink are composed of a specified polymer, and that the storage modulus of the dried ink film at 25°C is set within a specified range.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2018-150514

[0009] Patent Document 2: Japanese Patent Application Publication No. 2021-98835

[0010] Patent Document 3: Japanese Patent Application Publication No. 2022-156109 Summary of the Invention

[0011] The technical problem that the invention aims to solve

[0012] The aforementioned existing technologies are considered capable of meeting the market demands for high-quality and high-speed inkjet printing. However, according to the inventors' research, it has been determined that in these existing technologies, the filter filtration process, which is typically performed in the final stage of manufacturing inkjet ink, sometimes results in poor filterability, requiring increased filtration pressure, or the filter is prone to clogging, necessitating frequent filter replacement, and sometimes increases power consumption, filter usage, filter replacement workload, and ink loss.

[0013] Therefore, the object of the present invention is to provide a pigment dispersion capable of producing water-based inkjet ink with good filterability and a method for manufacturing the pigment dispersion.

[0014] Technical solutions for solving technical problems

[0015] To solve the aforementioned technical problems, the inventors of this invention conducted meticulous research. As a result, it was discovered that by using specific compounds as dispersants and organic solvents in a water-based inkjet pigment dispersion that uses colored organic pigments as pigments, and by setting the total amount of organic solvent within a specified range, a pigment dispersion capable of producing water-based inkjet inks with excellent filterability can be obtained. The main points of this invention are as follows.

[0016] The first aspect of the present invention relates to a water-based inkjet pigment dispersion containing a colored organic pigment, a dispersant, an organic solvent, and water. The dispersant is selected from at least one of benzyl acrylate-based polymeric dispersants and styrene-maleic anhydride-based polymeric dispersants with an acid value of less than 15 mg KOH / g. The organic solvent is selected from at least one of monoaliphatic alcohols and polyaliphatic alcohols. The total amount of the organic solvent is 3 to 8% by weight in the total water-based inkjet pigment dispersion.

[0017] In the first aspect of the present invention, the above-mentioned aliphatic alcohol may have a chain structure having 4 to 10 carbon atoms.

[0018] In a first aspect of the present invention, a surface tension modifier may be included.

[0019] The second aspect of the present invention relates to a method for manufacturing a pigment dispersion for water-based inkjet printing, characterized in that it includes: obtaining a mixture containing a colored organic pigment, a dispersant, an organic solvent and water, adjusting the content of the organic solvent in the mixture to 3-8% by weight, and then dispersing the mixture.

[0020] A third aspect of the present invention relates to an aqueous inkjet ink containing the above-mentioned aqueous inkjet pigment dispersion.

[0021] In the third aspect of the present invention, the filtration rate when filtering water-based inkjet ink with a pigment concentration of 4% by weight using a filter membrane with a mesh size of 1.2 μm can be 400 g / 3 minutes or more.

[0022] Invention Effects

[0023] The present invention provides a pigment dispersion capable of producing water-based inkjet ink with good filterability and a method for producing the pigment dispersion. Detailed Implementation

[0024] (Pigment dispersion for water-based inkjet printing)

[0025] The aqueous inkjet pigment dispersion (hereinafter, sometimes simply referred to as "pigment dispersion") according to embodiments of the present invention contains a colored organic pigment (hereinafter, sometimes simply referred to as "organic pigment" or "pigment"), a dispersant, an organic solvent, and water. The dispersant is at least one selected from benzyl acrylate-based polymeric dispersants and styrene-maleic anhydride-based polymeric dispersants with an acid value of 15 mg KOH / g or less. The organic solvent is at least one selected from monoaliphatic alcohols and polyaliphatic alcohols. The total amount of organic solvent in the aqueous inkjet pigment dispersion is 3 to 8% by weight.

[0026] Thus, by using specific compounds as dispersants and organic solvents relative to colored organic pigments, setting the total amount of organic solvent within a specified range, and performing dispersion treatment, the resulting pigment dispersion achieves good filterability when used to manufacture water-based inkjet inks. The filterability of water-based inkjet inks can be evaluated using the filterability of the pigment dispersion as an indicator, and the filterability of the pigment dispersion can be evaluated using the methods described in the Examples section below.

[0027] The composition of the pigment dispersion is described below.

[0028] Colored organic pigments are not specifically limited to any organic pigment other than black and white. Examples of pigment types include: anthraquinone pigments, aminoanthraquinone pigments, quinacridone pigments, quinacridonequinone pigments, diketopyrrolopyrrole pigments, perylene pigments, pyrene pigments, anthraquinone pigments, benzimidazolone pigments, diazo condensate pigments, azo pigments, thioindole pigments, pinantrone pigments, dioxazine pigments, quinophthalone pigments, isoindoline pigments, and phthalocyanine pigments, etc.

[0029] In addition, when using color index (CI) numbers to indicate applicable colored organic pigments, the following pigments can be listed for example.

[0030] As red pigments, the following can be listed: CI Pigment Red (PR) 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 21, 22, 23, 31, 32, 38, 41, 48, 48:1, 48:2, 48:3, 48:4, 48:5, 49, 52, 52:1, 52:2, 53:1, 54, 57:1, 58, 60:1, 63, 64:1, 68, 81:1, 83, 88, 89, 95, 112, 114, 119, 122, 123, 129, 136, 144, 146, 147, 149, 150, 164, 166, 168. 169, 170, 171, 172, 175, 176, 177, 178, 179, 181, 183, 184, 185, 187, 188, 190, 193, 194, 200, 202, 206, 207, 208, 209, 210, 211, 213, 214, 216, 220, 221, 224, 226, 237, 238, 239, 242, 245, 247, 248, 251, 253, 254, 255, 256, 257, 258, 260, 262, 263, 264, 266, 268, 269, 270, 271, 272, 279, 291, etc.

[0031] As blue pigments, examples include: CI Pigment Blue (PB) 1, 15, 15:1, 15:2, 15:3, 15:4, 15:5, 15:6, 16, 17:1, 24, 24:1, 25, 26, 56, 60, 61, 62, 63, 75, 79, 80, etc.

[0032] Examples of yellow pigments include: CI Pigment Yellow (PY) 74, 138, 139, 150, 155, 180, 185, etc.

[0033] Examples of orange pigments include: CI Pigment Orange (PO) 43, 71, 73, etc.

[0034] As green pigments, examples include: CI Pigment Green (PG) 1, 4, 7, 8, 10, 36, 58, 59, 63, etc.

[0035] As purple pigments, examples include: CI pigment purple (PV) 1, 2, 3, 3:1, 3:3, 5:1, 13, 17, 19, 23, 25, 27, 29, 31, 32, 36, 37, 38, 42, 50, etc.

[0036] The colored organic pigment may be one of the pigments mentioned above or two or more in combination. From the viewpoint of ensuring good dispersibility, the pigment content in the pigment dispersion is preferably 15.0 to 30.0% by weight, more preferably 15.0 to 28.0% by weight.

[0037] The average particle size of colored organic pigments only needs to be of a typical size used in applications such as inkjet inks for recording, typically 50–200 nm. The average particle size can be determined using common methods such as dynamic light scattering and laser diffraction.

[0038] The dispersant may be selected from at least one of the following, as mentioned above: benzyl acrylate-based polymeric dispersants and styrene-maleic anhydride-based polymeric dispersants with an acid value of less than 15 mg KOH / g.

[0039] Benzyl acrylate-based polymeric dispersants only require that they contain a benzyl acrylate copolymer as an active ingredient. The benzyl acrylate copolymer can be any copolymer composed of structural units derived from benzyl acrylate and structural units derived from monomers capable of polymerizing with benzyl acrylate. The copolymer can be a random copolymer or a block copolymer; from the viewpoint of further improving filter filtration performance, a block copolymer is preferred. Furthermore, as a block copolymer, it is preferable that it contains a hydrophilic block (hereinafter sometimes referred to as "block A") and a hydrophobic block (hereinafter sometimes referred to as "block B"). The hydrophilic block refers to a block with a relatively higher affinity for water compared to the hydrophobic block. For example, the copolymer may be constructed such that the hydrophilic block contains the hydrophilic monomer described later, and the hydrophobic block does not contain the hydrophilic monomer, or contains a smaller proportion of hydrophilic functional groups than the hydrophilic block. The hydrophobic block preferably does not contain hydrophilic monomers.

[0040] As monomers capable of polymerizing with benzyl acrylate, as mentioned above, examples include hydrophilic monomers and monomers other than hydrophilic monomers (hereinafter sometimes referred to as "hydrophobic monomers").

[0041] Examples of hydrophilic monomers include: unsaturated polycarboxylic acids such as (meth)acrylic acid and maleic acid; monomers containing carboxyl or anhydride groups such as maleic anhydride; monomers containing sulfonic acid groups such as styrene sulfonic acid and 4-(methacryloyloxy)butylsulfonic acid; and ethylene oxide-modified (meth)acrylate monomers such as ethylene oxide-modified (meth)acrylate alkyl esters. Among these, unsaturated polycarboxylic acids such as (meth)acrylic acid and maleic anhydride are preferred, and (meth)acrylic acid is more preferred. A single hydrophilic monomer can be used alone, or two or more monomers can be used in combination. When two or more monomers are combined, the structures of blocks A and B can be either random polymers or block polymers. Furthermore, (meth)acrylic acid refers to methacrylic acid and / or acrylic acid.

[0042] Examples of hydrophobic monomers include: styrene-based monomers such as styrene, α-methylstyrene, and vinyltoluene; α-olefin-based monomers such as ethylene, propylene, and 1-butene; and vinyl-based monomers containing phenyl, biphenyl, or naphthyl groups. A single hydrophobic monomer can be used alone, or in combination of two or more. When two or more monomers are combined, the structures of blocks A and B can be either random polymers or block polymers, respectively. Additionally, benzyl acrylate is a hydrophobic monomer.

[0043] The structure of the benzyl acrylate copolymer is preferably composed of block A, which is composed of structural units derived from benzyl acrylate and hydrophilic monomers, and block B, which is composed of structural units derived from hydrophobic monomers. More preferably, it is composed of block A, which is composed of structural units derived from benzyl acrylate and hydrophilic monomers, and block B, which is composed of structural units derived from one or more hydrophobic monomers containing benzyl acrylate. Even more preferably, it is composed of block A, which is composed of structural units derived from benzyl acrylate and (meth)acrylic acid, and block B, which is composed of structural units derived from benzyl acrylate. Particularly preferably, it is composed of block A, which is composed of structural units derived from benzyl acrylate, methacrylic acid, and acrylic acid, and block B, which is composed of structural units derived from benzyl acrylate.

[0044] The ratio of block A to block B (weight ratio A / B) can be appropriately determined according to the type and content of the components, and is preferably 30 / 70 to 70 / 30. Furthermore, when block A is composed of structural units from benzyl acrylate (X) and (meth)acrylic acid (Y), the ratio of block A (weight ratio X / Y) is preferably 1 / 1 to 3 / 1. Additionally, when the (meth)acrylic acid is acrylic acid (Y1) and methacrylic acid (Y2), the ratio of block A (weight ratio Y1 / Y2) is preferably 200 / 1 to 100 / 1.

[0045] The peak molecular weight of the benzyl acrylate copolymer is not particularly limited, but from the viewpoint of further improving filter filtration performance, it is preferably 5000 to 20000, more preferably 6000 to 15000. The peak molecular weight of the benzyl acrylate copolymer can be determined by GPC (gel permeation chromatography). The acid value of the benzyl acrylate copolymer is not particularly limited, but from the viewpoint of further improving filter filtration performance, it is preferably 100 to 200 mg KOH / g. The acid value (acid value converted from solids content) can be determined, for example, using a method based on DIN EN ISO 2114.

[0046] The amine value of benzyl acrylate copolymers is not particularly limited, but it is preferably below 10 mg KOH / g, and particularly preferably 0 mg KOH / g. The amine value (amine value converted from solid content) can be determined, for example, using a method based on DIN 16945.

[0047] There are no particular limitations on the method for synthesizing block copolymers composed of block A and block B. For example, after obtaining a polymer by living polymerization of the monomer of block A, the polymer and the monomer of block B can be obtained by living polymerization. The polymerization is not limited to living polymerization; it can also be free radical polymerization.

[0048] When the benzyl acrylate copolymer contains block A, it is preferable to mix the polymer, an alkali such as sodium hydroxide, and pure water after polymer separation to prepare a polymer solution with a pH of 7.5 to 10.0. This polymer solution can be used as a dispersant.

[0049] Styrene-maleic anhydride-based polymeric dispersants only need to contain a styrene-maleic anhydride copolymer as an active ingredient and have an acid value of less than 15 mg KOH / g, preferably 5–12 mg KOH / g. The styrene-maleic anhydride copolymer preferably has an amine value. The amine value is not particularly limited, but is preferably 10–50 mg KOH / g.

[0050] The content of the dispersant is not particularly limited, but is preferably 20 to 80 parts by weight of solids relative to 100 parts by weight of the colored organic pigment, more preferably 25 to 70 parts by weight, and even more preferably 30 to 70 parts by weight.

[0051] The organic solvent need to be at least one selected from monoaliphatic alcohols and polyaliphatic alcohols.

[0052] A monoaliphatic alcohol is an organic compound whose structure is formed by replacing one hydrogen atom of an aliphatic hydrocarbon with a hydroxyl group. Aliphatic alcohols can be chain-like or cyclic. In the chain form, the structure can be straight-chain or branched. Furthermore, it can be saturated or unsaturated, but saturated is preferred. The position of the hydroxyl group is not particularly limited and can be any alcohol selected from primary, secondary, and tertiary alcohols. The number of carbon atoms is not particularly limited, but is preferably 4 to 10.

[0053] Examples of monoaliphatic alcohols include: 1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, cyclobutanol, etc.; 1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol, 2-methyl-2-butanol, 3-methyl-2-butanol, 2,2-dimethyl-1-propanol, cyclopentanol, etc.; 1-hexanol, 2-hexanol, 3-hexanol, 2-methyl-1-pentanol, 3-methyl-1-pentanol, 4-methyl-1-pentanol, 2-methyl-2-pentanol, 3-methyl-2-pentanol, 4-methyl-2-pentanol, 2-methyl-3-pentanol, 3-methyl-3-pentanol, 2,2-dimethyl-1-butanol, 2,3 Hexanols such as dimethyl-1-butanol, 3,3-dimethyl-1-butanol, 2,3-dimethyl-2-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol, and cyclohexanol; heptanols such as 1-heptanol, 2-heptanol, 3-heptanol, 4-heptanol, 2-methyl-1-hexanol, and cycloheptanol; octanols such as 1-octanol, 2-octanol, 3-octanol, 2-methyl-1-heptanol, 2-ethyl-1-hexanol, and cyclooctanol; nonanols such as 1-nonanol, 2-nonanol, 3-nonanol, 2-methyl-1-octanol, 3-methyl-3-octanol, and cyclononanol; and decanols such as 1-decanol, 2-decanol, 3-decanol, 2-methyl-1-nonanol, 3-methyl-3-nonanol, and cyclodecanol.

[0054] As a monoallionic alcohol, 1-hexanol and cyclohexanol are preferred.

[0055] Polyaliphatic alcohols are organic compounds whose structure is formed by replacing two or more hydrogen atoms of an aliphatic hydrocarbon with hydroxyl groups. The structure of aliphatic alcohols can be chain-like or cyclic, but chain-like structures are preferred. In the case of a chain, the structure can be straight-chain or branched. Furthermore, it can be saturated or unsaturated, but saturated is preferred. The position of the hydroxyl groups is not particularly limited, but it is preferable that the hydroxyl groups are close together; in the case of a chain structure, it is more preferable that they are close together near the end. The number of carbon atoms is not particularly limited, but is preferably 4 to 10. The valence is not particularly limited, but is preferably divalent.

[0056] Examples of polyaliphatic alcohols include: 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2-methyl-1,3-propanediol, 1,2-cyclobutanediol, etc.; 1,2-pentanediol, 1,5-pentanediol, 2-methyl-1,3-butanediol, 1,2-cyclopentanediol, 1,3-cyclopentanediol, etc.; 1,2-hexanediol, 1,5-hexanediol, 3-methyl-1,5-pentanediol, 2-methyl-2,4-pentanediol, 1,2-cyclohexanediol, 1,3-cyclohexanediol, etc.; 1,2-heptanediol, 1,7-heptanediol, etc. 1,2-Cycloheptanediol and other heptanediols; 1,2-octanediol, 1,8-octanediol, 2-ethyl-1,3-hexanediol, 2,2,4-trimethyl-1,3-pentanediol, 2,2-dimethyl-1,3-propanediol, 1,2-cyclooctanediol, 1,5-cyclooctanediol and other octanediols; 1,2-nonanediol, 1,9-nonanediol, 2-butyl-2-ethyl-1,3-propanediol and other nonanediols; 1,2-decanediol, 1,10-decanediol, (1R,2R)-cyclodecane-1,2-diol and other decanediols; erythritol, threitol and other butylols; xylitol and other pentitols; mannitol and other hexitols, etc.

[0057] As polyaliphatic alcohols, 1,2-hexanediol and 1,2-octanediol are preferred.

[0058] Regarding the content of organic solvent, its total amount in the pigment dispersion is 3 to 8% by weight, preferably 2 to 5% by weight. By keeping the content in the pigment dispersion within this range, the effects brought about by the organic solvent in the manufacturing method described later can be obtained, and there is a tendency to easily obtain the improved filterability of water-based inkjet inks made from pigment dispersions.

[0059] There are no particular limitations on the water, but water after removing impurities is preferred. Examples include ion-exchanged water, distilled water, and RO water (purified water obtained using a reverse osmosis membrane). The water content can be appropriately determined, for example, it can be 50.0% to 80.0% by weight in the overall pigment dispersion. In addition, if water is contained in the dispersant or other components described later, it should also be included.

[0060] From the viewpoint of further improving the filterability of water-based inkjet inks, the pigment dispersion may contain a surface tension modifier. Examples of surface tension modifiers include nonionic surface tension modifiers such as acetylenic diols and alcohol alkoxylates. Examples of acetylenic diols include unmodified alkylene oxide acetylenic diols and alkylene oxide modified acetylenic diols.

[0061] Specific examples of surface tension modifiers include: SURFYNOL 82, 465, 485, 2502, OLFINE E1010, E1020, PD-002W, PD-004, EXP4001, EXP4002, EXP4123, EXP4300, etc. manufactured by Nissin Chemical Industry Co., Ltd.; ACETYLENOL E00, E103T, E40, E60, E100, E200, etc. manufactured by Kawaken Fine Chemicals Co., Ltd.; and BYK-DYNWET 800 manufactured by BYK Chemie Japan Co., Ltd.

[0062] The content of the surface tension modifier can be appropriately determined, for example, it can be 0.1 to 2.0 by weight in the overall pigment dispersion.

[0063] In addition to the components mentioned above, other components may be added to the pigment dispersion as needed. Examples of such other components include pH adjusters, pigment derivatives, antioxidants, anti-agglomeration agents, and defoamers. For example, aqueous solutions containing alkalis such as sodium hydroxide can be used as pH adjusters.

[0064] Pigment dispersions composed of the above-described components have a typical viscosity suitable for water-based inkjet inks. This viscosity can be measured, for example, using the method described in the Examples section below. Furthermore, in the filterability test described later, the pigment dispersion exhibits a high filter throughput of particles, thus imparting good filterability to the inkjet ink. This excellent filterability is achieved in the filter filtration process, typically performed at the final stage of inkjet ink manufacturing. Therefore, it reduces filtration pressure, filter usage and exchange frequency, ink loss, etc., making filtration less labor-intensive and providing inkjet ink in an environmentally friendly and inexpensive manner.

[0065] (Method for manufacturing pigment dispersions for water-based inkjet printing)

[0066] The pigment dispersions described above can be manufactured, for example, as described below.

[0067] First, a mixture containing the aforementioned colored organic pigment, dispersant, organic solvent, and water (and other components as needed) is prepared, with the organic solvent content in the mixture adjusted to 3-8% by weight. It is believed that by maintaining the organic solvent content within this specified range during the dispersion process described later, the organic solvent facilitates the dissolution of the pigment in water and promotes the adsorption of both the pigment and the dispersant. Regardless of the average particle size after dispersion, the pigment's permeability to the filter becomes good, resulting in improved filterability of the final water-based inkjet ink. It is speculated that when the organic solvent content exceeds 8% by weight, the adsorption of the pigment by the dispersant begins to desorb. As a result, although the mechanism is not yet clear, it is considered that the filterability of the water-based inkjet ink made from the obtained pigment dispersion decreases because the dispersion process hinders the breakdown of pigment aggregation, or because aggregation occurs after dispersion.

[0068] The organic solvent that can be used only needs to contain one or more of monoaliphatic alcohols and polyaliphatic alcohols as described above. From the viewpoint of the filterability of inkjet ink obtained by using pigment dispersion, it is preferable to select one or more of polyaliphatic alcohols.

[0069] Next, a dispersion process (dispersion process) is performed on the mixture 1, to which the organic solvent concentration has been adjusted. The dispersion process can be carried out using a general dispersion apparatus and conventional methods. Examples of dispersion apparatus include bead mills, sand mills, grinding mills, dispersers, paint conditioners, and kneaders. When using a dispersion medium, its type is not particularly limited; glass beads, zirconia beads, alumina beads, stainless steel beads, etc., can be used. The size of the medium is not particularly limited and can be appropriately selected according to various conditions; for example, a medium with a diameter of φ1.00 mm or less can be used. Furthermore, dispersion can be performed by progressively reducing the size of the medium as needed. Additionally, when performing dispersion as described above, a pre-dispersion process can be performed after preparing the mixture, at a load lower than that used during dispersion. During the pre-dispersion process, the concentration of the organic solvent in the mixture can be adjusted to the aforementioned range, or it can remain unchanged.

[0070] After the dispersion process, water can be added and stirred as needed to adjust the pigment concentration (adjustment process). Since the colored organic pigment has been uniformly dispersed through the dispersion process, the stirring at this stage can be a simple stirring operation or performed using the dispersion apparatus described above. Afterwards, if dispersion was performed using a medium, the medium can be removed to obtain the desired pigment dispersion. The medium can be removed before the adjustment process, but from the viewpoint of accuracy in concentration adjustment, it is preferable to remove it after the adjustment process.

[0071] (Water-based inkjet ink)

[0072] The aqueous inkjet ink according to embodiments of the present invention contains the above-described aqueous inkjet pigment dispersion. That is, it contains the aforementioned colored organic pigment, dispersant, organic solvent, and water constituting the pigment dispersion, and may contain other components added to the pigment dispersion as needed. Additionally, it may contain organic solvents commonly used in inkjet inks (hereinafter referred to as ink-grade organic solvents), and may contain surfactants and other additives. The concentration of the aforementioned colored organic pigment in the aqueous inkjet ink may, for example, be 1.0 to 10.0% by weight, preferably 3.0 to 7.0% by weight. The combination of the pigment dispersion and the ink-grade organic solvent, etc., is adjusted to ensure that the concentration of the colored organic pigment is within this range.

[0073] In addition to the mono- or poly-aliphatic alcohols specified above, general organic solvents used in ink applications may also be used as organic solvents for ink applications. Examples of such organic solvents for ink applications include water-soluble organic solvents, such as compounds described in Japanese Patent Documents 1-3. Specifically, examples include: monohydric alcohols; diols, diols other than diols, polyols such as glycerol; diol ethers such as alkylene glycol monoalkyl ethers; ketones such as acetone-acetone; esters such as γ-butyrolactone, glyceryl diacetate, and triethyl phosphate; lower alkoxy alcohols such as 2-methoxyethanol and 2-ethoxyethanol; amines such as ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, triethylenetetramine, tetraethylenepentamine, and pentamethyldiethylenetriamine; and formamides. Amides such as N,N-dimethylformamide, N-methylformamide, and N,N-dimethylacetamide; heterocyclic compounds such as 2-pyrrolidone, N-ethylpyrrolidone, N-methyl-2-pyrrolidone, cyclohexylpyrrolidone, morpholine, N-ethylmorpholine, 2-oxazolidinone, 1,3-dimethyl-2-imidazolinone, imidazole, methylimidazoline, hydroxyimidazoline, dimethylaminopyridine, 1,3-propanesulfonyllactone, hydroxyethylpiperazine, and piperazine; sulfoxides such as dimethyl sulfoxide; and sulfones such as sulfolane. These can be used individually or in combination of two or more.

[0074] The content of organic solvent in water-based inkjet ink can be appropriately determined taking into account factors such as pigment concentration. For example, the total amount of the specified organic solvent contained in the pigment dispersion mentioned above can be 10.0% to 30.0% by weight in water-based inkjet ink.

[0075] There are no particular limitations on the surfactant used; examples include anionic surfactants, cationic surfactants, amphoteric surfactants, nonionic surfactants, silicone surfactants, and fluorinated surfactants. Specific examples of these surfactants include, for instance, the compounds described in Japanese Patent Document 2. The surfactant content in the total water-based inkjet ink can be, for example, 0.01 to 5.0% by weight.

[0076] Other additives include antioxidants, anti-agglomeration agents, surface conditioners (leveling agents), preservatives, pH adjusters, rust inhibitors, and defoamers.

[0077] Water-based inkjet ink can be obtained by mixing the above-mentioned pigment dispersion with ink using an organic solvent, a surfactant as needed, and other additives, followed by uniform stirring and filtration through a conventional filter to remove particles of a certain size that may be present in the ink. The mesh size (pore size) and material of the filter can be appropriately determined according to the intended use.

[0078] The above-described water-based inkjet inks, by containing the aforementioned pigment dispersions, possess excellent filterability even after being formulated into inks. Therefore, as described above, the filtration process is labor-saving, and inkjet inks can be provided in an environmentally friendly and inexpensive manner.

[0079] Example

[0080] The embodiments of the present invention will be described in detail below based on examples.

[0081] (Example 1)

[0082] 20 parts by weight of a red pigment (manufactured by DCL Corporation, 1149 Perylene Red, CI Pigment Red 149), 33.33 parts by weight of dispersant A (described later) as a dispersant, 4.00 parts by weight of 1,2-hexanediol (1,2-HD) as an organic solvent, 0.45 parts by weight of a 30% by weight NaOH aqueous solution, and 17.69 parts by weight of pure water were mixed to obtain a mixture 1 containing 75.47 parts by weight. The concentration of the organic solvent in this mixture 1 was 5.3% by weight. 347 parts by weight of zirconia beads (bead diameter φ0.65 mm) were added to this mixture 1, and the mixture was dispersed using a sand mill at 2000 rpm for 120 minutes to obtain a dispersion (dispersion process).

[0083] Add 24.53 parts by weight of pure water to the obtained dispersion to obtain 100.00 parts by weight (excluding zirconia beads) of mixture 2. After stirring mixture 2, remove the zirconia beads to obtain a pigment dispersion in which the red pigment is uniformly dispersed (adjustment process). The concentration of organic solvent in the final pigment dispersion is 4.0% by weight.

[0084] (Examples 2-17, Comparative Examples 1-14)

[0085] The dispersion process was performed in the same manner as in Example 1, with the dispersion formulas set as shown in Tables 1 and 2. Next, the final pigment dispersion was obtained by setting the organic solvent concentrations as shown in Tables 1 and 2, except that the process was the same as in Example 1.

[0086] (Dispersant A)

[0087] Dispersant A contains a benzyl acrylate copolymer as its active ingredient. This benzyl acrylate copolymer, based on monomer composition ratios, is a diblock copolymer composed of blocks of benzyl acrylate / acrylic acid / methacrylic acid in a weight ratio of 34.6 / 15.3 / 0.1 and blocks of benzyl acrylate in a weight ratio of 50. The copolymer has an acid value of 128 mg KOH / g, an amine value of 0 mg KOH / g, and a peak molecular weight of 8000.

[0088] Dispersant A is an aqueous polymer solution composed of the copolymer, sodium hydroxide, and deionized water, adjusted to a solid content of 27.0% by weight and a pH of 8.2. Dispersant A does not contain organic solvents.

[0089] (Dispersant B)

[0090] Dispersant B contains a benzyl methacrylate-based copolymer polymerized using benzyl methacrylate instead of benzyl acrylate as its constituent monomer. This benzyl methacrylate-based copolymer, based on monomer composition ratios, is a diblock copolymer composed of blocks with a methyl methacrylate / acrylic acid ratio of 61.7 / 11.1 (by weight) and a benzyl methacrylate block ratio of 27.2 (by weight). The copolymer has an acid value of 93 mg KOH / g, an amine value of 0 mg KOH / g, and a peak molecular weight of 8920. Dispersant B is an aqueous polymer solution composed of this copolymer, sodium hydroxide, and deionized water, adjusted to a solids content of 21.4% by weight and a pH of 8.2. Dispersant B does not contain organic solvents.

[0091] (evaluate)

[0092] <Determination of average particle size>

[0093] The average particle size of the pigment dispersion was determined using a ZETA potential / particle size / molecular weight measurement system (manufactured by Otsuka Electronics Co., Ltd., ELSZ-2000ZS).

[0094] <Viscosity Measurement>

[0095] The viscosity of the pigment dispersion was measured at 25°C using an E-type viscometer (Toki Sangyo Co., Ltd., TV-22).

[0096] (evaluate)

[0097] <Filterability Test>

[0098] <<Preparation of Test Sample A>>

[0099] 400 parts by weight of pure water were added to each pigment dispersion obtained in the examples and comparative examples to make the pigment concentration equivalent to the ink composition 4%, and the mixture was stirred to prepare test sample A in which the colored organic pigment was uniformly dispersed.

[0100] <<Preparation of Test Sample B>>

[0101] 396 parts by weight of pure water and 4 parts by weight of 1,2-hexanediol were added to the pigment dispersion of Comparative Example 2 to make the pigment concentration equivalent to the ink composition 4%. The mixture was stirred to prepare test sample B, in which the red pigment was uniformly dispersed. Test sample B was compared with test sample A using the pigment dispersion of Example 5 to confirm whether the organic solvent had any effect when it was added later during the preparation of the ink composition.

[0102] <<Experiment>>

[0103] For the obtained test samples A and B, the filtration test was carried out according to the following method.

[0104] A 500mL sample bottle, with its pre-measured weight, was placed in a vacuum filtration system (Advantec, VT-500). The system included a membrane filter (PALL Corporation, NNG29325, 1.2μm mesh) and a filter holder (Advantec, KGS-47). 500g of the test sample was added under a suction pressure of 0.08MPa and filtered. After 3 minutes, suction was stopped, the pressure was reduced to normal, and the 500mL sample bottle was removed and its weight measured. The weight difference before and after filtration was calculated to determine the amount of filtrate passing through the filter every 3 minutes (filtration volume). The evaluation criteria for filtration volume were: 500g / 3 minutes or more was ◎ / Excellent, 400g / 3 minutes or more was ○ / Acceptable, and less than 400g / 3 minutes was × / Unacceptable. The evaluation results using test sample A are shown in Tables 1 and 2. Additionally, when using test sample B, the result was 130g / 3 minutes, and the filterability evaluation was × / unacceptable. Furthermore, for tests exceeding 500g / 3 minutes, the filtration time (seconds) was recorded and is shown in Tables 1 and 2.

[0105] [Table 1]

[0106]

[0107] [Table 2]

[0108]

[0109] As shown in Tables 1 and 2, it can be seen that when using colored organic pigments as pigments, by using a specified dispersant and organic solvent, and dispersing the pigment dispersion within a specified content range, the resulting pigment dispersion exhibits significantly higher filter throughput and superior filterability compared to the pigment dispersion of the comparative example. Furthermore, it can be seen that compared to the results using test sample B of Comparative Example 2, when no organic solvent was used in the preparation of the pigment dispersion, but an organic solvent was added for the first time in the preparation of the ink composition, the filterability was significantly worse than in Example 5, and the effect of the organic solvent could not be obtained.

Claims

1. An aqueous pigment dispersion for inkjet, characterized by: containing a colored organic pigment, a dispersant, an organic solvent, and water, wherein, the dispersant is at least one selected from the group consisting of a benzyl acrylate-based high-molecular dispersant and a styrene-maleic anhydride-based high-molecular dispersant having an acid value of 15 mgKOH / g or less, the organic solvent is at least one selected from the group consisting of a monohydric aliphatic alcohol and a polyhydric aliphatic alcohol, the total amount of the organic solvent is 3 to 8% by weight in the entire aqueous pigment dispersion for inkjet.

2. The aqueous pigment dispersion for inkjet according to claim 1, characterized by: the aliphatic alcohol has a chain structure having 4 to 10 carbon atoms.

3. The aqueous pigment dispersion for inkjet according to claim 1 or 2, characterized by: containing a surface tension adjusting agent.

4. A method for producing an aqueous pigment dispersion for inkjet, for producing the aqueous pigment dispersion for inkjet according to claim 1 or 2, the method being characterized by comprising: a step of obtaining a mixed solution containing a colored organic pigment, a dispersant, an organic solvent, and water, adjusting the content of the organic solvent in the entire mixed solution to 3 to 8% by weight, and then subjecting the mixed solution to a dispersion treatment.

5. An aqueous inkjet ink, characterized by: containing the aqueous pigment dispersion for inkjet according to claim 1 or 2.

6. The aqueous inkjet ink according to claim 5, characterized by: a filtration amount of 400 g / 3 minutes or more when an aqueous inkjet ink having a pigment concentration of 4% by weight is filtered using a filter membrane having a mesh size of 1.2 pm.

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