Water-based inkjet pigment dispersion and method for producing the same.

Abstract

To provide a pigment dispersion capable of producing water-based inkjet inks with good filterability, and a method for producing the pigment dispersion. [Solution] In a water-based inkjet pigment dispersion containing a colored organic pigment, a dispersant, an organic solvent, and water, The dispersant is at least one selected from benzyl acrylate-based polymer dispersants and styrene-maleic anhydride-based polymer dispersants with an acid value of 15 mg KOH / g or less. The organic solvent is at least one selected from monohydric aliphatic alcohols and polyhydric aliphatic alcohols. A water-based inkjet pigment dispersion wherein the total amount of the organic solvent is 3 to 8% by weight of the entire water-based inkjet pigment dispersion.

Description

Technical Field

[0005]

[0001] The present invention relates to a pigment dispersion for aqueous inkjet and a method for producing the same, and particularly to a pigment dispersion for aqueous inkjet using a colored organic pigment as a pigment and a method for producing the same.

Background Art

[0002] In recent years, inkjet printers have come to be used not only for printing at home but also for printing in various business operators such as the creation of business documents due to the improvement of image quality and printing speed. On the other hand, for example, in printing for business and industrial applications such as the latter, further improvement in image quality and high-speed printing are required, and various studies have been made to meet these requirements (for example, Patent Documents 1 to 3).

[0003] In Patent Document 1, in order to record an image excellent in character quality and also be able to record an image excellent in fastness such as marker resistance that is not easily soiled even when traced with a marker pen, in an aqueous ink for inkjet containing a pigment and a urethane resin, it is proposed to use a specific urethane resin and set the dynamic surface tension of the aqueous ink within a predetermined range.

[0004] In Patent Document 2, as the characteristics required for the coloring composition (ink) become more advanced, the dispersibility of the coloring material is not sufficient with conventional dispersants, and when the amount of the dispersant used is increased, problems such as changes in hue due to heating during or after ink ejection occur. Therefore, in order to provide a dispersant having excellent dispersibility even in a small amount, a polymerization product in which a specific block copolymer is contained at a predetermined range of content, and a coloring composition containing this have been proposed.

[0005] In Patent Document 3, in order to provide an aqueous ink for inkjet recording that improves the rub resistance during recording on coated paper, the resin particles contained in the aqueous ink for inkjet recording are composed of a predetermined polymer, and it is proposed to set the storage elastic modulus at 25°C of the dried ink film within a predetermined range. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2018-150514 [Patent Document 2] Japanese Patent Publication No. 2021-98835 [Patent Document 3] Japanese Patent Publication No. 2022-156109 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] According to the aforementioned prior art, it is considered possible to meet the market demands for higher image quality and faster printing in inkjet printing. However, the inventors have found that some of these prior arts have poor filter performance, such as requiring increased filtration pressure in the filter filtration process, which is normally performed in the final stage of inkjet ink manufacturing, or causing filters to clog easily and requiring frequent filter replacement. This can lead to increased power consumption, increased filter usage, increased filter replacement labor, and increased ink loss.

[0008] Therefore, the object of the present invention is to provide a pigment dispersion capable of producing an aqueous inkjet ink with good filterability, and a method for producing the pigment dispersion. [Means for solving the problem]

[0009] This invention was developed through diligent research to solve the aforementioned problems. As a result, it was found that by using specific dispersants and organic solvents, and by keeping the total amount of organic solvent within a predetermined range, a pigment dispersion for aqueous inkjet inks with good filterability can be obtained in a pigment dispersion for aqueous inkjet inks using colored organic pigments as pigments. The gist of this invention is as follows.

[0010] A first aspect of the present invention relates to an aqueous inkjet pigment dispersion comprising a colored organic pigment, a dispersant, an organic solvent, and water, wherein the dispersant is at least one selected from benzyl acrylate polymer dispersants and styrene-maleic anhydride polymer dispersants having an acid value of 15 mg KOH / g or less, the organic solvent is at least one selected from monohydric aliphatic alcohols and polyhydric aliphatic alcohols, and the total amount of the organic solvent is 3 to 8% by weight of the entire aqueous inkjet pigment dispersion.

[0011] In a first aspect of the present invention, the aliphatic alcohol may have a chain-like structure with 4 to 10 carbon atoms.

[0012] In the first aspect of the present invention, a surface tension modifier may be included.

[0013] A second aspect of the present invention relates to a method for producing a water-based inkjet pigment dispersion, comprising the steps of first obtaining a mixture containing a colored organic pigment, a dispersant, an organic solvent, and water, wherein the content of the organic solvent in the entire mixture is adjusted to 3 to 8% by weight, and then dispersing the mixture.

[0014] A third aspect of the present invention relates to an aqueous inkjet ink containing the aqueous inkjet pigment dispersion.

[0015] In a third aspect of the present invention, when an aqueous inkjet ink with a pigment concentration of 4% by weight is filtered through a filter membrane with a mesh size of 1.2 μm, the amount filtered may be 400 g / 3 min or more. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a pigment dispersion capable of producing an aqueous inkjet ink with good filterability, and a method for producing the pigment dispersion. [Modes for carrying out the invention]

[0017] (Pigment dispersion for water-based inkjet printers) A pigment dispersion for aqueous inkjet systems according to an embodiment of the present invention (hereinafter sometimes simply referred to as "pigment dispersion") comprises 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 polymer dispersants and styrene-maleic anhydride-based polymer dispersants with an acid value of 15 mg KOH / g or less. The organic solvent is at least one selected from monohydric aliphatic alcohols and polyhydric aliphatic alcohols. The total amount of the organic solvent is 3 to 8% by weight of the entire pigment dispersion for aqueous inkjet systems.

[0018] Thus, a pigment dispersion obtained by dispersing a colored organic pigment with a specific combination of a dispersant and organic solvent, and keeping the total amount of the organic solvent within a predetermined range, can achieve good filterability when used as an ink for 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 by the method described in the Examples section below.

[0019] The components of the pigment dispersion are described below.

[0020] Colored organic pigments are not particularly limited as long as they are organic pigments other than black and white. Examples of pigment types include anthraquinone pigments, aminoanthraquinone pigments, quinacridone pigments, quinacridone quinone pigments, diketopyrrolopyrrole pigments, perylene pigments, perinone pigments, anthanthrone pigments, benzimidazolon pigments, disazo condensate pigments, azo pigments, thioindigo pigments, pyrantrone pigments, dioxazine pigments, quinophthalone pigments, isoindoline pigments, phthalocyanine pigments, and the like.

[0021] Furthermore, applicable colored organic pigments, indicated by their color index (CI) numbers, include the following:

[0022] Examples of red pigments include C.I. 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,​​​​​​​​​​​​​​​​​​​Examples of purple pigments include CI Pigment Violet (PV) 1, 2, 3, 3:1, 3:3, 5:1, 13, 17, 19, 23, 25, 27, 29, 31, 32, 36, 37, 38, 42, and 50.

[0028] The colored organic pigment can be one or more of the aforementioned pigments in combination. From the viewpoint of ensuring good dispersibility, the pigment content is preferably 15.0 to 30.0% by weight in the pigment dispersion, and more preferably 15.0 to 28.0% by weight.

[0029] The average particle size of colored organic pigments should be of a size typical for applications such as inkjet inks for recording, and is usually between 50 and 200 nm. The average particle size can be measured by common methods such as dynamic light scattering and laser diffraction.

[0030] As mentioned above, the dispersant may be at least one selected from benzyl acrylate-based polymer dispersants and styrene-maleic anhydride-based polymer dispersants with an acid value of 15 mg KOH / g or less.

[0031] A benzyl acrylate-based polymer dispersant is acceptable as long as it contains a benzyl acrylate-based copolymer as an active ingredient. The benzyl acrylate-based copolymer is acceptable as a copolymer composed of structural units derived from benzyl acrylate and structural units derived from monomers polymerizable with benzyl acrylate. The copolymer may be a random copolymer or a block copolymer, but a block copolymer is preferred from the viewpoint of further improving filterability. Furthermore, it is preferable that the block copolymer contains hydrophilic blocks (hereinafter sometimes referred to as "block A") and hydrophobic blocks (hereinafter sometimes referred to as "block B"). Here, a hydrophilic block means a block that has a relatively greater affinity for water than a hydrophobic block. For example, the copolymer is configured such that the hydrophilic block contains hydrophilic monomers as described later, and the hydrophobic block either does not contain hydrophilic monomers or contains hydrophilic monomers in a proportion less than that of the hydrophilic block. It is preferable that the hydrophobic block does not contain hydrophilic monomers.

[0032] Examples of monomers that can polymerize with benzyl acrylate include hydrophilic monomers and monomers other than hydrophilic monomers (which may be referred to as "hydrophobic monomers" below), as mentioned above. Examples of hydrophilic monomers include unsaturated polycarboxylic acids such as (meth)acrylic acid and maleic acid, monomers containing carboxyl groups or acid anhydride groups such as maleic anhydride; monomers containing sulfonic acid groups such as styrene sulfonic acid and 4-(methacryloyloxy)butyl sulfonic acid; and ethylene oxide-modified (meth)acrylic acid ester monomers such as ethylene oxide-modified alkyl (meth)acrylic acid esters. Of these, unsaturated polycarboxylic acids such as (meth)acrylic acid and maleic anhydride are preferred, and (meth)acrylic acid is more preferred. Hydrophilic monomers may be used alone or in combination of two or more. When two or more are combined, the structures of block A and B may each be random polymers or block polymers. (Meth)acrylic acid means methacrylic acid and / or acrylic acid.

[0033] Examples of hydrophobic monomers include styrene monomers such as styrene, α-methylstyrene, and vinyltoluene; α-olefin monomers such as ethylene, propylene, and 1-butene; and vinyl monomers having phenyl groups, biphenyl groups, naphthyl groups, etc. Hydrophobic monomers may be used individually or in combination of two or more. When two or more are combined, the structures of blocks A and B may be random polymers or block polymers, respectively. Benzyl acrylate is an example of a hydrophobic monomer.

[0034] The structure of the benzyl acrylate copolymer is preferably composed of a block A composed of structural units derived from benzyl acrylate and hydrophilic monomers, and a block B composed of structural units derived from hydrophobic monomers; more preferably composed of a block A composed of structural units derived from benzyl acrylate and hydrophilic monomers, and a block B composed of structural units derived from one or more hydrophobic monomers including benzyl acrylate; even more preferably composed of a block A composed of structural units derived from benzyl acrylate and (meth)acrylic acid, and a block B composed of structural units derived from benzyl acrylate; and particularly preferably composed of a block A composed of structural units derived from benzyl acrylate, methacrylic acid, and acrylic acid, and a block B composed of structural units derived from benzyl acrylate.

[0035] The weight ratio (A / B) of Block A and Block B can be appropriately determined depending on the type and content of the components, but 30 / 70 to 70 / 30 is preferred. Furthermore, if Block A is composed of constituent units derived from benzyl acrylate (X) and (meth)acrylic acid (Y), the weight ratio (X / Y) in Block A is preferably 1 / 1 to 3 / 1. In addition, if (meth)acrylic acid is acrylic acid (Y1) and methacrylic acid (Y2), the weight ratio (Y1 / Y2) in Block A is preferably 200 / 1 to 100 / 1.

[0036] The peak top molecular weight of the benzyl acrylate copolymer is not particularly limited, but from the viewpoint of further improving filterability, 5000 to 20000 is preferred, and 6000 to 15000 is more preferred. The peak top molecular weight of the benzyl acrylate copolymer can be measured by GPC (gel permeation chromatography). The acid value of the benzyl acrylate copolymer is not particularly limited, but from the viewpoint of further improving filterability, 100 to 200 mg KOH / g is preferred. The acid value (acid value when converted to solid content) can be determined, for example, by a method in accordance with DIN EN ISO 2114. The amine value of the benzyl acrylate copolymer is not particularly limited, but an amine value of 10 mg KOH / g or less is preferred, and 0 mg KOH / g is particularly preferred. The amine value (amine value when calculated on a solid content basis) can be determined, for example, by a method in accordance with DIN 16945.

[0037] The method for synthesizing a block copolymer composed of block A and block B is not particularly limited, but for example, it can be obtained by first obtaining a polymer by living polymerization of the monomer of block A, and then by living polymerization of this polymer with the monomer of block B. Polymerization is not limited to living polymerization; radical polymerization may also be used.

[0038] If the benzyl acrylate copolymer contains block A, it is preferable to isolate the polymer and then mix the polymer with a base such as sodium hydroxide and pure water to obtain a polymer solution with a pH of 7.5 to 10.0. This polymer solution can be used as a dispersant.

[0039] The styrene-maleic anhydride polymer dispersant contains a styrene-maleic anhydride copolymer as an active ingredient and has an acid value of 15 mg KOH / g or less, but 5 to 12 mg KOH / g is preferred. The styrene-maleic anhydride copolymer is preferably one that has an amine value. The amine value is not particularly limited, but 10 to 50 mg KOH / g is preferred.

[0040] The amount of dispersant is not particularly limited, but is preferably 20 to 80 parts by weight, more preferably 25 to 70 parts by weight, and even more preferably 30 to 70 parts by weight, based on solid content, per 100 parts by weight of colored organic pigment.

[0041] The organic solvent may be at least one selected from monohydric aliphatic alcohols and polyhydric aliphatic alcohols.

[0042] Monohydric aliphatic alcohols are organic compounds in which one hydrogen atom of an aliphatic hydrocarbon is replaced by a hydroxyl group. The structure of an aliphatic alcohol may be linear or cyclic. If linear, the structure may be linear or branched. It may also be saturated or unsaturated, but saturated is preferred. There are no particular limitations on the position of the hydroxyl group, and it may be any primary to tertiary alcohol. There are no particular limitations on the number of carbon atoms, but 4 to 10 is preferred.

[0043] Examples of monohydric aliphatic alcohols include butanols such as 1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, and cyclobutanol; Pentanols such as 1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol, 2-methyl-2-butanol, 3-methyl-2-butanol, 2,2-dimethyl-1-propanol, and cyclopentanol; Hexanols such as 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-dimethyl-1-butanol, 3,3-dimethyl-1-butanol, 2,3-dimethyl-2-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol, cyclohexanol, etc. 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; Examples include decanols such as 1-decanol, 2-decanol, 3-decanol, 2-methyl-1-nonanol, 3-methyl-3-nonanol, and cyclodecanol.

[0044] As monohydric aliphatic alcohols, 1-hexanol and cyclohexanol are preferred.

[0045] Polyhydric aliphatic alcohols are organic compounds having a structure in which two or more hydrogen atoms of an aliphatic hydrocarbon are replaced by hydroxyl groups. The structure of an aliphatic alcohol may be linear or cyclic, but a linear structure is preferred. If it is linear, the structure may be linear or branched. It may also be saturated or unsaturated, but saturation is preferred. There are no particular limitations on the position of the hydroxyl groups, but it is preferable that the hydroxyl groups are close to each other, and in the case of a linear structure, it is more preferable that they are close to each other near the ends. There are no particular limitations on the number of carbon atoms, but 4 to 10 is preferred. There are no particular limitations on the valency, but divalent is preferred.

[0046] Examples of polyhydric aliphatic alcohols include butanediols such as 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2-methyl-1,3-propanediol, and 1,2-cyclobutanediol; Pentanediols such as 1,2-pentanediol, 1,5-pentanediol, 2-methyl-1,3-butanediol, 1,2-cyclopentanediol, and 1,3-cyclopentanediol; Hexanediols such as 1,2-hexanediol, 1,5-hexanediol, 3-methyl-1,5-pentanediol, 2-methyl-2,4-pentanediol, 1,2-cyclohexanediol, and 1,3-cyclohexanediol; Heptanediols such as 1,2-heptanediol, 1,7-heptanediol, and 1,2-cycloheptanediol; Octanediols such as 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, and 1,5-cyclooctanediol; Nonanediols such as 1,2-nonanediol, 1,9-nonanediol, and 2-butyl-2-ethyl-1,3-propanediol; Decanediols such as 1,2-decanediol, 1,10-decanediol, and (1R,2R)-cyclodecane-1,2-diol; Tetritoles such as erythritol and trethitol; Pentitol such as xylitol; Examples include hexitol such as mannitol; and others.

[0047] Preferred polyhydric aliphatic alcohols include 1,2-hexanediol and 1,2-octanediol.

[0048] The total content of the organic solvent is 3 to 8% by weight of the entire pigment dispersion. Preferably, it is 2 to 5% by weight. By setting the content in the pigment dispersion within this range, the effects of the organic solvent in the manufacturing method described later can be enjoyed, and an improvement in the filterability of the water-based inkjet ink produced from the pigment dispersion tends to be easily obtained.

[0049] While there are no particular limitations on the water used, water from which impurities have been removed is preferred. Examples include pure water such as ion-exchanged water, distilled water, and RO water (reverse osmosis purified water). The water content can be determined as appropriate, for example, it can be 50.0 to 80.0% by weight of the entire pigment dispersion. Note that if the dispersant or other components described later contain water, these are also included.

[0050] The pigment dispersion may contain a surface tension modifier to further improve the filterability of the water-based inkjet ink. Examples of surface tension modifiers include nonionic surface tension modifiers such as acetylene glycols and alcohol alkoxylates. Examples of acetylene glycols include unmodified alkylene oxide acetylene glycols and alkylene oxide-modified acetylene glycols.

[0051] Specific examples of surface tension modifiers include Surfinol 82, 465, 485, 2502, Orfin E1010, E1020, PD-002W, PD-004, EXP4001, EXP4002, EXP4123, EXP4300, etc. from Nisshin Chemical Industry Co., Ltd., Acetyleneol E00, E103T, E40, E60, E100, E200, etc. from Kawaken Fine Chemical Co., Ltd., and BYK-DYNWET 800, etc. from BYK Chemie Japan Co., Ltd.

[0052] The amount of surface tension modifier can be determined as appropriate, for example, 0.1 to 2.0% by weight of the entire pigment dispersion.

[0053] 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-coagulation agents, and defoaming agents. Examples of pH adjusters include aqueous solutions containing a base such as sodium hydroxide.

[0054] A pigment dispersion with the above-described component composition has a normal viscosity applicable to water-based inkjet inks. This viscosity can be measured, for example, by the method described in the Examples section below. Furthermore, in a filterability test, for example, described below, the pigment dispersion allows a large amount of particles in the pigment dispersion to pass through the filter, making it possible to impart good filterability to the inkjet ink. Because good filterability can be achieved in the filter filtration process, which is generally performed in the final stage of the inkjet ink manufacturing process, the pressure during filtration, the amount of filter used and the number of replacements, and the ink loss can be reduced, making it possible to save labor during the filtration process and to provide inkjet ink in an environmentally friendly and inexpensive manner.

[0055] (Method for manufacturing a pigment dispersion for aqueous inkjet printers) The aforementioned pigment dispersion can be manufactured, for example, as follows: First, a mixture containing the aforementioned colored organic pigment, dispersant, organic solvent, and water (and other components as needed) is prepared, and the organic solvent content in the mixture is adjusted to 3-8% by weight. It is believed that having the organic solvent content within this predetermined range allows the pigment to blend more easily with water during the dispersion process described later, further aiding in the adsorption of the pigment and dispersant, resulting in good pigment permeability through the filter regardless of the average particle size after dispersion, and ultimately improving the filterability of the resulting water-based inkjet ink. It is speculated that if the organic solvent content exceeds 8% by weight, the adsorption of the dispersant to the pigment begins to detach. As a result, although the mechanism is not entirely clear, it is thought that the filterability of the water-based inkjet ink made from the resulting pigment dispersion decreases due to factors such as hindering the disintegration of pigment aggregates during dispersion or causing aggregates to form after dispersion.

[0056] As mentioned above, the usable organic solvent may include one or more selected from monohydric aliphatic alcohols and polyhydric aliphatic alcohols, but from the viewpoint of the filterability of the inkjet ink obtained using the pigment dispersion, it is preferable that it be one or more selected from polyhydric aliphatic alcohols.

[0057] Next, a dispersion process is performed on the mixture 1, in which the concentration of the organic solvent has been adjusted (dispersion process). The dispersion process can be carried out according to a standard method using a general dispersion apparatus. Examples of dispersion apparatuses include bead mills, sand mills, attritors, dispersers, paint conditioners, kneaders, etc. When using a dispersion medium (media), there are no particular limitations on its type, and glass beads, zirconia beads, alumina beads, stainless steel beads, etc., can be used. There are no particular limitations on the size of the media, and it can be appropriately selected according to various conditions, for example, a size of φ1.00 mm or less can be used. Also, if necessary, the dispersion process may be carried out by gradually reducing the size of the media. Furthermore, when performing the dispersion process as described above, after preparing the mixture, a preliminary dispersion process may be performed using a load smaller than that used for the final dispersion process. When performing a preliminary dispersion process, the concentration of the organic solvent in the mixture may or may not be adjusted to be within the predetermined range described above.

[0058] After the dispersion process, if necessary, water may be added and the mixture stirred to adjust the pigment concentration, etc. (adjustment process). Since the colored organic pigment is uniformly dispersed by the dispersion process, a simple stirring operation is sufficient, but this may also be done using the aforementioned dispersion apparatus. Subsequently, if a media was used for the dispersion process, the media can be removed to obtain the desired pigment dispersion. The media may be removed before the adjustment process, but it is preferable to remove it after the adjustment process from the viewpoint of the accuracy of concentration adjustment, etc.

[0059] (Water-based inkjet ink) The aqueous inkjet ink according to an embodiment of the present invention includes the aforementioned aqueous inkjet pigment dispersion. That is, it includes the aforementioned colored organic pigment, dispersant, organic solvent, and water that constitute the pigment dispersion, and may include other components added to the pigment dispersion as needed. It also further includes an organic solvent (hereinafter referred to as the ink organic solvent) commonly used in inkjet inks, and may include surfactants and other additives. The concentration of the aforementioned colored organic pigment in the aqueous inkjet ink can be, for example, 1.0 to 10.0% by weight, preferably 3.0 to 7.0% by weight, in the aqueous inkjet ink. The formulation of the pigment dispersion and the ink organic solvent, etc., is adjusted so that the colored organic pigment concentration falls within this range.

[0060] As organic solvents for ink, in addition to the aforementioned specified monohydric or polyhydric aliphatic alcohols, general organic solvents used for inks can be used. Examples of such organic solvents for inks include water-soluble organic solvents, such as those described in Patent Documents 1 to 3. Specifically, these include: monohydric alcohols; polyhydric alcohols such as glycols, diols other than glycols, and glycerin; glycol ethers such as alkylene glycol monoalkyl ethers; ketones such as acetonylacetone; esters such as γ-butyrolactone, diacetin, 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 amides such as formamide, N,N-dimethylformamide, N-methylformamide, and N,N-dimethylacetamide. Examples include heterocyclic compounds such as 2-pyrrolidone, N-ethylpyrrolidone, N-methyl-2-pyrrolidone, cyclohexylpyrrolidone, morpholine, N-ethylmorpholine, 2-oxazolidone, 1,3-dimethyl-2-imidazolidinone, imidazole, methylimidazole, hydroxyimidazole, dimethylaminopyridine, 1,3-propanesultone, hydroxyethylpiperazine, and piperazine; sulfoxides such as dimethyl sulfoxide; and sulfones such as sulfolane. These may be used individually or in combination of two or more.

[0061] The content of organic solvent in water-based inkjet inks can be appropriately determined considering factors such as the pigment concentration. For example, the total amount of the organic solvent contained in the aforementioned pigment dispersion can be 10.0 to 30.0% by weight in the water-based inkjet ink.

[0062] The surfactant is not particularly limited and examples include anionic surfactants, cationic surfactants, amphoteric surfactants, nonionic surfactants, silicone-based surfactants, and fluorine-based surfactants. Specific examples of these are described in, for example, Patent Document 2. The surfactant content can be, for example, 0.01 to 5.0% by weight of the total water-based inkjet ink.

[0063] Other additives include antioxidants, anti-flocculation agents, surface modifiers (leveling agents), preservatives, pH adjusters, rust inhibitors, and defoamers.

[0064] Water-based inkjet inks can typically be obtained by mixing the aforementioned pigment dispersion with an organic solvent for the ink, and optionally a surfactant and other additives, stirring until uniform, and then filtering the mixture to remove any particles larger than a certain size that may be present in the ink. The mesh size (pore diameter) and material of the filter can be appropriately determined depending on the application.

[0065] The water-based inkjet ink described above, by containing the aforementioned pigment dispersion, maintains good filterability even after being formed into ink. Therefore, as mentioned above, it becomes possible to reduce labor during the filtration process, making it possible to provide inkjet ink in an environmentally friendly and inexpensive manner. [Examples]

[0066] The embodiments of the present invention will be described in detail below based on the examples provided.

[0067] (Example 1) Mixture 1 was prepared by mixing 20 parts by weight of red pigment (DCL Corporation, 1149 Perylene Red, CIPigment Red 149) as a colored organic pigment, 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 30% NaOH aqueous solution, and 17.69 parts by weight of pure water to obtain a mixture 1 of 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 dispersed using a sand mill at 2000 rpm for 120 minutes to obtain a dispersion (dispersion step). 24.53 parts by weight of pure water was added to the obtained dispersion to obtain a mixture 2 containing 100.00 parts by weight (excluding zirconia beads). After stirring this mixture 2, the zirconia beads were removed to obtain a pigment dispersion in which the red pigment was uniformly dispersed (preparation step). The concentration of the organic solvent in the final pigment dispersion was 4.0% by weight.

[0068] (Examples 2-17, Comparative Examples 1-14) The dispersion process was carried out in the same manner as in Example 1, except that the dispersion formulation was as shown in Tables 1 and 2. Then, a pigment dispersion with the final composition was obtained in the same manner as in Example 1, except that the organic solvent concentration was adjusted to the levels shown in Tables 1 and 2.

[0069] (Dispersant A) Dispersant A contains a benzyl acrylate copolymer as an active ingredient. This benzyl acrylate copolymer is a diblock copolymer composed of a block with a monomer composition ratio of benzyl acrylate / acrylic acid / methacrylic acid = 34.6 / 15.3 / 0.1 (by weight) and a block with benzyl acrylate = 50 (by weight). This copolymer had an acid value of 128 mgKOH / g, an amine value of 0 mgKOH / g, and a peak-top molecular weight of 8000. Dispersant A is a polymer aqueous solution composed of the copolymer, sodium hydroxide, and deionized water, adjusted to have a solid content of 27.0% by weight and a pH of 8.2. Dispersant A does not contain any organic solvents.

[0070] (Dispersant B) Dispersant B contains a benzyl methacrylate copolymer polymerized using benzyl methacrylate instead of benzyl acrylate as its constituent monomer. This benzyl methacrylate copolymer is a diblock copolymer composed of a block with a monomer composition ratio of methyl methacrylate / acrylic acid = 61.7 / 11.1 (by weight) and a block with benzyl methacrylate = 27.2 (by weight). This copolymer had an acid value of 93 mgKOH / g, an amine value of 0 mgKOH / g, and a peak-top molecular weight of 8920. Dispersant B is a polymer aqueous solution composed of the copolymer, sodium hydroxide, and deionized water, adjusted to have a solid content of 21.4% by weight and a pH of 8.2. Dispersant B does not contain any organic solvents.

[0071] (evaluation) <Measurement of average particle size> The average particle size of the particles in the pigment dispersion was measured using a zeta potential, particle size, and molecular weight measurement system (ELSZ-2000ZS, manufactured by Otsuka Electronics Co., Ltd.).

[0072] <Viscosity Measurement> The viscosity of the pigment dispersion was measured at 25°C using an E-type viscometer (TV-22, manufactured by Toki Sangyo Co., Ltd.).

[0073] (evaluation) <Filtration Test> <<Preparation of Test Sample A>> To each pigment dispersion obtained in the examples and comparative examples, 400 parts by weight of pure water were added to achieve a pigment concentration of 4%, equivalent to that of an ink composition, and the mixture was stirred to prepare test sample A, in which the colored organic pigment was uniformly dispersed.

[0074] <<Preparation of Test Sample B>> To the pigment dispersion of Comparative Example 2, 396 parts by weight of pure water and 4 parts by weight of 1,2-hexanediol were added to achieve a pigment concentration of 4%, corresponding to the ink composition. The mixture was then stirred to prepare Test Sample B, in which the red pigment was uniformly dispersed. Test Sample B was compared with Test Sample A, which used the pigment dispersion of Example 5, to confirm whether or not there is an effect of the organic solvent when the organic solvent is added later during the preparation of the ink composition.

[0075] <<Exam>> The following filterability tests were performed on each of the obtained test samples A and B. A 500 mL sample bottle, whose weight had been measured beforehand, was placed in a vacuum filtration system (Advantec VT-500). A vacuum filtration filter holder (Advantec KGS-47) equipped with a membrane filter (Nippon Pall Co., Ltd., NNG29325, mesh size 1.2 μm) was installed on top. With a suction pressure of 0.08 MPa, 500 g of the test sample was added and filtered. After 3 minutes, the suction was stopped and the pressure returned to atmospheric pressure, and the 500 mL sample bottle was removed and its weight was measured. The weight of the filtrate that passed through the filter per 3 minutes (filtration rate) was determined by calculating the difference in weight before and after filtration. The evaluation criteria for the filtration rate were: 500 g / 3 min or more = ◎, 400 g / 3 min or more = ○, and less than 400 g / 3 min = ×. The evaluation results using test sample A are shown in Tables 1 and 2. The result using test sample B was 130 g / 3 min, and the filterability was evaluated as ×. Furthermore, for filtration times of 500g / 3 minutes or longer, the filtration time (in seconds) was recorded and is shown in Tables 1 and 2.

[0076] [Table 1]

[0077] [Table 2]

[0078] As shown in Tables 1 and 2, when a colored organic pigment is used as the pigment, by using a predetermined dispersant and organic solvent, and incorporating the organic solvent within a predetermined content range, and performing a dispersion treatment, the resulting pigment dispersion shows a significantly higher filter pass-through rate and superior filterability compared to the pigment dispersion of the comparative example. Furthermore, from the results of test sample B using Comparative Example 2, it can be seen that when the organic solvent is not used during the preparation of the pigment dispersion and is only added when preparing the ink composition, the filterability is significantly worse compared to Example 5, and the effect of the organic solvent is not obtained.

Claims

1. In a water-based inkjet pigment dispersion containing a colored organic pigment, a dispersant, an organic solvent, and water, The dispersant is at least one selected from benzyl acrylate-based polymer dispersants and styrene-maleic anhydride-based polymer dispersants with an acid value of 15 mg KOH / g or less. The organic solvent is at least one selected from monohydric aliphatic alcohols and polyhydric aliphatic alcohols. A water-based inkjet pigment dispersion wherein the total amount of the organic solvent is 3 to 8% by weight of the entire water-based inkjet pigment dispersion.

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

3. A water-based inkjet pigment dispersion according to claim 1 or 2, comprising a surface tension modifier.

4. In a method for producing an aqueous inkjet pigment dispersion according to claim 1 or 2, A manufacturing method comprising the steps of obtaining a mixture containing a colored organic pigment, a dispersant, an organic solvent, and water, adjusted so that the content of the organic solvent in the entire mixture is 3 to 8% by weight, and then dispersing the mixture.

5. A water-based inkjet ink comprising the water-based inkjet pigment dispersion according to claim 1 or 2.

6. The aqueous inkjet ink according to claim 5, wherein the amount of water filtered when an aqueous inkjet ink with a pigment concentration of 4% by weight is filtered through a filter film with a mesh opening of 1.2 μm is 400 g / 3 min or more.

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