JP2012036251A - Aqueous pigment dispersion for inkjet, and ink - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aqueous pigment dispersion for inkjet excellent in long-term storage stability and ink discharge stability from an inkjet head, having a high color developing property and excellent printing quality on a plain paper, and to provide an ink using the dispersion.SOLUTION: This aqueous pigment dispersion for inkjet is described as follows: a pigment coated with a polymeric dispersant, an aqueous organic solvent and an alkali agent are contained; a content ratio between the pigment and the polymeric dispersant is 40/60-95/5; the polymeric dispersant is a block polymer comprising a methacrylate-based hydrophobic polymer block and a hydrophilic polymer block containing 30-5 mass% methacrylic acid; both of the degree of dispersion PDI of the polymer and PDI of the hydrophobic polymer block constituting the block polymer are each ≤1.5; and assuming that the molecular weight of the block polymer is Mnab, the molecular weight of the hydrophobic polymer is Mna, and the molecular weight of the hydrophilic polymer is Mnb, a requirement of Mna≥Mnb, Mna<Mnb≤2Mna or the like is satisfied.

Description

  The present invention relates to an aqueous pigment dispersion as a colorant for an inkjet ink and an ink for an inkjet printer using the aqueous pigment dispersion. More specifically, the present invention relates to an aqueous pigment dispersion for inkjet which is excellent in long-term storage stability of a pigment dispersion and ink, ink ejection stability from a printer head, print quality and the like, and ink using the same.

  Inkjet printers have been used for various purposes such as personal use, office use, business use, recording use, color display use, and color photography use due to their high functionality. In addition, in pigment ink, in order to improve the sharpness, color tone, color density, etc. of the pigment, the pigment particles in the ink are being miniaturized. On the other hand, miniaturization of ejected droplets (ink droplets) is also progressing in order to cope with higher speed and higher image quality by improving the printer device. These improvements provided excellent image quality for inkjet processed paper, especially photographic paper and wide format paper, but when printed on non-processed paper called plain paper, ink droplets were generated on the paper. As a result, there is another problem that the color density of the print, that is, the color developability is deteriorated.

  Therefore, when printing on plain paper, many of the aqueous pigment dispersion inks mainly suppress the penetration of paper, thereby preventing the ink from getting wet on the paper surface and keeping the ink droplets near the paper surface to reduce the print density. Improvements are being studied and put into practical use.

  For example, a pigment is encapsulated using a graft or block polymer that consists of a highly hydrophilic part and the other is a highly hydrophobic part, and the highly hydrophobic part is adsorbed on the pigment surface to coat the pigment. By preventing the pigment from penetrating into the paper and making it easier to exist on the surface of the paper, on the other hand, by making the highly hydrophilic portion take a form that is compatible with the dispersion medium and extending, Attempts have been made to improve the storage stability of ink by steric hindrance (see Patent Documents 1-3).

JP 2008-231130 A JP 2009-149912 A JP 2004-51971 A

  Along with the recent improvement in quality of ink jet printers, at the same time, higher quality of ink is required. The present invention, in response to these demands, has excellent long-term storage stability, excellent ejection stability of ink from an inkjet head, and has excellent color quality on plain paper and excellent print quality, and an aqueous pigment dispersion for inkjet An object of the present invention is to provide an ink using the ink.

  Another object of the present invention is to provide ink that solves the following problems. In the case of ink-jet ink applications, surfactants and glycol solvents for preventing drying and clogging contained therein are pigments or dispersants encapsulating aqueous pigment dispersions and dispersants in ink. Has the property of facilitating adsorption and desorption from the adsorbed pigment. As a result, the storage stability of the dispersion liquid and the ink is lowered, or the penetrability into the paper is increased during printing, resulting in a problem that the printing density is lowered. The object is to solve this problem.

According to the present invention, the following is provided.
1. In an aqueous pigment dispersion containing a pigment whose surface is coated with a polymer dispersant, an aqueous organic solvent, and an alkaline agent,
The content ratio of pigment to polymer dispersant (mass ratio of pigment / block polymer) is 50/50 to 95/5,
The polymer dispersant is a block polymer comprising a methacrylate-based hydrophobic polymer block and a hydrophilic polymer block containing at least 30 to 5% by mass of methacrylic acid. The degree of dispersion PDI of the block polymer and the block polymer Each of the PDIs of the hydrophobic polymer block constituting is 1.5 or less,
Furthermore, the number average molecular weight of the block polymer is Mnab, the number average molecular weight of the hydrophobic polymer block constituting the block polymer is Mna, and the number average molecular weight of the hydrophilic polymer block constituting the block polymer is Mnb (Mnb = Mnb− Mna), an aqueous pigment dispersion for inkjet which satisfies all the requirements (1) to (4).
(1) 2,000 ≦ Mna ≦ 10,000
(2) 1,500 ≦ Mnb ≦ 10,000
(3) 5,000 ≦ Mab ≦ 20,000
(4) Mna ≧ Mnb or Mna <Mnb ≦ 2Mna

2. As a constituent component of the hydrophobic polymer block, at least one selected from the group consisting of methyl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, and isobutyl methacrylate, and the constitution of the hydrophilic polymer block The aqueous pigment dispersion for inkjet according to 1 above, further comprising 2-hydroxyethyl methacrylate as a component.
3. 3. The aqueous pigment dispersion for inkjet according to 1 or 2 above, wherein the hydrophilic polymer block has an acid value of 50 to 250 mgKOH / g, and the block polymer has an acid value of 20 to 150 mgKOH / g.

4). The content ratio of the pigment whose surface is coated with the polymer dispersant and the polymer dispersant is 60/40 to 90/10, and the content ratio of the pigment in 100% by mass of the aqueous pigment dispersion The aqueous pigment dispersion for inkjet according to any one of the above 1 to 3, wherein is 30 to 5% by mass.

5. The pigment has a color index number (CI) pigment blue of 15: 3, 15: 4; I. Pigment red-122, 269; I. Pigment violet-19; I. Pigment yellow-74, 155, 180; C.I. I. Pigment green-36; I. Pigment orange-43; I. The aqueous pigment dispersion for inkjet according to any one of 1 to 4 above, which comprises one selected from the group consisting of CI Pigment Black-7, and wherein the average primary particle diameter of the pigment is 150 nm or less.
6). The aqueous organic solvent is a mixture of water and a water-soluble organic solvent, the ratio of water in the mixture is 81 to 73% by mass, and the water-soluble organic solvent is diethylene glycol monobutyl ether, triethylene glycol mono 6. The inkjet aqueous solution according to any one of 1 to 5 above, comprising at least one selected from the group consisting of butyl ether, glycerin, propylene glycol, 1,2-hexanediol, 2-pyrrolidone, and N-methyl-2-pyrrolidone. Pigment dispersion.

7). The pigment whose surface is coated with a polymer dispersant is a polymer dispersant-coated pigment obtained by mixing at least a polymer dispersant, a pigment, and an aqueous organic solvent and dispersing in water, and then adding an acid. The aqueous pigment dispersion for inkjet according to any one of 1 to 6 above.

8). 8. An ink-jet aqueous pigment ink comprising the ink-jet aqueous pigment dispersion described in any one of 1 to 7 above, wherein the content of the pigment in the ink is 4 to 10% by mass.
9. Furthermore, the aqueous | water-based pigment ink for inkjets of said 8 formed by containing 0.1-2 mass% of surfactant.

  INDUSTRIAL APPLICABILITY According to the present invention, an aqueous pigment dispersion for inkjet having excellent long-term storage stability, excellent ejection stability of ink from an inkjet head, high color development on plain paper, and excellent printing quality, and inkjet using the same Ink can be provided.

  More specifically, the present invention is characterized by a polymer dispersant (hereinafter referred to as AB block) comprising a hydrophobic block (hereinafter also referred to as A block) and a hydrophilic block (hereinafter also referred to as B block). (Sometimes referred to as a polymer). By making the pigment coated with a polymer dispersant having a structure in which the PDI of the polymer is controlled to be small and the molecular weight is adjusted, the hydrophilic / hydrophobic site is made clearer and more precise than before, Various ink characteristic effects can be obtained. That is, the hydrophobicity of the A block suppresses adsorption / desorption between the surfactant or penetrant added during preparation, for example, and the dispersant, and firmly coats the pigment surface. On the other hand, the hydrophilicity of the B block shows sufficient hydrophilicity even at a low acid value, thereby increasing the affinity of the pigment itself to the dispersion medium, suppressing aggregation of the pigments, and fixing to paper because of the low acid value. Occasionally suppresses paper penetration. Furthermore, the dispersing agent which suppresses the free dispersing agent and is not coated on the pigment also forms a polymer micelle in which the A block contracts inward with respect to the aqueous medium, takes a spherical shape and has an extended B block. Thus, a pigment dispersion having excellent ejection stability of ink from the inkjet head can be obtained. By using such an aqueous pigment dispersion, an inkjet ink having excellent long-term storage stability, moderately suppressed permeability to plain paper, and high color development is provided.

Hereinafter, the present invention will be described in more detail with reference to preferred embodiments.
[Polymer dispersant]
The polymer dispersant used in the present invention is described as a block polymer (hereinafter referred to as AB block polymer) comprising a methacrylate-based hydrophobic polymer block and a hydrophilic polymer block containing at least 30 to 5% by mass of methacrylic acid. ). Here, A and B are polymer blocks each composed of one or more addition polymerizable monomers. The A block is a polymer block composed of a hydrophobic monomer, and the B block is a hydrophilic polymer block including a monomer having at least a carboxyl group as an acid group. The AB block polymer used in the present invention is a methacrylate polymer composed of a methacrylate monomer. When a methacrylate monomer is polymerized, the conventional radical polymerization cannot obtain a polymer having a controlled PDI and a narrow PDI like the block polymer defined in the present invention. In recent years, a living radical polymerization method has been invented, and PDI is narrow (dispersion degree is small), and a structure like an AB block polymer can be controlled.

  The method for obtaining the AB block polymer used in the present invention is not particularly limited, but it is necessary to control the structure so as to be in a predetermined state. For example, using a metal complex such as copper or ruthenium as a catalyst, an organic halide as a starting compound, an atom transfer radical polymerization method from the oxidation reduction thereof, a nitroxide method in which a —NO— radical of nitroxide is transferred, a dithiocarboxylic acid ester Reversible addition-fragmentation chain transfer polymerization method using iodine, iodine transfer polymerization method, method using organic tellurium or organic bismuth, addition-cleavage type chain transfer polymerization method using cobalt compound as catalyst it can.

  Next, the effect | action in pigment dispersion of each polymer block of A and B which comprises the AB block polymer used by this invention is demonstrated. The hydrophobic A block is adsorbed on the pigment surface, while the hydrophilic B block is dissolved in water or an aqueous solution containing a water-soluble organic solvent (hereinafter referred to as an aqueous medium) to form a stable dispersion system. Specifically, since the A block is a polymer block that is insoluble in an aqueous medium, the A block is adsorbed, deposited, and coated on the hydrophobic surface of the pigment in the aqueous medium, and further, the A block does not dissolve in the aqueous medium. , Keep the pigment coated. Further, since the pigment is coated, when the printing medium is paper, the pigment is prevented from penetrating into the paper, and the pigment is likely to be present on the paper surface, thereby enhancing the color development of the image.

  On the other hand, the B block is a polymer block having a carboxyl group, and can be dissolved in an aqueous medium by neutralizing and ionizing the carboxyl group with an alkaline agent. As a result, the B block is ionized to exhibit the effects of electrical repulsion and steric hindrance to improve storage stability and prevent aggregation and sedimentation of pigment particles. In addition, the polymer block B increases the compatibility with the surfactant to be added, does not cause aggregation and the like, and is a dissolved resin, so it functions as a film component and acts as a binder. Further, when the printing medium is paper, for example, it has affinity with calcium atoms present in the paper to prevent penetration into the paper and enhance the color developability.

Each of the configurations of the AB block polymer will be described.
(A polymer block)
The A block is a hydrophobic polymer block insoluble in an aqueous medium composed of methacrylate monomers. The hydrophobic polymer block is adsorbed on the pigment surface to coat the pigment. The following are mentioned as a preferable form. The monomer component constituting the A block preferably includes at least one component selected from the group consisting of methyl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, and isobutyl methacrylate. The number average molecular weight (hereinafter abbreviated as Mna) of the A polymer block is 2,000 ≦ Mna ≦ 10,000, and its PDI is 1.5 or less.

  More preferably, the component of the A block may contain a pigment affinity component in the range not impairing its function, in addition to the above-mentioned methacrylate monomers. As the pigment affinity component, methacrylic acid or 2-hydroxyethyl methacrylate can be used. When a pigment affinity component is used, a carboxyl group and a hydroxyl group are introduced, and adsorption by hydrogen bonding with an organic pigment is preferable. Furthermore, use of a polyethylene glycol-based monomer, a sulfonic acid, or a carboxyl group-containing monomer other than methacrylic acid that can impart solubility in water is also preferred. However, it is necessary to reduce the addition amount of these as much as possible, and it is necessary to make it smaller than the total amount of acid components including methacrylic acid constituting the B block. On the condition that this requirement is satisfied, the total amount of the above components can be 30% by mass or less, more preferably 20% by mass or less in 100% by mass of the A block constituent monomer. If methacrylic acid is added, 0 to 20% by mass, and if 2-hydroxyethyl methacrylate is added, 0 to 30% by mass can be used. When the total amount of the above components is large, there is a problem that the solubility in an aqueous medium is increased and the coated dispersant is detached, resulting in a decrease in storage stability.

  As described above, the number average molecular weight (Mna) of the A block is 2,000 ≦ Mna ≦ 10,000. When Mna is less than 2,000, the adsorptivity and covering property of the pigment are weak and may be dissolved in an aqueous medium. On the other hand, when Mna is larger than 10,000, the molecular weight is too large, There is a possibility that fine dispersion is not obtained due to adsorption between multiple particles.

(B polymer block)
The B block is a hydrophilic polymer block containing at least 30 to 5% by mass of methacrylic acid, and is characterized by using methacrylic acid as a constituent component as an acid component. The number average molecular weight (hereinafter abbreviated as Mnb) of the B polymer block is 1,500 ≦ Mnb ≦ 10,000, and PDI is 1.5 or less.

  The AB block polymer used in the present invention introduces a monomer having a carboxyl group (acid group) as a component of the B block, neutralizes the carboxyl group, and ionizes the AB block. The polymer is soluble in an aqueous solvent (hereinafter sometimes referred to as “water-soluble”). At this time, for example, when acrylic acid having strong acidity is used from among acrylic monomers having a carboxyl group, although there is an effect as water solubility, PDI control becomes difficult. When polymerizing the block polymer, PDI can be controlled more effectively by polymerizing the hydrophilic B block after polymerizing the hydrophobic A block first. For this purpose, it is most effective to use methacrylic acid as the acid component of the B block. As a result, the PDI of the polymer is narrowed, the block efficiency is improved, and the useful effects of the present invention are obtained.

  The acid value of the polymer block of B is preferably 50 to 250 mgKOH / g. When this acid value is less than 50 mgKOH / g, there is a possibility that the dispersion stability and the ejection stability are lowered. On the other hand, when the acid value exceeds 250 mgKOH / g, the solubility in an aqueous medium is improved, the block polymer is detached from the pigment, becomes a free resin, and further becomes a normal water-soluble polymer from the polymer micelle. This is not preferable because the viscosity of the liquid tends to increase.

  Further, the B block is prepared by copolymerizing methacrylic acid and another methacrylate monomer so as to have the acid value described above. A conventionally well-known thing is used as another methacrylate type monomer. Examples of the monomer that can be used in this case include aliphatic methacrylates such as methyl methacrylate, butyl methacrylate, and cyclohexyl methacrylate, hydroxyl group-containing methacrylates such as 2-hydroxyethyl, 2-hydroxypropyl, and dimethylaminoethyl methacrylate. Various amino group-containing methacrylates and the like, and glycol-based methacrylates such as polyethylene glycol monomethyl ether methacrylate can be used, and one or more can be used. These monomers are very useful for assisting in controlling the strong hydrophilicity of the B block, and for improving the color development on paper by giving affinity to the hydroxyl group of the glucose unit of paper.

  Particularly preferable examples of the monomer composition constituting the B block include 5 to 30% by weight of methacrylic acid, 70 to 95% by weight of methyl methacrylate, and 0 to 30% of 2-hydroxyethyl methacrylate in 100% by weight of the monomer constituting the B block. % By mass. The B polymer block obtained by the above configuration has strong hydrophilicity, and the effect of the present invention can be obtained.

  The number average molecular weight (Mnb) of the B block is expressed by subtracting the number average molecular weight (Mna) of the A block from the number average molecular weight (hereinafter abbreviated as Mnab) of the AB block polymer (Mnb = Mnab). -Mna). The Mnb is 1,500 ≦ Mnb ≦ 10,000, preferably 2,000 ≦ Mnb ≦ 5,000. Furthermore, the PDI is required to be 1.5 or less. When Mnb is greater than 10,000, the hydrophilic B block has a strong property and takes the form of a normal water-soluble polymer from the polymer micelle, thereby increasing the viscosity of the pigment dispersion or entanglement of molecular chains. Agglomeration may occur or ejection stability may be impaired. On the other hand, if Mnb is smaller than 1,500, sufficient water solubility cannot be obtained, resulting in poor dispersion. Further, Mnb has a lower molecular weight or the same molecular weight as Mna, or when Mnb has a higher molecular weight than Mna, Mnb needs to be not more than twice Mna. When Mnb has a molecular weight more than twice that of Mna, the hydrophilicity of the B block becomes stronger and the ability to dissolve in the aqueous medium becomes stronger than the pigment adsorption force, and the AB block polymer dissolves from the pigment and the pigments aggregate. Problem arises.

(AB block polymer)
The AB block polymer used in the present invention may be obtained by purifying the A block after producing the A block, and then polymerizing the monomers constituting the B block. It may be obtained by polymerizing by adding a monomer constituting the above. That is, in any case, the monomer constituting the B block is not introduced into the A block. However, in the latter case where the A block is not purified, the monomer of A is introduced into the B block, and it may be adjusted within the range of the polymer composition of B described above, and both are preferable modes. .

  The AB block polymer needs to have a total number average molecular weight (Mnab) of 5,000 ≦ Mab ≦ 20,000 and a PDI of 1.5 or less. Moreover, it is preferable that the whole acid value is 20-150 mgKOH / g. If the number-average molecular weight (Mnab) of the AB block polymer that functions as a dispersant is greater than 20,000, aggregation due to entanglement of molecular chains, thickening, and further, ejection stability may be deteriorated. is there. On the other hand, if Mnab is less than 5,000, there is a possibility that the coating on the pigment surface and the affinity to the aqueous medium cannot be exhibited.

Therefore, in the present invention, the block polymer used as the polymer dispersant is the number average molecular weight Mnab, the hydrophobic polymer block constituting the block polymer Mna, and the hydrophilic polymer block constituting the block polymer. When the number average molecular weight of Mb is Mnb (Mnb = Mnab−Mna), all the requirements (1) to (4) are satisfied.
(1) 2,000 ≦ Mna ≦ 10,000
(2) 1,500 ≦ Mnb ≦ 10,000
(3) 5,000 ≦ Mab ≦ 20,000
(4) Mna ≧ Mnb or Mna <Mnb ≦ 2Mna

  On the other hand, when the PDI is larger than 1.5, it is difficult to control the balance between the hydrophilic and hydrophobic segments at the same time as the large molecular weight and the small molecular weight are varied, which causes a problem in solubility in an aqueous medium. Further, there is a possibility that viscosity increase or particle change may occur, for example, dissolution from a polymer having a low molecular weight or high hydrophilicity causes aggregation of pigment particles due to formation of free resin. Therefore, it is preferable that the molecular weight is uniform. For this reason, More preferably, it is 1.45 or less.

  The acid value is preferably 20 to 150 mgKOH / g for the entire AB block polymer. When the acid value is less than 20 mgKOH / g, the solubility in an aqueous medium is poor, and the storage stability may be lowered. On the other hand, when the acid value exceeds 150 mgKOH / g, the ink has poor water resistance, and the ink tends to penetrate into the paper, and the color developability may be lowered.

  The AB block polymer is preferably obtained by solution polymerization, and a conventionally known solvent is used. A water-soluble organic solvent is preferable. This is because the AB block polymer of the present invention is neutralized with an alkali agent to form an aqueous solution, and in this case, the solvent is miscible with water. The solvent is not particularly limited.

[Alkaline agent]
The alkaline agent constituting the pigment dispersion of the present invention is used for solubilizing the polymer dispersant comprising the block polymer in an aqueous medium. The alkali agent to be used is not particularly limited, but for example, from the group consisting of ammonia, primary, secondary or tertiary organic amines (including basic nitrogen-containing heterocyclic compounds) and alkali metal hydroxides. The selected compound can be preferably used.

[Pigment]
As the pigment used in the pigment dispersion liquid of the present invention, one or more organic pigments and inorganic pigments that have been conventionally used are used in the same manner as in the prior art. Examples thereof include carbon black pigments, quinacridone pigments, phthalocyanine pigments, benzimidazolone pigments, isoindolinone pigments, and azo pigments. Moreover, it is preferable to contain 5-30 mass% of the usage-amount in 100 mass% of aqueous pigment dispersions.

  More specifically, the following are preferable from the viewpoint of color developability, dispersibility, and weather resistance. For example, C.I. I. Pigment Blue-15: 3, 15: 4, C.I. I. Pigment red-122, 269, C.I. I. Pigment violet-19, C.I. I. Pigment yellow-74, 155, 180, C.I. I. Pigment green-36, C.I. I. Pigment orange-43, C.I. I. And CI Pigment Black-7. Furthermore, it is preferable that these average primary particle diameters are less than 150 nm.

  In addition, the average primary particle diameter said here is a particle diameter obtained at the time of preparation of the dispersion liquid which passed through the predetermined | prescribed dispersion | distribution process. The average primary particle diameter is preferably less than 150 nm, from the viewpoint of the color developability and print quality of the ink such as optical density and saturation, or the sedimentation property of the pigment in the ink. When the particle diameter is 150 nm or more, the color development and print quality of the ink are deteriorated, and the pigment is liable to settle and the storage stability of the dispersion may be deteriorated.

[Aqueous organic solvent]
The aqueous organic solvent used in the present invention is preferably a water-soluble organic solvent. For example, it is preferably at least one selected from the group consisting of diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, glycerin, propylene glycol, 1,2-hexanediol, 2-pyrrolidone, and N-methyl-2-pyrrolidone. . These aqueous organic solvents are for preventing the inkjet head from drying, and are essential in the present invention. The amount is preferably 5% by mass to 20% by mass with respect to 100% by mass of the aqueous pigment dispersion. If the amount is less than 5% by mass, the pigment is liable to be dried. If the amount exceeds 20% by mass, the penetration into the paper is increased, and bleeding is likely to occur, resulting in a decrease in print quality.

[Distributed processing]
In producing the pigment dispersion of the present invention, the dispersion treatment is preferably performed separately for pre-dispersion and main dispersion. More preferably, in order to obtain high dispersion stability and high performance, first, a conventionally known method using a polymer dispersant neutralized with a pigment and an alkali agent in the pre-dispersion, water, and an organic solvent. Disperse the pigment with Subsequently, it is preferable to deposit the polymer dispersant on the surface of the pigment particles and coat the pigment particles with the A block by a method described later. In the state of the aqueous pigment dispersion by the pre-dispersion, since there is an organic solvent that has an affinity for the A block, the B block of the polymer dispersant is in a state having an affinity for water. This is the same as a conventional aqueous pigment dispersion.

  The content ratio (mass ratio of pigment / block polymer) of the pigment and the polymer dispersant in the pigment whose surface is coated with the polymer dispersant (hereinafter sometimes referred to as polymer dispersion-treated pigment) is 50/50 to 95. / 5. More preferably, it is 60/40 to 90/10, and more preferably 70/30 to 90/10. If the amount of the polymer dispersant with respect to the pigment is as large as 60% or more, the resin that cannot be adsorbed on the pigment and liberated increases, which may reduce the printing density. On the other hand, when the amount is less than 5%, the polymer chain with respect to one particle decreases, and dispersion stability may not be ensured.

  The ratio of the polymer dispersant-treated pigment to 100% by mass of the aqueous pigment dispersion is preferably 35 to 5% by mass. When the amount is more than 35% by mass, the density of the pigment in the aqueous solution itself is high and the viscosity is high, and aggregation tends to occur. When the amount is less than 5% by mass, the pigment concentration becomes low at the time of ink preparation, and a high printing density may not be obtained.

[Coating]
Next, the precipitation process for coating the pigment particles with the polymer dispersant will be described in detail. An aqueous pigment dispersion obtained by dispersing a pigment by a conventionally known method using a polymer dispersant neutralized with a pigment and an alkali agent, water, and an organic solvent, or the pigment content is 10 masses with water. %, Or a neutralized polymer dispersant is added so as to obtain a desired pigment content if necessary. Next, the acid groups such as carboxyl groups that make up the polymer dispersant are neutralized with ions, ionized, and dissolved in water. By adding an acid to this, it becomes insoluble in water on the pigment particles. And the polymer dispersant can be deposited. Although the acid used in this case is not particularly limited, for example, inorganic acids such as hydrochloric acid, sulfuric acid and nitric acid, and organic acids such as acetic acid, propionic acid and toluenesulfonic acid can be used. The acid may be added as it is, but it is preferably used as an aqueous solution of 10% by mass or less or as a dispersion with an aqueous organic solvent. In addition to acid addition, as a method of depositing a polymer dispersant by insolubilizing in water, for example, a kneaded product of at least a polymer dispersant and a pigment may be added to a poor solvent for the polymer dispersant. Obtainable. Although it does not specifically limit as a poor solvent used in this case, For example, hydrocarbon solvents, such as alcohol solvents, such as isopropyl alcohol, and a cyclohexane can be used.

  While the aqueous pigment dispersion obtained by the dispersion is diluted with water and stirred by a conventionally known method, the acid is gradually added using a stirrer capable of high-speed stirring such as a dissolver. The amount of the acid added is at least equimolar to the alkali neutralizing the B block of the polymer dispersant, and more preferably at least 1.1 times the molar amount.

  After the polymer dispersant is precipitated on the pigment particles by addition of an acid, the precipitate is filtered. After the precipitation, the particles may be heated as necessary to aggregate the precipitated particles and facilitate filtration. It is preferable to sufficiently remove ionic substances and organic solvents adhering to the precipitate by this filtration.

  The water paste obtained by filtration may be dried and pulverized and used in the next step, but preferably the water paste is used as it is. This is because when the water paste is used as it is, there is no fusion of the polymer dispersant due to drying, there is no need for pulverization, and the particle diameter of the pigment remains as it is dispersed.

  As the disperser for performing the main dispersion, the following can be used. For example, a kneader such as a kneader, two-roll, three-roll, Miracle KCK (Asada Steel Corporation, trade name), an ultrasonic disperser, or a high-pressure homogenizer, a microfluidizer (Mizuho Industrial Co., Ltd., trade name) ), Nanomizer (Yoshida Kikai Kogyo Co., Ltd., trade name), Starburst (Sugino Machine Co., trade name), G-Smasher (Rix Corporation, trade name), and the like. In the case of using a bead medium such as glass or zircon, a ball mill, a sand mill, a horizontal media mill disperser, a colloid mill, or the like can be used.

  In the present invention, a desirable dispersed particle diameter of the pigment is not more than 150 nm in terms of a dispersion average particle diameter in consideration of ink color development and printing quality such as optical density and saturation, or sedimentation of the pigment in the ink. To obtain a pigment dispersion having a desired particle size distribution, reduce the size of the grinding media of the disperser, increase the filling rate of the grinding media, increase the processing time, slow the discharge speed, grinding A method such as classification using a post filter or a centrifuge is used. Or the combination of those methods is mentioned. Furthermore, it is possible to use a method of using a pigment in which the primary particle diameter of the pigment to be used is finely adjusted in advance by a conventionally known method, for example, a salt milling method.

  When the aqueous pigment dispersion according to the present invention is used in an aqueous pigment ink for inkjet, the amount added as a pigment is preferably 0.5 to 30% by mass, more preferably 4 to 10% by mass in 100% by mass of the ink. . If the addition amount is less than 0.5% by mass, the print density cannot be secured, and if the addition amount exceeds 30% by mass, the viscosity of the ink increases and structural viscosity is generated in the viscosity characteristics, so that the ink ejection from the inkjet head is stable. It tends to be worse.

  Further, in the aqueous pigment ink for inkjet of the present invention, a surfactant, an organic solvent, a humectant, and the like are further used in order to improve the dispersibility of the pigment, the dispersion stability, and the ink storage stability over time. For example, the surface tension of the ink is preferably in the range of 20 mN / m or more and 40 mN / m or less from the viewpoint of expanding the dot diameter printed by the ink jet to the optimum width. Therefore, it is preferable to add at least a surfactant so that the surface tension of the ink falls within the above range. As the surfactant, anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants as listed below can be used. As an anionic surfactant, alkyl sulfate ester salt, alkyl aryl sulfate ester salt, alkyl aryl sulfonate, alkyl naphthalene sulfonate, polyoxyethylene alkyl ether sulfonate, polyoxyethylene alkyl aryl ether sulfonate, Naphthalenesulfonic acid formalin condensate, polyoxyethylene alkyl phosphoric acid ester salt, polyoxyethylene alkyl aryl phosphoric acid ester salt and the like can be mentioned. Nonionic surfactants include polyoxyethylene alkyl ether, polyoxyethylene alkyl aryl ether, sorbitan fatty acid ester, polyoxyethylene alkylamine ether, fatty acid diethanoldiamide, sorbitan fatty acid ester, acetylene alcohols, acetylene glycols, etc. Can be mentioned. Examples of the cationic surfactant include alkylamine salts and quaternary ammonium salts, and examples of the amphoteric surfactant include alkylbetaines and amine oxides. If the amount added is too large, the dispersion stability of the pigment may be impaired. Therefore, the amount is preferably 0.01% by mass to 5% by mass, more preferably 0.1% by mass in 100% by mass of the ink. It is -2 mass%.

  The present invention will be described more specifically with reference to synthesis examples, examples and comparative examples, but the present invention is not limited to these examples. In the text, “part” or “%” is based on mass.

[Synthesis Example 1] Synthesis of Polymer Dispersant-1 (AB Block Polymer) (Synthesis of A Polymer Block)
In a 1-liter separable flask reactor equipped with a stirrer, a reverse flow condenser, a thermometer and a nitrogen introduction tube, 295.68 parts of diethylene glycol dimethyl ether (hereinafter referred to as diglyme), 3.03 parts of iodine, azobis (methoxydimethylvaleronitrile) ) (Hereinafter abbreviated as V-70) 14.8 parts, di-t-butylhydroxytoluene (hereinafter abbreviated as BHT) 0.66 parts, cyclohexyl methacrylate (hereinafter abbreviated as CHMA) 176.4 parts. The mixture was heated to 40 ° C. while flowing nitrogen, and polymerized for 6.5 hours to obtain an A polymer block. The polymerization conversion rate calculated from the nonvolatile content was 85%, the number average molecular weight (Mna) was 4,000, and the PDI was 1.37.

(Synthesis of AB block polymer)
To the reaction solution, a mixture of 75 parts of methyl methacrylate (hereinafter abbreviated as MMA), 25.8 parts of methacrylic acid (hereinafter abbreviated as MAA) and 0.45 part of V-70 was added and polymerized for 4.5 hours. It was. The polymerization rate obtained by sampling and conversion from the nonvolatile content was 97.6%, and it was confirmed that most of the monomers were polymerized. This B polymer block contains CHMA which has not been polymerized with the constituent monomer of the A polymer block, and the composition is MMA / MAA / CHMA = 59/20/21 in terms of mass from the above yield. . The number average molecular weight (Mnab) of the obtained AB block polymer was 6,200, and PDI was 1.41. The acid value was 61.3 mgKOH / g. The calculated value of the number average molecular weight (Mnb) of the B polymer block is 2,200. The acid value of the B polymer block was 137 mgKOH / g.

  Next, 147.8 parts of diethylene glycol monobutyl ether (hereinafter abbreviated as BDG) was added, and a mixed solution of 18.5 parts of sodium hydroxide and 129.3 parts of water was added for neutralization. The solution was clear and there was no precipitation. Subsequently, it was made to react at 50 degreeC for 2 hours, and the iodine of the polymer terminal was decomposed | disassembled. The solid content was 34.4%. As described above, Polymer Dispersant-1 was obtained.

  When this polymer dispersant-1 was diluted with 10 times water, it was almost transparent, but it became a blue fine dispersion, and it is considered that the dispersant was dispersed as polymer micelles.

[Comparative Synthesis Example 1] Synthesis of Random Polymer Polymer Dispersant Using the same apparatus as in Synthesis Example 1, 278 parts of diglyme was charged and heated to 75 ° C. Next, 76 parts of MMA, 25.8 parts of MAA, 176.4 parts of CHMA and 1.8 parts of azobisisobutyronitrile were mixed and homogenized in a separate container. Next, 1/2 of the monomer mixed solution was added to the reaction vessel, and the remaining monomer mixed solution was added dropwise over 2 hours to polymerize for 4 hours. This solution was sampled, and the polymerization rate calculated from the nonvolatile content was 98%, and it was confirmed that most of the monomers had reacted. Moreover, when measured by GPC, the number average molecular weight was 6,000 and PDI was 1.95.

  Next, an aqueous solution of sodium hydroxide in an amount to neutralize the carboxyl groups in the polymer was added, pure water was added to adjust the solid content to 40%, and a transparent polymer dispersant solution could be obtained. The resin obtained as described above is referred to as Comparative Polymer Dispersant-1.

  When this comparative polymer dispersant-1 was diluted with 10 times the amount of water, the aforementioned polymer dispersant-1 became a blue fine dispersion, but the comparative polymer dispersant-1 was transparently dissolved. This is because Comparative Polymer Dispersant-1 is a conventional random copolymer, the molecular weight and structure are not controlled, and hydrophilic carboxyl groups are randomly arranged. Therefore, it is considered that the polymer dispersant is uniformly dissolved in water and dissolved.

Example 1 Preparation of Red Aqueous Pigment Dispersion Polymer Dispersant-1 (solid content 40%) obtained in Synthesis Example 1 was combined with the pigment, solvent and water in the combinations shown in Table 1, and peptized with a disper. Millbase-1 was prepared and dispersed at a peripheral speed of 10 m / s using a horizontal medium disperser “Dynomill 0.6 liter ECM type” (trade name manufactured by Shinmaru Enterprises Co., Ltd., zirconia bead diameter 0.5 mm). Processed. Dispersion was terminated when dispersed for 2 hours.

  To 200 parts of the obtained millbase-1, 3,200 parts of ion-exchanged water was added so that the pigment content was 5%, and 5% acetic acid was added dropwise with stirring to precipitate a pigment dispersion. At this time, the initial pH was 9.5, and the pH was lowered to 4.5 by adding an acid. Next, this aqueous pigment dispersion was filtered and washed thoroughly with ion exchange water to obtain a polymer dispersant-coated pigment paste (solid content: 30.5%).

  Next, a solution obtained by dissolving 37.46 parts of BDG and 1.15 parts of sodium hydroxide in 10 parts of water was added to 700 parts of this pigment paste and stirred to stir. Subsequently, it was dispersed again for a predetermined time by the horizontal media dispersing machine. Furthermore, using an ultrahigh pressure homogenizer “Microfluidizer” (manufactured by Microfluidics Co., Ltd.), dispersion was carried out at 150 MPa for 3 passes.

  This dispersion is centrifuged (7,500 rpm, 20 minutes), the pigment solution is filtered through a 10 μm membrane filter, adjusted with ion-exchanged water, and a red aqueous pigment dispersion having a pigment concentration of 14%. It was set to 1. When the average particle size of this pigment dispersion was measured with a particle size measuring instrument “NICOMP 380ZLS-S” (manufactured by Particle Sizing System PSS), the average particle size was 108 nm and the viscosity was 2.22 mPa · s. When this was stored at 70 ° C. for one week, no changes were observed in the particle size and viscosity, and the storage stability was good.

[Comparative Examples 1 to 4] Preparation of Comparative Red Aqueous Pigment Dispersions 1 to 4 In the same manner as in Red Aqueous Pigment Dispersion 1 of Example 1, pigments and polymer dispersants were blended in combinations shown in Table 2, and each mill base -2 to 5 were obtained. Using each mill base obtained, the same red water pigment dispersions 1 to 4 for comparison were obtained through the same dispersion and treatment steps as in Example 1.

[Usage example 1]
Next, using the red aqueous pigment dispersion 1 obtained in Example 1, with respect to 40 parts of the red aqueous pigment dispersion, 1.8 parts of BDG, 5 parts of 1,2-hexanediol, and 10 parts of glycerin 1 part of “Surfinol 465” (trade name, manufactured by Air Products) and 60 parts of a mixed solution of water were added and sufficiently stirred. Thereafter, the mixture was filtered through a membrane filter having a pore size of 10 microns to obtain a red inkjet aqueous pigment ink 1.

[Comparative Usage Examples 1-4]
In the same manner as in Use Example 1, comparative red aqueous pigment dispersions 1 to 4 obtained in Comparative Examples 1 to 4 were used, and prepared according to the formulation shown in Table 3 to prepare an aqueous pigment ink for inkjet.

<Evaluation of ink>
(Evaluation of dispersion stability)
Table 4 shows changes in viscosity and particle diameter when the aqueous pigment ink for ink jet prepared as described above using the pigment dispersion prepared under each condition is left at 70 ° C. for 7 days. The dispersion stability is shown as the viscosity and the rate of change in particle diameter (%) when each aqueous pigment ink is left at 70 ° C. for 7 days. The rate of change is shown as 100% (%) of 1- (value after measurement date) / (initial value).

(Evaluation of plain paper OD)
As the plain paper, Xerox 4024 paper manufactured by Xerox Corporation in the United States was used. The printer used was an ink-jet printer EM930C manufactured by Seiko Epson Corporation, plain paper was printed in a printing mode with photo 720 dpi, and the printed matter was measured for optical density (“Macbeth RD-914” trade name, manufactured by Macbeth). An average value of 5 times for each recording sheet was used. The results are shown in Table 4.

[Evaluation criteria]
Hereinafter, the same evaluation criteria are used for similar test results.
(Particle size)
◎: Change rate ± 5% or less ○: Change rate ± 5% or more and less than 10% △: Change rate ± 10% or more and less than 15% ×: Change rate ± 15% or more (viscosity change)
◎: Low viscosity, change rate less than ± 10% ○: High viscosity, change rate less than ± 10% △: Low viscosity, change rate ± 10% or more ×: High viscosity, change rate ± 10% or more

  As is apparent from Table 4, the ink of Comparative Use Example 1 using the pigment dispersion of Comparative Example 1 using Comparative Polymer Dispersant-1 treated with a conventional random polymer has increased particle size and viscosity. Gelled. This is probably because the polymer dispersant was released from the pigment by the adsorption and desorption of the surfactant and the polymer dispersant, and became a gel by the adsorption of the pigment particles. Moreover, although the ink of the comparative use example 4 created from the pigment dispersion liquid of the comparative example 4 using the polymer dispersing agent covering pigment which increased the throughput of the comparative polymer dispersing agent-1 did not gelatinize, The particle density and viscosity increased, and the printing density was low. On the other hand, the ink of Example 1 prepared using the pigment dispersion of Example 1 using the polymer dispersant of the AB block polymer has little change over time in particle diameter and viscosity, and has storage stability. It was excellent and the printing density was higher than that of Comparative Use Example 4.

  On the other hand, the ink of Comparative Use Example 1 which is an AB block polymer but has a constitution in which the treatment amount of the polymer dispersant with respect to the pigment is large has increased in particle size and viscosity and has a low printing density. It is considered that this is because the polymer dispersant concentration is too high for the pigment, entanglement between the resins occurs, the pigments aggregate, and the particle size and viscosity increase. Further, since the pigment easily penetrates into the paper and hardly comes out on the surface, the printing density is lowered. In addition, in the case of the ink of Comparative Use Example 2, gelation occurred due to a small amount of the polymer dispersant, and storage stability could not be ensured.

[Synthesis Examples 2 to 4]
In the same manner as in Synthesis Example 1, polymer dispersants 2 to 5 that were AB block polymers with different molecular weights, acid values, and monomer components were prepared. These are summarized in Table 5.

[Comparative Synthesis Examples 2-6, Reference Synthesis Example 1]
Comparative polymer dispersants 2 to 6 which are AB block polymers in which the molecular weight, acid value, and monomer component were changed were prepared in the same manner as in Synthesis Example 1, and are listed in Table 6.

The meanings of the abbreviations in Tables 5 and 6 are as follows.
(1) MMA: methyl methacrylate (2) MAA: methacrylic acid (3) BzMA: benzyl methacrylate (4) CHMA: cyclohexyl methacrylate (5) HEMA: 2-hydroxyethyl methacrylate (6) IBXMA: isobornyl methacrylate (7) IBMA: Isobutyl methacrylate (8) (A): A block unreacted monomer

[Use Examples 2-5, Comparative Use Examples 5-9, Reference Use Example 1]
A pigment and a polymer dispersant were blended in the combinations shown in Table 7, and a pigment dispersion was obtained in the same manner as in Example 1. Further, using each of the obtained pigment dispersions, each of red water-based pigment inks for inkjet 2-5, comparative use examples 5-9 and reference use example 1 of use examples 2-5 in the same manner as use example 1 Ink was obtained.

<Evaluation of ink>
Table 8 shows the storage stability and the results of the printing test for each ink of Use Examples 2 to 5 and Comparative Use Examples 5 to 9.

  As is clear from Table 8, the acid density of the B polymer block was low, and the printing density was increased by appropriately containing a pigment-affinity monomer in the A polymer block. This is because the affinity for the pigment is maintained, while the acid value is low immediately after printing on plain paper, so the affinity for the paper is low and the paper does not easily permeate and the printing density is high. Furthermore, the ink of Use Example 5 prepared from the polymer dispersion 5 having a high number average molecular weight of the B polymer block has the above effect after printing on plain paper, and the B polymer block forms a coating film and penetrates into the paper. It is suppressed and the print density is further increased.

  On the other hand, since the ink of Comparative Use Example 5 had a low total acid value of the polymer dispersant used, the affinity for the ink medium was poor and a dispersion could not be prepared. On the other hand, in the ink of Comparative Use Example 6, the acid value of the polymer dispersant is too high, the affinity to the medium in the ink is increased, the dispersant is liberated from the pigment surface, the particle size is increased, and the storage stability is lowered. did.

In the ink of Comparative Use Example 7, the molecular weight of the used AB block polymer was as small as 5,000 or less, and the respective characteristic effects of the AB block polymer could not be sufficiently exhibited, and the viscosity increased.
In the ink of Comparative Use Example 8, the molecular weight of the B polymer block constituting the used polymer dispersant is too larger than that of the A polymer block, so that the affinity to the medium in the ink is the same as in Comparative Use Example 7. Strengthened, the dispersant was released from the pigment surface, the particle size increased, the shelf life decreased.

In the ink of Comparative Use Example 9, since the PDI of the used AB block polymer was wide, the intended polymer dispersant could not be obtained, and the storage stability and print quality when used as an ink could not be ensured.
As mentioned above, the effect with respect to the composition of the polymer dispersing agent which consists of AB block polymer prescribed | regulated by this invention was confirmed.

  Next, pigments and polymer dispersants were blended in the combinations shown in Table 9, and each red aqueous pigment dispersion was prepared in the same manner as in Example 1 through the dispersion and treatment steps. Further, by using the dispersion, the same red ink composition and method as those in Use Example 1, the red aqueous pigment ink-6, the red aqueous pigment ink-7, the comparative red aqueous pigment ink-10, and the comparative red aqueous pigment ink-11. Got.

  Table 10 shows the storage stability and the evaluation results of the printing test for each of the red inkjet aqueous pigment inks obtained above.

  From the results in Table 10, C.I. I. Similar to the red water pigment ink using CI Pigment Red-122, the pigment is C.I. I. Pigment violet-19 and C.I. I. Even in the aqueous pigment ink changed to Pigment Red-269, the inks of the comparative use examples using the random polymer polymer dispersant are both gelled or thickened and thickened, have poor storage stability and low printing density. there were. On the other hand, the ink of the usage example using the pigment dispersion liquid of the example using the AB block polymer as the polymer dispersant had good storage stability and high printing density.

  Next, pigments and polymer dispersants were blended in the combinations shown in Table 11, and each pigment dispersion was prepared by the same method as in Example 1 through the dispersion / treatment process.

  In addition, water-based pigment inks for ink jets having different ink colors of black, yellow, green, and orange with the same ink composition as in Use Example 1 were prepared from each color aqueous pigment dispersion. At that time, for the blue ink, 26.7 parts of the pigment dispersion, 1.8 parts of BDG, 5 parts of 1,2-hexanediol, 15 parts of glycerin, 1 part of Surfinol 465, and a mixed solution of water 73.3 The inks were prepared in the same manner as in Use Example 1 except for the above.

  Table 12 shows the storage stability and the results of the printing test of each of the aqueous pigment inks for ink jets obtained as described above.

From the results of Table 12, as in the case of the red water-based pigment ink, each of the water-based pigment inks of blue, black, yellow, green and orange has the pigment dispersion liquid of the example using the AB block polymer as the polymer dispersant. The ink used had good storage stability and high printing density. On the other hand, an ink using a pigment dispersion of a comparative example using a conventional random polymer as a polymer dispersant was gelled or increased in particle size and thickened, had poor storage stability, and had a low printing density.
As described above, the blue, black, yellow, green, and orange aqueous pigment inks according to the embodiments of the present invention have the same effects as the red aqueous pigment ink described above, and are defined in the present invention. This ink has excellent long-term storage stability, good density on plain paper, and excellent print quality.

  According to the present invention, an aqueous pigment dispersion for inkjet having excellent long-term storage stability, excellent ejection stability of ink from an inkjet head, high color development on plain paper, and excellent print quality, and the same are used. Ink jet ink is obtained.

Claims (9)

In an aqueous pigment dispersion containing a pigment coated with a polymer dispersant, an aqueous organic solvent, and an alkaline agent,
The content ratio of pigment to polymer dispersant (mass ratio of pigment / block polymer) is 50/50 to 95/5,
The polymer dispersant is a block polymer comprising a methacrylate-based hydrophobic polymer block and a hydrophilic polymer block containing at least 30 to 5% by mass of methacrylic acid,
The dispersity of the block polymer (weight average molecular weight (Mw) / number average molecular weight (Mn): hereinafter abbreviated as PDI) and the PDI of the hydrophobic polymer block constituting the block polymer are both 1.5 or less,
Furthermore, the number average molecular weight of the block polymer is Mnab, the number average molecular weight of the hydrophobic polymer block constituting the block polymer is Mna, and the number average molecular weight of the hydrophilic polymer block constituting the block polymer is Mnb (Mnb = Mnb− Mna), an aqueous pigment dispersion for inkjet which satisfies all the requirements (1) to (4).
(1) 2,000 ≦ Mna ≦ 10,000
(2) 1,500 ≦ Mnb ≦ 10,000
(3) 5,000 ≦ Mab ≦ 20,000
(4) Mna ≧ Mnb or Mna <Mnb ≦ 2Mna   As a constituent component of the hydrophobic polymer block, at least one selected from the group consisting of methyl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, and isobutyl methacrylate, and the constitution of the hydrophilic polymer block The aqueous pigment dispersion for inkjet according to claim 1, further comprising 2-hydroxyethyl methacrylate as a component.   The aqueous pigment dispersion for inkjet according to claim 1 or 2, wherein the hydrophilic polymer block has an acid value of 50 to 250 mgKOH / g and the block polymer has an acid value of 20 to 150 mgKOH / g.   The content ratio is 60/40 to 90/10, and the content ratio of the pigment in 100% by mass of the aqueous pigment dispersion is 30 to 5% by mass. The aqueous pigment dispersion for inkjet according to item.   The pigment has a color index number (CI) pigment blue of 15: 3, 15: 4; I. Pigment red-122, 269; I. Pigment violet-19; I. Pigment yellow-74, 155, 180; C.I. I. Pigment green-36; I. Pigment orange-43; I. The aqueous pigment dispersion for inkjet according to any one of claims 1 to 4, comprising one selected from the group consisting of CI Pigment Black-7 and having an average primary particle size of the pigment of 150 nm or less. .   The aqueous organic solvent is a mixture of water and a water-soluble organic solvent, the ratio of water in the mixture is 81 to 73% by mass, and the water-soluble organic solvent is diethylene glycol monobutyl ether, triethylene glycol mono 6. The method according to claim 1, comprising at least one selected from the group consisting of butyl ether, glycerin, propylene glycol, 1,2-hexanediol, 2-pyrrolidone, and N-methyl-2-pyrrolidone. Aqueous pigment dispersion for inkjet.   The pigment whose surface is coated with a polymer dispersant is a polymer dispersant-coated pigment obtained by mixing at least a polymer dispersant, a pigment, and an aqueous organic solvent and dispersing in water, and then adding an acid. The aqueous pigment dispersion for inkjet according to any one of claims 1 to 6.   An aqueous pigment ink for inkjet comprising the aqueous pigment dispersion for inkjet according to any one of claims 1 to 7, wherein the content of the pigment in the ink is 4 to 10% by mass. .   The aqueous pigment ink for inkjet according to claim 8, further comprising 0.1 to 2% by mass of a surfactant.