Pigment dispersion

Abstract

To provide a pigment dispersion that makes it possible to prepare an aqueous inkjet ink excellent in discharge stability, that is unlikely to generate sediment despite containing an inorganic pigment having a heavy specific gravity, and that is excellent in redispersibility. The pigment dispersion contains an inorganic pigment and a pigment dispersant and is used for preparing the aqueous inkjet ink. The pigment dispersant is an A-B block copolymer. Polymer block A contains 70 mass% or more of a constituent unit (A-1) derived from cyclohexyl methacrylate and a constituent unit (A-2) derived from methyl methacrylate or the like. Polymer block B contains 30-70 mass% of a constituent unit (B-1) derived from methacrylic acid and 20-50 mass% of a constituent unit (B-2) derived from cyclohexyl methacrylate. The amount of polymer block B in the A-B block copolymer is 0.5-1.5 mass parts per mass part of polymer block A.

Description

pigment dispersion

[0001] The present invention relates to a pigment dispersion.

[0002] Dyes have traditionally been used as colorants in aqueous inks for inkjet recording. However, in order to improve the water resistance and light resistance of the recorded images, pigments have begun to be used instead of dyes. To prepare aqueous inks for inkjet recording that contain pigments as colorants, it is necessary to use a pigment dispersion in which the pigment is finely dispersed in order to prevent clogging of the nozzles of the recording head and to improve the color development of the images.

[0003] The white color of an image has traditionally been expressed by the color of the paper itself used as a recording medium. To achieve a whiter white color, there has been a trend toward the use of white inks obtained using white pigment dispersions in which inorganic pigments such as titanium oxide pigments or hollow plastic beads are dispersed. Among these, titanium oxide pigments are used as inorganic pigments with high hiding power.

[0004] In order to maintain a good dispersion state of the pigment in the pigment dispersion and the aqueous ink obtained using the same, pigment dispersions and aqueous inks using various pigment dispersants have been proposed (Patent Documents 1 to 3). Also, an inkjet printer provided with a path for circulating the ink has been proposed (Patent Document 4) in order to suppress sedimentation of the pigment in the ink.

[0005] JP 60-123564 A JP 2009-24165 A JP 2017-39922 A JP 2011-121344 A

[0006] However, even in low-viscosity pigment dispersions obtained by dispersing inorganic pigments such as titanium oxide pigments in an aqueous medium, inorganic pigments tend to settle easily due to their high specific gravity. Furthermore, settled inorganic pigments tend to form hard cakes, making them difficult to re-disperse. Even with the pigment dispersions proposed in Patent Documents 1 to 3, the settling of inorganic pigments was not necessarily sufficiently suppressed, and there was room for improvement in re-dispersibility. Furthermore, the inkjet printer proposed in Patent Document 4 required the use of special components, etc., to improve the device, resulting in a lack of versatility.

[0007] The present invention has been made in view of the problems associated with the conventional techniques, and an object of the present invention is to provide a pigment dispersion liquid which is capable of preparing an aqueous inkjet ink having excellent ejection stability, which is less likely to produce sediment even when it contains an inorganic pigment with a high specific gravity, and has excellent re-dispersibility.

[0008] That is, according to the present invention, there is provided the following pigment dispersion: [1] A pigment dispersion used for preparing an aqueous inkjet ink, which contains an inorganic pigment, a pigment dispersant, water, a water-soluble organic solvent, and an alkali, in which the pigment dispersant is an A-B block copolymer having a polymer block A and a polymer block B, and having a number average molecular weight of 5,000 to 10,000 and a molecular weight distribution (weight average molecular weight / number average molecular weight) of 1.3 to 1.8, in which the polymer block A contains 70% by mass or more of structural units (A-1) derived from cyclohexyl methacrylate and at least one structural unit derived from at least one selected from the group consisting of methyl methacrylate, ethyl methacrylate, t-butyl methacrylate, and benzyl methacrylate. and (A-2), the polymer block B having a number average molecular weight of 3,000 to 6,000 and a molecular weight distribution of 1.2 to 1.6, wherein the polymer block B is a polymer block having a number average molecular weight of 1,000 to 6,000 that contains 30 to 70% by mass of structural units (B-1) derived from methacrylic acid, 20 to 50% by mass of structural units (B-2) derived from cyclohexyl methacrylate, and structural units (B-3) derived from at least one member selected from the group consisting of methyl methacrylate, ethyl methacrylate, t-butyl methacrylate, and benzyl methacrylate, and the amount of the polymer block B in the A-B block copolymer is 0.5 to 1.5 parts by mass per 1 part by mass of the polymer block A. [2] The pigment dispersion according to [1], wherein the content of the inorganic pigment is 50 to 70% by mass, the content of the pigment dispersant is 2.5 to 10 parts by mass relative to 100 parts by mass of the inorganic pigment, the average particle diameter of the inorganic pigment is 180 to 300 nm, and the viscosity at 25° C. is 5 to 20 mPa·s. [3] The pigment dispersion according to [1] or [2], wherein the inorganic pigment is a titanium oxide pigment surface-treated with alumina. [4] The pigment dispersion according to any one of [1] to [3], wherein the solids content (% by mass) of the upper layer after storage at 70° C. for one week is 70% or more based on the solids content (% by mass) before storage.

[0009] According to the present invention, it is possible to provide a pigment dispersion that is capable of preparing an aqueous inkjet ink with excellent ejection stability, that is less likely to produce sediment even when it contains an inorganic pigment with a high specific gravity, and that has excellent re-dispersibility.

[0010] <Pigment Dispersion> Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. One embodiment of the pigment dispersion of the present invention is used to prepare an aqueous inkjet ink, and contains an inorganic pigment, a pigment dispersant, water, a water-soluble organic solvent, and an alkali. Hereinafter, the pigment dispersion of this embodiment will be described in detail.

[0011] (Inorganic Pigment) As the inorganic pigment, any of the conventionally known inorganic pigments can be used. Examples of the inorganic pigment include titanium oxide pigments; zinc oxide pigments; iron oxide pigments such as ochre and red iron oxide; zirconium oxide, copper oxide, yttria, molybdenum oxide, indium oxide, tin oxide, tungsten oxide, bismuth oxide, antimony oxide, and composite oxide pigments thereof; composite oxides of alkali metals such as lithium, sodium, and potassium with iron, manganese, cobalt, and nickel; composite oxides of alkaline earth metals such as magnesium, strontium, and calcium with iron, manganese, cobalt, and nickel; composite oxide pigments of cobalt, chromium, titanium, and aluminum; natural minerals such as clay and mica; and hollow silica pigments.

[0012] Examples of the shape of the inorganic pigment include a plate shape such as a glass plate, an irregular shape, and a particulate shape. From the viewpoint of the ejection properties of the inkjet ink, the inorganic pigment is preferably particulate in shape. Furthermore, the average particle diameter of the primary particles of the particulate inorganic pigment is preferably 50 to 300 nm. In this specification, "average particle diameter" refers to the average particle diameter (number average particle diameter) of 100 or more primary particles of the inorganic pigment, measured and calculated using a transmission electron microscope.

[0013] The inorganic pigment may be previously surface-treated with a surface treatment agent, such as inorganic treatments including silica treatment, alumina treatment, silica-alumina treatment, and zirconia treatment; organic acid treatments including oleic acid treatment; and conventionally known silane coupling agent treatments and silazane treatments.

[0014] As an inorganic pigment for expressing white in inkjet printing, it is preferable to use a white pigment, and it is more preferable to use a titanium oxide pigment with excellent hiding power. As the titanium oxide pigment, any titanium oxide pigment can be used, regardless of the crystal system such as rutile type, anatase type, or brookite type, or the manufacturing method such as the sulfuric acid method or the chlorine method. Among them, it is preferable to use a rutile type titanium oxide pigment from the viewpoints of stability and availability.

[0015] Titanium oxide pigments that have been inorganically or organically treated, such as silica-treated, alumina-treated, zirconia-treated, zinc-treated, silica-alumina-treated, silane coupling agent-treated, and alkanoic acid-treated, can also be used. Among these, titanium oxide pigments that have been surface-treated with alumina are preferred. Examples of alumina surface treatments include alumina treatment, silica-alumina treatment, and zirconia-alumina treatment. The use of titanium oxide pigments that have been surface-treated with alumina improves the adsorption of pigment dispersants, further enhancing the dispersion stability of inorganic pigments.

[0016] From the viewpoint of enhancing the hiding power of the white color, the average particle size of the primary particles of the titanium oxide pigment is preferably 50 to 300 nm, and more preferably 100 to 280 nm. If the average particle size of the titanium oxide pigment is less than 50 nm, the hiding power may be somewhat insufficient and a transparent appearance may be obtained. On the other hand, if the average particle size of the titanium oxide pigment is more than 300 nm, the particles may not be easily refined even after dispersion treatment and may easily clog filters or recording heads. If the average particle size is too large, the particles may be more likely to settle, resulting in a slight decrease in dispersion stability.

[0017] (Pigment Dispersant) In conventional pigment dispersions, the storage stability of pigment dispersants dissolved in water was not necessarily good. As a result, the ejection stability, resolubility, and low viscosity of inkjet inks prepared using such pigment dispersions were often insufficient. Furthermore, inorganic pigments such as titanium oxide pigments tend to settle, and the settled inorganic pigments sometimes form hard cakes, making them difficult to re-disperse. Consequently, it was difficult to sufficiently improve the performance of inkjet inks prepared using such pigment dispersions. After extensive investigation, the present inventors discovered that a pigment dispersion capable of preparing an aqueous inkjet ink with sufficient performance can be obtained by using an A-B block copolymer having a specific block structure as the pigment dispersant. Specifically, the pigment dispersant used in the pigment dispersion of this embodiment is an A-B block copolymer having a polymer block A (polymer chain A) and a polymer block B (polymer chain B).

[0018] The polymer block A is a water-insoluble polymer block. On the other hand, the polymer block B is a polymer block having a structural unit (B-1) derived from methacrylic acid, in which at least a portion of the carboxyl groups are neutralized with an alkali to become water-soluble. Because the polymer block A is a water-insoluble polymer block, it is highly hydrophobic and easily interacts hydrophobically with a water-insoluble inorganic pigment. For this reason, the polymer block A is adsorbed to the inorganic pigment by hydrogen bonding or the like, and is hardly detached from the inorganic pigment. Furthermore, because the polymer block A is adsorbed to the inorganic pigment and the polymer block B is soluble in water, the finely dispersed inorganic pigments sterically repel each other, maintaining the finely dispersed state for a long period of time.

[0019] Furthermore, both polymer block A and polymer block B contain structural units derived from cyclohexyl methacrylate. The action of the cyclohexyl group can improve the dispersion stability, sedimentation stability, and sedimentation recovery of the inorganic pigment over a long period of time. Polymer block A adsorbs to the inorganic pigment due to the action of the hydrophobic cyclohexyl group, and is a water-insoluble polymer block, so it is difficult to detach from the inorganic pigment. While polymer block B is a polymer block that dissolves in water, the cyclohexyl group prevents the molecular chain from spreading into water. As a result, the inorganic pigment becomes encapsulated in the pigment dispersant, which is an A-B block copolymer.

[0020] The polymer block A contains a structural unit (A-1) derived from cyclohexyl methacrylate. Because the cyclohexyl group is highly hydrophobic, it adsorbs well to inorganic pigments such as titanium oxide pigments. Among cycloalkyl groups, the cyclohexyl group is stable, has a small molecular weight, and is commercially available. By using the cyclohexyl group, a large number of cycloalkyl groups can be introduced into the polymer block A. Furthermore, by introducing a large number of cycloalkyl groups into the polymer block A, the solubility in an aqueous medium containing a water-soluble organic solvent can be reduced. Furthermore, the glass transition point (Tg) can be increased (for example, to 50°C or higher), thereby suppressing detachment from the inorganic pigment. Note that methacrylates having cycloalkyl groups other than cyclohexyl groups (for example, cyclohexyl groups having an alkyl group, or cyclopolycyclic rings such as tricyclodecyl groups and isobornyl groups) may be too hydrophobic, making it difficult to introduce a large number of cycloalkyl groups into the polymer block A.

[0021] The content of the structural unit (A-1) derived from cyclohexyl methacrylate in the polymer block A is 70% by mass or more, and preferably 75 to 85% by mass. If the content of the structural unit (A-1) is less than 70% by mass, the hydrophobicity of the polymer block A decreases, and the structural unit (A-1) may be easily detached from the inorganic pigment or dissolved in an aqueous medium, resulting in a decrease in the dispersion stability of the inorganic pigment.

[0022] The polymer block A contains a structural unit (A-2) derived from at least one selected from the group consisting of methyl methacrylate, ethyl methacrylate, t-butyl methacrylate, and benzyl methacrylate. Among these, the structural unit (A-2) is preferably derived from at least one selected from the group consisting of methyl methacrylate, which has a small carbon number and is therefore less likely to interfere with the properties of the polymer block A, and benzyl methacrylate, which has an aromatic ring and therefore has high adsorption to inorganic pigments. It is preferable that the polymer block A is substantially composed of only the structural unit (A-1) and the structural unit (A-2). By including such a structural unit (A-2), the hydrophobicity of the cyclohexyl group, the solubility of the polymer block A in aqueous media, and the glass transition temperature (Tg) of the polymer block A are maintained, thereby enhancing the adsorption of the polymer block A to inorganic pigments. Using a methacrylate other than methyl methacrylate, ethyl methacrylate, t-butyl methacrylate, and benzyl methacrylate may result in a decrease in the glass transition temperature (Tg) of the polymer block A or excessive increase in affinity for water-soluble organic solvents. This may make it easier for the polymer block A to be detached from the inorganic pigment, resulting in a slight decrease in dispersion stability.

[0023] Furthermore, when polymer block A contains structural units derived from a hydrophilic group-containing methacrylate, such as a hydroxyl group-containing methacrylate such as 2-hydroxyethyl methacrylate, an ether group-containing methacrylate such as tetrahydrofurfuryl methacrylate and polyethylene glycol monomethyl methacrylate, or an amino group-containing methacrylate such as dimethylaminoethyl methacrylate, the hydrophobicity of polymer block A is likely to decrease. This may result in a decrease in the adsorption of the pigment dispersant to the inorganic pigment, which may lead to detachment of the pigment dispersant from the inorganic pigment in an aqueous medium and a decrease in the dispersibility of the inorganic pigment.

[0024] The number-average molecular weight of the polymer block A is 3,000 to 6,000, and preferably 3,500 to 4,500. If the number-average molecular weight of the polymer block A is less than 3,000, the molecular weight is too small, resulting in poor adsorption to inorganic pigments. On the other hand, if the number-average molecular weight of the polymer block A exceeds 6,000, it may be difficult for the polymer block A to be well adsorbed to inorganic pigments.

[0025] The molecular weight distribution (weight average molecular weight / number average molecular weight) of the polymer block A is 1.2 to 1.6, preferably 1.25 to 1.55. That is, the molecular weight of the polymer block A is relatively uniform. It is industrially and practically difficult to produce a polymer block A having a molecular weight distribution of less than 1.2. On the other hand, if the molecular weight distribution of the polymer block A exceeds 1.6, the dispersion stability of the inorganic pigment decreases. The number average molecular weight and weight average molecular weight in this specification are values ​​measured by gel permeation chromatography (GPC) in terms of polystyrene.

[0026] The polymer block B contains a structural unit (B-1) derived from methacrylic acid. The content of the structural unit (B-1) derived from methacrylic acid in the polymer block B is 30 to 70% by mass, and preferably 35 to 65% by mass. If the content of the structural unit (B-1) is less than 30% by mass, the hydrophilicity of the polymer block B will be insufficient, resulting in a decrease in the dispersion stability of the inorganic pigment in the aqueous medium, as well as a decrease in redispersibility and sedimentation recovery. On the other hand, if the content of the structural unit (B-1) is more than 70% by mass, the hydrophilicity of the polymer block B will be too high. As a result, the solubility in the aqueous medium will increase, the viscosity of the prepared ink will excessively increase, and the dispersion stability of the inorganic pigment will decrease.

[0027] The polymer block B contains a structural unit (B-2) derived from cyclohexyl methacrylate. The content of the structural unit (B-2) derived from cyclohexyl methacrylate in the polymer block B is 20 to 50% by mass, preferably 30 to 40% by mass. By setting the content of the structural unit (B-2) in the polymer block B within the above range, the polymer block B can be made a water-soluble polymer block, and can be endowed with affinity for water while maintaining hydrophobicity. This allows the inorganic pigment to be encapsulated by the adsorbed pigment dispersant, thereby exhibiting redispersibility and sedimentation recovery properties. If the content of the structural unit (B-2) is less than 20% by mass, it becomes difficult to exert the effects of the cyclohexyl group. On the other hand, if the content of the structural unit (B-2) is more than 50% by mass, even if a large number of carboxy groups are present, water solubility will be insufficient, and the dispersion stability of the inorganic pigment will be reduced.

[0028] Polymer block B contains a structural unit (B-3) derived from at least one selected from the group consisting of methyl methacrylate, ethyl methacrylate, t-butyl methacrylate, and benzyl methacrylate. Of these, structural unit (B-3) is preferably a structural unit derived from at least one selected from the group consisting of methyl methacrylate and benzyl methacrylate. It is preferable that polymer block B be composed essentially of only structural units (B-1), (B-2), and (B-3).

[0029] The number-average molecular weight of polymer block B is 1,000 to 6,000, preferably 1,500 to 4,500. The number-average molecular weight (MnB) of polymer block B is the total number-average molecular weight (Mn) of the A-B block copolymer minus the number-average molecular weight (MnA) of polymer block A (MnB = Mn - MnA). If the number-average molecular weight of polymer block B is less than 1,000, the molecular weight of the water-soluble polymer block is small, resulting in insufficient dispersibility of the inorganic pigment. On the other hand, if the number-average molecular weight of polymer block B is more than 6,000, the proportion of the water-soluble polymer block becomes excessive, resulting in an excessive increase in viscosity and a decrease in water resistance. That is, even if polymer block A is adsorbed to the inorganic pigment, the water-soluble polymer block B is excessively large, which promotes detachment of the A-B block copolymer (pigment dispersant) from the inorganic pigment, resulting in unstable dispersibility of the inorganic pigment.

[0030] The number-average molecular weight of the A-B block copolymer (pigment dispersant) is 5,000 to 10,000. If the number-average molecular weight of the A-B block copolymer is less than 5,000, it tends to be easily detached from the inorganic pigment. On the other hand, if the number-average molecular weight of the A-B block copolymer exceeds 10,000, the viscosity may increase excessively during polymerization or the viscosity of the pigment dispersion may increase excessively.

[0031] The molecular weight distribution of the A-B block copolymer (weight average molecular weight / number average molecular weight) is 1.3 to 1.8, and preferably 1.35 to 1.6. If the molecular weight distribution of the A-B block copolymer exceeds 1.8, many copolymers having number average molecular weights outside the range will be included, resulting in a decrease in the dispersibility of inorganic pigments.

[0032] The amount of polymer block B in the A-B block copolymer is 0.5 to 1.5 parts by mass, preferably 0.75 to 1.25 parts by mass, per part by mass of polymer block A. If the amount of polymer block B in the A-B block copolymer is less than 0.5 parts by mass per part by mass of polymer block A, the hydrophilicity of the A-B block copolymer will be insufficient, and the A-B block copolymer itself will become particulate in the aqueous medium, resulting in insufficient adsorption to the inorganic pigment. On the other hand, if the amount of polymer block B in the A-B block copolymer is more than 1.5 parts by mass per part by mass of polymer block A, even if the polymer block A is adsorbed to the inorganic pigment, the polymer block B will be large and will therefore be prone to detachment, resulting in reduced dispersion stability of the inorganic pigment.

[0033] (Method for Producing A-B Block Copolymer) An A-B block copolymer can be produced by conventionally known living radical polymerization. While it is difficult to obtain a copolymer having a block structure by ordinary radical polymerization, an A-B block copolymer having the desired block structure can be obtained by living radical polymerization.

[0034] Examples of this living radical polymerization method include the nitroxide method (NMP method) which utilizes the dissociation and bonding of amine oxide radicals; the atom transfer radical polymerization method (ATRP method) which uses heavy metals such as copper, ruthenium, nickel, and iron and ligands which form complexes with these heavy metals, and polymerizes using a halogen compound as an initiator compound; the reversible addition-fragmentation chain transfer polymerization method (RAFT method) which uses a dithiocarboxylic acid ester or the like as an initiator compound and polymerizes using an addition-polymerizable monomer and a radical initiator; the TERP method which uses an organotellurium compound as an initiator compound and ditelluride as a catalyst; and the reversible transfer catalyst polymerization method (RTCP method, RCMP method) which uses at least one of iodine and an iodine compound as a polymerization initiator compound and a commercially available organic compound which can form radicals as a catalyst. Among these, the TERP method, the RTCP method, and the RAFT method are suitable for controlling the molecular weight of a methacrylate polymer and forming a block structure, and the RTCP method, which uses inexpensive raw materials and an organic compound as a catalyst, is particularly preferred.

[0035] After forming polymer block A, the monomers for forming polymer block B are added to form polymer block B, thereby obtaining the target methacrylate polymer. Alternatively, polymer block A may be formed after polymer block B is formed.

[0036] The A-B block copolymer is preferably synthesized by solution polymerization. The solvent used during solution polymerization is preferably the same as the water-soluble organic solvent used in the aqueous inkjet ink. Examples of water-soluble organic solvents include alcohol-based solvents such as methanol, ethanol, and isopropanol; polyhydric glycol-based solvents such as ethylene glycol, propylene glycol, dipropylene glycol, glycerin, and 1,2-hexanediol; glycol ether-based solvents such as propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, propylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, and tripropylene glycol monomethyl ether; amide-based solvents such as pyrrolidone and 3-methoxy-N,N-dimethylpropionamide; and urea-based solvents such as tetramethylurea.

[0037] (Dispersion Medium) The pigment dispersion of this embodiment contains a liquid medium (aqueous medium) containing water and a water-soluble organic solvent as a dispersion medium for the inorganic pigment. Examples of water that can be used include pure water, ion-exchanged water, and soft water. Examples of water-soluble organic solvents include alcohol-based solvents such as methanol, ethanol, and isopropanol; polyhydric glycol-based solvents such as ethylene glycol, propylene glycol, dipropylene glycol, glycerin, and 1,2-hexanediol; glycol ether-based solvents such as propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, propylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, and tripropylene glycol monomethyl ether; amide-based solvents such as pyrrolidone and 3-methoxy-N,N-dimethylpropionamide; and urea-based solvents such as tetramethylurea. Among these, propylene glycol, glycerin, 1,2-hexanediol, diethylene glycol monobutyl ether, and triethylene glycol monobutyl ether are preferred.

[0038] (Alkali) The pigment dispersion of this embodiment contains an alkali. The carboxyl groups in the polymer block B are neutralized and ionized by the alkali. This makes the polymer block B hydrophilic, allowing the inorganic pigment to be dispersed using the pigment dispersant. Any conventionally known alkali can be used as the alkali. Examples of the alkali include ammonia; organic amines such as dimethylaminoethanol and triethanolamine; and alkali metal hydroxides such as sodium hydroxide and potassium hydroxide.

[0039] The amount of alkali is preferably an amount that neutralizes the carboxy groups in the pigment dispersant (A-B block copolymer) to a neutralization rate of 110% or less. If the neutralization rate exceeds 110%, the residual alkali may cause the pH of the pigment dispersion or ink to become too high, which may lead to deterioration of components that constitute the printer, etc.

[0040] After forming the A-B block copolymer by solution polymerization, it is preferable to add an alkaline aqueous solution to the polymerization reaction system to neutralize the formed A-B block copolymer and turn it into an aqueous solution, which allows the formed A-B block copolymer to be used as is in the pigment dispersion liquid without being removed from the polymerization reaction system.

[0041] (Other Additives) The pigment dispersion of this embodiment may contain other additives as needed. Examples of other additives include organic solvents other than the water-soluble organic solvents described above, surfactants, preservatives, leveling agents, surface tension adjusters, pH adjusters, UV absorbers, light stabilizers, antioxidants, dyes, fillers, waxes, thickeners, antifoaming agents, mildew inhibitors, antistatic agents, and binder components. Examples of surfactants include silicone-based, acetylene glycol-based, fluorine-based, alkylene oxide-based, and hydrocarbon-based surfactants. Examples of preservatives include sodium benzoate, benzimidazole, thiabendazole, potassium sorbitanate, sodium sorbitanate, sodium dehydroacetate, thiazosulfamide, and pyridine thiol oxide.

[0042] (Pigment Dispersion) The content of inorganic pigment in the pigment dispersion is preferably 50 to 70% by mass, and more preferably 55 to 65% by mass. By achieving such a high content of inorganic pigment, the amount of pigment dispersion to be blended when preparing an aqueous inkjet ink can be reduced, thereby increasing the degree of freedom in ink formulation design. If the content of inorganic pigment in the pigment dispersion is less than 50% by mass, the ink production cost increases and the degree of freedom in ink design tends to decrease. On the other hand, if the content of inorganic pigment exceeds 70% by mass, the ink tends to become highly viscous, making it difficult to disperse.

[0043] The content of the pigment dispersant in the pigment dispersion is preferably 2.5 to 10 parts by mass, and more preferably 3 to 7.5 parts by mass, per 100 parts of inorganic pigment. If the content of the pigment dispersant is too low, the dispersion stability of the inorganic pigment may be slightly reduced. On the other hand, if the content of the pigment dispersant is too high, components that do not contribute to dispersion will be present in excess, which may increase the viscosity of the pigment dispersion or ink. Furthermore, the excess pigment dispersant may easily become a binder component of the hard cake formed by sedimentation of the inorganic pigment, which may reduce redispersibility and sedimentation recovery.

[0044] The pigment dispersion can be produced according to a conventionally known method. Conventional mixers, stirrers, and dispersers can be used for mixing, stirring, and dispersion. Examples of stirrers include dissolvers and homogenizers. Examples of mixers and dispersers include kneaders, attritors, ball mills, vertical media dispersers and horizontal media dispersers loaded with glass beads or zirconia beads, colloid mills, jet mills, high-pressure homogenizers, and ultrasonic dispersers. Furthermore, it is preferable to use beads of 1 mm or less as the media loaded into the media disperser. By using such beads as media, the crystals and shape of the inorganic pigment are less likely to be destroyed, enabling so-called soft dispersion.

[0045] It is preferable to disperse the inorganic pigment into primary particles by the above-mentioned mixing, stirring, and dispersion treatment. However, the inorganic pigment may be aggregated to the required particle size. The dispersion state of the inorganic pigment can be confirmed by conventionally known methods. For example, it can be confirmed using an optical microscope or electron microscope, by measuring with a particle size distribution measuring device such as light scattering, or by measuring absorbance using a spectrophotometer. After the dispersion treatment, coarse particles may be removed by centrifugal filtration or filter filtration. Thereafter, other additives may be added as necessary to obtain a pigment dispersion.

[0046] The average particle size of the inorganic pigment in the pigment dispersion is preferably 180 to 300 nm, and more preferably 200 to 280 nm. By adjusting the average particle size of the inorganic pigment in the pigment dispersion within the above range, the hiding power of the resulting ink can be improved. If the average particle size of the inorganic pigment is less than 180 nm, the hiding power may be insufficient. On the other hand, if the average particle size of the inorganic pigment is more than 300 nm, clogging of the filter or recording head may occur. Furthermore, the inorganic pigment is more likely to settle in the pigment dispersion or ink, and the smoothness and sharpness of the resulting image (printed matter) may be reduced. The average particle size of the inorganic pigment in the pigment dispersion is the number-average particle size measured using a particle size measuring device that uses light scattering.

[0047] The viscosity of the pigment dispersion at 25°C is preferably 5 to 20 mPa·s, and more preferably 5 to 15 mPa·s. If the viscosity of the pigment dispersion is less than 5 mPa·s, sedimentation of the inorganic pigment may be easily promoted. On the other hand, if the viscosity of the pigment dispersion is more than 20 mPa·s, the viscosity of the prepared ink may be easily increased.

[0048] The surface tension of the pigment dispersion at 25° C. is preferably 20 to 40 mN / m. The pH of the pigment dispersion at 25° C. is preferably 7.5 to 10.0, and more preferably 8.0 to 9.9.

[0049] The pigment dispersion of this embodiment is resistant to sedimentation even though it contains an inorganic pigment with a high specific gravity. Specifically, the solids content (% by mass) of the upper layer after storage at 70°C for one week is preferably 70% or more, and more preferably 80% or more, based on the solids content (% by mass) before storage. Here, the "upper layer" refers to the portion above half the height of the pigment dispersion after storage.

[0050] Furthermore, the pigment dispersion of this embodiment is unlikely to produce sediment even after a long period of time has passed, and even if sediment does form, it can be returned to its original dispersed state by a simple operation such as stirring. It is believed that the use of an A-B block copolymer having a polymer block B with controlled water solubility as a pigment dispersant encapsulates the inorganic pigment and places ionized carboxy groups on the surface of the inorganic pigment, thereby improving sedimentation stability and sedimentation recovery.

[0051] (Use of Pigment Dispersion) The pigment dispersion of this embodiment is useful as a pigment dispersion to be incorporated into an aqueous inkjet ink. The printing method using the aqueous inkjet ink is not particularly limited, and examples thereof include a thermal method and a piezo method. The type of inkjet printer is also not particularly limited, and the inkjet printer can be used in an office inkjet printer, an industrial inkjet printer, an inkjet printer for textile printing, a high-speed printer, and the like.

[0052] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the examples and comparative examples, "parts" and "%" are by mass unless otherwise specified.

[0053] Synthesis of Pigment Dispersant (Synthesis Example 1) 141.3 parts of diethylene glycol monobutyl ether (BDG), 1.0 part of iodine, 3.8 parts of 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70, manufactured by Fujifilm Corporation), 44.1 parts of cyclohexyl methacrylate (CHMA), 18.5 parts of benzyl methacrylate (BzMA), and 0.05 parts of N-iodophthalimide (NIS) were placed in a reaction vessel. The mixture was heated to 45°C and stirred under a nitrogen stream, and polymerized for 4 hours to form polymer block A (polymer). A portion of the resulting liquid was sampled, and the polymerization conversion calculated from the nonvolatile content that reached a constant weight at 180°C was approximately 100%. The sampled liquid was dissolved in acetone and analyzed by gas chromatography, revealing that almost no monomer was detected. The molecular weight of the polymer was measured by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as a developing solvent. As a result, the number average molecular weight (Mn) of the polymer in terms of polystyrene was 4,700, and the molecular weight distribution (PDI = weight average molecular weight (Mw) / number average molecular weight (Mn)) was 1.31.

[0054] After cooling to 40°C, 11.8 parts of CHMA, 13.9 parts of MMA, and 22.5 parts of methacrylic acid (MAA) were added, and polymerization was carried out for 4 hours to form polymer block B, thereby obtaining an A-B block copolymer. It was confirmed that the polymerization system had increased in viscosity.

[0055] The Mn measured by GPC was 9,100, and the PDI was 1.39. Since the molecular weight increased compared to polymer block A, it is believed that an A-B block copolymer was formed. The Mn of polymer block B ("Mn of A-B block copolymer" - "Mn of polymer block A") was 4,400. The ratio of the amount of monomers constituting polymer block A to the amount of monomers constituting polymer block B (mass ratio = A / B) was calculated to be 1 / 0.77.

[0056] Next, a mixture of 10.7 parts of sodium hydroxide and 245.5 parts of ion-exchanged water (aqueous sodium hydroxide solution) was added to form an aqueous solution, thereby obtaining a transparent, viscous solution of pigment dispersant D-1. The resulting solution had a solids content of 22.9% and a pH of 9.9.

[0057] (Synthesis Examples 2 to 9) Solutions of pigment dispersants D-2 to D-9 were obtained in the same manner as in Synthesis Example 1, except that the types and amounts (unit: parts) of various materials shown in Tables 1 to 3 were used. The meanings of the abbreviations in the table are as follows: tBMA: t-butyl methacrylate EMA: ethyl methacrylate PPG: propylene glycol monopropyl ether

[0058]

[0059]

[0060]

[0061] Comparative Synthesis Example 1 An A-B block copolymer was formed in the same manner as in Synthesis Example 4 above, except that isobornyl methacrylate was used instead of BzMA. The resulting A-B block copolymer had an Mn of 9,200 and a PDI of 1.32. The Mn of polymer block B was 4,100. When an aqueous sodium hydroxide solution was added in the same manner as in Synthesis Example 4 above, the solution became cloudy and some of it precipitated. This is thought to be because the polymer block A was so hydrophobic that it could not be made into an aqueous solution. From the above, it was determined that the resulting A-B block copolymer could not be used as a pigment dispersant.

[0062] Comparative Synthesis Example 2: 141.3 parts of BDG were placed in a reaction vessel and heated to 80°C. A mixture of 55.9 parts of CHMA, 18.5 parts of BzMA, 13.9 parts of MMA, 22.5 parts of MAA, 3 parts of azobisisobutyronitrile, and 2.0 parts of thioglycerol was placed in a dropping device, and after adding one-third of the mixture, the remaining amount was added dropwise over 2 hours. Polymerization was carried out at 80°C for 7 hours to form a random copolymer. The resulting random copolymer had an Mn of 8,600 and a PDI of 1.89. Next, aqueous sodium hydroxide solution was added in the same manner as in Synthesis Example 1, to obtain a transparent, viscous solution of pigment dispersant HD-1. The resulting solution had a solids content of 22.9% and a pH of 9.8.

[0063] (Comparative Synthesis Examples 3 to 6) Solutions of pigment dispersants HD-2 to HD-5 were obtained in the same manner as in Synthesis Example 1, except that the types and amounts (unit: parts) of various materials shown in Table 4 were used. The abbreviations in the table have the following meanings: BMA: butyl methacrylate

[0064]

[0065] <Preparation of Pigment Dispersion (1)> (Example 1) 270 parts of water and 100 parts of a pigment dispersant D-1 solution were mixed and homogenized, followed by the addition of 15 parts of BDG and 15 parts of PPG. 400 parts of C.I. Pigment White 6 (trade name "JR-600A", manufactured by Teika Corporation, primary average particle size: 0.25 μm, alumina surface treated) were added, and the mixture was thoroughly stirred and mixed using a dissolver to obtain a mixture. The resulting mixture (mill base) was dispersed using a horizontal media disperser (trade name "Dynomill 0.6 Liter ECM Type", manufactured by Shinmaru Enterprises, zirconia bead diameter: 0.5 mm) at a peripheral speed of 7 m / s to thoroughly disperse the pigment. The mixture was filtered through a 10 μm membrane filter to remove coarse particles, yielding a white pigment dispersion W-1 with a pigment concentration of 50%. The amount of pigment dispersant per 100 parts of pigment was 5.75 parts. The average particle size of the pigment was 281 nm, and the viscosity was 9.9 mPa·s. The solid content calculated from the residue obtained by heating in a thermostatic chamber at 150° C. until a constant weight was reached was 52.7%.

[0066] (Examples 2 to 9, Comparative Examples 1 to 5) Pigment dispersions W-2 to 9 and HW-1 to 5 were obtained in the same manner as in Example 1, except that solutions of the types of pigment dispersants shown in Tables 5-1 and 5-2 were used.

[0067] <Evaluation> (Sedimentation Stability) The pigment dispersion was poured into a screw-capped test tube with a diameter of 10 mm and a length of 150 mm to a height of 100 mm and stored in a thermostatic chamber at 70°C for one week. After storage, a portion of the upper layer (above half the height) was sampled and the solid content was measured. The solid content retention rate was calculated using the following formula (1), and the sedimentation stability of the pigment dispersion was evaluated according to the following evaluation criteria. The results are shown in Tables 5-1 and 5-2. "Solid content retention rate (%)" = {(Solid content of upper layer after storage) / (Solid content immediately after production)} × 100 (1) ⊚: Solid content retention rate was 90% or more and 100% or less. ◯: Solid content retention rate was 70% or more and less than 90%. △: Solid content retention rate was 50% or more and less than 70%. ×: Solid content retention rate was less than 50%.

[0068] (Sedimentation Recovery Property) The pigment dispersion was poured into a screw-capped test tube with a diameter of 10 mm and a length of 150 mm to a height of 100 mm and stored in a thermostatic chamber at 70°C for one week. After storage, the tube was shaken at 150 rpm for 15 seconds using a mini shaker, and a portion was sampled and measured for solid content. The sedimentation recovery property of the pigment dispersion was then calculated using the following formula (2), and the sedimentation recovery property was evaluated according to the following evaluation criteria. The results are shown in Tables 5-1 and 5-2. "Sedimentation Recovery Property (%)" = {(Solid content after shaking) / (Solid content immediately after production)} × 100 (2) ○: Sedimentation recovery property was 90% or more and 100% or less. ×: Sedimentation recovery property was less than 90%.

[0069] (Redispersibility) The viscosity and average particle size of the pigment dispersion after shaking obtained in the evaluation of "sedimentation recovery" above were measured. Then, the redispersibility of the pigment dispersion was evaluated according to the evaluation criteria shown below. The results are shown in Tables 5-1 and 5-2. ∘: The average particle size after shaking was within ±20 nm of the average particle size immediately after production, and the viscosity change was less than ±1 mPa s. ×: The average particle size after shaking exceeded the range of ±20 nm of the average particle size immediately after production, or the viscosity change was ±1 mPa s or more.

[0070] (Ejection Stability) 16 parts of the pigment dispersion, 16.8 parts of the binder component, 5 parts of 1,2-hexanediol, 13 parts of propylene glycol, 1 part of a surfactant (trade name "Surfynol 465", manufactured by Nissin Chemical Co., Ltd.), and 49.2 parts of water were mixed, thoroughly stirred, and then filtered through a membrane filter with a pore size of 10 μm to prepare inkjet inks IW-1 to 9 and HIW-1 to 5. The polymethacrylate emulsion (emulsion containing polymethacrylate, solid content: 30.8%) described in "Example 14" of Japanese Patent No. 6967168 was used as the binder component. The resulting inks were each filled into cartridges and installed in an inkjet printer (trade name "MMP-813", manufactured by Mastermind), and a solid image was printed on a PET film (polyethylene terephthalate film, manufactured by Futamura Chemical Co., Ltd., thickness: 60 μm). The ink ejection performance during printing was visually confirmed, and the ink ejection stability was evaluated according to the following evaluation criteria. The results are shown in Tables 5-1 and 5-2. ◯: Ejection was successful without any problems, and a good image was printed. △: Scattering of minute droplets was observed. ×: When ejecting, droplets splashed and scattered, causing a disturbance to the image.

[0071] (Image Whiteness) The print obtained in the above evaluation of "ejection stability" was placed on black stripe-shaped paper. Then, using a spectrophotometer, the L value of the image in the area corresponding to the black stripe was measured, and the whiteness of the image was evaluated according to the evaluation criteria shown below. The higher the L value, the better the hiding power can be evaluated. The results are shown in Tables 5-1 and 5-2. ⊚: The L value was 75 or more. ◯: The L value was 60 or more and less than 75. ×: The L value was less than 60.

[0072]

[0073]

[0074] <Preparation of Pigment Dispersion (2)> (Example 10) A brown pigment dispersion with a pigment concentration of 50% was obtained in the same manner as in Example 1 described above, except that a red iron oxide pigment (product name "Todacolor 120ED", manufactured by Toda Kogyo Co., Ltd., average primary particle size: 0.14 μm) was used instead of C.I. Pigment White 6. The average particle size of the pigment was 290 nm, the viscosity was 15.6 mPa·s, and the solid content was 52.6%. Furthermore, when the "sedimentation stability" described above was evaluated, the solid content of the upper layer was 40.6%, and the solid content maintenance rate was 77.2%. This confirmed that the dispersion exhibited good sedimentation stability.

[0075] Example 11 A black pigment dispersion with a pigment concentration of 50% was obtained in the same manner as in Example 1, except that a black composite oxide (trade name "Dipyroxide TM3550", manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., average primary particle size: 60 nm) was used instead of C.I. Pigment White 6. The average particle size of the pigment was 198 nm, the viscosity was 18.4 mPa·s, and the solid content was 52.6%. Furthermore, when the "sedimentation stability" described above was evaluated, the solid content of the upper layer was 48.9%, and the solid content maintenance rate was 92.9%. This confirmed that the dispersion exhibited good sedimentation stability.

[0076] The pigment dispersion of the present invention is useful as a pigment dispersion used for preparing aqueous inkjet inks. Furthermore, inks prepared using the pigment dispersion of the present invention have excellent ejection stability, and when a white inorganic pigment such as a titanium oxide pigment is used, they are capable of recording images with high whiteness. Therefore, inks prepared using the pigment dispersion of the present invention are suitable not only for personal and office use, but also for industrial inkjet printing such as packaging film printing and high-speed printing. They are also applicable to printing inks such as aqueous paints for automobiles or building materials, water-based stationery, water-based gravure inks, and water-based flexo inks.

Claims

1. A pigment dispersion liquid used to prepare an aqueous inkjet ink, which contains an inorganic pigment, a pigment dispersant, water, a water-soluble organic solvent, and an alkali, wherein the pigment dispersant is an A-B block copolymer consisting of polymer block A and polymer block B, which has a number average molecular weight of 5,000 to 10,000 and a molecular weight distribution (weight average molecular weight / number average molecular weight) of 1.3 to 1.8, and the polymer block A is a polymer block containing 70% by mass or more of structural units (A-1) derived from cyclohexyl methacrylate and structural units (A-2) derived from at least one selected from the group consisting of methyl methacrylate, ethyl methacrylate, t-butyl methacrylate, and benzyl methacrylate, and has a number average molecular weight of 3,000 to 6,000 and a molecular weight distribution of 1.2 to 1.6, the polymer block B is a polymer block having a number average molecular weight of 1,000 to 6,000, and containing 30 to 70% by mass of structural units (B-1) derived from methacrylic acid, 20 to 50% by mass of structural units (B-2) derived from cyclohexyl methacrylate, and structural units (B-3) derived from at least one species selected from the group consisting of methyl methacrylate, ethyl methacrylate, t-butyl methacrylate, and benzyl methacrylate; and the amount of the polymer block B in the A-B block copolymer is 0.5 to 1.5 parts by mass per part by mass of the polymer block A.

2. The pigment dispersion liquid according to claim 1, wherein the content of the structural unit (A-1) in the polymer block A is 75% by mass or more.

3. The pigment dispersion according to claim 1 or 2, wherein the structural unit (A-2) is a structural unit derived from at least one selected from the group consisting of methyl methacrylate and benzyl methacrylate.

4. The pigment dispersion liquid according to any one of claims 1 to 3, wherein the content of the inorganic pigment is 50 to 70 mass %, the content of the pigment dispersant is 2.5 to 10 mass parts per 100 mass parts of the inorganic pigment, the average particle diameter of the inorganic pigment is 180 to 300 nm, and the viscosity at 25°C is 5 to 20 mPa·s.

5. The pigment dispersion according to any one of claims 1 to 4, wherein the inorganic pigment is a titanium oxide pigment whose surface has been treated with alumina.

6. A pigment dispersion liquid described in any one of claims 1 to 5, wherein the solid content (mass %) of the upper layer after storage at 70°C for one week is 70% or more based on the solid content (mass %) before storage.

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