High polymer dispersant and method for producing the same, aqueous pigment dispersion, and aqueous inkjet ink

Abstract

To provide a high polymer dispersant including, as an effective component, a structural unit derived from an itaconic acid as a biomass material, the dispersant enabling preparation of a pigment dispersion having excellent pigment dispersibility and an aqueous inkjet ink having excellent ejectability and allowing recording of an image with excellent image quality, adhesion to a substrate, and durability, as well as a method for producing the dispersant.SOLUTION: A polymer dispersant for dispersing a pigment, the dispersant comprising a structural unit (i) derived from itaconic acid or the like in an amount of 5-20 mass%, a structural unit (ii) derived from isobornyl acrylate in an amount of 15-45 mass%, a structural unit (iii) derived from tetrahydrofurfuryl acrylate or the like in an amount of 10-40 mass%, and a structural unit (iv) derived from styrene or the like in an amount of 15-45 mass%, the polymer being a carboxyl group-containing polymer having a number average molecular weight of 10,000-30,000, at least a part of the carboxyl groups being neutralized with an alkali and dissolving in water.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polymer dispersant and a method for producing the same, an aqueous pigment dispersion, and an aqueous inkjet ink. [Background technology]

[0002] Aqueous pigment dispersions used as colorants in aqueous inks and other media require that the pigment be finely dispersed in an aqueous medium and maintained in a stable dispersed state without aggregation for extended periods. The pigment dispersant contained in an aqueous pigment dispersion is an important factor for maintaining the fine dispersion and long-term dispersion stability of the pigment, and various dispersants have been developed to date. Furthermore, images produced with aqueous inks prepared using the aqueous pigment dispersion require durability, including adhesion to the substrate, abrasion resistance, water resistance, and solvent resistance. To improve image durability, a film-forming binder component is generally incorporated into aqueous pigment dispersions and aqueous inks. However, pigment dispersants can sometimes reduce the performance of the binder component. Therefore, pigment dispersants must also be designed with an eye toward improving the durability of the images to be printed.

[0003] One application of aqueous pigment dispersions is aqueous inkjet ink. Aqueous inkjet inks require particularly high levels of properties, such as pigment fine dispersion and stability, as well as durability of the recorded image. Due to their increasingly sophisticated functionality, printers and printing machines incorporating aqueous inkjet inks have been used for a wide variety of purposes, including personal, office, commercial, recording, color display, and color photography. In recent years, they have been increasingly used in industrial high-speed printing machines. Furthermore, to meet the demands of higher speeds and higher image quality, further improvements in performance, such as pigment fine dispersion and image durability, are required. To meet these demands, various pigment dispersions have been proposed, each incorporating a unique resin and surfactant composition (Patent Documents 1 to 5).

[0004] Furthermore, in recent years, from the perspective of carbon neutrality, there has been a growing movement to replace petroleum-derived organic materials with biomass-derived materials, and there is also a growing demand for the use of biomass-derived materials in water-based inkjet inks.

[0005] Examples of biomass-derived acrylic monomers that can be used to synthesize acrylic resins include tetrahydrofurfuryl (meth)acrylate, ethyl (meth)acrylate, isobornyl (meth)acrylate, lauryl (meth)acrylate, and stearyl (meth)acrylate. These acrylic monomers are compounds obtained using plant-derived alcohol components and are widely produced industrially.

[0006] Furthermore, 4-vinylguaiacol is known as a biomass-derived styrene-based monomer. 4-vinylguaiacol is a compound obtained by decarboxylation of ferulic acid. Ferulic acid is produced using rice bran residue, which is generated during the process of producing rice bran oil from rice bran. 4-vinylguaiacol contains a phenolic hydroxyl group in its molecule that exhibits radical scavenging activity. For this reason, during radical polymerization, a protected phenolic hydroxyl group, such as an acetylated product, may be used to avoid polymerization inhibition (Non-Patent Document 1).

[0007] Another material that has attracted attention from the perspective of carbon neutrality is itaconic acid. Itaconic acid is a biomass-derived material that is industrially produced by fermentation using plant-derived glucose as a raw material. Itaconic acid is a compound that exhibits radical polymerization, has a high polymer Tg, and contains two carboxyl groups per molecule. Using itaconic acid as a monomer can increase the content of biomass-derived materials in the resulting pigment dispersant (polymer dispersant). Furthermore, it is expected to improve film-forming properties, substrate adhesion due to the hydrogen bonding properties of the carboxyl groups, and pigment dispersibility due to the hydrophilic carboxyl groups neutralized by alkali.

[0008] However, radical polymerization using itaconic acid poses several challenges. Itaconic acid has two large functional groups attached to the α-position of its vinyl group. Therefore, its polymerization rate, polymer conversion rate, and copolymerizability with other monomers are lower than those of commonly used industrial monomers such as (meth)acrylates and styrene. Furthermore, due to the low lipophilicity of the carboxyl group, itaconic acid is poorly miscible with hydrophobic monomers. Therefore, when copolymerizing itaconic acid with hydrophobic monomers, it is necessary to homogenize the monomers using a solvent. However, while itaconic acid dissolves in highly polar solvents such as water and ethanol, hydrophobic monomers and polymers are less soluble in highly polar solvents. For these reasons, copolymerizing itaconic acid with hydrophobic monomers is generally difficult. However, hydrophobic monomers are important for molecular design of polymeric dispersants that exhibit affinity for pigment particles.

[0009] Various measures have been proposed to solve the problems associated with radical polymerization using itaconic acid. For example, itaconic acid is known to have a higher copolymerizability with styrene and acrylates than other monomers (Non-Patent Document 2).

[0010] Because acrylates are highly reactive, when conducting free-radical polymerization on a large scale, it is necessary to add the monomers dropwise to the reaction system in parallel with the polymerization progress, rather than adding all the monomers at once, to prevent thermal runaway caused by a sudden reaction. However, itaconic acid is poorly miscible with hydrophobic acrylates, making it difficult to add itaconic acid and acrylate in a homogeneous solution. Therefore, if the monomers are polymerized while being added dropwise to the reaction solution, it is difficult to achieve a uniform content of itaconic acid-derived structural units in each resulting polymer molecule.

[0011] It is also known that, in order to improve solubility in solvents, itaconic acid can be replaced with an itaconic acid monoester or itaconic acid diester in which the carboxyl group of itaconic acid is protected with a fat-soluble functional group. For example, a copolymer obtained by emulsion polymerization of an itaconic acid diester and a fat-soluble acrylate is known (Patent Document 6). [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Patent No. 5403313 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-45023 [Patent Document 3] Patent No. 4157868 [Patent Document 4] Special Publication No. 2009-515007 [Patent Document 5] International Publication No. 2013 / 008691 [Patent Document 6] Special Publication No. 2018-514634 [Non-patent literature]

[0013] [Non-Patent Document 1] Macromolecues,2017,50,4206-4216 [Non-patent document 2] Organic Synthetic Chemistry, Vol. 21, No. 7 (1963), pp. 542-547 Summary of the Invention [Problem to be solved by the invention]

[0014] However, there are problems such as an increase in the number of steps required to protect the carboxy group of itaconic acid, and a decrease in the polymerization rate, conversion rate, and polymerizability due to the steric hindrance of the introduced protecting group. Furthermore, when an itaconic acid diester is used, it is necessary to deprotect the carboxy group after polymerization in order to utilize the water solubility of the carboxy group. However, not only does this increase the number of steps, but the protecting group generated by deprotection does not function as a pigment dispersant, which poses a problem from the perspective of carbon neutrality.

[0015] 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 polymer dispersant that effectively contains a constituent unit derived from itaconic acids, which are biomass materials, and that can be used to prepare a pigment dispersion that has excellent pigment dispersibility, or an aqueous inkjet ink that has excellent ejection properties and is capable of recording images that are excellent in image quality, adhesion to a substrate, and durability, as well as a method for producing the same.

[0016] Another object of the present invention is to provide a pigment dispersion liquid using a polymer dispersant that effectively contains a constituent unit derived from itaconic acids, which is a biomass material, and that can prepare an aqueous inkjet ink that has excellent pigment dispersibility, is capable of recording images that are excellent in image quality, adhesion to the substrate, and durability, and has excellent jetting properties.

[0017] Another object of the present invention is to provide an aqueous inkjet ink that uses a polymer dispersant that effectively contains structural units derived from itaconic acids, which are biomass materials, and that is capable of recording images that are excellent in image quality, adhesion to substrates, and durability, and that has excellent ejection properties. [Means for solving the problem]

[0018] That is, according to the present invention, the following polymer dispersant is provided. [1] A polymer dispersant for dispersing a pigment, which is blended in an aqueous pigment dispersion used to prepare an aqueous inkjet ink, comprising 5 to 20% by mass of structural units (i) derived from at least one selected from the group consisting of itaconic acid and itaconic acid monoesters, 15 to 45% by mass of structural units (ii) derived from isobornyl acrylate, 10 to 40% by mass of structural units (iii) derived from at least one selected from the group consisting of tetrahydrofurfuryl acrylate, ethyl acrylate, dimethyl itaconate, and diethyl itaconate, and styrene. and 15 to 45 mass% of structural units (iv) derived from at least one selected from the group consisting of ethylene, 4-vinylguaiacol, acetylated 4-vinylguaiacol, and methylated 4-vinylguaiacol, wherein the total content of the structural units (i) to (iv) is 90 mass% or more, the number average molecular weight is 10,000 to 30,000, and the molecular weight distribution (weight average molecular weight / number average molecular weight) is 1.7 to 2.7, and the polymer dispersant is soluble in water when at least a portion of the carboxy groups are neutralized with an alkali.

[0019] The present invention also provides the following method for producing a polymer dispersant. [2] A method for producing the polymer dispersant according to [1] above, comprising the steps of mixing all of the monomers constituting the structural units (i) to (iv) with a water-soluble organic solvent, heating the mixture to 40°C or higher, adding a polymerization initiator having a 10-hour half-life temperature of 50 to 80°C, and carrying out solution polymerization at 60 to 80°C. [3] The method for producing a polymer dispersant according to [2] above, wherein the amount of the polymerization initiator used is 2.0 to 5.0 parts by mass per 100 parts by mass of the total of the monomers.

[0020] Furthermore, according to the present invention, there is provided the following aqueous pigment dispersion. [4] An aqueous pigment dispersion used to prepare an aqueous inkjet ink, the aqueous pigment dispersion containing a pigment, water, a water-soluble organic solvent, and a polymer dispersant for dispersing the pigment, wherein the polymer dispersant is the polymer dispersant described in [1] above. [5] The aqueous pigment dispersion according to [4], wherein the content of the pigment is 5 to 60% by mass, the content of the water is 20 to 80% by mass, the content of the water-soluble organic solvent is 30% by mass or less, and the content of the polymer dispersant is 0.5 to 20% by mass.

[0021] The present invention also provides the following aqueous inkjet ink. [6] An aqueous inkjet ink containing the aqueous pigment dispersion liquid according to [4] or [5] above. [7] The aqueous inkjet ink according to [6] above, which is for printing on plastic media or for textile printing. [Effects of the Invention]

[0022] According to the present invention, it is possible to provide a polymer dispersant that effectively contains a constituent unit derived from itaconic acids, which are biomass materials, and that can be used to prepare a pigment dispersion liquid with excellent pigment dispersibility, or an aqueous inkjet ink with excellent ejection properties and that can record images with excellent image quality, adhesion to substrates, and durability, as well as a method for producing the same.

[0023] Furthermore, according to the present invention, it is possible to provide a pigment dispersion liquid using a polymer dispersant that effectively contains a constituent unit derived from itaconic acids, which are biomass materials, and which can prepare an aqueous inkjet ink that has excellent pigment dispersibility, is capable of recording images that are excellent in image quality, adhesion to the substrate, and durability, and has excellent jetting properties.

[0024] Furthermore, according to the present invention, it is possible to provide an aqueous inkjet ink that uses a polymer dispersant that effectively contains structural units derived from itaconic acids, which are biomass materials, and that is capable of recording images that are excellent in image quality, adhesion to substrates, and durability, and that has excellent ejection properties. DETAILED DESCRIPTION OF THE INVENTION

[0025] <Polymer dispersant> Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. Various physical property values ​​in this specification are values ​​at room temperature (25°C) unless otherwise specified. Hereinafter, "aqueous pigment dispersion" will also be referred to simply as "pigment dispersion," and "aqueous inkjet ink" will also be referred to simply as "ink."

[0026] One embodiment of the polymer dispersant of the present invention is a polymer dispersant (pigment dispersant) for dispersing pigments, which is blended into an aqueous pigment dispersion used to prepare an aqueous inkjet ink. The polymer dispersant of this embodiment is a polymer having a carboxy group, which includes 5 to 20 mass% of structural units (i) derived from at least one selected from the group consisting of itaconic acid and itaconic acid monoesters, 15 to 45 mass% of structural units (ii) derived from isobornyl acrylate, 10 to 40 mass% of structural units (iii) derived from at least one selected from the group consisting of tetrahydrofurfuryl acrylate, ethyl acrylate, dimethyl itaconate, and diethyl itaconate, and 15 to 45 mass% of structural units (iv) derived from at least one selected from the group consisting of styrene, 4-vinylguaiacol, acetylated 4-vinylguaiacol, and methylated 4-vinylguaiacol. In the polymer dispersant (polymer), the total content of the structural units (i) to (iv) is 90 mass% or more, the number average molecular weight of the polymer is 10,000 to 30,000, and the molecular weight distribution (weight average molecular weight / number average molecular weight) is 1.7 to 2.7. The polymer dispersant is a polymer in which at least a portion of the carboxyl groups are neutralized with an alkali and is soluble in water. The polymer dispersant of this embodiment will be described in detail below.

[0027] The polymeric dispersant is a polymer containing structural unit (i) derived from at least one selected from the group consisting of itaconic acid and itaconic acid monoesters. The content of structural unit (i) in the polymer is 5 to 20% by mass, preferably 10 to 15% by mass. The inclusion of structural unit (i) allows at least a portion of the carboxyl groups to be neutralized with alkali to dissolve in an aqueous medium, thereby improving the adhesion of the coating film, such as an image, to the substrate. Furthermore, the use of itaconic acid and itaconic acid monoesters (hereinafter collectively referred to as "itaconic acids") as monomers can suppress the reaction rate of acrylates. Therefore, even if the monomers are charged into a reaction vessel all at once for polymerization rather than by dropwise addition, thermal runaway is unlikely, enabling safe and reproducible production. If the content of structural unit (i) is less than 5% by mass, the water solubility of the polymeric dispersant is insufficient. On the other hand, if the content of structural unit (i) is more than 20% by mass, the hydrophilicity of the polymeric dispersant becomes excessively high, which may result in reduced water resistance of the coating film formed.

[0028] The polymer dispersant is a polymer containing structural unit (ii) derived from isobornyl acrylate. The content of structural unit (ii) in the polymer is 15 to 45% by mass, preferably 20 to 40% by mass. The inclusion of structural unit (ii) improves the hardness of the polymer dispersant and can enhance the abrasion resistance of the coating film formed. Furthermore, the hydrophobicity of the polymer dispersant is increased, thereby improving the water resistance and solvent resistance of the coating film formed and enhancing the hydrophobic interaction between the polymer dispersant and the pigment. If the content of structural unit (ii) is less than 15% by mass, the desired effect cannot be achieved. On the other hand, if the content of structural unit (ii) is more than 45% by mass, the polymer becomes excessively hard and hydrophobic, resulting in reduced pigment dispersibility and reduced substrate adhesion and followability of the coating film.

[0029] The polymer dispersant is a polymer containing structural unit (iii) derived from at least one selected from the group consisting of tetrahydrofurfuryl acrylate, ethyl acrylate, dimethyl itaconate, and diethyl itaconate. The content of structural unit (ii) in the polymer is 10 to 40% by mass, preferably 15 to 30% by mass. The inclusion of this structural unit (iii) can improve the substrate adhesion and flexibility of the coating film formed. If the content of structural unit (iii) is less than 10% by mass, the desired effect cannot be obtained. On the other hand, if the content of structural unit (iii) is more than 40% by mass, the proportions of other structural units are relatively reduced, making it impossible to obtain the desired effect.

[0030] The polymer dispersant is a polymer containing structural unit (iv) derived from at least one selected from the group consisting of styrene, 4-vinylguaiacol, acetylated 4-vinylguaiacol, and methylated 4-vinylguaiacol. The content of structural unit (iv) in the polymer is 15 to 45% by mass, preferably 20 to 45% by mass. Using styrene or the like as a monomer can suppress the reaction rate of acrylate. Therefore, even if the monomers are charged into a reaction vessel all at once for polymerization instead of dropping the monomers, thermal runaway is unlikely, allowing for safe and reproducible production. Furthermore, the aromatic ring and the pigment form strong bonds through ππ stacking, resulting in high dispersibility. Furthermore, the hydrophobic interaction improves the adhesion of the coating film to highly hydrophobic substrates such as OPP film. Furthermore, the ππ stacking improves the adhesion of the coating film to aromatic polymer substrates such as PET film. If the content of structural unit (iv) is less than 15% by mass, the desired effect cannot be achieved. On the other hand, if the content of the structural unit (iv) exceeds 45 mass %, the proportion of other structural units will be relatively reduced, making it impossible to obtain the desired effect.

[0031] The total content of the structural units (i) to (iv) in the polymer dispersant (polymer) is 90% by mass or more, and preferably 95% by mass or more. That is, the polymer dispersant may further contain structural units other than the structural units (i) to (iv) at a ratio of less than 10% by mass. It is preferable that the polymer dispersant (polymer) is substantially composed only of the structural units (i) to (iv).

[0032] Examples of the monomers constituting the other structural units include conventionally known vinyl-based monomers. Other monomers include itaconic acid diesters such as dibutyl itaconate; styrene-based monomers such as alpha-methylstyrene; vinyl-based monomers such as vinyltoluene, vinyl acetate, and vinylpyrrolidone; alkyl esters of (meth)acrylic acid such as butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, cetyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, and behenyl (meth)acrylate; aromatic ring-containing (meth)acrylic acid esters such as benzyl (meth)acrylate, phenyl (meth)acrylate, naphthyl (meth)acrylate, and phenoxyethyl (meth)acrylate; cyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, 3,3,5-trimethyl (meth)acrylate, and methyl methyl acrylate. Examples of (meth)acrylic acid esters include alicyclic-containing (meth)acrylic acid esters such as methylcyclohexyl, dicyclopentanyl (meth)acrylate, and isobornyl methacrylate; hydroxyl group-containing (meth)acrylic acid esters such as hydroxyethyl (meth)acrylate, hydroxybutyl (meth)acrylate, polyethylene glycolyl (meth)acrylate, and polypropylene glycol (meth)acrylate; (meth)acrylic acid esters of glycol monoethers such as methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, ethoxyethoxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, and dicyclopentenyloxyethyl (meth)acrylate; and (meth)acrylic acid esters containing an amino group or a quaternary ammonium salt such as dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, t-butylaminoethyl (meth)acrylate, and trimethylethylammonium chloride (meth)acrylate.

[0033] The polymer dispersant is a polymer in which at least a portion of the carboxy groups derived from itaconic acids, preferably carboxy groups corresponding to 8% by mass or more of the structural unit (i), are neutralized with an alkali and ionized. Neutralizing at least a portion of the carboxy groups with an alkali improves the hydrophilicity of the polymer, making it possible to produce a polymer dispersant that is soluble in water.

[0034] The alkali used to neutralize the carboxyl groups is preferably at least one selected from the group consisting of ammonia, dimethylaminoethanol, sodium hydroxide, and potassium hydroxide. When neutralized with ammonia or dimethylaminoethanol, the ammonia or dimethylaminoethanol volatilizes upon drying to form a coating film, resulting in deionization and the formation of carboxyl groups. This improves the water resistance of the coating film. When neutralized with sodium hydroxide or sodium hydroxide, the ammonia or dimethylaminoethanol does not deionize upon drying to form a coating film, improving the resolubility of the polymer dispersant. The alkali can be selected and used depending on the printing process, the durability of the coating film (image), and the like.

[0035] The polymer dispersant is a polymer having a number-average molecular weight (Mn) of 10,000 to 30,000, preferably 13,000 to 25,000. If the Mn of the polymer is less than 10,000, it becomes difficult to improve the durability of the coating film that is formed. On the other hand, if the Mn of the polymer is more than 30,000, the viscosity of the pigment dispersion may become too high. Note that the number-average molecular weight (Mn) and weight-average molecular weight (Mw) in this specification are both values ​​calculated as polystyrene equivalents measured by gel permeation chromatography (GPC).

[0036] The polymer dispersant is a polymer having a molecular weight distribution (PDI = weight average molecular weight (Mw) / number average molecular weight (Mn)) of 1.7 to 2.7, preferably 1.8 to 2.5. Polymers having a molecular weight distribution (PDI) within the above range are preferred because they contain a good balance of low molecular weight components and high molecular weight components.

[0037] <Method of manufacturing polymer dispersants> One embodiment of the method for producing a polymeric dispersant of the present invention is a method for producing the aforementioned polymeric dispersant, and includes a step of mixing all of the monomers constituting the structural units (i) to (iv) and a water-soluble organic solvent, heating the mixture to 40°C or higher, adding a polymerization initiator having a 10-hour half-life temperature of 50 to 80°C, and carrying out solution polymerization at 60 to 80°C (polymerization step).

[0038] In the case of ordinary radical polymerization, it is desirable to suppress thermal runaway due to a rapid reaction by adding monomers dropwise to the polymerization reaction system in parallel with the progress of the polymerization. In contrast, in the production method of the present embodiment, the entire amounts of all monomers are charged into a polymerization reaction system, such as a reaction vessel, all at once, and then the polymerization reaction is initiated. This allows the itaconic acids constituting the structural unit (i) and the styrene or the like constituting the structural unit (iv) to suppress the rapid progress of the acrylate reaction, thereby enabling polymerization while preventing thermal runaway. Furthermore, even itaconic acids that are poorly compatible with other monomers and difficult to add dropwise to a polymerization reaction system can be easily used as monomers, and a polymer effectively containing the structural unit (i) can be easily produced.

[0039] In the polymerization step, all of the monomers and the water-soluble organic solvent are mixed in a polymerization reaction system such as a reaction vessel, and then heated to 40° C. or higher, preferably 60 to 80° C. Heating to a predetermined temperature allows the itaconic acids to dissolve.

[0040] The polymerization initiator used should have a 10-hour half-life temperature of 50 to 80°C. Using a polymerization initiator with a 10-hour half-life temperature of less than 50°C can result in an insufficient polymerization rate, resulting in a polymer with a number-average molecular weight (Mn) of less than 10,000. Furthermore, when itaconic acid is exposed to high temperatures, side reactions such as radical abstraction of the methylene hydrogen at the α-position of the vinyl group and cyclodehydration can easily occur. Therefore, using a polymerization initiator with a 10-hour half-life temperature of more than 80°C can result in a polymer with a number-average molecular weight (Mn) of less than 10,000.

[0041] Examples of polymerization initiators with a 10-hour half-life temperature of 50 to 80°C include peroxide-based radical polymerization initiators such as lauroyl peroxide (62°C) and benzoyl peroxide (74°C); azo-based radical initiators such as azobisisobutyronitrile (65°C), 2,2'-azobis(2-methylbutyronitrile) (67°C), 2,2'-azobis(isobutyrate)dimethyl (66°C), and 2,2'-azobis(2,4-dimethylvaleronitrile) (51°C); and the like (the temperature in parentheses is the 10-hour half-life temperature). When 4-vinylguaiacol is used as the monomer constituting the structural unit (iv), the oxygen radicals generated by the peroxide-based radical polymerization initiator are easily inactivated by the phenolic hydroxyl group, which may prevent the polymerization from proceeding sufficiently. For this reason, when 4-vinylguaiacol is used as the monomer constituting the structural unit (iv), it is preferable to use an azo-based radical polymerization initiator.

[0042] The amount of polymerization initiator used per 100 parts by mass of the total monomers is preferably 2.0 to 5.0 parts by mass, and more preferably 3.0 to 4.5 parts by mass. If the amount of polymerization initiator used per 100 parts by mass of the total monomers is less than 2.0 parts by mass, the polymer conversion rate will decrease and the number-average molecular weight (Mn) of the resulting polymer may exceed 30,000. On the other hand, if the amount of polymerization initiator used is more than 5.0 parts by mass per 100 parts by mass of the total monomers, the number-average molecular weight (Mn) of the resulting polymer will likely be less than 10,0000 and it may be somewhat difficult to control thermal runaway due to a rapid reaction.

[0043] The temperature during solution polymerization is preferably 5 to 15°C higher than the 10-hour half-life temperature of the polymerization initiator used. Therefore, the temperature during solution polymerization is preferably 60 to 80°C.

[0044] In the polymerization step, a water-soluble organic solvent is used as the polymerization solvent. Poorly water-soluble organic solvents are not suitable as polymerization solvents because they have poor solubility for itaconic acid. Furthermore, when a poorly water-soluble organic solvent is used as the polymerization solvent, the poorly water-soluble organic solvent must be removed when preparing an aqueous pigment dispersion using the resulting polymer, which makes the process complicated.

[0045] Examples of the water-soluble organic solvent include alcohol solvents such as methanol, ethanol, isopropanol, propyl alcohol, butanol, and isobutanol; glycol solvents such as ethylene glycol, propylene glycol, and glycerin; diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, and tripropylene glycol monomethyl ether. Examples of the solvent include glycol ethers such as propylene glycol monopropyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, dipropylene glycol dimethyl ether, and 3-methoxy-3-methyl-1-butanol; amide solvents such as dimethylformamide, dimethylacetamide, pyrrolidone, N-methylpyrrolidone, 3-methoxy-N,N-dimethylpropanamide, and 3-butoxy-N,N-dimethylpropanamide; carbonate solvents such as ethylene carbonate, propylene carbonate, and dimethyl carbonate; dimethyl sulfoxide; tetramethylurea; and dimethylimidazolidinone.

[0046] Among these, glycol ethers are preferably used as the polymerization solvent, and it is more preferable to use at least one glycol ether selected from the group consisting of diethylene glycol monobutyl ether, propylene glycol monopropyl ether, and propylene glycol monomethyl ether as the polymerization solvent. Among water-soluble organic solvents, these glycol ethers are preferred because they have relatively high fat solubility and can uniformly dissolve monomer components containing a large amount of hydrophobic monomers. Note that when the above glycol ethers, which have relatively high fat solubility, are used as the polymerization solvent, itaconic acids may be difficult to dissolve, but they can be uniformly dissolved by heating to a predetermined temperature.

[0047] It is preferable to use 50 parts by mass or more of the polymerization solvent per 100 parts by mass of the polymer to be produced, and more preferably 100 parts by mass or more of the polymerization solvent. After the solution polymerization, it is preferable to add the aforementioned alkali to neutralize at least a portion of the carboxyl groups derived from itaconic acid and convert it into an aqueous solution. This allows for a substantially transparent aqueous solution containing the polymer dispersant.

[0048] Alternatively, the solid polymer removed from the polymerization reaction system after solution polymerization may be dissolved in water. Specifically, the polymerization reaction system after solution polymerization is added to a poor solvent to precipitate the resulting polymer. Alternatively, an alkali may be added after solution polymerization to form an aqueous solution, which may then be added to a large amount of water to homogenize, and an acid may then be added to precipitate the polymer. Water and, if necessary, a water-soluble organic solvent may be added to the precipitated solid polymer, and an aqueous alkali solution may be added to neutralize at least a portion of the carboxy groups to form an aqueous solution, thereby obtaining an aqueous solution containing a polymer dispersant.

[0049] <Water-based pigment dispersion> One embodiment of the aqueous pigment dispersion of the present invention is a pigment dispersion used for preparing an aqueous inkjet ink, and contains a pigment, water, a water-soluble organic solvent, and the above-mentioned polymer dispersant for dispersing the pigment.

[0050] (pigment) The pigment may be an organic pigment or an inorganic pigment. Examples of the organic pigment include soluble azo pigments, insoluble azo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, isoindoline pigments, perylene pigments, perinone pigments, dioxazine pigments, anthraquinone pigments, dianthraquinonyl pigments, anthrapyrimidine pigments, anthanthrone pigments, indanthrone pigments, flavanthrone pigments, pyranthrone pigments, and diketopyrrolopyrrole pigments. Examples of the inorganic pigment include titanium dioxide, iron oxide, antimony pentoxide, zinc oxide, silica, cadmium sulfide, calcium carbonate, barium carbonate, barium sulfate, clay, talc, yellow lead, and carbon black.

[0051] Suitable pigments, indicated by Color Index Number (CI), include pigments commonly used in inkjet inks, such as CI Pigment Blue 15:3, 15:4, 15:6; CI Pigment Red 122, 176, 202, 209, 254, 269, 291; CI Pigment Violet 19, 23; CI Pigment Yellow 74, 155, 180; CI Pigment Green 36, 58; CI Pigment Orange 43, 48, 49, 71; CI Pigment Black 7; and CI Pigment White 6.

[0052] The number average particle size (primary particle size) of the organic pigment is preferably 150 nm or less. The number average particle size (primary particle size) of the inorganic pigment is preferably 300 nm or less. By using a pigment having a number average particle size within the above range, the optical density, saturation, color development, and print quality of the recorded image can be improved, and sedimentation of the pigment in the ink can be appropriately suppressed. The number average particle size of the pigment can be measured using, for example, an electron microscope or a light scattering particle size distribution analyzer.

[0053] The pigment may be surface-treated with a surface treatment agent such as a polymer dispersant, a silane coupling agent, an inorganic substance (e.g., silica, zirconia, sulfuric acid), or a pigment derivative (synergist). For example, these surface treatment agents may be added or present when synthesizing, pigmenting, or micronizing the pigment. Furthermore, the quinacridone pigment may also be a composite such as a mixed crystallized product of different pigments or a solid solution pigment.

[0054] (liquid medium) The aqueous pigment dispersion contains an aqueous liquid medium containing water and a water-soluble organic solvent as a dispersion medium for the pigment. The water-soluble organic solvent may be the alcohol-based solvent, glycol-based solvent, or amide-based solvent described above that can be used as a polymerization solvent.

[0055] (Water-based pigment dispersion) The content of the pigment in the aqueous pigment dispersion is preferably 5 to 60% by mass. When the pigment is an organic pigment, the content of the organic pigment in the aqueous pigment dispersion is preferably 5 to 30% by mass, and more preferably 10 to 25% by mass. When the pigment is an inorganic pigment, since inorganic pigments have a large specific gravity, the content of the inorganic pigment in the aqueous pigment dispersion is preferably 20 to 60% by mass, and more preferably 30 to 50% by mass.

[0056] The water content in the aqueous pigment dispersion is preferably 20 to 80% by mass. By preparing an aqueous pigment dispersion containing an appropriate amount of water, it is possible to easily prepare an aqueous inkjet ink.

[0057] The content of the water-soluble organic solvent in the aqueous pigment dispersion is preferably 30% by mass or less, and more preferably 0.5 to 20% by mass. If the content of the water-soluble organic solvent exceeds 30% by mass, the recorded image may become difficult to dry.

[0058] The content of the polymer dispersant in the aqueous pigment dispersion is preferably 0.5 to 20% by mass. If the content of the polymer dispersant is less than 0.5% by mass, it may be somewhat difficult to stably disperse the pigment. On the other hand, if the content of the polymer dispersant is more than 20% by mass, the viscosity may become too high and the dispersion may exhibit non-Newtonian viscosity, making it somewhat difficult to eject the dispersion linearly using an inkjet method.

[0059] The content of the polymer dispersant in the aqueous pigment dispersion is preferably set according to the type of pigment, surface properties, particle size, etc. Specifically, the content of the polymer dispersant is preferably 5 to 50 parts by mass, more preferably 10 to 30 parts by mass, per 100 parts by mass of the organic pigment. Furthermore, the content of the polymer dispersant is preferably 1 to 20 parts by mass, more preferably 3 to 10 parts by mass, per 100 parts by mass of the inorganic pigment.

[0060] (Other ingredients) The aqueous pigment dispersion may further contain an alkali to neutralize the polymer dispersant or to adjust the pH. The alkali may be at least one selected from the group consisting of ammonia, dimethylaminoethanol, sodium hydroxide, and potassium hydroxide. The content of the alkali in the aqueous pigment dispersion is preferably 0.5 to 5% by mass.

[0061] The aqueous pigment dispersion may further contain at least one emulsion of an acrylic resin emulsion or a urethane resin emulsion as a binder component. The content of the emulsion in the aqueous pigment dispersion, calculated as solid content, is preferably 5 to 20 mass %. By adding such an emulsion, the aqueous pigment dispersion can be prepared into an aqueous inkjet ink capable of recording images with improved durability, such as abrasion resistance, and gloss.

[0062] The acrylic resin emulsion can be an emulsion with a dispersed particle size (number average particle size) of 50 to 200 nm obtained by polymerizing styrene and acrylic acid monomers in the presence of a surfactant.The urethane resin emulsion can be an emulsion with a dispersed particle size (number average particle size) of 50 to 200 nm obtained by reacting a diisocyanate such as isophorone diisocyanate, a polyol such as polycarbonate diol, a diol such as diethylene glycol, and a diol monocarboxylic acid such as dimethylolpropanoic acid, and then chain-extending the mixture with isophorone diamine while self-emulsifying with alkaline water.

[0063] (Physical properties of aqueous pigment dispersion) The viscosity of the aqueous pigment dispersion can be appropriately set depending on the properties of the pigment, the viscosity of the aqueous inkjet ink to be prepared, etc. When an organic pigment is used, the viscosity of the aqueous pigment dispersion at 25°C is preferably 3 to 20 mPa·s. When an inorganic pigment is used, the viscosity of the aqueous pigment dispersion at 25°C is preferably 5 to 30 mPa·s.

[0064] The surface tension of the aqueous pigment dispersion at 25° C. is preferably 15 to 45 mN / m, and more preferably 20 to 40 mN / m. The surface tension of the aqueous pigment dispersion can be adjusted, for example, by changing the type and amount of the water-soluble organic solvent, or by adding a surfactant or the like.

[0065] (Method for preparing aqueous pigment dispersion) Aqueous pigment dispersions can be prepared according to conventional methods. For example, a mixture of pigment and polymeric dispersant is prepared by adding water and, if necessary, a water-soluble organic solvent. The pigment is then finely dispersed using a paint shaker, ball mill, attritor, sand mill, horizontal media mill, colloid mill, roll mill, or the like to prepare a dispersion. Water and a water-soluble organic solvent are added to the prepared dispersion, and, if necessary, a binder component (emulsion) and other additives are added to adjust the concentration to the desired level. Furthermore, alkali or the like may be added to adjust the pH. Furthermore, various additives such as surfactants and preservatives may be added as necessary to obtain the desired aqueous pigment dispersion. After mixing and dispersing the components, it is preferable to remove coarse particles using a centrifuge or filter.

[0066] In order to adjust the number average particle size (particle size distribution) of the pigment to fall within the desired range, techniques such as reducing the size of the grinding media used, increasing the packing ratio of the grinding media, lengthening the processing time, slowing the discharge speed, and classifying the powder after grinding using a filter or centrifuge, etc. It is also preferable to use a pigment that has been previously micronized by a conventionally known method such as salt milling.

[0067] <Water-based inkjet ink> One embodiment of the ink of the present invention is an aqueous inkjet ink suitable for printing on plastic media or textile printing, containing the aqueous pigment dispersion described above. The ink of this embodiment can be prepared according to a conventionally known method, except for using the aqueous pigment dispersion described above. The content of the pigment in the ink is preferably 4 to 20% by mass.

[0068] The ink may contain various additives used in conventional aqueous inkjet inks, such as surfactants, organic solvents, humectants, pigment derivatives, dyes, leveling agents, antifoaming agents, UV absorbers, binder components such as emulsions, preservatives, and antibacterial agents.

[0069] The physical properties of the ink are appropriately set depending on the performance of the inkjet printer, etc. For example, the surface tension of the ink at 25° C. is preferably 20 to 40 mN / m.

[0070] The ink of this embodiment can be suitably used in an inkjet printer for printing on plastic media. Examples of media (recording media) to be printed on include plastic films such as olefin film, vinyl chloride film, polyester film, and polyamide film. Of course, it is also possible to print on media such as paper and metal that are commonly used in inkjet printing.

[0071] By using the ink of this embodiment, it is possible to record highly colored images that have excellent adhesion and durability to substrates at high speed on various media, including non-absorbent media such as plastic media. Furthermore, since the ink of this embodiment is suitable for high-speed printing, it is suitable for inkjet printers for mass printing used to print on food packages, packaging materials, etc. [Example]

[0072] 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.

[0073] <Production of polymer dispersants> Example 1 100 parts of diethylene glycol monobutyl ether (BDG), 15 parts of itaconic acid (ITA), 35 parts of isobornyl acrylate (IBXA), 30 parts of tetrahydrofurfuryl acrylate (THFA), and 20 parts of styrene (St) were placed in a reaction vessel and heated to 72.5°C under a nitrogen atmosphere. 2.0 parts of 2,2'-azobis(2-methylbutyronitrile) (V59) were added to initiate polymerization. The temperature of the polymerization solution changed within 0.5°C, and no significant exotherm was observed. The liquid temperature was maintained, and after 3 hours, 1.0 part of V59 was added. Polymerization continued for an additional 4 hours to form a polymer, yielding a polymer-containing liquid. A portion of the liquid was sampled, and the molecular weight of the polymer was measured by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as the developing solvent. As a result, the number average molecular weight (Mn) of the polymer was 17,500, and the molecular weight distribution (PDI = weight average molecular weight (Mw) / number average molecular weight (Mn)) was 2.42. The solid content of the obtained liquid was 50.5%, and the polymerization rate was approximately 100%. The solid content was calculated from the residue obtained by weighing a portion of the obtained liquid into an aluminum dish and drying it for 2 hours in a blast dryer at 150°C. After cooling the polymer-containing liquid to 50°C, an alkaline aqueous solution containing 15.4 parts of 28% ammonia water and 184.6 parts of water was added to neutralize it, yielding an aqueous solution of polymer dispersant-1 (colorless, transparent liquid). The content (solid content) of polymer dispersant-1 in the obtained aqueous solution was 25.1%.

[0074] Examples 2 to 7 Aqueous solutions of polymer dispersants-2 to -7 were obtained in the same manner as in Example 1, except that the types and amounts (units: parts) of monomers shown in Table 1 were used. In all cases, the temperature change of the polymerization solution after the addition of the polymerization initiator was within 0.5°C, and no significant heat generation was observed. The physical properties of the obtained polymer dispersants are shown in Table 1. The meanings of the abbreviations in Table 1 are as follows: ITDM: Dimethyl itaconate 4VGAc: 4-vinylguaiacol acetylated 4VG: 4-vinylguaiacol ·LA: Lauryl acrylate

[0075] TIFF2025166315000001.tif136170

[0076] (Comparative Example 1) 100 parts of BDG, 15 parts of ITA, 45 parts of IBXA, and 40 parts of THFA were placed in a reaction vessel. 0.25 parts of a chain transfer agent (thioglycerol (TGL)) was added, and the mixture was heated to 65°C under a nitrogen atmosphere. 0.025 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (V65) was added to initiate polymerization. After 1.5 hours and 3 hours, 0.025 parts of V65 were added, and the polymerization was continued for another 4 hours to form a polymer, yielding a liquid containing the polymer. The Mn of the polymer was 18,900 and the PDI was 2.30. The solids content of the resulting liquid was 50.8%, and the polymerization rate was approximately 100%. The polymer-containing solution was cooled to 50°C and neutralized with an alkaline solution containing 15.4 parts of 28% ammonia water and 184.6 parts of water, yielding an aqueous solution of polymeric dispersant-8 (a colorless, transparent liquid). The content (solid content) of polymer dispersant-8 in the obtained aqueous solution was 25.1%.

[0077] (Comparative Example 2) An aqueous solution of polymer dispersant-9 was obtained in the same manner as in Comparative Example 1, except that the types and amounts (units: parts) of monomers shown in Table 2 were used. The physical properties of the obtained polymer dispersant are shown in Table 2.

[0078] TIFF2025166315000002.tif142170

[0079] (Comparative Example 3) A transparent liquid containing a polymer was obtained in the same manner as in Example 1 above, except that the amount of ITA was 3 parts and the amount of IBXA was 47 parts. The polymer had an Mn of 19,900 and a PDI of 2.36. The solids content of the obtained liquid was 50.3%, and the polymerization rate was approximately 100%. When 10.3 parts of aqueous ammonia was added, the liquid became cloudy and solids precipitated, making it impossible to obtain a transparent solution of the polymer dispersant. It is believed that the amount of ITA was too small, which resulted in insufficient water solubility of the obtained polymer.

[0080] Comparative Example 4 100 parts of BDG and 15 parts of ITA were placed in a reaction vessel and heated to 72.5°C under a nitrogen atmosphere. 35 parts of IBXA, 30 parts of THFA, 20 parts of St, and 2.0 parts of V59 were placed in a separate vessel and mixed to obtain a homogeneous monomer solution. The resulting monomer solution was placed in a dropping funnel and added dropwise to the reaction vessel over 1 hour while maintaining the internal temperature at 72.5°C. Two hours after the completion of the dropping, 1.0 part of V59 was added, and polymerization was continued for an additional 4 hours to form a polymer, yielding a transparent liquid containing the polymer. The polymer had an Mn of 19,800 and a PDI of 2.65. The solids content of the resulting liquid was 50.9%, and the polymerization rate was approximately 100%. The polymer-containing solution was then cooled to 50°C and neutralized with an alkaline aqueous solution containing 15.4 parts of 28% ammonia water and 184.6 parts of water, yielding an aqueous solution of polymeric dispersant-10 (a pale white, transparent liquid). The content (solid content) of polymer dispersant-10 in the obtained aqueous solution was 25.1%.

[0081] <Preparation of pigment dispersion and ink> Example 8 A clear solution was obtained by mixing 175 parts of an aqueous solution of polymer dispersant-1 and 350 parts of water. 175 parts of pigment (CI Pigment Blue 15:3, product name "A-220JC", manufactured by Dainichiseika Color & Chemicals Co., Ltd.) was added to the resulting liquid and stirred for 30 minutes using a disperser to prepare a millbase. A horizontal media disperser (product name "Dynomill 0.6 Liter ECM Type", manufactured by Shinmaru Enterprises, zirconia bead diameter: 0.5 mm) was used to disperse the pigment thoroughly in the millbase at a peripheral speed of 10 m / s. 272.2 parts of water was added to adjust the pigment concentration to 18%. The millbase was centrifuged (7,500 rpm, 20 minutes) and then filtered through a 10 μm pore membrane filter. The mixture was diluted with water to obtain a pigment dispersion for inkjet ink (IJD-1) with a pigment concentration of 14%.

[0082] The number-average particle size of the pigment in IJD-1 was measured using a particle size analyzer (trade name "NICOMP 380ZLS-S" manufactured by International Business Co., Ltd.) and was found to be 128 nm, confirming that the pigment was finely dispersed. The viscosity of IJD-1 was 2.90 mPa·s and the pH was 9.0. After storage at 70°C for one week, the number-average particle size of the pigment in IJD-1 was 129.1 nm and the viscosity of IJD-1 was 2.89 mPa·s. This confirmed that IJD-1 has excellent storage stability.

[0083] 40 parts of IJD-1, 42.2 parts of water, 5 parts of 1,2-hexanediol, 10 parts of glycerin, and 1 part of a surfactant (trade name "Surfynol 465", manufactured by Air Products Co., Ltd.) were mixed and thoroughly stirred, and then filtered through a membrane filter with a pore size of 10 μm to obtain an inkjet ink (IJI-1).

[0084] (Examples 9 to 14, Comparative Examples 5 to 7) Pigment dispersions for inkjet inks (IJD-2 to 7 and CIJD-1 to 3) were obtained in the same manner as in Example 8, except that the types of polymer dispersants shown in Table 2 were used. The properties of each pigment dispersion (number average particle size and viscosity of the pigment immediately after dispersion and after storage at 70°C for 1 week) are shown in Table 3.

[0085] In the case of CIJD-3 prepared in Comparative Example 7, the pigment was finely dispersed immediately after dispersion, but after one week of storage at 70°C, pigment aggregation was confirmed. Polymer Dispersant-10 (Comparative Example 4) was prepared by a method in which a monomer other than itaconic acid was added dropwise to the reaction system to polymerize it. Therefore, it is believed that the structural unit (i) derived from itaconic acid was not uniformly introduced into the polymer dispersant molecule, and sufficient water solubility was not ensured, resulting in a loss of pigment dispersibility. CIJD-3 was not used in the following studies.

[0086] Furthermore, water-based pigment inks for inkjet use (IJI-2 to 7 and CIJI-1 to 2) were obtained in the same manner as in Example 8 above, except that the prepared IJD-2 to 7 and CIJD-1 to 2 were used, respectively.

[0087] <Evaluation> The prepared aqueous pigment inks were filled into cartridges and installed in an inkjet printer equipped with a plate heater (product name "MMP825H", manufactured by Mastermind Co., Ltd.). A solid image was then printed on a PET film (product name "FE22001#50", manufactured by Futamura Chemical Co., Ltd.) that had been heated with the plate heater to a surface temperature of 50°C, to obtain a printed product.

[0088] (dispersibility) The dispersibility of the pigment in the prepared pigment dispersion was evaluated according to the following evaluation criteria, and the results are shown in Table 3. ◯: Finely dispersed and had good storage stability. Δ: Finely dispersed, but storage stability was not good. ×: Not finely dispersed, and storage stability was poor.

[0089] (Dischargeability) The ink ejection state during printing was visually observed, and the ink ejection properties were evaluated according to the following evaluation criteria. The results are shown in Table 3. ○: Discharge was possible without any problems. △: Scattering of minute droplets was observed. ×: Droplets splashed and scattered during ejection, causing image distortion.

[0090] (image quality) The printed images were visually observed and the image quality was evaluated according to the following criteria. The results are shown in Table 3. Good: A good image was recorded. ×: The image was distorted due to streaks or the like.

[0091] (adhesion) After drying the printed material at 100°C for 10 minutes, cellophane tape was firmly pressed against the image and then peeled off. The degree of peeling of the image from the PET film was visually observed, and the adhesion of the image to the PET film was evaluated according to the following criteria. The results are shown in Table 3. ◎: No peeling at all. ○: Slight peeling. △: The peeled area was smaller than the non-peeled area. ×: The peeled area was larger than the non-peeled area.

[0092] (Rubbing resistance (dry and wet friction resistance)) After drying the print at 100°C for 10 minutes, a Gakushin-type rub fastness tester (product name "RT-300", manufactured by Daiei Scientific Co., Ltd.) was used to conduct a rub test, in which the print was rubbed 10 times with a dry white cloth and a wet white cloth, each with a load of 100g. After the rub test, the degree of peeling of the image was visually observed, and the rub resistance of the image (dry rub resistance and wet rub resistance) was evaluated according to the following evaluation criteria. The results are shown in Table 3. ◎: No peeling at all. ○: Slight peeling. △: The peeled area was smaller than the non-peeled area. ×: The peeled area was larger than the non-peeled area.

[0093] TIFF2025166315000003.tif155170 [Industrial Applicability]

[0094] The polymeric dispersant of the present invention can be used to prepare an aqueous inkjet ink suitable for printing on plastic media or textile printing, and this aqueous inkjet ink can be used to print high-quality images on demand at high speed on, for example, various containers, packages, labels, sign displays, and the like.

Claims

1. A polymer dispersant for dispersing a pigment, which is blended in an aqueous pigment dispersion used to prepare an aqueous inkjet ink, comprising: 5 to 20% by mass of structural units (i) derived from at least one selected from the group consisting of itaconic acid and itaconic acid monoesters; 15 to 45% by mass of structural units (ii) derived from isobornyl acrylate; 10 to 40% by mass of structural units (iii) derived from at least one selected from the group consisting of tetrahydrofurfuryl acrylate, ethyl acrylate, dimethyl itaconate, and diethyl itaconate; and 15 to 45% by mass of structural units (iv) derived from at least one selected from the group consisting of styrene, 4-vinylguaiacol, acetylated 4-vinylguaiacol, and methylated 4-vinylguaiacol, the total content of the structural units (i) to (iv) is 90% by mass or more, a polymer having a carboxy group, which has a number average molecular weight of 10,000 to 30,000 and a molecular weight distribution (weight average molecular weight / number average molecular weight) of 1.7 to 2.7; A polymer dispersant in which at least a portion of the carboxyl groups is neutralized with an alkali and is soluble in water.

2. A method for producing the polymer dispersant according to claim 1, comprising: A method for producing a polymer dispersant, comprising the steps of mixing all of the monomers constituting the structural units (i) to (iv) with a water-soluble organic solvent, heating the mixture to 40°C or higher, adding a polymerization initiator having a 10-hour half-life temperature of 50 to 80°C, and carrying out solution polymerization at 60 to 80°C.

3. 3. The method for producing a polymer dispersant according to claim 2, wherein the amount of the polymerization initiator used is 2.0 to 5.0 parts by mass based on 100 parts by mass of the total of the monomers.

4. 1. An aqueous pigment dispersion used to prepare an aqueous inkjet ink, comprising: The ink contains a pigment, water, a water-soluble organic solvent, and a polymer dispersant for dispersing the pigment, An aqueous pigment dispersion, wherein the polymer dispersant is the polymer dispersant according to claim 1.

5. The content of the pigment is 5 to 60% by mass, The water content is 20 to 80% by mass, The content of the water-soluble organic solvent is 30% by mass or less, 5. The aqueous pigment dispersion according to claim 4, wherein the content of the polymer dispersant is 0.5 to 20% by mass.

6. An aqueous inkjet ink containing the aqueous pigment dispersion according to claim 4 or 5.

7. The aqueous inkjet ink according to claim 6, which is for use in printing on plastic media or textile printing.

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