JP2026002493A - Water-based pigment dispersions - Google Patents

Abstract

The present invention relates to an aqueous pigment dispersion that, when used in an aqueous ink, has excellent storage stability and continuous dischargeability, and also has excellent substrate adhesion (tape peel resistance) of the ink coating film of a printed material on a resin film, and to an aqueous ink containing the aqueous pigment dispersion.
[Solution] [1] An aqueous pigment dispersion of acrylic resin particles containing a pigment, wherein the acrylic resin particles containing the pigment contain an acrylic resin A containing structural units derived from (meth)acrylic acid (a-1), structural units derived from a cycloalkyl acrylate (a-2), and structural units derived from a styrene (a-3), and the mass ratio (a-2)/(a-3) of the structural units derived from the cycloalkyl acrylate (a-2) to the structural units derived from the styrene (a-3) in the acrylic resin A is 2.1 or more and 36 or less; and [2] an aqueous ink containing the aqueous pigment dispersion according to [1] above and a water-soluble organic solvent.
[Selection diagram] None

Description

The present invention relates to an aqueous pigment dispersion and an aqueous ink containing the aqueous pigment dispersion.

Printing methods using inkjet recording eject ink droplets from minute nozzles and deposit them directly onto a printing substrate, producing printed matter with recorded characters and images. This method has become extremely popular due to its many advantages: it is easy to produce full color, is inexpensive, can be used with a variety of printing substrates such as plain paper, label paper, and resin film, and does not come into contact with the substrate. In particular, from the perspective of weather resistance and water resistance of printed matter, the use of pigments as colorants has become mainstream.

Patent Document 1 discloses a pigment dispersion that contains a pigment having a specific structure and a resin that serves as a dispersant for the pigment and contains monomer units having an anionic group, with the aim of providing a pigment dispersion and inkjet ink that has excellent dispersion stability and enables stable ejection, particularly when used as an inkjet ink.

Japanese Patent Application Laid-Open No. 2016-216568

In the industrial printing market, such as packaging printing for food products, resin films are primarily used as printing substrates from the perspective of durability. A particular issue with using resin films as printing substrates is the substrate adhesion, particularly tape peel resistance, of the ink coating film of the resulting printed matter. If the tape peel resistance of the ink coating film of the printed matter is insufficient, the ink coating film is likely to peel off from the resin film, impairing the aesthetic appeal and information of the printed matter. Therefore, there is a demand for improved substrate adhesion (tape peel resistance) of the ink coating film of the printed matter obtained when printing on a resin film. In particular, when printing on a resin film using a water-based ink, the ink coating film formed by the water-based ink needs to adhere sufficiently to the resin film.
Furthermore, in the industrial printing market, such as packaging printing for food products, there is a demand for inks that enable efficient printing from the viewpoint of packaging productivity, and there is a demand for inks that can be stably ejected when inkjet printing is performed continuously, i.e., so-called continuous ejection properties. Furthermore, from the same viewpoint, there is also a demand for further improvement in the storage stability of inks.
The present invention relates to an aqueous pigment dispersion that, when used in an aqueous ink, has excellent storage stability and continuous dischargeability, and also has excellent substrate adhesion (tape peel resistance) of an ink coating film of a printed material on a resin film, and to an aqueous ink containing the aqueous pigment dispersion.

The present inventors have found that, in an aqueous pigment dispersion of pigment-containing acrylic resin particles, the above-described problems can be solved by ensuring that the mass ratio of cycloalkyl acrylate-derived structural units to styrene-derived structural units in acrylic resin A constituting the pigment-containing acrylic resin particles is within a specific range.
The present invention relates to the following [1] and [2].
[1] An aqueous pigment dispersion of acrylic resin particles containing a pigment, wherein the acrylic resin particles containing the pigment contain an acrylic resin A containing structural units derived from (meth)acrylic acid (a-1), structural units derived from a cycloalkyl acrylate (a-2), and structural units derived from a styrene (a-3), and the mass ratio (a-2)/(a-3) of the structural units derived from the cycloalkyl acrylate (a-2) to the structural units derived from the styrene (a-3) in the acrylic resin A is 2.1 or more and 36 or less.
[2] A water-based ink containing the water-based pigment dispersion according to [1] and a water-soluble organic solvent.

The present invention provides an aqueous pigment dispersion that, when used in an aqueous ink, has excellent storage stability and continuous dischargeability, and also provides excellent substrate adhesion (tape peel resistance) of the ink coating film of a printed material on a resin film, as well as an aqueous ink containing the aqueous pigment dispersion.

[Water-based pigment dispersion]
The aqueous pigment dispersion of the present invention is an aqueous pigment dispersion of acrylic resin particles containing a pigment (hereinafter also referred to as "pigment-containing acrylic resin particles"). The acrylic resin particles containing the pigment contain an acrylic resin A which contains structural units derived from (meth)acrylic acid (a-1), structural units derived from a cycloalkyl acrylate (a-2), and structural units derived from a styrene (a-3). In the acrylic resin A, the mass ratio (a-2)/(a-3) of the structural units derived from the cycloalkyl acrylate (a-2) to the structural units derived from the styrene (a-3) is 2.1 or more and 36 or less.

In the present invention, the term "aqueous system" means that water accounts for the largest proportion by mass of the liquid components. As the water, deionized water or distilled water is preferably used.
In the present invention, the term "(meth)acrylic acid" refers to acrylic acid and/or methacrylic acid, and the term "alkyl (meth)acrylate" refers to alkyl acrylate and/or alkyl methacrylate.
The term "liquid-non-absorbent" in the context of the printing substrate of the present invention means that the amount of water absorbed by the printing substrate when the printing substrate is in contact with pure water for 100 ms is 1 g/m ² or less.
The term "hydrophobic" in the context of the printing substrate according to the present invention means that the surface free energy (wetting tension) is 45 mN/m or less. The surface free energy (wetting tension) of the printing substrate is measured using a wetting tension test mixture (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in accordance with the wetting tension test method of JIS K6768:1999.
In the present invention, "adhesion to substrate" means the tape peel resistance of the ink coating film of the printed matter obtained when used for printing on a hydrophobic, non-liquid-absorbing resin film, and may be simply referred to as "adhesion to substrate."

The aqueous pigment dispersion of the present invention, when used in an aqueous ink, is excellent in storage stability and continuous dischargeability, and also in substrate adhesion (tape peel resistance) of the ink coating film of a printed material on a resin film. The reasons for this are not necessarily clear, but are thought to be as follows.
In the present invention, the acrylic resin A contained in the pigment-containing acrylic resin particles contains a structural unit derived from (meth)acrylic acid (a-1), a structural unit derived from a cycloalkyl acrylate (a-2), and a structural unit derived from styrene (a-3).
In the acrylic resin A, the structural unit of (meth)acrylic acid (a-1) functions as a dispersing group in an aqueous medium. In addition, in the acrylic resin A, the structural unit derived from styrene (a-3) is highly hydrophobic, and therefore the acrylic resin A can be strongly adsorbed to the pigment surface, thereby preventing the acrylic resin A from being detached from the pigment. Therefore, the pigment-containing acrylic resin particles of the present invention contain the acrylic resin A, and therefore can be dispersed in an aqueous pigment dispersion.
Here, if the ratio of the structural unit derived from styrene (a-3) in the acrylic resin A is high, when the ink dries and concentrates inside the nozzle or print head, the pigment-containing acrylic resin particles strongly aggregate with each other, originating from the structural unit derived from styrene (a-3). Therefore, even if the print head is refilled with the ink, the aggregates are not redispersed, which may result in a decrease in continuous ejection properties.
In the water-based pigment dispersion of the present invention, the mass ratio of the structural units derived from the cycloalkyl acrylate (a-2) to the structural units derived from the styrene (a-3) in the acrylic resin A is within a specific range.
The structural unit derived from cycloalkyl acrylate (a-2) has high mobility because it does not have a methyl group in the main chain of the acrylic resin. The cycloalkyl ester moiety is bulky and does not have the π-π stacking property of aromatic rings, so the formation of robust aggregates is suppressed, and when water-based ink (water) is supplied again during continuous ejection, the aggregates are easily redispersed. This is thought to improve both the storage stability and continuous ejection properties of the water-based ink.
Furthermore, the cycloalkyl ester moiety of the structural unit derived from the cycloalkyl acrylate (a-2) is highly hydrophobic and therefore has a high affinity for the resin film used as the printing substrate, and by using the aqueous pigment dispersion of the present invention as an aqueous ink, the wettability of the pigment-containing acrylic resin particles to the resin film is improved. Furthermore, van der Waals interactions can be efficiently expressed between the resin film and the main chain of the acrylic resin A containing the structural unit derived from the cycloalkyl acrylate (a-2), which is thought to improve the substrate adhesion of the ink coating to the resin film.

<Pigment-containing acrylic resin particles>
The form of the pigment-containing acrylic resin particles in the water-based ink of the present invention includes a form in which the pigment is encapsulated by the acrylic resin A, a form in which the pigment is uniformly dispersed in the acrylic resin A, a form in which the pigment is exposed on the surface of the acrylic resin A particles, a form in which the acrylic resin A is adsorbed to the pigment, and mixtures of these.

(Pigment)
The pigment constituting the pigment-containing acrylic resin particles according to the present invention may be either an inorganic pigment or an organic pigment.
Examples of inorganic pigments include carbon black and metal oxides, and carbon black is preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Examples of inorganic pigments for white inks include titanium dioxide, zinc oxide, silica, alumina, magnesium oxide, and other metal oxides.
Examples of organic pigments include azo pigments, diazo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxazine pigments, perylene pigments, perinone pigments, thioindigo pigments, anthraquinone pigments, and quinophthalone pigments.
In the achromatic ink, achromatic pigments such as white, black, and gray can be used, while in the chromatic ink, chromatic pigments such as yellow, magenta, cyan, red, blue, orange, and green can be used.
The pigments may be used alone or in combination of two or more.

(Acrylic Resin A)
The acrylic resin A constituting the pigment-containing acrylic resin particles contains an acrylic resin A containing structural units derived from (meth)acrylic acid (a-1), structural units derived from a cycloalkyl acrylate (a-2), and structural units derived from a styrene (a-3), and the mass ratio (a-2)/(a-3) of the structural units derived from the cycloalkyl acrylate (a-2) to the structural units derived from the styrene (a-3) in the acrylic resin A is 2.1 or more and 36 or less. Because the acrylic resin A has the above-described structure, the aqueous pigment dispersion of the present invention, when used as an aqueous ink, exhibits excellent storage stability and continuous dischargeability, and exhibits excellent substrate adhesion of the printed coating film.

[(Meth)acrylic acid (a-1)]
The (meth)acrylic acid (a-1) is at least one selected from the group consisting of acrylic acid and methacrylic acid, preferably acrylic acid, from the viewpoint of improving storage stability, continuous dischargeability, and substrate adhesion.

[Cycloalkyl acrylate (a-2)]
The number of carbon atoms in the cycloalkyl group of the cycloalkyl acrylate (a-2) is preferably 4 or more, more preferably 5 or more, from the viewpoint of improving the dispersion stability of the pigment, and thereby improving storage stability and continuous dischargeability, as well as improving adhesion to a substrate, and is preferably 12 or less, more preferably 8 or less, and even more preferably 7 or less.
The cycloalkyl acrylate (a-2) is preferably at least one selected from the group consisting of cyclopentyl acrylate, cyclohexyl acrylate, and cycloheptyl acrylate, and more preferably cyclohexyl acrylate, from the viewpoint of improving the dispersion stability of the pigment, thereby improving storage stability and continuous dischargeability, and improving adhesion to the substrate.

[Styrene (a-3)]
In the water-based pigment dispersion of the present invention, the acrylic resin A has a structural unit derived from styrene (a-3). By having a structural unit derived from styrene (a-3), storage stability is improved.

Acrylic resin A may contain structural units derived from monomers other than the monomers (a-1) to (a-3) described above, as long as the effects of the present invention are not impaired. Examples of other monomers include ionic monomers other than the monomers (a-1) to (a-3), hydrophobic monomers having aromatic groups, and nonionic monomers.

The acrylic resin A may be crosslinked as needed, that is, the acrylic resin A may have a crosslinked structure.
When the acrylic resin A has a crosslinked structure, the acrylic resin A having a crosslinked structure preferably has a structural unit derived from the monomer (a-1), a structural unit derived from the monomer (a-2), a structural unit derived from the monomer (a-3), and a structural unit derived from a crosslinking agent.

When the acrylic resin A has a crosslinked structure, the crosslinking agent is preferably a polyfunctional epoxy crosslinking agent.
The polyfunctional epoxy crosslinking agent is preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having from 3 to 8 carbon atoms, from the viewpoint of improving the dispersion stability of the pigment and further improving storage stability and continuous dischargeability.

From the viewpoint of efficient crosslinking reaction in an aqueous medium, the molecular weight of the crosslinking agent is preferably 120 or more, more preferably 150 or more, and preferably 2,000 or less, more preferably 1,500 or less, and even more preferably 500 or less.

The epoxy equivalent weight (g/eq.) of the crosslinker is preferably 90 or more, more preferably 100 or more, and preferably 300 or less, more preferably 200 or less.

From the viewpoint of efficient crosslinking reaction in an aqueous medium, the water solubility of the crosslinking agent is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. Here, the water solubility refers to the solubility (% by mass) when 10 parts by mass of the crosslinking agent is dissolved in 90 parts by mass of water at 25°C.

Preferred examples of crosslinking agents include one or more polyglycidyl ethers selected from the group consisting of trimethylolpropane polyglycidyl ether, 1,6-hexanediol diglycidyl ether, and 1,4-cyclohexanedimethanol diglycidyl ether, with trimethylolpropane polyglycidyl ether being more preferred.

When acrylic resin A has a crosslinked structure, the crosslinking rate of acrylic resin A having a crosslinked structure, expressed as the ratio of the molar equivalents of epoxy groups in the crosslinking agent to the molar equivalents of carboxy groups in acrylic resin A before crosslinking, is preferably 5 mol% or more, more preferably 10 mol% or more, and even more preferably 15 mol% or more, from the viewpoints of improving the dispersion stability of the pigment and further improving storage stability and continuous dischargeability. From the same viewpoints as above, the crosslinking rate is preferably 60 mol% or less, more preferably 55 mol% or less, and even more preferably 50 mol% or less.

(Content of each structural unit in acrylic resin A)
The content of (meth)acrylic acid (a-1) in the raw material monomers constituting the acrylic resin A, or the content of structural units derived from (meth)acrylic acid (a-1) in all structural units of the acrylic resin A, is, from the viewpoint of improving storage stability, continuous dischargeability, and substrate adhesion, preferably 15% by mass or more, more preferably 18% by mass or more, even more preferably 20% by mass or more, still more preferably 23% by mass or more, and is preferably 35% by mass or less, more preferably 32% by mass or less, even more preferably 30% by mass or less, and still more preferably 28% by mass or less.

From the viewpoint of improving storage stability, continuous dischargeability, and substrate adhesion, the content of cycloalkyl acrylate (a-2) in the raw material monomers constituting acrylic resin A, or the content of structural units derived from cycloalkyl acrylate (a-2) in all structural units of acrylic resin A, is preferably 20% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and even more preferably 55% by mass or more, and is preferably 80% by mass or less, more preferably 75% by mass or less, even more preferably 70% by mass or less, and even more preferably 65% by mass or less.

From the viewpoint of improving storage stability, continuous dischargeability, and substrate adhesion, the content of styrene (a-3) in the raw material monomers constituting acrylic resin A, or the content of structural units derived from styrene (a-3) in all structural units of acrylic resin A, is preferably 2% by mass or more, more preferably 4% by mass or more, even more preferably 6% by mass or more, still more preferably 10% by mass or more, and is preferably 24% by mass or less, more preferably 22% by mass or less, even more preferably 18% by mass or less, and still more preferably 15% by mass or less.

The mass ratio of the content of (meth)acrylic acid (a-1) to the content of cycloalkyl acrylate (a-2) in the raw material monomers constituting acrylic resin A, or the mass ratio (a-1)/(a-2) of the content of structural units derived from (meth)acrylic acid (a-1) to the content of structural units derived from cycloalkyl acrylate (a-2) in acrylic resin A, is preferably 0.2 or more, more preferably 0.25 or more, even more preferably 0.3 or more, from the viewpoint of improving storage stability, continuous dischargeability, and substrate adhesion, and is preferably 1.5 or less, more preferably 1.0 or less, even more preferably 0.75 or less, and even more preferably 0.5 or less.

The mass ratio of the content of (meth)acrylic acid (a-1) to the content of styrene (a-3) in the raw material monomers constituting acrylic resin A, or the mass ratio (a-1)/(a-3) of the content of structural units derived from (meth)acrylic acid (a-1) to the content of structural units derived from styrene (a-3) in acrylic resin A, is preferably 0.65 or more, more preferably 1.0 or more, even more preferably 1.2 or more, still more preferably 1.5 or more, from the viewpoint of improving storage stability, continuous dischargeability, and substrate adhesion, and is preferably 15 or less, more preferably 10 or less, even more preferably 6.0 or less, and still more preferably 3.0 or less.

The mass ratio of the content of cycloalkyl acrylate (a-2) to the content of styrene (a-3) in the raw material monomers constituting acrylic resin A, or the mass ratio (a-2)/(a-3) of the content of structural units derived from cycloalkyl acrylate (a-2) to the content of structural units derived from styrene (a-3) in acrylic resin A, is 2.1 or more, preferably 2.5 or more, more preferably 3.0 or more, even more preferably 4.0 or more, still more preferably 5.0 or more, and is 36 or less, preferably 25 or less, more preferably 15 or less, even more preferably 10 or less, and still more preferably 6.0 or less, from the viewpoint of improving storage stability, continuous dischargeability, and substrate adhesion.

(Physical properties of acrylic resin A)
From the viewpoint of improving storage stability, continuous dischargeability, and substrate adhesion, the acid value of the acrylic resin A is preferably 50 mgKOH/g or more, more preferably 100 mgKOH/g or more, even more preferably 150 mgKOH/g or more, and is preferably 300 mgKOH/g or less, more preferably 260 mgKOH/g or less, even more preferably 230 mgKOH/g or less.
The acid value of the acrylic resin A is measured by the method described in the examples.

The weight average molecular weight of the acrylic resin A is preferably 5,000 or more, more preferably 10,000 or more, and is preferably 100,000 or less, more preferably 50,000 or less, and even more preferably 25,000 or less, from the viewpoint of improving storage stability, continuous dischargeability, and substrate adhesion.
The weight average molecular weight of the acrylic resin A is measured by the method described in the examples.

(Production of acrylic resin A)
The acrylic resin A can be produced by copolymerizing raw material monomers including the above-mentioned monomers (a-1) to (a-3) by a known polymerization method.

(Neutralizing agent)
The acid groups of the acrylic resin A may be neutralized with a neutralizing agent. This increases the charge repulsion force of the acid groups that occurs after neutralization, which is thought to suppress aggregation of the pigment-containing acrylic resin particles in the aqueous dispersion of the present invention, further improving dispersion stability, storage stability, and continuous dischargeability.
In the case of neutralization, it is preferable to neutralize so that the pH is 7 or more and 11 or less.

Examples of the neutralizing agent include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, ammonia, and bases such as organic amines. Among these, preferred is one or more selected from the group consisting of sodium hydroxide, potassium hydroxide, and organic amines, and more preferred is sodium hydroxide.
Suitable examples of organic amines used as neutralizing agents include alkylamines, alkanolamines, aminoalkanediols, alkoxyamines, and heterocyclic amines. Among these, alkanolamines are more preferred.
The alkanolamine is preferably at least one selected from the group consisting of triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, N,N-dimethylethanolamine, and N,N-diethylethanolamine, and more preferably at least one selected from the group consisting of triethanolamine and N,N-dimethylethanolamine.

From the viewpoint of improving continuous dischargeability, the degree of neutralization of the acid groups of the acrylic resin A is preferably 20 mol % or more, more preferably 30 mol % or more, and is preferably 100 mol % or less, more preferably 90 mol % or less.
Here, the degree of neutralization can be calculated by the following formula (1) as the equivalent amount of neutralizer used relative to the acid groups of acrylic resin A. When the equivalent amount of neutralizer used is 100 mol% or less, it is synonymous with the degree of neutralization. When the equivalent amount of neutralizer used exceeds 100 mol%, it means that the neutralizer is in excess relative to the carboxy groups of acrylic resin A, and in this case, the degree of neutralization of acrylic resin A is considered to be 100 mol%.
Equivalent amount of neutralizing agent used (mol %)=[{weight (g) of neutralizing agent added/equivalent amount of neutralizing agent}/[{acid value of acrylic resin A (mg KOH/g) × weight (g) of acrylic resin A}/(56 × 1000)]] × 100 (1)

<Method for producing water-based pigment dispersion>
The water-based pigment dispersion of the present invention can be efficiently produced by a method including the following steps 1 and 2.
Step 1: A step of dispersing a pigment mixture containing a pigment, acrylic resin A, an organic solvent, and water to obtain a pigment dispersion. Step 2: A step of removing the organic solvent from the pigment dispersion obtained in Step 1 to obtain an aqueous pigment dispersion of acrylic resin particles containing the pigment.

Furthermore, when the acrylic resin A has a crosslinked structure, the method for producing the water-based pigment dispersion in the present invention preferably further includes the following step 3 after step 2.
Step 3: A step of adding a polyfunctional epoxy crosslinking agent to the aqueous pigment dispersion obtained in Step 2, and crosslinking the acrylic resin A with the polyfunctional epoxy crosslinking agent.

(Step 1)
The pigment mixture in step 1 is preferably obtained by a method in which a pigment, water, and, if necessary, a neutralizing agent, a surfactant, and the like are added to an organic solvent solution of acrylic resin A obtained by dissolving acrylic resin A in an organic solvent, and then the mixture is mixed.
Although there are no restrictions on the organic solvent used in step 1, preferred are ketones, ethers, esters, aliphatic alcohols having from 1 to 3 carbon atoms, and from the viewpoint of improving the wettability to the pigment and the adsorption of the water-dispersible polymer to the pigment, more preferred are ketones having from 4 to 8 carbon atoms, even more preferred are methyl ethyl ketone and methyl isobutyl ketone, and even more preferred is methyl ethyl ketone. When acrylic resin A is synthesized by solution polymerization, the solvent used in polymerization may be used as is.
The acrylic resin A may be previously neutralized with a neutralizing agent.
When a surfactant is used in step 1, the surfactant may be the same as that used in the water-based ink of the present invention, which will be described later.

In the dispersion treatment in step 1, the pigment can be atomized to a desired particle size by main dispersion using shear stress alone, but from the viewpoint of obtaining a uniform aqueous pigment dispersion, it is preferable to pre-disperse the pigment mixture and then further carry out main dispersion.
As a dispersing machine used for preliminary dispersion, a commonly used mixing and stirring device such as an anchor blade or a disperser blade can be used.
Examples of means for applying shear stress used in the dispersion include kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersers such as paint shakers and bead mills. Among these, from the viewpoint of reducing the particle size of the pigment, it is preferable to use one or more types selected from the group consisting of high-pressure homogenizers and bead mills.

(Step 2)
The organic solvent can be removed by a known method in step 2. It is preferable that the organic solvent is substantially removed from the obtained aqueous pigment dispersion, but it may remain in an amount of, for example, 0.1% by mass or less, as long as the object of the present invention is not impaired.
Furthermore, in order to remove coarse particles and the like, it is preferable that the aqueous pigment dispersion from which the organic solvent has been removed is further centrifuged, and then the liquid phase portion is recovered and filtered with a filter or the like, and the portion that has passed through the filter or the like is obtained as the aqueous pigment dispersion.

(Step 3)
Step 3 is a step of adding a polyfunctional epoxy crosslinking agent to the aqueous pigment dispersion obtained in step 2, and crosslinking the acrylic resin A with the polyfunctional epoxy crosslinking agent.
In the present invention, when the acrylic resin A has a crosslinked structure, a crosslinked structure can be formed in the surface layer portion of the pigment-containing acrylic resin particles formed in step 2. This is thought to cause the polymer formed by crosslinking the acrylic resin A with the polyfunctional epoxy crosslinking agent to be firmly adsorbed or fixed to the pigment surface, thereby suppressing aggregation of the pigment. As a result, it is thought that the dispersion stability of the pigment in the resulting water-based ink is improved, and the storage stability and continuous ejection ability are further improved.
From the viewpoint of reaction efficiency, the temperature of the crosslinking treatment in step 3 is preferably 40° C. or higher, more preferably 50° C. or higher, and preferably 100° C. or lower, more preferably 95° C. or lower.

<Physical properties of water-based pigment dispersion>
[0047] The solids concentration of the aqueous pigment dispersion of the present invention is, from the viewpoint of productivity of the aqueous ink, improving storage stability and continuous ejection property, and improving adhesion to a substrate, preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and is preferably 35% by mass or less, more preferably 30% by mass or less, even more preferably 25% by mass or less.
The solid content concentration is measured by the method described in the Examples.

From the viewpoints of productivity of the water-based ink, improving storage stability and continuous ejection, and print density, the pigment content in the water-based pigment dispersion of the present invention is preferably 3% by mass or more, more preferably 7% by mass or more, even more preferably 10% by mass or more, and is preferably 28% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less.

The mass ratio of the pigment content to the total content of the pigment and acrylic resin in the aqueous pigment dispersion of the present invention [pigment/(pigment + acrylic resin)] is preferably 0.45 or more, more preferably 0.50 or more, even more preferably 0.55 or more, from the viewpoints of improving storage stability and continuous dischargeability, and improving substrate adhesion, and is preferably 0.80 or less, more preferably 0.75 or less, even more preferably 0.70 or less, and even more preferably 0.65 or less.

[0044] The average particle size of the pigment-containing acrylic resin particles in the aqueous pigment dispersion of the present invention is, from the viewpoint of improving continuous dischargeability and improving adhesion to a substrate, preferably 50 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, still more preferably 90 nm or more, and is preferably 400 nm or less, more preferably 300 nm or less, even more preferably 200 nm or less, and still more preferably 150 nm or less.
The average particle size of the pigment-containing acrylic resin particles in the water-based pigment dispersion of the present invention is measured by the method described in the Examples.

From the viewpoint of productivity of the aqueous ink, the viscosity of the aqueous pigment dispersion of the present invention at 20°C is preferably 1.0 mPa s or more, more preferably 1.5 mPa s or more, even more preferably 2.0 mPa s or more, and is preferably 15 mPa s or less, more preferably 10 mPa s or less, even more preferably 5 mPa s or less.
The viscosity at 20° C. of the water-based pigment dispersion of the present invention can be determined by the method described in the examples.

[Water-based ink]
The water-based ink of the present invention contains the water-based pigment dispersion of the present invention and a water-soluble organic solvent. Because the water-based ink of the present invention contains the water-based pigment dispersion of the present invention and a water-soluble organic solvent, the water-based ink of the present invention has excellent storage stability and continuous dischargeability, and also has excellent substrate adhesion of the ink coating film of a printed material onto a resin film.
<Water-soluble organic solvent>
The water-soluble organic solvent is an organic solvent that can be mixed with water in any ratio. The water-soluble organic solvents can be used alone or in combination of two or more.
Examples of the water-soluble organic solvent include polyhydric alcohols, polyhydric alcohol alkyl ethers, nitrogen-containing heterocyclic compounds, amides, amines, and sulfur-containing compounds. Among these, the water-soluble organic solvent preferably contains at least one selected from the group consisting of polyhydric alcohols and polyhydric alcohol alkyl ethers, from the viewpoints of improving storage stability and continuous dischargeability, and improving substrate adhesion.

Examples of polyhydric alcohols include alkanediols such as ethylene glycol, propylene glycol (1,2-propanediol), 1,2-pentanediol, 1,2-hexanediol, and 1,2-octanediol; polyalkylene glycols such as diethylene glycol and triethylene glycol; glycerin; and trimethylolpropane. Among these, the polyhydric alcohol is preferably one or more selected from the group consisting of propylene glycol, 1,2-hexanediol, triethylene glycol, and glycerin, and more preferably propylene glycol.

Examples of the polyhydric alcohol ether include (poly)alkylene glycol monoalkyl ethers such as monoalkylene glycol monoalkyl ethers, dialkylene glycol monoalkyl ethers, and trialkylene glycol monoalkyl ethers; and (poly)alkylene glycol dialkyl ethers such as monoalkylene glycol dialkyl ethers and dialkylene glycol dialkyl ethers. Among these, the polyhydric alcohol ether is preferably a (poly)alkylene glycol monoalkyl ether, and more preferably a dialkylene glycol monoalkyl ether.
The alkylene oxide group of the polyhydric alcohol ether may be at least one selected from the group consisting of an ethylene oxide group and a propylene oxide group, with an ethylene oxide group being more preferred.
The polyhydric glycol ether has at least one hydrocarbon group having 2 to 8 carbon atoms.

Preferred examples of (poly)alkylene glycol monoalkyl ethers include one or more selected from the group consisting of ethylene glycol monoalkyl ethers such as ethylene glycol monoisopropyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol monoisobutyl ether, and ethylene glycol mono-n-hexyl ether; diethylene glycol monoalkyl ethers such as diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol mono-n-butyl ether, diethylene glycol monoisobutyl ether, and diethylene glycol mono-n-hexyl ether; triethylene glycol monoalkyl ethers such as triethylene glycol monobutyl ether; propylene glycol monoalkyl ethers such as propylene glycol mono-n-propyl ether; dipropylene glycol monoalkyl ethers; tripropylene glycol monoalkyl ethers; and ethylene glycol aryl ethers such as ethylene glycol monobenzyl ether. Among these, preferred (poly)alkylene glycol monoalkyl ethers are diethylene glycol monoalkyl ethers, and more preferably diethylene glycol mono-n-butyl ether.

(Poly)alkylene glycol dialkyl ethers include diethylene glycol dialkyl ethers such as diethylene glycol methyl ethyl ether and diethylene glycol diethyl ether.

From the viewpoints of improving storage stability and continuous dischargeability, and improving substrate adhesion, the water-soluble organic solvent preferably contains one or more solvents selected from the group consisting of polyhydric alcohols and polyhydric alcohol alkyl ethers, more preferably one or more solvents selected from the group consisting of polyhydric alcohols and (poly)alkylene glycol monoalkyl ethers, even more preferably one or more solvents selected from the group consisting of polyhydric alcohols and dialkylene glycol monoalkyl ethers, and even more preferably one or more solvents selected from the group consisting of propylene glycol and diethylene glycol mono-n-butyl ether.

(Various additives)
The water-based ink of the present invention may contain various additives, such as a fixing resin, a surfactant, a humectant, a wetting agent, a wetting/penetrating agent, a viscosity adjuster, an antifoaming agent, an antiseptic, an antifungal agent, and an antirust agent, as needed.

[Surfactant]
Examples of surfactants include nonionic surfactants, anionic surfactants, and amphoteric surfactants. Of these, nonionic surfactants are preferred.
Examples of nonionic surfactants include polyoxyalkylene alkyl ether surfactants, acetylene glycol surfactants, polyhydric alcohol surfactants, fatty acid alkanolamides, silicone surfactants, and fluorine surfactants.
Among these, from the viewpoints of improving storage stability and continuous ejection property, defoaming property, and wettability to a resin film, one or more surfactants selected from the group consisting of acetylene glycol-based surfactants and silicone-based surfactants are preferred, and acetylene glycol-based surfactants are more preferred.
Examples of acetylene glycol surfactants include acetylene diols such as 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol, 3,5-dimethyl-1-hexyn-3-ol, and 2,4-dimethyl-5-hexyn-3-ol, and ethylene oxide adducts of these acetylene diols.
Examples of commercially available acetylene glycol surfactants include the "Surfynol" series and the "Olfine" series manufactured by Nissin Chemical Industry Co., Ltd.
As the silicone surfactant, polyether-modified silicone is preferred.
Commercially available examples of silicone surfactants include the KF series (KF-353, KF-355A, KF-642, KF-6011, etc.) manufactured by Shin-Etsu Chemical Co., Ltd., the Silface SAG series manufactured by Nissin Chemical Industry Co., Ltd., and the BYK series manufactured by BYK Japan K.K.

<Method of manufacturing water-based ink>
The water-based ink of the present invention can be produced by mixing pigment-containing acrylic resin particles, a water-soluble organic solvent, water, and, if necessary, various additives such as a surfactant and a fixing resin.

<Contents and properties of each component of water-based ink>
From the viewpoints of improving continuous jetting ability, improving adhesion to the substrate, and print density, the content of pigment-containing acrylic resin particles in the water-based ink of the present invention is preferably 2% by mass or more, more preferably 4% by mass or more, even more preferably 6% by mass or more, and is preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less.

From the viewpoint of print density, the pigment content in the water-based ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, and preferably 12% by mass or less, more preferably 9% by mass or less, even more preferably 6% by mass or less.

From the viewpoint of improving continuous jetting properties and improving adhesion to the substrate, the content of acrylic resin A in the water-based ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, and preferably 8% by mass or less, more preferably 6% by mass or less, even more preferably 4% by mass or less.

From the viewpoint of improving substrate adhesion, the mass ratio of the pigment content to the total content of pigment and acrylic resin A in the water-based ink of the present invention [pigment/(pigment + acrylic resin A)] is preferably 0.45 or more, more preferably 0.50 or more, even more preferably 0.55 or more, and is preferably 0.80 or less, more preferably 0.75 or less, even more preferably 0.70 or less, and even more preferably 0.65 or less.

From the viewpoint of improving continuous jetting properties and improving adhesion to the substrate, the content of the water-soluble organic solvent in the water-based ink of the present invention is preferably 5% by mass or more, more preferably 8% by mass or more, even more preferably 10% by mass or more, and is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less.

When the water-based ink of the present invention contains a surfactant, the content of the surfactant in the water-based ink of the present invention is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, from the viewpoint of improving continuous ejection properties and improving adhesion to the substrate, and is preferably 2% by mass or less, more preferably 1.5% by mass or less, even more preferably 1% by mass or less.

From the viewpoint of improving continuous jetting properties and improving adhesion to the substrate, the water content in the water-based ink of the present invention is preferably 35% by mass or more, more preferably 45% by mass or more, even more preferably 55% by mass or more, and is preferably 93% by mass or less, more preferably 90% by mass or less, even more preferably 85% by mass or less.

The average particle size of the pigment-containing acrylic resin particles in the water-based ink of the present invention is preferably 50 nm or more, more preferably 70 nm or more, from the viewpoint of improving continuous jetting properties and improving adhesion to a substrate, and is preferably 400 nm or less, more preferably 200 nm or less, and even more preferably 150 nm or less.
The average particle size of the pigment-containing acrylic resin particles in the water-based ink of the present invention is measured by the method described in the Examples.

The viscosity of the water-based ink of the present invention at 20° C. is preferably 1 mPa·s or more, more preferably 2 mPa·s or more, and is preferably 12 mPa·s or less, more preferably 9 mPa·s or less.
The viscosity of the water-based ink is measured by the method described in the examples.

The water-based ink of the present invention can be used as an ink for various printing purposes such as inkjet printing, gravure printing, and flexographic printing. From the viewpoints of improving continuous ejection properties and improving adhesion to a substrate, it is preferably used as a water-based ink for inkjet printing.
When the aqueous ink of the present invention is used as an aqueous ink for inkjet printing, the aqueous ink can be loaded into a known inkjet printing device and ejected as ink droplets onto a printing substrate such as a resin film to print an image, etc. The ink droplet ejection method can be any of a piezoelectric, thermal, or electrostatic method.

From the viewpoint of substrate adhesion, a resin film is preferred as a printing substrate used in printing with the water-based ink of the present invention. That is, the water-based ink of the present invention is preferably used in printing using a resin film as a printing substrate.
Examples of the resin film include transparent synthetic resin films, such as polyester films such as polyethylene terephthalate films; vinyl chloride films; polyolefin films such as polypropylene films and polyethylene films; and polyamide films such as nylon films. These resin films may be stretched films such as biaxially stretched films and uniaxially stretched films, or unstretched films. Furthermore, it is preferable to use these resin films that have been subjected to surface treatment such as corona discharge treatment, in order to impart polar functional groups to the resin film surface, improve the wettability of the resin film with aqueous ink, and form hydrogen bonds between the polar functional groups and the cycloalkyl ester moieties to improve substrate adhesion.
Among these, a resin film having a surface free energy of 45 mN/m or less is preferred, and one or more selected from the group consisting of polyester film, oriented polypropylene film, and polyethylene film is more preferred, and one or more selected from the group consisting of corona discharge-treated polyethylene terephthalate (PET) film, corona discharge-treated biaxially oriented polypropylene (OPP) film, corona discharge-treated polyethylene (PE) film, etc. is even more preferred.

In the following production examples, examples, and comparative examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. The methods for measuring and calculating each physical property are as follows:

(Measurement method)
[Measurement of weight average molecular weight of acrylic resin A]
The measurement was performed by gel permeation chromatography under the following conditions.
GPC device: Tosoh Corporation "HLC-8320GPC"
Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolum Super AW-H" manufactured by Tosoh Corporation
Eluent: N,N-dimethylformamide dissolved with phosphoric acid and lithium bromide at concentrations of 60 mmol/L and 50 mmol/L, respectively. Flow rate: 0.5 mL/min
Standard material: Monodisperse polystyrene kit with known molecular weight [PStQuick B (F-550, F-80, F-10, F-1, A-1000), PStQuick C (F-288, F-40, F-4, A-5000, A-500)] (Tosoh Corporation)
Measurement sample: 0.1 g of acrylic resin A was mixed with 10 mL of the eluent in a glass vial, stirred with a magnetic stirrer at 25°C for 10 hours, and filtered through a syringe filter "DISMIC-13HP" (PTFE, 0.2 µm, manufactured by ADVANTEC) and used.

(Measurement method)
[Measurement of Acid Value of Acrylic Resin A]
Acrylic resin A was dissolved in a titration solvent consisting of a mixture of toluene and acetone (2:1) in an automatic potentiometric titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with a 0.1 N potassium hydroxide/ethanol solution by potentiometric titration, with the inflection point on the titration curve being the endpoint. The acid value (mg KOH/g) was calculated from the titration amount of the potassium hydroxide solution up to the endpoint.

[Measurement of solids concentration]
Approximately 10 g of sodium sulfate was weighed out in a 30 mL polypropylene container (φ = 40 mm, height = 30 mm) at a constant weight in a desiccator, and approximately 1 g of the sample was added and mixed, after which the mass was precisely weighed. This was then maintained at 105°C for 2 hours to remove volatiles, and the container was left in the desiccator for a further 15 minutes before being precisely weighed. The mass of the sample after volatiles removal was taken as the solid content and divided by the mass of the added sample to obtain the solid content concentration (%).

[Measurement of average particle size of water-based pigment dispersion and water-based ink]
Cumulant analysis was performed using a laser particle analysis system "ELS-8000" (manufactured by Otsuka Electronics Co., Ltd.), and the obtained cumulant average particle size was taken as the average particle size of the water-based pigment dispersion or water-based ink. The measurement sample used was a dispersion diluted with water so that the concentration of the particles to be measured was 5 x ^10-3 % (converted to solids concentration). The measurement conditions were a temperature of 25°C, an angle between the incident light and the detector of 90°, and 100 cumulative measurements, and the refractive index of water (1.333) was entered as the refractive index of the dispersion solvent.

[Viscosity of Water-Based Pigment Dispersion and Water-Based Ink]
The aqueous pigment dispersion and the aqueous ink were measured at 20° C. using an E-type viscometer "TV-25" (manufactured by Toki Sangyo Co., Ltd., using a standard cone rotor 1°34' x R24, rotation speed 50 rpm).

(Production of acrylic resin A)
Production Example 1 (Production of Acrylic Resin A1) A reaction vessel equipped with a stirrer, a reflux condenser, and a dropping tank was initially charged with 2.6 parts of acrylic acid, 7.1 parts of cyclohexyl acrylate, and 0.3 parts of styrene as raw material monomers, 13.5 parts of methyl ethyl ketone (hereinafter referred to as "MEK") as a polymerization solvent, and 1.5 parts of deionized water (one-tenth of the total amount of the raw material monomers and the polymerization solvent), and the mixture was stirred for 10 minutes while maintaining the temperature of the reaction vessel at 77°C.
Next, 23.4 parts of acrylic acid, 63.9 parts of cyclohexyl acrylate, and 2.7 parts of styrene as raw material monomers, 121.5 parts of MEK as a polymerization solvent, and 13.5 parts of deionized water (9/10 of the total amount of raw material monomers and polymerization solvent), 0.56 parts of 4,4'-azobis(4-cyanovaleric acid) as a polymerization initiator, and a mixture of 0.41 parts of 3-mercaptopropionic acid as a chain transfer agent were continuously added to the reaction vessel over 5 hours. After completion of the addition, the polymerization reaction was carried out for 1 hour, and then the polymerization reaction was terminated by cooling to room temperature, yielding a solution of acrylic resin A1 (resin solids concentration 40%). The physical properties of the obtained acrylic resin A1 are shown in Table 1.

Production Examples 2 to 10, 21 to 23 (Production of Acrylic Resins A2 to A10, A21 to A23)
A solution of acrylic resin A (resin solids concentration 40%) was obtained in the same manner as in Production Example 1, except that the monomer composition constituting acrylic resin A in Production Example 1 was changed to the conditions shown in Table 1. Acrylic resin A23 obtained in Production Example 23 did not use a raw material monomer for a structural unit derived from cycloalkyl acrylate, but instead used cyclohexyl methacrylate. The physical properties of each acrylic resin A obtained are shown in Table 1.

(Production of Water-Based Pigment Dispersion)
Example 1-1 (Production of Water-Based Pigment Dispersion D1)
To 41.7 parts of the acrylic resin A1 solution (resin solids concentration: 40%) obtained in Production Example A1, 0.6 parts of MEK, 8.5 parts of a 5N aqueous sodium hydroxide solution (degree of neutralization: 60 mol%), and 163.7 parts of deionized water were added, and 25.0 parts of a cyan pigment (manufactured by DIC Corporation, trade name: Fastogen Blue CA5380 15:3) were further added to obtain a pigment mixed liquid (ratio of the pigment mass to the total mass of the pigment and acrylic resin A [pigment/(pigment + acrylic resin A)] = 0.60).
The obtained pigment mixture was mixed for 1 hour using a disper blade at 7000 rpm and 20°C, and then further subjected to a dispersion treatment of 10 passes at a pressure of 180 MPa using a Microfluidizer (high-pressure homogenizer, manufactured by Microfluidics, product name: M-140K), to obtain a pigment dispersion.
From the obtained pigment dispersion, MEK was removed at 60°C under reduced pressure, and then some of the water was removed and the mixture was centrifuged. The liquid layer was then filtered through a membrane filter (manufactured by Sartorius, trade name: Minisart Syringe Filter, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, yielding an aqueous pigment dispersion D1 of pigment-containing acrylic resin particles (total concentration of pigment and acrylic resin A: 20%). The physical properties of the obtained aqueous pigment dispersion D1 are shown in Table 2.

Examples 1-2 to 1-13 and Comparative Examples 1-1 to 1-3 (Production of Water-Based Pigment Dispersions D2 to D13 and D21 to D23)
Aqueous dispersions of pigment-containing acrylic resin particles (total concentration of pigment and acrylic resin A: 20%, degree of neutralization of resin: 60 mol%) were obtained in the same manner as in Example 1-1, except that the type of acrylic resin A and the type of neutralizing agent in Example 1-1 were changed as shown in Table 2. The physical properties of the obtained aqueous pigment dispersions are shown in Table 2.

Example 1-14 (Preparation of Water-Based Pigment Dispersion D14)
94.32 parts of the obtained aqueous pigment dispersion D3 was transferred to a screw-cap glass bottle, and 1.14 parts of trimethylolpropane polyglycidyl ether (Denacol EX-321LT, manufactured by Nagase ChemteX Corporation, molecular weight: 302, epoxy value: 139 g/eq., water solubility: 27%) as a polyfunctional epoxy crosslinker and deionized water were added so that the total amount of the aqueous pigment dispersion was 100 parts. The bottle was then sealed and heated at 90°C for 5 hours while stirring with a stirrer. The temperature was then lowered to room temperature, and the bottle was filtered through a membrane filter (manufactured by Sartorius GmbH, trade name: Minisart Syringe Filter, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, yielding aqueous pigment dispersion D14 (total concentration of pigment and acrylic resin A3 having a crosslinked structure: 20%) containing pigment-containing acrylic resin particles having a crosslinked structure. The physical properties of each aqueous pigment dispersion D14 obtained are shown in Table 2.

(Production of water-based ink)
Example 2-1 (Production of Water-Based Ink 1)
To obtain the ink composition shown in Table 3, Water-Based Pigment Dispersion D1 of pigment-containing acrylic resin particles was mixed with 8.3 parts (pigment content 5.0 parts) as the total of the pigment and acrylic resin A, 8.0 parts of propylene glycol, 5.0 parts of diethylene glycol monobutyl ether, 0.5 parts of an acetylene glycol surfactant (manufactured by Nissin Chemical Industry Co., Ltd., trade name: Surfynol 440 (ethylene oxide (9 moles) adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol, active content 100%)), and deionized water so that the total amount of the water-based ink was 100 parts. The mixture was stirred and then filtered through a membrane filter (trade name: Minisart Syringe Filter, pore size: 5 μm, material: cellulose acetate), to obtain Water-Based Ink 1. The physical properties of the resulting Water-Based Ink 1 are shown in Table 3.

Examples 2-2 to 2-14 and Comparative Examples 2-1 to 2-3 (Production of Water-Based Inks 2 to 14, 21 to 23)
Each water-based ink was obtained in the same manner as in Example 2-1, except that the composition of the water-based ink was changed to the conditions shown in Table 3. The physical properties of each water-based ink obtained are shown in Table 3.

(Evaluation method)
The storage stability, continuous ejection property, and substrate adhesion of the water-based inks 1 to 14 and 21 to 23 obtained in the above Examples and Comparative Examples were evaluated by the methods described below. The evaluation results are shown in Table 3.

<Evaluation of storage stability>
The water-based inks of the Examples and Comparative Examples were stored in a thermostatic chamber at 70° C. for 7 days, and the viscosity of the water-based ink after storage was measured in the same manner as in the above "Viscosity of water-based pigment dispersion and water-based ink."
The viscosity change rate was calculated from the viscosity before and after storage using the following formula (decimals are discarded). The smaller the viscosity change rate, the better the storage stability.
Viscosity change rate (%) = [[viscosity after storage/viscosity before storage] - 1] x 100

<Evaluation of continuous ejection>
The ink supply tube of an inkjet recording device "LPP-6010N" (manufactured by LG Electronics) equipped with a thermal inkjet head was pulled out and inserted into an ink tank, and the device was modified to enable printing. Each water-based ink was filled into the ink tank, and the printing conditions were set to best mode (resolution: 1,600 dpi vertical x 1,600 dpi horizontal) and a duty of 100%.
The device was placed in an environment of a temperature of 25±1°C and a relative humidity of 30±5%, one sheet of A4 size plain paper was printed, the print density was measured with an optical densitometer, and then printing was continued while measuring the print density with the optical densitometer every 100 sheets, and the number of sheets that could be printed when the print density of the first sheet was 0.04 lower for two consecutive times was used as an index of ejection property, and this number was evaluated according to the following evaluation criteria. In the following evaluation criteria, the more sheets that could be printed, the better the continuous ejection property.
HP All-in-One printing paper (manufactured by Hewlett-Packard) was used as the plain paper, and SpectroEye (manufactured by X-Rite) was used as the optical densitometer.
(Evaluation criteria)
A: 15,000 or more sheets of A4 size plain paper B: 10,000 or more sheets but less than 15,000 sheets of A4 size plain paper C: 7,500 or more sheets but less than 10,000 sheets of A4 size plain paper D: 1,000 or more sheets but less than 7,500 sheets of A4 size plain paper

<Evaluation of Adhesion to Substrate>
A printing evaluation device (Seiko Epson Corporation, inkjet printer, PX105, piezo type) was used, and an A4 size film heater (Kawai Electric Works, Ltd.) was fixed to the printing substrate discharge section so that the area where the ink was injected could be heated to 50°C.
The printing substrate used was a resin film shown below cut to A4 size.
In an environment of a temperature of 25±1°C and a relative humidity of 30±5%, the cartridges of the printing evaluation device were filled with each of the aqueous inks of the Examples and Comparative Examples, and a 50 mm x 50 mm solid image was printed with the ink at 100% duty. The image was then dried for 3 minutes on a film heater set to heat at 50°C to obtain a printed product. A 50 mm long, 15 mm wide tape "Nice Tack No. 4" (registered trademark) (manufactured by Nichiban Co., Ltd.) was attached to the printed surface of the obtained printed product over a 4 cm area, leaving a 1 cm margin, and a T-peel test was performed using a Tensilon universal material testing machine (manufactured by A&D Co., Ltd., product name: RTC-1150A). The peel strength (N/15 mm), which is an index of substrate adhesion, was measured, and the substrate adhesion was evaluated according to the following evaluation criteria.
The larger the value, the better the adhesion to the substrate.
(Printing base material)
OPP film: Polypropylene film, manufactured by Futamura Chemical Co., Ltd., product number: FOR-AQ, thickness: 20 μm

From Table 3, it was confirmed that the water-based inks of Examples 2-1 to 2-14, which contained the water-based pigment dispersions of Examples 1-1 to 1-14, were excellent in storage stability and continuous dischargeability, and also in substrate adhesion of the ink coating film of the printed matter to the resin film.
On the other hand, the water-based ink of Comparative Example 2-1 containing the water-based pigment dispersion of Comparative Example 1-1 had a mass ratio (a-2)/(a-3) of the structural units derived from cycloalkyl acrylate (a-2) to the structural units derived from styrene (a-3) in the acrylic resin A of more than 36, and therefore the storage stability and continuous ejection properties were poor.
Furthermore, the water-based ink of Comparative Example 2-2, which contained the water-based pigment dispersion of Comparative Example 1-2, had a mass ratio (a-2)/(a-3) of the structural units derived from cycloalkyl acrylate (a-2) to the structural units derived from styrene (a-3) in the acrylic resin A of less than 2.1, and therefore exhibited poor continuous ejection properties.
Furthermore, the aqueous ink of Comparative Example 2-3 containing the aqueous pigment dispersion of Comparative Example 1-3 had poor continuous ejection properties because the acrylic resin A did not have a structural unit derived from cycloalkyl acrylate (a-1).

According to the present invention, it is possible to obtain an aqueous pigment dispersion that, when used in an aqueous ink, has excellent storage stability and continuous dischargeability, and also has excellent substrate adhesion (tape peel resistance) of an ink coating film of a printed material on a resin film, and an aqueous ink containing the aqueous pigment dispersion.

Claims (8)

An aqueous pigment dispersion of acrylic resin particles containing a pigment,
the acrylic resin particles containing the pigment contain an acrylic resin A that includes a structural unit derived from (meth)acrylic acid (a-1), a structural unit derived from a cycloalkyl acrylate (a-2), and a structural unit derived from styrene (a-3);
the mass ratio (a-2)/(a-3) of the structural units derived from the cycloalkyl acrylate (a-2) to the structural units derived from the styrene (a-3) in the acrylic resin A is 2.1 or more and 36 or less. The aqueous pigment dispersion according to claim 1, wherein the content of structural units derived from the cycloalkyl acrylate (a-2) in all structural units of the acrylic resin A is 20% by mass or more and 80% by mass or less. The aqueous pigment dispersion according to claim 1, wherein the content of structural units derived from the (meth)acrylic acid (a-1) in all structural units of the acrylic resin A is 15% by mass or more and 35% by mass or less. The aqueous pigment dispersion according to claim 1, wherein the acid value of the acrylic resin A is 50 mg KOH/g or more and 300 mg KOH/g or less. The aqueous pigment dispersion according to claim 1, wherein the cycloalkyl acrylate (a-2) is one or more selected from the group consisting of cyclopentyl acrylate, cyclohexyl acrylate, and cyclopentyl acrylate. A water-based ink containing the water-based pigment dispersion according to any one of claims 1 to 5 and a water-soluble organic solvent. The water-based ink according to claim 6, which is used for printing on a resin film as the printing substrate. The water-based ink of claim 6 for ink-jet printing.