JP7521950B2 - Water-based ink for inkjet printing - Google Patents

Description

The present invention relates to a water-based ink for inkjet printing and an inkjet printing method using the same.

2. Description of the Related Art In the field of commercial printing for product packaging, advertisements, and the like, inkjet printing methods using water-based inks have attracted attention for printing media such as resin films of polyethylene terephthalate, polyethylene, polypropylene, and the like, from the viewpoints of reducing environmental load, saving energy, and the like.
Inkjet printing is a printing method in which ink droplets are ejected directly from tiny nozzles and deposited on a print medium to produce a printed matter with characters and images. It has many advantages, including the ease of producing full color, low cost, and no contact with the print medium.
In addition, there is an increasing demand for printing on printing media that are not white, such as cardboard, paperboard, and resin films, and white inks are used to express the color white and to improve visibility. Titanium oxide, which has high hiding power, is commonly used as a pigment for white inks, and a polymer dispersant is used in the ink to improve the dispersibility of titanium oxide.

On the other hand, as the quality of printed images improves, the resolution of inkjet printers is also increasing. Accordingly, the nozzles of inkjet print heads are becoming denser and the droplets are becoming finer, i.e., the nozzle diameters for ejecting ink are becoming finer and more highly integrated.
As nozzle diameters become smaller, the nozzles become more susceptible to clogging, and with conventional water-based inks, the ejection properties (hereinafter also referred to as "intermittent ejection properties") significantly deteriorate when printing is completed, a predetermined period of pause is then performed, and the ink is ejected and printed again.
The tendency for intermittent ejection to deteriorate due to the miniaturization of the nozzle diameter is particularly noticeable with white ink.

Various proposals have been made regarding water-based inks for ink-jet printing that use titanium oxide as a pigment.
For example, Patent Literature 1 discloses an aqueous pigment dispersion having low viscosity and excellent storage stability, and an aqueous inkjet ink having excellent hiding property, the aqueous pigment dispersion containing titanium oxide surface-treated with an organic compound and a pigment dispersing resin having a long-chain alkyl group, and an ink containing the aqueous pigment dispersion and an alkanediol-based solvent and/or an alkylene glycol alkyl ether-based solvent.
Patent Literature 2 discloses an aqueous dispersion and an aqueous ink for inkjet recording having high dispersion stability and redispersibility, in which poorly water-soluble vinyl polymer particles containing titanium dioxide are dispersed, the vinyl polymer particles contain a silicone component, and the vinyl polymer is a polymer obtained by polymerizing a monomer mixture containing a specific aromatic vinyl compound, a (meth)acrylic acid ester, and an unsaturated monomer having a salt-forming group.
Patent Literature 3 discloses a water-based ink for inkjet printing having excellent redispersibility, which contains titanium oxide and a pigment dispersant, the pigment dispersant having a constituent unit derived from an anionic group-containing monomer (a) and a constituent unit derived from a polyalkylene glycol (meth)acrylate (b), and the average number of moles added of alkylene oxide of the component (b), and the acid value and content of the pigment dispersant are within specific ranges.

JP 2014-43492 A JP 2015-108063 A JP 2017-39922 A

However, the techniques disclosed in Patent Documents 1 to 3 are insufficient to accommodate the trend toward smaller nozzle diameters and higher integration in recent inkjet printing.
Furthermore, when printing on low-liquid-absorbency printing media such as coated paper or resin film, water-based inks have difficulty penetrating into the printing medium, resulting in the problem of low adhesion of the ink to the printing medium (hereinafter also referred to as "substrate adhesion").
An object of the present invention is to provide a water-based ink for ink-jet printing that has excellent intermittent ejection properties and adhesion to a substrate, and an ink-jet printing method using the same.

The present inventors have found that the above-mentioned problems can be solved by providing an aqueous ink containing water-insoluble polymer particles A containing a pigment, polymer particles B not containing a pigment, and a water-soluble organic solvent, in which the pigment is hydrophobically treated titanium oxide, the water-insoluble polymer particles A contain one or more types selected from water-insoluble (meth)acrylic resins and polyester resins, the glass transition temperature of the polymer constituting the polymer particles B and the mass ratio of the pigment to the water-insoluble polymer particles A containing a pigment are within specific ranges, and an alkylene glycol ether is included as the water-soluble organic solvent.

That is, the present invention provides the following [1] and [2].
[1] A water-based ink for ink-jet printing, comprising water-insoluble polymer particles A containing a pigment, polymer particles B not containing a pigment, a water-soluble organic solvent, and water,
The pigment is titanium dioxide that has been hydrophobically treated,
the water-insoluble polymer particles A contain at least one type selected from a water-insoluble (meth)acrylic resin and a water-insoluble polyester resin, and the glass transition temperature of the polymer constituting the polymer particles B is −10° C. or more and 40° C. or less;
a mass ratio of the pigment to the mass of the water-insoluble polymer particles A containing the pigment (pigment/water-insoluble polymer particles A containing the pigment) is 0.91 or more and less than 1;
The water-soluble organic solvent contains an alkylene glycol ether.
Water-based ink for inkjet printing.
[2] An inkjet printing method, comprising printing on a low liquid-absorbent print medium using the water-based ink according to [1] above.

The present invention provides a water-based ink for inkjet printing that has excellent intermittent ejection properties and adhesion to substrates, and an inkjet printing method using the same.

[Water-based ink]
The water-based ink for ink-jet printing of the present invention is a water-based ink for ink-jet printing that contains water-insoluble polymer particles A containing a pigment (hereinafter also referred to as "polymer particles A"), polymer particles B containing no pigment (hereinafter also referred to as "polymer particles B"), a water-soluble organic solvent, and water, wherein the pigment is titanium oxide that has been subjected to a hydrophobic treatment,
the water-insoluble polymer particles A contain at least one type selected from a water-insoluble (meth)acrylic resin and a water-insoluble polyester resin, and the glass transition temperature of the polymer constituting the polymer particles B is −10° C. or more and 40° C. or less;
a mass ratio of the pigment to the mass of the water-insoluble polymer particles A containing the pigment (pigment/water-insoluble polymer particles A containing the pigment) is 0.91 or more and less than 1;
The water-soluble organic solvent contains an alkylene glycol ether.
This is a water-based ink for ink-jet printing (hereinafter also referred to as "the ink of the present invention").
The term "aqueous" means that water accounts for the largest proportion of the medium.
"Low liquid absorption" is a concept that includes low liquid absorption and non-absorption of ink. Low liquid absorption can be evaluated by the water absorption of pure water. More specifically, it means that the water absorption amount per surface area of the printing medium when the printing medium is in contact with pure water for 100 ms is 0 g/m2 or ^more and 10 g/ ^m2 or less.
"Printing" is a concept that includes printing to record characters and images, and printing.
The meaning of "water-insoluble" will be described in the section on polymers below.

According to the present invention, it is possible to provide a water-based ink for ink-jet printing that has excellent intermittent ejection properties and adhesion to a substrate. The reason for this is not clear, but is thought to be as follows.
When printing on a low-absorbency print medium such as a synthetic resin film using a water-based ink, a drying process is usually required. However, low-absorbency print media are generally prone to deformation due to heat, and problems such as conveyance deviation are likely to occur, so the drying temperature is limited, and the drying time becomes short as the printing speed increases. Under such an environment, the ink is required to have good adhesion to the substrate.
In the ink of the present invention, the fixability of the pigment particles to a low-liquid-absorbing print medium is improved by using polymer particles B, which are a fixing agent, having a glass transition temperature of −10° C. or more and 40° C. or less. Furthermore, since the water-soluble organic solvent contains an alkylene glycol ether, it is easy to plasticize the polymer that constitutes the polymer particles B, and this, combined with the mass ratio of the pigment to the mass of the water-insoluble polymer particles A that contain the pigment being 0.91 or more and less than 1, is thought to improve adhesion to the substrate.
In addition, the alkylene glycol ether relaxes the electric charge of the polymer that constitutes the polymer particles B and further swells the polymer appropriately, thereby enhancing the dispersion effect of the water-insoluble polymer particles A that contain a pigment, and further imparts appropriate wettability to the ink in the flow path of the inkjet printing head, which is thought to improve intermittent ejection properties.

<Polymer particles A containing pigment>
(Pigment)
The titanium oxide pigment used in the present invention is hydrophobically treated titanium oxide (hereinafter also referred to as "hydrophobically treated titanium oxide" or simply "pigment"), and is contained as water-insoluble polymer particles A containing a pigment from the viewpoint of improving the intermittent ejection properties and substrate adhesion of the resulting ink of the present invention (white ink).
In this specification, "water-insoluble polymer particles A containing a pigment" includes particles in a form in which the polymer encompasses the pigment, particles made of the polymer and the pigment, part of the pigment is exposed on the surface of the particle, and particles in a form in which the polymer is adsorbed to part of the pigment.

(Hydrophobic titanium oxide)
Hydrophobized titanium oxide is obtained by subjecting a raw material, titanium oxide powder (TiO ₂ ) (hereinafter also referred to as "raw titanium oxide"), to a hydrophobization treatment.
Here, "hydrophobization" means treating the surface of raw titanium oxide with a hydrophobic organic treatment agent. The hydrophilic parts of the surface of raw titanium oxide are sealed by the hydrophobization treatment, improving water resistance and water repellency, and when blended in the ink of the present invention, gel formation between the hydrophobized titanium oxide and the polymer particles B is suppressed, which is thought to improve the intermittent ejection properties and substrate adhesion of the ink.

The particle shape of the raw material titanium oxide is not particularly limited, and examples thereof include granular, approximately spherical, spindle-shaped, rod-shaped, needle-shaped, plate-shaped, amorphous, etc., with granular, approximately spherical, and spindle-shaped being preferred.
The shape can be observed using a scanning electron microscope or the like.
The average primary particle diameter of the raw material titanium oxide is, from the viewpoint of whiteness, preferably 8 nm or more, more preferably 10 nm or more, and even more preferably 12 nm or more, and, from the viewpoint of hiding power, preferably 100 nm or less, more preferably 50 nm or less, and even more preferably 30 nm or less.
The average primary particle size of the raw material titanium oxide is measured by the method described in the Examples.
Titanium oxide has a crystal structure of rutile type (tetragonal), anatase type (tetragonal), or brookite type (orthorhombic). From the viewpoints of crystal stability, whiteness, hiding power, and availability, rutile type titanium oxide is preferred.

The raw titanium oxide may be a composite powder whose surface has been treated or coated with another inorganic material. The other inorganic material may be one or more selected from alumina, silica, zinc oxide, zirconia, magnesium oxide, iron oxide, etc., and preferably one or more selected from alumina, silica, zinc oxide, and zirconia. The amount of the surface treatment or coating with these inorganic materials is usually preferably 25% by mass or less, more preferably 20% by mass or less, and preferably 0% by mass or more, more preferably 1% by mass or more, based on 100% by mass of the total amount of titanium oxide after hydrophobization treatment.
Examples of commercially available raw material titanium oxide products include Titan Kogyo Co., Ltd. (product name: KURONOS KR series), Ishihara Sangyo Co., Ltd. (product name: Typek R, CR, PF series), Sakai Chemical Industry Co., Ltd. (product name: R series), Teika Corporation (product name: MT series), and Huntsmann Co., Ltd. (product name: TR series).

The hydrophobic organic treatment agent may be any one that is commonly used in the hydrophobic treatment of inorganic particles, and is not particularly limited. Examples of the hydrophobic organic treatment agent include alkylsilane, silicone, fatty acid salt, fluorine compound, oil such as wax or squalane oil, and N-acylated amino acid such as Nε-lauroyl-L-lysine. Among these, the hydrophobic treatment is preferably one or more selected from alkylsilane treatment, silicone treatment, fatty acid salt treatment, and fluorine compound treatment, more preferably one or more selected from alkylsilane treatment, silicone treatment, and fatty acid salt treatment, and more preferably alkylsilane treatment.
Examples of alkylsilanes that are hydrophobic organic treatment agents include alkyltriethoxysilane, alkyltrimethoxysilane, phenyltrimethoxysilane, and phenyltriethoxysilane. Of these, one or more selected from alkyltriethoxysilanes and alkyltrimethoxysilanes having an alkyl group with 1 to 14 carbon atoms, preferably 3 to 12 carbon atoms, are preferred, alkyltriethoxysilanes having an alkyl group with 4 to 10 carbon atoms are more preferred, and one or more selected from isobutyltriethoxysilane, hexyltriethoxysilane, octyltriethoxysilane, and decyltriethoxysilane are even more preferred.

Examples of silicones that are hydrophobic organic treatment agents include methylhydrogenpolysiloxanes, methylpolysiloxanes, dimethylpolysiloxanes, trimethylsiloxysilicic acid, copolymers of methylhydrogenpolysiloxanes and dimethylpolysiloxanes, organopolysiloxanes containing reactive trialkoxysilyl groups such as trimethoxysilyl groups and triethoxysilyl groups, silicone resins, and various modified products thereof.
Examples of fatty acids include myristic acid, palmitic acid, stearic acid, and the like, which have from 12 to 20 carbon atoms, and examples of their salts include metal salts, ammonium salts, organic amine salts, etc. Among these, metal salts are preferred, and examples of the metal salts include monovalent metal salts such as sodium, lithium, potassium, etc.; divalent metal salts such as zinc, magnesium, calcium, etc.; and trivalent metal salts such as aluminum, etc., and one or more selected from sodium salts, potassium salts, ammonium salts, zinc salts, and magnesium salts are preferred.
The fluorine compound may be an organic compound or polymer containing a trifluoromethyl group ( _-CF3 group), and is preferably one or more selected from alkoxysilanes, alkylsilanes and chlorosilane compounds having a perfluoroalkyl group, perfluoroalkyl phosphate esters, perfluoroalkyl acrylate resins, etc. Specific examples thereof include a toluene dispersion of an acrylate resin containing a _CF3- ( _CF2 ) _7- group.

The hydrophobic treatment can be carried out by a known method such as a dry method, a wet method using a solvent such as a solvent-based, aqueous, or solvent-water mixed solvent, a mechanochemical method, etc. For example, the hydrophobic organic treatment agent can be dissolved in an organic solvent such as toluene or alcohol, thoroughly mixed with the raw material titanium oxide, the organic solvent is removed by distillation or the like, and the resulting mixture is then heated and crushed.
The amount of the hydrophobization treatment of the raw titanium oxide varies depending on the type of hydrophobic organic treatment agent, but from the viewpoint of improving the intermittent ejection properties and substrate adhesion of the resulting white ink, the amount is preferably 1 part by mass or more, more preferably 2 parts by mass or more, even more preferably 5 parts by mass or more, and is preferably 80 parts by mass or less, more preferably 70 parts by mass or less, and even more preferably 60 parts by mass or less, per 100 parts by mass of the total amount of titanium oxide after hydrophobization treatment.
From the same viewpoints as above, the hydrophobicity of the hydrophobized titanium oxide is preferably 30% or more, more preferably 50% or more, even more preferably 70% or more, and is preferably 100% or less, more preferably 95% or less, even more preferably 90% or less.
Here, "hydrophobicity" refers to the degree of methanol hydrophobicity (%) calculated by the following formula when water is added to hydrophobized titanium oxide, the mixture is stirred, and then methanol is added dropwise to the mixture until the hydrophobized titanium oxide floating on the liquid surface has completely disappeared.
Hydrophobicity (%)=[methanol titration (mL)/(methanol titration (mL)+water mass)]×100
The details of the method for measuring the hydrophobicity of the hydrophobic treated titanium oxide are as described in the Examples.

The average primary particle size of the hydrophobized titanium oxide is substantially the same as the average primary particle size of the raw material titanium oxide.
The hydrophobic treated titanium oxide can be prepared by a known method, for example, by mixing raw titanium oxide, a hydrophobic organic treating agent, and an organic solvent such as alcohol, heating the resulting pigment cake to remove the solvent, and then pulverizing the mixture, etc. Commercially available products can also be used.
Commercially available examples of hydrophobized titanium oxide include T-805 from Nippon Aerosil Co., Ltd., the ST series from Titan Kogyo Co., Ltd., the TAF series (anatase type) from Fuji Titan Kogyo Co., Ltd., the JMT series from Teika Corporation, and the IT-S series from Ishihara Sangyo Kaisha, Ltd.

<Water-insoluble polymer a constituting the pigment-containing water-insoluble polymer particles A>
The water-insoluble polymer a (hereinafter, also referred to as "polymer a") constituting the water-insoluble polymer particles A containing a pigment is not particularly limited as long as it has at least the ability to disperse the pigment.
From the viewpoint of the dispersion stability of the pigment, the polymer a is preferably present in the ink in a pigment-encapsulated state in which the polymer a contains the pigment.
The water-insoluble polymer particles containing a pigment are preferably crosslinked with a crosslinking agent as described below. In this case, even if the polymer used for dispersing the pigment is a water-soluble polymer, it becomes a water-insoluble polymer by crosslinking with the crosslinking agent.
Here, the term "water-insoluble" for polymer a means that when the polymer is dried at 105° C. for 2 hours and reaches a constant weight, and then dissolved in 100 g of water at 25° C., the amount of dissolution is 10 g or less, and the amount of dissolution of polymer a is preferably 5 g or less, and more preferably 1 g or less. When polymer a is an anionic polymer, the amount of dissolution is the amount of dissolution when the anionic groups of the polymer are 100% neutralized with sodium hydroxide.
From the viewpoints of substrate adhesion and intermittent dischargeability, the polymer a includes at least one type selected from water-insoluble (meth)acrylic resins and polyester resins.

[(Meth)acrylic resin]
From the viewpoints of improving the dispersion stability of the pigment and improving the intermittent ejection property and the adhesion to the substrate, the (meth)acrylic resin preferably contains a structural unit derived from a hydrophilic monomer. Examples of the hydrophilic monomer include an ionic monomer and a nonionic monomer, and an ionic monomer is preferable.
In the present invention, "hydrophilic" means that when a monomer is dissolved in 100 g of ion-exchanged water at 25° C. until it is saturated, the amount of dissolution is 10 g or more, and the amount of dissolution is preferably 50 g or more. In the case of an ionic monomer, "hydrophilic" means that the amount of dissolution is within the above range when the ionic functional group is neutralized.

[Ionic Monomer (a-1)]
The ionic monomer (a-1) may be an anionic monomer or a cationic monomer, with anionic monomers being preferred, monomers having an acid group being more preferred, and monomers having a carboxy group being even more preferred.
Specific examples of the ionic monomer (a-1) include those described in paragraph [0017] of JP-A-2018-80255. Among these, (meth)acrylic acid is preferred.
Here, "(meth)acrylic acid" means at least one selected from acrylic acid and methacrylic acid, and "(meth)acrylate" described below means at least one selected from acrylate and methacrylate.

[Nonionic Monomer (a-2)]
Examples of the nonionic monomer (a-2) include polyalkylene glycol (meth)acrylate, alkoxy polyalkylene glycol (meth)acrylate, and phenoxy (ethylene glycol/propylene glycol copolymer) (meth)acrylate.
Specific examples of the nonionic monomer (a-2) include those described in paragraphs [0022] to [0023] of JP-A-2018-80255.
Of these, alkoxypolyalkylene glycol (meth)acrylate is preferred, and methoxypolyethylene glycol (meth)acrylate is more preferred.
Specific examples of commercially available nonionic monomers include NK Ester M-40G, 90G, 230G, and the like (all trade names of Shin-Nakamura Chemical Co., Ltd.), Blemmer PE-90, 200, 350, and the like, Blemmer PME-100, 200, 400, and the like, Blemmer 50PEP-300, Blemmer 50POEP-800B, and the like (all trade names of NOF Corporation).

From the viewpoints of improving the dispersion stability of the pigment and improving the intermittent ejection property and the adhesion to the substrate, it is preferable that the (meth)acrylic resin used in the present invention further contains a structural unit derived from a hydrophobic monomer, in addition to a structural unit derived from a hydrophilic monomer.
In the present invention, "hydrophobic" means that when a monomer is dissolved to saturation in 100 g of ion-exchanged water at 25° C., the amount of the monomer dissolved is less than 10 g. From the viewpoint of adsorption to the pigment, the amount of the hydrophobic monomer dissolved is preferably 5 g or less, more preferably 1 g or less.

(Hydrophobic Monomer)
Examples of the hydrophobic monomer (a-3) include alkyl (meth)acrylates, aromatic ring-containing monomers, and macromonomers. Of these, at least one selected from alkyl (meth)acrylates and aromatic ring-containing monomers is preferred, and aromatic ring-containing monomers are more preferred.
Specific examples of the hydrophobic monomer (a-3) include those described in paragraphs [0018] to [0021] of JP-A-2018-80255. Among these, one or more selected from styrene, α-methylstyrene, and benzyl (meth)acrylate are preferred.

(Content of each component or structural unit in the monomer mixture or (meth)acrylic resin)
The contents of hydrophilic monomers and hydrophobic monomers in the monomer mixture during production of the (meth)acrylic resin (content as unneutralized amount; the same applies hereinafter) or the contents of structural units derived from hydrophilic monomers and structural units derived from hydrophobic monomers in the (meth)acrylic resin are as follows, from the viewpoint of improving the dispersion stability of the pigment.
The content of the hydrophilic monomer component is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and preferably 75% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less.
The content of the hydrophobic monomer component is preferably 25% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less.

The mass ratio of [hydrophilic monomer component/hydrophobic monomer component] is preferably 0.1 or more, more preferably 0.15 or more, even more preferably 0.25 or more, and is preferably 3 or less, more preferably 2 or less, even more preferably 1 or less.

(Production of (meth)acrylic resin)
The (meth)acrylic resin is produced by copolymerizing the monomer mixture by a known polymerization method such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, etc. Among these polymerization methods, the solution polymerization method is preferred.
The solvent used in the solution polymerization method is not particularly limited, but a polar organic solvent is preferred. Examples of the polar organic solvent include aliphatic alcohols having 1 to 3 carbon atoms, ketones having 3 to 5 carbon atoms, ethers, esters such as ethyl acetate, etc. Among these, acetone, methyl ethyl ketone, or a mixed solvent of one or more of these with water is preferred.
In the polymerization, a polymerization initiator and a polymerization chain transfer agent can be used.
As the polymerization initiator, known polymerization initiators such as azo compounds such as 2,2'-azobisisobutyronitrile and 2,2'-azobis(2,4-dimethylvaleronitrile) and organic peroxides can be used.
As the polymerization chain transfer agent, known polymerization chain transfer agents such as carboxy group-containing mercaptans; alkyl mercaptans; hydroxy group-containing mercaptans such as 2-mercaptoethanol and 3-mercapto-1,2-propanediol can be used.
Furthermore, there is no limitation on the chain form of the polymerized monomers, and any of the polymerization forms such as random, block, and graft may be used.

Preferred polymerization conditions vary depending on the type of polymerization initiator, monomer, solvent, etc. used, but usually the polymerization temperature is preferably 30° C. or higher, more preferably 50° C. or higher, and preferably 95° C. or lower, more preferably 80° C. or lower. The polymerization time is preferably 1 hour or longer, more preferably 2 hours or longer, and preferably 20 hours or shorter, more preferably 10 hours or shorter. The polymerization atmosphere is preferably a nitrogen gas atmosphere, an inert gas atmosphere such as argon, etc.
After the polymerization reaction is completed, the produced polymer can be isolated from the reaction solution by a known method such as reprecipitation or solvent distillation.

From the viewpoint of intermittent ejection, the acid value of the (meth)acrylic resin used in the present invention is preferably 50 mgKOH/g or more, more preferably 70 mgKOH/g or more, even more preferably 100 mgKOH/g or more, and is preferably 300 mgKOH/g or less, more preferably 280 mgKOH/g or less, even more preferably 260 mgKOH/g or less. If the acid value is within the above range, the amount of acid groups and neutralized acid groups is sufficient, and the dispersion stability of the pigment is ensured. In addition, it is also preferable from the viewpoint of the balance between the affinity of the polymer a with the aqueous medium and the interaction between the polymer a and the pigment.
The acid value of the (meth)acrylic resin can be calculated from the mass ratio of the constituent monomers. It can also be determined by a method in which the (meth)acrylic resin is dissolved or swollen in an appropriate organic solvent (e.g., MEK) and then titrated.

The number average molecular weight of the (meth)acrylic resin is preferably 500 or more, more preferably 1,000 or more, and even more preferably 3,000 or more, from the viewpoint of intermittent ejection property and adhesion to the substrate, and is preferably 30,000 or less, more preferably 20,000 or less, and even more preferably 15,000 or less, from the viewpoint of dispersion stability of the pigment. The number average molecular weight can be measured by the method described in the Examples.

[Polyester Resin]
The polyester resin used in the present invention contains a constituent unit derived from an alcohol component and a constituent unit derived from a carboxylic acid component, and can be obtained by polycondensation of the alcohol component and the carboxylic acid component.

(Alcohol content)
The alcohol component, which is a raw material monomer for the polyester resin, preferably contains an aromatic diol from the viewpoints of improving the dispersion stability of the pigment and improving intermittent ejection properties and adhesion to the substrate. As the aromatic diol, an alkylene oxide adduct of bisphenol A is preferable. The alkylene oxide adduct of bisphenol A refers to the entire structure in which an oxyalkylene group is added to 2,2-bis(4-hydroxyphenyl)propane.
Specifically, the alkylene oxide adduct of bisphenol A is preferably a compound represented by the following general formula (I), and two or more kinds of such compounds may be used in combination.

In formula (I), OR ¹ and R ² O are both oxyalkylene groups, preferably each independently an oxyalkylene group having from 1 to 4 carbon atoms, more preferably an oxyethylene group or an oxypropylene group.
x and y correspond to the number of moles of alkylene oxide added. Furthermore, from the viewpoint of reactivity with the carboxylic acid component, the average value of the sum of x and y is preferably 2 or more, and preferably 7 or less, more preferably 5 or less, and even more preferably 3 or less.
Furthermore, the x number of OR ¹ and the y number of R ² O may be the same or different, but are preferably the same from the viewpoint of improving intermittent ejection properties and substrate adhesion to the printing medium.
The alkylene oxide adduct of bisphenol A is preferably a propylene oxide adduct of bisphenol A or an ethylene oxide adduct of bisphenol A, and more preferably a propylene oxide adduct of bisphenol A.
The content of the alkylene oxide adduct of bisphenol A in the alcohol component is preferably 50 mol % or more, more preferably 60 mol % or more, and even more preferably 70 mol % or more, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection reliability and bending resistance, and the upper limit thereof is preferably 100 mol % or less.

The alcohol component may contain the following other alcohol components in addition to the alkylene oxide adduct of bisphenol A. Examples include ethylene glycol, propylene glycol (1,2-propanediol), glycerin, pentaerythritol, trimethylolpropane, hydrogenated bisphenol A, sorbitol, or their alkylene oxide adducts (having a carbon number of 2 to 4) (average number of moles added: 1 to 16).

(Carboxylic Acid Component)
The carboxylic acid component, which is a raw material monomer for the polyester resin, includes carboxylic acid, its acid anhydride, and its alkyl (having 1 to 3 carbon atoms) ester.
Examples of the carboxylic acid component include aromatic dicarboxylic acids, aliphatic dicarboxylic acids, alicyclic dicarboxylic acids, and trivalent or higher polycarboxylic acids.
As the aromatic dicarboxylic acid, phthalic acid, isophthalic acid and terephthalic acid are preferred, and terephthalic acid is more preferred.
The aliphatic dicarboxylic acid may be unsaturated or saturated, and the unsaturated aliphatic dicarboxylic acid is preferably fumaric acid or maleic acid, more preferably fumaric acid, and the saturated aliphatic dicarboxylic acid is preferably adipic acid or succinic acid.
As the alicyclic dicarboxylic acid, cyclohexanedicarboxylic acid, decalindicarboxylic acid, and tetrahydrophthalic acid are preferable, and as the polyvalent carboxylic acid having three or more valences, trimellitic acid and pyromellitic acid are preferable.
Among the above carboxylic acids, aromatic dicarboxylic acids and aliphatic dicarboxylic acids are preferred from the viewpoints of improving intermittent discharge properties and substrate adhesion, and of reactivity with the alcohol component.
The carboxylic acid components may be used alone or in combination of two or more kinds.

(Production of polyester resin)
The polyester resin can be obtained by polycondensing the alcohol component and the carboxylic acid component in an appropriate combination, for example, by polycondensing the alcohol component and the carboxylic acid component in an inert gas atmosphere at a temperature of 150° C. to 250° C. using an esterification catalyst as necessary.
Examples of the esterification catalyst include tin catalysts, titanium catalysts, and metal compounds such as antimony trioxide, zinc acetate, and germanium dioxide, and from the viewpoint of esterification reaction efficiency, tin catalysts are preferred. As the tin catalyst, dibutyltin oxide, tin(II) di(2-ethylhexanoate), and salts thereof are preferred, and tin(II) di(2-ethylhexanoate is more preferred. If necessary, an esterification promoter such as gallic acid may be further used.
A radical polymerization inhibitor such as 4-t-butylcatechol may also be used in combination.

From the viewpoint of intermittent ejection property and substrate adhesion, the polyester resin preferably has an acid group. This provides the same effect as when the (meth)acrylic resin has an acid group. From the viewpoint of intermittent ejection property and substrate adhesion, the acid value of the polyester resin is preferably 5 mgKOH/g or more, more preferably 15 mgKOH/g or more, and is preferably 40 mgKOH/g or less, more preferably 30 mgKOH/g or less, and even more preferably 25 mgKOH/g or less.
The weight average molecular weight of the polyester resin is preferably 2,000 or more, more preferably 5,000 or more, and even more preferably 10,000 or more, from the viewpoint of intermittent ejection property and substrate adhesion, and is preferably 40,000 or less, more preferably 30,000 or less, and even more preferably 20,000 or less, from the viewpoint of pigment dispersion stability.
The acid value and number average molecular weight (Mn) can be both obtained as desired by appropriately adjusting the types and blending ratios of the monomers used, the polycondensation temperature, and the reaction time.

[Preparation of Pigment-Containing Water-Insoluble Polymer Particles A]
The water-insoluble polymer particles A containing a pigment can be efficiently produced as a pigment water dispersion by a method having the following step 1. From the viewpoint of improving the intermittent ejection properties and substrate adhesion of the resulting ink, it is preferable to further carry out step 2 (crosslinking step).
Step 1: A step of dispersing a pigment mixture containing a pigment, a polymer a1, water, and, as necessary, a neutralizing agent, a surfactant, and the like, to obtain an aqueous dispersion of polymer particles a1 containing a pigment (hereinafter, also referred to as "aqueous pigment dispersion (i)"). Here, the polymer a1 contains one or more types selected from a (meth)acrylic resin and a polyester resin.
Step 2: A step of adding a crosslinking agent to the pigment water dispersion (i) obtained in step 1 to crosslink a part or all of the pigment-containing polymer particles a1 to obtain a pigment-containing water dispersion (I) of water-insoluble polymer particles a2 (hereinafter also referred to as "pigment water dispersion (I)"). The pigment-containing water-insoluble polymer particles A according to the present invention include the pigment-containing polymer particles a1 and the pigment-containing water-insoluble polymer particles a2 after crosslinking treatment. That is, even if the polymer a1 is a water-soluble polymer, it becomes a water-insoluble polymer by crosslinking with a crosslinking agent, and therefore the polymer particles A contain one or more types selected from water-insoluble (meth)acrylic resins and polyester resins.

(Step 1)
As described above, it is preferable that the polymer a has an acid group, particularly a carboxy group, and it is more preferable that at least a part of the acid group is neutralized with a neutralizing agent. This is believed to improve the intermittent ejection property of the ink of the present invention.
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 bases such as sodium hydroxide, potassium hydroxide, ammonia, and various amines, with sodium hydroxide and ammonia being preferred. Also, the polymer a may be neutralized in advance.
From the viewpoint of improving storage stability, the amount of the neutralizing agent used is preferably 10 mol % or more, more preferably 20 mol % or more, even more preferably 30 mol % or more, and is preferably 150 mol % or less, more preferably 120 mol % or less, even more preferably 100 mol % or less.
Here, the equivalent amount of the neutralizing agent used can be calculated by the following formula, where polymer a before neutralization is "polymer a'".
Equivalent amount of neutralizing agent used (mol %)=[{weight (g) of neutralizing agent added/equivalent amount of neutralizing agent}/[{acid value of polymer a' (mg KOH/g) x weight (g) of polymer a'}/(56 x 1,000)]] x 100

The dispersion process in step 1 can be carried out by a known method. Examples of dispersing machines include kneading machines such as anchor blades, dispersing blades, and kneaders, and media-type dispersing machines such as paint shakers and bead mills.

(Step 2)
By forming a crosslinked structure in part or all of the pigment-containing polymer particles a1 in step 2, the polymer is firmly adsorbed or fixed to the pigment surface, suppressing pigment aggregation, and it is believed that the resulting ink of the present invention has improved substrate adhesion to the printing medium and also improved intermittent ejection properties.

The crosslinking agent used is, from the viewpoint of efficiently carrying out a crosslinking reaction and improving storage stability, preferably a compound having 2 to 6, and preferably 4 or less, epoxy groups in the molecule, more preferably a compound having 2 or more glycidyl ether groups, and even more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 8 carbon atoms.
The epoxy equivalent of the crosslinking agent is preferably 90 or more, more preferably 100 or more, even more preferably 110 or more, and is preferably 300 or less, more preferably 200 or less, even more preferably 150 or less.
Specific examples of the crosslinking agent include polyglycidyl ethers such as polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, pentaerythritol polyglycidyl ether, 1,6-hexanediol diglycidyl ether, etc. Among these, preferred is one or more selected from trimethylolpropane polyglycidyl ether and pentaerythritol polyglycidyl ether, more preferred is trimethylolpropane polyglycidyl ether.

From the viewpoint of improving storage stability, the crosslinking rate, expressed as the ratio of the molar equivalent number of the crosslinkable functional group of the crosslinking agent to the molar equivalent number of the carboxy group of the polymer a, is preferably 10 mol % or more, more preferably 20 mol % or more, even more preferably 30 mol % or more, and is preferably 80 mol % or less, more preferably 70 mol % or less, even more preferably 60 mol % or less.
From the same viewpoint as above, the temperature of the crosslinking treatment is preferably 40° C. or higher, more preferably 50° C. or higher, and is preferably 95° C. or lower, more preferably 85° C. or lower.
From the viewpoints of completion of the crosslinking reaction and economic efficiency, the time for the crosslinking treatment is preferably 0.5 hours or more, more preferably 1 hour or more, and is preferably 12 hours or less, more preferably 10 hours or less.

From the viewpoint of improving the dispersion stability of the pigment water dispersion (I) and facilitating the production of the ink, the solid content concentration of the pigment water dispersion (I) is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 40% by mass or less, more preferably 35% by mass or less. The solid content concentration is measured by the method described in the examples.
From the viewpoint of dispersion stability, the content of the pigment in the pigment water dispersion (I) is preferably 5% by mass or more, more preferably 8% by mass or more, and is preferably 40% by mass or less, more preferably 30% by mass or less.
From the viewpoint of improving storage stability, the average particle size of the water-insoluble polymer particles a2 containing the pigment in the pigment water dispersion (I) is preferably 100 nm or more, more preferably 150 nm or more, even more preferably 200 nm or more, still more preferably 250 nm or more, and is preferably 600 nm or less, more preferably 500 nm or less, and even more preferably 400 nm or less.
The average particle size is measured by the method described in the Examples.

The mass ratio of the pigment to the mass of the water-insoluble polymer particles A containing a pigment (pigment/water-insoluble polymer particles A containing a pigment) is, from the viewpoint of improving the adhesion to the substrate and the intermittent ejection properties of the resulting ink, 0.91 or more, preferably 0.92 or more, more preferably 0.92 or more, even more preferably 0.93 or more, and is less than 1, preferably 0.99 or less, more preferably 0.98 or less.
The mass ratio (pigment/pigment-containing water-insoluble polymer particles A) can be calculated from the charge amount ratio.

<Polymer particles B not containing pigment>
From the viewpoint of improving the adhesion to the substrate, the ink of the present invention contains pigment-free polymer particles B. The pigment-free polymer particles B are preferably capable of taking the form of a dispersion in a medium mainly composed of water.
From the viewpoint of improving the adhesion of the ink of the present invention to a substrate, the polymer b constituting the pigment-free polymer particles B (hereinafter also referred to as "polymer b") is preferably a water-insoluble polymer.
Here, the "water-insoluble" of polymer b has the same meaning as that of polymer a described above.
The polymer b is preferably at least one selected from a (meth)acrylic resin and a polyurethane resin, and among them, the polymer b is preferably a (meth)acrylic resin containing a structural unit derived from (meth)acrylic acid (b-1) and a structural unit derived from a (meth)acrylic acid ester (b-2).

[(Meth)acrylic resin]
[(Meth)acrylic acid (b-1)]
From the viewpoint of improving the adhesion to the substrate, the (meth)acrylic acid (b-1) is preferably at least one selected from acrylic acid and methacrylic acid, and more preferably methacrylic acid.

[(Meth)acrylic acid ester (b-2)]
From the viewpoint of improving adhesion to a substrate, the (meth)acrylic acid ester (b-2) is preferably at least one selected from methyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and cyclohexyl (meth)acrylate, more preferably at least one selected from methyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and cyclohexyl (meth)acrylate, and more preferably 2-ethylhexyl (meth)acrylate.

[Aromatic Group-Containing Monomer (b-3)]
The polymer b may contain a structural unit derived from a monomer other than the (meth)acrylic acid (b-1) and the acrylic acid ester (b-2). Examples of the other monomer include an ionic monomer other than (meth)acrylic acid, a hydrophobic monomer having an aromatic group, or a nonionic monomer, and among these, a hydrophobic monomer having an aromatic group (hereinafter also referred to as "aromatic group-containing monomer (b-3)") is preferable.
Examples of the aromatic group-containing monomer (b-3) include the above-mentioned styrene-based monomers, aromatic group-containing (meth)acrylates, and aromatic group-containing monomer-based macromonomers.

From the viewpoint of improving adhesion to a substrate, the content of the structural unit derived from (meth)acrylic acid (b-1) in the polymer b is preferably 1 mass % or more, more preferably 2 mass % or more, even more preferably 3 mass % or more, and is preferably 20 mass % or less, more preferably 12 mass % or less, even more preferably 8 mass % or less.
From the viewpoint of improving adhesion to a substrate, the content of the structural unit derived from the (meth)acrylic acid ester (b-2) in the polymer b is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and is preferably 99% by mass or less, more preferably 98% by mass or less, even more preferably 96% by mass or less.
The mass ratio of the (meth)acrylic acid (b-1) to the (meth)acrylic acid ester (b-2) [(meth)acrylic acid (b-1)/(meth)acrylic acid ester (b-2)] is preferably 0.01 or more, more preferably 0.02 or more, and is preferably 2 or less, more preferably 1 or less, and even more preferably 0.1 or less.

From the viewpoint of improving the adhesion to the substrate, the weight average molecular weight of polymer b is preferably 10,000 or more, more preferably 50,000 or more, and even more preferably 100,000 or more, and is preferably 2,000,000 or less, more preferably 1,500,000 or less, and even more preferably 1,200,000 or less.
From the viewpoint of improving the adhesion to the substrate and the intermittent ejection properties of the resulting ink, the glass transition temperature (Tg) of polymer b is 40° C. or lower, preferably 35° C. or lower, more preferably 30° C. or lower, and even more preferably 28° C. or lower, and is −10° C. or higher, preferably −5° C. or higher, more preferably 0° C. or higher, and even more preferably 5° C. or higher.
The glass transition temperature is measured by the method described in the Examples.
From the viewpoint of improving adhesion to a substrate, the average particle size of the water-insoluble polymer particles B not containing a pigment is preferably 10 nm or more, more preferably 30 nm or more, even more preferably 50 nm or more, and is preferably 250 nm or less, more preferably 200 nm or less, even more preferably 150 nm or less.

[Polyurethane resin]
The polyurethane resin used in the present invention can be obtained by subjecting a polyol having two or more alcoholic hydroxyl groups in one molecule to a polyaddition reaction with a polyisocyanate.

(Polyol)
The polyol is not particularly limited, and examples thereof include polycarbonate polyol, polyester polyol, polyether polyol, etc., and one or more selected from polycarbonate polyol and polyester polyol are preferable.
That is, the polyurethane resin is preferably at least one selected from polycarbonate-based polyurethanes and polyester-based polyurethanes.

The polycarbonate polyol can be obtained by reacting a carbonate compound with a diol.
Examples of the carbonate compound include dimethyl carbonate, diethyl carbonate, diphenyl carbonate, ethylene carbonate, and diethylene carbonate.
Examples of the diol include aliphatic diols which may be substituted with a lower alcohol, alicyclic diols such as cyclohexanediol and hydrogenated xylylene glycol, and aromatic diols such as xylylene glycol. Among these, aliphatic diols are preferred, and aliphatic diols having a carbon chain length of 4 to 9, such as 1,4-butanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, heptanediol, octanediol, and nonanediol, are more preferred.

The polyester polyol can be obtained by condensing a low molecular weight diol and a dicarboxylic acid.
Examples of low molecular weight diols include ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, 1,4-butanediol, etc. Among these, ethylene glycol, propylene glycol, 1,4-butanediol, etc. are preferred.
Examples of dicarboxylic acids include aliphatic dibasic acids such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, brassylic acid, etc., and aromatic dibasic acids such as isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid, etc. Among these, aliphatic dibasic acids are preferred, and dibasic acids with a methylene chain length of 4 to 8, such as adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, etc., are more preferred.

Examples of polyether polyols include polymers obtained by ring-opening polymerization of cyclic ether compounds such as ethylene oxide, propylene oxide, butylene oxide, styrene oxide, tetrahydrofuran, and epichlorohydrin, either alone or in combination, using a compound having an active hydrogen atom as a catalyst. Specific examples include polyethylene glycol, polypropylene glycol, and polytetramethylene glycol.

(Polyisocyanate)
Examples of polyisocyanates include chain-like aliphatic diisocyanates such as tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, dodecamethylene diisocyanate, trimethylhexamethylene diisocyanate, and lysine diisocyanate; aliphatic diisocyanates having a cyclic structure such as isophorone diisocyanate, hydrogenated xylylene diisocyanate, and dicyclohexylmethane 4,4'-diisocyanate; aliphatic diisocyanates having an aromatic ring such as xylylene diisocyanate and tetramethylxylylene diisocyanate; aromatic diisocyanates such as tolylene diisocyanate and diphenylmethane diisocyanate; modified products of these diisocyanates (carbodiimide, uretdione, uretoimine-containing modified products, etc.), etc. Among these, aliphatic diisocyanates and aromatic diisocyanates are preferred.

Examples of the reaction solvent for the polyaddition reaction include acetone, methyl ethyl ketone, tetrahydrofuran, dioxane, ethyl acetate, toluene, and xylene. In the polyaddition reaction, a chain extender or a reaction terminator may be used in combination as necessary. The molecular weight can be further increased by using a chain extender. Examples of the chain extender include polyols and polyamines, and examples of the reaction terminator include monoalcohols and monoamines.
The polyurethane resin is preferably used as an emulsion, and the emulsion may contain a dispersing agent such as a surfactant, if necessary.

From the viewpoint of substrate adhesion to the printing medium, the acid value of the polyurethane resin is preferably 5 mgKOH/g or more, more preferably 10 mgKOH/g or more, even more preferably 20 mgKOH/g or more, and is preferably 50 mgKOH/g or less, more preferably 40 mgKOH/g or less, even more preferably 35 mgKOH/g or less.
Examples of commercially available dispersions of water-insoluble polyurethane resin particles include Superflex 210 and 420 (ester-based and carbonate-based polyurethane emulsions) manufactured by Daiichi Kogyo Seiyaku Co., Ltd.

<Water-soluble organic solvent>
The water-soluble organic solvent used in the present invention may be either liquid or solid at 25° C., but when the organic solvent is dissolved in 100 ml of water at 25° C., the amount of the dissolved organic solvent is 10 ml or more.
The water-soluble organic solvent contains an alkylene glycol ether from the viewpoint of improving the intermittent ejection properties and substrate adhesion of the resulting ink of the present invention. It is believed that the alkylene glycol ether mainly maintains an appropriate liquid surface shape of the ink in the nozzle, thereby improving the intermittent ejection properties of the ink.

Examples of the alkylene glycol ether include one or more selected from ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monopropyl ether, diethylene glycol monoisobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monoisobutyl ether, tetraethylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol monobutyl ether, dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether, and tripropylene glycol monobutyl ether.
Among these, from the viewpoint of improving the intermittent ejection properties and substrate adhesion of the resulting ink of the present invention, one or more types selected from diethylene glycol monoisobutyl ether and diethylene glycol monobutyl ether are preferred.

In the present invention, it is preferable to further contain propylene glycol in addition to the alkylene glycol ether.
Propylene glycol is believed to function primarily by suppressing the evaporation of water from the ink nozzle, and by quickly volatilizing it after printing, forming a strong ink film with a minimum drying process, and by suppressing adhesion between the printed surface and the reverse side of the print.
The ink of the present invention may contain alkylene glycol ethers, polyhydric alcohols other than propylene glycol, amide compounds, and the like, as necessary.
Examples of polyhydric alcohols include glycerin, ethylene glycol, various butanediols, various pentanediols, various hexanediols, various octanediols, diethylene glycol, and dipropylene glycol.
The amide compound includes various 2-pyrrolidone compounds.

(Surfactant)
The ink of the present invention preferably contains a silicone surfactant from the viewpoints of maintaining the surface tension of the ink at an appropriate level and improving the wettability of the ink to the printing medium.
(Silicone surfactant)
Examples of silicone surfactants include dimethylpolysiloxane, polyether-modified silicone, amino-modified silicone, and carboxy-modified silicone, with polyether-modified silicone being preferred from the same viewpoint as above.
Polyether-modified silicone surfactants have a structure in which the side chain and/or terminal hydrocarbon group of silicone oil is replaced with a polyether group.As the polyether group, polyethyleneoxy group, polypropyleneoxy group, polyalkyleneoxy group in which ethyleneoxy group (EO) and propyleneoxy group (PO) are added in block form or randomly are suitable, and compounds in which polyether group is grafted to silicone main chain, compounds in which silicone and polyether group are bonded in block form, etc. can be used.

The HLB (hydrophilic lipophilic balance) value of the polyether-modified silicone surfactant is preferably at least 2, more preferably at least 3, and even more preferably at least 4. Here, the HLB value can be determined by the Griffin method.
The polyether-modified silicone surfactant preferably has a kinetic viscosity of 50 mm ² /s or more, more preferably 80 mm ² /s or more, and preferably 500 mm ² /s or less, more preferably 300 mm ² /s or less at 25° C. The kinetic viscosity can be determined by an Ubbelohde viscometer.
Specific examples of polyether-modified silicone surfactants include KF series (KF-353, KF-355A, KF-642, etc.) manufactured by Shin-Etsu Chemical Co., Ltd., Silface SAG005 manufactured by Nissin Chemical Industry Co., Ltd., FZ-2191 manufactured by NUC Corporation, and BYK-348 manufactured by BYK Japan K.K.

In the present invention, it is also preferable to use an acetylene glycol surfactant in addition to the silicone surfactant.
(Acetylene glycol surfactant)
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-hexyne-3-ol, 2,4-dimethyl-5-hexyne-3-ol, 2,5-dimethyl-3-hexyne-2,5-diol, and 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol, and ethylene oxide adducts thereof.
The sum (n) of the average number of moles of ethyleneoxy groups (EO) added in the ethylene oxide adduct is preferably 1 or more, more preferably 1.5 or more, and is preferably 20 or less, more preferably 10 or less.
Commercially available examples of acetylene glycol surfactants include the "Surfynol" series and "Olfine" series manufactured by Nissin Chemical Industry Co., Ltd., and the "Acetylenol" series manufactured by Kawaken Fine Chemical Co., Ltd. Among these, preferred are Surfynol 420 (n:1.3), Surfynol 440 (n:3.5), Surfynol 465 (n:10.0), Surfynol 485 (n:30.0), Olfine E1010, Acetylenol E100, Acetylenol E200, Acetylenol E40 (n:4), Acetylenol E60 (n:6), Acetylenol E81 (n:8), Acetylenol E100 (n:10), and the like.

[Production of Water-Based Ink]
The ink of the present invention can be efficiently produced by mixing the pigment water dispersion (I), the water dispersion of polymer particles B, a water-soluble organic solvent, and, if necessary, a surfactant, and further various additives such as a humectant, a wetting agent, a viscosity modifier, a defoamer, a preservative, an antifungal agent, and an antirust agent. There is no particular limitation on the method of mixing them.

The contents of the various components in the ink of the present invention and the ink properties are as follows.
(Pigment content)
The content of the pigment (hydrophobically treated titanium oxide) in the ink of the present invention is, from the viewpoint of print density, preferably 2% by mass or more, more preferably 4% by mass or more, and even more preferably 6% by mass or more, and from the viewpoint of improving storage stability, is preferably 25% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less.
(Content of Pigment-Containing Water-Insoluble Polymer Particles A)
The content of polymer particles A containing a pigment (hydrophobically treated titanium oxide) in the ink of the present invention is preferably 2% by mass or more, more preferably 4% by mass or more, and even more preferably 6% by mass or more, from the viewpoint of improving print density, and is preferably 25% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less, from the viewpoint of improving storage stability.

(Content of Pigment-Free Polymer Particles B)
The content of the water-insoluble polymer particles B which do not contain a pigment in the ink of the present invention is preferably 1 mass % or more, more preferably 2 mass % or more, and even more preferably 4 mass % or more from the viewpoint of improving adhesion to the substrate, and is preferably 25 mass % or less, more preferably 20 mass % or less, and even more preferably 15 mass % or less, from the viewpoint of improving storage stability.
(Content of Water-soluble Organic Solvent)
From the viewpoint of improving the intermittent ejection properties of the ink and its adhesion to a substrate, the content of the water-soluble organic solvent in the ink of the present invention is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and is preferably 50% by mass or less, more preferably 45% by mass or less, even more preferably 40% by mass or less.

(Surfactant content)
From the viewpoint of improving the intermittent ejection properties of the ink and its adhesion to the substrate, the content of the silicone-based surfactant in the ink of the present invention is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, even more preferably 0.05% by mass or more, and is preferably 3% by mass or less, more preferably 2% by mass or less, even more preferably 1% by mass or less.
From the same viewpoints as above, the content of the acetylene glycol surfactant in the ink of the present invention is preferably 0.2 mass% or more, more preferably 0.4 mass% or more, even more preferably 0.6 mass% or more, and is preferably 4 mass% or less, more preferably 3 mass% or less, even more preferably 2 mass% or less.
(Water Content)
From the same viewpoints as above, the water content in the ink of the present invention is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less.

(mass ratio of (pigment-containing water-insoluble polymer particles A/total water-soluble organic solvents))
From the same viewpoints as above, the mass ratio of the water-insoluble polymer particles A containing a pigment to the total water-soluble organic solvent (water-insoluble polymer particles A containing a pigment/total water-soluble organic solvent) is preferably 0.05 or more, more preferably 0.1 or more, even more preferably 0.2 or more, and is preferably 0.7 or less, more preferably 0.6 or less, even more preferably 0.5 or less.

(Physical properties of the ink of the present invention)
From the viewpoint of improving the intermittent ejection properties and adhesion to a substrate of the ink, the viscosity of the ink of the present invention at 32°C is preferably 2 mPa·s or more, more preferably 2.5 mPa·s or more, and is preferably 12 mPa·s or less, more preferably 9 mPa·s or less, and even more preferably 7 mPa·s or less.
The viscosity of the ink of the present invention is measured by the method described in the examples.
From the viewpoints of storage stability, reduced corrosiveness, and the like, the pH of the ink of the present invention is preferably 7.0 or more, more preferably 7.2 or more, even more preferably 7.5 or more, and is preferably 11 or less, more preferably 10 or less, even more preferably 9.5 or less.
The ink of the present invention is particularly suitable for printing on flexible packaging. The term "flexible packaging printing" refers to printing on thin, flexible materials such as resin films, either alone or in combination. Flexible packaging printed using the ink of the present invention is suitable for packaging food, daily necessities, etc.

[Inkjet printing method]
The inkjet printing method of the present invention is a method of printing on a low-liquid-absorbent print medium using the ink of the present invention, in which the ink of the present invention is loaded into a known inkjet printing device and ejected as ink droplets onto the low-liquid-absorbent print medium to print an image, etc.
There are three types of ink droplet ejection methods: piezo, thermal, and electrostatic, with the piezo method being the most preferable. In the piezo method, a large number of nozzles are each connected to a pressure chamber, and ink droplets are ejected from the nozzles by vibrating the walls of the pressure chambers with a piezo element.

(Low liquid absorption printing media)
Low liquid-absorbent print media include low liquid-absorbent coated papers and resin films.
Examples of coated paper include general-purpose glossy paper and multi-color form glossy paper.
Examples of the resin film include transparent synthetic resin films, such as polyester films, vinyl chloride films, polypropylene films, polyethylene films, nylon films, etc. These films may be biaxially oriented films, uniaxially oriented films, or inorganic films. Among these, polyester film and stretched polypropylene film are preferred, and corona discharge-treated polyethylene terephthalate (PET) film and corona discharge-treated biaxially stretched polypropylene (OPP) film are more preferred. preferable.

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

(1) Measurement of number average molecular weight and weight average molecular weight of polymer a and polymer b Using a solution of phosphoric acid and lithium bromide dissolved in N,N-dimethylformamide to a concentration of 60 mmol/L and 50 mmol/L, respectively, as an eluent, the measurements were performed by gel permeation chromatography (GPC apparatus (HLC-8320GPC) manufactured by Tosoh Corporation, columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolum Super AW-H) manufactured by Tosoh Corporation, flow rate: 0.5 mL/min) using a 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), manufactured by Tosoh Corporation) as a standard substance.
The measurement sample was prepared by mixing 0.1 g of polymer with 10 mL of the eluent in a glass vial, stirring with a magnetic stirrer at 25° C. for 10 hours, and filtering with a syringe filter (Advantec Co., Ltd., DISMIC-13HP PTFE 0.2 μm).

(2) Measurement of average primary particle diameter of titanium oxide The average primary particle diameter of titanium oxide was measured by extracting 500 primary particles of titanium oxide by image analysis using a transmission electron microscope "JEM-2100" (manufactured by JEOL Ltd.), measuring their particle diameters, and calculating the average of the particle diameters to obtain the number average particle diameter. In addition, when titanium oxide has a major axis and a minor axis, the major axis was used for the calculation.

(3) Measurement of average particle size of titanium oxide dispersion particles Using a laser diffraction/scattering type particle size distribution analyzer LA950 manufactured by Horiba, Ltd., the refractive index of titanium oxide was set to 2.75, and water having a refractive index of 1.333 was used as the dispersion medium, and the measurement was performed after 1 minute of irradiation with ultrasound at 3 at a circulation speed of 5. The volume median particle size ( _D50 ) value at this time was taken as the average particle size of the particles in the dispersion.

(4) Measurement of solid content concentration Using an infrared moisture meter "FD-230" (Kett Electric Laboratory Co., Ltd.), 5 g of a measurement sample was dried under conditions of a drying temperature of 150°C and measurement mode 96 (monitoring time 2.5 minutes/fluctuation range 0.05%), and the moisture content (%) of the measurement sample was measured and the solid content concentration was calculated by the following formula.
Solid content (%) = 100 - moisture content (%) of measurement sample

(5) Viscosity of Ink The viscosity was measured at 32° C. using an E-type viscometer “TV-25” (manufactured by Toki Sangyo Co., Ltd., using a standard cone rotor 1°34′×R24, rotation speed 50 rpm).

(6) Glass transition temperature (Tg) of resin
Using a differential scanning calorimeter "Q-100" (manufactured by TA Instruments Japan, Inc.), 0.01 to 0.02 g of an aqueous dispersion of polymer particles B not containing a pigment is weighed into an aluminum pan, and the temperature is increased from room temperature to 200° C. at a heating rate of 10° C./min, and then cooled from 200° C. to −50° C. at a heating rate of 10° C./min to prepare a measurement sample. Thereafter, the temperature is increased at a heating rate of 10° C./min, and the glass transition temperature is determined as the temperature at the intersection of an extension of the baseline below the maximum endothermic peak temperature and a tangent line showing the maximum slope from the rising part of the peak to the apex of the peak.

(7) Hydrophobicity of Titanium Oxide At 25° C., 0.1 g of hydrophobized titanium oxide was weighed out into a 200 mL beaker, 50 g of ion-exchanged water was added, and the mixture was stirred with a magnetic stirrer. 2 mL of methanol was dropped into the beaker from a burette every 10 seconds, and the end point was the state where the hydrophobized titanium oxide floating on the liquid surface had completely disappeared. The hydrophobicity (%) was calculated by the following formula.
Hydrophobicity (%)=[amount of methanol added to the end point (mL)/(amount of methanol added to the end point (mL)+50)]×100

Production Example 1 (Production of polymer dispersant a1: water-insoluble acrylic resin)
A monomer mixture was prepared by mixing 61.6 parts of acrylic acid, 128.8 parts of styrene, and 9.6 parts of α-methylstyrene. 20 parts of methyl ethyl ketone (MEK), 0.3 parts of a polymerization chain transfer agent (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 2-mercaptoethanol), and 10% of the monomer mixture were placed in a reaction vessel and mixed, followed by thorough nitrogen gas replacement.
Meanwhile, a mixture of the remaining 90% of the monomer mixture, 0.27 parts of the polymerization chain transfer agent, 60 parts of MEK, and 2.2 parts of an azo-based radical polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile), manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., V-65) was placed in a dropping funnel, and the monomer mixture in the reaction vessel was heated to 65°C while being stirred under a nitrogen atmosphere, and the mixture in the dropping funnel was dropped over 3 hours. After 2 hours had elapsed at 65°C from the end of the dropping, a solution in which 0.3 parts of the polymerization initiator was dissolved in 5 parts of MEK was added, and the mixture was further aged at 65°C for 2 hours and at 70°C for 2 hours, to obtain an MEK solution of a water-insoluble polymer a1 (number average molecular weight of polymer a1: 10,500).
The resulting polymer solution was dried under reduced pressure to give a water-insoluble polymer a1 (25 parts), which was then mixed with 97.6 parts of ion-exchanged water, and 15.2 parts of a 5N aqueous sodium hydroxide solution (solid content concentration: 16.9%) was added to neutralize the water-insoluble polymer a1 so that the ratio of the moles of sodium hydroxide to the moles of carboxyl groups in the water-insoluble polymer a1 became 60% (acid value: 240 mg KOH/g, degree of neutralization: 60 mol%), thereby obtaining a polymer dispersant a1 (solid content concentration: 20%).

Production Example 2 (Production of polymer dispersant a2: water-insoluble polyester resin)
The raw material monomers (alcohol component and carboxylic acid component) and the esterification catalyst were mixed in the ratio shown in Table 1, and the mixture was placed in a 10 L four-neck flask equipped with a thermometer, a stirrer, a downflow condenser, and a nitrogen inlet tube. The mixture was reacted for 10 hours at 210° C. using a mantle heater in a nitrogen atmosphere to obtain a water-insoluble polymer a2.
25 parts of the obtained water-insoluble polymer a2 was mixed with 97.6 parts of ion-exchanged water, and 2.9 parts of a 5N aqueous sodium hydroxide solution was further added to neutralize the mixture so that the ratio of the moles of sodium hydroxide to the moles of carboxyl groups in the water-insoluble polymer a2 became 85% (neutralization degree 85 mol%), and a polymer dispersant a2 (solid concentration 20%) was obtained.
The polymer dispersant a2 has a crosslinked structure due to trimellitic acid.

Preparation Example A1 (Preparation of Titanium Oxide Dispersion A-1)
In a 500 mL polyethylene bottle, 20.5 g of polymer dispersant a1 (water-insoluble acrylic resin, solid content concentration 20%) obtained in Production Example 1 as a polymer dispersant, 95.9 g of hydrophobized titanium oxide (manufactured by Teika Co., Ltd., JMT-150IB, isobutyltriethoxysilane treatment, average primary particle size 15 nm, hydrophobicity 74%), and 216.9 g of ion-exchanged water were added, and finally 1000 g of 2 mm zirconia beads were added, and the mixture was dispersed for 8 hours at 250 rpm on a benchtop pot mill stand (As One Co., Ltd.). After dispersion, the zirconia beads were removed using a mesh, and the solid content was adjusted with water to obtain a titanium oxide dispersion with an average particle size of 325 nm (titanium oxide content 95.9%, solid content concentration 30%).
To 300 g of the obtained dispersion, 0.81 g of a crosslinking agent (Denacol EX-321L, trimethylolpropane polyglycidyl ether, epoxy equivalent 129, water solubility: 27%, manufactured by Nagase ChemteX Corporation) was added, and the mixture was stirred at 70° C. for 3 hours. After cooling to 25° C., the mixture was filtered through the 5 μm filter, and ion-exchanged water was added to give a solids concentration of 30%, to obtain an aqueous dispersion A-1 (crosslinking rate 40 mol%) of polymer particles a1 containing titanium oxide.
The average particle size of the titanium oxide-containing polymer particles a1 in the obtained aqueous dispersion was 326 nm. The titanium oxide content of the titanium oxide dispersion A-1 after crosslinking was 95%.

Preparation Example A2 (Preparation of Titanium Oxide Dispersion A-2)
A water dispersion A-2 (crosslinking rate 40 mol%, solid content concentration 30%) of polymer particles a2 containing titanium oxide was obtained in the same manner as in Preparation Example A1, except that the amount of polymer dispersant a1 (water-insoluble acrylic resin) obtained in Production Example 1 was changed to 33.5 g, the amount of hydrophobized titanium oxide (JMT-150IB) was changed to 93.3 g, the amount of ion-exchanged water was changed to 206.5 g, and the amount of crosslinking agent was changed to 1.30 g.
The average particle size of the titanium oxide-containing polymer particles a2 in the obtained aqueous dispersion was 330 nm.

Preparation Example A3 (Preparation of Titanium Oxide Dispersion A-3)
A water dispersion A-3 (crosslinking rate 40 mol%, solid content concentration 30%) of polymer particles a3 containing titanium oxide was obtained in the same manner as in Preparation Example A1, except that the amount of polymer dispersant a1 (water-insoluble acrylic resin) obtained in Production Example 1 was changed to 8.0 g, the amount of hydrophobized titanium oxide (JMT-150IB) was changed to 98.4 g, the amount of ion-exchanged water was changed to 226.9 g, and the amount of crosslinking agent was changed to 0.32 g.
The average particle size of the titanium oxide-containing polymer particles a3 in the obtained aqueous dispersion was 328 nm.

Preparation Example A4 (Preparation of Titanium Oxide Dispersion A-4)
In a 500 mL polyethylene bottle, 25.0 g of polymer dispersant a2 (water-insoluble polyester resin, solid content concentration 20%) obtained in Production Example 2 as a polymer dispersant, 95.9 g of hydrophobized titanium oxide (JMT-150IB), and 216.9 g of ion-exchanged water were added, and finally 1000 g of 2 mm zirconia beads were added, and dispersion was performed for 8 hours at 250 rpm using a benchtop pot mill stand (As One Corporation). After dispersion, the zirconia beads were removed using a mesh, and the solid content concentration was adjusted to 30% with water to obtain an aqueous dispersion A-4 (with a crosslinked structure, solid content concentration 30%) of polymer particles a4 containing titanium oxide.
The average particle size of the titanium oxide-containing polymer particles a4 in the obtained aqueous dispersion was 340 nm.

Comparative Preparation Example 1 (Preparation of Titanium Oxide Dispersion A-5)
In a 500 mL polyethylene bottle, 25.0 g of polymer dispersant a2 (water-insoluble polyester resin, solid content concentration 20%), 95.0 g of titanium oxide (manufactured by Titanium Kogyo Co., Ltd., Al-Si treatment, average primary particle size: 355 nm), and 213.3 g of ion-exchanged water were added in advance, and finally 1000 g of 2 mm zirconia beads were added, and the mixture was dispersed for 8 hours at 250 rpm on a tabletop pot mill stand (As One Corporation). After dispersion, the zirconia beads were removed using a mesh, and the solid content concentration was adjusted to 30% with water to obtain an aqueous dispersion A-5 (without crosslinking structure) of polymer particles a5 containing titanium oxide (without hydrophobization treatment).
The average particle size of the titanium oxide-containing polymer particles a5 in the obtained aqueous dispersion was 310 nm.

Preparation Example B1 (Preparation of Water Dispersion B-1 of Pigment-Free Polymer Particles)
In a reaction vessel equipped with a dropping funnel, the monomers shown in "Initially charged monomer solution" in Table 2, sodium polyoxyethylene alkyl ether sulfate (Latemul E-118B, emulsifier, manufactured by Kao Corporation), potassium persulfate (polymerization initiator), and ion-exchanged water were charged and mixed, and nitrogen gas replacement was performed to obtain an initially charged monomer solution. Also, the monomers shown in "Dropped monomer solution" in Table 2, surfactant, polymerization initiator, and ion-exchanged water were mixed to obtain a dropped monomer solution, which was placed in a dropping funnel and nitrogen gas replacement was performed.
Under a nitrogen atmosphere, the initial monomer solution in the reaction vessel was heated from room temperature to 80° C. over 30 minutes while stirring, and while maintaining the temperature at 80° C., the monomer in the dropping funnel was gradually dropped into the reaction vessel over 3 hours. After the dropping was completed, the mixture was stirred for 1 hour while maintaining the temperature in the reaction vessel. The mixture was then filtered through a 200 mesh to obtain an aqueous dispersion B-1 (solid content concentration 44.1%) of polymer particles b1 not containing a pigment.

Preparation Examples B2 to B5 (Preparation of Water Dispersions B-2 to B-5 of Pigment-Free Polymer Particles)
Using the monomer ratios shown in Table 2 below, aqueous dispersions B-2 to B-5 of polymer particles b2 to b5 not containing a pigment were obtained in the same manner as in Preparation Example B1.

Example 1 (Preparation of Water-Based Ink)
The following were added: 34.6 g of an aqueous dispersion A-1 (solid concentration 30%, pigment 28.9%, polymer 1.1%) of titanium oxide-containing polymer particles a1 obtained in Preparation Example A1; 18.1 g of an aqueous dispersion B-1 (solid concentration 44.1%) of pigment-free polymer particles b1 obtained in Preparation Example B1; 20.0 g of propylene glycol; 3.5 g of diethylene glycol monoisobutyl ether; 1.0 g of an acetylene glycol surfactant (manufactured by Nissin Chemical Industry Co., Ltd., Surfynol 440; an ethylene oxide (EO) (40 mol%) adduct of Surfynol 104, average EO added moles: 3.5); and 0.1 g of a polyether-modified silicone surfactant (manufactured by Nissin Chemical Industry Co., Ltd., Silface SAG005, HLB: 7, kinetic viscosity: 170 mm2/s (25°C)); and ion-exchanged water was added so that the total amount was 100 g.

Examples 2 to 9 and Comparative Examples 1 to 5 (Production of Water-Based Inks)
A water-based ink was obtained in the same manner as in Example 1, except that the composition was changed to that shown in Table 3.
SF210 and SF420 used in Examples 8 and 9 are self-emulsifying water-based urethane resins (product name: Superflex) manufactured by Daiichi Kogyo Seiyaku Co., Ltd., and the details thereof are as follows.
SF210: Ester-based urethane resin, solid content 35%
SF420: Carbonate-based urethane resin, solids concentration 32%

The resulting water-based ink was used to print a printed matter obtained by the printing method described below, and the intermittent ejection property and adhesion to a substrate were evaluated by the methods described below.
Inkjet printing method
In an environment of temperature 25±1° C. and relative humidity 30±5%, ink was filled into the cartridge of a print evaluation device (Seiko Epson Corporation, inkjet printer, PX105, piezoelectric type), an A4 size film heater (Kawai Electric Works, Ltd.) was attached to the print medium outlet section so that the print medium described below could be heated, and a corona discharge treated PET film (Futamura Chemical Co., Ltd.) cut to A4 size was placed in the paper storage area.

<Evaluation of intermittent ejection>
Using the printing evaluation device, ten 20 mm x 20 mm solid images were printed with 100% ink duty, and then the print was left for five minutes without printing. After five minutes had elapsed, one 20 mm x 20 mm solid image was printed, and from the state of the solid print obtained, the ejection recovery rate (%) was calculated using the following formula to evaluate the intermittent ejection property. In this evaluation, no current was applied to the film heater.
Discharge recovery rate (%)=[(discharge area of solid print obtained by command to print 1 sheet)/(discharge area of 10th solid print obtained by command to print 10 sheets)]×100
(Evaluation criteria)
5: The ejection recovery rate is 90% or more.
4: The ejection recovery rate is 80% or more and less than 90%.
3: The ejection recovery rate is 70% or more and less than 80%.
2: The ejection recovery rate is 60% or more and less than 70%.
1: The ejection recovery rate is less than 60%.
If the evaluation result is 3 or more, there is no problem in practical use, and if it is 4 or more, it is excellent in practical use.

<Evaluation of Adhesion to Substrate>
Using the printing evaluation device, a 50 mm x 50 mm solid image was printed with 100% ink duty and dried with a dryer to prepare a sample. Then, a 50 mm long, 15 mm wide tape "Cellotape CT15" (registered trademark) (manufactured by Nichiban Co., Ltd.) was attached to the printed surface of the sample for 4 cm with a 1 cm margin, and the tape was peeled off at an angle of 90° at a speed of 10 cm/sec, and the remaining area of the coated surface of the sample was visually evaluated for substrate adhesion according to the following 5-point scale.
The higher the score, the better the adhesion to the PET film substrate.
<Evaluation criteria>
5: No peeling.
4: Peeling occurred, but the peeled area was less than 20%.
3: The peeled area is 20% or more and less than 40%.
2: The peeled area is 40% or more and less than 60%.
1: Peeled area is 60% or more.
If the evaluation result is 3 or more, there is no problem in practical use, and if it is 4 or more, it is excellent in practical use.

From Table 3, it can be seen that the water-based ink obtained in the examples achieves a high level of both intermittent ejection properties and adhesion to the film substrate compared to the water-based ink obtained in the comparative examples.

Claims (8)

A water-based ink for ink-jet printing, comprising water-insoluble polymer particles A containing a pigment, polymer particles B not containing a pigment, a water-soluble organic solvent, and water,
The pigment is titanium dioxide that has been hydrophobically treated,
the pigment-containing water-insoluble polymer particles A contain at least one type selected from the group consisting of water-insoluble (meth)acrylic resins and polyester resins;
the (meth)acrylic resin contains a structural unit derived from an anionic monomer, and the content of the structural unit derived from the anionic monomer in the (meth)acrylic resin is 50 mass% or less;
the glass transition temperature of the polymer constituting the pigment-free polymer particles B is −10° C. or more and 40° C. or less;
a mass ratio of the pigment to the mass of the water-insoluble polymer particles A containing the pigment (pigment/water-insoluble polymer particles A containing the pigment) is 0.91 or more and less than 1;
The water-soluble organic solvent contains an alkylene glycol ether,
the mass ratio of the water-insoluble polymer particles A containing a pigment to the total water-soluble organic solvents (water-insoluble polymer particles A containing a pigment/total water-soluble organic solvents) is 0.5 or less;
Water-based ink for inkjet printing. The water-based ink according to claim 1, in which the pigment-containing polymer particles A have a crosslinked structure. 3. The water-based ink according to claim 1, wherein the polymer constituting the pigment-free polymer particles B comprises at least one type selected from the group consisting of (meth)acrylic resins and polyurethane resins. The water-based ink according to any one of claims 1 to 3, wherein the alkylene glycol ether is at least one selected from diethylene glycol monoisobutyl ether and diethylene glycol monobutyl ether. The water-based ink according to any one of claims 1 to 4, further comprising propylene glycol. The water-based ink according to any one of claims 1 to 5, further comprising a silicone surfactant. An inkjet printing method in which the water-based ink according to any one of claims 1 to 6 is used to print on a low-absorbency print medium. 8. The inkjet printing method according to claim 7, wherein the low liquid-absorbent printing medium is a resin film made of polyethylene terephthalate or polypropylene.