JP7709905B2 - Water-based ink for ink-jet printing - Google Patents

Description

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

Inkjet recording is a recording method in which ink droplets are directly ejected from fine nozzles and deposited on a recording medium to obtain printed matter on which characters and images are recorded. This method has become extremely popular due to its many advantages, including the ease and low cost of full coloring, the ability to use plain paper as a recording medium, and the fact that there is no contact with the recording medium. In particular, from the viewpoint of the weather resistance and water resistance of the recorded matter, the use of pigments as colorants has become mainstream.
In pigment inks, pigment particles are dispersed in the ink liquid, so after the ink is applied to a recording medium, the stable dispersion system is easily broken down and the pigments are likely to aggregate when the water and organic solvents evaporate during the drying process. When the aggregated pigment precipitates on the recording medium, it causes problems such as a loss of surface smoothness, a decrease in gloss, and a deterioration of the recorded image.

On the other hand, in order to realize the color and texture that the user envisions, various recording media with different surface characteristics are used. Plain paper, coated paper, etc. are widely used as recording media, and for coated paper, which is in high demand for high image quality and high grade, various types of textures are used, such as glossy paper with a mirror surface that has excellent glossiness and matte paper with a matte finish.
When inkjet recording is performed using a pigment ink, the gloss can be adjusted by appropriately changing the recording conditions depending on the surface characteristics of the recording medium used, but the range of adjustment is limited, and changing the recording conditions every time the type of recording medium is changed has a negative effect on productivity.
Therefore, there is a demand for inkjet recording inks that are highly versatile and can be used regardless of the type of recording medium, such as matte paper, and various proposals have been made.

For example, Patent Document 1 discloses a water-based ink that contains water-insoluble crosslinked polymer particles (A) containing a pigment and a polymer emulsion (B) with the aim of improving ejection stability and image fastness, in which the crosslinked polymer constituting the crosslinked polymer particles (A) is a polymer having a constituent unit derived from a carboxylic acid monomer and a constituent unit derived from a hydrophobic monomer, the constituent polymer having an acid value of 200 mg KOH/g or more, crosslinked with an epoxy compound, and the polymer constituting the polymer emulsion (B) contains a constituent unit derived from a carboxylic acid monomer and a constituent unit derived from a hydrophobic monomer.

JP 2019-14883 A

Matte paper is widely used for brochures, posters, catalogs, business cards, etc. Matte paper is paper with a matte coating on the surface, which has the advantage of reducing the glossiness and making text easier to read. Matte paper also has the advantage of having a smooth texture, ink is easily absorbed, and has high color reproducibility, so when text and photos are used together, the photos do not stand out unnaturally and have a balanced and calm atmosphere. However, there is a demand for the glossiness of the image parts printed on matte paper to be as high as possible.
The water-based inks disclosed in Patent Document 1 and the like do not satisfy the above demands because the gloss of the images obtained by inkjet recording on matte paper or the like is insufficient.
An object of the present invention is to provide a water-based ink for ink-jet printing that can provide recorded matter having excellent gloss even on recording media such as matte paper, and an ink-jet recording method that uses the water-based ink.

The present inventors have found that the above problems can be solved by a water-based ink containing a pigment and a polymer emulsion containing crosslinked polymer particles having polyalkylene oxide chains.
That is, the present invention provides the following [1] and [2].
[1] A water-based ink for ink-jet printing, comprising a pigment and a polymer emulsion, the polymer emulsion containing crosslinked polymer particles crosslinked with a polyalkylene glycol diglycidyl ether.
[2] An inkjet recording method for recording on matte paper using the water-based ink according to [1] above.

The present invention provides a water-based ink for inkjet printing that can produce records with excellent gloss even on recording media such as matte paper, and an inkjet recording method that uses the water-based ink.

[Water-based ink for ink-jet printing]
The water-based ink for ink-jet printing of the present invention (hereinafter also referred to as "the ink of the present invention") is a water-based ink containing a pigment and a polymer emulsion, and is characterized in that the polymer emulsion contains crosslinked polymer particles crosslinked with polyalkylene glycol diglycidyl ether.
Here, "water-based" means that water accounts for the largest proportion of the medium in which the pigment is dispersed. Also, "recording" is a concept that includes printing and printing characters and images.

The ink of the present invention makes it possible to obtain a recorded matter having excellent glossiness when performing inkjet recording on a recording medium such as matte paper. The reason for this is not clear, but is thought to be as follows.
When a water-based ink is ejected onto a recording medium in ink-jet recording, the water in the ink on the recording medium evaporates, and the concentrated ink becomes hydrophobic, containing a large amount of organic solvent.
Here, since the ink of the present invention contains a polymer emulsion containing crosslinked polymer particles crosslinked with polyalkylene glycol diglycidyl ether, a polymer having a polyalkylene oxide chain is present. The polyalkylene oxide chain is localized in the outermost shell of the polymer emulsion particle, and water is solvated in the polyalkylene oxide chain, so that the steric repulsion suppresses aggregation of the pigment and polymer particles.
Furthermore, the polyalkylene oxide chain has an affinity for both the water and the organic solvent in the ink, and even in the ink after the water has evaporated and become concentrated, the steric repulsion caused by solvation is maintained, suppressing aggregation of the pigment and polymer particles.
As a result, when recording on matte paper or the like, with conventional inks, the pigment particles and polymer particles quickly aggregate and solidify during the drying process on the recording surface, whereas with the ink of the present invention, aggregation is less likely to occur, which results in more time for the polymer particles to be smoothed by the gravity of the recording surface, improving the ability to fill in unevenness on the surface of the recording medium such as matte paper, forming a uniform polymer film on the recording medium, and it is believed that this improves the glossiness of recording media such as matte paper.

<Pigments>
The pigment used in 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, channel black, etc. Examples of 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.
The hue is not particularly limited, and 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 above pigments can be used alone or in combination of two or more kinds.

Suitable forms of the pigment used in the ink of the present invention include (i) a pigment that can maintain a dispersed state without a dispersant, i.e., a self-dispersing pigment, (ii) a pigment particle form in which the pigment is dispersed with a surfactant, and (iii) a polymer particle form containing the pigment. Of these, the polymer particle form containing the pigment is preferred from the viewpoint of improving the gloss of the resulting recorded matter.
Here, "polymer particles containing a pigment" (hereinafter also referred to as "pigment-containing polymer particles") refers to particles in which a polymer encapsulates a pigment, particles consisting of a polymer and a pigment, on the surface of which part of the pigment is exposed, particles in which a polymer is adsorbed to part of the pigment, or mixtures thereof. Of these, particles in which a polymer encapsulates a pigment are more preferred.

[Pigment-containing polymer particles]
The polymer constituting the pigment-containing polymer particles (hereinafter, also referred to as "polymer a") is not particularly limited as long as it has at least the ability to disperse the pigment.
Examples of the polymer a include vinyl resins obtained by addition polymerization of vinyl monomers, polyester resins, polyurethane resins, etc. Among these, vinyl resins are preferred from the viewpoints of pigment dispersion stability, storage stability, etc. The polymer a may be appropriately synthesized or may be a commercially available product.
The pigment-containing polymer particles are more preferably crosslinked polymer particles containing a pigment (hereinafter also referred to as "pigment-containing crosslinked polymer particles") obtained by further crosslinking polymer particles containing a pigment with a crosslinking agent.

The polymer a before crosslinking may be either a water-soluble polymer or a water-insoluble polymer, but is preferably a water-insoluble polymer. Even if the polymer used is a water-soluble polymer, the polymer becomes a water-insoluble polymer by subjecting the polymer to a crosslinking treatment.
In this specification, the term "water-insoluble" of a polymer means that when a polymer that has been dried at 105° C. for 2 hours and has reached a constant weight is dissolved in 100 g of water at 25° C., the amount of dissolution is less than 10 g. When the polymer is an anionic polymer, the amount of dissolution is the amount of dissolution when 100% of the anionic groups of the polymer are neutralized with sodium hydroxide.

[Polymer a]
When the polymer a is a vinyl resin, the polymer a preferably contains (a-1) a structural unit derived from an ionic monomer, more preferably contains (a-2) a structural unit derived from a hydrophobic monomer and/or (a-3) a nonionic monomer, and further preferably contains (a-1) a structural unit derived from an ionic monomer and (a-2) a structural unit derived from a hydrophobic monomer.

[(a-1) Ionic Monomer]
As the ionic monomer (a-1), an anionic monomer is preferable. Examples of the anionic monomer include a carboxylic acid monomer and a sulfonic acid monomer, and a carboxylic acid monomer is more preferable.
The carboxylic acid monomer may be at least one selected from (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, and is preferably (meth)acrylic acid. "(Meth)acrylic acid" means at least one selected from acrylic acid and methacrylic acid.

[(a-2) Hydrophobic Monomer]
The term "hydrophobic" in the (a-2) hydrophobic monomer means that when the monomer is dissolved in 100 g of ion-exchanged water at 25° C. until it is saturated, the amount of the monomer that dissolves is less than 10 g.
Specific examples of the hydrophobic monomer (a-2) include those described in paragraphs [0020] to [0022] of JP-A-2018-83938. Among these, at least one selected from alkyl (meth)acrylates having an alkyl group having 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms, and aromatic group-containing monomers having an aromatic group having 6 to 22 carbon atoms, preferably 6 to 18 carbon atoms, are preferred, and at least one selected from ethyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, styrene, α-methylstyrene, and benzyl (meth)acrylate are more preferred.

[(a-3) Nonionic Monomer]
The nonionic monomer (a-3) is a monomer that has a high affinity for water or a water-soluble organic solvent, and is, for example, a monomer that contains a hydroxyl group or a polyalkylene glycol chain.
Specific examples of the component (a-3) include those described in paragraph [0018] of JP-A-2018-83938. Among these, one or more selected from methoxypolyethylene glycol (n = 1 to 30) (meth)acrylate and polypropylene glycol (n = 2 to 30) (meth)acrylate are preferred.
The monomer components contained in each of the above components (a-1) to (a-3) can be used alone or in combination of two or more kinds.

(Content of each structural unit in vinyl resin)
From the viewpoint of improving the gloss of the resulting recorded matter, the content of the structural units derived from the components (a-1) to (a-3) in the vinyl resin is as follows.
The content of the (a-1) component is preferably 5 mass% or more, more preferably 10 mass% or more, even more preferably 15 mass% or more, and is preferably 45 mass% or less, more preferably 40 mass% or less, even more preferably 35 mass% or less.
The content of the (a-2) component is preferably 35% by mass or more, more preferably 40% by mass or more, even more preferably 45% by mass or more, and is preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less.
When the component (a-3) is contained, the content thereof is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 5% by mass or less.
The mass ratio of the component (a-1) to the component (a-2) [component (a-1)/component (a-2)] is preferably 0.05 or more, more preferably 0.1 or more, and is preferably 0.8 or less, more preferably 0.6 or less.

(Production of Polymer a)
The polymer a can be produced by copolymerizing a mixture of the above monomer components (a-1) to (a-3) by a known polymerization method, preferably a solution polymerization method.
The solvent used in the solution polymerization method is not limited, but polar solvents such as water, aliphatic alcohols, ketones, ethers, and esters are preferred, and water, methanol, ethanol, acetone, methyl ethyl ketone, and the like are more preferred.
In the polymerization, a polymerization initiator and a polymerization chain transfer agent can be used. Examples of the polymerization initiator include persulfates such as ammonium persulfate and potassium persulfate, and water-soluble azo polymerization initiators, and examples of the polymerization chain transfer agent include mercaptans.
The polymerization temperature varies depending on the types of polymerization initiator, monomer, solvent, etc. used, but is preferably 40° C. or higher, more preferably 50° C. or higher, and is preferably 95° C. or lower, more preferably 90° C. or lower.
The polymerization atmosphere is preferably a nitrogen gas or inert gas atmosphere.
The vinyl resin is preferably neutralized with a neutralizing agent as described below.

From the viewpoint of improving the glossiness of the resulting recorded matter, the weight average molecular weight of polymer a is preferably 5,000 or more, more preferably 10,000 or more, even more preferably 15,000 or more, and is preferably 80,000 or less, more preferably 60,000 or less, even more preferably 40,000 or less.
The weight average molecular weight of the polymer is measured by the method described in the Examples.
From the viewpoint of improving the glossiness of the resulting recorded matter, the acid value of polymer 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 350 mgKOH/g or less, more preferably 300 mgKOH/g or less, even more preferably 250 mgKOH/g or less.
The acid value of the polymer is measured by the method described in the Examples.

[Preparation of pigment-containing polymer particles]
The pigment-containing polymer particles can be efficiently produced as a pigment aqueous dispersion by a method including the following steps 1 and 2, and, if necessary, further step 3.
Step 1: A step of dispersing a pigment mixture containing a pigment, polymer a, an organic solvent, and water to obtain a dispersion. Step 2: A step of removing the organic solvent from the dispersion obtained in step 1 to obtain an aqueous dispersion of pigment-containing polymer particles A (hereinafter also referred to as "pigment aqueous dispersion (i)"). Step 3: A step of adding a crosslinking agent to the pigment aqueous dispersion (i) obtained in step 2 to crosslink the pigment-containing polymer particles to obtain an aqueous dispersion (I) of pigment-containing crosslinked polymer particles (hereinafter also referred to as "pigment aqueous dispersion (I)"). The pigment-containing polymer particles according to the present invention also include the pigment-containing crosslinked polymer particles obtained in step 3.

(Step 1)
The pigment mixture in step 1 is preferably obtained by a method in which polymer a is dissolved in an organic solvent, and pigment, water, and, if necessary, a neutralizing agent, a surfactant, and the like are added to the obtained organic solvent solution and mixed to obtain an oil-in-water dispersion.
Although there is no limitation 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 polymer a to the pigment, more preferred are ketones having from 4 to 8 carbon atoms, and even more preferred is methyl ethyl ketone. When a vinyl resin is synthesized as polymer a by solution polymerization, the solvent used in the polymerization may be used as it is.

When the polymer a has acid groups, it is preferable that at least a part of the acid groups is neutralized with a neutralizing agent, which is believed to increase the charge repulsion force exhibited after neutralization, suppress the aggregation of pigment particles in the water-based ink, and improve the dispersion stability of the pigment.
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.
The polymer a may be neutralized in advance.
From the viewpoint of improving dispersion stability, the amount of the neutralizing agent used is preferably 20 mol % or more, more preferably 30 mol % or more, even more preferably 40 mol % or more, and is preferably 120 mol % or less, more preferably 100 mol % or less, even more preferably 80 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

In the dispersion treatment in step 1, the pigment particles can be atomized to a desired particle size only by main dispersion using shear stress. However, 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 dispersing blade can be used.
Examples of means for applying shear stress used in the present 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, it is preferable to use high-pressure homogenizers and bead mills from the viewpoint of reducing the particle size of the pigment.
When the dispersion treatment is carried out using a high-pressure homogenizer, the average particle size of the pigment particles in the pigment aqueous dispersion can be adjusted by controlling the treatment pressure and the number of passes.
From the viewpoints of productivity and economy, the treatment pressure is preferably 60 MPa or more and 300 MPa or less, and the number of passes is preferably 3 or more and 30 or less.

(Step 2)
The organic solvent can be removed by a known method in step 2. It is preferable that the organic solvent in the obtained pigment water dispersion (i) has been substantially removed, but it may remain as long as the object of the present invention is not impaired.
In addition, for the purpose of removing coarse particles and the like, it is preferable to further centrifuge the aqueous dispersion from which the organic solvent has been removed, and then filter the liquid layer portion through a filter or the like, and obtain the pigment aqueous dispersion (i) that has passed through the filter or the like.

(Step 3)
In step 3, which is optional, a crosslinking agent is added to the pigment water dispersion (i) obtained in step 2 to crosslink a portion of the carboxy groups of the polymer a constituting the pigment-containing polymer particles, forming a crosslinked structure in the surface layer portion of the pigment-containing polymer particles, thereby obtaining a pigment water dispersion (I) of pigment-containing crosslinked polymer particles. This allows the polymer formed by crosslinking the polymer a to be firmly adsorbed or fixed on the pigment surface, suppressing the aggregation of the pigment, and as a result, it is believed that the dispersion stability of the obtained ink is further improved.

The crosslinking agent is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule, more preferably a compound having two or more glycidyl ether groups, and even more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 4 carbon atoms.
The epoxy equivalent (g/eq) of the crosslinking agent is preferably 90 or more, more preferably 100 or more, and preferably 300 or less, more preferably 200 or less.
Suitable examples of the crosslinking agent include one or more selected from polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether.

From the viewpoint of improving the gloss of the resulting recorded matter, the degree of crosslinking in step 3 is preferably 8 mol % or more, more preferably 10 mol % or more, and is preferably 70 mol % or less, more preferably 50 mol % or less, in terms of the ratio of the molar equivalent number of the crosslinkable functional groups of the crosslinking agent to the molar equivalent number of the carboxy groups of polymer a.
From the viewpoint of crosslinking reaction efficiency, the temperature of the crosslinking treatment is preferably 40° C. or higher, more preferably 50° C. or higher, and is preferably 90° C. or lower, more preferably 85° C. or lower.
The acid value of the polymer a after crosslinking is preferably 40 mgKOH/g or more, more preferably 60 mgKOH/g or more, even more preferably 80 mgKOH/g or more, and is preferably 250 mgKOH/g or less, more preferably 200 mgKOH/g or less, even more preferably 150 mgKOH/g or less.

From the viewpoint of improving the dispersion stability of the pigment water dispersion, the non-volatile component concentration (solid content concentration) of the resulting pigment water dispersion (I) is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 45% by mass or less, more preferably 35% by mass or less.
The solids 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 7% by mass or more, more preferably 10% by mass or more, and preferably 30% by mass or less, more preferably 20% by mass or less.
From the viewpoint of dispersion stability, the content of the (crosslinked) polymer in the pigment water dispersion (I) is preferably 4% by mass or more, more preferably 5% by mass or more, and is preferably 15% by mass or less, more preferably 10% by mass or less.
The mass ratio of the (crosslinked) polymer constituting the pigment-containing (crosslinked) polymer particles in the pigment water dispersion (I) to the pigment [(crosslinked) polymer/pigment] is preferably 0.1 or more, more preferably 0.2 or more, and is preferably 1.2 or less, more preferably 0.8 or less.
From the viewpoint of improving the gloss of the resulting recorded matter, the average particle size of the pigment-containing (crosslinked) polymer particles is preferably 50 nm or more, more preferably 80 nm or more, and is preferably 300 nm or less, more preferably 200 nm or less.
The average particle size is measured by the method described in the Examples.

<Polymer emulsion>
The polymer emulsion is used to fill in voids in the unevenness of the surface of the recording medium as the aqueous ink dries after being discharged, thereby improving the smoothness and gloss.
In the present invention, the polymer emulsion contains crosslinked polymer particles crosslinked with polyalkylene glycol diglycidyl ether. That is, the crosslinked polymer particles crosslinked with polyalkylene glycol diglycidyl ether are composed of a component derived from a polymer (hereinafter also referred to as "polymer b") and a component derived from a polyalkylene glycol diglycidyl ether.
Specific examples of the polymer b include vinyl resins, urethane resins, and polyester resins. Among these, vinyl resins and urethane resins are preferred, and vinyl resins are more preferred.
The vinyl resin preferably contains (b-1) a structural unit derived from an ionic monomer and (b-2) a structural unit derived from a hydrophobic monomer, and may further contain (b-3) a structural unit derived from a nonionic monomer.
Specific examples and suitable examples of the components (b-1) to (b-3) are the same as the components (a-1) to (a-3) described in the section on the polymer (polymer a) that constitutes the pigment-containing polymer particles.

The method for producing the vinyl resin is the same as the method for producing the polymer a.
When a polymer emulsion is produced by emulsion polymerization, the pH tends to be acidic due to the presence of carboxyl groups on the surface of the polymer particles, which tends to cause an increase in viscosity and aggregation. Therefore, neutralization is usually performed with a basic substance such as sodium hydroxide. The amount of neutralizing agent added is appropriately determined so that the pH of the polymer emulsion is in the range of 7.5 to 9.5, preferably 7.5 to 8.5.
From the viewpoints of reaction completion and economic efficiency, the reaction temperature is preferably 40° C. or higher, more preferably 50° C. or higher, and preferably 95° C. or lower, more preferably 90° C. or lower.

Examples of the urethane resin which is the polymer b include polyester-based urethane resins having an ester bond, polycarbonate-based urethane resins having a carbonate bond, and polyether-based urethane resins having an ether bond, with polyester-based urethane resins being preferred.
The polyester-based urethane resin means a urethane resin obtained by reacting a polyester polyol with a polyisocyanate. The polyester-based urethane resin may be a synthesized product or a commercially available product.
Examples of methods for producing polyester-based urethane resins include a method in which a polyol containing a polyester polyol is reacted with a polyisocyanate to produce an isocyanate-terminated urethane prepolymer, and then the anionic groups in the isocyanate-terminated urethane prepolymer are neutralized with a neutralizing agent such as ammonia or an organic amine, and then the prepolymer is further reacted with a polyamine (chain extender) and, if necessary, a reaction terminator.
Commercially available examples of polyester-based urethane resins include Vylon UR-3500 and UR-3600 manufactured by Toyobo Co., Ltd., and NeoRez R-9660 and R-2170 manufactured by DSM Coating Resins.

From the viewpoint of improving the glossiness of the resulting recorded matter, the acid value of polymer b is preferably 5 mgKOH/g or more, more preferably 20 mgKOH/g or more, even more preferably 35 mgKOH/g or more, still more preferably 90 mgKOH/g or more, and is preferably 250 mgKOH/g or less, more preferably 200 mgKOH/g or less.
When polymer b is a vinyl resin, the acid value is, from the viewpoint of improving the gloss of the resulting recorded matter, preferably 50 mgKOH/g or more, more preferably 90 mgKOH/g or more, even more preferably 100 mgKOH/g or more, still more preferably 180 mgKOH/g or more, and is preferably 250 mgKOH/g or less, more preferably 230 mgKOH/g or less, even more preferably 200 mgKOH/g or less.
When polymer b is a urethane resin, the acid value is, from the viewpoint of improving the gloss of the resulting recorded matter, preferably 10 mgKOH/g or more, more preferably 20 mgKOH/g or more, even more preferably 30 mgKOH/g or more, and is preferably 100 mgKOH/g or less, more preferably 50 mgKOH/g or less, even more preferably 40 mgKOH/g or less.

The polyalkylene glycol diglycidyl ether as the crosslinking agent may be any diglycidyl ether capable of introducing a polyalkylene oxide chain into the polymer b, but it is preferable that the diglycidyl ether contains an alkylene group having preferably 2 to 8 carbon atoms, more preferably 2 to 4 carbon atoms.
From the viewpoints of making the polymer emulsion hydrophilic and stabilizing it, imparting flexibility, and forming a smooth polymer coating film on a recording medium, the number of repeating units n of the alkylene oxide group is preferably 2 or more and 30 or less, more preferably 4 or more and 20 or less, even more preferably 6 or more and 15 or less, and still more preferably 8 or more and 12 or less.

From the same viewpoints as above, the epoxy equivalent (g/eq) of the polyalkylene glycol diglycidyl ether is preferably 200 g/eq or more, more preferably 215 g/eq or more, even more preferably 250 g/eq or more, still more preferably 270 g/eq or more, and is preferably 600 g/eq or less, more preferably 560 g/eq or less, even more preferably 530 g/eq or less, still more preferably 500 g/eq or less, and still more preferably 350 g/eq or less.
Specific examples of polyalkylene glycol diglycidyl ethers include, preferably, one or more selected from polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, polytetramethylene glycol diglycidyl ether, and the like, and more preferably, one or more selected from polyethylene glycol diglycidyl ether and polypropylene glycol diglycidyl ether.

Examples of commercially available polyalkylene glycol diglycidyl ethers include polyethylene glycol diglycidyl ethers manufactured by Nagase ChemteX Corporation, trade names: EX-821 (n=4, epoxy equivalent: 185), EX-830 (n=9, epoxy equivalent: 268), EX-841 (n=14, epoxy equivalent: 372), EX-861 (n=22, epoxy equivalent: 551), polypropylene glycol diglycidyl ether, trade name: EX-931 (n=11, epoxy equivalent: 471), and polyethylene glycol diglycidyl ethers manufactured by Kyoeisha Chemical Co., Ltd., trade names: Epolite 200E (n=4, epoxy equivalent: 215), Epolite 400E (n=9, epoxy equivalent: 290).

The mass ratio of the polyalkylene glycol diglycidyl ether (crosslinking agent) to the polymer constituting the polymer emulsion (crosslinking agent/polymer) is preferably 0.05 or more, more preferably 0.1 or more, even more preferably 0.2 or more, still more preferably 0.5 or more, and is preferably 1.5 or less, more preferably 1.2 or less, even more preferably 1.1 or less, and still more preferably 1 or less.
The temperature for the crosslinking treatment of the polymer emulsion is preferably 40° C. or higher, more preferably 50° C. or higher, and preferably 90° C. or lower, more preferably 85° C. or lower, from the viewpoint of crosslinking reaction efficiency.

From the viewpoint of improving the gloss of the resulting recorded matter, the degree of crosslinking of the polymer constituting the polymer emulsion is preferably 8 mol % or more, more preferably 10 mol % or more, even more preferably 15 mol % or more, still more preferably 30 mol % or more, still more preferably 40 mol % or more, and is preferably 90 mol % or less, more preferably 85 mol % or less, still more preferably 80 mol % or less, and still more preferably 60 mol % or less.
The degree of crosslinking is an apparent degree of crosslinking calculated from the acid value of the polymer and the equivalent of the epoxy group of the crosslinking agent, i.e., the degree of crosslinking is "the number of molar equivalents of the epoxy group of the epoxy compound / the number of molar equivalents of the carboxy group of the polymer before crosslinking".

From the viewpoint of improving the gloss of the resulting recorded matter, the acid value of the crosslinked polymer after crosslinking that constitutes the polymer emulsion is preferably 4 mgKOH/g or more, more preferably 8 mgKOH/g or more, and is preferably 180 mgKOH/g or less, more preferably 150 mgKOH/g or less.
When the crosslinked polymer is a vinyl resin, its acid value is preferably 8 mgKOH/g or more, more preferably 15 mgKOH/g or more, even more preferably 20 mgKOH/g or more, still more preferably 45 mgKOH/g or more, and preferably 180 mgKOH/g or less, more preferably 160 mgKOH/g or less, even more preferably 140 mgKOH/g or less, still more preferably 120 mgKOH/g or less, still more preferably 100 mgKOH/g or less, still more preferably 80 mgKOH/g or less.
When the crosslinked polymer is a urethane resin, its acid value is preferably 4 mgKOH/g or more, more preferably 6 mgKOH/g or more, and preferably 60 mgKOH/g or less, more preferably 40 mgKOH/g or less, and even more preferably 30 mgKOH/g or less.

From the viewpoint of improving the gloss of the resulting recorded matter, the average particle size of the crosslinked polymer particles in the polymer emulsion is preferably 30 nm or more, more preferably 50 nm or more, even more preferably 70 nm or more, and is preferably 200 nm or less, more preferably 150 nm or less, even more preferably 120 nm or less.
The average particle size of the crosslinked polymer particles is measured by the method described in the Examples.

[Method of manufacturing the ink of the present invention]
The ink of the present invention can be produced by mixing the aqueous pigment dispersion containing the pigment-containing (crosslinked) polymer particles obtained by the above-mentioned method, the above-mentioned polymer emulsion, an organic solvent, water, and, as necessary, various additives such as a surfactant.
There is no particular limitation on the method for mixing the above components.

From the viewpoint of improving the storage stability and the like of the water-based ink, it is preferable that the organic solvent contains one or more organic solvents having a boiling point of preferably 110° C. or higher, preferably 150° C. or higher, and preferably 240° C. or lower, more preferably 220° C. or lower.
Specific examples of the organic solvent include polyhydric alcohols, polyhydric alcohol alkyl ethers, nitrogen-containing heterocyclic compounds, amides, amines, sulfur-containing compounds, etc. Among these, one or more selected from polyhydric alcohols and polyhydric alcohol alkyl ethers are preferred, one or more selected from ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, glycerin, trimethylolpropane, and diethylene glycol diethyl ether are more preferred, and one or more selected from propylene glycol, glycerin, triethylene glycol, and trimethylolpropane are even more preferred.
Examples of the surfactant include nonionic surfactants, anionic surfactants, amphoteric surfactants, silicone-based surfactants, and fluorine-based surfactants, with nonionic surfactants being more preferred.
Examples of the nonionic surfactant include acetylene glycol surfactants, polyoxyalkylene alkyl ether surfactants, polyhydric alcohol surfactants, fatty acid alkanolamides, and the like, with acetylene glycol surfactants and polyoxyalkylene alkyl ether surfactants being more preferred.
Other additives that can be used in the method for producing the ink of the present invention include a humectant, a wetting agent, a penetrating agent, a viscosity adjusting agent, a defoaming agent, a preservative, an antifungal agent, and an antirust agent.

(Content of each component in the ink of the present invention)
The content of each component in the ink of the present invention is as follows, from the viewpoint of improving the glossiness and the like of the resulting recorded matter.
(Pigment content)
[0043] From the viewpoint of improving the printing density of the water-based ink, the content of the pigment in the ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, and from the viewpoint of improving the storage stability, it is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less.
(Content of (crosslinked) polymer particles containing pigment)
The content of the pigment-containing (crosslinked) polymer particles in the ink of the present invention, in terms of the total of the pigment and the (crosslinked) polymer, is preferably 2% by mass or more, more preferably 4% by mass or more, even more preferably 5% by mass or more, and is preferably 17% by mass or less, more preferably 12% by mass or less, even more preferably 10% by mass or less.

(Polymer Emulsion Content)
The polymer emulsion is present in an amount, in terms of solids content in the water-based ink, of preferably 2% by mass or more, more preferably 4% by mass or more, even more preferably 6% by mass or more, and is preferably 12% by mass or less, more preferably 10% by mass or less, even more preferably 9% by mass or less.
From the viewpoint of improving the gloss of the obtained recorded matter, the mass ratio of the pigment to the polymer (the total amount of polymer a constituting the pigment-containing polymer particles and polymer b constituting the polymer emulsion) in the ink of the present invention (pigment/polymer) is preferably 0.1 or more, more preferably 0.2 or more, even more preferably 0.3 or more, and is preferably 2 or less, more preferably 1 or less, even more preferably 0.8 or less.
From the viewpoint of improving the gloss of the obtained recorded matter, the mass ratio of the pigment-containing (crosslinked) polymer particles in the ink of the present invention to the crosslinked polymer particles in the polymer emulsion (solid content of the polymer emulsion) [pigment-containing (crosslinked) polymer particles/crosslinked polymer particles] is preferably 0.1 or more, more preferably 0.15 or more, even more preferably 0.18 or more, and is preferably 1 or less, more preferably 0.8 or less, even more preferably 0.6 or less, and still more preferably 0.4 or less.

(Organic solvent content)
From the viewpoint of improving the ejection stability of the ink, the content of the organic solvent in the 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 50% by mass or less, more preferably 30% by mass or less, even more preferably 20% by mass or less.
(Water Content)
From the viewpoint of improving the ejection stability and the like, the water content in the ink of the present invention is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, and is preferably 85% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less.

(Physical properties of the ink of the present invention)
From the viewpoint of improving the storage stability of the ink, the viscosity of the ink of the present invention at 32°C is preferably 2 mPa·s or more, more preferably 4 mPa·s or more, and is preferably 12 mPa·s or less, more preferably 8 mPa·s or less.
The pH of the ink of the present invention is preferably 7 or more, more preferably 7.5 or more, from the viewpoint of improving the storage stability of the ink, etc. Also, from the viewpoints of member resistance and skin irritation, the pH is preferably 11 or less, more preferably 10 or less.

[Inkjet recording method]
The ink-jet recording method of the present invention is characterized in that recording is performed on matte paper using the ink of the present invention.
The ink of the present invention can be loaded into a known ink jet recording device and ejected as ink droplets onto a recording medium to record images and the like having excellent gloss.
Ink jet recording apparatuses include thermal and piezoelectric types, and it is more preferable to use water-based inks for piezoelectric type ink jet printing.
From the viewpoint of high speed recording efficiency, the voltage applied to the recording head during ink jet recording is preferably 10 V or more, more preferably 20 V or more, and is preferably 40 V or less, more preferably 30 V or less.
From the viewpoint of reducing the viscosity of the ink, the temperature inside the recording head is preferably 20° C. or higher, more preferably 25° C. or higher, and is preferably 45° C. or lower, more preferably 40° C. or lower.
The ink of the present invention can be applied to recording paper such as plain paper, coated paper, and resin film, but in the inkjet recording method of the present invention, coated paper is preferred, and the effects of the present invention are more pronounced when recording on matte paper.

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 polymer weight average molecular weight (Mw) The weight average molecular weight was measured by gel permeation chromatography (GPC apparatus (HLC-8320GPC) manufactured by Tosoh Corporation, columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolumn Super AW-H) manufactured by Tosoh Corporation, flow rate: 0.5 mL/min) using a solution in which phosphoric acid and lithium bromide were dissolved in N,N-dimethylformamide to concentrations of 60 mmol/L and 50 mmol/L, respectively, as an eluent, and 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 particle size of pigment-containing polymer particles and crosslinked polymer particles Using a laser particle analysis system (manufactured by Otsuka Electronics Co., Ltd., product name: ELS-8000), particle size was measured by dynamic light scattering method, and calculated by cumulant method analysis.
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 input as the refractive index of the dispersion solvent. The measurement sample was prepared by weighing an aqueous dispersion into a screw tube (Maruemu Co., Ltd., No. 5), adding water so that the solids concentration was 2 x ^10-4 mass%, and stirring with a magnetic stirrer at 25°C for 1 hour.

(3) Measurement of acid value of polymer 2 g of the polymer before crosslinking or 2 g of the aqueous dispersion of the polymer after crosslinking was diluted with 50 g of ion-exchanged water, and 3 ml of 0.1 N sodium hydroxide solution was added. 0.1 N hydrochloric acid was gradually added dropwise thereto, and the inflection points of pH were measured at two points. The number of moles of acid calculated from the difference in the amount of 0.1 N hydrochloric acid added between the two points corresponds to the number of moles of carboxylic acid in the polymer, and this number of moles was converted into the acid value.

(4) Measurement of solid content concentration of aqueous dispersion 10.0 g of sodium sulfate, which had been brought to a constant weight in a desiccator, was weighed out into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm), and about 1.0 g of the sample was added thereto and mixed, then weighed, and maintained at 105° C. for 2 hours to remove volatile matter, and further left in the desiccator for 15 minutes, and the mass was measured. The mass of the sample after removing the volatile matter was taken as the solid content, and was divided by the mass of the sample added to obtain the solid content concentration.

Production Example 1 (Preparation of Aqueous Dispersion of Vinyl Polymer P1)
(1) Production of Polymer P1 A monomer shown in the "initial charge monomer solution" in Table 1 and an organic solvent (methyl ethyl ketone (MEK)) were placed in a reaction vessel equipped with a dropping funnel, mixed, and the atmosphere was replaced with nitrogen gas to obtain an "initial charge monomer solution."
On the other hand, the monomer shown in the "monomer solution to be dropped" in Table 1, MEK, a polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile), manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., product name: V-65), and a polymerization chain transfer agent (2-mercaptoethanol: manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., reagent) were mixed to obtain a "monomer solution to be dropped", which was placed in a dropping funnel and replaced with nitrogen gas.
Under a nitrogen atmosphere, the "initial charge monomer solution" in the reaction vessel was maintained at 77° C. with stirring, and the "dropped monomer solution" in the dropping funnel was gradually dropped into the reaction vessel over a period of 3 hours.
After the dropwise addition was completed, the mixed solution in the reaction vessel was stirred for 0.5 hours at 77° C. Next, a polymerization initiator solution was prepared by dissolving 0.6 parts of the polymerization initiator in 27.0 parts of MEK, and added to the mixed solution, followed by stirring at 77° C. for 1 hour for aging.
The preparation, addition and aging of the polymerization initiator solution were repeated five more times. Next, the reaction solution in the reaction vessel was maintained at 80° C. for 1 hour, and then the reaction solution in the reaction vessel was transferred to a stainless steel tray and MEK was distilled off using a vacuum dryer to obtain a polymer P1.
The weight average molecular weight of the obtained polymer P1 was 28,700 and the acid value was 200 mgKOH/g. The results are shown in Table 1.

(2) Preparation of aqueous dispersion of polymer P1 128.6 parts of polymer P1 obtained in (1) above was dissolved in 192.9 parts of MEK to obtain an MEK solution of polymer P1 (solid concentration: 40%). The MEK solution of polymer P1 was charged into a 3L reaction vessel, and a mixture of 1091.2 parts of ion-exchanged water and 48.9 parts of 5N aqueous sodium hydroxide solution (neutralizing agent equivalent: 45 mol%) was added dropwise over 2 hours while stirring the solution.
After the dropwise addition, the content of the reaction vessel was placed in a 3 L eggplant flask, and the organic solvent was removed by keeping the temperature at 32° C. for 3 hours at a pressure of 0.09 MPa in a warm bath adjusted to 32° C. at a rotation speed of 50 rpm using a rotary distillation apparatus (manufactured by Tokyo Rikakikai Co., Ltd., product name: Rotary Evaporator N-1000S). The temperature of the warm bath was further adjusted to 62° C., the pressure was reduced to 0.07 MPa, and the mixture was concentrated until the solid content reached 25%, to obtain an aqueous dispersion of polymer P1.

Production Example 2 (Preparation of Aqueous Dispersion of Vinyl Polymer P2)
Polymer P2 was obtained in the same manner as in Production Example 1 (1), except that the monomer compositions of the initially charged monomer solution and the dropped monomer solution were changed to the amounts shown in Table 1. The weight average molecular weight of the obtained polymer P2 was 20,200 and the acid value was 100 mgKOH/g.
Further, in Production Example 1(2), the above-mentioned polymer P2 was used instead of polymer P1, and 24.5 parts of a 5N aqueous sodium hydroxide solution (neutralizing agent equivalent: 45 mol%) was used, and an aqueous dispersion of polymer P2 was obtained in the same manner as in Production Example 1(2).

Production Example 3 (Preparation of Water Dispersion of Vinyl Polymer P3)
Into a glass vessel, 96.34 parts of ion-exchanged water, 18.46 parts (solids) of a surfactant (sodium polyoxyethylene alkyl ether sulfate, manufactured by Kao Corporation, emulsifier for emulsion polymerization, product name: LATEMUL E-118B, solids concentration: 26%), 0.32 parts of a water-soluble polymerization initiator (potassium persulfate: manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent), 10.0 parts of a polymerization chain transfer agent (n-dodecyl mercaptan, manufactured by Kao Corporation, product name: Thiokalcol 20), 30.0 parts of methacrylic acid as monomers, 40.0 parts of 2-ethylhexyl acrylate, and 130.0 parts of styrene were added, and the mixture was stirred at 500 rpm for 30 minutes using a stirring blade made of Teflon (registered trademark) to obtain a pre-emulsion.
Next, 96.3 parts of ion-exchanged water, 4.6 parts (solid content) of the surfactant, 5.3 parts of the water-soluble polymerization initiator, and 15.5 parts of the pre-emulsion prepared in advance were added to a glass reactor equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube.
After replacing with nitrogen, the temperature was raised to 80° C. in a water bath while stirring at 250 rpm. After the temperature reached 80° C., the remaining pre-emulsion (309.62 parts) was added dropwise over 2 hours to carry out emulsion polymerization. The acid value of the polymer constituting the obtained emulsion was 97 mgKOH/g.
After the dropwise addition was completed, the mixture was aged at 80° C. for 2 hours and then cooled to room temperature. Then, 36.9 parts of a 5N aqueous sodium hydroxide solution (amount equivalent to 45 mol % based on the total amount of methacrylic acid in the obtained polymer emulsion: neutralization degree 45%) was added, and ion-exchanged water was further added so that the polymer solids concentration became 25%, thereby obtaining an aqueous dispersion of polymer P3.

Preparation Example 1 (Preparation of Emulsion A1 of Crosslinked Polymer P1)
227 parts of the aqueous dispersion of polymer P1 obtained in Production Example 1 (2) (50.0 parts as polymer P1) and 25.8 parts (crosslinking rate: 50 mol%) of crosslinking agent X1 (polyethylene glycol diglycidyl ether, manufactured by Kyoeisha Chemical Co., Ltd., product name: Epolite 400E, n = 9, epoxy equivalent: 290 g / eq) were stirred in a sealable container at 80 ° C. for 2 hours. After cooling to 5 ° C., the mixture was filtered through the 5 μm filter to remove coarse particles, and ion-exchanged water was added so that the solid content concentration was 22%, to obtain an emulsion A1 of crosslinked polymer P1 (acid value: 66 mg KOH / g) that does not contain a pigment.
The results are shown in Table 2. In addition, the composition (parts) in Table 2 is the amount of solids (effective content).

Preparation Examples 2 to 11, Comparative Preparation Example 1 (Preparation of Crosslinked Polymer Emulsions A2 to A11, AC1)
Instead of the aqueous dispersion of polymer P1 obtained in Production Example 1(2), the aqueous dispersion of polymer P2 obtained in Production Example 2, the aqueous dispersion of polymer P3 obtained in Production Example 3, or the emulsions of commercially available urethane resins P4 and P5 were used in the amounts shown in Table 2, and the type and amount of the crosslinking agent were used in the amounts shown in Table 2. Except for this, the same procedure as in Preparation Example 1 was repeated to obtain pigment-free crosslinked polymer emulsions A2 to A11 and AC1.
The crosslinked vinyl polymers P2 and P3 each had an acid value of 40 mgKOH/g.
In addition, commercially available urethane resins are as follows:
Urethane resin P4: polyester urethane resin (manufactured by Toyobo Co., Ltd., product name: Vylon UR-3500, acid value 35 mg KOH/g (catalog value), solid content concentration: 40%)
Urethane resin P5: polyester urethane resin (manufactured by Toyobo Co., Ltd., product name: Vylon UR-3600, acid value 20 mg KOH/g (catalog value), solid content concentration: 40%)

Details of the crosslinking agents X2 to X6 listed in Table 2 are as follows.
X2: polyethylene glycol diglycidyl ether (manufactured by Nagase ChemteX Corporation, product name: Denacol EX-841, n = 14, epoxy equivalent: 372 g/eq)
X3: Polypropylene glycol diglycidyl ether (manufactured by Nagase ChemteX Corporation, product name: Denacol EX-931, n = 11, epoxy equivalent: 471 g / eq)
X4: polyethylene glycol diglycidyl ether (manufactured by Kyoeisha Chemical Co., Ltd., product name: Epolite 200E, n = 4, epoxy equivalent: 215 g / eq)
X5: polyethylene glycol diglycidyl ether (manufactured by Nagase ChemteX Corporation, product name: Denacol EX-861, n = 22, epoxy equivalent: 551 g/eq)
X6: 1,6-hexanediol diglycidyl ether (manufactured by Nagase ChemteX Corporation, product name: Denacol EX212, epoxy equivalent: 151 g/eq)

Preparation Example 20 (Preparation of Pigment Water Dispersion)
128.6 parts of the polymer P1 obtained in Production Example 1(1) was mixed with 308 parts of MEK to obtain a MEK solution of the polymer P1.
The MEK solution of polymer P1 was charged into a high-speed disperser (manufactured by Primix Corporation, product name: T.K. Robomix, stirring part: Homo Disper 2.5 type (blade diameter 40 mm)) having a receiver with a volume of 3 L, and while stirring at 1,400 rpm, 893.8 parts of ion-exchanged water and 56.6 parts of a 5N aqueous sodium hydroxide solution (neutralization degree: 52 mol%) were added, and while cooling in a water bath at 0°C, stirring was continued for 15 minutes at 1,400 rpm.
Next, 300 parts of carbon black (manufactured by Cabot Corporation, product name: MONARCH717) was added as a pigment, and the mixture was stirred at 20° C. and 7,000 rpm for 3 hours. Further, 399.6 parts of ion-exchanged water was added, and the resulting pigment mixture was subjected to a dispersion treatment using a Microfluidizer (manufactured by Microfluidics, product name: M-110EH-30XP) for 20 passes at a pressure of 150 MPa to obtain a dispersion treatment product.
2,000 parts of the obtained dispersion was placed in a 4 L eggplant flask, 800 parts of ion-exchanged water was added, and the organic solvent was removed by holding for 3 hours at a pressure of 0.09 MPa in a warm bath adjusted to 32° C. at a rotation speed of 50 rpm using a rotary distillation apparatus (manufactured by Tokyo Rikakikai Co., Ltd., product name: Rotary Evaporator N-1000S). The warm bath was then adjusted to 62° C., the pressure was reduced to 0.07 MPa, and the mixture was concentrated until the solid content reached 25%.
The obtained concentrate was placed in a 500 mL angle rotor and centrifuged at 7,000 rpm for 20 minutes using a high-speed refrigerated centrifuge (manufactured by Hitachi Koki Co., Ltd., product name: himac CR22G, set temperature 20° C.), and the liquid layer was then filtered through a 5 μm membrane filter (manufactured by Sartorius, product name: Minisart) to obtain an aqueous dispersion (i) of pigment-containing polymer particles.
13.7 parts of ion-exchanged water was added to 100 parts of the obtained aqueous dispersion (containing 17.2% pigment and 7.4% polymer P1), and 0.22 parts of Proxel LVS (manufactured by Arch Chemicals Japan Co., Ltd.: antifungal agent, active content 20%) and 0.27 parts of a crosslinking agent (trimethylolpropane polyglycidyl ether, manufactured by Nagase Chemtex Corporation, trade name: Denacol EX321L, epoxy equivalent: 129 g/eq) were added so that the degree of crosslinking was 40 mol%, and the mixture was stirred at 80 ° C. for 2 hours. After cooling to 25 ° C., the mixture was filtered through the 5 μm filter, and ion-exchanged water was added so that the solid content concentration was 22% (15.4% pigment, 6.6% crosslinked polymer), to obtain an aqueous dispersion (I) of pigment-containing polymer particles. The average particle size of the pigment-containing polymer particles in the aqueous pigment dispersion (I) was 94.7 nm, and the acid value of the crosslinked polymer constituting the particles was 120 mg KOH / g.

Example 1 (Preparation of Water-Based Ink 1)
26.3 parts of the pigment aqueous dispersion obtained in Preparation Example 20 (4.5 parts of pigment, 1.9 parts of polymer P1), 34.5 parts of a pigment-free crosslinked polymer aqueous dispersion (A1) (5.0 parts of polymer P1, 2.6 parts of crosslinking agent), 12.0 parts of propylene glycol, 0.6 parts of a nonionic surfactant (a 50% solution of 2,4,7,9-tetramethyl-5-decyne-4,7-diol in propylene glycol, manufactured by Nissin Chemical Industry Co., Ltd., product name: Surfynol 104PG50), and 0.6 parts of a nonionic surfactant (polyoxyethylene lauryl ether, manufactured by Kao Corporation, product name: Emulgen 120) were mixed, and ion-exchanged water was added to make up a total of 100 parts.
The resulting mixture was filtered using a 25 mL needleless syringe equipped with a 1.2 μm filter (acetyl cellulose membrane, outer diameter: 2.5 cm, manufactured by FUJIFILM Corporation) to remove coarse particles, thereby obtaining water-based ink 1 (pigment in ink: 4.5%, crosslinked polymer not containing pigment in ink: 7.6%).
The results are shown in Table 3. The numerical values of the compositions in Table 3 are the solid content (effective content).

Examples 2 to 11 and Comparative Example 1 (Preparation of Water-Based Inks 2 to 11 and 21)
Water-based inks 2 to 11 and 21 were prepared in the same manner as in Example 1, except that the type and amount of the pigment-free crosslinked polymer emulsion obtained in Preparation Examples 2 to 11 and Comparative Preparation Example 1 were changed as shown in Table 3.

<Evaluation method>
The matte paper gloss of the water-based inks obtained in Examples 1 to 11 and Comparative Example 1 was evaluated by the following method. The results are shown in Table 3.

(Printing method)
A solid print was made on matte paper (UPM Finesse Matt, basis weight: 115 g/m ² , 60° gloss: 5.6, water absorption: 4.4 g/m ² ) using a piezoelectric printer (Kyocera Corporation, product name: KJ4B-1200).
The recording conditions of the printer were set as follows: recording head voltage 26 V, recording head temperature 32° C., resolution 600 dpi, number of pre-ejection flushings 200, and negative pressure −4.0 kPa. The printed matter was then left to stand at 25° C. for 24 hours to obtain a final printed matter.

(Measurement of gloss of matte paper)
The 60° gloss of the final print was measured five times using a gloss meter (manufactured by Nippon Denshoku Industries Co., Ltd., trade name: HANDY GLOSSMETER PG-1), and the average value was recorded as the gloss.
[Evaluation Criteria]
5: 60° glossiness is 30 or more.
4: The 60° gloss level is 27 or more and less than 30.
3: The 60° gloss level is 24 or more and less than 27.
2: The 60° gloss level is 21 or more and less than 24.
1: 60° gloss level is less than 21.
A gloss evaluation result of 3 or more is satisfactory for practical use.

From Table 3, it can be seen that the water-based inks of Examples 1 to 11 have superior matte paper gloss compared to the water-based ink of Comparative Example 1.

Claims (9)

The water-based ink for ink-jet printing contains a pigment and a polymer emulsion, the polymer emulsion containing crosslinked polymer particles made of a urethane resin crosslinked with polyalkylene glycol diglycidyl ether. The water-based ink for ink-jet printing according to claim 1, wherein the number of repeating units n of the alkylene oxide group of the polyalkylene glycol diglycidyl ether is 2 or more and 30 or less. The water-based ink for ink-jet printing according to claim 1 or 2, wherein the epoxy equivalent of the polyalkylene glycol diglycidyl ether is 200 g/eq or more and 600 g/eq or less. The water-based ink for ink-jet printing according to any one of claims 1 to 3, wherein the polyalkylene glycol diglycidyl ether is at least one selected from polyethylene glycol diglycidyl ether and polypropylene glycol diglycidyl ether. The water-based ink for ink-jet printing according to any one of claims 1 to 4, wherein the acid value of the crosslinked polymer constituting the polymer emulsion is 4 mgKOH/g or more and 180 mgKOH/g or less. The water-based ink for ink-jet printing according to any one of claims 1 to 5, wherein the pigment is in the form of polymer particles containing the pigment. The water-based ink for ink-jet printing according to claim 6, wherein the pigment-containing polymer particles are crosslinked polymer particles containing a pigment. The water-based ink for ink-jet printing according to claim 7, wherein the acid value of the crosslinked polymer constituting the pigment-containing crosslinked polymer particles is 100 mg KOH/g or more and 140 mg KOH/g or less. An inkjet recording method for recording on matte paper using the water-based ink according to any one of claims 1 to 8.