JP7675645B2 - Water-based ink for plate printing - Google Patents

Description

The present invention relates to a water-based ink for plate printing.

2. Description of the Related Art Gravure printing, flexographic printing, letterpress printing, and the like, which use printing plates such as intaglio, lithographic, and letterpress plates, are widely used because they allow the quality of printing to be controlled by changing the form of the plate and enable high-definition printing.
Conventionally, oil-based inks have been widely used in plate printing such as gravure printing, but there are problems with the working environment, the global environment, and disaster prevention. Therefore, plate printing using water-based inks has been attracting attention.

However, water-based inks containing pigments have the problem that the pigments settle over time and the pigment particles aggregate, making them difficult to redisperse. Various attempts have been made to improve this problem, but the reality is that they have not produced satisfactory results.
Furthermore, in recent years, resin films have come to be widely used as printing substrates, but water-based inks have a low affinity for highly lipophilic printing substrates such as resin films, resulting in poor image density.

Patent Document 1 discloses a printing ink composition that contains a solvent, a polyurethane resin, and an acetylene glycol compound, and the solvent contains water and propylene glycol ether, and the propylene glycol ether accounts for 10% by weight or less of the total amount of the ink composition, as an aqueous printing ink composition for gravure printing that has good printing suitability such as leveling and trapping properties.

JP 2016-44282 A

The aqueous ink described in Patent Document 1 has insufficient redispersibility when the pigment settles or aggregates, and the image density is also unsatisfactory, particularly when plate printing is performed on a low water-absorbent printing substrate such as a resin film.
An object of the present invention is to provide a water-based ink for plate printing which has excellent redispersibility and can give printed matter having excellent image density.

The present inventors have discovered that the above-mentioned problems can be solved by adding a small amount of fumed silica having a relatively large average secondary particle diameter to an aqueous ink for plate printing, which contains a pigment, a fixing resin, fumed silica, a water-soluble organic solvent, and water.
That is, the present invention provides an aqueous ink for plate printing, comprising a pigment, a fixing resin, fumed silica, a water-soluble organic solvent, and water,
The average secondary particle size of the fumed silica is 5 μm or more and 100 μm or less,
The present invention provides a water-based ink for plate printing, in which the content of the fumed silica in the ink is 0.005% by mass or more and 0.4% by mass or less.

The present invention provides a water-based ink for plate printing that has excellent redispersibility and can produce printed matter with excellent image density.

[Water-based ink for plate printing]
The aqueous ink for plate printing of the present invention is an aqueous ink for plate printing (hereinafter also referred to as "the ink of the present invention") containing a pigment, a fixing resin, fumed silica, a water-soluble organic solvent, and water, wherein the average secondary particle diameter of the fumed silica is 5 μm or more and 100 μm or less, and the content of the fumed silica in the ink is 0.005 mass % or more and 0.4 mass % or less.
The term "aqueous" means that water accounts for the majority of the medium.
The ink of the present invention can be suitably used for printing using a printing plate, such as gravure printing, flexographic printing, and letterpress printing, but is more preferably used for gravure printing.

The ink of the present invention has excellent redispersibility and can give printed matter having excellent image density. The reason for this is not clear, but is thought to be as follows.
Pigment particles contained in water-based inks settle over time. In the case of organic solvent-based inks, the spaces between the settled pigment particles can be sufficiently wetted, so they can be redispersed with a short period of stirring. However, in the case of water-based inks, once the pigment particles settle, they cannot be wetted, so a long period of stirring is required to redisperse them.
Here, the present invention contains a specific amount of fumed silica having an average secondary particle diameter of 5 μm to 100 μm in the aqueous ink, so that when the pigment particles in the aqueous ink settle over time, the fumed silica also settles at the same time. Since this fumed silica forms aggregated secondary particles having a three-dimensional beaded network structure, it is believed that it efficiently penetrates into the gaps between the settled pigment particles and forms layers. As a result, it is believed that high sedimentation and redispersibility can be achieved with a small amount of fumed silica.
In addition, after drying, the ink coating takes on a three-dimensional structure, which increases the apparent surface area of the pigment and increases light reflection. At the same time, the surface smoothness is not significantly impaired, which is thought to result in improved image density.

<Pigments>
The pigment used in the ink of the present invention is preferably one that can maintain a dispersed state in an aqueous ink with a particle diameter of 250 nm or less. Suitable forms of the pigment 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 low-molecular or high-molecular surfactant, and (iii) a polymer particle form containing the pigment. Among these, from the viewpoint of improving the redispersibility of the ink and improving the image density of the resulting printed matter, the form of the polymer particle containing the pigment is preferred, and the form of the water-insoluble polymer particle containing the pigment is more preferred.
In this specification, the polymer particles containing a pigment refer to particles in which the pigment is encapsulated in a polymer, particles in which part of the pigment is exposed on the surface of a particle consisting of a polymer and a pigment, particles in which the polymer is adsorbed on part of the pigment, or mixtures of these. Of these, particles in which the pigment is encapsulated in a polymer are more preferred.

(Pigment)
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, thermal 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 case of chromatic inks, any chromatic pigment such as yellow, magenta, cyan, red, blue, orange, green, etc. can be used.
The above pigments can be used alone or in combination of two or more kinds.

(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.
The pigment-containing polymer particles (hereinafter also referred to as "pigment-containing polymer particles") are more preferably pigment-containing crosslinked polymer particles (hereinafter also referred to as "pigment-containing crosslinked polymer particles") obtained by further crosslinking the pigment-containing polymer particles 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, and the amount of dissolution of the polymer is preferably 5 g or less, more preferably 1 g or less. When the polymer 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.

(Polymer a)
The polymer a is a polymer having a pigment dispersing ability to disperse a pigment in an aqueous medium mainly composed of water. The polymer a may have any structure, but from the viewpoint of improving the storage stability of the ink of the present invention, a vinyl-based polymer obtained by addition polymerization of a vinyl monomer such as a vinyl compound, a vinylidene compound, or a vinylene compound is preferred.
The vinyl polymer preferably contains a constituent unit derived from the ionic monomer (a-1), and more preferably is a vinyl polymer obtained by copolymerizing a monomer mixture A (hereinafter also referred to as "monomer mixture A") containing the component (a-1) and a hydrophobic monomer (a-2). The vinyl polymer has a constituent unit derived from the component (a-1) and a constituent unit derived from the component (a-2).

[(a-1) Ionic Monomer]
From the viewpoint of improving the dispersion stability of the pigment in the ink of the present invention, the ionic monomer (a-1) is preferably used as a monomer component of the polymer a. Examples of the ionic monomer (a-1) include anionic monomers and cationic monomers, and anionic monomers are preferred.
Examples of the anionic monomer include carboxylic acid monomers, sulfonic acid monomers, phosphoric acid monomers, etc. Examples of the carboxylic acid monomer include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, etc.
Among these, from the same viewpoint as above, carboxylic acid monomers are more preferred, and one or more selected from acrylic acid and methacrylic acid are even more preferred.

[(a-2) Hydrophobic Monomer]
From the viewpoint of improving the dispersion stability of the pigment in the ink of the present invention, it is preferable to use the hydrophobic monomer (a-2) as a further monomer component in addition to the component (a-1).
Specific examples of the component (a-2) include those described in paragraphs [0020] to [0022] of JP 2018-83938 A. Among these, one or more selected from alkyl (meth)acrylates having an alkyl group having 1 to 22 carbon atoms, styrene, α-methylstyrene, and benzyl (meth)acrylate are preferred.

[(a-3) Nonionic Monomer]
The nonionic monomer (a-3) (hereinafter also referred to as "component (a-3)") can be used from the viewpoint of further improving the dispersion stability of the pigment in the ink of the present invention.
The component (a-3) is a monomer that has a high affinity for water or a water-soluble organic solvent, such as 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 component in monomer mixture A or each structural unit in polymer a)
The contents of components (a-1) to (a-3) in monomer mixture A (content as unneutralized amount; the same applies hereinafter) during production of polymer a, or the contents of structural units derived from components (a-1) to (a-3) in polymer a, are as follows, from the viewpoint of improving the dispersion stability of the pigment and the redispersibility of the ink.
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 40 mass% or less, more preferably 30 mass% or less, even more preferably 20 mass% or less.
The content of the (a-2) component 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 75% by mass or less.
When the component (a-3) is contained, the content thereof is preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less.
The mass ratio of the component (a-1) to the component (a-2), [(a-1)/(a-2)], is preferably 0.1 or more, more preferably 0.2 or more, even more preferably 0.25 or more, and is preferably 1.2 or less, more preferably 1.0 or less, even more preferably 0.8 or less.
In the present invention, the content of the structural units derived from the components (a-1) to (a-3) in the polymer a can be determined by measurement, or can be substituted by the charging ratio of the raw material monomers including the components (a-1) to (a-3) during the production of the polymer a.

(Production of Polymer a)
The polymer a can be produced by copolymerizing the monomer mixture A 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, and solvent used, but is preferably 30° C. or higher, more preferably 50° C. or higher, and is preferably 95° C. or lower, more preferably 80° C. or lower.
The polymerization atmosphere is preferably a nitrogen gas or inert gas atmosphere.
It is preferable that the polymer a is neutralized with a neutralizing agent as described below.

From the viewpoint of improving the dispersion stability in the ink of the pigment dispersed in the polymer, the weight average molecular weight of polymer a is preferably 10,000 or more, more preferably 15,000 or more, and is preferably 200,000 or less, more preferably 100,000 or less, and even more preferably 50,000 or less.
From the same viewpoint as above, the acid value of polymer a is preferably 50 mgKOH/g or more, more preferably 70 mgKOH/g or more, even more preferably 80 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.
The weight average molecular weight and the acid value can be measured by the method described in the Examples.

Polymer a may be a commercially available product as long as it has a structural unit derived from component (a-1) and a structural unit derived from component (a-2). Commercially available examples of vinyl polymers include styrene-acrylic resins such as Joncryl 67, 611, 678, 680, 690, and 819 manufactured by BASF Corporation.

[Preparation of pigment-containing polymer particles]
From the viewpoint of efficient production, the pigment-containing polymer particles are preferably produced as pigment dispersion A in which the pigment-containing polymer particles are dispersed in an aqueous medium by a method including the following step I and, if necessary, step II:
Step I: A step of dissolving polymer a in a solvent to obtain a solution of polymer a, and then adding and mixing a pigment, and if necessary, a neutralizing agent, a surfactant, and the like, to obtain a pigment dispersion A consisting of a pigment mixture. In order to efficiently incorporate the pigment into the polymer particles, it is preferable that the solvent contains an organic solvent in Step I. When the solvent contains an organic solvent, in addition to Step I, the following Step II may be further included.
Step II: A step of removing the organic solvent from the pigment mixture obtained in Step I to obtain a pigment dispersion A. In addition to Steps I and II, it is preferable to further carry out the following Step III.
Step III: A step of mixing the pigment dispersion A obtained in step I or step II with a crosslinking agent a to obtain crosslinked pigment-containing crosslinked polymer particles. In this specification, pigment dispersion A refers to both a dispersion in which pigment-containing polymer particles are dispersed in an aqueous medium and a dispersion in which pigment-containing crosslinked polymer particles are dispersed in an aqueous medium.

(Process I)
In step I, the solvent in which the polymer a is dissolved is not limited, but from the viewpoints of wettability to the pigment, solubility of the polymer a, and adsorption to the pigment, it is preferable to use one or more selected from water, aliphatic alcohols having from 1 to 3 carbon atoms, ketones, ethers, esters, etc. When the polymer a is synthesized by a solution polymerization method, the solvent used in the polymerization may be used as it is.
When the polymer a is an anionic polymer, it is preferable to neutralize the anionic groups using a neutralizing agent so that the pH is from 7 to 11. Examples of the neutralizing agent include bases such as sodium hydroxide, potassium hydroxide, ammonia, and various amines. The polymer a may be neutralized in advance.
From the viewpoint of improving the storage stability of the ink of the present invention, the degree of neutralization of the anionic groups of polymer a, expressed as the ratio of the molar equivalent of the neutralizing agent to the molar equivalent of the anionic groups of polymer a, is preferably 30 mol % or more, more preferably 40 mol % or more, even more preferably 50 mol % or more, and is preferably 300 mol % or less, more preferably 200 mol % or less, even more preferably 150 mol % or less.

In step I, it is preferable to apply mechanical force to the obtained pigment mixture to perform a dispersion treatment. There is no particular limitation on the method of applying mechanical force, but for example, the method described in paragraph [0032] of JP-A-2018-83938 can be mentioned. As a device for applying mechanical force, a media type dispersing machine is preferable from the viewpoint of efficiently reducing the particle size of the pigment.
When carrying out the dispersion treatment, the pigment can be adjusted to have a desired particle size by controlling the dispersion pressure and the like.

(Process II)
Step II is an optional step. It is preferable that the organic solvent in the obtained pigment dispersion A is substantially removed, but as long as the object of the present invention is not impaired, the residual organic solvent may remain in an amount of 0.1% by mass or less.
From the viewpoint of improving storage stability and performing high-definition printing, the average particle size of the pigment-containing polymer particles in Pigment Dispersion A is preferably 50 nm or more, more preferably 100 nm or more, even more preferably 120 nm or more, and is preferably 350 nm or less, more preferably 320 nm or less, even more preferably 300 nm or less.
Even when crosslinking treatment is carried out in the below-described step III, the average particle size of the pigment-containing crosslinked polymer particles in the resulting pigment dispersion A is the same as the average particle size of the pigment-containing polymer particles.
The average particle size is measured by the method described in the Examples.

(Process III)
In step III, a part of the carboxyl groups of the polymer a constituting the pigment-containing polymer particles is crosslinked to form a crosslinked structure in a part or all of the surface layer of the pigment-containing polymer particles, thereby converting the pigment-containing polymer particles into pigment-containing crosslinked polymer particles. This crosslinking treatment is believed to suppress swelling and shrinkage of the polymer a, which adversely affect the dispersion stability of the pigment, and to further improve the redispersibility of the ink and the image density of the resulting printed matter.

When polymer a is an anionic polymer having an anionic group, crosslinking agent a is preferably a compound having a functional group capable of reacting with the anionic group, and more preferably a compound having 2 to 6 such functional groups in the molecule.
Suitable examples of the crosslinking agent a include compounds having two or more epoxy groups, oxazoline groups, or isocyanate groups in the molecule. Among these, compounds that are water-insoluble with a water solubility of 50% by mass or less, preferably 40% by mass or less, and have 2 to 4 epoxy groups in the molecule are preferred, and one or more selected from trimethylolpropane polyglycidyl ether and pentaerythritol polyglycidyl ether are more preferred, and trimethylolpropane polyglycidyl ether (water solubility of 27% by mass) is even more preferred. Here, the "water solubility" refers to the solubility (mass%) when 10 parts by mass of the crosslinking agent is dissolved in 90 parts by mass of ion-exchanged water at 25°C.

From the viewpoint of improving the redispersibility of the ink and the image density of the resulting printed matter, the crosslinking rate of the pigment-containing polymer particles in step III 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, 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 improving the redispersibility of the ink and the image density of the resulting printed matter.
From the viewpoints of completion of the crosslinking reaction and economic efficiency, the temperature of the crosslinking treatment is preferably 40° C. or higher, more preferably 55° C. or higher, and is preferably 95° C. or lower, more preferably 80° C. or lower.
From the viewpoint of improving the dispersion stability of the pigment dispersion, the solid content concentration of the obtained pigment dispersion A is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 45% by mass or less, more preferably 40% by mass or less.
The solids concentration is measured by the method described in the Examples.

<Fixing resin>
The ink of the present invention contains a fixing resin from the viewpoint of improving the image density of the resulting print.
It is more preferable that the fixing resin is composed of resin only, that is, the fixing resin is preferably resin particles that do not contain a pigment (hereinafter, also referred to as "resin particles B").
The resin particles B may be subjected to a crosslinking treatment, if necessary.
Examples of the polymer constituting the resin particles B (hereinafter also referred to as "polymer b") include vinyl polymers, urethane polymers, polyester polymers, etc. Among these, vinyl polymers are preferred from the viewpoint of improving the image density of the resulting printed matter.

The polymer b can be produced by copolymerizing a monomer mixture containing (b-1) an ionic monomer, (b-2) a hydrophobic monomer, and the like, by a known solution polymerization method or the like.
The components (b-1) and (b-2) are the same as the components (a-1) and (a-2), and preferred examples thereof are also the same.
From the viewpoint of improving fixability, the weight average molecular weight of polymer b is preferably 50,000 or more, more preferably 100,000 or more, even more preferably 150,000 or more, and is preferably 800,000 or less, more preferably 600,000 or less, even more preferably 400,000 or less.
From the viewpoint of improving fixation, the acid value of polymer b is preferably 10 mgKOH/g or more, more preferably 20 mgKOH/g or more, even more preferably 30 mgKOH/g or more, and is preferably 80 mgKOH/g or less, more preferably 70 mgKOH/g or less, even more preferably 60 mgKOH/g or less.
The weight average molecular weight and acid value of the polymer b are measured by the method described in the examples.

From the viewpoint of improving the redispersibility of the ink and the image density, the average particle size of resin particles B in the ink of the present invention is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more, and is preferably 300 nm or less, more preferably 200 nm or less, and even more preferably 150 nm or less.
The average particle size of the resin particles B is measured by the method described in the Examples.

In addition, polymer a and polymer b may be the same or different. That is, polymer a and b may have different compositions, or may be the same polymer including the composition, and differ only in the presence or absence of a pigment.

Examples of commercially available dispersions of fixing resins include Neocryl A1127 (anionic self-crosslinking water-based acrylic resin) manufactured by DSM Neo Resins, Joncryl 390 (acrylic resin) manufactured by BASF, Joncryl 7100, 7600, 734, 780, 537J, 538J, PDX-7164, PDX-7775 (styrene-acrylic resin), SR-100, SR-102 (styrene-butadiene resin) manufactured by Nippon A&L Co., Ltd., and Vinyblan 700, 701 (vinyl chloride-acrylic resin) manufactured by Nissin Chemical Industry Co., Ltd.

<Fumed Silica>
In the present invention, "fumed silica" means silica made of amorphous, nearly spherical, and small-pore primary particles produced by flame hydrolysis. Fumed silica can be produced by vaporizing highly purified silicon tetrachloride and subjecting it to high-temperature hydrolysis in an oxyhydrogen flame. Fumed silica is also called "dry silica" or "vapor-phase silica" to distinguish it from silica produced by wet methods such as precipitation silica.
However, silica other than fumed silica is unlikely to form aggregated secondary particles having a three-dimensional beaded network structure in the ink, as is the case with fumed silica, and even if these are used, excellent ink redispersibility and improved image density cannot be obtained.

Fumed silica includes hydrophilic fumed silica and hydrophobic fumed silica.
Hydrophobic fumed silica refers to particles that are hydrophobized by reacting the hydroxyl groups on the surface of the fumed silica with an organic compound having a functional group, such as an organosilicon compound or silicone oil, to chemically fix the functional group to the surface of the fumed silica particles.Hydrophilic fumed silica refers to fumed silica that has not been subjected to the above surface treatment.

The fumed silica is preferably hydrophilic fumed silica from the viewpoint of finely dispersing it in the aqueous ink and maintaining the optical density.
The average primary particle size of the fumed silica is preferably 50 nm or less, more preferably 30 nm or less, and even more preferably 30 nm or less, and is preferably 5 nm or more, more preferably 7 nm or more. The average primary particle size of the fumed silica can be referred to the catalog value of the product.
The average secondary particle diameter of the fumed silica (the average particle diameter of agglomerated particles of the fumed silica) is, from the viewpoint of improving the redispersibility of the ink and improving the image density of the obtained printed matter, 5 μm or more, preferably 8 μm or more, more preferably 10 μm or more, even more preferably 15 μm or more, still more preferably 25 μm or more, and is 100 μm or less, preferably 80 μm or less, more preferably 60 μm or less, and even more preferably 50 μm or less.
The average secondary particle size of the fumed silica is measured by the method described in the Examples.
The average secondary particle size of the fumed silica can be adjusted by mixing the fumed silica with water and controlling the rotation speed and time using a dispersing machine such as a disperser. For example, a water dispersion having a fumed silica concentration of about 10% by mass can be dispersed using a disperser at a rotation speed of 2000 to 4000 rpm for about 5 to 40 minutes to obtain fumed silica having an average secondary particle size of 5 μm to 100 μm.

The BET specific surface area of the fumed silica is preferably 50 m ² /g or more, more preferably 80 m ² /g or more, even more preferably 100 m ² /g or more, and preferably 340 m ² /g or less, more preferably 310 m ² /g or less, even more preferably 280 m ² /g or less.

The mass ratio of fumed silica to pigment (fumed silica/pigment) is preferably 0.0001 or more, more preferably 0.0005 or more, even more preferably 0.001 or more, and is preferably 0.05 or less, more preferably 0.04 or less, even more preferably 0.035 or less, from the viewpoint of improving the redispersibility of the ink and improving the image density of the resulting print.

Examples of commercially available hydrophilic silica products include Aerosil W7520, Aerosil 90, Aerosil 130, Aerosil 200, Aerosil 300, and Aerosil R816 manufactured by Nippon Aerosil Co., Ltd., CAB-O-SIL MS-5 and CAB-O-SIL MS-7 manufactured by Cabot Corporation, Reoloseal QS-102 and Reoloseal 103 manufactured by Tokuyama Corporation, and Nipsil LP manufactured by Tosoh Silica Corporation.

<Water-soluble organic solvent>
The water-soluble organic solvent used in the ink of the present invention may be liquid or solid at 25°C. 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.
From the viewpoint of improving the wetting and spreading properties of the ink, the boiling point of the water-soluble organic solvent is preferably 100° C. or higher, more preferably 120° C. or higher, even more preferably 140° C. or higher, and is preferably 260° C. or lower, more preferably 250° C. or lower, even more preferably 240° C. or lower.
Examples of the water-soluble organic solvent include glycol ethers such as alkylene glycol ethers, polyhydric alcohols such as propylene glycol, amide compounds, etc. Among these, alkylene glycol ethers are preferred.

Examples of the alkylene glycol ether include one or more selected from ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monopropyl ether, diethylene glycol monoisobutyl ether, diethylene glycol monobutyl 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, tripropylene glycol monobutyl ether, and the like.
Of these, from the viewpoints of improving the redispersibility of the ink and improving the image density of the resulting printed matter, one or more selected from diethylene glycol monoisobutyl ether, diethylene glycol monobutyl ether, and dipropylene glycol monomethyl ether are preferred, and diethylene glycol monoisobutyl ether is more preferred.

<Surfactant>
The ink of the present invention may contain a surfactant from the viewpoint of improving the wetting and spreading properties of the ink on the printing substrate and improving redispersibility and image density. As the surfactant, a silicone-based surfactant is preferred.
The silicone surfactant refers to a surfactant having a polysiloxane structure, and may have a hydrophilic group, a hydrophilic polymer chain, or the like on a side chain, terminal, or the like.
The silicone surfactant preferably contains a silicone surfactant represented by the following general formula (1):

In formula (1), R ¹ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, preferably a methyl group or an ethyl group, and more preferably a methyl group.
^R2 represents a group represented by the following formula (a), and ^R3 represents a group represented by the following formula (b).
k is 1 to 500, preferably 5 to 400, and more preferably 10 to 300.
m is 1 to 500, preferably 1 to 300, and more preferably 2 to 200.
n is an integer of 0 to 100, preferably 0 to 75, and more preferably 0 to 50.

In formula (a), each R ⁴ independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and is preferably a hydrogen atom.
Each ^R5 independently represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms or an acyl group having 1 to 4 carbon atoms, and is preferably a hydrogen atom.
a is 1 to 20, preferably 1 to 3, and more preferably 1.
b is an integer of 0 to 50, preferably 1 to 3, and more preferably 1.
c is 0 to 50, preferably 0 to 3, and more preferably 0.

In formula (b), R ⁶ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and is preferably a hydrogen atom.
d is 2 to 20, preferably 2 to 16, and more preferably 2 to 12.
e is an integer of 1 to 20, preferably 2 to 16, and more preferably 2 to 12.
In the silicone surfactant represented by general formula (1) in the present invention, from the viewpoint of improving the redispersibility of the ink, the ratio of k:m:n is preferably 60 to 90:40 to 10:10 to 0, more preferably 65 to 85:35 to 13:0 to 5, and even more preferably 70 to 85:30 to 15:0 to 2.
From the viewpoint of improving the redispersibility of the ink, it is preferable that the silicone surfactant represented by the general formula (1) has n of 0 or more, i.e., has a group represented by formula (b).

The silicone surfactant represented by the general formula (1) may be a commercially available product, such as TEGO twin 4000 and TEGO twin 4100 (gemini surfactants) manufactured by Evonik.
The ink of the present invention may further contain, as necessary, a polyether silicone surfactant other than the silicone surfactant represented by the general formula (1), an acetylene glycol surfactant, a known nonionic surfactant, and the like.

(Content of each component in the ink of the present invention)
The contents of pigment, fixing resin, fumed silica, water-soluble organic solvent, surfactant, and water in the ink of the present invention are as follows, from the viewpoint of improving the redispersibility of the ink and improving the image density of the obtained printed matter.
The content of each component below refers to the content of each component in the ink at the time of printing. The ink of the present invention may be used as is after adjusting the content of each component to the content at the time of printing, or a previously prepared base ink may be diluted with water or the like and adjusted to the content at the time of printing.

The content of the pigment in the ink of the present invention is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 8% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 18% by mass or less.
The content of the pigment-containing polymer particles in the ink of the present invention is preferably 3% by mass or more, more preferably 5% by mass or more, even more preferably 8% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 20% by mass or less.

The content of the fixing resin in the ink of the present invention is preferably 1 mass% or more, more preferably 2 mass% or more, even more preferably 3 mass% or more, and preferably 20 mass% or less, more preferably 18 mass% or less, even more preferably 15 mass% or less.
The mass ratio of the polymer (total amount of polymer a and polymer b) to the pigment in the ink of the present invention [polymer/pigment] is preferably 0.2 or more, more preferably 0.3 or more, even more preferably 0.4 or more, and is preferably 3 or less, more preferably 2 or less, even more preferably 1.5 or less.

The content of fumed silica in the ink of the present invention is 0.005% by mass or more, preferably 0.01% by mass or more, more preferably 0.02% by mass or more, even more preferably 0.05% by mass or more, and is 0.4% by mass or less, preferably 0.3% by mass or less, more preferably 0.25% by mass or less, even more preferably 0.2% by mass or less.
The content of the water-soluble organic solvent in the ink of the present invention is preferably 12% by mass or less, more preferably 10% by mass or less, even more preferably 8% by mass or less, and preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 2% by mass or more.

The content of the surfactant, in particular the silicone-based surfactant, in the ink of the present invention is preferably 0.1 mass% or more, more preferably 0.2 mass% or more, even more preferably 0.3 mass% or more, and is preferably 3 mass% or less, more preferably 2 mass% or less, even more preferably 1 mass% or less.
The water content in the ink of the present invention is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 65% by mass or more, and preferably 85% by mass or less, more preferably 80% by mass or less, even more preferably 77% by mass or less.
The ink of the present invention may contain various additives such as a pH adjuster, a viscosity adjuster, a defoamer, a preservative, and a rust inhibitor as optional components depending on the application.

The Zahn cup viscosity at 20°C of the ink of the present invention is, from the viewpoint of improving the redispersibility of the ink and improving the image density of the obtained prints, preferably 10 seconds or more, more preferably 12 seconds or more, even more preferably 13 seconds or more, and is preferably 25 seconds or less, more preferably 20 seconds or less, even more preferably 18 seconds or less.
The pH of the ink of the present invention at 20°C is preferably 5.5 or more, more preferably 6.0 or more, and even more preferably 6.5 or more, from the viewpoint of improving dispersion stability, and is preferably 11.0 or less, more preferably 10.5 or less, and even more preferably 10.0 or less, from the viewpoints of component resistance and skin irritation.

[Gravure printing]
The aqueous ink for plate printing of the present invention has excellent redispersibility and can be suitably used in gravure printing using a gravure plate. By printing the ink of the present invention on a printing substrate by gravure printing, a high-definition gravure print having excellent image density can be obtained.
Examples of printing substrates used in gravure printing include papers such as coated paper, art paper, synthetic paper, and processed paper; and resin films such as polyester film, polyethylene film, polypropylene film, polystyrene film, polyvinyl chloride film, and nylon film.
Among the resin films, polyester films and polypropylene films are preferred from the viewpoint of suitability for post-processing such as punching after the production of the printed matter. These resin films may be biaxially stretched films, uniaxially stretched films, or non-stretched films.
From the viewpoint of improving suitability for gravure printing, a resin film that has been subjected to a surface treatment by electrical discharge processing such as corona treatment or plasma treatment may be used.

In the following manufacturing examples, 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 the weight average molecular weight of the polymer The weight average molecular weight was measured by gel 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 weights (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 resin with 10 mL of the eluent in a glass vial, stirring the mixture with a magnetic stirrer at 25° C. for 10 hours, and filtering the mixture with a syringe filter (DISMIC-13HP, PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.).

(2) Measurement of acid value of polymer The polymer was dissolved in a titration solvent of toluene and acetone (2:1) in an automatic potentiometric titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with 0.1 N potassium hydroxide/ethanol solution by potentiometric titration. The inflection point on the titration curve was taken as the end point. The acid value was calculated from the titration amount of the potassium hydroxide solution up to the end point.

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

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

(5) Measurement of Epoxy Equivalent of Epoxy Compound The epoxy equivalent of an epoxy compound was measured by potentiometric titration using an automatic potentiometric titrator, AT-610, manufactured by Kyoto Electronics Manufacturing Co., Ltd., in accordance with JIS K7236.

(6) Measurement of average secondary particle size of fumed silica Using a laser diffraction/scattering type particle size distribution analyzer LA950 manufactured by Horiba, Ltd., measurements were made with the refractive index of fumed silica set to 1.46, water with a refractive index of 1.333 used as a dispersion medium, and a circulation speed of 5. The volume median particle size ( _D50 ) value at this time was taken as the average secondary particle size of the particles in the dispersion.

Production Example A1 (Production of Pigment Dispersion A1)
(1) 236 parts of ion-exchanged water was weighed out into a 2 L flask, and 60 parts of a water-insoluble styrene-acrylic polymer (manufactured by BASF, product name: JONCRYL 690, weight average molecular weight: 16500, acid value: 240 mgKOH/g) as a pigment dispersion polymer and 36.5 parts of a 5N sodium hydroxide solution (degree of sodium neutralization: 60 mol%) were added. The mixture was stirred for 2 hours at 200 rpm using an anchor blade, to obtain 332.5 parts of an aqueous styrene-acrylic polymer solution (solids concentration: 19.9%).
331.7 parts of the above aqueous solution and 448.3 parts of ion-exchanged water were placed in a 2 L vessel equipped with a disper blade, and the mixture was stirred at 1,400 rpm for 15 minutes using a disper (Ultra Disper: product name, manufactured by Asada Iron Works Co., Ltd.) while cooling in a water bath at 0°C.
(2) Then, 220 parts of cyan pigment (C.I. Pigment Blue 15:3) was added and stirred at 6400 rpm for 1 hour. The obtained dispersion was put into a wet disperser (Hiroshima Metal & Machinery Co., Ltd., product name: Ultra Apex Mill UAM05) filled with 80% by volume of zirconia beads (Nikkato Co., Ltd., product name: XTZ Ball, 0.3 mmφ), and dispersed five times at a peripheral speed of 8 m/s and a flow rate of 200 g/min while cooling with cooling water at 5°C, and then filtered using a 200 mesh wire net.
(3) To 500 parts of the filtrate obtained above (110 parts of pigment, 33 parts of polymer), 7.3 parts of Denacol EX-321L (manufactured by Nagase ChemteX Corporation, trimethylolpropane polyglycidyl ether, epoxy equivalent: 129) (corresponding to 40 mol% of the carboxylic acid that serves as a crosslinking reaction site contained in the acrylic acid in the polymer) and 1 part of Proxel LV (S) (manufactured by Lonza Japan Co., Ltd., antifungal agent, active ingredient concentration: 20%) were added, and 17.9 parts of ion-exchanged water was added so that the solids concentration became 28.6%, and the mixture was stirred at 70°C for 3 hours, and then filtered through a 200 mesh wire net to obtain 526.2 parts of pigment dispersion A1 of pigment-containing crosslinked polymer particles (content of pigment-containing crosslinked polymer particles: 28.6%, average particle size: 164 nm).

Production Examples B1 to B6, Comparative Production Example B1 (Production of Fumed Silica Dispersions B1 to B6, and Gel Silica Dispersion B11)
450 parts of ion-exchanged water and 50 parts of fumed silica or the like shown in Table 1 were added to a 2 L flask, and the mixture was stirred at 3000 rpm using a Disper (Ultra Disper: product name, manufactured by Asada Iron Works Co., Ltd.) for the time shown in Table 1. The mixture was then filtered through a 100 mesh wire screen to obtain fumed silica dispersions B1 to B6 and gel silica dispersion B11. The results are shown in Table 1.
The details of the fumed silica and other materials used are as follows:
Aerosil 130: hydrophilic fumed silica, BET specific surface area: 130 m ² /g, average primary particle size: 16 nm
Aerosil 200: hydrophilic fumed silica, BET specific surface area: 200 m ² /g, average primary particle size: 12 nm
Aerosil 300: hydrophilic fumed silica, BET specific surface area: 300 m ² /g, average primary particle size: 7 nm
Aerosil 90: hydrophilic fumed silica, BET specific surface area: 90 m ² /g, average primary particle size: 20 nm
(Aerosil is manufactured by Nippon Aerosil Co., Ltd.)
Silica 470, manufactured by Fuji Silysia Chemical Ltd., gel-like porous silica, BET specific surface area: 350 m ² /g

Example 1 (Preparation of Water-Based Ink 1)
In a production vessel, 47.5 parts of the pigment dispersion A1 obtained in Preparation Example 1 (pigment content in water-based ink: 10.0 parts, pigment dispersing polymer content: 3.6 parts) were added to 22 parts of a styrene-acrylic polymer emulsion (polymer content in water-based ink: 10.0 parts, manufactured by BASF Japan Ltd., product name: Joncryl PDX-7775, weight average molecular weight: 200,000, acid value: 55 mgKOH/g, solids concentration: 45%, average particle size: 80 nm) as a fixing resin, and the mixture was stirred at 150 rpm.
Further, 5.0 parts of diethylene glycol monobutyl ether (BDG: manufactured by Nippon Nyukazai Co., Ltd., boiling point 230° C.), 0.5 parts of a silicone surfactant (manufactured by Evonik, product name: TEGO Twin 4100), 1 part of the fumed silica dispersion B1 obtained in Production Example 1, and ion-exchanged water were added so that the total blend amount was 100 parts, and the mixture was stirred at room temperature for 30 minutes and then filtered through a stainless steel wire mesh (100 mesh) to obtain water-based ink 1. The results are shown in Table 2.
The amount of each component in Table 2 is the effective amount.
The silicone-based surfactant used, trade name: TEGO Twin 4100, has the following general formula (1): in which R ¹ is a methyl group, k = 1 to 500, m = 1 to 500, n = 1 to 100, and k:m:n = 71:27:2; in formula (a), R ⁴ is a hydrogen atom, R ⁵ is a hydrogen atom, a = 1, b = 1, c = 0; and in formula (b), R ⁶ is a hydrogen atom, d = 3, and e = 1 to 20.

Examples 2 to 8, Comparative Examples 1 to 3 (Production of Water-Based Inks 2 to 8, 11 to 13)
Water-based inks 2 to 8 and 11 to 13 were obtained in the same manner as in Example 1, except that the blending compositions in Example 1 were changed to those shown in Table 2. The results are shown in Table 2.
Details of the surfactants in Table 2 are as follows:

(1) Evaluation of redispersibility The initial absorbance of the obtained ink and the absorbance after the redispersion test were measured as described below, and the redispersibility of the ink was evaluated as described below.
(i) Measurement of initial absorbance 1 g of the water-based ink obtained in the Examples and Comparative Examples was diluted 2500 times with ion-exchanged water and measured over an absorption wavelength range of 400 to 600 nm using a spectrophotometer (manufactured by Hitachi, Ltd., model number: U-3010). The initial absorbance was calculated using the absorbance at the maximum absorption wavelength within the measurement range according to the following formula.
Initial absorbance = (absorbance at maximum absorption wavelength x 2500)
(ii) Measurement of absorbance after redispersion test 300 g of the ink obtained in the Examples and Comparative Examples was placed in a 500 mL centrifuge tube, set in an angle rotor, and centrifuged at 1500 rpm for 45 minutes in a high-speed refrigerated centrifuge (manufactured by Hitachi Koki Co., Ltd., product name: himac CR22G, set temperature 20° C.). Thereafter, the centrifuge tube was placed on a tabletop pot mill (manufactured by AS ONE Corporation, product name: PM-001) and stirred at 200 rpm.
The time when stirring at 200 rpm was started was set as 0 seconds, and 1 g of the liquid phase was sampled every 30 seconds thereafter and diluted 2,500 times with ion-exchanged water. Using the absorbance at the maximum absorption wavelength within the above measurement range using a spectrophotometer, the absorbance after the redispersion test was calculated according to the following formula.
Absorbance after redispersion test = (absorbance at maximum absorption wavelength x 2500)
The time (seconds) required for a sample to be taken after the redispersion test to reach 98% of the initial absorbance was measured, and the redispersibility of the ink was evaluated.
The shorter the time, the better the redispersibility, and if the time is less than 400 seconds, there is no practical problem with the redispersibility.

(2) Evaluation of Image Density Using the aqueous inks obtained in the Examples and Comparative Examples, gravure printing was performed on the corona discharge treated surface of an OPP film (FOR-AQ#20, laminate grade, manufactured by Futamura Chemical Co., Ltd.) The printing was performed using a tabletop gravure printing test machine (K Printing Proofer, manufactured by Matsuo Sangyo Co., Ltd.) and a laser platemaking gravure 250 line, 12 μm plate (manufactured by Nabe Process Co., Ltd.) to perform solid printing.
After the solid printing, the image was dried for 10 minutes in a dryer (Drying Oven DSV402, manufactured by Yamato Scientific Co., Ltd.) set at 60°C.
The image density of the printed portion was measured using a spectrophotometer (manufactured by GretagMacbeth, product name: SpectroEye) in the measurement mode (DIN, Abs). The results are shown in Table 2.
If the image density is 1.7 or more, there is no practical problem.

From Table 2, it can be seen that the water-based inks obtained in the Examples have superior redispersibility and superior image density of the plate prints even on resin film substrates compared to the water-based inks obtained in the Comparative Examples.

Claims (8)

A water-based ink for plate printing, comprising a pigment, a fixing resin, fumed silica, a water-soluble organic solvent, and water,
The average secondary particle size of the fumed silica is 5 μm or more and 100 μm or less,
The water-based ink for plate printing, wherein the content of the fumed silica in the ink is 0.005% by mass or more and 0.4% by mass or less. The water-based ink for plate printing according to claim 1, in which the mass ratio of fumed silica to pigment (fumed silica/pigment) is 0.0001 or more and 0.05 or less. The water-based ink for plate printing according to claim 1 or 2, wherein the pigment is in the form of polymer particles containing the pigment. The water-based ink for plate printing according to any one of claims 1 to 3, wherein the boiling point of the water-soluble organic solvent is 100°C or higher and 260°C or lower. The water-based ink for plate printing according to any one of claims 1 to 4, wherein the content of the water-soluble organic solvent in the ink is 12 mass% or less. The water-based ink for plate printing according to any one of claims 1 to 5, wherein the water content in the ink is 50% by mass or more. The water-based ink for plate printing according to any one of claims 1 to 6, further comprising a silicone surfactant. The water-based ink for plate printing according to any one of claims 1 to 7, which is for gravure printing.