JP7834501B2 - White pigment dispersion, aqueous inkjet ink composition, and printed material - Google Patents

Description

This invention relates to a white pigment dispersion, an aqueous inkjet ink composition, and printed materials.

Inkjet printing and recording is a printing and recording method that uses extremely fine nozzles to directly eject droplets of water-based inkjet ink composition onto a printing and recording substrate, thereby obtaining characters and images.

Examples of aqueous inkjet ink compositions include those containing pigments, alkali-soluble resins (pigment dispersion resins), surfactants, specific water-soluble solvents, and water (Patent Documents 1-4). These documents disclose aqueous inkjet ink compositions with good storage stability and ejection stability.

Since the pigments described above typically cannot be dispersed in an ink composition on their own, as described in the aforementioned patent document, an alkali-soluble resin (pigment dispersion resin) is physically adsorbed or chemically bonded to the pigment before the other components of the ink composition are combined, and the pigment is then used in the form of a pigment dispersion.

As an example of such a pigment dispersion, a white pigment composition is known that comprises a pigment consisting of titanium dioxide particles and/or zinc oxide particles whose surfaces are not hydrophilized by silicon dioxide hydrate, and sodium polyacrylate as a pigment dispersant with a mass-average molecular weight of 10,000 or less, wherein the ratio of the average primary particle diameter of the titanium dioxide particles or the zinc oxide particles to the average dispersed particle diameter D50 of the dispersed titanium dioxide particles or zinc oxide particles is in the range of 1:0.3 to 1:3 (Patent Document 5).

Japanese Patent Publication No. 2020-56018 Japanese Patent Publication No. 2019-1955 Japanese Patent Publication No. 2018-104582 Japanese Patent Publication No. 2015-137319 Japanese Patent Publication No. 2018-188518

The above-mentioned white pigment composition exhibits excellent dispersibility of titanium dioxide and is suggested to prevent its re-aggregation. However, because titanium dioxide has a high specific gravity, it typically settles over time (e.g., during long-term storage). Settled titanium dioxide pigment becomes hard and difficult to redisperse. Conventional methods for suppressing settling include, for example, increasing viscosity. However, ink compositions with viscosity high enough to achieve effective settling suppression tend to have poor inkjet printability and storage stability.

This invention has been made in view of the above circumstances, and aims to provide a white pigment dispersion that yields an aqueous inkjet ink composition having redispersibility, ejection stability, printability, abrasion resistance, and storage stability for the white pigment.

Furthermore, the present invention aims to provide an aqueous inkjet ink composition containing the white pigment dispersion, and a printed material obtained by printing with the composition.

In other words, the present invention relates to a white pigment dispersion comprising a white pigment, an alkali-soluble resin, a basic compound, and water, wherein the white pigment is titanium dioxide having an average primary particle diameter (D) of 100 nm to 190 nm, the average dispersed particle diameter (D50) of the white pigment dispersion is 110 nm to 250 nm, the ratio of the average dispersed particle diameter (D50) to the average primary particle diameter (D) (D50/D) is 1.1 to 2.0, and the alkali-soluble resin is present in an amount of 5 to 40 parts by mass per 100 parts by mass of the white pigment.

Furthermore, the present invention relates to an aqueous inkjet ink composition comprising the white pigment dispersion, a water-soluble solvent, resin fine particles, and a surfactant.

Furthermore, the present invention relates to printed materials obtained by printing with the aqueous inkjet ink composition described above.

While the detailed mechanism of action of the white pigment dispersion of the present invention remains unclear, it is presumed to be as follows. However, the present invention is not limited to this mechanism of action.

The white pigment dispersion of the present invention comprises a white pigment, an alkali-soluble resin, a basic compound, and water, wherein the white pigment is titanium dioxide having an average primary particle diameter (D) of 100 nm or more and 190 nm or less, the average dispersed particle diameter (D50) of the white pigment dispersion is 110 nm or more and 250 nm or less, the ratio of the average dispersed particle diameter (D50) to the average primary particle diameter (D) (D50/D) is 1.1 or more and 2.0 or less, and the alkali-soluble resin is 5 to 40 parts by mass per 100 parts by mass of the white pigment. The white pigment dispersion of the present invention has the above-described average primary particle diameter (D), average dispersed particle diameter (D50), and ratio of the average dispersed particle diameter (D50) to the average primary particle diameter (D) (D50/D). Therefore, it is presumed that the majority of the pigment is stably dispersed in a state close to primary particles. Furthermore, because it contains a specific amount of alkali-soluble resin, an aqueous inkjet ink composition with good redispersibility, ejection stability, printability, abrasion resistance, and storage stability for the white pigment is obtained.

<White pigment dispersion>
The white pigment dispersion of the present invention comprises a white pigment, an alkali-soluble resin, a basic compound, and water.

<White Pigment>
The white pigment is titanium dioxide having an average primary particle diameter (D) of 100 nm or more and 190 nm or less. From the viewpoint of ensuring the opacity of the ink coating film, the average primary particle diameter (D) of the white pigment is preferably 105 nm or more, more preferably 110 nm or more, and from the viewpoint of improving redispersibility, it is preferably 180 nm or less, and more preferably 160 nm or less. The white pigment may have a crystalline form such as rutile or anatase, and its surface may be surface-treated with various materials such as alumina or silica. Furthermore, the white pigment may be a self-dispersing white pigment in which polar functional groups are introduced on its surface, or a coated pigment in which its surface is coated with polymer particles. The white pigment can be used alone or in combination of two or more types.

The average primary particle diameter (D) refers to the average particle diameter of the primary particles. Primary particles generally refer to the smallest particles constituting the powder, and include particles formed by the aggregation of single crystals or near-single crystallites. Furthermore, the average primary particle diameter (D) is the arithmetic mean diameter obtained by observing titanium oxide particles with an electron microscope. The particle size distribution may be monodisperse or polydisperse, and two or more types of titanium oxide with monodisperse particle size distributions may be mixed and used. The shape of the titanium oxide particles is not particularly limited; any shape such as spherical, rod-shaped, needle-shaped, spindle-shaped, or plate-shaped can be used. For commercially available products, the average primary particle diameter (D) should be based on the catalog value.

The proportion of the white pigment in the white pigment dispersion is preferably 20% by mass or more, more preferably 30% by mass or more, from the viewpoint of ensuring the opacity of the ink coating film. From the viewpoint of manufacturability and dispersion stability, it is preferably 70% by mass or less, and more preferably 60% by mass or less.

<Alkali-soluble resin>
The alkali-soluble resin of the present invention is an alkali-soluble resin that can be used as a pigment dispersion or binder for ordinary inks and paints, and is not particularly limited as long as it can dissolve in an aqueous medium in the presence of a basic compound, but resins containing one or more anionic groups such as a carboxyl group, a sulfonic acid group, or a phosphonic acid group (-P(=O)( _OH2 )) are preferred.

The alkali-soluble resin preferably further contains hydrophobic moieties within its molecule, primarily to improve its adsorption to pigments. Examples of hydrophobic moieties introduced into the molecule include long-chain alkyl groups, alicyclic groups, and aromatic cyclic hydrocarbon groups.

The acid value of the alkali-soluble resin is preferably 40 mg KOH/g or higher, and more preferably 70 mg KOH/g or higher, from the viewpoint of improving solubility in aqueous media. Furthermore, the acid value of the alkali-soluble resin is preferably 300 mg KOH/g or lower, and more preferably 250 mg KOH/g or lower, from the viewpoint of improving the water resistance of printed materials. Note that the acid value is a theoretical acid value calculated arithmetically based on the composition of the monomers used to synthesize the alkali-soluble resin, determining the amount of potassium hydroxide (in mg) theoretically required to neutralize 1 g of the alkali-soluble resin.

The glass transition temperature of the alkali-soluble resin is preferably 0°C or higher, and more preferably 10°C or higher, from the viewpoint of improving the blocking resistance of the printed material. The glass transition temperature of the alkali-soluble resin is preferably 100°C or lower, and more preferably 80°C or lower, from the viewpoint of improving the bending resistance of the printed material.

The glass transition temperature of the alkali-soluble resin is the theoretical glass transition temperature obtained by the following Wood equation when the alkali-soluble resin is an acrylic copolymer resin.
Wood's formula: 1/Tg = W1/Tg1 + W2/Tg2 + W3/Tg3 + ... + Wx/Tgx
[In the formula, Tg1 to Tgx represent the glass transition temperatures of the respective homopolymers of monomers 1, 2, 3, ... x that constitute the alkali-soluble resin, W1 to Wx represent the polymerization fractions of monomers 1, 2, 3, ... x, and Tg represents the theoretical glass transition temperature. However, the glass transition temperature in Wood's formula is an absolute temperature.]

The glass transition temperature of the alkali-soluble resin is the theoretical glass transition temperature determined by thermal analysis, in cases where the alkali-soluble resin is not an acrylic copolymer resin. The thermal analysis method follows JIS K7121 (Method for Measuring Transition Temperature of Plastics). As an example, the glass transition temperature can be measured using a PerkinElmer Pyris1 DSC under conditions of a heating rate of 20°C/min and a nitrogen gas flow rate of 20 ml/min.

The weight-average molecular weight of the alkali-soluble resin is preferably 5,000 or more, more preferably 10,000 or more, and even more preferably 12,000 or more, from the viewpoint of improving the water resistance of the printed material. The weight-average molecular weight of the alkali-soluble resin is preferably 100,000 or less, and more preferably 50,000 or less, from the viewpoint of increasing solubility in aqueous media.

The weight-average molecular weight can be measured by gel permeation chromatography (GPC). For example, using a Water 2690 GPC instrument (Waters), a PLgel, 5μ, MIXED-D column (Polymer Laboratories), tetrahydrofuran as the developing solvent, a column temperature of 25°C, a flow rate of 1 ml/min, an RI detector, a sample injection concentration of 10 mg/ml, and an injection volume of 100 microliters, chromatography can be performed to determine the weight-average molecular weight in polystyrene terms.

Examples of the alkali-soluble resins include acrylic copolymer resins, maleic acid copolymer resins, polyester resins obtained by condensation polymerization, and polyurethane resins. Materials for synthesizing such alkali-soluble resins are disclosed, for example, in Japanese Patent Application Publication No. 2000-94825. Acrylic copolymer resins, maleic acid copolymer resins, polyester resins, polyurethane resins, etc., obtained using the materials described in that publication are usable. Furthermore, resins obtained using other materials are also usable. The alkali-soluble resins can be used alone or in combination of two or more types.

As the acrylic copolymer resin, for example, one obtained by polymerizing a mixture of an anionic group-containing monomer and other copolymerizable monomers in a solvent in the presence of a common radical generator (e.g., benzoyl peroxide, tert-butyl peroxybenzoate, azobisisobutyronitrile, etc.) can be used.

Examples of the anionic group-containing monomer include monomers having at least one anionic group selected from the group consisting of carboxyl groups, sulfonic acid groups, and phosphonic acid groups, and among these, monomers having a carboxyl group are particularly preferred.

Examples of monomers having a carboxyl group include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, 2-carboxyethyl (meth)acrylate, 2-carboxypropyl (meth)acrylate, maleic anhydride, fumaric anhydride, and maleic acid half-esters. Examples of monomers having a sulfonic acid group include sulfoethyl methacrylate. Examples of monomers having a phosphonic acid group include phosphonoethyl methacrylate.

From the viewpoint of improving adsorption with pigments, it is preferable that other monomers copolymerizable with the aforementioned anionic group-containing monomer include hydrophobic group-containing monomers.

Examples of the hydrophobic group-containing monomers include, as monomers having long-chain alkyl groups, alkyl esters of radically polymerizable unsaturated carboxylic acids such as (meth)acrylic acid (e.g., 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, 2-hydroxystearyl (meth)acrylate, etc.), alkyl vinyl ethers having 8 or more carbon atoms (e.g., dodecyl vinyl ether, etc.), vinyl esters of fatty acids having 8 or more carbon atoms (e.g., vinyl 2-ethylhexanoate, vinyl laurate, vinyl stearate, etc.); as monomers having alicyclic hydrocarbon groups, cyclohexyl (meth)acrylate, etc.; and as monomers having aromatic hydrocarbon groups, styrene monomers such as benzyl (meth)acrylate, styrene, α-methylstyrene, vinyltoluene, etc. The hydrophobic group-containing monomers can be used individually or in combination of two or more types.

Other monomers copolymerizable with the aforementioned anionic group-containing monomer may include hydrophilic group-containing monomers, from the viewpoint of suppressing aggregation of alkali-soluble resins in aqueous media.

Examples of the hydrophilic group-containing monomers include monomers having a (poly)oxyalkylene chain, such as esters of (poly)alkylene glycols with one-terminated alkyl-bound (poly)alkylene glycols such as methoxypolyethylene glycol, methoxypolyethylene polypropylene glycol, ethoxypolyethylene glycol, ethoxypolyethylene polypropylene glycol, propoxypolyethylene glycol, and propoxypolyethylene polypropylene glycol, and radical polymerizable unsaturated carboxylic acids such as (meth)acrylic acid, or ethylene oxide adducts and/or propylene oxide adducts to radical polymerizable unsaturated carboxylic acids such as (meth)acrylic acid; examples of basic group-containing monomers include vinylpyrrolidones such as 1-vinyl-2-pyrrolidone and 1-vinyl-3-pyrrolidone, vinylpyridines such as 2-vinylpyridine, 4-vinylpyridine, 5-methyl-2-vinylpyridine, and 5-ethyl-2-vinylpyridine, and 1- Examples of nitrogen-containing monomers include: vinylimidazoles such as nilimidazole and 1-vinyl-2-methylimidazole; vinylbiperidins such as 3-vinylpiperidine and N-methyl-3-vinylpiperidine; dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, tertiary butylaminoethyl (meth)acrylate, (meth)acrylamide, N-methylol (meth)acrylamide, N-butoxymethyl (meth)acrylamide, N-methoxy (meth)acrylamide, N-ethoxy (meth)acrylamide, N-dimethylacrylamide, N-propylacrylamide, and other nitrogen-containing derivatives of (meth)acrylic acid; monomers having a hydroxyl group, such as hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate, and other hydroxyalkyl esters of (meth)acrylic acid; and monomers having an epoxy group, such as glycidyl (meth)acrylate. These hydrophilic group-containing monomers can be used individually or in combination of two or more.

Other copolymerizable monomers besides the hydrophobic group-containing monomer and the hydrophilic group-containing monomer include, for example, alkyl esters of (meth)acrylic acid with fewer than 8 carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, and hexyl (meth)acrylate. These other copolymerizable monomers can be used individually or in combination of two or more.

The content of the alkali-soluble resin is 5 to 40 parts by mass per 100 parts by mass of the white pigment. From the viewpoint of improving the dispersibility of the pigment, the content of the alkali-soluble resin is preferably 6 parts by mass or more, and more preferably 8 parts by mass or more. From the viewpoint of reducing the viscosity of the aqueous inkjet composition, the content of the alkali-soluble resin is preferably 35 parts by mass or less, and more preferably 30 parts by mass or less, per 100 parts by mass of the white pigment.

<Basic compounds>
The basic compound should be capable of dissolving the alkali-soluble resin in water. Examples include inorganic basic compounds such as sodium hydroxide and potassium hydroxide; and organic basic compounds such as ammonia, methylamine, ethylamine, monoethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, N,N-dibutylethanolamine, diethanolamine, N-methyldiethanolamine, triethanolamine, morpholine, N-methylmorpholine, and N-ethylmorpholine. The basic compound can be used alone or in combination of two or more.

The proportion of the basic compound in the white pigment dispersion may be such that it dissolves the alkali-soluble resin in the medium. However, from the viewpoint of improving the dispersion stability of the alkali-soluble resin, it is generally preferable that the proportion be 0.05% by mass or more, and from the viewpoint of improving the water resistance of the printed material, it is preferable that the proportion be 1.0% by mass or less.

<Water>
Examples of the water mentioned above include ion-exchanged water, pure water, distilled water, and industrial water. The water can be used alone or in combination of two or more types.

The proportion of water in the white pigment dispersion is preferably 20% by mass or more, more preferably 30% by mass or more, from the viewpoint of manufacturability and dispersion stability, and preferably 70% by mass or less, more preferably 60% by mass or less, from the viewpoint of ensuring opacity.

Furthermore, the white pigment dispersion may contain any components depending on the purpose, such as the components found in the aqueous inkjet ink composition described later.

<Method for preparing a white pigment dispersion>
The method for preparing (manufacturing) the white pigment dispersion is not particularly limited, and the above components can be added in order or simultaneously and mixed. For example, (1) a method in which an aqueous resin varnish obtained by dissolving an alkali-soluble resin in water in the presence of a basic compound, a white pigment, and a pigment dispersant if necessary are mixed, and then a white pigment dispersion (white ink base) is prepared using various dispersers, such as a ball mill, attritor, roll mill, sand mill, agitator mill, etc.; or (2) a method in which, after dispersing the pigment by the above method, an alkali-soluble resin is precipitated on the surface of the pigment by an acid precipitation method or an ion exchange means described in the republished patent WO2005/116147, etc., a resin-coated pigment is obtained, the obtained resin-coated pigment is neutralized with a basic compound, and then redispersed in water using various dispersers (high-speed stirring device, etc.).

The average dispersed particle size (D50) of the white pigment dispersion is between 110 nm and 250 nm. From the viewpoint of opacity and dispersion stability, the average dispersed particle size (D50) of the white pigment dispersion is preferably 120 nm or larger, more preferably 150 nm or larger, and from the viewpoint of suppressing sedimentation, it is preferably 240 nm or smaller, more preferably 220 nm or smaller. The average dispersed particle size (D50) of the white pigment dispersion was measured using dynamic light scattering with Nikkiso Co., Ltd. product name "9340-UPA150".

Furthermore, the white pigment dispersion has a ratio (D50/D) of the average dispersed particle diameter (D50) to the average primary particle diameter (D) of 1.1 or more and 2.0 or less. From the viewpoint of dispersion stability, the ratio (D50/D) of the average dispersed particle diameter (D50) to the average primary particle diameter (D) is preferably 1.2 or more, and preferably 1.5 or less.

<Water-based inkjet ink composition>
The aqueous inkjet ink composition of the present invention comprises the white pigment dispersion, a water-soluble solvent, resin fine particles, and a surfactant.

<Water-soluble solvents>
The aforementioned water-soluble solvent can be any known water-soluble solvent used in water-based inkjet ink compositions, such as monoalcohols, polyhydric alcohols, lower alkyl ethers of polyhydric alcohols, nitrogen-containing compounds, ketones, ethers, esters, and the like. The aforementioned water-soluble solvent can be used alone or in combination of two or more types.

Examples of the aforementioned monoalcohols include methanol, ethanol, 1-propanol, 1-butanol, and 3-methoxy-3-methyl-1-butanol.

Examples of the aforementioned polyhydric alcohols include ethylene glycol, propylene glycol, glycerin, diethylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, 1,2-pentanediol, 1,5-pentanediol, neopentyl glycol, 1,2-hexanediol, 1,6-hexanediol, 1,2-cyclohexanediol, heptanediol, and 1,8-octanediol.

Examples of the lower alkyl ethers of the polyhydric alcohols include ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, ethylene glycol diethyl ether, ethylene glycol monopropyl ether, ethylene glycol isopropyl ether, ethylene glycol monobutyl ether, ethylene glycol isobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, dipropylene glycol mono-n-propyl ether, and dipropylene glycol mono-n-butyl ether.

Examples of the nitrogen-containing compounds include pyrrolidone and N-methyl-2-pyrrolidone.

Examples of the aforementioned ketones include acetone, methyl ethyl ketone, methyl butyl ketone, methyl isobutyl ketone, diisopropyl ketone, cyclopentanone, and cyclohexanone.

Examples of the aforementioned ethers include isopropyl ether, n-butyl ether, tetrahydrofuran, tetrahydropyran, and 1,4-dioxane.

Examples of the aforementioned esters include propylene carbonate, methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate, amyl acetate, ethyl lactate, ethyl butyrate, dibutyl phthalate, dioctyl phthalate; and cyclic esters such as ε-caprolactone and ε-caprolactam.

The aforementioned water-soluble solvent preferably contains at least one selected from the group consisting of monoalcohols, polyhydric alcohols, lower alkyl ethers of polyhydric alcohols, and nitrogen-containing compounds, from the viewpoint of suppressing drying of the ink composition within the inkjet nozzle and facilitating the formation of an ink layer (film) on the substrate. More preferably, it contains at least one selected from the group consisting of propylene glycol, glycerin, diethylene glycol, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, and 3-methoxy-3-methyl-1-butanol.

<Resin fine particles>
The resin fine particles can be known resin fine particles used in aqueous inkjet ink compositions, such as polyurethane resin fine particles, acrylic resin fine particles, wax fine particles, and polyester resin fine particles. Examples of the resin fine particles include polyurethane resin emulsions such as polyester-based polyurethane resin emulsion, polyether-based polyurethane resin emulsion, and polycarbonate-based polyurethane resin emulsion; acrylic resin emulsions such as styrene-acrylic resin emulsion; wax emulsions such as paraffin wax and polyethylene wax; and various other resin emulsions such as polyester resin emulsion. From the viewpoint of improving the drying properties and adhesion to the substrate of the printed material, it is preferable that the glass transition temperature of the resin emulsion is 20°C or lower. The glass transition temperature is determined by differential scanning calorimetry (DSC) and is usually calculated as the midpoint of the temperature range in which glass transition occurs. The resin fine particles can be used alone or in combination of two or more types.

<Surfactants>
The surfactant can be any known surfactant used in aqueous inkjet ink compositions without particular limitations, such as nonionic surfactants, cationic surfactants, anionic surfactants, and betaine surfactants. Specific examples of the surfactant include, for example, silicone-based surfactants, fluorine-based surfactants, and acetylene-based surfactants. The surfactant can be used alone or in combination of two or more.

Furthermore, the aqueous inkjet ink composition may contain any components depending on the purpose. Examples of additives include known resins, pigment dispersants, antifungal agents, rust inhibitors, thickeners, antioxidants, UV absorbers, preservatives, defoamers, and pH adjusters.

The proportions of each component contained in the aforementioned aqueous inkjet ink composition will be described below.

The proportion of the white pigment in the aqueous inkjet ink composition is preferably 4% by mass or more, more preferably 8% by mass or more, preferably 30% by mass or less, and more preferably 20% by mass or less.

The proportion of the water-soluble solvent in the aqueous inkjet ink composition is preferably 15% by mass or more, more preferably 20% by mass or more, from the viewpoint of improving ejection stability, and preferably 60% by mass or less, more preferably 50% by mass or less, from the viewpoint of improving coating film drying properties.

The solid content of the resin fine particles in the aqueous inkjet ink composition is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more, from the viewpoint of imparting coating film resistance. From the viewpoint of inkjet printability, it is preferably 25% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less.

The proportion of the surfactant in the aqueous inkjet ink composition is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, from the viewpoint of improving dot expandability and the uniformity of solid areas in printed materials. Furthermore, from the viewpoint of improving storage stability, it is preferably 2% by mass or less, and more preferably 1% by mass or less.

The proportion of water (including water contained in each component) in the aqueous inkjet ink composition is preferably 40% by mass or more, more preferably 50% by mass or more, from the viewpoint of improving the drying properties of the coating film, and preferably 70% by mass or less, more preferably 60% by mass or less, from the viewpoint of improving the ejection stability.

<Preparation of water-based inkjet ink composition>
The method for preparing the aqueous inkjet ink composition is not particularly limited, and for example, it can be produced by adding the white pigment dispersion, the water-soluble solvent, the resin fine particles, and the surfactant, and optionally the above-mentioned various additives, to water, and stirring and mixing with a disperser or the like.

The aforementioned aqueous inkjet ink composition has an initial viscosity after manufacturing of 2.0 to 15.0 mPa·s, preferably in the range of 3.0 to 12.0 mPa·s. Viscosity can be measured, for example, using an E-type viscometer (product name "RE100L Viscometer," manufactured by Toki Sangyo Co., Ltd.).

<Printed matter>
The printed material of the present invention is obtained by printing the aqueous inkjet ink composition. Specifically, the aqueous inkjet ink composition is obtained by coating (printing) it onto a substrate using an inkjet printer.

Examples of the aforementioned substrates include non-absorbent printing media such as coated paper (art paper, inkjet-specific paper, inkjet glossy paper, etc.), plastic substrates such as polypropylene film and polyvinyl chloride sheet; uncoated paper such as plain paper and offset paper; and fabrics such as cotton.

<Inkjet Printing Method>
The inkjet printing method of the present invention can employ conventionally known conditions as appropriate, but for example, one method is to house the aqueous inkjet ink composition in an ink cartridge, install the ink cartridge in an inkjet recording device such as a single-pass system, and perform inkjet printing by spraying the ink from a nozzle onto a substrate.

The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

<Example 1>
<Preparation of White Pigment Dispersion>
30 parts by mass of alkali-soluble resin (acrylic acid/lauryl acrylate/styrene copolymer, weight-average molecular weight 15,000, acid value 150 mg KOH/g, glass transition temperature 80°C) was dissolved in a mixed solution of 3.8 parts by mass of potassium hydroxide and 66.2 parts by mass of water to obtain an aqueous resin varnish with a solid content of 30% by mass. Subsequently, 16.7 parts by mass of water was added to 33.3 parts by mass of the above aqueous resin varnish and mixed to prepare a resin varnish for pigment dispersion. 50 parts by mass of titanium dioxide (trade name "PT-501R", average primary particle size (D): 180 nm, manufactured by Ishihara Sangyo Co., Ltd.) was added to this varnish, stirred and mixed, and then kneaded in a wet circulation mill to prepare a white pigment dispersion (white pigment concentration 50%). The average dispersed particle size (D50) of the white pigment dispersion was 218 nm.

<Preparation of water-based inkjet ink composition>
The aqueous inkjet ink composition of Example 1 was produced by stirring and mixing the white pigment dispersion, propylene glycol as a water-soluble solvent, wax emulsion (product name "AQUACER 531", solid content 45%, manufactured by BYK) as resin fine particles, acetylene-based surfactant (product name "Surfinol 440 (solid content 100%, HLB: 8, manufactured by EVONIK)) as a surfactant, and water in the mass ratios shown in Table 1.

<Examples 2-20, Comparative Examples 1-9>
<Preparation of white pigment dispersion, preparation of aqueous inkjet ink composition>
In each example and comparative example, the white pigment dispersions and aqueous inkjet ink compositions for each example and comparative example were prepared in the same manner as in Example 1, except that the raw materials used and their quantities were changed as shown in Table 1-3. The average dispersed particle size (D50) of each white pigment dispersion is shown in Table 1-3.

The white pigment dispersions and aqueous inkjet ink compositions of each example and comparative example were evaluated using the following method, and the results are shown in Table 1-3. A composition with even one "×" in the evaluation below will be considered unacceptable.

<Evaluation of the softness of the sediment>
100 mL of the aqueous inkjet ink composition prepared above was filled into a 100 mL sample bottle and left to stand for one week at 60°C. After one week, the appearance of the ink composition was observed, and titanium dioxide had settled at the bottom. The softness of the settled material was evaluated by scraping the bottom with a spatula.
○: The sediment is soft and not sticky, and there is no resistance when scraping it with a spatula.
△: The sediment has a medium consistency, is not sticky, and offers slight resistance when scraped with a spatula.
×: The sediment is hard and sticky, creating significant resistance when scraped with a spatula, and the spatula tends to pierce the sediment.

<Evaluation of redistribution>
100 mL of the aqueous inkjet ink composition prepared above was filled into a 100 mL sample bottle and left to stand for one week at 60°C. After one week, the appearance of the ink composition was observed to show that titanium dioxide had settled at the bottom. The sample bottle was shaken up and down 50 times by hand (to a height of approximately 30 cm), and the extent to which the settled material had redispersed was visually evaluated.
○: There is almost no sediment at the bottom, and it will redisperse.
△: Most of it will be redispersed, but some sediment will remain at the bottom.
×: The sediment remains unchanged and does not redisperse.

<Evaluation of discharge stability>
The aqueous inkjet ink composition prepared above was introduced into an Epson PX105 printer, and its ejection stability was evaluated.
○: Printing is consistent and stable.
△: Although there is some printing irregularity, it can still be dispensed.
×: There are printing irregularities, and the material cannot be ejected consistently.

<Evaluation of printability>
The aqueous inkjet ink composition prepared above was introduced into an Epson PX105 printer, and fine lines approximately 0.3 mm wide and solid images were printed on shielding paper. The ink deposition accuracy was evaluated on the fine lines, and the degree of solid color coverage was evaluated on the solid images.
○: The thin lines are straight, and the solid areas are filled well.
×: Thin lines are not straight, or streaks are observed in solid areas.

<Evaluation of abrasion resistance>
The aqueous inkjet ink composition prepared above was packed into a cartridge of an Epson PX105 printer, a solid image (100% duty cycle) was printed on shielding paper, and the printed material was dried in a 60°C hot air dryer for 3 minutes. The printed material was left overnight, cut into 2.5 cm x 25 cm pieces to make test pieces, and the abrasion resistance was evaluated using a JSPS friction tester (manufactured by Daiei Kagaku Seiki Seisakusho) by placing a bleached cloth on the printed surface and moving it back and forth 100 times with a load of 500 g. The degree to which the ink came off was used to evaluate the abrasion resistance.
○: The ink coating did not peel off at all.
△: The ink coating has peeled off slightly.
×: The ink coating has almost completely peeled off.

<Evaluation of storage stability>
The aqueous inkjet ink composition prepared above was filled into a glass bottle and left to stand at 60°C for 4 weeks. After that, the viscosity of the aqueous pigment-type inkjet ink composition was measured.
○: The rate of change from the initial viscosity is less than 10%.
△: The rate of change from the initial viscosity is 10% or more but less than 20%.
×: The rate of change from the initial viscosity is 20% or more.

In Table 1-3, PT-501R is titanium dioxide (product name "PT-501R", average primary particle size (D): 180 nm, manufactured by Ishihara Sangyo Co., Ltd.);
PT-401L is titanium dioxide (product name "PT-401L", average primary particle size (D): 130 nm, manufactured by Ishihara Sangyo Co., Ltd.);
MPT-141 is titanium dioxide (product name "MPT-141", average primary particle size (D): 110 nm, manufactured by Ishihara Sangyo Co., Ltd.);
W-2641 is titanium dioxide (product name "W-2641", average primary particle size (D): 150 nm, manufactured by Sakai Chemical Co., Ltd.);
A-190 is titanium dioxide (product name "A-190", average primary particle size (D): 150 nm, manufactured by Sakai Chemical Co., Ltd.);
SA-1 is titanium dioxide (product name "SA-1", average primary particle size (D): 150 nm, manufactured by Teika Corporation);
CR-90 is titanium dioxide (product name "CR-90", average primary particle size (D): 250 nm, manufactured by Ishihara Sangyo Co., Ltd.);
CR-50 is titanium dioxide (product name "CR-50", average primary particle size (D): 250 nm, manufactured by Ishihara Sangyo Co., Ltd.);
PF-726 is titanium dioxide (product name "PF-726", average primary particle size (D): 210 nm, manufactured by Ishihara Sangyo Co., Ltd.);
PF-736 is titanium dioxide (product name "PF-736", average primary particle size (D): 250 nm, manufactured by Ishihara Sangyo Co., Ltd.);
PF-671 is titanium dioxide (product name "PF-671", average primary particle size (D): 210 nm, manufactured by Ishihara Sangyo Co., Ltd.);
TTO-55(B) is titanium dioxide (product name "TTO-55(B)", average primary particle size (D): 35 nm, manufactured by Ishihara Sangyo Co., Ltd.);
The wax emulsion is polyethylene wax emulsion (product name "AQUACER 531", solids content 45%, manufactured by BYK Corporation);
The acrylic resin emulsion is acrylic resin emulsion (product name "Neocryl A-1105", solids content 50%, manufactured by DSM);
The polyurethane resin emulsion is a polyurethane resin emulsion (product name "NeoRez R-966", solids content 33%, manufactured by DSM).
The polyester resin emulsion is a polyester resin emulsion (product name "Seporjon ES", solids content 50%, manufactured by Sumitomo Seika Co., Ltd.);
The acetylene-based surfactant is an acetylene-based surfactant (product name "Surfinol 440", 100% solids, HLB: 8, manufactured by EVONIK).
The term "silicone-based surfactant" refers to the silicone-based surfactant (product name "Tego WET280", 100% solids content, manufactured by EVONIK).

Claims (4)

A white pigment dispersion comprising a white pigment, an alkali-soluble resin, a basic compound, and water,
The aforementioned white pigment is titanium dioxide having an average primary particle size (D) of 100 nm or more and 190 nm or less.
The average dispersed particle size (D50) of the white pigment dispersion is 110 nm or more and 250 nm or less.
The ratio (D50/D) of the average dispersed particle diameter (D50) to the average primary particle diameter (D) is 1.1 or more and 2.0 or less.
The alkali-soluble resin is characterized by having an acid value of 250 mgKOH/g or less and being present in an amount of 5 to 40 parts by mass per 100 parts by mass of the white pigment. The white pigment dispersion according to claim 1, characterized in that the proportion of white pigment in the white pigment dispersion is 20% by mass or more and 70% by mass or less. A water-based inkjet ink composition characterized by comprising the white pigment dispersion, water-soluble solvent, resin fine particles, and surfactant described in claim 1 or 2. A printed article characterized by being obtained by printing the aqueous inkjet ink composition described in claim 3.