Inkjet ink set

Abstract

In one embodiment, the present invention provides an inkjet recording ink set, etc., that can suppress the bleeding of color inks. [Solution] In one aspect, the present invention is an inkjet recording ink set comprising a color ink containing a color pigment, a pigment dispersion resin A, a water-soluble organic solvent, and water, and a white ink containing titanium dioxide particles, a pigment dispersion resin B, a water-soluble organic solvent, and water. The pigment dispersion resin A is a polymer A or a neutralized product obtained by neutralizing the carboxyl groups of polymer A, which comprises a hydrophobic unit A derived from (meth)acrylic acid, a hydrophobic unit B derived from (meth)acrylic acid, and a hydrophilic unit derived from (meth)acrylic acid. The pigment dispersion resin B is a polymer B containing a hydrophilic unit derived from (meth)acrylic acid, or a neutralized product obtained by neutralizing the carboxyl groups of polymer B. The value obtained by subtracting the acid value of polymer A from the acid value of polymer B is 30 mgKOH / g or more.

Description

Technical Field

[0001] The present invention relates to an inkjet recording ink set containing color ink and white ink, an inkjet recording method using the same, and the like.

Background Art

[0002] A printing method using an inkjet recording method is a method of discharging ink droplets from fine nozzles and directly attaching them to a printing substrate to obtain a printed matter or the like on which characters and images are recorded. This printing method is easy to full-colorize and inexpensive, can use various printing substrates such as plain paper, label paper, and plastic films, and has many advantages such as being non-contact with the printing substrate, so it has become extremely popular. In particular, from the viewpoints of the weather resistance and water resistance of printed matter, inks using pigments as colorants have become mainstream. In recent years, in particular, the demand for printing on printing substrates such as cardboard, paperboard, and resin films that are not white backgrounds has been increasing from printing on conventional printing substrates such as white paper. In the case of printing on a printing substrate that is not a white background, white ink is used for the purpose of expressing white or enhancing visibility. As the pigment used in white ink, titanium oxide particles, which are inorganic pigments with high hiding power, are widely used. In addition, a pigment dispersant is used for the purpose of improving the dispersibility of titanium oxide particles.

[0003] For example, Patent Document 1 discloses an inkjet coloring dispersion liquid containing a colorant, an A-B block polymer, a tellurium compound, and water for the purpose of obtaining an inkjet coloring dispersion liquid excellent in redispersibility after drying and also excellent in water resistance of a recorded image. The A-B block copolymer is a dispersant, the monomers constituting the A block are (meth)acrylic acid and butyl (meth)acrylate, and the content of (meth)acrylic acid in the A block is 26 to 42% by mass, and the monomers constituting the B block include cyclohexyl (meth)acrylate. The acid value of the above dispersant is 100 to 159 mgKOH / g, and the mass average molecular weight of the above dispersant is 10,000 to 50,000.

[0004] Patent Document 2 discloses an inkjet ink copolymer containing 10 to 99% by weight of cyclohexyl methacrylate units and 1 to 90% by weight of acrylic acid units, with the aim of providing an inkjet ink copolymer that can productively provide an inkjet ink capable of forming printed materials with excellent weather resistance, print density (color development), and fixability (abrasion resistance), and can also productively provide a pigment dispersion with excellent dispersibility. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] WO2022 / 025166 publication [Patent Document 2] WO2012 / 118078 publication [Overview of the project] [Problems that the invention aims to solve]

[0006] Solid white images using white ink require a very deep white color to improve the visibility and appearance of color images on transparent printing substrates such as resin films, and to enhance the concealment and light-blocking properties (preventing damage to contents from sunlight). Therefore, printing white requires the use of inks with a higher pigment concentration compared to color inks. However, because titanium dioxide particles have a high specific gravity, they tend to settle, which can cause nozzle clogging and printing defects. Therefore, there are limits to how high a concentration of titanium dioxide particles can be in white ink. As a result, a large amount of white ink needs to be printed on top of the color ink. In this process, if the colored ink applied earlier is redispersed into the white ink applied later, the colored ink will bleed, leading to a decrease in image quality.

[0007] This invention provides an ink set that can suppress the bleeding of colored inks. [Means for solving the problem]

[0008] The present inventors have found that the above problems can be solved by using a specific pigment dispersion resin as the dispersant for the color ink and the white ink, respectively, in an inkjet recording ink set containing a color ink and a white ink, and by having an acid value of 30 mgKOH / g or more higher for the dispersant of the white ink than for the dispersant of the color ink. That is, the present invention provides the following [1] to [3]. [1] A color ink containing a color pigment, a pigment dispersion resin A, a water-soluble organic solvent, and water, This inkjet recording ink set includes a white ink containing titanium dioxide particles, pigment dispersion resin B, a water-soluble organic solvent, and water. The aforementioned pigment dispersion resin A is Hydrophobic unit A derived from (meth)cyclohexyl acrylate, Hydrophobic unit B derived from (meth)acrylate, A polymer A containing hydrophilic units derived from (meth)acrylic acid, or a neutralized product obtained by neutralizing some or all of the carboxyl groups of polymer A, The pigment dispersion resin B is a polymer B containing hydrophilic units derived from (meth)acrylic acid, or a neutralized product in which some or all of the carboxyl groups of the polymer B are neutralized. An inkjet recording ink set in which the difference between the acid value of polymer B and the acid value of polymer A is 30 mg KOH / g or higher. . [2] An inkjet recording method using the inkjet recording ink set described in [1] above, Applying the aforementioned color ink to the printing substrate, An inkjet recording method comprising printing a solid color with white ink on top of the color ink applied to the printing substrate. [3] A method for manufacturing printed materials using the inkjet recording ink set described in [1] above, Applying the aforementioned color ink to the printing substrate, A method for manufacturing a printed material, comprising printing a solid color with white ink on top of the color ink applied to the printing substrate. [Effects of the Invention]

[0009] According to the present invention, an ink set capable of suppressing the bleeding of color inks can be provided. [Modes for carrying out the invention]

[0010] [Ink Set] The inkjet recording ink set of the present invention (hereinafter sometimes abbreviated as "ink set") includes a color ink and a white ink. The color ink includes a color pigment, a pigment dispersion resin A, a water-soluble organic solvent (hereinafter, for convenience of explanation, the water-soluble organic solvent contained in the color ink will also be referred to as "water-soluble organic solvent A"), and water. The white ink includes titanium dioxide particles, a pigment dispersion resin B, a water-soluble organic solvent (hereinafter, for convenience of explanation, the water-soluble organic solvent contained in the white ink will also be referred to as "water-soluble organic solvent B"), and water. Titanium dioxide particles are a white pigment. The pigment dispersion resin A is a polymer A containing hydrophobic units A derived from (meth)cyclohexyl acrylate, hydrophobic units B derived from (meth)butyl acrylate, and hydrophilic units derived from (meth)acrylic acid, or a neutralized product obtained by neutralizing some or all of the carboxyl groups of polymer A. The pigment dispersion resin B is polymer B containing hydrophilic units derived from (meth)acrylic acid, or a neutralized product obtained by neutralizing some or all of the carboxyl groups of polymer B. The acid value of polymer B is at least 30 mgKOH / g higher than that of polymer A.

[0011] The value obtained by subtracting the acid value of polymer A from the acid value of polymer B is 30 mg KOH / g or more from the viewpoint of suppressing bleeding, preferably 50 mg KOH / g or more, more preferably 80 mg KOH / g or more, and preferably 800 mg KOH / g or less from the viewpoint of adhesion to the substrate, dispersion stability, and suppression of bleeding.

[0012] According to the present invention, an ink set capable of suppressing bleeding of color ink can be provided. Although the reason is not clear, it is considered as follows.

[0013] The pigment dispersion resin A contained in the color ink is a polymer A containing a hydrophobic unit A derived from cyclohexyl (meth)acrylate and a hydrophobic unit B derived from butyl (meth)acrylate, or a neutralized product in which part or all of the carboxy groups of the polymer A are neutralized. However, these hydrophobic units, particularly the hydrophobic unit A derived from cyclohexyl (meth)acrylate, have low affinity for water, so it is considered that they promote drying of moisture in the color ink, which is an aqueous ink, and accelerate drying and solidification of the color ink. Also, a pigment dispersion resin B contained in the white ink applied to the printing substrate after the color ink is selected to have a different composition from the pigment dispersion resin A, and the difference between the acid value of the polymer B and the acid value of the polymer A is set to 30 mgKOH / g or more. As a result, there is a difference in the dispersion stability of the pigments in both inks. When both inks are mixed, thickening due to aggregation of both pigments occurs, and it is considered that mutual diffusion of the pigments between the two inks is suppressed. More specifically, the carboxy group that becomes carboxylate (-COO - ) in an aqueous system is a source of electrostatic repulsion of particles. The fact that the acid values of the polymer A and the polymer B are different means that the dispersion stabilities of the pigments (pigment resin composite particles) dispersed by them are also different. When pigment resin composite particles with different dispersion stabilities come into contact with each other, heteroaggregation is likely to occur. As a result, it is considered that mutual diffusion of the pigments between the two inks is suppressed and bleeding of the color ink into the white ink is suppressed. In the present invention, the pigment dispersion resin A and the pigment dispersion resin B may be the polymer A and the polymer B, respectively. However, from the viewpoint of the production of carboxylate (-COO - ), it is more advantageous that the carboxy groups are neutralized because the pigment dispersion resin A and the pigment dispersion resin B are more easily dissolved in an aqueous system than when they are not neutralized. Thus, in the ink set of the present invention, since the drying and solidification of the color ink are promoted and the mutual diffusion of pigments between the two inks is suppressed, it is considered that bleeding of the color ink can be suppressed. In addition, the hydrophilic units constituting Polymer A and the hydrophilic units constituting Polymer B are each adsorbed to the pigment. Aggregation of the pigment is suppressed by the electrostatic repulsive force of the hydrophilic unit and the steric hindrance effect of the pigment dispersion resin itself, and the dispersion stability of the pigment in each ink is improved.

[0014] Definitions of various terms in this specification are shown below. "Aqueous system" means that water occupies the largest ratio on a mass basis in the liquid component. As water, deionized water, ion-exchanged water, or distilled water is preferably used. More preferably, it is ion-exchanged water. "Recording" means an operation of fixing characters or images in a state recognizable by a human visually on a printing substrate (also simply referred to as "substrate"), and this is a concept including printing and typing. In the inkjet recording method using the ink set of the present invention, an operation of applying a liquid having an effect of enhancing the fixing property of the ink or enhancing the color development property of the color, that is, a liquid not containing a colorant, to the substrate using an inkjet head does not correspond to "recording". "Solid printing" means uniformly printing the entire area to be printed with a single color, and an image obtained by solid printing is called a "solid image". For the substrate, in order to improve the sharpness of characters and suppress color mixing, a pretreatment liquid containing a flocculant may be applied to the substrate to form a pretreatment layer before printing the ink on the substrate. "Substrate adhesion" in the present invention means the adhesion of an ink coating film formed directly on the printing substrate or via a pretreatment layer to the printing substrate or the pretreatment layer. "Low liquid absorbency" of the low liquid absorbency substrate is a concept including low liquid absorbency and non-liquid absorbency, and the water absorption amount of the recording medium at a contact time of 100 msec between the substrate and pure water is 0 g / m 2 10 g / m or more 2 Preferably 0 g / m or less 2 6 g / m or more 2This means the following: The amount of water absorbed can be measured using an automatic scanning liquid absorber (for example, "KM500win" manufactured by Kumagai Riki Kogyo Co., Ltd.) as the amount of transfer over a contact time of 100 milliseconds with pure water under conditions of 23°C and 50% relative humidity. "Intermittent ejection" refers to the ink ejection properties when ink is ejected again after a predetermined period of time has elapsed without ink being ejected from the inkjet nozzle. "Containing ingredient X" also means "being made by combining ingredient X." In this invention, "pigment resin composite particles" is a concept that includes both pigment-containing polymer particles (pigment-containing polymer particles A, pigment-containing polymer particles B) and pigment-containing crosslinked polymer particles (pigment-containing crosslinked polymer particles A, pigment-containing crosslinked polymer particles B), as described below, and is a general term for these. In this invention, "titanium oxide particles" include not only untreated titanium oxide particles but also titanium oxide particles that have been surface-treated with one or more selected from alumina, silica, zinc oxide, and zirconia. Unless otherwise specified, cyclohexyl (meth)acrylate means one or more selected from cyclohexyl acrylate and cyclohexyl methacrylate. (Meth)butyl acrylate means one or more selected from butyl acrylate and butyl methacrylate. (Meth)acrylic acid means one or more selected from acrylic acid and methacrylic acid.

[0015] [Color Ink] <Color Pigments> The color pigments may be either inorganic or organic pigments, and lake pigments and fluorescent pigments may also be used. Furthermore, these may be used in combination with extender pigments as needed. Specific examples of inorganic pigments include metal oxides such as carbon black, titanium dioxide, iron oxide, red iron oxide, and chromium oxide, as well as pearlescent pigments. Carbon black is particularly preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Specific examples of organic pigments include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; and polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and slene pigments. For achromatic inks, achromatic pigments such as white, black, and gray can be used, while for chromatic inks, chromatic pigments such as yellow, magenta, cyan, blue, red, orange, and green can be used. Specific examples of preferred organic pigments include one or more products from each product code selected from CI Pigment Yellow, CI Pigment Red, CI Pigment Orange, CI Pigment Violet, CI Pigment Blue, and CI Pigment Green. Examples of extender pigments include silica, calcium carbonate, and talc. The above pigments can be used individually or in combination of two or more.

[0016] It is preferable that the color pigments are held in a dispersed state in the ink by pigment dispersion resin A, which will be described in detail later.

[0017] In color inks, the preferred form of the color pigment and pigment dispersion resin A is polymer particles containing the color pigment (hereinafter also referred to as "pigment-containing polymer particles A"). Pigment-containing polymer particles A refers to particles in which pigment dispersion resin A encapsulates color pigment, particles in which a portion of the color pigment is exposed on the surface of particles composed of pigment dispersion resin A and color pigment, particles in which pigment dispersion resin A is adsorbed onto a portion of the color pigment, and mixtures thereof. Among these, particles in which pigment dispersion resin A encapsulates color pigment are more preferred, and for example, particles in which pigment dispersion resin A is densely adsorbed onto the surface of the color pigment are even more preferred. Furthermore, the pigment-containing polymer particles may also be pigment-containing crosslinked polymer particles A, which are obtained by further crosslinking them with a crosslinking agent as needed. Pigment-containing crosslinked polymer particles A are formed by crosslinking the pigment dispersion resin A that constitutes the pigment-containing polymer particles A with a crosslinking agent. Pigment-containing polymer particles A and pigment-containing crosslinked polymer particles A can be produced in the manufacturing process of the aqueous pigment dispersion A described later.

[0018] <Pigment-dispersed resin A> The polymer A contained in the above-mentioned color ink may be either a water-soluble resin or a water-insoluble resin. Here, the "water-soluble" and "water-insoluble" properties of the resin are determined as follows: when the resin, dried at 105°C for 2 hours and reaching a constant weight, is dissolved in 100g of 25°C water until saturated, it is judged to be "water-soluble" if the amount dissolved exceeds 10g, and "water-insoluble" if it is 10g or less. If the carboxyl groups of polymer A in the color ink are neutralized with a neutralizing agent, it is determined whether the ink is "water-soluble" or "water-insoluble" based on the degree of neutralization and the amount of polymer A that dissolves.

[0019] The pigment dispersion resin A contained in the color ink of the present invention is a polymer A comprising a hydrophobic unit A derived from (meth)cyclohexyl acrylate (a-1), a hydrophobic unit B derived from (meth)butyl acrylate (a-2), and a hydrophilic unit derived from (meth)acrylic acid (a-3), or a neutralized product obtained by neutralizing some or all of the carboxyl groups of polymer A.

[0020] [(meth)cyclohexyl acrylate (a-1)] (Meth)cyclohexyl acrylate (a-1) (hereinafter also referred to as "monomer (a-1)", "raw material monomer (a-1)", or "hydrophobic monomer (a-1)") is at least one monomer selected from the group consisting of cyclohexyl acrylate and cyclohexyl methacrylate, and is preferably cyclohexyl methacrylate, from the viewpoint of ensuring appropriate hardness of the ink coating film and improving flexibility.

[0021] [(meth)acrylate(a-2)] Butyl (meth)acrylate (a-2) (hereinafter also referred to as "monomer (a-2)", "raw material monomer (a-2)", or "hydrophobic monomer (a-2)") is at least one monomer selected from the group consisting of butyl acrylate and butyl methacrylate, and is preferably butyl methacrylate, from the viewpoint of improving the flexibility of the ink coating film.

[0022] [(meth)acrylic acid (a-3)] (Meth)acrylic acid (a-3) (hereinafter also referred to as "monomer (a-3)" or "raw material monomer (a-3)") is at least one monomer selected from the group consisting of acrylic acid and methacrylic acid, and is preferably methacrylic acid, from the viewpoint of improving the dispersion stability of the pigment.

[0023] Each of the above monomers (a-1) to (a-3) can be used individually or as a mixture of two or more. Furthermore, the term "hydrophobic" in hydrophobic monomers (a-1) and hydrophobic monomers (a-2) means that when the monomer is dissolved in 100g of deionized water at 25°C until saturated, the amount dissolved is less than 10g.

[0024] The mass ratio (hydrophobic unit B / hydrophobic unit A) of the constituent unit (hydrophobic unit A) derived from monomer (a-1) to the constituent unit (hydrophobic unit A) derived from monomer (a-2) is, expressed in mass%, 25% or more, preferably 40% or more, and more preferably 50% or more, from the viewpoint of improving the flexibility of the ink coating film, and from the same viewpoint, 100% or less, preferably 98% or less, and more preferably 95% or less. Furthermore, the above mass ratio can be considered as the mass ratio (mass%) of raw material monomer (a-2) to raw material monomer (a-1) of polymer A.

[0025] From the viewpoint of storage stability of the color ink, the mass ratio of the constituent units (hydrophilic units) derived from monomer (a-3) to the constituent units (hydrophobic units A) derived from monomer (a-1) (hydrophilic units / hydrophobic units A) is preferably 25% or more, more preferably 30% or more, and even more preferably 35% or more, expressed in mass%, and from the same viewpoint, preferably 70% or less, more preferably 65% ​​or less, even more preferably 60% or less, and even more preferably 55% or less. Furthermore, the above mass ratio can be considered as the mass ratio (mass%) of raw material monomer (a-3) to raw material monomer (a-1) of polymer A.

[0026] Polymer A may contain units (constituent units) derived from other monomers other than monomers (a-1) to (a-3), as long as they do not impede the effects of the present invention. Examples of other monomers include ionic monomers other than monomer (a-3), hydrophobic monomers having aromatic groups, and nonionic monomers.

[0027] The content of cyclohexyl (meth)acrylate (a-1) in the raw material monomers of polymer A, or the content of constituent units derived from cyclohexyl (meth)acrylate (a-1) in all constituent units of polymer A, is preferably 25% by mass or more, more preferably 30% by mass or more, even more preferably 35% by mass or more, even more preferably 40% by mass or more, and preferably 80% by mass or less, more preferably 70% by mass or less, even more preferably 60% by mass or less, and even more preferably 50% by mass or less.

[0028] The content of (meth)butyl(a-2) in the raw material monomers of polymer A, or the content of constituent units derived from (meth)butyl(a-2) in all constituent units of polymer A, is preferably 0.5% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, even more preferably 15% by mass or more, and preferably 55% by mass or less, more preferably 50% by mass or less, even more preferably 45% by mass or less, and even more preferably 40% by mass or less.

[0029] The content of (meth)acrylic acid (a-3) in the raw material monomer of polymer A, or the content of constituent units derived from (meth)acrylic acid (a-3) in all constituent units of polymer A, is preferably 9% by mass or more, more preferably 11% by mass or more, even more preferably 13% by mass or more, even more preferably 15% by mass or more, and preferably 40% by mass or less, more preferably 36% by mass or less, even more preferably 32% by mass or less, and even more preferably 28% by mass or less.

[0030] (Manufacturing of polymer A) Polymer A can be produced by copolymerizing raw material monomers containing the monomers (a-1) to (a-3), etc., using a known polymerization method. Solution polymerization is preferred as the polymerization method. There are no restrictions on the solvent used in solution polymerization, but polar solvents such as aliphatic alcohols, ketones, ethers, and esters are preferred, and methanol, ethanol, acetone, methyl ethyl ketone, etc., are more preferred. Polymerization initiators and chain transfer agents can be used during polymerization. Examples of polymerization initiators include persulfates such as ammonium persulfate and potassium persulfate; and water-soluble azo polymerization initiators. Examples of chain transfer agents include mercaptans. The polymerization temperature varies depending on the type of polymerization initiator, monomer, and solvent used, but is preferably 30°C or higher, more preferably 50°C or higher, and preferably 95°C or lower, more preferably 80°C or lower. The polymerization atmosphere is preferably a nitrogen gas or inert gas atmosphere.

[0031] (Physical properties of polymer A) The weight-average molecular weight of polymer A is preferably 6,000 or more, more preferably 9,000 or more, even more preferably 12,000 or more, and preferably 100,000 or less, more preferably 75,000 or less, and even more preferably 50,000 or less, from the viewpoint of improving adhesion to the substrate. The weight-average molecular weight of polymer A is measured by the method described in the examples.

[0032] The acid value of polymer A is preferably 80 mg KOH / g or more, more preferably 90 mg KOH / g or more, even more preferably 100 mg KOH / g or more, even more preferably 150 mg KOH / g or more, and preferably 260 mg KOH / g or less, more preferably 240 mg KOH / g or less, even more preferably 220 mg KOH / g or less, and even more preferably 200 mg KOH / g or less, from the viewpoint of improving adhesion to the substrate. The acid value of the polymer is measured by the method described in the examples.

[0033] <Water-soluble organic solvent A> The water-soluble organic solvent A contained in the color ink is an organic solvent that can be mixed with water in any proportion, and when dissolved in 100 mL of water at 25°C, its dissolution volume is 5 mL or more. Water-soluble organic solvent A can be used alone or in combination of two or more types. The boiling point of water-soluble organic solvent A is preferably 90°C or higher, more preferably 120°C or higher, even more preferably 150°C or higher, and preferably 260°C or lower, more preferably 250°C or lower, even more preferably 240°C or lower, and even more preferably 230°C or lower. When two or more types of water-soluble organic solvent A are used, the boiling point value is a weighted average value weighted by the content (mass%) of each water-soluble organic solvent A. Examples of water-soluble organic solvent A include polyhydric alcohols, polyhydric alcohol alkyl ethers, nitrogen-containing heterocyclic compounds, amides, amines, and sulfur-containing compounds. Among these, water-soluble organic solvent A preferably contains one or more selected from the group consisting of polyhydric alcohols and polyhydric alcohol alkyl ethers, from the viewpoint of improving adhesion to the substrate.

[0034] Examples of polyhydric alcohols include ethylene glycol, propylene glycol, alkanediols such as 1,2-pentanediol, 1,2-hexanediol, and 1,2-octanediol; polyalkylene glycols such as diethylene glycol and triethylene glycol; glycerin; and trimethylolpropane. Among these, the polyhydric alcohol is preferably one or more selected from the group consisting of propylene glycol, 1,2-hexanediol, diethylene glycol, triethylene glycol, and glycerin, and more preferably propylene glycol.

[0035] Examples of polyhydric alcohol alkyl ethers include (poly)alkylene glycol monoalkyl ethers such as monoalkylene glycol monoalkyl ethers, dialkylene glycol monoalkyl ethers, and trialkylene glycol monoalkyl ethers; and (poly)alkylene glycol dialkyl ethers such as monoalkylene glycol dialkyl ethers and dialkylene glycol dialkyl ethers. The alkylene oxide group of the polyhydric alcohol alkyl ether can be one or more selected from the group consisting of ethylene oxide group and propylene oxide group, with the ethylene oxide group being more preferred. A polyhydric alcohol alkyl ether has at least one hydrocarbon group having 2 to 8 carbon atoms.

[0036] Examples of (poly)alkylene glycol monoalkyl ethers include ethylene glycol monoalkyl ethers such as ethylene glycol monoisopropyl ether (boiling point: 142°C), ethylene glycol mono-n-butyl ether (boiling point: 171°C), ethylene glycol monoisobutyl ether (boiling point: 161°C), and ethylene glycol mono-n-hexyl ether (boiling point: 208°C); diethylene glycol monoethyl ether (boiling point: 202°C), diethylene glycol monoisopropyl ether (boiling point: 207°C), diethylene glycol mono-n-butyl ether (boiling point: 231°C), and diethylene glycol monoisobutyl ether ( Preferably, one or more are selected from the group consisting of diethylene glycol monoalkyl ethers such as diethylene glycol mono-n-hexyl ether (boiling point: 259°C), triethylene glycol monoalkyl ethers such as triethylene glycol monobutyl ether (boiling point: 271°C), propylene glycol monoalkyl ethers such as propylene glycol mono-n-propyl ether (boiling point: 150°C), dipropylene glycol monoalkyl ether, tripropylene glycol monoalkyl ether, and ethylene glycol aryl ethers such as ethylene glycol monobenzyl ether (boiling point: 256°C). Examples of (poly)alkylene glycol dialkyl ethers include diethylene glycol dialkyl ethers such as diethylene glycol methyl ethyl ether (176°C) and diethylene glycol diethyl ether (189°C).

[0037] From the viewpoint of improving adhesion to the substrate, it is preferable that the water-soluble organic solvent A contains one or more polyhydric alcohols and one or more polyhydric alcohol alkyl ethers. When the water-soluble organic solvent A contains one or more polyhydric alcohols and one or more polyhydric alcohol alkyl ethers, the mass ratio of the polyhydric alcohol alkyl ether content to the polyhydric alcohol content in the color ink [total polyhydric alcohol alkyl ether content / total polyhydric alcohol content] is preferably 0.05 or more, more preferably 0.10 or more, even more preferably 0.15 or more, and preferably 2.0 or less, more preferably 1.7 or less, even more preferably 1.4 or less, and even more preferably 1.1 or less.

[0038] The color ink may contain various additives as needed, such as fixing resins, surfactants, humectants, wetting agents, wetting / penetrating agents, viscosity modifiers, defoamers, preservatives, fungicides, and rust inhibitors. Examples of surfactants include nonionic surfactants, anionic surfactants, and amphoteric surfactants. Among these, nonionic surfactants are preferred. Examples of nonionic surfactants include polyoxyalkylene alkyl ether type surfactants, acetylene glycol-based surfactants, polyhydric alcohol-type surfactants, fatty acid alkanolamides, silicone-based surfactants, and fluorine-based surfactants. Among these, from the viewpoint of improving substrate adhesion, the nonionic surfactant preferably contains one or more selected from the group consisting of polyoxyalkylene alkyl ether type surfactants, acetylene glycol-based surfactants, and silicone-based surfactants.

[0039] (Content of each component in the color ink) The color ink contains a color pigment, a pigment dispersion resin A, a water-soluble organic solvent A, and water. The content of color pigment in the color ink is preferably 3.0% by mass or more, more preferably 3.5% by mass or more, even more preferably 4.0% by mass or more, and preferably 10% by mass or less, more preferably 9.0% by mass or less, and even more preferably 8.0% by mass or less, from the viewpoint of print density.

[0040] From the viewpoint of improving adhesion to the substrate, the content of pigment dispersion resin A in the color ink is preferably 0.5% by mass or more, more preferably 0.8% by mass or more, and preferably 2.5% by mass or less, more preferably 2.0% by mass or less, and even more preferably 1.8% by mass or less.

[0041] The content of water-soluble organic solvent A in the color ink is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, and preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less.

[0042] The water content in the color ink is the remaining amount after removing components other than water, and from the viewpoint of reducing environmental impact, it is preferably 40% by mass or more, more preferably 45% by mass or more, and preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 60% by mass or less.

[0043] When the color ink contains a fixing resin, the content of the fixing resin in the color ink is preferably 1.5% by mass or more, more preferably 2.0% by mass or more, even more preferably 3% by mass or more, and preferably 8.0% by mass or less, more preferably 7.0% by mass or less, and even more preferably 6.0% by mass or less.

[0044] If the color ink contains a surfactant, the surfactant content in the color ink is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, even more preferably 0.7% by mass or more, and preferably 3.0% by mass or less, more preferably 2.5% by mass or less, and even more preferably 2.0% by mass or less.

[0045] (Physical properties of colored inks) The viscosity of the color ink at 32°C is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, even more preferably 4 mPa·s or more, and preferably 12 mPa·s or less, more preferably 9 mPa·s or less, and even more preferably 7 mPa·s or less. The viscosity of the ink can be measured using an E-type viscometer.

[0046] The pH of the color ink at 20°C is preferably 7.0 or higher, more preferably 7.2 or higher, and even more preferably 7.5 or higher. Furthermore, from the viewpoint of resistance to printing equipment components and skin irritation, the pH is preferably 11 or lower, more preferably 10 or lower, and even more preferably 9.5 or lower. The pH of the color ink can be measured by conventional methods.

[0047] (Method of manufacturing color inks) In one embodiment, a color ink can be produced by preparing an aqueous pigment dispersion A containing a color pigment, a pigment dispersion resin A, and water; adding a water-soluble organic solvent A, and optionally an aqueous dispersion of fixing polymer particles, various additives such as surfactants and pH adjusters, and additional water to the aqueous pigment dispersion A; stirring these together; and removing coarse particles as necessary.

[0048] Aqueous pigment dispersion A can be efficiently produced, for example, by a method comprising the following steps I and II. Step I: A step to obtain an aqueous dispersion of the pigment dispersion resin A by neutralizing at least a portion of the carboxyl groups of polymer A with a neutralizing agent. Step II: Adding a color pigment to the aqueous dispersion of pigment dispersion resin A obtained in Step I to obtain a pigment mixture, and then performing a dispersion treatment on the pigment mixture to obtain an aqueous pigment dispersion A in which the color pigment is dispersed in pigment dispersion resin A and which contains resin particles containing the color pigment (also called "pigment-containing polymer particles A"). Furthermore, the method for producing the aqueous pigment dispersion A may optionally include a step III in which the pigment dispersion resin A in the aqueous pigment dispersion A obtained in step II is crosslinked with a crosslinking agent.

[0049] (Process I) In step I, it is preferable that at least a portion of the carboxyl groups of polymer A are neutralized using a neutralizing agent. This allows the negative charge (COO) that emerges after neutralization to be neutralized. -The increased charge repulsion between the particles is thought to suppress aggregation of the pigment (pigment resin composite particle A) in the color ink and improve the dispersion stability of the pigment (pigment resin composite particle A). Examples of neutralizing agents include alkali metal hydroxides, organic amine compounds, and inorganic amine compounds. Specific examples of preferred alkali metal hydroxides include sodium hydroxide and potassium hydroxide. Specific examples of preferred organic amine compounds include dimethylaminoethanol and triethanolamine. Specific examples of preferred inorganic amine compounds include ammonia. The above neutralizing agents can be used individually or in combination of two or more.

[0050] Furthermore, in step I, polymer A, which is mixed with the neutralizing agent, is dissolved in the organic solvent (polymerization solvent) used for the polymerization of polymer A, depending on its type. The organic solvent is preferably an organic solvent that dissolves in water, such as aliphatic alcohols with 1 to 4 carbon atoms, such as methanol, ethanol, and 2-propanol; ketones with 3 to 8 carbon atoms, such as acetone and methyl ethyl ketone (MEK); or ethers, such as tetrahydrofuran. The amount of neutralizing agent added varies depending on the type of neutralizing agent. When using alkali metal hydroxides or organic amine compounds as neutralizing agents, it is preferable that the amount is such that the degree of neutralization of the carboxyl groups of polymer A is 10 mol% or more and less than 100 mol%. When using inorganic amine compounds as neutralizing agents, it is preferable that the amount is such that the degree of neutralization of the carboxyl groups of polymer A is 20 mol% or more and less than 200 mol%. In step I, after neutralizing at least some of the carboxyl groups of polymer A, water such as deionized water and, if necessary, an organic solvent are added to the dispersion of pigment dispersion resin A to form an aqueous dispersion of the pigment dispersion resin. The organic solvent is preferably the same organic solvent as the polymerization solvent.

[0051] (Process II) In step II, the dispersion treatment can be performed solely by shear stress to finely atomize the pigment-containing polymer particles A to the desired particle size. However, from the viewpoint of obtaining a uniform aqueous pigment dispersion A, it is preferable to pre-disperse a pigment mixture containing an aqueous dispersion of pigment dispersion resin A and a color pigment, and then perform the main dispersion. As the disperser used for pre-dispersion, commonly used mixing and stirring devices such as anchor blades and disperser blades can be used. The above organic solvent is removed by distillation using an evaporator or the like before adding the color pigment to the aqueous dispersion of pigment dispersion resin A or before crosslinking the pigment dispersion resin.

[0052] Dispersion machines used for this dispersion include kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, paint shakers, and media-type dispersants such as bead mills. Among these, bead mills are preferred from the viewpoint of reducing the particle size of the pigment.

[0053] When performing dispersion processing using a high-pressure homogenizer, the average particle size of pigment-containing polymer particles A in the aqueous pigment dispersion can be adjusted by controlling the processing pressure and the number of passes. From the viewpoint of productivity and economic efficiency, the processing pressure is preferably 60 MPa to 300 MPa, and the number of passes is preferably 3 to 30. Furthermore, the pigment mixture dispersed in step II may contain organic solvents, surfactants, etc., as needed.

[0054] (Process III) If the method for producing an aqueous pigment dispersion includes step III, in which the pigment dispersion resin A in the aqueous pigment dispersion A obtained in step II is crosslinked with a crosslinking agent, then the pigment dispersion resin A in which the color pigment is dispersed is crosslinked by the crosslinking agent to form a crosslinked resin, and an aqueous pigment dispersion A can be obtained in which particles of the crosslinked resin containing the color pigment (also called "pigment-containing crosslinked polymer particles") are dispersed in water.

[0055] The crosslinking agent used in step III is preferably an epoxy compound, and more preferably a compound having two or more epoxy groups in its molecule. The compound having two or more epoxy groups in its molecule is preferably a polyfunctional epoxy compound having two or more glycidyl ether groups in its molecule, and more preferably a polyglycidyl ether compound of a polyhydric alcohol having hydrocarbon groups with 3 to 8 carbon atoms.

[0056] Preferred crosslinking agents include one or more polyglycidyl ethers selected from the group consisting of trimethylolpropane polyglycidyl ether, 1,6-hexanediol diglycidyl ether, and 1,4-cyclohexanedimethanol diglycidyl ether.

[0057] The epoxy equivalent (g / eq.) of the crosslinking agent is preferably 90 or more, more preferably 100 or more, and preferably 300 or less, more preferably 200 or less.

[0058] From the viewpoint of efficiently carrying out the crosslinking reaction in water, the water solubility of the crosslinking agent is preferably less than 50% by mass, more preferably 40% by mass or less, and even more preferably 35% by mass or less. Here, water solubility refers to the dissolution rate (mass%) when 10 parts by mass of the crosslinking agent is dissolved in 90 parts by mass of water at 25°C.

[0059] The common logarithm logPow (hereinafter referred to as "logPow") of the partition coefficient P(1-octanol / water) between the crosslinking agent 1-octanol and water is preferably -1.5 or higher, more preferably -1.0 or higher, even more preferably -0.5 or higher, and preferably 2.0 or lower, more preferably 1.5 or lower, and even more preferably 1.0 or lower, from the viewpoint of improving the dispersion stability of the pigment in the ink containing the hydrophobic water-soluble organic solvent A and improving intermittent ejection properties. logPow is measured and calculated by the following method according to the flask shaking method described in JIS Z7260-107.

[0060] [Measurement of partition coefficient P(1-octanol / water)] 10 g of deionized water and 10 g of 1-octanol are placed in a 50 mL separatory funnel and shaken at 25°C to equilibrate. 1 g of crosslinking agent is added and shaken well, then the 1-octanol phase and aqueous phase are separated by centrifugation. The amount of crosslinking agent dissolved in each phase is quantified by gas chromatography, and the common logarithm of the partition coefficient P between the two phases is defined as logPow.

[0061] The crosslinking ratio of the crosslinked pigment dispersion resin A is the ratio of the number of molar equivalents of epoxy groups in the crosslinking agent (total number of moles of epoxy groups in the crosslinking agent) to the number of molar equivalents of carboxyl groups in the pigment dispersion resin A before crosslinking (total number of moles of carboxyl groups (-COOH) in the pigment dispersion resin A before crosslinking), from the viewpoint of effectively suppressing the detachment of pigment dispersion resin A from the color pigment.

[0062] When the pigment dispersion resin A is crosslinked with a crosslinking agent, the crosslinking rate of the crosslinked pigment dispersion resin A is preferably 15 mol% or more, more preferably 20 mol% or more, even more preferably 25 mol% or more, even more preferably 30 mol% or more, and even more preferably 35 mol% or more, from the viewpoint of ink storage stability and discharge stability, and also preferably 85 mol% or less, more preferably 75 mol% or less, even more preferably 70 mol% or less, and even more preferably 65 mol% or less.

[0063] (Content and physical properties of each component of aqueous pigment dispersion A) The solid content of the aqueous pigment dispersion A is preferably 15% by mass or more, more preferably 18% by mass or more, and even more preferably 20% by mass or more, from the viewpoint of improving the storage stability of the aqueous pigment dispersion A and facilitating the addition of other components when formulating ink. Similarly, from the same viewpoint, it is preferably 40% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less.

[0064] The content of color pigment in aqueous pigment dispersion A is preferably 10% by mass or more, more preferably 13% by mass or more, and even more preferably 15% by mass or more, from the viewpoint of printing density, and similarly, preferably 25% by mass or less, more preferably 22% by mass or less, and even more preferably 20% by mass or less.

[0065] The ratio (mass ratio) of the content of color pigment to the content of pigment dispersion resin A in aqueous pigment dispersion A [color pigment / pigment dispersion resin A] is preferably 1.5 or higher, more preferably 2.0 or higher, even more preferably 2.5 or higher, and preferably 7.0 or lower, more preferably 6.0 or lower, from the viewpoint of improving the dispersion stability of pigment resin composite particles A.

[0066] The water content in aqueous pigment dispersion A is the amount remaining after removing components other than water, and from the viewpoint of reducing environmental impact, it is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less.

[0067] The average particle size of the pigment resin composite particles A in the aqueous pigment dispersion A is preferably 60 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, and preferably 160 nm or less, more preferably 150 nm or less, even more preferably 140 nm or less, and even more preferably 130 nm or less, from the viewpoint of ink color development and storage stability. The average particle size of the pigment resin composite particles in the aqueous pigment dispersion A of the present invention is measured by the method described in the examples.

[0068] [White ink] <Titanium dioxide particles> The white ink comprising the ink set of the present invention comprises titanium dioxide particles, pigment dispersion resin B, water-soluble organic solvent B, and water, and the white ink contains titanium dioxide particles as a white pigment. Titanium dioxide has three crystalline structures: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic). However, rutile titanium dioxide is preferred from the viewpoint of crystal stability, opacity, and availability. While untreated titanium dioxide particles can be used, surface-treated particles are preferred from the viewpoint of obtaining good dispersibility. Surface treatments for titanium dioxide particles include surface treatment with inorganic substances, surface treatment with titanium coupling agents, silane coupling agents, and organic substances such as silicone oil, but surface treatment with inorganic substances is preferred.

[0069] Inorganic surface treatment methods for titanium oxide particles include treatment with one or more substances selected from alumina (Al2O3), silica (SiO2), zinc oxide (ZnO), zirconia (ZrO2), magnesium oxide (MgO), etc.

[0070] Since titanium dioxide particles have the ability to decompose organic matter through photocatalytic activity, it is preferable that their surface be surface-treated with alumina or the like from the viewpoint of suppressing photocatalytic activity and improving the wettability of the titanium dioxide particles during dispersion. Furthermore, it is preferable that silica be used in combination for surface treatment from the viewpoint of adjusting the acid-base state of the titanium dioxide particle surface and improving durability. From the above viewpoints, it is more preferable that the titanium dioxide particles are treated with one or more selected from alumina, silica, zinc oxide, and zirconia, and even more preferable that they are treated with one or more selected from alumina and silica. Surface-treated titanium oxide particles can also be fired at 800-1000°C to suppress inter-particle sintering and improve the fluidity and dispersibility of secondary particles.

[0071] The shape of the titanium dioxide particles is not particularly limited and can be granular, needle-shaped, etc., but the average primary particle diameter is preferably 50 nm or more, more preferably 100 nm or more, even more preferably 150 nm or more, and preferably 450 nm or less, more preferably 400 nm or less, and even more preferably 350 nm or less, based on the arithmetic mean of the major axis of the primary particles from the viewpoint of whiteness.

[0072] Examples of commercially available rutile-type titanium dioxide include the Typeque® R, CR, and PF series from Ishihara Sangyo Co., Ltd., the R series from Sakai Chemical Industry Co., Ltd., the JR and M series from Teika Co., Ltd., the KURONOS KR series from Titanium Industry Co., Ltd., and the TR series from Fuji Titanium Industry Co., Ltd.

[0073] It is preferable that the titanium dioxide particles are held in a dispersed state in the white ink using pigment dispersion resin B.

[0074] In the white ink, the titanium dioxide particles and pigment dispersion resin B are preferably in the form of polymer particles containing titanium dioxide particles (hereinafter also referred to as "pigment-containing polymer particles B"). Pigment-containing polymer particles B refer to particles in which the pigment dispersion resin B encapsulates titanium dioxide particles, particles in which a portion of the titanium dioxide particles is exposed on the surface of particles composed of the pigment dispersion resin B and titanium dioxide particles, particles in which the pigment dispersion resin B is adsorbed onto a portion of the titanium dioxide particles, and mixtures thereof. Among these, particles in which the pigment dispersion resin B encapsulates titanium dioxide particles are more preferred, and for example, particles in which the pigment dispersion resin B is densely adsorbed onto the surface of titanium dioxide particles are even more preferred. Furthermore, the pigment-containing polymer particles B may be further crosslinked with a crosslinking agent as needed, resulting in pigment-containing crosslinked polymer particles B. In pigment-containing crosslinked polymer particles B, the pigment dispersion resin B constituting the pigment-containing polymer particles B is crosslinked with a crosslinking agent. Pigment-containing polymer particles B and pigment-containing crosslinked polymer particles B can be produced in the manufacturing process of the aqueous pigment dispersion B described later.

[0075] <Pigment-dispersed resin B> The pigment dispersion resin B is polymer B containing structural units derived from the ionic monomer (meth)acrylic acid, or a neutralized product obtained by neutralizing some or all of the carboxyl groups of polymer B. Polymer B may be a homopolymer or copolymer of the ionic monomer (meth)acrylic acid. Polymer B may also be a polymer containing structural units derived from the hydrophobic monomer (b-2) and / or nonionic monomer (b-3) in addition to structural units derived from the ionic monomer (b-1).

[0076] [Ionic monomer (b-1)] The ionic monomer (b-1) is (meth)acrylic acid.

[0077] [Hydrophobic monomer (b-2)] The term "hydrophobic" in hydrophobic monomer (b-2) means that when the monomer is dissolved in 100g of deionized water at 25°C until saturated, the amount dissolved is less than 10g. Specific examples of hydrophobic monomers (b-2) include those described in paragraphs

[0020] to

[0022] of Japanese Patent Publication No. 2018-83938. Among these, alkyl (meth)acrylates having an alkyl group with 1 to 18 carbon atoms, particularly 1 to 10 carbon atoms, aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms, and macromonomers having a polymerizable functional group at one end are preferred, and one or more selected from styrene, α-methylstyrene, and benzyl (meth)acrylate are more preferred.

[0078] [Nonionic monomer (b-3)] Nonionic monomers (b-3) are monomers that have a high affinity for water and water-soluble organic solvent B, and are, for example, monomers containing hydroxyl groups or polyalkylene glycol chains. Specific examples of nonionic monomers (b-3) include the nonionic monomers described in paragraph

[0018] of Japanese Patent Publication No. 2018-83938. Among these, one or more selected from methoxypolyethylene glycol (n=1~30) (meth)acrylate and polypropylene glycol (n=2~30) (meth)acrylate are preferred.

[0079] (Content of each constituent unit in polymer B) The content of constituent units derived from monomers (b-1) to (b-3) in polymer B is as follows, from the viewpoint of improving discharge stability, substrate adhesion, abrasion resistance, etc. The content of ionic monomer (b-1) in the raw material monomers of polymer B, or the content of constituent units derived from ionic monomer (b-1) in all constituent units of polymer B, is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and preferably 100% by mass or less. When polymer B contains at least one or both of the constituent units derived from hydrophobic monomer (b-2) and nonionic monomer (b-3), the content of such one or both constituent units in the total constituent units of polymer B, or the content of at least one or both of the hydrophobic monomer (b-2) and nonionic monomer (b-3) in the raw material monomers of polymer B, is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less. If polymer B contains constituent units derived from hydrophobic monomer (b-2) but does not contain constituent units derived from nonionic monomer (b-3), the content of constituent units derived from hydrophobic monomer (b-2) in the total constituent units of polymer B, or the content of hydrophobic monomer (b-2) in the raw material monomers of polymer B, is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less. From the viewpoint of suppressing bleeding of color inks, the content of units derived from (meth) cyclohexyl acrylate in polymer B is preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0% by mass, of the total constituent units of polymer B.

[0080] (Production of polymer B) Polymer B can be produced by known methods, either by homopolymerizing the above monomer components or copolymerizing a mixture of monomers. Solution polymerization is preferred as the polymerization method. There are no restrictions on the solvent used in solution polymerization, but polar solvents such as water, aliphatic alcohols, ketones, ethers, and esters are preferred, and water and ketones such as MEK are more preferred. During polymerization, polymerization initiators such as persulfates and water-soluble azo compounds, as well as polymerization chain transfer agents such as mercaptans, can be used. The polymerization temperature varies depending on the type of polymerization initiator, monomer, and solvent used, but is preferably 30 to 95°C, and more preferably 50 to 80°C.

[0081] (Physical properties of polymer B) The weight-average molecular weight of polymer B is preferably 2,000 or more, more preferably 3,000 or more, even more preferably 4,000 or more, and preferably 50,000 or less, more preferably 30,000 or less, and even more preferably 25,000 or less, from the viewpoint of improving discharge stability, substrate adhesion, abrasion resistance, etc. The weight-average molecular weight of polymer B is measured by the method described in the examples.

[0082] The acid value of polymer B is preferably 160 mg KOH / g or more, more preferably 180 mg KOH / g or more, even more preferably 200 mg KOH / g or more, and preferably 800 mg KOH / g or less, from the viewpoint of dispersion stability and other factors.

[0083] (Water-soluble organic solvent B) The water-soluble organic solvent B contained in the white ink is an organic solvent that can be mixed with water in any ratio, similar to the water-soluble organic solvent A contained in the color ink, and is an organic solvent whose dissolution amount is 5 mL or more when dissolved in 100 mL of water at 25°C. Specific examples of preferred water-soluble organic solvent B contained in the white ink are the same as specific examples of preferred water-soluble organic solvent A contained in the color ink.

[0084] White ink may contain various additives such as fixing resins and surfactants, as needed. Specific examples of such additives are the same as those that may be included in colored inks.

[0085] (Content of each component in white ink) The content of each component in the white ink is as follows, from the viewpoint of improving ejection stability, whiteness of solid images, substrate adhesion, abrasion resistance, and water resistance.

[0086] The titanium dioxide particle content in the white ink is preferably 3% by mass or more, more preferably 4% by mass or more, even more preferably 5% by mass or more, even more preferably 6% by mass or more, and preferably 14% by mass or less, more preferably 13% by mass or less, even more preferably 12% by mass or less, and even more preferably 11% by mass or less.

[0087] From the viewpoint of improving adhesion to the substrate, the content of pigment dispersion resin B in the white ink is preferably 0.1% by mass or more, more preferably 0.15% by mass or more, and preferably 1.0% by mass or less, more preferably 0.7% by mass or less, and even more preferably 0.5% by mass or less.

[0088] The content of water-soluble organic solvent B in the white ink is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and preferably 45% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less.

[0089] The water content in the white ink is the remaining amount after removing components other than water, and is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 45% by mass or more, and preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 60% by mass or less.

[0090] If the white ink contains a fixing resin, the content of the fixing resin in the white ink is preferably 1.5% by mass or more, more preferably 2.0% by mass or more, even more preferably 3% by mass or more, and preferably 8.0% by mass or less, more preferably 7.0% by mass or less, and even more preferably 6.0% by mass or less.

[0091] If the white ink contains a surfactant, the surfactant content in the white ink is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, even more preferably 0.8% by mass or more, and preferably 2.5% by mass or less, more preferably 2.0% by mass or less, and even more preferably 1.8% by mass or less.

[0092] (Physical properties of white ink) The viscosity of the white ink at 32°C is preferably 2 mPa·s or higher, more preferably 3 mPa·s or higher, even more preferably 4 mPa·s or higher, and preferably 12 mPa·s or lower, more preferably 9 mPa·s or lower, and even more preferably 7 mPa·s or lower, from the viewpoint of improving ejection stability and image density. The viscosity of the aqueous ink can be measured using an E-type viscometer.

[0093] The pH of the white ink at 20°C is preferably 7.0 or higher, more preferably 7.2 or higher, and even more preferably 7.5 or higher, from the viewpoint of storage stability and improving image density. Furthermore, from the viewpoint of material resistance and skin irritation, the pH is preferably 11 or lower, more preferably 10 or lower, and even more preferably 9.5 or lower. The pH of the white ink can be measured by conventional methods.

[0094] (Method of manufacturing white ink) In one embodiment, a method for producing color ink involves preparing an aqueous pigment dispersion B containing titanium dioxide particles, pigment dispersion resin B, and water; adding a water-soluble organic solvent B, and optionally an aqueous dispersion of fixing polymer particles, various additives such as surfactants and pH adjusters, and additional water to the aqueous pigment dispersion B; stirring these together; and removing coarse particles as necessary.

[0095] Aqueous pigment dispersion B can be efficiently produced by a method comprising the following steps I and II. Step I: A step to obtain an aqueous dispersion of the pigment dispersion resin B by neutralizing at least a portion of the carboxyl groups of polymer B with a neutralizing agent. Step II: Adding titanium dioxide particles to the aqueous dispersion of pigment dispersion resin B obtained in Step I to obtain a pigment mixture, and then performing a dispersion treatment on the pigment mixture to obtain an aqueous pigment dispersion B in which titanium dioxide particles are dispersed in pigment dispersion resin B and resin particles containing titanium dioxide particles (also called "pigment-containing polymer particles B"). Furthermore, the method for producing the aqueous pigment dispersion B described above may optionally include a step III in which the pigment dispersion resin B in the aqueous pigment dispersion B obtained in step II is crosslinked with a crosslinking agent.

[0096] Step I of the method for producing aqueous pigment dispersion B can be carried out in the same manner as step I of the method for producing aqueous pigment dispersion A, except that the types of pigment and pigment dispersion resin are different. The dispersion treatment in step II of the method for producing aqueous pigment dispersion B can be carried out in the same manner as the dispersion treatment in step II of the method for producing aqueous pigment dispersion A. The crosslinking treatment in step III of the method for producing aqueous pigment dispersion B can be carried out in the same manner as the crosslinking treatment in step III of the method for producing aqueous pigment dispersion A. The preferred examples of the crosslinking agent, the water solubility of the crosslinking agent, and the crosslinking rate can also be the same as those in the method for producing aqueous pigment dispersion A.

[0097] (Content and physical properties of each component of aqueous pigment dispersion B) The solid content of the aqueous pigment dispersion B is preferably 20% by mass or more, more preferably 25% by mass or more, and even more preferably 30% by mass or more, from the viewpoint of improving the storage stability of the aqueous pigment dispersion B and facilitating the addition of other components when formulating ink. Similarly, it is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 42% by mass or less.

[0098] The titanium dioxide pigment content in aqueous pigment dispersion B is preferably 20% by mass or more, more preferably 25% by mass or more, and even more preferably 30% by mass or more, from the viewpoint of printing density, and similarly, preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less.

[0099] The ratio (mass ratio) of titanium dioxide particles to the pigment dispersion resin B content in aqueous pigment dispersion B [titanium dioxide particles / pigment dispersion resin B] is preferably 10 or more, more preferably 12 or more, even more preferably 15 or more, and preferably 100 or less, more preferably 70 or less, and even more preferably 55 or less, from the viewpoint of improving the dispersion stability of pigment resin composite particles B.

[0100] The water content in aqueous pigment dispersion B is the amount remaining after removing components other than water, and from the viewpoint of reducing environmental impact, it is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 65% ​​by mass or less.

[0101] The average particle size of the pigment resin composite particles in the aqueous pigment dispersion B is preferably 200 nm or more, more preferably 240 nm or more, even more preferably 260 nm or more, and preferably 450 nm or less, more preferably 400 nm or less, even more preferably 350 nm or less, and even more preferably 320 nm or less, from the viewpoint of ink color development and storage stability. The average particle size of the pigment resin composite particles in the aqueous pigment dispersion B of the present invention is measured by the method described in the examples.

[0102] The ink set of the present invention can be used as an ink for various printing applications such as inkjet printing, gravure printing, and flexographic printing, but it is preferable to use it as a water-based white ink for inkjet printing from the viewpoint of ink ejection performance and substrate adhesion.

[0103] (Inkjet recording method and method for manufacturing printed materials) In one embodiment, the present invention is an inkjet recording method using the ink set of the present invention. Applying the color ink to the side of the printed substrate placed on the heater that is opposite to the side facing the heater, This includes printing a solid color with white ink on the opposite side of the printing substrate, over the color ink applied to the printing substrate.

[0104] Furthermore, in one embodiment, the present invention is a method for manufacturing printed materials using the ink set of the present invention. Applying the color ink to the side of the printed substrate placed on the heater that is opposite to the side facing the heater, This includes printing a solid color with white ink on the opposite side of the printing substrate, over the color ink applied to the printing substrate. The method for manufacturing printed materials according to the present invention is what is known as reverse printing.

[0105] In the inkjet recording method and printing method of the present invention, the white ink is applied on top of the color ink which has been applied to the printing substrate and is in an incompletely dried state. In the present invention, "incomplete drying state" refers to a state in which the liquid component in the color ink has not been completely removed, and the liquid component is retained on the surface or inside the applied color ink to the extent that the color pigment can diffuse into the white ink that is applied later. Specifically, if the total amount of water-soluble organic solvent and water in the color ink before it is applied to the printing substrate is considered to be 100% by mass, then the total amount of water-soluble organic solvent and water (remaining amount) in the color ink on the printing substrate immediately before the white ink droplets are applied is 5% by mass or more and 100% by mass or less. Furthermore, the inkjet recording method and method for manufacturing printed materials of the present invention include drying the color ink and the white ink after solid printing with white ink.

[0106] The present invention's inkjet recording method and method for manufacturing printed materials uses, for example, a known inkjet recording device such as a piezo type. This device is equipped with multiple inkjet heads. In the present invention's inkjet recording method and method for manufacturing printed materials, color ink is loaded into the inkjet head, ejected from the inkjet head as ink droplets, and deposited onto a printing substrate such as a low-liquid-absorbent substrate (described later) to record images such as characters. Similarly, white ink is loaded into the inkjet head, ejected from the inkjet head as ink droplets, and deposited onto the printing substrate on which the color ink has been deposited, on top of the deposited color ink, to record a solid image.

[0107] (Low liquid absorption base material) Preferred low-liquid-absorbent substrates include low-liquid-absorbent coated paper and non-liquid-absorbent resin films. Preferred coated papers include general-purpose glossy paper and multi-color foam gloss paper. Preferred resin films include polyester film, polyvinyl chloride film, polypropylene film, and polyethylene film. When using a resin film as a low-liquid-absorbent substrate, it is more preferable to use a resin film that has undergone surface treatment such as corona treatment, and it is also preferable to use a resin film that has undergone one type of stretching treatment selected from the group consisting of uniaxial stretching and biaxial stretching. In the inkjet recording method of the present invention, the low liquid absorption substrate may be in a continuous shape wound into a roll, or it may be in a shape cut into predetermined sizes such as A3 to A6 or B3 to B5. [Examples]

[0108] In the following manufacturing examples, embodiments, and comparative examples, "parts" and "%" refer to "parts by mass" and "mass%" unless otherwise specified. The measurement and calculation methods for each physical property are as follows.

[0109] (1) Measurement of the weight-average molecular weight (Mw) of polymer A and polymer B The weight-average molecular weights of the polymers shown in Table 1 were determined by gel permeation chromatography. The measurement conditions are as follows. GPC device: Tosoh Corporation "HLC-8320GPC" Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolum Super AW-H" manufactured by Tosoh Corporation. Eluent: A solution prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively. Flow rate: 0.5mL / min Standard materials: Monodisperse polystyrene kits with known molecular weights: "PStQuick B (F-550, F-80, F-10, F-1, A-1000)" and "PStQuick C (F-288, F-40, F-4, A-5000, A-500)" (all manufactured by Tosoh Corporation). Measurement sample: 0.1 g of polymer was mixed with 10 mL of the eluent in a glass vial, and the mixture was stirred with a magnetic stirrer at 25°C for 10 hours. The filtrate obtained by filtering through a syringe filter "DISMIC-13HP" (PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.) was used.

[0110] (2) Measurement of the acid value of polymer A and polymer B A potentiometric automatic titrator (manufactured by Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610) was used to measure the acid value of the polymer. Specifically, the polymer was dissolved in a titration solvent of toluene and acetone (toluene:acetone = 2:1 (volume ratio)), and titrated with a 0.1N potassium hydroxide / ethanol solution by potentiometric titration, with the inflection point on the titration curve as the endpoint. The acid value (mgKOH / g) was calculated from the amount of potassium hydroxide solution titrated to the endpoint.

[0111] (3) Measurement of solid content concentration 10.0 g of sodium sulfate, which had been stabilized in a desiccator, was weighed into a 30 mL polypropylene container (φ=40 mm, height=30 mm). Approximately 1.0 g of the sample was added and mixed, and then accurately weighed. Next, the mixture was maintained at 105°C for 2 hours to remove volatile components, and then left in a desiccator for another 15 minutes before the mass was measured. The mass of the sample after removal of volatile components was taken as the solid content, and the solid content concentration (%) was obtained by dividing it by the mass of the sample added to the polypropylene container.

[0112] (4) Measurement of the average particle size of particles (pigment resin composite particles) in aqueous pigment dispersions Cumulant analysis was performed on the aqueous pigment dispersion using the laser particle analysis system "ELS-8000" (manufactured by Otsuka Electronics Co., Ltd.), and the resulting average cumulant particle size was taken as the average particle size of the pigment resin composite particles (pigment-containing polymer particles) in the aqueous pigment dispersion. The sample measured had a particle concentration of 5 × 10⁻⁶ -3 A dispersion was used, obtained by diluting an aqueous pigment dispersion with water to a concentration of % (converted to solid content). The measurement conditions were a dispersion temperature of 25°C, an angle of 90° between the incident light and the detector, and 100 cumulative measurements. The refractive index of water (1.333) was input as the refractive index of the dispersion solvent.

[0113] (8) Measurement of ink viscosity The viscosity of the ink at 32°C was measured using an E-type viscometer (TV-25, manufactured by Toki Sangyo Co., Ltd., with a standard cone rotor of 1°34'×R24 and a rotation speed of 50 rpm).

[0114] <Polymer manufacturing> [Manufacturing Example 1-1] A monomer mixture was prepared by mixing 46.0 parts of cyclohexyl methacrylate, 30.0 parts of butyl methacrylate, 24.0 parts of methacrylic acid, and 75.0 parts of methyl ethyl ketone (hereinafter referred to as "MEK"). In a reaction vessel equipped with a stirrer, reflux condenser, and dropping tank, 10% of the monomer mixture was added as an initial charge, and the mixture was thoroughly purged with nitrogen gas. Meanwhile, the remaining 90% of the monomer mixture was mixed with 1.2 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65) as a polymerization initiator and 1.0 part of 3-mercaptopropionic acid as a chain transfer agent, and then placed in a dropping tank. Under a nitrogen atmosphere, the contents of the reaction vessel were heated to 77°C while stirring, and the contents of the dropper tank were continuously added to the reaction vessel over 3 hours. After the addition was complete, the polymerization reaction was carried out for 2 hours, then a mixture of 0.3 parts V-65 and 2.7 parts MEK was added to the reaction vessel, and the reaction was carried out for another 1 hour. Subsequently, 70.0 parts MEK was added to the reaction vessel, and the polymerization reaction was terminated by cooling to room temperature, yielding polymer solution 1 (resin solids concentration: 40%).

[0115] [Manufacturing Examples 1-2 to 1-3] Solutions 2 and 3 of the polymer (resin solids concentration 40%) were obtained in the same manner as in Production Example 1-1, except that the monomer composition constituting the polymer was changed to the conditions shown in Table 1. The acid value and weight-average molecular weight of the polymer are shown in Table 1, respectively.

[0116] [Table 1]

[0117] <Manufacturing of aqueous pigment dispersions> [Manufacturing Example 2-1] 2.0 parts of polyacrylic acid (PAA; manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Mw: 5,000 (catalog value), acid value 779 (mgKOH / g)) were added to 1.9 parts of 28% aqueous ammonia and 148.1 parts of deionized water so that the degree of neutralization with respect to the carboxyl group was 100 mol%, and these were stirred at 80°C for 1 hour to dissolve. After that, it was cooled to 25°C, and 98.0 parts of titanium dioxide particles (manufactured by Ishihara Industries Co., Ltd., product name: Typeque CR-97, rutile type, average primary particle size 250 nm) were added to obtain a pigment mixture. Next, the obtained pigment mixture and 1000g of 1.5mm zirconia beads were placed in a 1L plastic container and dispersed for 8 hours at a rotation speed of 250rpm using a tabletop pot mill stand (AS ONE Corporation). After that, the zirconia beads were removed using a metal mesh to obtain an aqueous pigment dispersion D1 containing pigment-containing polymer particles (total concentration of pigment and pigment dispersion resin: 40% by mass). The average particle size of the obtained pigment-containing polymer particles is shown in Table 2.

[0118] [Manufacturing Example 2-2] To 5.0 parts of solution 2 (resin solids concentration 40%) of the polymer obtained in Production Example 1-2, 0.6 parts of 28% aqueous ammonia were added so that the degree of neutralization of the carboxyl groups of polymer II contained in solution 2 was 100 mol%, and these were stirred at 25°C. Then, 176.4 parts of deionized water were added to obtain an aqueous dispersion of pigment dispersion resin. Next, 98.0 parts of titanium dioxide particles (manufactured by Ishihara Sangyo Co., Ltd., product name: Typeque CR-97, rutile type, average primary particle diameter 250 nm) were added to the aqueous dispersion of pigment dispersion resin to obtain a pigment mixture. Next, the obtained pigment mixture and 1000g of 1.5mm zirconia beads were placed in a 1L plastic container and dispersed for 8 hours at a rotation speed of 250rpm using a benchtop pot mill stand (manufactured by AS ONE Corporation). After that, the zirconia beads were removed using a metal mesh, and MEK was removed by distillation using an evaporator to obtain an aqueous pigment dispersion D2 containing pigment-containing polymer particles (total concentration of pigment and pigment dispersion resin: 40% by mass).

[0119] [Manufacturing Example 2-3] and [Manufacturing Example 2-4] Aqueous pigment dispersions D3 and D4 containing pigment-containing polymer particles (both with a total concentration of pigment and pigment dispersion resin of 40% by mass) were prepared in the same manner as in [Production Example 2-2], except that the type of polymer solution, the amount of 28% ammonia aqueous solution added, and the amount of deionized water added were changed.

[0120] [Manufacturing Example 2-5] To 62.5 parts of solution 1 (resin solids concentration 40%) of the polymer obtained in Production Example 1-1, 4.7 parts of 28% aqueous ammonia were added so that the degree of neutralization of the carboxyl groups of polymer I was 100 mol%, and the mixture was stirred at 25°C. Then, 30.0 g of MEK and 228.0 parts of deionized water were added, and further 75.0 parts of magenta pigment (manufactured by Dainichi Seika Kogyo Co., Ltd., product name: CIPigment Red 150) were added to obtain a pigment mixture. The resulting pigment mixture was mixed using a disperser blade at 7000 rpm and 20°C for 1 hour. It was then subjected to a 10-pass dispersion treatment at a pressure of 180 MPa using a microfluidizer (Microfluidics, high-pressure homogenizer, product name: M-140K) to obtain a pigment dispersion. 200.0 parts of deionized water were added to the obtained pigment dispersion, and after stirring, MEK was removed under reduced pressure at 60°C, and some of the water was further removed. The liquid phase of the obtained dispersion was recovered using a centrifuge, and the dispersion was filtered through a membrane filter (Sartorius, product name: Minisart syringe filter, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, obtaining an aqueous pigment dispersion D5 containing pigment-containing polymer particles (total concentration of pigment and pigment dispersion resin: 22% by mass).

[0121] [Manufacturing Example 2-6] Except for changing the type of pigment dispersion resin solution, the amount of 28% ammonia aqueous solution added, the amount of MEK added, and the amount of ion-exchanged water added, an aqueous pigment dispersion D6 containing pigment-containing polymer particles (total concentration of pigment and pigment dispersion resin: 22% by mass) was obtained in the same manner as in [Production Example 2-5].

[0122] [Table 2]

[0123] <Preparation of aqueous dispersion of fixed polymer particles> In a reaction vessel equipped with a dropping funnel, 0.5 g of methacrylic acid, 25.5 g of methyl methacrylate, 9 g of 2-ethylhexyl acrylate, 11.1 g of sodium polyoxyethylene alkyl ether sulfate (manufactured by Kao Corporation, Latemul® E-118B, emulsifier), 0.2 g of potassium persulfate (polymerization initiator), and 382.8 g of deionized water were added and mixed. The reaction vessel was then purged with nitrogen gas to obtain the initial monomer solution. Furthermore, 9.5 g of methacrylic acid, 34.5 g of methyl methacrylate, 171 g of 2-ethylhexyl acrylate, 35.1 g of sodium polyoxyethylene alkyl ether sulfate (manufactured by Kao Corporation, Latemul® E-118B, emulsifier), 0.6 g of potassium persulfate (polymerization initiator), and 183 g of deionized water were mixed to obtain a dropper monomer solution, which was then placed in the dropping funnel, and the inside of the dropping funnel was purged with nitrogen gas. Next, under a nitrogen atmosphere, the initial monomer solution in the reaction vessel was stirred while the temperature was raised from room temperature to 80°C over 30 minutes. While maintaining the temperature at 80°C, the dropping monomer solution in the dropping funnel was gradually added to the reaction vessel over 3 hours. After the dropping was complete, the mixture was stirred for 1 hour while maintaining the temperature in the reaction vessel. The mixture was then filtered through a 200-mesh filter, and deionized water was added to obtain a fixed polymer particle aqueous dispersion with a solid content of 30% by mass.

[0124] <Manufacturing of white ink> [Manufacturing Example 3-1] To achieve the ink composition shown in Table 3, the following ingredients were used: 25.5 parts of aqueous pigment dispersion D1 containing pigment-containing polymer particles (10.2 parts total of pigment and pigment dispersion resin, 10.0 parts pigment content), 30.0 parts of propylene glycol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent), 5.0 parts of diethylene glycol monoisobutyl ether (manufactured by Nippon Emulsifier Co., Ltd.), 16.7 parts of the above-mentioned aqueous dispersion of fixed polymer particles, 0.3 parts of Surfinol 104PG50 (manufactured by Nisshin Chemical Industry Co., Ltd., propylene glycol solution of 2,4,7,9-tetramethyl-5-decine-4,7-diol, 50% effective content), 1.0 part of KF-6011 (manufactured by Shin-Etsu Chemical Co., Ltd., polyether-modified silicone surfactant, 100% effective content), and triethanolamine (manufactured by Tokyo Chemical Industry Co., Ltd.) The reagents were mixed so that the ink pH (at 20°C) was 8.5, and then deionized water was added to make a total volume of 100 parts. After stirring, the mixture was filtered through a membrane filter (product name: Minisart syringe filter, pore size: 5 μm, material: cellulose acetate) to obtain aqueous pigment ink W-1 (white ink).

[0125] [Manufacturing Example 3-2]~[Manufacturing Example 3-6] Except for the change in the formulation shown in Table 3, water-based pigment inks W-2 to W-4 (white ink) and M-1 to M-2 (magenta ink) were prepared in the same manner as in [Production Example 3-1]. The pH at 20°C and viscosity at 32°C for each water-based pigment ink are shown in Table 3.

[0126] [Table 3]

[0127] [Production of printed materials] A printing evaluation apparatus was prepared, equipped with three inkjet heads (Kyocera Corporation's "KJ4B-1200", piezo type) and an underheater that heats the printing substrate from the back side of the surface (front side) of the printing substrate facing the inkjet heads. Of the three inkjet heads, the frontmost one was filled with magenta ink, and the two rearmost heads were filled with white ink. The combinations of magenta and white ink in Examples 1-3 and Comparative Examples 1-2 are shown in Table 4. The distance between the underheater and the printing substrate was set to 0.2 mm, the distance between the inkjet head and the printing substrate was set to 1.0 mm, and the surface temperature of the underheater was set to 40°C. For the printing substrate, we prepared a PET film (manufactured by Futamura Chemical Co., Ltd., product number: FE2001, film thickness: 20 μm) that had undergone corona discharge treatment, cut to A4 size. Under conditions of 25±1℃ temperature and 30±5% relative humidity, the head voltage was set to 26V, the appropriate amount of ejected liquid to 3pl, the head temperature to 32℃, the resolution to 1200dpi, and the number of pre-ejection flushing cycles to 200. The printing substrate was fixed to the printing evaluation device with its longitudinal direction and its transport direction being the same.

[0128] Printing was performed as follows using the ink sets of the examples and comparative examples shown in Table 4. Specifically, first, an image of the letter "a" inverted was recorded on the front surface of the printing substrate using magenta ink at 100% duty cycle (font size 8pt). Then, after a 20-second interval, solid white ink was printed over the letter "a" on the front surface of the printing substrate. For the solid printing, two inkjet heads were used, with each head using 100% duty cycle ink, for a total duty cycle of 200%. Next, the printed ink was heated and dried in a 60°C hot air dryer for 2 minutes, and then left to stand for 24 hours to obtain the printed material.

[0129] The obtained printed materials were observed from the back side of the printing substrate, and the clarity of the letter "a" was evaluated according to the following criteria. The results are shown in Table 4. (Evaluation Criteria) 5: There is no blurring of the letter "a", and the quality of the letter "a" is not diminished. 4: The letter "a" is slightly blurred, but there is virtually no decrease in the quality of the letter "a". 3: The letter "a" is slightly blurred, and its quality is reduced, but it is still recognizable and usable for practical purposes. 2: The letter "a" is blurred, and there is a noticeable decrease in the quality of the letter "a," but the letter "a" is still recognizable. 1: The letter "a" is blurred and cannot be recognized.

[0130] [Table 4]

[0131] As can be seen from Table 4, the types of pigment dispersion resins used to prepare the color inks and the white inks were different. It can be seen that if the acid value of polymer B used to prepare the white inks is 30 mgKOH / g or more higher than the acid value of polymer A used to prepare the color inks, the bleeding of the color inks can be significantly suppressed. [Industrial applicability]

[0132] The inkjet recording ink set of the present invention effectively suppresses the bleeding of color ink when white ink is printed as a solid layer on top of color ink. Therefore, the inkjet recording ink set of the present invention can maintain sharp outlines and vivid colors, and can be suitably used for package printing, label printing, and the like, using transparent resin film as the printing substrate.

Claims

1. A color ink containing a color pigment, a pigment dispersion resin A, a water-soluble organic solvent, and water. This is an inkjet recording ink set containing titanium dioxide particles, pigment dispersion resin B, a water-soluble organic solvent, and a white ink containing water. The aforementioned pigment dispersion resin A is Hydrophobic unit A derived from (meth) cyclohexyl acrylate, Hydrophobic unit B derived from (meth)acrylate, A polymer A containing a hydrophilic unit derived from (meth)acrylic acid, or a neutralized product obtained by neutralizing some or all of the carboxyl groups of polymer A. The pigment dispersion resin B is a polymer B containing hydrophilic units derived from (meth)acrylic acid, or a neutralized product in which some or all of the carboxyl groups of the polymer B are neutralized. An inkjet recording ink set in which the difference between the acid value of polymer B and the acid value of polymer A is 30 mg KOH / g or more.

2. The inkjet recording ink set according to claim 1, wherein the content of units derived from (meth)acrylate cyclohexyl in polymer B is 1% by mass or less.

3. The inkjet recording ink set according to claim 1, wherein the content of the hydrophobic unit A in the polymer A is 40% by mass or more and 60% by mass or less.

4. The inkjet recording ink set according to claim 1, wherein the mass ratio of hydrophobic unit B to hydrophobic unit A in polymer A (hydrophobic unit B / hydrophobic unit A) is 25% by mass or more and 100% by mass or less, expressed in mass percent.

5. The inkjet recording ink set according to claim 1, wherein the acid value of polymer A is 150 mg KOH / g or more.

6. The inkjet recording ink set according to claim 1, wherein the acid value of polymer B is 160 mg KOH / g or more.

7. An inkjet recording ink set according to claim 1, used for printing on a resin film as a printing substrate.

8. An inkjet recording method using an inkjet recording ink set according to any one of claims 1 to 7, Applying the aforementioned color ink to the printing substrate, An inkjet recording method comprising printing a solid color with white ink on top of the color ink applied to the printing substrate.

9. A method for manufacturing printed materials using an inkjet recording ink set according to any one of claims 1 to 7, Applying the aforementioned color ink to the printing substrate, A method for manufacturing a printed material, comprising printing a solid color with white ink on top of the color ink applied to the printing substrate.

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