Dispersion for ink, ink, ink set, and method for producing ink

Abstract

A dispersion for ink according to one aspect of the present disclosure comprises: a water-insoluble or poorly water-soluble ultraviolet absorber that is solid at 25°C; a polymer attached to at least part of the ultraviolet absorber; an antioxidant; and an aqueous medium. The ultraviolet absorber is dispersed in the aqueous medium.

Description

Ink dispersion, ink, ink set, and method for producing ink

[0001] The present invention relates to an ink dispersion, an ink, an ink set, and a method for making the ink.

[0002] Examples of inks for forming images on print media include water-based inks in which a coloring material such as a pigment is dispersed in an aqueous medium such as water. After applying such water-based inks to a print medium, the aqueous medium contained in the water-based ink is allowed to penetrate the print medium or evaporate, thereby forming an image on the print medium. While water-based inks are safer than inks containing organic solvents, they tend to dry more slowly. For this reason, improving the ink's drying properties is required when printing with water-based inks. Improving the ink's drying properties can, for example, reduce the power consumption and size of printing devices, including drying devices. Techniques for improving the ink's drying properties include, for example, incorporating an ultraviolet absorber into the water-based ink and then irradiating it with ultraviolet light to heat the water-based ink when drying the ink on the print medium. UV absorbers are also added to improve the weather resistance of dried inks.

[0003] As a composition containing an ultraviolet absorber, such as an ink containing an ultraviolet absorber, for example, Patent Document 1 describes an ink containing an ultraviolet absorber.

[0004] International Publication No. 2019 / 188522

[0005] An ink dispersion according to one aspect of the present disclosure includes a water-insoluble or poorly water-soluble ultraviolet absorber that is solid at 25° C., a polymer attached to at least a portion of the ultraviolet absorber, an antioxidant, and an aqueous medium. The ultraviolet absorber is dispersed in the aqueous medium.

[0006] In another aspect of the present disclosure, a method for producing an ink includes dispersing, in an aqueous medium, a water-insoluble or poorly water-soluble ultraviolet absorber that is solid at 25°C and has a polymer attached to at least a portion thereof, adding an antioxidant to the aqueous medium to prepare an ink dispersion, and mixing the ink dispersion with a colorant.

[0007] FIG. 1 is a schematic diagram showing the configuration of an example of an inkjet recording apparatus.

[0008] Hereinafter, embodiments of the present disclosure will be described, but the present disclosure is not limited to these.

[0009] [Ink Dispersion] The ink dispersion according to an embodiment of the present disclosure is an ink dispersion comprising: a water-insoluble or poorly water-soluble ultraviolet absorber that is solid at 25° C.; a polymer attached to at least a portion of the ultraviolet absorber; an antioxidant; and an aqueous medium, wherein the ultraviolet absorber is dispersed in the aqueous medium. The attachment of the polymer can be, for example, attachment to at least a portion of the surface of the ultraviolet absorber.

[0010] The present inventors have found that, in producing an aqueous ink containing the above-mentioned UV absorber, it is preferable to use a dispersion in which the UV absorber has been pre-dispersed in an aqueous medium, rather than using the UV absorber as is, because this improves the dispersibility of the UV absorber. The present inventors have also noted that when producing an ink (aqueous ink) using such a dispersion, i.e., when using the dispersion as an ink dispersion, the dispersion may be stored after production until it is used to produce the ink. Based on these findings, the present inventors have found that the ink dispersion is required to exhibit little change over time that would adversely affect the performance of the ink produced thereafter, specifically, to suppress discoloration, etc.

[0011] The ink dispersion is used in the production of ink to obtain an ink (water-based ink) containing an ultraviolet absorber. The ink dispersion may be stored, for example, at 25°C for several tens of days before being used in the production of ink. The present inventors have found that, for example, in the case of a dispersion in which an ultraviolet absorber is simply dispersed in water, the dispersion may discolor if stored as described above. Furthermore, when ink is produced using this discolored dispersion, the color tone of the print may be adversely affected, and suitable ink may not be obtained.

[0012] Therefore, the present inventors conducted various studies and found that an ink dispersion according to one embodiment of the present disclosure can achieve the above object.

[0013] The ink dispersion according to an embodiment of the present disclosure is less susceptible to changes over time that would adversely affect the performance of ink, even after storage as described above, and for example, discoloration that would adversely affect the performance of inks produced thereafter can be sufficiently suppressed. This is believed to be due to the following reasons.

[0014] First, it is believed that the discoloration over time of a dispersion in which an ultraviolet absorber is simply dispersed in water is due to oxidative decomposition of the ultraviolet absorber. The antioxidant contained in the ink dispersion contributes to suppressing the oxidative decomposition. On the other hand, the inclusion of the antioxidant tends to reduce the dispersibility of the ultraviolet absorber in the ink dispersion, but the attachment of the polymer to the ultraviolet absorber increases the dispersibility. For these reasons, the ink dispersion can suppress the oxidative decomposition by the antioxidant while maintaining the dispersibility of the ultraviolet absorber. Therefore, the effect of suppressing discoloration of the ink dispersion by the antioxidant can be suitably achieved. For these reasons, it is believed that the ink dispersion can suppress discoloration and suppress changes over time that adversely affect ink performance.

[0015] (UV Absorber) The UV absorber is not particularly limited as long as it is a water-insoluble or poorly water-soluble UV absorber that is solid at 25°C. Here, "water-insoluble" means that the absorber is insoluble or almost insoluble in water, specifically, that the solubility in water at 25°C is less than 1 mg / L (less than 0.001 g / L). "Poorly water-soluble" means that the absorber is only slightly soluble in water, specifically, that the solubility in water at 25°C is 1 mg / L or more and 100 mg / L or less (0.001 g / L or more and 0.1 g / L or less). That is, a water-insoluble or poorly water-soluble UV absorber is a UV absorber that has a solubility in water at 25°C of 100 mg / L or less. Examples of the UV absorber include UV absorbers that can be contained in ink and heat the ink by irradiating the ink with UV light. Specific examples of the ultraviolet absorber include dihydroxybenzophenone-based ultraviolet absorbers, benzotriazole-based ultraviolet absorbers, hydroxyphenyltriazine-based ultraviolet absorbers, dipropylene glycol-based ultraviolet absorbers, cyanoacrylate-based ultraviolet absorbers, benzotriazine-based ultraviolet absorbers, and benzoate-based ultraviolet absorbers. Examples of the dihydroxybenzophenone-based ultraviolet absorbers include 2,2',4,4'-tetrahydroxybenzophenone, 2,4-dihydroxybenzophenone, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, and 2-hydroxy-4-methoxybenzophenone. Examples of the benzotriazole-based ultraviolet absorbers include 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole, 2-(2'-hydroxy-5'-tert-octylphenyl)benzotriazole, 2,2'-methylenebis[6-(2H-benzotriazol-2-yl)-4-tert-octylphenol], 6-(2-benzotriazolyl)-4-tert-octyl-6'-tert-butyl-4'-methyl-2,2'-methylenebisphenol, bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate, and bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate.The ultraviolet absorber may have a phenol group in the molecule. This is because, when an ultraviolet absorber is irradiated with ultraviolet light, the electronic state of the phenol group moiety tends to become excited by absorbing the ultraviolet light, and heat is generated when this returns to the ground state. The ultraviolet absorber may also have a carbonyl group in the molecule. Therefore, the ultraviolet absorber may have a phenol group and a carbonyl group in the molecule, and examples thereof include the dihydroxybenzophenone ultraviolet absorbers, more specifically 2,2',4,4'-tetrahydroxybenzophenone. The ultraviolet absorbers may be used alone or in combination of two or more.

[0016] As described above, the ultraviolet absorber is solid at 25°C. The size of the ultraviolet absorber may be such that the particle diameter of the ultraviolet absorber to which the polymer is attached is the particle diameter described below (median diameter 150 nm or less). Specifically, the median diameter of the ultraviolet absorber may be 120 nm or less, 10 nm to 100 nm, or 30 nm to 80 nm. When the size of the ultraviolet absorber is within the above range, the ink can be more effectively heated when the ultraviolet absorber is contained in the ink and the ink is irradiated with ultraviolet light. The median diameter is the 50% particle diameter (D50) in a volume-based cumulative particle size distribution, and examples thereof include the median diameter measured using a light scattering particle size distribution analyzer (e.g., ELSZneo manufactured by Otsuka Electronics Co., Ltd.).

[0017] (Polymer) The polymer is not particularly limited as long as it is a polymer that is attached to at least a portion of the UV absorber. Examples of the polymer include polymers that can enhance the dispersibility of the UV absorber in the aqueous medium by adhering to the surface of the UV absorber. By adhering the polymer to at least a portion of the surface of the UV absorber, the dispersibility of the UV absorber in the aqueous medium is enhanced, thereby enhancing the dispersibility of the UV absorber in the ink dispersion. More specifically, examples of the polymer include polymers that can form an emulsion by dispersing the UV absorber to which the polymer is attached in the aqueous medium as emulsion particles. Furthermore, the polymer may be attached to at least a portion of the surface of the UV absorber, or may coat the surface of the UV absorber. In other words, the emulsion particles may be particles to which the polymer is attached to at least a portion of the surface of the UV absorber, and may be particles in which the surface of the UV absorber is coated with the polymer. Examples of the polymer include resins capable of forming emulsion particles in an aqueous medium. More specifically, examples include resins having a carbonyl group in the molecule (carbonyl group-containing resins), such as styrene-acrylic acid copolymers, polyvinyl alcohol, polyurethane, and ethylene-vinyl acetate copolymers. Examples of the carbonyl group-containing resin include (meth)acrylic acid resins, styrene-(meth)acrylic acid copolymers, styrene-maleic acid copolymers, styrene-(meth)acrylic acid-maleic acid copolymers, and styrene-(meth)acrylic acid-(meth)acrylic acid ester-maleic acid copolymers. (Meth)acrylic acid includes acrylic acid and methacrylic acid. The copolymers herein may be random copolymers or block copolymers. These polymers may be used alone or in combination of two or more. The polymer may also be the carbonyl group-containing resin, such as a styrene-(meth)acrylic acid copolymer, more specifically a styrene-acrylic acid copolymer.As the polymer, an amine-based polymer or a phosphoric acid-based polymer can also be used.

[0018] The molecular weight of the polymer is not particularly limited, but may be, for example, a weight average molecular weight Mw of 5,000 to 100,000, or 8,000 to 50,000. When the molecular weight of the polymer is within the above range, the dispersibility of the ultraviolet absorber in the aqueous medium can be further improved and the high dispersibility can be maintained. The weight average molecular weight may be measured by a general molecular weight measurement method, and specifically, a value measured using gel permeation chromatography (GPC) can be used.

[0019] The acid value of the polymer is not particularly limited, and may be, for example, 30 mgKOH / g or more and 500 mgKOH / g or less, or 50 mgKOH / g or more and 300 mgKOH / g or less. When the acid value of the polymer is within the above range, the dispersibility of the ultraviolet absorber in the aqueous medium can be further improved and the high dispersibility can be further maintained. Here, the acid value refers to the amount (mg) of potassium hydroxide (KOH) required to neutralize free acid in 1 g of sample.

[0020] The polymer may be crosslinked to prevent the polymer from being released from the UV absorber. As described above, the polymer may adhere to the surface of the UV absorber, thereby increasing the dispersibility of the UV absorber in the aqueous medium. For example, in such cases, a crosslinked polymer is less likely to release from the UV absorber, thereby maintaining the dispersibility of the UV absorber in the aqueous medium at a high level. Crosslinking refers to the formation of a bond between a portion of the hydrophilic group moiety of the polymer and a crosslinking agent, and the degree of crosslinking indicates the proportion of such bonds. The degree of crosslinking of the polymer may be 10 mol% to 70 mol%, 20 mol% to 60 mol%, 20 mol% to 50 mol%, or 30 mol% to 50 mol%. When the degree of crosslinking of the polymer is within the above range, the dispersibility of the UV absorber in the aqueous medium can be further increased and the high dispersibility can be maintained. In other words, the release can be suppressed while preventing the polymer from being excessively crosslinked, which would otherwise hinder the effect of increasing the dispersibility of the UV absorber in the aqueous medium. The degree of crosslinking herein refers to the ratio (mol %) of crosslinked structural units (repeating units) to all structural units (repeating units) of the polymer.

[0021] Examples of crosslinking of the polymer include crosslinking using a crosslinking agent. Specifically, crosslinking of the polymer attached to the UV absorber using a crosslinking agent can be exemplified. The crosslinking agent is not particularly limited as long as it can crosslink the polymer. The crosslinking agent varies depending on the polymer, and examples include compounds having an epoxy group in the molecule, and may also be compounds having multiple epoxy groups in the molecule (polyfunctional epoxy compounds) to facilitate crosslinking. Examples of the polyfunctional epoxy compounds include trimethylolpropane polyglycidyl ether.

[0022] (Particles with a polymer attached to an ultraviolet absorber: emulsion particles) As described above, in the ink dispersion, the ultraviolet absorber with the polymer attached is dispersed in the aqueous medium. The particle diameter of the particles (emulsion particles) with the polymer attached to the dispersed ultraviolet absorber may be 150 nm or less, 10 nm to 120 nm, or 30 nm to 100 nm in terms of median diameter. When the particle diameter of the emulsion particles is within the above range, the ink can be more effectively heated when the emulsion particles are contained in the ink and the ink is irradiated with ultraviolet light. The median diameter is the 50% particle diameter (D50) in a volume-based cumulative particle size distribution, and examples thereof include the median diameter measured using a light-scattering particle size distribution analyzer (e.g., ELSZneo manufactured by Otsuka Electronics Co., Ltd.).

[0023] (Antioxidant) The antioxidant is not particularly limited as long as it is an antioxidant that may be contained in the ink. Examples of the antioxidant include ascorbic acid (vitamin C) ((R)-3,4-dihydroxy-5-((S)-1,2-dihydroxyethyl)furan-2(5H)-one), tocopherol (vitamin E), catechin, sodium sulfite, potassium sulfite, tris(3,5-di-tert-butyl-4-hydroxybenzyl isocyanurate), pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tris(2,4-di-tert-butylphenyl)phosphite, 3,9-bis(octadecyloxy)-2,4 ,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane, triisodecyl phosphite, sodium hypophosphite, potassium pyrosulfite (potassium metabisulfite), dibutylhydroxytoluene (butylated hydroxytoluene: BHT) (2,6-di-tert-butyl-p-cresol), butylhydroxyanisole (BHA), propyl gallate, sulfur dioxide, rosmarinic acid, rutin, ascorbyl palmitate (ascorbyl palmitate), ascorbyl dipalmitate, sodium erythorbate, and sodium pyrosulfite. Of these, sodium sulfite, ascorbic acid, and tris(2,4-di-tert-butylphenyl) phosphite are preferred. These antioxidants may be used alone or in combination of two or more.

[0024] (Aqueous Medium) The aqueous medium is not particularly limited as long as it contains water. The aqueous medium may be, for example, an aqueous medium containing water as a main component (e.g., 50% by mass or more) or an aqueous medium consisting of water, but the aqueous medium may be water alone. The content of the water may be, for example, 50% by mass or more, 90% by mass or more, or even 100% by mass, relative to the aqueous medium. The aqueous medium may also contain water and an organic solvent. That is, the aqueous medium may be an aqueous medium containing water and an organic solvent. The aqueous medium may also contain components that may be generated when the emulsion particles are formed in the aqueous medium.

[0025] (Other Components) The ink dispersion only needs to contain the UV absorber, the polymer, the antioxidant, and the aqueous medium, and may also contain other components. On the other hand, since necessary components can be added to the ink produced using the ink dispersion during ink production, the ink dispersion does not need to contain the other components. The ink dispersion may contain the other components, but may also be a dispersion consisting of the UV absorber, the polymer, the antioxidant, and the aqueous medium.

[0026] (Dispersion) In the ink dispersion, the ultraviolet absorber having the polymer attached thereto (particles in which the polymer is attached to the ultraviolet absorber) is dispersed in the aqueous medium. This dispersion may be such that the ultraviolet absorber is uniformly spread in the aqueous medium, and examples thereof include a dispersion in which an emulsion is formed in which particles in which the polymer is attached to the ultraviolet absorber are dispersed as emulsion particles.

[0027] (Content) The content of the UV absorber is not particularly limited, and may be, for example, 1% by mass or more and 7% by mass or less, or 2% by mass or more and 6% by mass or less, relative to the ink dispersion. If the content of the UV absorber in the ink dispersion is low, a larger amount of the ink dispersion is added during ink production to ensure that the necessary amount of UV absorber is contained in the ink. In other words, a larger amount of the aqueous medium is added as a component of the ink dispersion. If a larger amount of the aqueous medium is added as a component of the ink dispersion, a smaller amount of the aqueous medium alone is added during ink production, which may make ink production difficult. By increasing the content of the UV absorber in the ink dispersion, the amount of the aqueous medium alone can be increased during ink production. As a result, ink production becomes easier.

[0028] The content of the ultraviolet absorber (the emulsion particles) to which the polymer is attached is not particularly limited, and may be, for example, 5% by mass or more and 35% by mass or less, or 10% by mass or more and 30% by mass or less, relative to the ink dispersion. When the contents of the ultraviolet absorber and the emulsion particles are within the above ranges, the ink dispersion can be incorporated into ink and the ink can be irradiated with ultraviolet light, while maintaining suitable dispersibility of the ultraviolet absorber.

[0029] The content of the polymer is not particularly limited, but may be, for example, 4% by mass to 28% by mass, or 8% by mass to 24% by mass, relative to the mass of the ink dispersion. When the content of the polymer is within this range, the dispersibility of the ultraviolet absorber in the aqueous medium can be further increased and the high dispersibility can be maintained.

[0030] The content of the antioxidant is not particularly limited, but may be, for example, 5% by mass or more and 30% by mass or less, or 8% by mass or more and 15% by mass or less, relative to the UV absorber. As described above, the antioxidant is used to suppress discoloration of the ink dispersion caused by the UV absorber. Therefore, the higher the content of the antioxidant, the more effectively discoloration of the ink dispersion can be suppressed. Therefore, when the content of the antioxidant is within the above range, changes over time such as discoloration can be more effectively suppressed.

[0031] The content of the aqueous medium is not particularly limited, but may be 70% by mass or more and 90% by mass or less, or 75% by mass or more and 85% by mass or less, relative to the mass of the ink dispersion. When the content of the aqueous medium is within the above range, the dispersibility of the ultraviolet absorber in the aqueous medium can be further increased and the high dispersibility can be further maintained.

[0032] When the contents of the ultraviolet absorber, the polymer, the antioxidant, and the aqueous medium are each within the above ranges, a more suitable ink dispersion can be obtained, specifically, the ink dispersion has better dispersibility and can be more effectively prevented from changing over time, such as discoloration.

[0033] (Ink Dispersion) As described above, discoloration of a dispersion containing the UV absorber is believed to be due to oxidative decomposition of the UV absorber. Specifically, when a UV absorber having a phenol group in the molecule, such as a dihydroxybenzophenone-based UV absorber, is used as the UV absorber, oxidative decomposition is believed to occur via the Dakin oxidation reaction or the like. When 2,2',4,4'-tetrahydroxybenzophenone is used as the UV absorber, it is oxidatively decomposed into 2,4-hydroxybenzoic acid and 3,6-dihydroxy-9H-xathen-9-one, for example. This oxidative decomposition is believed to proceed more rapidly in a basic environment. Furthermore, when a carbonyl group-containing resin, such as a styrene-(meth)acrylic acid copolymer, is used as the polymer in the ink dispersion, a base, such as sodium hydroxide, may be added to enhance the dispersibility of emulsion particles. In such cases, the oxidative decomposition is more likely to proceed. Even in such a dispersion, the discoloration can be suppressed by including an antioxidant in the ink dispersion.

[0034] When an ultraviolet absorber contains a phenol group in its molecule, it is easily excited by ultraviolet light and tends to have high ultraviolet absorption ability. However, when an ultraviolet absorber contains a phenol group in its molecule, it tends to be easily decomposed, as described above. If an antioxidant is added to an ink dispersion containing such an ultraviolet absorber, the ultraviolet absorption ability of the ink can be increased and discoloration can be suppressed when the ink is made. Furthermore, when an ultraviolet absorber contains a carbonyl group in its molecule, it tends to be easily excited by ultraviolet light and tends to have high ultraviolet absorption ability.

[0035] As described above, the ink dispersion can further suppress discoloration due to the resistance of the UV absorber to decomposition. For example, the amount of components decomposed from the UV absorber after storage at 25°C for 30 days may be 5% by mass or less, 3% by mass or less, or 2% by mass or less, relative to the UV absorber contained in the ink dispersion before storage. The smaller the amount of decomposition, the more effectively discoloration can be suppressed. In practice, however, the limit is considered to be approximately 1% by mass, and therefore the lower limit of the amount of decomposition may be, for example, 1% by mass or more. Here, the components decomposed from the UV absorber vary depending on the type of UV absorber. Therefore, the method for measuring the amount of decomposition varies depending on the type of UV absorber and the type of decomposed components, but can be analyzed by creating a calibration curve using liquid chromatography-mass spectrometry (LC-MS).

[0036] The ink dispersion has low absorbance in the visible light region, thereby suppressing discoloration so as to reduce the effect on the color of the ink produced thereafter. A low absorbance in the visible light region specifically refers to the absorbance at a wavelength of 450 nm, and the value may be, for example, 0.9 or less, 0.8 or less, or 0.5 or less. A lower absorbance at a wavelength of 450 nm can better suppress discoloration, but in practice, a value of about 0.3 is considered to be the limit, so the lower limit of the absorbance at a wavelength of 450 nm can be, for example, 0.3 or more.

[0037] As described above, the ink dispersion can suppress discoloration. For example, the absorbance at a wavelength of 450 nm after storage at 25°C for 30 days (absorbance after storage) may be 1.5 or less, 1.2 or less, or 1.0 or less. The lower the absorbance after storage, the better the suppression of discoloration. However, in practice, the limit is about 0.3, so the absorbance may be 0.3 or more. Furthermore, the rate of change of the height of the absorbance peak at a wavelength of 300 to 400 nm after storage relative to the height of the absorbance peak at a wavelength of 300 to 400 nm before storage may be 20% or less, 15% or less, or 10% or less. The lower the rate of change, the better the suppression of discoloration. However, in practice, the limit is considered to be about 3%, so the lower limit of the rate of change may be, for example, 3% or more. The absorbance may be, for example, the absorbance measured using a spectrophotometer such as an ultraviolet-visible-near-infrared spectrophotometer. The absorbance at a wavelength of 450 nm may be measured using a spectrophotometer, etc. The height of the absorbance peak at wavelengths of 300 to 400 nm may be measured using a spectrophotometer, etc.

[0038] The viscosity of the ink dispersion at 32°C is not particularly limited, but may be, for example, 10 mPa·s or less, 8 mPa·s or less, or 6 mPa·s or less, from the viewpoint of ease of use in ink production, etc. A lower viscosity has the advantage of ease of use in ink production, but in reality, the viscosity of water (approximately 0.89 mPa·s) is considered to be the limit, so the lower limit of the viscosity may be, for example, 0.89 mPa·s or more. Note that the viscosity is measured under conditions of 32°C and can be measured using, for example, a general viscometer or rheometer such as a vibration viscometer or a falling-ball viscometer. Furthermore, the rate of change of the viscosity after 30 days of storage at 25°C relative to the viscosity before storage may be 100% or less, 50% or less, or 30% or less.

[0039] (Method for Producing Ink Dispersion) The method for producing the ink dispersion is not particularly limited as long as it can produce the ink dispersion. Examples of methods for producing the ink dispersion include a method in which the UV absorber having the polymer attached to at least a portion of its surface (particles of the UV absorber having the polymer attached to at least a portion of its surface) is dispersed in an aqueous medium, and the antioxidant is added to the aqueous medium. More specifically, the method for producing the ink dispersion includes first dissolving the UV absorber and the polymer in a solvent capable of dissolving them. The solvent is not particularly limited as long as it can dissolve the UV absorber and the polymer, and varies depending on the type of UV absorber and the type of polymer. Examples of the solvent include organic solvents such as methyl ethyl ketone (MEK). Next, a basic aqueous solution is added to the solution in which the UV absorber and the polymer are dissolved while stirring. This results in the UV absorber having the polymer attached thereto. The basic aqueous solution is not particularly limited as long as it can adhere the polymer dissolved in the solvent to the UV absorber, and examples thereof include a sodium hydroxide aqueous solution. The liquid to which the basic aqueous solution has been added is subjected to a dispersion treatment using a disperser or the like. This improves the dispersibility of the UV absorber to which the polymer has been attached. The disperser is not particularly limited as long as it can improve the dispersibility of the UV absorber, and examples thereof include an ultrasonic homogenizer. The solvent is then removed from the dispersion-treated liquid using an evaporator or the like. This results in an emulsion (dispersion) in which the UV absorber to which the polymer has been attached is dispersed in an aqueous medium derived from the basic aqueous solution. The crosslinking agent may then be added to this emulsion to crosslink the polymer attached to the UV absorber. The antioxidant is then added to the resulting emulsion. This results in the ink dispersion.

[0040] [Ink] An ink according to another embodiment of the present disclosure is an ink containing a colorant and the ink dispersion. As long as the ink contains the colorant and the ink dispersion, it may contain other components such as components contained in ink.

[0041] (Coloring Material) The coloring material is not particularly limited as long as it can be used as a coloring material contained in ink. Examples of the coloring material include coloring materials contained in inkjet inks, and specific examples include pigments.

[0042] The pigment is not particularly limited as long as it can form a desired image (i.e., can express the colors that constitute the desired image), and examples thereof include known organic pigments and inorganic pigments. Examples of organic pigments include azo pigments, polycyclic pigments, nitro pigments, nitroso pigments, and aniline black. Examples of azo pigments include azo lake pigments, insoluble azo pigments, condensed azo pigments, and chelate azo pigments. Examples of polycyclic pigments include phthalocyanine pigments, perylene pigments, anthraquinone pigments, quinacridone pigments, dioxandinone pigments, thioindigo pigments, isoindolinone pigments, and quinophthaloni pigments. Examples of inorganic pigments include carbon blacks such as acetylene black and lamp black.

[0043] The pigments can also be classified according to color, and examples thereof include yellow pigments, orange pigments, red pigments, blue pigments, green pigments, and black pigments.

[0044] Examples of the yellow pigment include C.I. Pigment Yellow 12, C.I. Pigment Yellow 13, C.I. Pigment Yellow 14, C.I. Pigment Yellow 15, C.I. Pigment Yellow 17, C.I. Pigment Yellow 74, C.I. Pigment Yellow 93, C.I. Pigment Yellow 94, C.I. Pigment Yellow 95, C.I. Pigment Yellow 109, C.I. Pigment Yellow 110, Pigment Yellow 120, C.I. Pigment Yellow 128, C.I. Pigment Yellow 138, C.I. Pigment Yellow 139, C.I. Pigment Yellow 151, C.I. Pigment Yellow 154, C.I. Pigment Yellow 155, C.I. Examples of pigments that can be used include C.I. Pigment Yellow 173, C.I. Pigment Yellow 180, C.I. Pigment Yellow 185, and C.I. Pigment Yellow 193.

[0045] Examples of the orange pigment include C.I. Pigment Orange 31, C.I. Pigment Orange 34, C.I. Pigment Orange 36, C.I. Pigment Orange 43, C.I. Pigment Orange 61, C.I. Pigment Orange 63, and C.I. Pigment Orange 71.

[0046] Examples of the red pigment include C.I. Pigment Red 2, C.I. Pigment Red 3, C.I. Pigment Red 5, C.I. Pigment Red 6, C.I. Pigment Red 7, C.I. Pigment Red 15, C.I. Pigment Red 16, C.I. Pigment Red 48:1, C.I. Pigment Red 53:1, C.I. Pigment Red 57:1, C.I. Pigment Red 122, C.I. Pigment Red 123, C.I. Pigment Red 139, C.I. Pigment Red 144, C.I. Pigment Red 149, C.I. Pigment Red 150, C.I. Pigment Red 166, C.I. Examples of pigments include C.I. Pigment Red 177, C.I. Pigment Red 178, C.I. Pigment Red 202, and C.I. Pigment Red 222.

[0047] Examples of the blue pigment include C.I. Pigment Blue 15, C.I. Pigment Blue 15:2, C.I. Pigment Blue 15:3, and C.I. Pigment Blue 16.

[0048] Examples of the green pigment include C.I. Pigment Green 7.

[0049] Examples of the black pigment include C.I. Pigment Black 7.

[0050] As the coloring material, the pigments may be used alone or in combination of two or more.

[0051] The content of the coloring material (pigment) is not particularly limited, but from the viewpoint of increasing image density while maintaining high ink fluidity, it may be, for example, 2% by mass or more and 10% by mass or less, or 3% by mass or more and 8% by mass or less, relative to the total amount of the ink. A high content of the coloring material (pigment) (for example, 2% by mass or more) tends to increase image density, while a low content of the coloring material (pigment) (for example, 10% by mass or less) increases ink fluidity.

[0052] (Other Components: Aqueous Medium) As described above, the ink contains the ink dispersion, and therefore includes the aqueous medium contained in the ink dispersion. The ink may further include an aqueous medium (aqueous medium for ink) in addition to the aqueous medium contained in the ink dispersion. The aqueous medium for ink is not particularly limited as long as it is a medium containing water, and examples thereof include the same aqueous medium as the aqueous medium contained in the pretreatment liquid, such as an organic solvent contained in the aqueous medium and the content thereof. The content of the aqueous medium (the total content of the aqueous medium contained in the ink dispersion and the aqueous medium for ink) is not particularly limited and may be the remainder [the remainder of components other than the aqueous components (e.g., colorant, binder resin, and surfactant described below)], or may be, for example, 70% by mass or more and 90% by mass or less, or 75% by mass or more and 80% by mass or less, relative to the total amount of the ink.

[0053] (Other Components: Binder Resin) The ink may contain a binder resin for the purpose of improving image fixation, etc. The binder resin may exist in the form of particles dispersed in an aqueous medium. The binder resin can function as a binder that binds the print medium (printed material) and coloring materials such as pigments. Therefore, by including the binder resin in the ink, a printed material with excellent image fixation can be obtained.

[0054] The binder resin is not particularly limited, and examples thereof include urethane resin, acrylic resin, methacrylic resin, styrene-acrylic acid copolymer, styrene-methacrylic acid copolymer, styrene-maleic acid copolymer, vinylnaphthalene-acrylic acid copolymer, vinylnaphthalene-methacrylic acid copolymer, and vinylnaphthalene-maleic acid copolymer. The content of the binder resin is not particularly limited, but from the viewpoint of improving image fixation while maintaining ink fluidity, it may be, for example, 2% by mass or more and 20% by mass or less, or 5% by mass or more and 15% by mass or less, relative to the total amount of the ink. A high content of the binder resin (for example, 2% by mass or more) tends to improve image fixation, while a low content of the binder resin (for example, 20% by mass or less) improves ink fluidity.

[0055] (Other Components: Surfactants) The ink may contain a surfactant for the purpose of adjusting the surface tension, etc. Furthermore, by containing a surfactant in the ink, the wettability of the ink to the print medium (substrate to be printed) is also improved. The surfactant is not particularly limited, but may be any of a nonionic surfactant, a cationic surfactant, an anionic surfactant, an amphoteric surfactant, etc. The content of the surfactant is not particularly limited, but may be, for example, 0.1% by mass or more and 5% by mass or less, or 0.3% by mass or more and 3% by mass or less, relative to the total amount of the ink, in order to further suppress the occurrence of bleeding.

[0056] (Other Components: Additives) The ink may further contain known additives (more specifically, a dissolution stabilizer, an anti-drying agent, an antioxidant, a viscosity adjuster, a pH adjuster, an anti-mold agent, etc.) as needed.

[0057] (Applications) The ink may be any ink used to form an image on a print medium (substrate), and may be used, for example, as an inkjet ink. That is, the ink may be used when forming an image using an inkjet recording device. Furthermore, since the ink contains the ultraviolet absorber, it is heated by irradiating ultraviolet light, resulting in an ink with excellent drying properties. Therefore, when the ink is used, for example, when forming an image using an inkjet recording device, problems caused by poor ink drying properties can be suppressed, and a suitable image can be formed on the print medium. The print medium on which an image is formed using the ink is not particularly limited, and examples thereof include paper, film, and fabric. Examples of paper include plain paper, high-quality paper, matte-coated paper, cast paper, and photo paper. Examples of film include resins such as polyethylene terephthalate (PET) film. Examples of fabric include woven fabrics, knitted fabrics, and nonwoven fabrics. Printing using the ink exhibits excellent drying properties, allowing images to be formed favorably even on print media with low ink permeability. In this respect, the print medium may be the film, and the ink may also be used when forming an image on a film using an inkjet recording device, for example.

[0058] The ink is not particularly limited as long as it can be used to form an image on a print medium, and examples thereof include inkjet recording devices. The inkjet recording device is also not particularly limited as long as it can form an image using the ink. An example of the inkjet recording device is the inkjet recording device shown in FIG. 1. FIG. 1 is a schematic diagram showing the configuration of an example of an inkjet recording device 10.

[0059] 1 , the inkjet recording apparatus 10 includes a discharge unit 12 having at least one inkjet head 21, a transport unit 11 that transports a print medium (substrate) 101 to which inkjet ink discharged from the inkjet head 21 is to be adhered, and an ultraviolet irradiation unit 14 that irradiates ultraviolet light onto the print medium 101 to which the ink has been adhered. The inkjet recording apparatus 10 may further include a first heating unit 13 that heats the print medium 101 before the ink discharged from the discharge unit 12 adheres to the print medium 101, and a second heating unit 15 that heats the print medium 101 after the ink discharged from the discharge unit 12 has adhered to the print medium. The inkjet recording apparatus 10 also includes a control unit 16 that controls the transport unit 11, the discharge unit 12, the first heating unit 13, the ultraviolet irradiation unit 14, the second heating unit 15, etc.

[0060] The transport unit 11 transports the print medium 101 from a supply roller 11A to a collection roller 11B. The inkjet recording device 10 further includes the discharge unit 12, the first heating unit 13, the ultraviolet irradiating unit 14, and the second heating unit 15 along a transport path of the print medium 101 transported by the transport unit 11. The supply roller 11A supplies the print medium 101 to the discharge unit 12, onto which ink discharged from the inkjet head 21 adheres. The collection roller 11B collects the print medium 101 supplied to the discharge unit 12. The transport unit 11 may include rollers 19A to 19D for passing the print medium 101 along a predetermined transport path when transporting the print medium 101 from the supply roller 11A to the collection roller 11B. The supply roller 11A, the collection roller 11B, and the rollers 19A to 19D may each be a drive roller or a driven roller, as long as they can transport the print medium 101 from the supply roller 11A to the collection roller 11B.

[0061] The print medium 101 transported by the transport unit 11 may be in a long shape or in a sheet-like shape. When the print medium 101 is in a sheet-like shape, the transport unit 11 may include a transport belt. In this case, the transport unit 11 may transport the print medium 101 by placing the print medium 101 on the transport belt and transporting the transport belt in that state. Furthermore, as described above, the print medium 101 may be paper or fabric, or may be a print medium with low permeability, specifically, a film.

[0062] The ejection unit 12 ejects ink droplets toward the print medium 101, causing the ink to adhere to the print medium 101. The ejection unit 12 includes at least one inkjet head 21 that faces the print medium 101 and is directly responsible for ejecting ink, and an ink tank 17 that stores ink to be supplied to the inkjet head 21. The inkjet head 21 ejects ink supplied from the ink tank 17 toward the print medium 101. The inkjet head 21 is held so that an ejection surface 21a that ejects ink corresponds to the print medium 101. Note that an image may be printed by dividing it into a plurality of sections (for example, a grid pattern) and forming or not forming pixels (dots) in each section, or by forming pixels of varying sizes in stages.

[0063] The first heating unit 13 heats the print medium 101 to promote evaporation of the aqueous medium contained in the ink applied to the print medium 101. The first heating unit 13 may include a first heating roller 13A that contacts the print medium 101 and directly heats the print medium 101. The first heating roller 13A heats the print medium 101 and also functions as a roller in the transport unit 11. The first heating unit 13 may heat the print medium 101 by blowing hot air onto the print medium 101 instead of or in addition to heating with the first heating roller 13A. The heating of the print medium 101 by the first heating unit 13 may be performed before or after ink is applied to the print medium 101, or may be performed both before and after ink is applied to the print medium 101. That is, the first heating section 13 may be provided on the upstream side of the ejection section 12 in the transport direction of the printing medium 101, or on the downstream side, or on both sides.

[0064] The ultraviolet irradiation unit 14 irradiates the ink attached to the printing medium 101 with ultraviolet (UV) rays to heat the ink. The ink is easily heated because it contains the ultraviolet absorber. The ultraviolet irradiation unit 14 irradiates ultraviolet rays and therefore includes at least a light source 14A for irradiating ultraviolet rays. The ultraviolet irradiation unit 14 irradiates the ink attached to the printing medium 101 with ultraviolet rays, and therefore is performed after the ink has been attached to the printing medium 101. In other words, the ultraviolet irradiation unit 14 is provided downstream of the ejection unit 12 in the transport direction of the printing medium 101.

[0065] The second heating unit 15 heats the print medium 101, thereby heating the ink that has been attached to the print medium 101 and heated by the ultraviolet irradiation unit. This heating fixes an image formed with the colorant contained in the ink onto the print medium 101. The second heating unit 15 heats the ink that has been heated by the ultraviolet irradiation unit 14, and is therefore provided near the ultraviolet irradiation unit 14. The second heating unit 15 is located, for example, on the opposite side of the print medium 101 from the ultraviolet irradiation unit 14. The second heating unit 15 may also include a second heating roller 15A that contacts the print medium 101 and directly heats the print medium 101. The second heating roller 15A not only heats the print medium 101, but also functions as a roller in the transport unit 11. In addition, the second heating section 15 may heat the printing medium 101 by blowing hot air onto the printing medium 101 instead of or in addition to heating by the second heating roller 15A.

[0066] The control unit 16 controls the transport unit 11, the ejection unit 12, the first heating unit 13, the ultraviolet ray irradiation unit 14, the second heating unit 15, etc. Specifically, the control unit 16 also controls the transport speed of the print medium 101 in the transport unit 11 and the drive frequency at which the inkjet ink is ejected in the ejection unit 12. The control unit 16 includes a computer. The control unit 16 includes, for example, a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), an external storage device, etc., although not particularly shown.

[0067] As described above, the inkjet recording device is not particularly limited as long as it can form an image using the ink. As an example, the inkjet recording device shown in FIG. 1 can fix a high-quality image on the printing medium using the ink.

[0068] (Ink Manufacturing Method) The ink manufacturing method is not particularly limited as long as it can manufacture the ink. Examples of the ink manufacturing method include a method of mixing the ink dispersion (e.g., the ink dispersion manufactured by the ink dispersion manufacturing method) with the colorant. That is, examples of the ink manufacturing method include a method of dispersing the UV absorber having the polymer attached to at least a portion of its surface (particles of the UV absorber having the polymer attached to at least a portion of their surface) in an aqueous medium, adding an antioxidant to the aqueous medium to manufacture an ink dispersion, and then mixing the ink dispersion with the colorant. The mixing method is not particularly limited as long as it can manufacture the ink, and examples include mixing using a mixer such as a bead mill. The mixing time is also not particularly limited as long as it can manufacture the ink, and examples include 10 minutes to 180 minutes. Furthermore, the ink manufacturing method may involve manufacturing the ink by directly mixing the ink dispersion with the colorant without storing the manufactured ink dispersion. That is, the storage time may be 0 hours. On the other hand, since discoloration of the ink dispersion can be suppressed even during storage, the ink dispersion may be stored after its production and before mixing the ink dispersion with a colorant. The storage temperature is not particularly limited, but examples include 25°C or less, and may be 15°C or less. The storage time is not particularly limited, but may be, for example, 360 days or less, 180 days or less, 90 days or less, or 30 days or less. Furthermore, in terms of obtaining a suitable ink, the ink may be produced without the storage, i.e., with a storage time of 0 days. On the other hand, since the ink dispersion is suppressed from changing over time, a suitable ink can be produced even after the storage.

[0069] [Ink Set] The inks are formed into a plurality of inks of different colors by using colorants of different colors as the colorants. Different colors refer to differences in at least one of hue and brightness. These inks of different colors may be used as an ink set. That is, an ink set according to another embodiment of the present disclosure includes a plurality of inks of different colors, each of which is the ink (including the colorant and the ink dispersion). Furthermore, if the inks included in the ink set have different UV absorber contents, the ink with a higher UV absorber content may have a higher antioxidant content than the ink with a lower UV absorber content, for the following reasons. Even if the UV absorber content is high, discoloration can be sufficiently suppressed because the antioxidant content is high. Furthermore, an ink set with such a combination can also suppress bleeding. When forming an image using the ink set, for example, when using the inkjet recording apparatus shown in FIG. 1, the image may be formed by ejecting the inks included in the ink set from multiple inkjet heads 21, respectively.

[0070] As described above, this specification discloses various aspects of the technology, the main technologies of which are summarized below.

[0071] The ink dispersion according to a first aspect includes a water-insoluble or poorly water-soluble ultraviolet absorber that is solid at 25°C, a polymer attached to at least a portion of the ultraviolet absorber, an antioxidant, and an aqueous medium, wherein the ultraviolet absorber is dispersed in the aqueous medium.

[0072] The ink dispersion according to the second aspect is the ink dispersion according to the first aspect, in which the absorbance at a wavelength of 450 nm is 0.9 or less.

[0073] An ink dispersion according to a third aspect is the ink dispersion according to the first or second aspect, wherein the ultraviolet absorber has a phenol group in the molecule.

[0074] The ink dispersion according to a fourth aspect is the ink dispersion according to any one of the first to third aspects, wherein the degree of crosslinking of the polymer is 20 mol % or more and 50 mol % or less.

[0075] An ink dispersion according to a fifth aspect is the ink dispersion according to any one of the first to fourth aspects, wherein the content of the aqueous medium in the ink dispersion is 70% by mass or more and 90% by mass or less.

[0076] An ink dispersion according to a sixth aspect is the ink dispersion according to any one of the first to fifth aspects, wherein the content of the ultraviolet absorber in the ink dispersion is 1% by mass or more and 7% by mass or less.

[0077] An ink dispersion according to a seventh aspect is the ink dispersion according to any one of the first to sixth aspects, wherein the content of the antioxidant is 5% by mass or more and 30% by mass or less relative to the ultraviolet absorber.

[0078] An ink dispersion according to an eighth aspect is the ink dispersion according to any one of the first to seventh aspects, wherein the aqueous medium is water.

[0079] The ink dispersion according to a ninth aspect is the ink dispersion according to any one of the first to eighth aspects, wherein the viscosity at 32° C. is 10 mPa·sec or less.

[0080] An ink dispersion according to a tenth aspect is the ink dispersion according to any one of the first to ninth aspects, wherein the absorbance at a wavelength of 450 nm after storage at 25° C. for 30 days is 1.5 or less.

[0081] An ink dispersion according to an eleventh aspect is the ink dispersion according to any one of the first to tenth aspects, wherein after storage at 25°C for 30 days, the rate of change in the height of the absorbance peak at a wavelength of 300 to 400 nm relative to the height of the absorbance peak at a wavelength of 300 to 400 nm before storage is 20% or less.

[0082] An ink dispersion according to a twelfth aspect is the ink dispersion according to any one of the first to eleventh aspects, wherein the rate of change in viscosity after storage at 25°C for 30 days relative to the viscosity before storage is 100% or less.

[0083] The ink dispersion according to a thirteenth aspect is the ink dispersion according to any one of the first to twelfth aspects, wherein the amount of components decomposed from the ultraviolet absorber when stored at 25°C for 30 days is 5% by mass or less relative to the ultraviolet absorber contained in the ink dispersion before the storage.

[0084] The ink dispersion according to a fourteenth aspect is the ink dispersion according to any one of the first to thirteenth aspects, wherein the median diameter of the particles in which the polymer is attached to the ultraviolet absorber is 150 nm or less.

[0085] The ink according to the fifteenth aspect is an ink containing a colorant and the ink dispersion according to any one of the first to fourteenth aspects.

[0086] An ink set according to a sixteenth aspect is an ink set comprising a plurality of inks of different colors, each of which is an ink according to the fifteenth aspect.

[0087] An ink set according to a seventeenth aspect is the ink set according to the sixteenth aspect, wherein the plurality of inks include inks with different contents of the ultraviolet absorber, and the ink with a higher content of the ultraviolet absorber has a higher content of the antioxidant than the ink with a lower content of the ultraviolet absorber.

[0088] A method for producing an ink according to an eighteenth aspect includes dispersing, in an aqueous medium, a water-insoluble or poorly water-soluble ultraviolet absorber that is solid at 25°C and has a polymer attached to at least a portion thereof, and adding an antioxidant to the aqueous medium to prepare an ink dispersion, and mixing the ink dispersion with a colorant.

[0089] A method for producing an ink according to a nineteenth aspect is the method for producing an ink according to the eighteenth aspect, wherein after preparing the ink dispersion, the ink dispersion is stored before mixing the ink dispersion with a colorant.

[0090] According to the present disclosure, it is possible to provide an ink dispersion that is inhibited from changing over time in a way that adversely affects the performance of the ink. Furthermore, according to the present disclosure, it is possible to provide an ink containing the ink dispersion, an ink set including the ink, and a method for producing an ink containing the ink dispersion.

[0091] The present disclosure will be explained in more detail below using examples, but the scope of the present disclosure is not limited to these examples.

[0092] [Ink Dispersion] First, the ultraviolet absorber, polymer, crosslinking agent, and antioxidant used in the ink dispersion in the test examples will be described.

[0093] (Ultraviolet absorber) 2,2',4,4'-tetrahydroxybenzophenone (Uvinul 3050 manufactured by BASF Japan Ltd.) (solid at 25°C, slightly soluble in water)

[0094] (Polymer) Styrene-acrylic acid copolymer (JONCRYL 819 manufactured by BASF Japan Ltd., weight average molecular weight 14,500, acid value 75 mgKOH / g)

[0095] (Crosslinking agent) Multifunctional epoxy compound (trimethylolpropane polyglycidyl ether, Denacol EX-321 manufactured by Nagase ChemteX Corporation)

[0096] (Antioxidants) Antioxidant 1: Sodium sulfite (special grade reagent, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) Antioxidant 2: Ascorbic acid (special grade reagent, vitamin C, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) Antioxidant 3: Tris(2,4-di-tert-butylphenyl) phosphite (>98.0% reagent, manufactured by Tokyo Chemical Industry Co., Ltd.)

[0097] (Preparation of Ink Dispersion) First, the UV absorber and four times the mass of the polymer (g) were added to methyl ethyl ketone (MEK) and stirred to dissolve the UV absorber and the polymer in MEK. Next, while stirring the solution in which the UV absorber and the polymer were dissolved in a solvent, an aqueous solution of sodium hydroxide (NaOH) was added dropwise to the solution. The liquid to which the NaOH aqueous solution had been added was subjected to a dispersion treatment using an ultrasonic homogenizer (US-300T, manufactured by Nippon Seiki Seisakusho Co., Ltd.). This resulted in the UV absorber having the polymer attached thereto. Then, MEK was removed from the dispersion-treated liquid using an evaporator. This resulted in a dispersion in which the UV absorber having the polymer attached thereto was dispersed in water (aqueous medium) derived from the NaOH. The mass ratio of the UV absorber to the polymer in this dispersion was 1:4. This mass ratio may be between 1:2 and 1:6. The crosslinking agent was then added to this dispersion. This crosslinking caused the polymer attached to the UV absorber to be crosslinked. The degree of crosslinking [the ratio (mol %) of crosslinked structural units (repeating units) to all structural units (repeating units) of the polymer] was calculated from the acid value of the polymer, the chemical structure of the crosslinking agent, and the blending ratio of the polymer and the crosslinking agent, and the values ​​shown in Tables 1 to 3 were obtained. The antioxidant was added to the resulting dispersion. The ink dispersion was thus obtained. The contents of the UV absorber (emulsion particles) to which the polymer was attached and the aqueous medium are shown in Tables 1 to 3 as the contents relative to the total of the UV absorber, the polymer, and the aqueous medium (emulsion: liquid before the antioxidant was added). The content of the antioxidant is also shown in Tables 1 to 3 as the content relative to the total of the UV absorber, the polymer, and the aqueous medium (emulsion: liquid before the antioxidant was added). In Tables 1 to 3, test examples in which the antioxidant content is 0 mass % are examples in which the antioxidant was not added.

[0098] (Dispersion state) The dispersion state of the ink dispersion in each test example was visually confirmed. As a result, if it was confirmed that the ink dispersion was dispersed, it is indicated as "dispersed" in Tables 1 to 3, and if it was confirmed that the ink dispersion was not dispersed, it is indicated as "non-dispersed" in Table 1.

[0099] (Viscosity) The viscosity of the ink dispersion in each test example was measured using a rheometer (MCR302 manufactured by Anton Paar) at 32° C. The results obtained are shown in Tables 1 to 3.

[0100] (Absorbance at a wavelength of 450 nm) The absorbance at a wavelength of 450 nm of the ink dispersion in each test example was measured using an ultraviolet-visible-near-infrared spectrophotometer (V-670 manufactured by JASCO Corporation). The obtained results are shown in Tables 1 to 3.

[0101] (Average particle diameter D50 of emulsion particles) The average particle diameter D50 (median diameter) of the emulsion particles dispersed in the ink dispersion in each test example was measured using a light scattering particle size distribution measuring device (ELSZneo manufactured by Otsuka Electronics Co., Ltd.) The results obtained are shown in Tables 1 to 3.

[0102]

[0103]

[0104]

[0105] The ink dispersion in each test example was stored under the storage conditions (storage temperature and storage days) shown in Tables 4 to 6, and the ink dispersion after storage was evaluated as follows.

[0106] (Viscosity) The viscosity of the ink dispersion after storage in each test example was measured using a rheometer (MCR302 manufactured by Anton Paar) under the condition of 32°C, as described above. The results obtained are shown in Tables 4 to 6. When the ink dispersion gelled after storage, this is indicated as "gelled" in Tables 4 to 6.

[0107] (Change in viscosity) The change in viscosity of the ink dispersion after storage (viscosity after storage) in each test example relative to the viscosity of the ink dispersion before storage (viscosity before storage) in each test example was calculated using the following formula. The results are shown in Tables 4 to 6. When the ink dispersion gelled after storage, the viscosity after storage could not be measured, and therefore the change in viscosity could not be calculated, and this is indicated as "-" in Tables 4 to 6. Change in viscosity (%) = (viscosity after storage - viscosity before storage) / viscosity before storage x 100

[0108] (Absorbance at a wavelength of 450 nm) In the same manner as described above, the absorbance at a wavelength of 450 nm of the ink dispersion in each test example after storage was measured using an ultraviolet-visible-near-infrared spectrophotometer (V-670 manufactured by JASCO Corporation). The results obtained are shown in Tables 4 to 6. Note that when the absorbance of the ink dispersion after storage could not be measured, for example because the ink dispersion gelled after storage, this is indicated as "-" in Tables 4 to 6.

[0109] (Rate of Change in Absorbance Peak) First, the maximum absorbance value at wavelengths of 300 to 400 nm (height of the absorbance peak at wavelengths of 300 to 400 nm) of the ink dispersion in each test example before storage was measured using an ultraviolet-visible-near-infrared spectrophotometer (V-670, manufactured by JASCO Corporation). Similarly, the maximum absorbance value at wavelengths of 300 to 400 nm (height of the absorbance peak at wavelengths of 300 to 400 nm) of the ink dispersion in each test example after storage was measured using an ultraviolet-visible-near-infrared spectrophotometer (V-670, manufactured by JASCO Corporation). The rate of change in the absorbance peak height (height after storage) of the ink dispersion in each test example after storage relative to the absorbance peak height (height before storage) of the ink dispersion in each test example was calculated using the following formula. The results obtained are shown in Tables 4 to 6. In addition, when the absorbance of the ink dispersion after storage could not be measured due to gelation or the like, this is indicated as "-" in Tables 4 to 6. Absorbance peak change rate (%) = (height after storage - height before storage) / height before storage x 100

[0110] (Average particle diameter D50 of emulsion particles) The average particle diameter D50 (median diameter) of emulsion particles dispersed in the ink dispersion after storage in each test example was measured using a light scattering particle size distribution measuring device (ELSZneo manufactured by Otsuka Electronics Co., Ltd.) in the same manner as above. The results obtained are shown in Tables 4 to 6. Note that when the ink dispersion had gelled after storage, the D50 could not be measured, and this is indicated as "-" in Tables 4 to 6.

[0111] (Amount of decomposition components produced) The amount of decomposition components produced was measured by analysis using liquid chromatography mass spectrometry (LC-MS) with the aid of a calibration curve. The results are shown in Tables 4 to 6.

[0112]

[0113]

[0114]

[0115] (Ink) Ink was produced using the ink dispersion after storage in each test example as follows: A predetermined amount of ink dispersion containing propylene glycol as a humectant, a styrene-acrylic acid-based polymer as a fixing polymer, an acetylene-based substance as a surfactant, and an ultraviolet absorber in a pigment dispersion of each color (magenta, cyan, yellow, and black) was weighed out and stirred for 1 hour with a magnetic stirrer to produce the ink.

[0116] (ΔE) An ink film was formed on OK topcoat paper (manufactured by Oji Paper Co., Ltd.) using a bar coater, and the ink film was dried using a UV-LED irradiator. Subsequently, (L*, a*, b*) were measured using a spectrophotometer (eXact manufactured by X-Rite Corporation) to calculate ΔE. Products with ΔE<5 were judged to be good products.

[0117] (Printing Condition) Using the ink, solid printing was performed on a flatbed printing jig (an inkjet recording apparatus corresponding to FIG. 1 , equipped with the inkjet head, a prototype machine manufactured by Kyocera Corporation) in which inkjet heads (1200 dpi non-circulating heads (KJ4B-122) manufactured by Kyocera Corporation) were arranged in the transport direction, and an ink film was formed on a printing medium (OK topcoat paper manufactured by Oji Paper Co., Ltd.). The ink film was dried using a UV-LED irradiator, and samples that were visually free of streaks were rated "good (good product)," samples that had streaks but were judged to pose no problems in use were rated "passable," and samples that had streaks but were judged to pose problems in use were rated "unacceptable (defective)."

[0118] (Drying property) An ink film was formed on OK topcoat paper (manufactured by Oji Paper Co., Ltd.) using a bar coater. The ink film was dried using a UV-LED irradiator, and after drying, a peeling test was performed on the ink film using cellophane tape (No. 405-1P manufactured by Nichiban Co., Ltd.). The optical density (OD value) was measured using a spectrophotometer (eXact manufactured by X-Rite) before and after tape peeling, and a difference in OD value of 5% or less was rated as "good (good product)", 3% or less was rated as "passable", and more than 3% was rated as "unacceptable (defective)".

[0119]

[0120]

[0121]

[0122] Next, inks were produced in the same manner as the inks described above, except that the contents of the ink dispersion (the ink dispersion after storage) were adjusted so that the contents of the ultraviolet absorber and antioxidant 1 in the ink were the contents shown in Table 10. Specifically, inks using magenta pigments (magenta inks, Nos. 68 to 70) were produced in the same manner as ink No. 7, except that the contents of the ink dispersion were adjusted so that the contents of the ultraviolet absorber and antioxidant 1 in the ink were the contents shown in Table 10. Inks using cyan pigments (cyan inks, Nos. 71 to 73) were produced in the same manner as ink No. 23, except that the contents of the ink dispersion were adjusted so that the contents of the ultraviolet absorber and antioxidant 1 in the ink were the contents shown in Table 10. Inks using a yellow pigment (yellow inks, Nos. 74 to 76) were produced in the same manner as ink No. 24, except that the contents of the ink dispersion were adjusted so that the contents of the ultraviolet absorber and antioxidant 1 in the ink were as shown in Table 10. Inks using a black pigment (black inks, Nos. 77 to 79) were produced in the same manner as ink No. 25, except that the contents of the ink dispersion were adjusted so that the contents of the ultraviolet absorber and antioxidant 1 in the ink were as shown in Table 10.

[0123] The drying properties of each of the inks thus obtained were evaluated according to the above-mentioned method for evaluating drying properties. The results are shown in Table 10.

[0124]

[0125] (Ink Set) First, an ink set was prepared using each of the inks described above in the combination shown in Table 11. Using the ink set, an image for evaluation was formed on a printing medium (OK topcoat paper manufactured by Oji Paper Co., Ltd.) on a flatbed printing jig (an inkjet recording apparatus corresponding to FIG. 1 , equipped with the inkjet head, a prototype manufactured by Kyocera Corporation) in which inkjet heads (1200 dpi non-circulating heads (KJ4B-122) manufactured by Kyocera Corporation) were arranged in the transport direction.

[0126] (Bleeding) The formed image was checked under a microscope at 300x magnification. If bleeding was observed in the formed image but the bleeding was within 5 μm, the image was evaluated as "excellent", if the bleeding was within 10 μm, the image was evaluated as "good", if the bleeding was within 21 μm, the image was evaluated as "fair", and if the bleeding was more than 21 μm, the image was evaluated as "poor".

[0127]

[0128] Tables 1 to 9 reveal that the ink dispersions (Nos. 2 to 16 and 18 to 67) containing the antioxidant in an aqueous medium in which particles having a polymer attached to the UV absorber are dispersed exhibited a lower rate of change in absorbance and a lower amount of decomposition components, even after a specified period of storage, compared to the ink dispersion (No. 1) that did not contain the antioxidant. Furthermore, when the ink dispersions Nos. 2 to 16 and 18 to 67 were stored under specified conditions and then images were formed using inks produced using the ink dispersions after storage, the resulting images exhibited a small ΔE and excellent print quality and drying properties. Therefore, it was found that by including the antioxidant in an aqueous medium in which particles having a polymer attached to the UV absorber are dispersed, ink dispersions can be obtained that are inhibited from undergoing changes over time that adversely affect ink performance. This indicates that the ink dispersions containing the antioxidant and having the storage period of No. This was further evidenced by the fact that even when the storage period was longer than the 30 days specified in Example 1 (Nos. 8 to 10, 32 to 34, and 53 to 55), the rate of change in absorbance was lower and the amount of decomposition products was also lower compared to when the antioxidant was not contained (No. 1). Furthermore, when particles in which the polymer was attached to the UV absorber were not dispersed in an aqueous medium (No. 17), good ink dispersions and inks were not obtained. Furthermore, it was found that ink dispersions in which deterioration over time that adversely affects ink performance was suppressed were obtained regardless of whether sodium sulfite was used as the antioxidant (Nos. 2 to 16 and 18 to 25), ascorbic acid was used (Nos. 26 to 46), or tris(2,4-di-tert-butylphenyl) phosphite was used (Nos. 47 to 67). It was also found that regardless of the color, whether a magenta pigment (e.g., No. 4), a cyan pigment (No. 23), a yellow pigment (No. 24), or a black pigment (No. 25) was used, ΔE was small and images with excellent print quality and drying properties could be formed.

[0129] It was found that when the content of the aqueous medium relative to the ultraviolet absorber is 70% by mass or more and 90% by mass or less (Nos. 12, 13, 36, 37, 57, and 58), not only does the rate of change in absorbance become low and the amount of decomposition components generated become small, but also ink dispersions with a smaller rate of change in viscosity can be obtained compared to when the content of the aqueous medium is less than 70% by mass (Nos. 11, 35, and 56) and when it exceeds 90% by mass (Nos. 14, 38, and 59).

[0130] It was found that when the content of the antioxidant relative to the UV absorber was 5% by mass or more and 30% by mass or less (Nos. 5, 14 to 16, 27, 38 to 40, 48, and 59 to 61), ink dispersions were obtained that were more inhibited from changing over time, which would adversely affect ink performance, compared to when the content was less than 5% by mass (Nos. 2, 26, and 47).It was also found that when the content was 5% by mass or more and 30% by mass or less, ink dispersions with excellent dispersibility were obtained compared to when the content exceeded 30% by mass (Nos. 17, 41, and 62).

[0131] It was found that when the degree of crosslinking of the polymer was 20 mol % or more and 50 mol % or less (Nos. 19 to 21, 43 to 45, and 64 to 66), the dispersibility of the ultraviolet absorber was better and ink dispersions with excellent long-term storage stability were obtained, compared to when the degree of crosslinking was less than 20 mol % (Nos. 18, 42, and 63).It was also found that when the degree of crosslinking was 20 mol % or more and 50 mol % or less, the dispersibility of the ultraviolet absorber was better and ink dispersions with excellent long-term storage stability were obtained, compared to when the degree of crosslinking exceeded 50 mol % (Nos. 22, 46, and 67).

[0132] Table 10 shows that inks containing 8 to 12% by mass of the ultraviolet absorber (Nos. 68, 69, 71, 72, 74, 75, 77, and 78) exhibited superior drying properties compared to inks containing less than 8% by mass (Nos. 7, 23, 24, and 25) and inks containing more than 12% by mass (Nos. 70, 73, 76, and 79). This indicates that a high ultraviolet absorber content results in sufficient heating of the ink due to the ultraviolet ray irradiation, thereby improving drying properties. On the other hand, if the ultraviolet absorber content is too high, although the ink is heated on the side irradiated with ultraviolet rays, the ink may not be sufficiently heated internally, making it difficult to improve drying properties. These results indicate that a content of the ultraviolet absorber of 8 to 12% by mass of the ink further improves drying properties.

[0133] From Table 11, it can be seen that when the inks are used in combination (B) so that the ink with a higher content of the ultraviolet absorber has a higher content of the antioxidant than the ink with a lower content of the ultraviolet absorber, bleeding of the formed image can be further suppressed compared to when this is not the case (A and C).

[0134] This application is based on Japanese Patent Application No. 2024-147517 filed on August 29, 2024, the contents of which are incorporated herein by reference.

[0135] In order to express the present invention, the present invention has been properly and sufficiently described through the embodiments in the above, but it should be recognized that those skilled in the art can easily change and / or improve the above-mentioned embodiments. Therefore, unless the changes or improvements made by those skilled in the art are at a level that causes departure from the scope of the claims described in the claims, such changes or improvements are interpreted as being included in the scope of the claims.

[0136] The present disclosure provides an ink dispersion that is inhibited from changing over time in a manner that adversely affects ink performance. The present disclosure also provides an ink containing the ink dispersion, an ink set including the ink, and a method for producing an ink containing the ink dispersion.

[0137] REFERENCE SIGNS LIST 10 Inkjet recording device 11 Conveying section 11A Supply roller 11B Recovery roller 12 Discharge section 13 First heating section 13A First heating roller 14 Ultraviolet ray irradiation section 14A Light source 15 Second heating section 15A Second heating roller 16 Control section 17 Ink tank 21 Inkjet head 21a Discharge surface 101 Printing medium

Claims

1. A dispersion for ink comprising: an ultraviolet absorber that is solid at 25°C and is water-insoluble or poorly water-soluble; a polymer attached to at least a portion of the ultraviolet absorber; an antioxidant; and an aqueous medium, wherein the ultraviolet absorber is dispersed in the aqueous medium.

2. The ink dispersion according to claim 1, which has an absorbance of 0.9 or less at a wavelength of 450 nm.

3. The ink dispersion according to claim 1 or 2, wherein the ultraviolet absorber has a phenol group in the molecule.

4. The ink dispersion according to any one of claims 1 to 3, wherein the degree of crosslinking of the polymer is 20 mol % or more and 50 mol % or less.

5. The ink dispersion according to any one of claims 1 to 4, wherein the content of the aqueous medium is 70% by mass or more and 90% by mass or less relative to the ink dispersion.

6. The ink dispersion according to any one of claims 1 to 5, wherein the content of the ultraviolet absorber is 1% by mass or more and 7% by mass or less relative to the ink dispersion.

7. The ink dispersion according to any one of claims 1 to 6, wherein the content of the antioxidant is 5% by mass or more and 30% by mass or less relative to the ultraviolet absorber.

8. The ink dispersion according to any one of claims 1 to 7, wherein the aqueous medium is water.

9. The ink dispersion according to any one of claims 1 to 8, which has a viscosity at 32°C of 10 mPa·sec or less.

10. The ink dispersion according to any one of claims 1 to 9, which has an absorbance of 1.5 or less at a wavelength of 450 nm after storage at 25°C for 30 days.

11. The ink dispersion according to any one of claims 1 to 10, wherein the rate of change in the height of the absorbance peak at a wavelength of 300 to 400 nm after storage at 25°C for 30 days is 20% or less relative to the height of the absorbance peak at a wavelength of 300 to 400 nm before storage.

12. The ink dispersion according to any one of claims 1 to 11, wherein the rate of change in viscosity after storage at 25°C for 30 days relative to the viscosity before storage is 100% or less.

13. The ink dispersion according to any one of claims 1 to 12, wherein the amount of components decomposed from the ultraviolet absorber after storage at 25°C for 30 days is 5 mass% or less relative to the ultraviolet absorber contained in the ink dispersion before storage.

14. The ink dispersion according to any one of claims 1 to 13, wherein the median diameter of the particles in which the polymer is attached to the ultraviolet absorber is 150 nm or less.

15. An ink comprising a colorant and the ink dispersion according to any one of claims 1 to 14.

16. An ink set comprising a plurality of inks of different colors, each of the inks being the ink described in claim 15.

17. The ink set according to claim 16, wherein the plurality of inks include inks with different contents of ultraviolet absorber, and the ink with a higher content of ultraviolet absorber has a higher content of antioxidant than the ink with a lower content of ultraviolet absorber.

18. A method for producing ink, comprising dispersing in an aqueous medium a water-insoluble or poorly water-soluble ultraviolet absorber that is solid at 25°C and has a polymer attached to at least a portion thereof, and adding an antioxidant to the aqueous medium to prepare an ink dispersion; and mixing the ink dispersion with a colorant.

19. The method for producing an ink according to claim 18, wherein after preparing the ink dispersion, the ink dispersion is stored before mixing the ink dispersion with a colorant.

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