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

Abstract

An ink dispersion according to one aspect of the present disclosure comprises: emulsion particles containing a polymer and a water-insoluble or poorly water-soluble ultraviolet absorber that is solid at 25°C; an aqueous medium; and a moisturizing solvent having a hydroxyl group in a molecule.

Description

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

[0001] The present disclosure relates to a dispersion for ink, an ink, an ink set, and a method for producing an ink.

[0002] Examples of inks for forming an image on a print medium include aqueous inks obtained by dispersing a coloring material such as a pigment in an aqueous medium such as water. Such an aqueous ink is an ink that forms an image on a print medium by, after being adhered to the print medium, allowing the aqueous medium contained in the aqueous ink to penetrate into the print medium or evaporating the aqueous medium. Although aqueous inks have higher safety than inks containing organic solvents, they tend to be inferior in drying property. For this reason, in printing using aqueous inks, it is required to improve the drying property of the ink. By improving the drying property of the ink, for example, power saving and size reduction of a printing apparatus including a drying device can be achieved. As a technique for improving the drying property of an ink, for example, there is a technique in which a compound that reacts to light such as an ultraviolet absorber is included in an aqueous ink, and when the aqueous ink adhered to a print medium is dried, the aqueous ink is heated by irradiation with light such as ultraviolet rays. Examples of such an aqueous ink include an inkjet ink composition containing water, and particles containing a polymer having an anionic group, a photoacid generator, and a sensitizer, as described in Patent Document 1.

[0003] International Publication No. 2019 / 188522

[0004] The dispersion for ink according to one aspect of the present disclosure includes emulsion particles that are solid at 25°C and contain a water-insoluble or poorly water-soluble ultraviolet absorber and a polymer, an aqueous medium, and a moisturizing solvent having a hydroxyl group in the molecule.

[0005] A method for producing an ink according to another aspect of the present disclosure includes: preparing a dispersion for ink that includes emulsion particles that are solid at 25°C and contain a water-insoluble or poorly water-soluble ultraviolet absorber and a polymer, an aqueous medium, and a moisturizing solvent having a hydroxyl group in the molecule; and mixing the dispersion for ink and a coloring material.

[0006] FIG. 1 is a schematic diagram showing the configuration of an example of an inkjet recording apparatus.

[0007] The embodiments relating to this disclosure will be described below, but this disclosure is not limited to these embodiments.

[0008] [Ink Dispersion] An ink dispersion according to one embodiment of the present disclosure comprises emulsion particles containing an insoluble or poorly soluble ultraviolet absorber and polymer that are solid at 25°C, an aqueous medium, and a moisturizing solvent having hydroxyl groups in its molecule. The ink dispersion is used in the manufacture of ink to obtain an aqueous ink containing an ultraviolet absorber.

[0009] The Disclosers have found that in the manufacture of water-based inks containing UV absorbers, using a dispersion of UV absorbers in which they are pre-dispersed in an aqueous medium, rather than using the UV absorbers directly, improves the dispersibility of the UV absorbers. The Disclosers have also noted that when manufacturing water-based inks using such dispersions, that is, when using the dispersions as ink dispersions, the dispersions may be stored after manufacture until they are used in ink production (for example, for several tens of days at 25°C). The Disclosers have found that, for example, in the case of a dispersion in which UV absorbers are simply dispersed in water, the dispersion may discolor if stored in the manner described above. When water-based ink is manufactured using these discolored dispersions, it can negatively affect the color tone of the print, and a suitable water-based ink may not be obtained. From these findings, the Disclosers have found that the ink dispersions must have minimal changes over time that negatively affect the performance of the ink produced thereafter; specifically, discoloration must be suppressed in the dispersions.

[0010] Therefore, after various studies, the Disclosers have found that an ink dispersion according to one embodiment of the Disclosed Disclosure can suppress changes over time that adversely affect the performance of the ink. In other words, even after storage as described above, the ink dispersion according to one embodiment of the Disclosed Disclosure can sufficiently suppress changes over time such as discoloration that adversely affect the performance of ink manufactured thereafter. This is thought to be due to the following:

[0011] First, the discoloration of a dispersion of UV absorber in water over time is thought to be due to degradation of the UV absorber, such as oxidative decomposition or dehydration. The moisturizing solvent contained in the ink dispersion is thought to contribute to suppressing this degradation. Therefore, it is thought that the ink dispersion can suppress discoloration and prevent changes over time that would adversely affect the performance of the ink.

[0012] (Emulsion Particles) The emulsion particles are not particularly limited as long as they contain the ultraviolet absorber and the polymer. Examples of the emulsion particles include emulsion particles containing the ultraviolet absorber and the polymer attached to at least a portion of the ultraviolet absorber. The attachment of the polymer can be, for example, to at least a portion of the surface of the ultraviolet absorber. The emulsion particles are particles dispersed in the aqueous medium. The emulsion particles are dispersed not only in the aqueous medium but also in a liquid (mixture) obtained by mixing the aqueous medium with the moisturizing solvent. Since the ink dispersion contains the aqueous medium and the moisturizing solvent, the ink dispersion has the emulsion particles dispersed in the aqueous medium and the moisturizing solvent. This dispersion means that the emulsion particles are spread uniformly in the aqueous medium and the moisturizing solvent, for example, a dispersion that forms an emulsion.

[0013] The particle size of the emulsion particles is a median diameter and may be 150 nm or less, 10 nm to 120 nm, or 30 nm to 100 nm. If the particle size of the emulsion particles is within the above range, the ink can be more preferably heated when it is contained in the ink and irradiated with ultraviolet light. The median diameter is the 50% particle size (D50) in the volume-based cumulative particle size distribution, and examples include the median diameter measured using a light scattering particle size distribution analyzer (for example, ELSZneo manufactured by Otsuka Electronics Co., Ltd.).

[0014] (Ultraviolet Absorber) The ultraviolet absorber is not particularly limited as long as it is a solid at 25°C and is non-water soluble or poorly water soluble. Here, non-water soluble means that it does not dissolve in water or is almost insoluble, and specifically, its solubility in water at 25°C is less than 1 mg / L (less than 0.001 g / L). Poorly water soluble means that it dissolves only slightly in water, and specifically, its 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). In other words, a non-water soluble or poorly water soluble ultraviolet absorber is an ultraviolet absorber whose solubility in water at 25°C is 100 mg / L or less. Examples of the ultraviolet absorber include an ultraviolet absorber that can be contained in an ink and used to heat the ink by irradiating the ink with ultraviolet light. Examples of the aforementioned ultraviolet absorbers 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 aforementioned 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], and 6-(2-benzotriazolyl)-4-tert-octyl-6'-tert-butyl-4'-methyl-2,2'-methylenebisphenol. The ultraviolet absorbers may have a phenol group in their molecules.When an ultraviolet absorber is irradiated with ultraviolet light, it tends to absorb the ultraviolet light, causing the electronic state of the phenol group portion to become excited, and heat is generated when it returns to the ground state. Furthermore, the ultraviolet absorber may have a carbonyl group in its molecule. Thus, the ultraviolet absorber may have both a phenol group and a carbonyl group in its molecule, for example, the dihydroxybenzophenone-based ultraviolet absorber, more specifically, 2,2',4,4'-tetrahydroxybenzophenone. Furthermore, these ultraviolet absorbers may be used individually or in combination of two or more types.

[0015] 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 emulsion particles (the ultraviolet absorber to which the polymer is attached) is the particle diameter (median diameter of 150 nm or less). Specifically, the size of the ultraviolet absorber may be 120 nm or less in median diameter, 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 preferably heated when it is contained in an ink and irradiated with ultraviolet light. The median diameter is the 50% particle diameter (D50) in the volume-based cumulative particle size distribution, and examples include the median diameter measured using a light scattering particle size distribution analyzer (for example, ELSZneo manufactured by Otsuka Electronics Co., Ltd.).

[0016] (Polymer) The polymer is not particularly limited as long as it is a polymer that becomes the emulsion particles together with the ultraviolet absorber. For example, the polymer is a polymer that adheres to at least a part of the ultraviolet absorber. By adhering the polymer to at least a part of the surface of the ultraviolet absorber, the dispersibility of the emulsion particles (particles to which the polymer is attached, at least a part of the surface of the ultraviolet absorber) in the mixture (a mixed liquid of the aqueous medium and the moisturizing solvent) can be increased. When the dispersibility of the emulsion particles in the mixture is increased, the dispersibility of the emulsion particles in the ink dispersion is increased. And since the emulsion particles contain the ultraviolet absorber, the dispersibility of the ultraviolet absorber in the ink dispersion is also increased. More specifically, the polymer is a polymer that allows the emulsion particles to disperse in the mixture to form an emulsion. Furthermore, as mentioned above, the polymer is a polymer that adheres to at least a part of the surface of the ultraviolet absorber, and may also be a polymer that covers the entire surface of the ultraviolet absorber. In other words, the emulsion particles include, for example, particles on which the polymer is attached to at least a part of the surface of the ultraviolet absorber, or particles on which the surface of the ultraviolet absorber is coated with the polymer. The polymer includes resins that can form emulsion particles in the aqueous medium, and more specifically, resins having carbonyl groups in their molecules (carbonyl group-containing resins) such as styrene-acrylic acid copolymers, polyvinyl alcohol, polyurethane, and ethylene-vinyl acetate copolymers. Examples of carbonyl group-containing resins 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 here may be random copolymers or block copolymers.Furthermore, the polymers may be used individually or in combination of two or more. The polymer may be the carbonyl group-containing resin, for example, a styrene-(meth)acrylic acid copolymer, and more specifically, a styrene-acrylic acid copolymer. Amine-based polymers and phosphoric acid-based polymers can also be used as the polymer.

[0017] The molecular weight of the polymer is not particularly limited, but for example, its weight-average molecular weight Mw may be 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 emulsion particles can be further enhanced and its high dispersibility can be better maintained. The weight-average molecular weight can be measured by any general molecular weight measurement method, specifically, values ​​measured using gel permeation chromatography (GPC), etc.

[0018] The acid value of the polymer is not particularly limited, but may be, for example, 30 mg KOH / g or more and 500 mg KOH / g or less, or 50 mg KOH / g or more and 300 mg KOH / g or less. When the acid value of the polymer is within the above range, the dispersibility of the emulsion particles can be further enhanced and its high dispersibility can be better maintained. Here, the acid value is the amount (mg) of potassium hydroxide (KOH) required to neutralize the free acid in 1 g of the sample.

[0019] The polymer may be crosslinked in order to make it less likely for the polymer to be released from the ultraviolet absorber. As described above, the polymer may be able to improve the dispersibility of the emulsion particles by adhering to the surface of the ultraviolet absorber. For example, in such a case, the crosslinked polymer is less likely to be released from the ultraviolet absorber, and the dispersibility of the emulsion particles can be maintained at a high level. Crosslinking is a process in which a part of the hydrophilic group portion of the polymer forms a bond with a crosslinking agent, and the degree of crosslinking indicates the proportion of this bond. 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 emulsion particles can be further improved and its high dispersibility can be maintained more effectively. In other words, it is possible to suppress the release of the polymer while preventing it from being crosslinked too much, which would make it difficult to achieve the effect of improving the dispersibility of the emulsion particles. Here, the degree of crosslinking refers to the ratio (mol%) of crosslinked constituent units (repeating units) to the total constituent units (repeating units) of the polymer.

[0020] Crosslinking of the polymer can be done, for example, by using a crosslinking agent. Specifically, this can be done by crosslinking the polymer attached to the ultraviolet absorber with a crosslinking agent. The crosslinking agent is not particularly limited as long as it can crosslink the polymer. Furthermore, the crosslinking agent varies depending on the polymer, and examples include compounds having epoxy groups in their molecules, and to facilitate crosslinking, compounds having multiple epoxy groups in their molecules (polyfunctional epoxy compounds) may also be used. Examples of polyfunctional epoxy compounds include trimethylolpropane polyglycidyl ether.

[0021] (Moisturizing Solvent) The moisturizing solvent is a moisturizing solvent having hydroxyl groups in its molecule and is not particularly limited as long as it is a moisturizing solvent that may be contained in the ink. Examples of the moisturizing solvent include polyol compounds (polyhydric alcohols), polyhydric alcohol alkyl ethers, and polyhydric alcohol aryl ethers. Examples of the polyol compounds include diol compounds, triol compounds, and compounds having four or more hydroxyl groups in their molecule. Examples of the diol compounds include ethylene glycol, propanediols such as 1,3-propanediol (1,3-propane glycol), butanediols such as 1,3-butanediol (1,3-butane glycol, 1,3-butylene glycol) and 1,4-butanediol, pentanediols such as 3-methyl-1,3-butaneglycol and 1,5-pentanediol, hexanediols such as 1,6-hexanediol, 2-ethyl-1,3-hexanediol, diethylene glycol, triethylene glycol, propylene glycol, polyethylene glycol, and polypropylene glycol. Examples of the triol compounds include glycerin, butanetriols such as 1,2,3-butanetriol and 1,2,4-butanetriol, hexanetriols such as 1,2,6-hexanetriol, 3-methyl-1,3,5-pentanetriol, and trimethylolpropane. Examples of compounds having four or more hydroxyl groups in the molecule include sorbitol. Examples of the polyhydric alcohol alkyl ethers include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol monomethyl ether, and propylene glycol monoethyl ether. Examples of the polyhydric alcohol aryl ethers include ethylene glycol monophenyl ether and ethylene glycol monobenzyl ether. Among these, the moisturizing solvent may be a glycol compound or the like due to its high moisturizing properties.More specifically, the aforementioned moisturizing solvent may be propylene glycol because it has a high moisturizing effect and maintains the fluidity of the ink. It may also be ethylene glycol because it has a good balance of moisturizing properties and volatility and improves the stability of the ink. It may also be diethylene glycol because it has high moisturizing properties and prevents the ink from drying out. It may also be triethylene glycol because it has moisturizing and antibacterial properties and maintains the quality of the ink. It may also be polyethylene glycol because it has high moisturizing properties and adjusts the viscosity of the ink. Furthermore, the aforementioned moisturizing solvent may be used alone or in combination of two or more types. In addition to the moisturizing solvent having a hydroxyl group in its molecule, the ink dispersion may also contain the following moisturizing compounds. Examples of the aforementioned moisturizing compounds include compounds having an amide group in their molecule, compounds having an ester group in their molecule, compounds having an amino group in their molecule, compounds having a sulfoxide group in their molecule, compounds having a sulfone group in their molecule, and compounds having a thioether group in their molecule. Examples of compounds having the amide group in their molecule include 2-pyrrolidone, N-methyl-2-pyrrolidone, N-hydroxyethyl-2-pyrrolidone, 1,3-dimethylimidazolidinone, ε-caprolactam, formamide, N-methylformamide, and N,N-dimethylformamide. Examples of compounds having the ester group in their molecule include γ-butyrolactone and propylene carbonate. Examples of compounds having the amino group in their molecule include monoethanolamine, diethanolamine, triethanolamine, monoethylamine, diethylamine, and triethylamine. Examples of compounds having the sulfoxide group in their molecule include dimethyl sulfoxide. Examples of compounds having the sulfone group in their molecule include sulfolane. Examples of compounds having the thioether group in their molecule include thiodiethanol.

[0022] (Aqueous medium) The aqueous medium is not particularly limited as long as it is a medium containing water. For example, the aqueous medium may be an aqueous medium containing water as its main component (for example, 50% by mass or more), or an aqueous medium consisting of water, but the aqueous medium may be water. The water content may be, for example, 50% by mass or more, 90% by mass or more, or 100% by mass relative to the aqueous medium. Furthermore, the aqueous medium may contain water and further contain an organic solvent other than the humectant solvent. That is, the aqueous medium may be an aqueous medium containing water and an organic solvent, or it may be water. Furthermore, the aqueous medium may contain components that may be generated when the emulsion particles are formed in the aqueous medium.

[0023] (Content) The content of the ultraviolet absorber is not particularly limited, but for example, it may be 0.6% by mass or more and 9.2% by mass or less, 0.8% by mass or more and 1.9% by mass or less, or 1.2% by mass or more and 1.7% by mass or less, relative to the total mass of the ultraviolet absorber and the moisturizing solvent. Also, the content of the ultraviolet absorber may be 0.5% by mass or more and 2.5% by mass or less, or 0.6% by mass or more and 1.2% by mass or less, relative to the ink dispersion. If the content ratio of the ultraviolet absorber in the ink dispersion is low, the amount of the ink dispersion added in order to include the required amount of ultraviolet absorber in the ink will increase during ink production. That is, the amount of the aqueous medium added as a component of the ink dispersion will increase. If the amount of the aqueous medium added as a component of the ink dispersion increases, the amount of aqueous medium added as a standalone component will decrease during ink production, which may make ink production difficult. By increasing the content ratio of the ultraviolet absorber in the ink dispersion, the amount of aqueous medium alone can be increased in the ink manufacturing process. As a result, ink can be manufactured.

[0024] The content of the moisturizing solvent is not particularly limited, but for example, it may be 25% by mass or more and 90% by mass or 50% by mass or 85% by mass or 65% by mass or 80% by mass, relative to the total mass of the ink dispersion (for example, the total mass of the emulsion particles, the aqueous medium and the moisturizing solvent). As described above, the moisturizing solvent is for suppressing discoloration of the ink dispersion due to the ultraviolet absorber. Therefore, the higher the content of the moisturizing solvent, the more effectively discoloration of the ink dispersion can be suppressed. Accordingly, if the content of the moisturizing solvent is within the above range, changes over time such as discoloration can be more effectively suppressed.

[0025] The content of the emulsion particles is not particularly limited, but may be, for example, 6% by mass or more and 16% by mass or 4.2% by mass or more and 7.3% by mass, relative to the total mass of the ink dispersion (for example, the total mass of the emulsion particles, the aqueous medium, and the moisturizing solvent). When the content of the emulsion particles is within the above range, the ink dispersion can be incorporated into the ink, and the ink can be suitably heated when irradiated with ultraviolet light, while maintaining the suitable dispersibility of the emulsion particles.

[0026] The polymer content is not particularly limited, but may be, for example, 4.9% by mass or more and 13.2% by mass or 3.5% by mass or more and 6.1% by mass, relative to the total mass of the ink dispersion (for example, the emulsion particles, the aqueous medium, and the moisturizing solvent). When the polymer content is within the above range, the dispersibility of the emulsion particles can be further enhanced and its high dispersibility can be better maintained.

[0027] The content of the aqueous medium is not particularly limited, but may be 22% by mass or more and 59% by mass or 15% by mass or more and 28% by mass, relative to the total mass of the ink dispersion (for example, the total mass of the emulsion particles, the aqueous medium, and the moisturizing solvent). When the content of the aqueous medium is within the above range, the dispersibility of the emulsion particles can be further enhanced and its high dispersibility can be better maintained.

[0028] A more suitable ink dispersion can be obtained if the content of each of the UV absorber, polymer, emulsion particles, moisturizing solvent, and aqueous medium is within the above range. Specifically, it can be made more dispersible and more resistant to changes over time such as discoloration.

[0029] (Other Components) The ink dispersion may contain the emulsion particles, the aqueous medium, and the moisturizing solvent, and may also contain other components. Examples of these other components include antioxidants. The antioxidant is thought to contribute to suppressing deterioration such as oxidative decomposition or dehydration of the ultraviolet absorber. Therefore, by including the antioxidant, the ink dispersion is thought to be able to further suppress discoloration and further suppress changes over time that adversely affect the performance of the ink. The antioxidant is not particularly limited as long as it is an antioxidant that may be contained in the ink. Examples of the aforementioned antioxidants 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, and 3,9-bis(octadecyloxy)-2,4 Examples of antioxidants include 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 (ascorbic acid palmitate ester), ascorbyl dipalmitate, sodium erythorbate, and sodium pyrosulfite. Among these antioxidants, sodium sulfite, ascorbic acid, and tris(2,4-di-tert-butylphenyl) phosphite are preferred. Furthermore, these antioxidants may be used individually or in combination of two or more.

[0030] When the ink dispersion contains the antioxidant, its content is not particularly limited, but for example, it may be 5% by mass or more and 30% by mass or 8% by mass or more and 15% by mass relative to the ultraviolet absorber. As described above, the antioxidant is for suppressing discoloration of the ink dispersion caused by the ultraviolet absorber. Therefore, the higher the content of the antioxidant, the more effectively discoloration of the ink dispersion can be suppressed. Accordingly, if the content of the antioxidant is within the above range, changes over time such as discoloration can be more effectively suppressed.

[0031] (Ink Dispersion) In the dispersion containing the UV absorber, discoloration of the dispersion is thought to be due to deterioration such as oxidative decomposition or dehydration of the UV absorber, as described above. Specifically, when a UV absorber having a phenol group in its molecule, such as a dihydroxybenzophenone-based UV absorber, is used as the UV absorber, oxidative decomposition by the Dakin oxidation reaction and dehydration of the phenol group are considered to occur. When 2,2',4,4'-tetrahydroxybenzophenone is used as the UV absorber, oxidative decomposition by the Dakin oxidation reaction produces 2,4-hydroxybenzoic acid and 3,6-dihydroxy-9H-xathene-9-one, etc. This deterioration such as oxidative decomposition and dehydration is thought to progress more rapidly in basic conditions. Furthermore, when the ink dispersion uses a carbonyl group-containing resin such as a styrene-(meth)acrylic acid copolymer as the polymer, a base such as sodium hydroxide may be added to improve the dispersibility of the emulsion particles. In such cases, the oxidative decomposition and dehydration tend to progress more easily. Even in the case of such a dispersion, the ink dispersion can suppress discoloration by containing the moisturizing solvent.

[0032] UV absorbers containing a phenol group in their molecule tend to be easily excited by ultraviolet light, resulting in higher UV absorption capacity. However, UV absorbers containing a phenol group in their molecule also tend to be easily decomposed, as mentioned above. By including the aforementioned moisturizing solvent in an ink dispersion containing such a UV absorber, the UV absorption capacity of the resulting ink can be increased while simultaneously suppressing discoloration. Furthermore, UV absorbers containing a carbonyl group in their molecule tend to be easily excited by ultraviolet light, resulting in higher UV absorption capacity.

[0033] The aforementioned ink dispersion can further suppress discoloration by having low absorbance in the visible light region, thereby minimizing its impact on the color of subsequently manufactured inks. The absorbance can be measured using a cell with a cell thickness of 10 mm with the ink dispersion diluted 3500 times. Specifically, low absorbance in the visible light region refers to the absorbance at a wavelength of 450 nm, and its value (absorbance before storage) may be, for example, 0.3 or less, or 0.7 or less. A lower absorbance at a wavelength of 450 nm can further suppress discoloration, but in practice, it is considered that around 0.1 is the limit, so the lower limit of absorbance at a wavelength of 450 nm can be, for example, 0.1 or more. Therefore, the absorbance of the ink dispersion at a wavelength of 450 nm before storage may be between 0.1 and 0.3.

[0034] As described above, the ink dispersion can further suppress discoloration by suppressing degradation such as oxidative decomposition and dehydration of the ultraviolet absorber. For example, the absorbance at a wavelength of 450 nm after storage at 25°C for 30 days (absorbance after storage) may be 1.4 or less, 1.1 or less, 1.0 or less, or 0.7 or less. A lower absorbance after storage can further suppress discoloration, but in reality, the limit is around 0.1, so it may be 0.1 or higher. That is, the absorbance at a wavelength of 450 nm after storage at 25°C for 30 days of the ink dispersion may be 0.1 or more and 1.4 or less, 0.1 or more and 1.1 or less, 0.1 or more and 1.0 or less, or 0.1 or more and 0.7 or less. Also, the change in absorbance A2 after storage compared to absorbance A1 before storage is 10 Ａ１－Ａ２ The evaluation was based on the following indicators: Ａ１－Ａ２ The value of may be between 0.15 and 1, or between 0.63 and 1. The absorbance A1 before storage is A1 = -log(L1 / L0) (log is the common logarithm), where L0 is the incident light and L1 is the transmitted light. Similarly, after storage, A2 = -log(L2 / L0). Since L0 can be considered the same, from these, L2 / L1 = 10 Ａ１－Ａ２ This is the result.

[0035] Absorbance can be measured using a spectrophotometer such as an ultraviolet-visible-near-infrared spectrophotometer. Absorbance at a wavelength of 450 nm can be measured using a spectrophotometer.

[0036] The viscosity of the ink dispersion at 32°C (viscosity before storage) is not particularly limited, but may be, for example, 20 mPa·s or less, 15 mPa·s or less, or 12 mPa·s or less, for ease of use in ink manufacturing. Furthermore, the viscosity of the ink dispersion after storage at 25°C for 30 days (viscosity after storage) may be 20 mPa·s or less, 15 mPa·s or less, or 14 mPa·s or less. A lower viscosity has advantages such as ease of use in ink manufacturing, but in reality, the viscosity of water (approximately 0.89 mPa·s) is considered to be the limit, so a lower limit of the viscosity can be, for example, 0.89 mPa·s or more. Furthermore, the rate of change of the viscosity after storage from the viscosity before storage may be within ±20%, within ±10%, or 0% or more and +10% or less. The viscosity measured under conditions of 32°C can be measured using a general-purpose viscometer or rheometer, such as a vibrating viscometer or a falling ball viscometer.

[0037] (Method for Manufacturing Ink Dispersion) The method for manufacturing the ink dispersion is not particularly limited as long as it can produce the ink dispersion. Examples of methods for manufacturing the ink dispersion include adding the moisturizing solvent to an emulsion in which the emulsion particles are dispersed in the aqueous medium. More specifically, the method for manufacturing the ink dispersion involves first dissolving the ultraviolet absorber and the polymer in a solvent capable of dissolving them. The solvent is not particularly limited as long as it can dissolve the ultraviolet absorber and the polymer, and varies depending on the type of ultraviolet absorber and the type of polymer. Examples of the solvent include organic solvents such as methyl ethyl ketone (MEK). Next, while stirring the solution in which the ultraviolet absorber and the polymer are dissolved in the solvent, a basic aqueous solution is added. This yields the emulsion particles (the ultraviolet absorber with the polymer attached). The basic aqueous solution is not particularly limited as long as it can attach the polymer dissolved in the solvent to the ultraviolet absorber, and examples include 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 emulsion particles. The disperser is not particularly limited as long as it can improve the dispersibility of the emulsion particles, and examples include an ultrasonic homogenizer. After that, the solvent is removed from the liquid that has undergone the dispersion treatment using an evaporator or the like. By doing so, an emulsion (dispersion) is obtained in which the emulsion particles are dispersed in an aqueous medium derived from the basic aqueous solution. Subsequently, the crosslinking agent may be added to this emulsion to crosslink the polymer attached to the ultraviolet absorber. The moisturizing solvent is added to the obtained emulsion. By doing so, the ink dispersion is obtained.

[0038] [Ink] An ink according to another embodiment of the present disclosure is an ink comprising a colorant and the ink dispersion. The ink may contain other components, such as components contained in the ink, as long as it comprises the colorant and the ink dispersion.

[0039] (Colorants) The colorants are not particularly limited as long as they can be used as colorants included in ink. Examples of colorants include colorants included in inkjet inks, and specifically, pigments.

[0040] The pigment is not particularly limited as long as it can form a desired image (express the colors that constitute the desired image), and examples 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, dioxandine pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments. Examples of inorganic pigments include acetylene black and carbon black such as lamp black.

[0041] The aforementioned pigments can also be classified according to their color, and examples include yellow pigments, orange pigments, red pigments, blue pigments, green pigments, and black pigments.

[0042] Examples of the aforementioned yellow pigments 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. Examples include I. Pigment Yellow 173, C. I. Pigment Yellow 180, C. I. Pigment Yellow 185, and C. I. Pigment Yellow 193.

[0043] 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.

[0044] 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. Pigment Red 177, C.I. Pigment Red 178, C.I. Pigment Red 202, and C.I. Pigment Red 222.

[0045] 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.

[0046] Examples of the green pigment include C.I. Pigment Green 7.

[0047] Examples of the black pigment include C.I. Pigment Black 7.

[0048] As the colorant, one of the pigments may be used alone, or two or more pigments may be used in combination.

[0049] The content of the colorant (pigment) is not particularly limited, but in order to increase the image density while maintaining high ink fluidity, it may be, for example, 2% by mass or more and 10% by mass or 3% by mass or more and 8% by mass relative to the total amount of ink. When the content of the colorant (pigment) is high (for example, 2% by mass or more), the image density tends to be increased, while when the content of the colorant (pigment) is low (for example, 10% by mass or less), the ink fluidity can be increased.

[0050] (Other components: Aqueous medium) As described above, the ink contains the ink dispersion, and therefore contains the aqueous medium contained in the ink dispersion. In addition to the aqueous medium contained in the ink dispersion, the ink may further contain an aqueous medium (aqueous medium for ink). The aqueous medium for ink is not particularly limited as long as it is a medium containing water, and similar examples include organic solvents contained in the aqueous medium and their content. The content of the aqueous medium (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 component (colorant, binder resin and surfactant described later, etc.)], or it may be, for example, 70% by mass or more and 90% by mass or 75% by mass or more and 80% by mass or less relative to the total amount of ink.

[0051] (Other components: binding resin) The ink may contain a binding resin for purposes such as improving image fixation. The binding resin may exist in the form of particles dispersed in an aqueous medium. The binding resin can function as a binder that binds the printing medium (substrate) to colorants such as pigments. Therefore, by including the binding resin in the ink, printed materials with excellent image fixation can be obtained.

[0052] The binder resin is not particularly limited, and examples 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 in order to improve image fixation while maintaining ink fluidity, it may be, for example, 2% by mass or more and 20% by mass or 5% by mass or more and 15% by mass, relative to the total amount of ink. When the binder resin content is high (for example, 2% by mass or more), image fixation tends to be improved, while when the binder resin content is low (for example, 20% by mass or less), ink fluidity can be improved.

[0053] (Other components: surfactants) The ink may contain surfactants for purposes such as adjusting surface tension. Furthermore, the inclusion of surfactants in the ink improves the wettability of the ink to the printing medium (substrate). The surfactant is not particularly limited, but may be any of nonionic surfactants, cationic surfactants, anionic surfactants, and amphoteric surfactants. The amount of surfactant is not particularly limited, but in order to further suppress the occurrence of bleeding, it may be, for example, 0.1% by mass or more and 5% by mass or 0.3% by mass or more and 3% by mass relative to the total amount of ink.

[0054] (Other components: additives) The ink may further contain known additives as needed (more specifically, dissolution stabilizers, drying inhibitors, antioxidants, viscosity modifiers, pH adjusters, and fungicides, etc.).

[0055] (Applications) The ink can be any ink used to form an image on a printing medium (substrate), and can be used as, for example, an inkjet ink. That is, the ink can be used when forming an image with an inkjet recording device. Furthermore, because the ink contains the ultraviolet absorber, it is heated by irradiation with ultraviolet light, resulting in an ink with excellent drying properties. For this reason, when the ink is used, for example, when forming an image with an inkjet recording device, the occurrence of problems due to the low drying properties of the ink can be suppressed, and an image suitable for the printing medium can be formed. The printing medium on which the image is formed with the ink is not particularly limited, but examples include paper, film, and fabric. Examples of paper include plain paper, fine paper, matte coated paper, cast paper, and photographic paper. Examples of film include polyethylene terephthalate (PET) film and other resins. Examples of fabric include woven fabrics, knitted fabrics, and nonwoven fabrics. Printing using the ink results in excellent drying properties, so an image can be suitably formed even on a printing medium with low ink permeability. From this perspective, the printing medium may be the film. That is, the ink can also be used, for example, when forming an image on a film using an inkjet recording device.

[0056] The ink is not particularly limited as long as it can be used to form an image on a printing medium, and examples include inkjet recording devices. Furthermore, the inkjet recording device is not particularly limited as long as it can be used to form an image. An example of the inkjet recording device is the inkjet recording device shown in Figure 1. That is, Figure 1 is a schematic diagram showing the configuration of an example of an inkjet recording device 10.

[0057] As shown in Figure 1, the inkjet recording device 10 includes an ejection unit 12 having at least one inkjet head 21, a transport unit 11 for transporting a printing medium (substrate) 101 to which inkjet ink ejected from the inkjet head 21 is to be deposited, and an ultraviolet irradiation unit 14 for irradiating the ink-deposited printing medium 101 with ultraviolet light. The inkjet recording device 10 may further include a first heating unit 13 for heating the printing medium 101 before the ink ejected from the ejection unit 12 is deposited, and a second heating unit 15 for heating the printing medium 101 after the ink ejected from the ejection unit 12 has been deposited. The inkjet recording device 10 also includes a control unit 16 for controlling the transport unit 11, the ejection unit 12, the first heating unit 13, the ultraviolet irradiation unit 14, and the second heating unit 15, etc.

[0058] The transport unit 11 transports the printing medium 101 from the supply roller 11A to the recovery roller 11B. The inkjet recording device 10 is also equipped with the discharge unit 12, the first heating unit 13, the ultraviolet irradiation unit 14, and the second heating unit 15, etc., along the transport path of the printing medium 101 by the transport unit 11. The supply roller 11A supplies the printing medium 101 to which the ink discharged from the inkjet head 21 will adhere to the discharge unit 12. The recovery roller 11B recovers the printing medium 101 supplied to the discharge unit 12. The transport unit 11 may also be equipped with rollers 19A to 19D to allow the printing medium 101 to pass through a predetermined transport path when transporting the printing medium 101 from the supply roller 11A to the recovery roller 11B. The supply roller 11A, the recovery roller 11B, and the rollers 19A to 19D can each be either a driven roller or a driven roller, as long as they can transport the printing medium 101 from the supply roller 11A to the recovery roller 11B.

[0059] The printing medium 101 transported by the transport unit 11 may be in the form of a long sheet or a single sheet. If the printing medium 101 is in the form of a single sheet, the transport unit 11 may be equipped with a transport belt. In this case, the transport unit 11 may transport the printing medium 101 by placing it on the transport belt and transporting the transport belt in that state. Furthermore, as described above, the printing medium 101 may be paper or cloth, or a printing medium with low permeability, and specifically, it may be a film.

[0060] The ejection unit 12 ejects ink droplets toward the printing medium 101, causing the ink to adhere to the printing medium 101. The ejection unit 12 includes at least one inkjet head 21 that directly ejects ink facing the printing medium 101, and an ink tank 17 that stores the ink supplied to the inkjet head 21. The inkjet head 21 ejects the ink supplied from the ink tank 17 toward the printing medium 101. The inkjet head 21 is held such that the ejection surface 21a that ejects the ink corresponds to the printing medium 101. The image may be printed by dividing it into a plurality of sections (for example, a grid) and forming pixels (dots) in each section or not, or by forming pixels of gradually different sizes.

[0061] The first heating unit 13 promotes the evaporation of the aqueous medium contained in the ink adhering to the printing medium 101 by heating the printing medium 101. The first heating unit 13 may also include a first heating roller 13A that contacts the printing medium 101 and directly heats it. The first heating roller 13A heats the printing medium 101 and also functions as a roller in the transport unit 11. Alternatively, the first heating unit 13 may heat the printing medium 101 by applying hot air to it, instead of or in addition to heating by the first heating roller 13A. Furthermore, the heating of the printing medium 101 by the first heating unit 13 may be performed before the ink adheres to the printing medium 101, after the ink adheres, or both. In other words, the first heating unit 13 may be provided on the upstream side of the discharge unit 12 in the transport direction of the printing medium 101, or on the downstream side, or on both sides.

[0062] The ultraviolet irradiation unit 14 irradiates the ink adhering to the printing medium 101 with ultraviolet light (UV: Ultraviolet) to heat the ink. Since the ink contains the ultraviolet absorber, it is easily heated. Furthermore, since the ultraviolet irradiation unit 14 irradiates with ultraviolet light, it is equipped with at least an ultraviolet light source 14A. Also, since the ultraviolet irradiation by the ultraviolet irradiation unit 14 is performed on the ink adhering to the printing medium 101, it is performed after the ink has adhered to the printing medium 101. That is, the ultraviolet irradiation unit 14 is provided on the downstream side of the discharge unit 12 in the transport direction of the printing medium 101.

[0063] The second heating unit 15 heats the ink that is attached to the printing medium 101 and heated by the ultraviolet irradiation unit by heating the printing medium 101. This heating fixes the image formed by the colorants contained in the ink onto the printing medium 101. Since the second heating unit 15 heats the ink heated by the ultraviolet irradiation unit 14, it is provided in the vicinity of the ultraviolet irradiation unit 14. For example, the second heating unit 15 is located on the opposite side of the printing medium 101 from the ultraviolet irradiation unit 14. The second heating unit 15 may include a second heating roller 15A that contacts the printing medium 101 and directly heats the printing medium 101. The second heating roller 15A heats the printing medium 101 and also functions as a roller in the transport unit 11. Furthermore, the second heating unit 15 may heat the printing medium 101 by applying hot air to the printing medium 101, instead of or in addition to heating by the second heating roller 15A.

[0064] The control unit 16 controls the transport unit 11, the ejection unit 12, the first heating unit 13, the ultraviolet irradiation unit 14, and the second heating unit 15, etc. Specifically, the control unit 16 also controls the transport speed of the printing medium 101 in the transport unit 11, and the drive frequency for ejecting the inkjet ink in the ejection unit 12, etc. The control unit 16 is equipped with a computer. The control unit 16 includes, for example, a CPU (central processing unit), ROM (read-only memory), RAM (random access memory), and an external storage device, etc., although these are not shown in the figures.

[0065] As mentioned above, the inkjet recording device is not particularly limited as long as it can form an image using the ink, but the inkjet recording device shown in Figure 1, which is given as an example, can fix a high-quality image onto the printing medium using the ink.

[0066] (Method for manufacturing ink) The method for manufacturing the ink is not particularly limited as long as it can produce the ink. Examples of the method for manufacturing the ink include mixing the ink dispersion (for example, the ink dispersion produced by the method for manufacturing the ink dispersion) with the colorant. That is, a method of preparing an ink dispersion containing emulsion particles containing the ultraviolet absorber and the polymer, an aqueous medium, and a moisturizing solvent, and mixing the ink dispersion with the colorant. Examples of the method for manufacturing the ink include producing the ink dispersion by adding the moisturizing solvent to an emulsion in which the emulsion particles are dispersed in the aqueous medium, and mixing the ink dispersion with the colorant. The mixing is not particularly limited as long as it can produce the ink, and examples include mixing using a mixer such as a bead mill. The time for performing the mixing is not particularly limited as long as it can produce the ink, and examples include 10 minutes or more and 180 minutes or less. Furthermore, the method for manufacturing the ink may involve manufacturing the ink dispersion and then mixing it directly with the colorant without storing it, thereby producing the ink. In other words, the storage time may be zero hours. On the other hand, since the ink dispersion can be stored without discoloration, the ink dispersion may be stored after manufacturing it but before mixing it with the colorant. The storage temperature is not particularly limited, but for example, it may be 25°C or lower, or 15°C or lower. The storage time is not particularly limited, but for example, it may be 360 ​​days or less, 180 days or less, 90 days or less, or 30 days or less. Also, in terms of obtaining a suitable ink, the ink may be manufactured without any storage, i.e., with a storage time of zero days. On the other hand, since the ink dispersion is subject to aging changes, a suitable ink can be manufactured even with storage.

[0067] [Ink Set] The ink can be made into multiple inks of different colors by using colorants of different colors as the colorants. Different colors mean that at least one of the hue and lightness is different. These multiple inks of different colors may be used as an ink set. That is, an ink set according to another embodiment of the present disclosure is an ink set comprising multiple inks of different colors, wherein each of the multiple inks is the ink (an ink containing the colorant and the ink dispersion). Furthermore, if there are inks with different amounts of the ultraviolet absorber among the multiple inks provided in the ink set, then, for the following reasons, the ink set with a higher amount of the ultraviolet absorber may have a higher amount of the moisturizing solvent than the ink with a lower amount of the ultraviolet absorber. Even if the amount of the ultraviolet absorber is high, discoloration can be sufficiently suppressed because of the high amount of the moisturizing solvent, and furthermore, with such a combination of ink sets, bleeding can also be suppressed. Furthermore, when forming an image using the ink set, for example, when using the inkjet recording device shown in Figure 1, the multiple inks provided in the ink set may be ejected from each of the multiple inkjet heads 21 to form an image.

[0068] As described above, this specification discloses various aspects of technology, the main technologies being summarized below.

[0069] The ink dispersion according to the first embodiment is an ink dispersion comprising emulsion particles containing an insoluble or poorly soluble ultraviolet absorber and polymer that are solid at 25°C, an aqueous medium, and a moisturizing solvent having hydroxyl groups in its molecule.

[0070] The second embodiment of the ink dispersion is the ink dispersion according to the first embodiment, wherein the moisturizing solvent is a polyol compound.

[0071] The ink dispersion according to the third embodiment is an ink dispersion according to the first or second embodiment in which the absorbance at a wavelength of 450 nm after storage at 25°C for 15 days is 1.1 or less.

[0072] The fourth embodiment of the ink dispersion is an ink dispersion according to any one of the first to third embodiments, wherein the ultraviolet absorber has a phenol group in its molecule.

[0073] The fifth embodiment of the ink dispersion is an ink dispersion according to any one of the first to fourth embodiments, wherein the degree of crosslinking of the polymer is 20 mol% or more and 50 mol% or less.

[0074] The ink dispersion according to the sixth embodiment is an ink dispersion according to any one of the first to fifth embodiments, wherein the content of the ultraviolet absorber is 0.8% by mass or more and 1.9% by mass or less, relative to the total mass of the ultraviolet absorber and the moisturizing solvent.

[0075] The ink dispersion according to the seventh embodiment is an ink dispersion according to any one of the first to sixth embodiments, wherein the content of the moisturizing solvent is 65% by mass or more and 80% by mass or less with respect to the ink dispersion.

[0076] The ink dispersion according to the eighth embodiment is an ink dispersion according to any one of the first to seven embodiments, wherein the aqueous medium is water.

[0077] The ink according to the ninth embodiment is an ink comprising a colorant and an ink dispersion according to any one of the first to eight embodiments.

[0078] The ink set according to the tenth embodiment is an ink set comprising a plurality of inks of different colors, wherein each of the plurality of inks is an ink according to the ninth embodiment.

[0079] The ink set according to the 11th embodiment is an ink set according to the 10th embodiment in which, among the plurality of inks, there are inks with different amounts of the ultraviolet absorber, and the ink with a higher amount of the ultraviolet absorber has a higher amount of the moisturizing solvent than the ink with a lower amount of the ultraviolet absorber.

[0080] The twelfth embodiment of the method for producing ink is a method for producing ink which involves preparing an ink dispersion comprising emulsion particles containing an insoluble or poorly soluble ultraviolet absorber and polymer that are solid at 25°C, an aqueous medium, and a moisturizing solvent having hydroxyl groups in its molecule, and mixing the ink dispersion with a colorant.

[0081] The method for manufacturing ink according to the 13th embodiment is the method for manufacturing ink according to the 12th embodiment, wherein after preparing the ink dispersion, the ink dispersion is stored before mixing the ink dispersion with the colorant.

[0082] According to this disclosure, it is possible to provide an ink dispersion in which changes over time that adversely affect the performance of the ink are suppressed. Furthermore, according to this disclosure, it is possible to provide an ink containing the ink dispersion, an ink set comprising the ink, and a method for manufacturing the ink containing the ink dispersion.

[0083] The present disclosure will be further illustrated below by examples, but the scope of the present disclosure is not limited thereto.

[0084] [Ink Dispersions] First, the ultraviolet absorber, polymer, crosslinking agent, and moisturizing solvent used in the ink dispersions in the test examples will be explained.

[0085] (UV absorber) 2,2',4,4'-tetrahydroxybenzophenone (Uvinul 3050, manufactured by BASF Japan Ltd.) (solid at 25°C, slightly soluble in water)

[0086] (Polymer) Styrene-acrylic acid copolymer (JONCRYL 819 manufactured by BASF Japan Ltd., weight-average molecular weight 14,500, acid value 75 mgKOH / g)

[0087] (Crosslinking agent) Polyfunctional epoxy compound (trimethylolpropane polyglycidyl ether, Denacol EX-321 manufactured by Nagase ChemteX Corporation)

[0088] (Moisturizing solvents) Moisturizing solvent 1: Propylene glycol (PG) (Wako Special Grade manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) Moisturizing solvent 2: Diethylene glycol (DEG) (Wako Special Grade manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)

[0089] (Preparation of Dispersion for Ink) First, the ultraviolet absorber and the polymer were added to methyl ethyl ketone (MEK) and stirred to dissolve the ultraviolet absorber and the polymer in MEK, so that the content of the ultraviolet absorber in the resulting emulsion particles was the composition (mass%) shown in Table 1. Next, while stirring the solution in which the ultraviolet absorber and the polymer were dissolved in the solvent, an aqueous sodium hydroxide (NaOH) solution was added dropwise to the solution. The liquid to which the NaOH aqueous solution was added was subjected to dispersion treatment using an ultrasonic homogenizer (US-300T manufactured by Nippon Seiki Seisakusho Co., Ltd.). As a result, the ultraviolet absorber with the polymer attached was obtained as emulsion particles. After that, MEK was removed from the dispersed liquid using an evaporator. In this way, a dispersion was obtained in which the emulsion particles were dispersed in water derived from NaOH (aqueous medium). Water was added as an aqueous medium as necessary so that the content of the aqueous medium was the composition (mass%) shown in Tables 1 and 2. The aqueous medium is water. Subsequently, the crosslinking agent was added to this dispersion. As a result, the polymer attached to the ultraviolet absorber was crosslinked. The degree of crosslinking [the ratio (mol%) of crosslinked constituent units (repeating units) to the total constituent 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 degree of crosslinking was the value shown in Tables 1 and 2. The moisturizing solvent (moisturizing solvent 1 and moisturizing solvent 2) was added to the obtained dispersion so that the content of the moisturizing solvent was the composition (mass%) shown in Tables 1 and 2. By doing so, the ink dispersion was obtained. The content of the emulsion particles, the content of the ultraviolet absorber contained in the emulsion particles, the aqueous medium, and the moisturizing solvent are shown in Tables 1 and 2 as their content relative to the ink dispersion. Furthermore, the content of the ultraviolet absorber relative to the total mass of the ultraviolet absorber and the moisturizing solvent is shown in Tables 1 and 2 as "ultraviolet absorber / (ultraviolet absorber + moisturizing solvent)".Furthermore, in Tables 1 and 2, the test examples where the content of the moisturizing solvent is 0% by mass are examples in which the moisturizing solvent was not added.

[0090] (Dispersion State) The dispersion state of the ink dispersions in each test example was visually confirmed. If dispersion was confirmed, it is indicated as "dispersed" in Tables 1 and 2; if non-dispersion was confirmed, it is indicated as "non-dispersed" in Tables 1 and 2. In this test, the non-dispersed materials ("non-dispersed") had trace amounts of solid matter in the prepared ink dispersions. Although the non-dispersed materials did not differ significantly in physical properties from the others, when used in inkjet inks, they may clog nozzles and cause ejection problems.

[0091] (Viscosity) The viscosity of the ink dispersion in each test example was measured at 32°C using a rheometer (Anton Paar MCR302). The results are shown in Tables 1 and 2.

[0092] (Absorbance at 450 nm wavelength) The absorbance of the ink dispersion in each test example at a wavelength of 450 nm was measured using an ultraviolet-visible near-infrared spectrophotometer (V-670, manufactured by JASCO Corporation). The results are shown in Tables 1 and 2.

[0093] (Average particle size D50 of emulsion particles) The average particle size D50 (median diameter) of emulsion particles dispersed in the ink dispersion in each test example was measured using a light scattering particle size distribution analyzer (ELSZneo manufactured by Otsuka Electronics Co., Ltd.). The results obtained are shown in Tables 1 and 2.

[0094]

[0095]

[0096] The ink dispersions in each test example were stored under the storage conditions (storage temperature and storage days) described in Tables 3 and 4, and the following evaluations were performed on the ink dispersions after storage.

[0097] (Dispersion State) The dispersion state of the ink dispersion after storage in each test example was visually inspected in the same manner as described above. If dispersion was confirmed, it is indicated as "dispersed" in Tables 3 and 4; if non-dispersion was confirmed, it is indicated as "non-dispersed" in Tables 3 and 4.

[0098] (Viscosity) The viscosity of the ink dispersion after storage in each test example was measured using a rheometer (Anton Paar MCR302) under the same conditions as above, at 32°C. The results obtained are shown in Tables 3 and 4.

[0099] (Absorbance at 450 nm wavelength) The absorbance at 450 nm wavelength of the ink dispersion after storage in each test example was measured using a UV-Vis-Near-Infrared spectrophotometer (V-670, manufactured by JASCO Corporation), in the same manner as described above. The ink dispersion was diluted 3500 times and measured in a cell with a cell thickness of 10 mm. The obtained results are shown in Tables 3 and 4 as "after storage". The difference from the absorbance before storage (absorbance after storage - absorbance before storage) is shown in Tables 3 and 4 as "difference from before storage".

[0100] (Average particle size D50 of emulsion particles) The average particle size 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 analyzer (ELSZneo manufactured by Otsuka Electronics Co., Ltd.) in the same manner as described above. The results obtained are shown in Tables 3 and 4.

[0101]

[0102]

[0103] (Ink) Ink was produced by adding the ink dispersion after storage in each test example to the pigment dispersion of each color (magenta, cyan, yellow, and black) in amounts of 10% by mass, 20% by mass, 30% by mass, 40% by mass, and 50% by mass, and stirring with a magnetic stirrer for 1 hour.

[0104] (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 with a UV-LED irradiator. Then, (L*, a*, b*) were measured using a spectrophotometer (eXact, manufactured by X-Rite), and ΔE was calculated. The evaluation was performed using ink prepared with an ink dispersion addition amount of 30% by mass. A result of ΔE < 2 was evaluated as "Excellent," a result of 2 ≤ ΔE < 3 was evaluated as "Good," a result of 3 ≤ ΔE < 5 was evaluated as "Acceptable," and a result of 5 ≤ ΔE was evaluated as "Unacceptable." The results are shown in Tables 5 and 6.

[0105] (Printing Condition) Using the ink, a solid print was made on a flatbed printing jig (an inkjet recording device corresponding to Figure 1, a prototype manufactured by Kyocera Corporation, equipped with the inkjet heads, corresponding to Figure 1) with inkjet heads (1200 dpi non-circulating heads (KJ4B-122) manufactured by Kyocera Corporation) arranged in the transport direction, forming an ink film on the printing medium (OK Topcoat paper manufactured by Oji Paper Co., Ltd.). The ink film was dried with a UV-LED irradiator, and those without streaks were judged as "Good (Good Product)". Each ink, prepared with ink dispersion amounts of 10% by mass, 20% by mass, 30% by mass, 40% by mass, and 50% by mass, was evaluated. If four or more inks were judged as "Good Product", the evaluation was "Excellent", if three were judged as "Good", if two were judged as "Acceptable", and if one or fewer were judged as "Unacceptable". The results are shown in Tables 5 and 6.

[0106] (Drying properties) 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 irradiation machine, and after drying, a peel test was performed on the ink film using cellophane tape (No. 405-1P, manufactured by Nichiban Co., Ltd.). Before and after peeling the tape, the optical density (OD value) was measured using a spectrophotometer (eXact, manufactured by X-Rite Inc.), and inks with a difference of OD value of 5% or less were classified as "Good (Good product)". Each ink prepared with an additive amount of ink dispersion of 10% by mass, 20% by mass, 30% by mass, 40% by mass, and 50% by mass was evaluated, and if four or more inks were judged as "Good product", the evaluation was "Excellent", if three were judged as "Good", if two were judged as "Acceptable", and if one or fewer were judged as "Unacceptable". The results are shown in Tables 5 and 6.

[0107]

[0108]

[0109] (Ink Set) Next, the inks were manufactured in the same manner as the inks, except that the content of the ink dispersion (the ink dispersion after storage) was adjusted so that the content of the ultraviolet absorber and moisturizing solvent 1 in the ink were as shown in Table 7. All of the moisturizing solvent 1 contained in the inks was originally present in the ink dispersion. Then, using each of the above inks, ink sets were prepared in the combinations shown in Table 7. Using the ink sets, evaluation images were formed on a printing medium (OK Topcoat paper manufactured by Oji Paper Co., Ltd.) on a flatbed printing jig (an inkjet recording device corresponding to Figure 1, a prototype manufactured by Kyocera Corporation, on which the inkjet heads are mounted, corresponding to Figure 1) with inkjet heads (1200 dpi non-circulating heads (KJ4B-122) manufactured by Kyocera Corporation) arranged in the transport direction.

[0110] (Blurring) The formed images were examined under a 300x microscope. If blurring was observed in the formed images, it was evaluated as "Excellent" if the blurring was within 5 μm, "Good" if the blurring was within 10 μm, "Acceptable" if the visible blurring was within 21 μm, and "Unacceptable" if the blurring exceeded 21 μm.

[0111]

[0112] Tables 1-6 show that when the emulsion particles (particles with the polymer attached to the UV absorber) are dispersed in the aqueous medium and the moisturizing solvent in an ink dispersion (Nos. 2-22, 24-27, 29-49, and 51-54), the increase in absorbance is suppressed even after storage, compared to cases without the moisturizing solvent (Nos. 1 and 28). Furthermore, when an image was formed using ink produced from the ink dispersions Nos. 2-22, 24-27, 29-49, and 51-54 after storing them under predetermined conditions, it was possible to produce an ink with a small ΔE, and furthermore, inks with excellent print quality and drying properties could be obtained over a wide range of ink dispersion addition amounts. Therefore, it was found that by including the moisturizing solvent, an ink dispersion can be obtained in which changes over time that adversely affect the performance of the ink are suppressed. This is because when the moisturizing solvent is included, the storage period is No. This was further clarified by the fact that even when the storage period was longer than 30 days (Nos. 7-22, 24-26, 34-49, and 51-54), the increase in absorbance due to storage was suppressed compared to cases where the moisturizing solvent was not included (Nos. 1 and 28). Furthermore, when the emulsion particles were not dispersed in the aqueous medium and the moisturizing solvent (Nos. 23 and 50), as described above, trace amounts of solid matter were observed in the prepared ink dispersion. In addition, when the ink obtained using this ink dispersion was used as inkjet ink, there was a risk of clogging the nozzles and causing ejection failure. Moreover, it was found that whether propylene glycol was used as the moisturizing solvent (Nos. 2-22 and 24-27) or diethylene glycol was used (Nos. 29-49 and 51-54), an ink dispersion was obtained in which changes over time that adversely affected the performance of the ink were suppressed.Furthermore, it was found that regardless of the color used as the pigment, whether magenta pigment (e.g., No. 10), cyan pigment (e.g., No. 11), yellow pigment (e.g., No. 12), or black pigment (e.g., No. 13), an image with a small ΔE can be formed, and an ink with excellent print quality and drying properties can be obtained, and the amount of ink dispersion added can be adjusted over a wide range.

[0113] The increase in viscosity during storage tended to be smaller when propylene glycol was used than when diethylene glycol was used. The reason for the viscosity increase is not well understood, but it is possible that factors other than viscosity may be involved. From this perspective, propylene glycol may be used instead of diethylene glycol.

[0114] It was also found that when the amount of the ultraviolet absorber is 0.8% by mass or more and 1.9% by mass or less relative to the total mass of the ultraviolet absorber and the moisturizing solvent (Nos. 6-18 and 33-45), an image with a smaller ΔE can be formed, and an ink with excellent print quality and drying properties can be obtained, and the amount of ink dispersion added can be made over a wider range.

[0115] It was also found that when the content of the moisturizing solvent is 65% by mass or more and 80% by mass or less relative to the ink dispersion (Nos. 6-18 and 33-45), an image with a smaller ΔE can be formed, and an ink with excellent print quality and drying properties can be obtained over a wider range of addition amounts to the ink dispersion.

[0116] Table 7 shows that when the inks are used in combination such that the ink with a high content of the UV absorber has a higher content of the moisturizing solvent than the ink with a low content of the UV absorber (A), the blurring of the formed image can be more suppressed compared to cases where this is not the case (B and C).

[0117] This application is based on Japanese Patent Application No. 2025-057014, filed on 28 March 2025, the contents of which are included in this application.

[0118] To represent this disclosure, the embodiments have been adequately and sufficiently described above, but those skilled in the art will recognize that it is easy to modify and / or improve upon the embodiments described above. Therefore, unless such modifications or improvements implemented by those skilled in the art fall outside the scope of the claims, such modifications or improvements will be construed as being included within the scope of the claims.

[0119] This disclosure provides an ink dispersion in which changes over time that adversely affect the performance of the ink are suppressed, an ink containing the ink dispersion, an ink set comprising the ink, and a method for manufacturing the ink containing the ink dispersion.

[0120] 10 Inkjet recording device 11 Transport unit 11A Supply roller 11B Recovery roller 12 Discharge unit 13 First heating unit 13A First heating roller 14 Ultraviolet irradiation unit 14A Light source 15 Second heating unit 15A Second heating roller 16 Control unit 17 Ink tank 21 Inkjet head 21a Discharge surface 101 Printing medium

Claims

1. An ink dispersion comprising emulsion particles containing an insoluble or poorly soluble ultraviolet absorber and polymer that are solid at 25°C, an aqueous medium, and a moisturizing solvent having hydroxyl groups in its molecule.

2. The ink dispersion according to claim 1, wherein the moisturizing solvent is a polyol compound.

3. The ink dispersion according to claim 1 or claim 2, wherein the absorbance at a wavelength of 450 nm after storage at 25°C for 15 days is 1.1 or less.

4. The ultraviolet absorber is an ink dispersion according to any one of claims 1 to 3, wherein the ultraviolet absorber has a phenol group in its molecule.

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

6. The ink dispersion according to any one of claims 1 to 5, wherein the content of the ultraviolet absorber is 0.8% by mass or more and 1.9% by mass or less, based on the total mass of the ultraviolet absorber and the moisturizing solvent.

7. The ink dispersion according to any one of claims 1 to 6, wherein the content of the moisturizing solvent is 65% by mass or more and 80% by mass or less with respect to the ink dispersion.

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

9. An ink comprising a colorant and an ink dispersion according to any one of claims 1 to 8.

10. An ink set comprising multiple inks of different colors, wherein each of the multiple inks is the ink described in claim 9.

11. The ink set according to claim 10, wherein among the plurality of inks, there are inks with different amounts of the ultraviolet absorber, and the ink with a higher amount of the ultraviolet absorber has a higher amount of the moisturizing solvent than the ink with a lower amount of the ultraviolet absorber.

12. A method for producing ink, comprising: preparing an ink dispersion comprising emulsion particles containing an insoluble or poorly soluble ultraviolet absorber and polymer that are solid at 25°C; an aqueous medium; and a moisturizing solvent having hydroxyl groups in its molecule; and mixing the ink dispersion with a colorant.

13. The method for producing ink according to claim 12, wherein the ink dispersion is stored after it has been prepared, and before it is mixed with a colorant.

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