Water-based ink, ink cartridge, inkjet recording method, titanium oxide particle dispersion liquid, method for producing the same, and method for producing water-based ink

Abstract

The present invention relates to an aqueous ink, an ink cartridge, an inkjet recording method, a titanium oxide particle dispersion liquid and a production method thereof, and a production method of an aqueous ink. The aqueous ink for inkjet recording includes titanium oxide particles containing titanium oxide, at least a part of the surface of the titanium oxide being coated with a specific ratio of alumina and silica; and a compound serving as a dispersant for the titanium oxide particles, the compound being represented by the following general formula (1): #imgabs0# wherein in the general formula (1), R1, R2 and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, each R4 is independently an alkylene group having 2 to 4 carbon atoms, X is a single bond or an alkylene group having 1 to 6 carbon atoms, n is 6 to 24, a is 1 to 3, b is 0 to 2, and a + b = 3.

Description

Technical Field

[0001] The present disclosure relates to aqueous ink, an ink cartridge, an inkjet recording method, a titanium oxide particle dispersion, a method for producing a titanium oxide particle dispersion, and a method for producing aqueous ink. Background Art

[0002] In recent years, inkjet recording devices have been widely used to output advertisements and displays using recording media such as paper and resin films. For example, in order to exhibit vivid color images even on a transparent recording medium, white ink is used in combination with black ink and inks of primary colors (hereinafter, sometimes collectively referred to as "color inks"). Specifically, the following recording method is employed: in which white ink is previously applied to a part of a transparent recording medium including a region where an image is to be recorded for an undercoat treatment, and color ink is applied thereon, or the inks are applied in the reverse order (so-called back printing).

[0003] Titanium oxide is widely used as a coloring material for white ink because of its low cost and excellent properties such as whiteness and hiding power required for white ink. In order to stably disperse titanium oxide in aqueous ink, a dispersant is required. The dispersant exists by repeating adsorption and desorption to titanium oxide. When environmental changes such as temperature change occur, in some cases, once the desorbed dispersant is difficult to adsorb on titanium oxide again, thereby impairing the dispersion stability. Even assuming the temperature during ink conveyance and various positions where the inkjet recording device is placed, it is necessary to maintain the stable dispersion of titanium oxide in a manner that does not affect the ejection stability of the ink.

[0004] So far, methods for stably dispersing titanium oxide while maintaining inkjet stability have been studied. For example, PCT Japanese Translation Patent Publication No. 2017-521348 discloses a method for producing a dried titanium oxide product in which the surface of titanium oxide particles is treated with silica, then further treated with a silane coupling agent, and the resulting titanium oxide particles are dried so that a part of the silane coupling agent is covalently bonded to the surface of the titanium oxide particles. International Publication No. 2018 / 190848 discloses an ink containing titanium oxide surface-treated with alumina, a monovalent metal salt, and alumina fine particles. Japanese Patent Application Laid-Open No. 2011-225867 discloses an ink containing titanium oxide surface-treated with alumina and silica, and then surface-treated with a silane coupling agent, a resin containing an anionic group, a water-soluble organic solvent, and a basic compound. Summary of the Invention

[0005] The present inventors studied the ejection stability of aqueous inks prepared using the dried titanium dioxide disclosed in PCT Japanese Translation Patent Publication No. 2017-521348 and aqueous inks disclosed in International Publication No. 2018 / 190848 and Japanese Patent Application Laid-Open No. 2011-225867. As a result, it was found that the ejection stability was insufficient, and there was room for improvement in the current situation where good performance was required even under more severe environmental changes assumed.

[0006] Accordingly, the present invention provides an aqueous ink containing titanium oxide for inkjet recording and having excellent ejection stability, an ink cartridge accommodating the aqueous ink, and an inkjet recording method. The present invention also provides a method for producing a titanium oxide particle dispersion liquid that can be used for producing an aqueous ink for inkjet recording having excellent ejection stability, and a method for producing an aqueous ink using the titanium oxide particle dispersion liquid obtained by this production method.

[0007] One aspect of the present invention aims to provide an aqueous ink for inkjet recording, the aqueous ink containing titanium oxide particles and a dispersant for the titanium oxide particles, wherein the titanium oxide particles contain titanium oxide, at least a part of the surface of the titanium oxide is coated with aluminum oxide and silicon dioxide, the ratio of aluminum oxide in the titanium oxide particles is 0.50 times or more and 1.00 times or less the ratio of silicon dioxide in the titanium oxide particles in terms of mass ratio, and the dispersant for the titanium oxide particles is represented by the following general formula (1):

[0008]

[0009] In the general formula (1), R1, R2, and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, each R4 is independently an alkylene group having 2 to 4 carbon atoms, X is a single bond or an alkylene group having 1 to 6 carbon atoms, n is 6 to 24, a is 1 to 3, b is 0 to 2, and a + b = 3.

[0010] With reference to the accompanying drawings, further features of the present invention will become apparent from the following description of exemplary embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 A cross-sectional view schematically showing an exemplary embodiment of an ink cartridge of the present invention.

[0012] Figure 2A A perspective view schematically showing an example of a main part of an inkjet recording apparatus used in an inkjet recording method according to the present invention, and Figure 2B A perspective view schematically showing an example of a head cartridge used in an inkjet recording method according to the present invention. DETAILED DESCRIPTION

[0013] The present invention will be described in more detail with reference to preferred exemplary embodiments. In the embodiments of the present invention, when the compound is a salt, the salt exists in the ink in a state of being dissociated into ions, but for convenience, it is expressed as "the ink contains a salt". Titanium oxide and titanium oxide particles may be simply referred to as "pigments". The aqueous ink for inkjet recording may be simply referred to as "ink". Unless otherwise specified, the physical property values are those at room temperature (25 °C).

[0014] Inorganic oxides such as titanium oxide react with water molecules contained in the aqueous medium in the aqueous ink to form hydroxyl groups (hereinafter, sometimes referred to as "surface hydroxyl groups") on the surface of the inorganic oxide. For this reason, in the aqueous ink for inkjet recording, in order to further improve the storage stability of the ink while utilizing the formed surface hydroxyl groups, the inorganic oxide is usually used in a state where it is surface-treated with a different inorganic oxide such as alumina or silica. The surface hydroxyl groups of the titanium oxide particles have the properties inherent to the inorganic oxide corresponding to the inorganic compound used for surface treatment, and the isoelectric point as an index of acid strength varies depending on the type of the inorganic compound. Therefore, although titanium oxide itself is an inorganic oxide, the surface of the titanium oxide particles exhibits the properties of the inorganic oxide corresponding to the inorganic compound used for surface treatment, and the surface charge of the titanium oxide particles strongly depends on the pH of the aqueous medium, the type of the surface treatment agent, and the amount of the surface treatment agent used.

[0015] The present inventors have studied how to improve the ejection stability of the ink by using the components contained in the ink. In order to stably disperse titanium oxide, a dispersant is required. At the same time, it has been found that using a dispersant having a high affinity for titanium oxide can stably disperse titanium oxide, but it affects the ejection stability of the ink. Although titanium oxide has a certain degree of dispersion stability due to charge repulsion through surface hydroxyl groups, it is difficult to maintain a stable dispersion state for a long time. For this reason, generally, the storage stability of the ink is improved by coating titanium oxide with silica and / or alumina, and the dispersion state is stably maintained by using a dispersant suitable for the surface state of the titanium oxide particles.

[0016] As a result of the inventors' research, when titanium oxide is coated only with silica, as described in PCT Japanese Translation Patent Publication No. 2017-521348, sufficient ejection stability of the ink cannot be obtained even when using a dispersant having a low affinity for titanium oxide, such as a silane coupling agent. As described in International Publication No. 2018 / 190848, when titanium oxide is coated only with alumina, sufficient dispersion stability of titanium oxide particles cannot be obtained even when using a dispersant having a low affinity for titanium oxide, such as a silane coupling agent. It has also been found that when a dispersant having a high affinity for titanium oxide particles is used to improve the dispersion stability of titanium oxide particles, in some cases, the ejection stability of the ink is affected. In addition, it has been found that titanium oxide particles surface-treated with alumina and silica as described in Japanese Patent Application Laid-Open No. 2011-225867 and then surface-treated with a silane coupling agent exhibit the same tendency as described above.

[0017] Under such circumstances, the present inventors conducted research to obtain a good balance between the dispersion stability of titanium oxide particles and the ejection stability of the ink by adjusting the coating amounts of alumina and silica on the surface of the titanium oxide particles. The present inventors also conducted research on a dispersant suitable for such titanium oxide particles. The present inventors found that by using a specific compound to disperse titanium dioxide particles whose surface is at least partially coated with a specific ratio of alumina and silica, the ejection stability of the ink can be improved without deteriorating the dispersion stability of the titanium oxide particles.

[0018] That is, the ink and the pigment dispersion according to an embodiment of the present invention have the following characteristics. Titanium oxide particles are used in which the ratio of the aluminum element in the titanium oxide particles to the silicon element in the titanium oxide particles is 0.57 times or more and 1.13 times or less in terms of mass ratio, and this ratio is obtained by inductively coupled plasma emission spectrometry. That is, titanium oxide particles are used in which the ratio of alumina in the titanium oxide particles to silica in the titanium oxide particles is 0.50 times or more and 1.00 times or less in terms of mass ratio. As a dispersant for dispersing the titanium oxide particles, a compound represented by the following general formula (1) is used. The mechanism by which the ejection stability of the ink is improved by the above configuration is speculated by the present inventors as follows.

[0019] The ink or pigment dispersion contains a compound represented by the general formula (1) as a dispersant for dispersing titanium oxide particles. Some of the compounds represented by the general formula (1) are known as silane coupling agents. In the general formula (1), each OR1 independently represents a hydroxyl group or an alkoxy group having 1 to 4 carbon atoms. A part of more than one OR1 group bonded to the silicon atom can be partially hydrolyzed in an aqueous medium to form a silanol group, and the silanol group can dissociate into ions. Therefore, by forming a hydrogen bond between the surface hydroxyl group of the titanium oxide particles and the silanol group of the compound represented by the general formula (1), a "weak affinity" is exhibited. A part between the surface hydroxyl group of the titanium oxide particles and the silanol group of the compound represented by the general formula (1) can be in a covalent bond state due to a dehydration reaction. That is, the surface hydroxyl group of the titanium oxide particles and the silanol group in the compound represented by the general formula (1) have an affinity for each other due to hydrogen bonds and covalent bonds. The compound represented by the general formula (1) can exist near the titanium oxide particles by repeating desorption and adsorption. When OR1 is an alkoxy group having more than 4 carbon atoms, it is difficult to form a silanol group by hydrolysis. As a result, the affinity for the surface hydroxyl group of the titanium oxide particles cannot be obtained, the titanium oxide particles cannot be stably dispersed, or the ejection stability of the ink cannot be obtained.

[0020] In addition to the part capable of forming a silanol group described above, the compound represented by the general formula (1) also has other parts as repeating units, including n alkylene oxide groups each having 2 to 4 carbon atoms ( (OR4) in the general formula (1)) n ) through X as a linking group. Here, n represents the number (average value) of alkylene oxide groups as repeating units, and is 6 to 24. Hereinafter, the above part will also be referred to as an "alkylene oxide chain". The alkylene oxide chain is hydrophilic. Therefore, the alkylene oxide chain extends moderately in an aqueous medium and exhibits a repulsive force due to steric hindrance. Therefore, the presence of the compound represented by the general formula (1) near the titanium oxide particles can stably disperse the titanium oxide particles.

[0021] Titanium oxide particles surface-treated only with silica have an affinity for the above dispersant which is also a silicon-containing compound. Therefore, the dispersant tends to be present in excess near the titanium oxide particles, and the surface properties of the titanium oxide particles are dominated by the properties of the dispersant. In a state where the excess dispersant repeats adsorption and desorption, the energy applied during inkjet ejection is consumed to desorb the excess dispersant. Therefore, the ejection speed of the ink decreases, and the ejection stability of the ink is insufficient.

[0022] Titanium oxide particles that have been surface-treated only with alumina have weak affinity for the above-mentioned dispersant that is a silicon-containing compound, and it is difficult for the dispersant to exist near the titanium oxide particles. For this reason, although the repulsive force of the electric double layer obtained by the ionization of some surface hydroxyl groups of the titanium oxide particles is obtained as described above, in the case of long-term storage or environmental changes such as temperature changes, a stable dispersion state of the titanium oxide particles cannot be maintained.

[0023] The titanium oxide particles contained in the ink and the pigment dispersion according to the embodiment of the present invention are titanium oxide coated with alumina and silica. The ratio of the aluminum element in the titanium oxide particles is 0.57 times or more and 1.13 times or less the ratio of the silicon element in the titanium oxide particles in terms of mass ratio, and this ratio is obtained by inductively coupled plasma emission spectrometry. That is, regarding the ratio of alumina and silica calculated by converting the result values of each element based on their oxides, the ratio of alumina in the titanium oxide particles is 0.50 times or more and 1.00 times or less the ratio of silica in the titanium oxide particles in terms of mass ratio. When the ratio of alumina (mass%) is within the above range, the titanium oxide particles can be stably dispersed and do not affect the ejection stability of the ink. This is because, since the titanium oxide particles are coated with alumina in a specific ratio, the amount of the dispersant present near the titanium oxide particles can be controlled within a range where dispersion stability can be obtained. When the mass ratio is less than 0.50 times, the ratio of silica to alumina is too high. This results in high affinity for the compound represented by the general formula (1), and the same as in the case of titanium oxide particles coated only with silica, an excessive amount of the compound is adsorbed on the titanium oxide particles. As a result, sufficient ink ejection stability cannot be obtained. When the mass ratio is greater than 1.00 times, the ratio of silica to alumina is too low. This results in insufficient affinity for the compound represented by the general formula (1), and the titanium oxide particles cannot be stably dispersed. As a result, ink ejection stability cannot be obtained.

[0024] In the embodiment of the present invention, by the constitution in which the compound represented by the general formula (1) is used to disperse titanium oxide particles having a surface coated with alumina and silica, due to the above-mentioned affinity, the compound can be widely present around the titanium oxide particles. Therefore, the titanium oxide particles can be stably dispersed. In addition, since the titanium oxide particles are coated with alumina and silica in a predetermined ratio, adsorption of an excessive amount of the compound can be suppressed. Therefore, the energy for desorbing the excessive compound used during ejection does not increase, and a decrease in the ejection speed can be suppressed. By satisfying these constitutions, the ejection stability of the ink can be improved.

[0025] Water-based ink, production method of water-based ink

[0026] The ink of the embodiments of the present invention is an aqueous ink for inkjet recording, which contains titanium oxide particles coated with a specific inorganic oxide and a specific compound for dispersing the titanium oxide particles. Since titanium oxide is a white pigment, the ink can be a white ink. The ink according to the embodiments of the present invention does not have to be a so-called "curable ink". Therefore, the ink according to the embodiments of the present invention does not need to contain compounds such as polymerizable monomers that can be polymerized by applying external energy such as heat or light. The components contained in the ink according to the embodiments of the present invention, the physical properties of the ink, the production method, etc. are described in detail below.

[0027] Colorant

[0028] The ink contains titanium oxide particles as a coloring material (pigment), and the titanium oxide particles contain titanium oxide surface-treated with a specific inorganic oxide. That is, the ink contains titanium oxide particles containing titanium oxide coated with a specific inorganic oxide on the surface. The content of titanium oxide particles in the ink is preferably 0.10% by mass or more and 20.00% by mass or less based on the total mass of the ink. The content of titanium oxide particles in the ink is more preferably 1.00% by mass or more and 20.00% by mass or less based on the total mass of the ink. The content of titanium oxide particles in the ink is particularly preferably 1.00% by mass or more and 15.00% by mass or less based on the total mass of the ink.

[0029] Titanium oxide is a white pigment and has three crystal forms: rutile type, anatase type, and brookite type. Among them, rutile type titanium oxide can be used. The industrial production methods of titanium dioxide include the sulfuric acid method and the chlorine method. The titanium oxide used in the embodiments of the present invention can be produced by any production method.

[0030] The volume-based 50% cumulative particle size (hereinafter, also referred to as "average particle size") of the titanium oxide particles is preferably 200 nm or more and 500 nm or less. In particular, the volume-based 50% cumulative particle size of the titanium oxide particles is more preferably 200 nm or more and 400 nm or less. The volume-based 50% cumulative particle size (D 50 ) is the particle size when the cumulative volume is 50% when integrating from the small particle size side based on the total volume of the measured particles in the cumulative particle size curve. The D 50 of titanium oxide can be measured, for example, under the conditions of SetZero: 30 seconds, number of measurements: 3 times, measurement time: 180 seconds, shape: non-spherical, refractive index: 2.60. As a particle size distribution measuring device, a particle size analyzer based on the dynamic light scattering method can be used. Of course, the measurement conditions, etc. are not limited to the above.

[0031] Titanium oxide surface-treated with alumina and silica. The surface treatment should suppress photocatalytic activity and improve dispersibility. In this specification, "alumina" is a general term for oxides of aluminum such as aluminum oxide. In this specification, "silica" is a general term for silicon dioxide and substances composed of silicon dioxide. Most of the alumina and silica coating the titanium oxide exist in the form of alumina and silica.

[0032] The proportion of titanium oxide in the titanium oxide particles, based on the total mass of the titanium oxide particles, can be 90.00 mass% or more. The proportion of titanium oxide in the titanium oxide particles, based on the total mass of the titanium oxide particles, can be 98.50 mass% or less. The proportion of alumina in the titanium oxide particles by mass ratio needs to be 0.50 times or more and 1.00 times or less the proportion of silica in the titanium oxide particles. A mass ratio less than 0.50 times or greater than 1.00 times does not result in sufficient ink ejection stability. The proportion of silica in the titanium oxide particles, based on the total mass of the titanium oxide particles, can be 1.00 mass% or more and 4.00 mass% or less. When the proportion of silica is less than 1.00 mass%, sufficient affinity for the compound represented by the general formula (1) may not be obtained, and sufficient ink ejection stability may not be obtained. When the proportion of silica exceeds 4.00 mass%, even with surface treatment with alumina, the amount of the compound represented by the general formula (1) adsorbed on the titanium oxide particles may not be suppressed, and sufficient ink ejection stability may not be obtained. The proportion of alumina in the titanium oxide particles, based on the total mass of the titanium oxide particles, can be 0.50 mass% or more and 4.00 mass% or less.

[0033] As a method for measuring the proportion of alumina and silica in the titanium oxide particles, that is, the coating amounts of alumina and silica, for example, quantitative analysis of aluminum and silicon elements by inductively coupled plasma (ICP) emission spectrometry can be performed. In this case, the calculation can be carried out by assuming that all atoms on the coating surface are in the form of oxides and converting the obtained values of aluminum and silicon into their oxides, that is, alumina and silica. The proportion of aluminum element in the titanium oxide particles by mass ratio is 0.57 times or more and 1.13 times or less the proportion of silicon element in the titanium oxide particles, which is obtained by inductively coupled plasma emission spectrometry. When these values are converted based on their oxides, that is, alumina and silica, the proportion of alumina in the titanium oxide particles by mass ratio is 0.50 times or more and 1.00 times or less the proportion of silica in the titanium oxide particles.

[0034] Examples of the surface treatment method of titanium oxide include wet treatment and dry treatment. For example, surface treatment can be carried out by dispersing titanium oxide in a liquid medium and then reacting titanium oxide with surface treatment agents such as sodium aluminate and sodium silicate. By appropriately changing the ratio of the surface treatment agent, the surface treatment can be adjusted to the desired properties. In addition to alumina and silica, inorganic oxides such as zinc oxide and zirconia, and organic substances such as polyols can also be used as long as the advantageous effects of the present invention are not impaired.

[0035] The ink may contain other pigments other than titanium oxide as long as the advantageous effects of the present invention are not impaired. In this case, colored inks other than white ink can also be used. The content of other pigments in the ink is preferably 0.10% by mass or more and 5.00% by mass or less, more preferably 0.10% by mass or more and 1.00% by mass or less, based on the total mass of the ink.

[0036] Compound represented by general formula (1)

[0037] The ink contains a compound represented by the following general formula (1) as a dispersant for dispersing titanium oxide particles. The amount of the compound represented by the general formula (1) in the ink is preferably 0.01% by mass or more and 1.00% by mass or less, more preferably 0.02% by mass or more and 0.50% by mass or less, based on the total mass of the ink.

[0038]

[0039] In the general formula (1), R1, R2, and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, each R4 is independently an alkylene group having 2 to 4 carbon atoms, X is a single bond or an alkylene group having 1 to 6 carbon atoms, n is 6 to 24, a is 1 to 3, b is 0 to 2, and a + b = 3.

[0040] In the general formula (1), R1, R2, and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. Examples of the alkyl group having 1 to 4 carbon atoms include methyl, ethyl, n-propyl, isopropyl, and n-butyl. Among them, methyl can be used from the viewpoint of easy hydrolysis. When each of R1, R2, and R3 is an alkyl group having more than 4 carbon atoms, the compound is not easily hydrolyzed to form a silanol group, so that the affinity for titanium oxide particles cannot be obtained. Therefore, the titanium oxide particles cannot be stably dispersed, and sufficient ejection stability of the ink cannot be obtained. Here, a representing the number of R1O is 1 to 3, b representing the number of R2 is 0 to 2, and a + b = 3. In particular, a can be 3 and b can be 0, that is, all three substituents on the silicon atom can be R1O.

[0041] In general formula (1), each R4 is independently an alkylene group having 2 to 4 carbon atoms. Examples of the alkylene group having 2 to 4 carbon atoms include ethylene, n-propylene, isopropylidene, and n-butylene. In particular, ethylene can be used. The number of OR4, that is, n (average value) representing the number of alkylene oxide groups is 6 to 24. When n is less than 6, the length of the alkylene oxide chain is too short, and thus, the repulsive force caused by steric hindrance cannot be sufficiently obtained, and sufficient ejection stability cannot be obtained. When n is greater than 24, the length of the alkylene oxide chain is too long, making the compound have higher hydrophilicity and more likely to exist in a free form in the aqueous medium. Therefore, the affinity for the surface hydroxyl groups of titanium oxide particles cannot be sufficiently obtained, and the aggregation of titanium oxide particles cannot be inhibited. Therefore, titanium oxide particles cannot be stably dispersed, and sufficient ejection stability of the ink cannot be obtained.

[0042] In general formula (1), X is a single bond or an alkylene group having 1 to 6 carbon atoms. When X is a single bond, it means that the silicon atom and OR4 are directly bonded to each other. Examples of the alkylene group having 1 to 6 carbon atoms include methylene, ethylene, n-propylene, isopropylidene, n-butylene, n-pentylene, and n-hexylene. In particular, n-propylene can be used. When X is an alkylene group having more than 6 carbon atoms, the hydrophobicity of the compound represented by general formula (1) is too high; thus, titanium oxide particles cannot be stably dispersed, and sufficient ejection stability of the ink cannot be obtained.

[0043] The compound represented by general formula (1) used as a dispersant for titanium oxide particles can be a compound represented by the following general formula (2). The compound represented by general formula (2) has three OR1 groups bonded to the silicon atom. Therefore, this compound can be partially hydrolyzed in the aqueous medium to form three hydroxyl groups bonded to the silicon atom, thereby increasing the moieties each having an affinity for titanium oxide particles. In addition, the compound represented by the following general formula (2) has a repeating unit of an oxyethylene group. Therefore, the oxyethylene chain is appropriately elongated in the aqueous medium, and the repulsive force caused by steric hindrance can be obtained.

[0044]

[0045] In general formula (2), R1 and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and m is 8 to 24.

[0046] The amount of the compound represented by the general formula (1) contained in the ink in terms of mass ratio may be 0.002 times or more and 0.10 times or less the amount of titanium oxide particles contained in the ink. When the mass ratio is less than 0.002 times, the effect of stably dispersing titanium oxide particles is weak. Therefore, in some cases, the ejection stability of the ink cannot be sufficiently obtained. When the mass ratio is greater than 0.10 times, the proportion of the compound represented by the general formula (1) is too high; therefore, intermolecular condensation (self-condensation) of the compound represented by the general formula (1) easily occurs. Therefore, the compound represented by the general formula (1) is not consumed as a dispersant; therefore, the effect of stably dispersing titanium oxide particles is weak, and sometimes the ejection stability of the ink cannot be sufficiently obtained.

[0047] The compound represented by the general formula (1) forms a hydrogen bond with the surface hydroxyl group of the titanium oxide particles, and it is considered that some of them form a covalent bond due to a dehydration reaction. However, in an embodiment of the present invention, the compound represented by the general formula (1) can disperse the titanium oxide particles even without forming a covalent bond with the titanium oxide particles. That is, the amount of the compound represented by the general formula (1) covalently bonded to the titanium oxide particles is very small and negligible. Therefore, the amount of the compound represented by the general formula (1) covalently bonded to the titanium oxide particles is not included in the content of the titanium oxide particles. As a result of the study by the present inventors, it has been found that when the amount of the compound represented by the general formula (1) covalently bonded to the titanium oxide particles is excessive, the ejection stability of the ink deteriorates. The reason is considered as follows: Generally, in a liquid medium having a high dielectric constant such as water, the electrostatic attraction hardly works, and therefore, the titanium oxide particles move freely without being greatly affected by the surrounding environment. However, when the compound represented by the general formula (1) is covalently bonded to the titanium oxide particles, the hydrophilic part (OR4 part) of the general formula (1) structure forms a hydrogen bond with water molecules, which in some cases affects the movement of the titanium oxide particles. Therefore, when a deformation due to an instantaneous pressure is applied to the liquid as in the case of inkjet ejection, the above characteristics are manifested as a difference in ejection characteristics. For this reason, the amount of the compound represented by the general formula (1) covalently bonded to the titanium oxide particles in terms of mass ratio may be 0.001 times or less the amount of titanium oxide particles contained in the aqueous ink. When the mass ratio is greater than 0.001 times, the ejection stability of the ink cannot be sufficiently obtained in some cases. This mass ratio may be 0.000 times. The amount of the compound represented by the general formula (1) covalently bonded to the titanium oxide particles can be calculated by, for example, thermogravimetric analysis.

[0048] Resin

[0049] The ink may contain a resin. Examples of the resin include acrylic resins, polyurethane resins, and urea resins. In particular, acrylic resins can be used. The resin content in the ink is preferably 1.00% by mass or more and 25.00% by mass or less, more preferably 3.00% by mass or more and 15.00% by mass or less, and particularly preferably 5.00% by mass or more and 15.00% by mass or less, based on the total mass of the ink.

[0050] In order to improve various properties of the recorded image, such as scratch resistance and masking properties, a resin can be included in the ink. Examples of the form of the resin include block copolymers, random copolymers, graft copolymers, and combinations of these copolymers. In addition, the resin can be a water-soluble resin that is soluble in an aqueous medium, or can be resin particles dispersed in an aqueous medium. The resin particles do not need to contain a coloring material.

[0051] In this specification, a "water-soluble resin" means that when the resin is neutralized with an amount of base equivalent to the acid value of the resin, the resin exists in the aqueous medium in a state where no particles with a particle size measurable by the dynamic light scattering method are formed. Whether the resin is soluble in water can be determined by the following method: First, a liquid containing the resin neutralized with a base equivalent to the acid value (e.g., sodium hydroxide or potassium hydroxide) is prepared (resin solid content: 10% by mass). The prepared liquid is diluted 10-fold (by volume) with deionized water to prepare a sample solution. Then, when the particle size of the resin in the sample solution is measured by the dynamic light scattering method, if no particles with a particle size are measured, it can be determined that the resin is soluble in water. The measurement conditions at this time can be set as follows: for example, SetZero: 30 seconds, number of measurements: 3 times, and measurement time: 180 seconds. As a particle size distribution measuring device, for example, a particle size analyzer by the dynamic light scattering method (e.g., trade name "UPA-EX150", available from Nikkiso Co., Ltd.) can be used. Of course, the particle size distribution measuring device and measurement conditions used are not limited to the above.

[0052] The acid value of the water-soluble resin is preferably 80 mgKOH / g or more and 250 mgKOH / g or less, more preferably 100 mgKOH / g or more and 200 mgKOH / g or less. When using resin particles, the acid value of the resin particles is preferably 0 mgKOH / g or more and 50 mgKOH / g or less. The weight average molecular weight of the resin is preferably 1,000 or more and 30,000 or less, more preferably 5,000 or more and 15,000 or less. The weight average molecular weight of the resin is a value in terms of polystyrene converted by gel permeation chromatography (GPC).

[0053] Aqueous medium

[0054] The ink is an aqueous ink containing water as an aqueous medium. The ink may contain water or an aqueous medium as a mixed solvent of water and a water-soluble organic solvent. As the water, deionized water (ion-exchanged water) can be used. The water content in the ink can be 50.00% by mass or more and 95.00% by mass or less based on the total mass of the ink.

[0055] There is no particular limitation on the water-soluble organic solvent as long as it is water-soluble (it can be dissolved in water at any ratio at 25°C). Specific examples of the water-soluble organic solvent that can be used include monohydric or polyhydric alcohols, alkylene glycols, glycol ethers, nitrogen-containing polar compounds, and sulfur-containing polar compounds. The content of the water-soluble organic solvent in the ink is preferably 3.00% by mass or more and 50.00% by mass or less, more preferably 10.00% by mass or more and 40.00% by mass or less based on the total mass of the ink. When the content of the water-soluble organic solvent is less than 3.00% by mass, the ink may adhere in the inkjet recording device and may have insufficient adhesion resistance. When the content of the water-soluble organic solvent is greater than 50.00% by mass, poor ink supply may occur.

[0056] Other additives

[0057] In addition to the above additives, the ink may further contain various additives such as surfactants, pH adjusters, rust inhibitors, preservatives, antifungal agents, antioxidants, reduction inhibitors, evaporation promoters, and chelating agents as needed. In particular, the ink may contain a surfactant. The content of the surfactant in the ink is preferably 0.10% by mass or more and 5.00% by mass or less, more preferably 0.10% by mass or more and 2.00% by mass or less based on the total mass of the ink. Examples of the surfactant include anionic surfactants, cationic surfactants, and nonionic surfactants. Among them, a nonionic surfactant with low affinity for titanium oxide particles and effective even in a small amount can be used because this surfactant is used to adjust various physical properties of the ink.

[0058] Physical properties of ink

[0059] The ink is used for an inkjet recording method; thus, its physical properties can be appropriately controlled. The surface tension of the ink at 25°C is preferably 10 mN / m or more and 60 mN / m or less, more preferably 20 mN / m or more and 40 mN / m or less. The surface tension of the ink can be adjusted by appropriately determining the type and amount of surfactant in the ink. The viscosity of the ink at 25°C can be 1.0 mPa·s or more and 10.0 mPa·s or less. The pH value of the ink at 25°C can be 7.0 or more and 9.0 or less. When the pH of the ink is within the above range, a silanol group is formed by hydrolyzing the compound represented by the general formula (1), and thus, the weak affinity between the titanium oxide particles and the compound represented by the general formula (1) is effectively exhibited. The pH of the ink can be measured using a general pH meter equipped with, for example, a glass electrode.

[0060] Production method of ink

[0061] The method for producing the ink according to the present invention includes mixing a dispersion of titanium oxide particles and other ink components. As the dispersion of titanium oxide particles, a dispersion of titanium oxide particles produced by the production method described below is used. Examples of other ink components include water, water-soluble organic solvents, resins, and the above-mentioned "other additives" to be further added. The method for producing the ink can be carried out, for example, by putting the dispersion of titanium oxide particles and other ink components into an appropriate container and stirring the mixture. Conditions such as stirring speed, temperature, and time can be appropriately set according to the desired conditions. In addition, known production steps can be combined.

[0062] Production method of titanium oxide particle dispersion

[0063] The method for producing a dispersion of titanium oxide particles according to an embodiment of the present invention is used for producing an aqueous ink for inkjet recording. The method includes a step of reacting a pretreatment agent with the surface of titanium oxide to prepare titanium oxide particles having a surface coated with alumina and silica (first step). The method further includes, after the first step, a dispersion step (second step) of dispersing the titanium oxide particles in a liquid medium with a compound represented by the general formula (1) to disperse the titanium oxide particles. Hereinafter, each step will be described in detail.

[0064] First step

[0065] The first step is a step of preparing titanium oxide particles having titanium oxide coated with alumina and silica on the surface. As the titanium oxide particles, titanium oxide particles pre-coated with alumina and silica can be used, or untreated titanium oxide particles subjected to a coating treatment can be used. Examples of the coating treatment method include wet treatment and dry treatment. Among them, wet treatment can be used because uniform surface treatment can be performed. Specific examples thereof include a method in which a surface treatment agent is added to a titanium oxide dispersion as a raw material. Examples of the surface treatment agent include sodium aluminate and sodium silicate. The conditions for the surface treatment can be ordinary. In addition, by further subjecting the titanium oxide particles pre-coated with alumina and silica to a coating treatment, various properties of the titanium oxide particles can be adjusted.

[0066] After the first step, the surface-treated titanium oxide (titanium oxide particles) can be in a dry state or in a state in which it is contained in a liquid medium such as water. For the convenience of the subsequent treatment in the second step, the titanium oxide particles can be in a state in which they are contained in a liquid medium. The concentration of the titanium oxide particles in the liquid medium can be set from the viewpoints of the efficiency of the second step and the ease of treatment. Specifically, the content of the titanium oxide particles in the liquid containing the titanium oxide particles can be 20.00% by mass or more and 60.00% by mass or less based on the total mass of the liquid.

[0067] Second step

[0068] The second step is a dispersion step of dispersing the titanium oxide particles in a liquid medium with a compound represented by the general formula (1) to disperse the titanium oxide particles. If necessary, a pH adjuster and various additives can be used. In the second step, a compound represented by the general formula (1) used as a dispersant is used to disperse the titanium oxide particles by a treatment such as applying a shear force required to obtain a desired particle size distribution. In the second step, known dispersion methods such as medium dispersion or medium-free dispersion can be used. Examples of the disperser for medium dispersion include paint stirrers, bead mills, sand mills, ball mills, and roll mills. Examples of the disperser for medium-free dispersion include ultrasonic homogenizers and high-pressure homogenizers. The second step can be carried out using a combination of two or more of the above-mentioned dispersers.

[0069] The titanium oxide particle dispersion obtained in the second step is stored for a certain period of time as needed, and then used to prepare an aqueous ink for inkjet recording. From such a viewpoint, the concentration of the titanium oxide particles in the dispersion can be set. Specifically, the content of the titanium oxide particles in the titanium oxide particle dispersion can be 20.00% by mass or more and 60.00% by mass or less based on the total mass of the dispersion.

[0070] The temperature in the second step can be freely set. The second step is carried out in an aqueous liquid medium; therefore, from the viewpoints of heat generation during this step and the reliability of the medium when using the medium dispersion method, the temperature is preferably 0 °C or higher and 100 °C or lower, more preferably 10 °C or higher and 40 °C or lower. The time of the second step can be adjusted according to, for example, the equipment used and the concentration of the dispersion liquid, and can be freely set as long as the titanium oxide particles are not over-dispersed. For example, when carrying out the dispersion step at 25 °C using a paint stirrer with 0.5 mm zirconia beads, the dispersion time can be 10 hours or more and 20 hours or less.

[0071] A preliminary dispersion step can be carried out to mix the components including titanium oxide particles, wet the components in the liquid medium, and promote dispersion. In the preliminary dispersion step, the dispersion methods and equipment as described available in the second step can be used.

[0072] pH regulator

[0073] In the second step, a pH regulator can be used to control the state of the surface hydroxyl groups of the titanium oxide particles. The pH regulator can be an acidic compound or a basic compound. Among them, a basic compound can be used because the surface hydroxyl groups of the titanium oxide particles can be maintained in an anionic state. When the liquid medium is alkaline, a part of the surface hydroxyl groups of the titanium oxide particles are ionized to form a double electric layer, and good dispersion stability due to charge repulsion can be obtained. Examples of the basic compound include ammonia; organic ammonium compounds; and alkali metal hydroxides such as potassium hydroxide and sodium hydroxide. Among them, considering loading the resulting dispersion liquid into the ink, potassium hydroxide can be used. The pH of the liquid medium in the second step adjusted using the pH regulator can be 7.0 or higher and 12.5 or lower. When the pH is greater than 12.5, the silica coating the surface of the titanium oxide particles may dissolve. In order to maintain the pH of the liquid medium within the above range, the pH regulator can be continuously added at a freely selected timing.

[0074] Liquid medium

[0075] The second step can be carried out in a liquid medium such as an aqueous liquid medium, for example. As the aqueous liquid medium, water can be used alone, or an aqueous medium mainly composed of water and combined with a protonic or aprotic organic solvent can be used. The aqueous medium is a mixed solvent of water and an organic solvent. As the organic solvent, an organic solvent that is miscible or soluble with water in any proportion can be used. Among them, as the aqueous medium, a homogeneous mixed solvent containing water in an amount of 50 mass% or more can be used. As the water, ion-exchanged water or deionized water can be used.

[0076] Protic organic solvents are organic solvents having a hydrogen atom (acidic hydrogen atom) bonded to oxygen or nitrogen. Aprotic organic solvents are organic solvents not having acidic hydrogen atoms. Examples of organic solvents include alcohols; alkylene glycols; polyalkylene glycols; glycol ethers; glycol ether esters; carboxylic acid amides; ketones; keto alcohols; cyclic ethers; nitrogen-containing compounds; and sulfur-containing compounds.

[0077] Post-treatment

[0078] The produced titanium oxide particle dispersion can be subjected to usual post-treatment methods such as purification and then used for producing an aqueous ink for inkjet recording. In the case of using only water as a liquid medium without using an organic solvent, the obtained dispersion can be directly used for preparing an ink, or the dispersion can be used as a final dispersion after washing or adjusting the content of titanium oxide particles. When using a liquid medium containing an organic solvent, the organic solvent can be removed. Examples of methods for removing the organic solvent include a method of adding water while heating and removing the organic solvent under reduced pressure or by using an evaporator, etc. to prepare an aqueous titanium oxide particle dispersion. In addition, there is also a method of repeatedly performing operations such as removing the organic solvent by ultrafiltration and then adding water. In particular, in the case of preparing a dispersion in which titanium oxide particles are dispersed in an aqueous medium, by dissociating the surface hydroxyl ions of the titanium oxide particles, the dispersed state can be more stably maintained due to the repulsive force based on electrostatic repulsion. Therefore, for example, a pH adjuster can be added as needed to make the dispersion alkaline.

[0079] Ink cartridge

[0080] The ink cartridge according to an embodiment of the present invention includes ink and an ink storage portion for storing the ink. The ink accommodated in the ink storage portion is the above-described aqueous ink (white ink) according to an embodiment of the present invention. Figure 1 It is a cross-sectional view schematically showing an exemplary embodiment of the ink cartridge of the present invention. As Figure 1 shown, an ink supply port 12 for supplying ink to a recording head is provided on the bottom surface of the ink cartridge. The inside of the ink cartridge is an ink storage portion for storing ink. The ink storage portion includes an ink storage chamber 14 and an absorber storage chamber 16 that communicate with each other through a communication port 18. The absorber storage chamber 16 communicates with the ink supply port 12. The ink storage chamber 14 accommodates liquid ink 20. The absorber storage chamber 16 receives absorbers 22 and 24 that hold the ink in an impregnated state. The ink storage portion can be configured to include an absorber that holds the total amount of stored ink without having an ink storage chamber for storing liquid ink. The ink storage portion does not have an absorber and can be configured to store the total amount of liquid ink. In addition, the ink cartridge can be configured to include an ink storage portion and a recording head.

[0081] Inkjet recording method

[0082] The inkjet recording method according to an embodiment of the present invention is a method of recording an image on a recording medium by ejecting the above-described aqueous ink according to an embodiment of the present invention from an inkjet recording head. Examples of the method for ejecting the ink include a method of applying mechanical energy to the ink and a method of applying thermal energy to the ink. In one embodiment of the present invention, a method of ejecting the ink by applying thermal energy to the ink can be employed. Except for using the ink according to an embodiment of the present invention, the steps included in the inkjet recording method can be the same as known steps. For example, when recording an image with white ink, a general inkjet recording method can be employed as it is. When performing an undercoat treatment on color ink with white ink, an image can be recorded by applying color ink (e.g., black, cyan, magenta, or yellow ink) so as to overlap at least a part of the area where the white ink is applied. In addition, the undercoat treatment can also be used for reverse printing of applying white ink so as to overlap at least a part of the area where the color ink is applied. There is no particular limitation on the recording medium, but since the aqueous ink according to an embodiment of the present invention can be used as white ink, a transparent or colored recording medium can be used. The recording medium can be a medium with poor absorbency (non-absorbent medium), such as a resin film with low absorbency of a liquid medium.

[0083] Figure 2A FIG. is a perspective view schematically showing an example of the main part of an inkjet recording apparatus used in the inkjet recording method according to the present invention, and Figure 2B FIG. is a perspective view schematically showing an example of a head cartridge used in the inkjet recording method according to the present invention. The inkjet recording apparatus includes a conveyance unit (not shown) configured to convey a recording medium 32; and a carriage shaft 34. The head cartridge 36 can be mounted to the carriage shaft 34. The head cartridge 36 includes recording heads 38 and 40, and is configured in such a manner that an ink cartridge 42 is provided therein. While the head cartridge 36 is conveyed in the main scanning direction along the carriage shaft 34, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. Then, the recording medium 32 is conveyed in the sub-scanning direction by a conveyance unit (not shown), thereby recording an image on the recording medium 32.

[0084] Multi-pass recording in which ink is applied to a unit area of the recording medium in a plurality of relative scans between the recording head and the recording medium can be used. In particular, white ink and color ink can be applied to the unit area in different relative scans. This allows more time for the inks to contact each other and makes it more likely to suppress mixing. The unit area can be set to any area, such as one pixel or one band.

[0085] Examples

[0086] Hereinafter, the present invention will be described in more detail by way of examples and comparative examples. The present invention is not limited to the following examples as long as it is within the scope of the present invention. Regarding the amounts of components, "parts" and "%" are based on mass unless otherwise specified. The titanium oxide particle dispersion is referred to as "pigment dispersion".

[0087] Preparation of titanium oxide

[0088] Commercially available titanium oxide particles that have been surface-treated in advance and titanium oxide particles prepared by surface-treating untreated titanium oxide are used. The volume-based 50% cumulative particle size (D 50 ) of the titanium oxide particles is measured by dynamic light scattering using a particle size analyzer (trade name: "UPA-EX150", available from Nikkiso Co., Ltd.). Table 1 shows the properties of the titanium oxide particles. In Table 1, TITANIX: JR, JR-403, JR-800, JR-806, JR-805, JR-301, JR-405, and JR-600A are the trade names of rutile-type titanium oxides available from Tayca Corporation. TITONE: R-62N and R-7E are the trade names of rutile-type titanium oxides available from Sakai Chemical Industry Co., Ltd. TIPAQUE: PFC-208, R-780-2, PFC-211, and R-780 are the trade names of rutile-type titanium oxides available from Ishihara Sangyo Kaisha, Ltd.

[0089] Measurement of coating amounts of alumina and silica

[0090] The ratios of alumina and silica in the titanium oxide particles, that is, the coating amounts of alumina and silica, are measured as follows: The liquid obtained by adding the prepared titanium oxide particles to nitric acid is used as a sample. Inductively coupled plasma (ICP) emission spectrometry is used for quantitative analysis of aluminum and silicon elements. At this time, assuming that all atoms on the surface of the coated titanium oxide are in the form of oxides, the obtained values of aluminum and silicon are converted to values based on oxides, that is, alumina and silica values, and the mass ratio is calculated.

[0091] Titanium oxide particles 1 to 8

[0092] The surface treatment of titanium oxide is carried out by a wet method to produce titanium oxide particles 1 to 8. In the surface treatment by the wet method, untreated titanium oxide is brought into contact with surface treatment agents (for example, sodium aluminate and sodium silicate). In the surface treatment, the amounts and ratios of the surface treatment agents used are appropriately adjusted to achieve the desired ratios.

[0093] Specifically, 300 parts of untreated rutile titanium oxide (trade name: "TITANIX JR", available from Tayca Corporation) and 700 parts of deionized water were mixed using a homogenizer. While stirring, the temperature was raised to 90 °C. Potassium hydroxide (pH regulator) was added to adjust the pH to 10.5. Sodium silicate was added thereto. Dilute sulfuric acid (pH regulator) was added over about 1 hour to adjust the pH to 5.0. The reaction was continued for about 1 hour. Then, a small amount of sodium aluminate was added at 90 °C. To maintain the pH, dilute sulfuric acid was used in combination to keep the pH above 6.0 and below 8.0. After adding sodium aluminate, the reaction was continued for about 1 hour to obtain a dispersion. The dispersion was cooled to 25 °C, purified by repeating sedimentation and redispersion in ion-exchanged water using a centrifugal separator, and dried at 120 °C to obtain each titanium oxide particle surface-treated with at least one of alumina and silica. Table 1 shows the properties of titanium oxide particles 1 to 8.

[0094] Titanium oxide particles 9 to 22

[0095] As titanium oxide particles 9 to 22, commercially available titanium oxide particles (including those pre-surface-treated with alumina and / or silica) were used. Table 1 also shows the properties of titanium oxide particles 9 to 22. Some commercially available titanium oxide particles contain inorganic oxides such as zinc oxide and zirconium oxide, and organic compounds such as polyols, in addition to alumina and silica, but the proportion is at most about 1.0%. Therefore, for convenience, these are collectively included as the proportion T (%) of titanium oxide in the titanium oxide particles ("titanium oxide T (%)" in Table 1).

[0096] Table 1 Properties of Titanium Oxide Particles

[0097]

[0098] Preparation of compound represented by general formula (1)

[0099] The compound represented by the general formula (1) was synthesized through the following steps. The synthesis conditions for the compound represented by the general formula (1) and the comparative compound are given in Table 2, and its structure is given in Table 3. The compound represented by the general formula (1) can be synthesized by allylation and hydrosilylation of a raw material (e.g., polyalkylene glycol monoalkyl ether).

[0100] Compounds 1 to 13 and comparative compound 14

[0101] The raw materials, base, and solvent listed in Table 2 were fed into a three-necked flask equipped with a stir bar and a nitrogen inlet. The mixture was stirred at 25 °C for 30 minutes. As the "sodium hydride", a 60% dispersion of sodium hydride in paraffin was used. This dispersion was used to achieve the amount of sodium hydride given in Table 2. The mixture was stirred at 25 °C while dropping the bromide described in Table 2. After the dropping was completed, stirring was continued for an additional 12 hours to prepare a mixture containing the reaction product. After separating unreacted sodium hydride and the neutralization product (sodium bromide) from the mixture containing the reaction product by filtration, THF was removed under reduced pressure to obtain a concentrate. The concentrate was dissolved in 500 parts of deionized water. This aqueous solution was extracted 3 times with 200 mL of hexane and then with 200 mL of dichloromethane. The solvent containing the product was dried by adding magnesium sulfate and concentrated under reduced pressure to obtain each allylated compound (allylation step).

[0102] The allylation raw materials and silane compound given in Table 2 were fed into a passivated and dried round-bottom flask equipped with a stir bar and an argon inlet. The mixture was stirred at 85 °C. Then, a 0.54 part 65 mmol / L solution of chloroplatinic acid monohydrate in isopropanol and water was added thereto. The mixture was heated at 85 °C for 5 hours. After the reaction was completed, the mixture was cooled to 25 °C. The excess silane compound was removed under reduced pressure. The residue was purified by column chromatography using silica gel passivated with triethoxysilane as the carrier to obtain each compound (hydrosilylation step). In the purification by column chromatography, an eluent of ethyl acetate / hexane / ethanol = 85 / 15 / 5 (by volume) was used.

[0103] Comparative compound 15

[0104] The raw materials and solvent described in Table 2 were fed into a three-necked flask equipped with a stir bar, a reflux condenser, and an argon inlet, and the raw materials were dissolved. The bromide described in Table 2 was dropped with stirring at 80 °C over about 1 hour, and the resulting mixture was refluxed for 30 minutes. After the reaction was completed, THF was removed under reduced pressure, and 300 parts of deionized water and the base described in Table 2 were added. The resulting liquid containing the reaction product was cooled to 25 °C and extracted twice with 200 mL of ether. The solvent containing the product was dried over magnesium sulfate and evaporated under reduced pressure to obtain the allylated compound. The hydrosilylation step was carried out in the same manner as for Compounds 1 to 13 and Comparative Compound 14 to obtain Comparative Compound 15.

[0105] Comparative compound 16

[0106] The raw materials and solvents described in Table 2 were fed into a three-necked flask equipped with a stir bar and a nitrogen inlet, and the raw materials were dissolved. The mixture was stirred at 55 °C for 3 hours. After the reaction was completed, the solvent was removed under reduced pressure to obtain a concentrate. Then, 100 parts of ethanol was added to the concentrate, and then, filtration was carried out. The residue was washed with ethanol to remove impurities. The liquid components in the filtrate were removed under reduced pressure to obtain the allylated compound. The hydrosilylation step was carried out in the same manner as for Compounds 1 to 13 and Comparative Compound 14 to obtain Comparative Compound 16.

[0107] Synthesis conditions of the compounds represented by the general formula (1) in Table 2

[0108]

[0109] Table 3 Structures of the synthesized compounds represented by the general formula (1)

[0110]

[0111] Preparation of pigment dispersion

[0112] The pigment dispersion was produced by the following steps. The production conditions of the pigment dispersion are given in Table 4.

[0113] Pigment dispersions 1 to 22, 27 to 29, 31 to 47 and 50

[0114] First, 40.00 parts of titanium oxide particles of the kind given in Table 4, the dispersant of the kind and the amount used (parts) given in Table 4, and ion-exchanged water were mixed in an amount such that the total amount of each component was 100.00 parts. Preliminary dispersion treatment was carried out using a homogenizer. Thereafter, dispersion treatment (main dispersion treatment) was carried out at 25 °C for 12 hours using a paint stirrer with 0.5 mm zirconia beads. The zirconia beads were separated by filtration. An appropriate amount of ion-exchanged water was added as needed to prepare each pigment dispersion having a titanium oxide particle content of 40.00%.

[0115] Pigment dispersions 23 to 26 and 30

[0116] First, 40.00 parts of titanium oxide particles of the type given in Table 4, a dispersant of the type and in the amount (parts) given in Table 4, and ion-exchanged water, which are used in an amount such that the total amount of each component is 100.00 parts, are mixed. Preliminary dispersion treatment is carried out using a homogenizer. Thereafter, dispersion treatment (main dispersion treatment) is carried out at 25 °C for 12 hours using a paint stirrer with 0.5 mm zirconia beads. The zirconia beads are separated by filtration. Appropriate amounts of ion-exchanged water are added as needed to prepare each pigment dispersion. Each prepared pigment dispersion is dried by blowing air at 35 °C with stirring to remove water. Then, the dehydration condensation reaction between the silanol groups and the surface hydroxyl groups of the titanium oxide particles is promoted in an oven at 120 °C to obtain titanium oxide particle powder in which a part of the compound represented by the general formula (1) is covalently bonded to the surface hydroxyl groups of the titanium oxide particles. The heating time in the oven at 120 °C is adjusted to meet the "mass ratio (times) of the covalently bonded compound" described in Tables 5 to 9. The titanium oxide particle powder is redispersed in an appropriate amount of ion-exchanged water to prepare each pigment dispersion having a titanium oxide particle content of 40.00%.

[0117] Pigment dispersion 48

[0118] First, 40.00 parts of titanium oxide particles of the type given in Table 4 and 60.00 parts of deionized water are mixed and pre-dispersed using a homogenizer. Thereafter, a resin dispersant (trade name: "Floren G700", acid value: 60 mgKOH / g, available from Kyoeisha Chemical Co., Ltd.) is added thereto to obtain a mixture. The mixture is subjected to dispersion treatment for 12 hours using a paint stirrer containing 0.5 mm zirconia beads to prepare Pigment Dispersion 48 having a titanium oxide particle content of 40.00%.

[0119] Pigment dispersion 49

[0120] A pigment dispersion is prepared according to the method for preparing Pigment 3k in Example 3 of PCT Japanese Translation Patent Publication No. 2017-521348. Specifically, except for using titanium oxide particles 4 and Compound 4 in place of titanium oxide particles 1 and Compound 5, the pigment dispersion is prepared in the same manner as Pigment Dispersion 1. The obtained pigment dispersion is dried by blowing air at 35 °C with stirring, and then dried in an oven at 105 °C for 4 hours and 15 minutes to obtain titanium oxide particle powder. The titanium oxide particle powder is redispersed in an appropriate amount of ion-exchanged water to prepare Pigment Dispersion 49 having a titanium oxide particle content of 40.0%.

[0121] Pigment dispersion 51

[0122] A pigment dispersion was prepared according to the method for preparing aqueous pigment dispersion A in Example 1 of Japanese Patent Laid-Open No. 2011-225867. Specifically, except that 1.2 parts of vinyltriethoxysilane (a silane coupling agent) was used instead of Compound 5, the pigment dispersion was prepared in the same manner as Pigment Dispersion 1. The obtained pigment dispersion was heated to dryness to obtain titanium oxide particle powder surface-treated with a silane coupling agent. Then, 40.00 parts of titanium oxide particle powder, 3.20 parts of styrene-acrylic resin, and 56.80 parts of ion-exchanged water were mixed. The styrene-acrylic resin was synthesized by a known method, with styrene / acrylic acid / methacrylic acid = 77 / 10 / 13 and an acid value of 150 mg / KOH. Thereafter, dispersion treatment was performed again in the same steps as Pigment Dispersion 1 to prepare Pigment Dispersion 51 with a titanium oxide particle content of 40.00% and a resin content of 3.20%.

[0123] Calculation of mass ratio of compound represented by general formula (1) covalently bonded to titanium oxide particles

[0124] The mass ratio of the compound represented by the general formula (1) covalently bonded to the surface hydroxyl groups of titanium oxide particles was measured as follows: First, each of the prepared pigment dispersions was centrifuged using a centrifuge to sediment the solid components containing titanium oxide particles. The supernatant component was removed. Water was added to the solid components to redisperse the solid components. The above series of operations were repeated 3 times to remove the compound represented by the general formula (1) that was not covalently bonded to the surface hydroxyl groups of titanium oxide particles, thereby obtaining titanium oxide particles covalently bonded to the compound represented by the general formula (1). The mass ratio was calculated by quantitatively analyzing the titanium oxide particles by thermogravimetric analysis (TGA). The obtained mass ratio is shown as "Mass ratio of covalently bonded compound (times)" in Tables 5 to 9. The results of the same measurement for the ink containing each pigment dispersion showed that the mass ratio did not change. However, when a compound that does not satisfy the general formula (1) was used as a dispersant, "0.000" was given in the column of "Mass ratio of covalently bonded compound (times)".

[0125] Table 4 Production conditions of pigment dispersions

[0126]

[0127] Preparation of liquid containing alumina particles

[0128] The liquid containing alumina particles was prepared according to the preparation method of Ink 3 in Example 1 of International Publication No. 2018 / 190848. Specifically, an alumina particle dispersion liquid containing amphoteric alumina particles (trade name: "Dispal 23N4-80", dispersed particle size: 90 nm, available from Sasol) in an amount of 10% was provided. The pH of the alumina particle dispersion liquid was adjusted to 4.0 with a strong acid (1 mol / L hydrochloric acid). The alumina particle dispersion liquid was mixed with a propeller mixer until it became uniform, and then pulverized with a bead mill to prepare a liquid containing alumina particles (alumina particle content: 10%).

[0129] Preparation of ink

[0130] The components of the types and amounts given in the upper rows of Tables 5 to 9 were mixed and stirred. Vinyblan 2685 (trade name) is the trade name of an acrylic emulsion (acrylic resin particle content: 30%) available from Nissin Chemical Industry Co., Ltd. Acetylenol E60 (trade name) is a nonionic surfactant available from Kawaken Fine Chemicals Co., Ltd. The ion-exchanged water containing potassium hydroxide is ion-exchanged water containing potassium hydroxide for adjusting the pH of each ink to the value given in Tables 5 to 9, and is added in such a way that the total amount of each component is 100%. Thereafter, pressure filtration was carried out with a membrane filter having a pore size of 5.0 μm (available from Sartorius) to prepare each ink. The pH of each ink was measured using a pH meter (trade name: "Portable pH Meter D-74", available from Horiba, Ltd).

[0131] Evaluation

[0132] The following items were evaluated for each of the inks obtained above. In the examples of the present invention, in the evaluation criteria of the following items, "A", "B", "C", "D", and "E" were defined as acceptable levels, and "F" was defined as an unacceptable level. The inks of Comparative Examples 16 and 17 could not be ejected. The evaluation results of the ejection stability of these inks were partially indicated as "non-ejectable". Although the inks of Examples 17 and 18 were evaluated in the same manner, the ink of Example 17 was excellent in ejection stability. The evaluation results are given in the lower half of Tables 5 to 9. In Tables 5 to 9, in the "mass ratio (times) of the covalently bonded compound" section, the amount of the compound represented by the general formula (1) covalently bonded to the surface of the titanium oxide particles is expressed as a mass ratio based on the titanium oxide particle content.

[0133] Ejection stability

[0134] Place each of the inks obtained above in a sealed container and store at 70 °C for 1 week. When the pH of the ink is higher than that before storage, add potassium hydroxide to adjust the pH to the same as before storage. This operation is an accelerated condition assuming long-term storage and environmental changes (such as temperature changes). Thereafter, fill each ink into an ink cartridge. Set the ink cartridge in an inkjet recording apparatus (trade name: "PIXUS PRO-10S", available from CANON KABUSHIKI KAISHA) equipped with a recording head that ejects ink by thermal energy. The ink is continuously ejected from five randomly selected ejection ports of the recording head at a frequency of 50,000 drops / second. Horizontally photograph the ejected ink droplets. Calculate the ejection speed of the ink droplets by image processing. Calculate the ejection speed of the ink droplets after a predetermined period of time (after 1 minute and 3 hours) from the start of continuous ejection. Calculate the "ejection speed change" of each ejection port based on the equation ("ejection speed of ink droplets after 1 minute" - "ejection speed of ink droplets after 3 hours") / ("ejection speed of ink droplets after 1 minute"). Determine the average "ejection speed change" for each of the five ejection ports and calculate the change rate of the ejection speed. Evaluate the ejection stability of the ink according to the following evaluation criteria. The lower the change rate of the ejection speed, the better the ink ejection stability. Conversely, for example, in a state where an excessive amount of dispersant is adsorbed to the titanium oxide particles, the ejection energy is consumed for the desorption of the dispersant that is excessively adsorbed during ejection. This results in a decrease in the ejection speed, thereby increasing the change rate of the ejection speed.

[0135] A: The change rate of the ejection speed is 0.02 or less.

[0136] B: The change rate of the ejection speed is greater than 0.02 and 0.05 or less.

[0137] C: The change rate of the ejection speed is greater than 0.05 and 0.08 or less.

[0138] D: The change rate of the ejection speed is greater than 0.08 and 0.10 or less.

[0139] E: The change rate of the ejection speed is greater than 0.10 and 0.20 or less.

[0140] F: The change rate of the ejection speed is greater than 0.20.

[0141] Table 5 Composition, properties, and evaluation results of the ink

[0142]

[0143] Table 6 Composition, properties, and evaluation results of the ink

[0144]

[0145] Table 7 Composition, properties, and evaluation results of the ink

[0146]

[0147] Table 8 Composition, properties, and evaluation results of the ink

[0148]

[0149] Table 9 Composition, properties, and evaluation results of the ink

[0150]

[0151] According to an embodiment of the present invention, there can be provided an aqueous ink containing titanium oxide for inkjet and having excellent ejection stability, an ink cartridge accommodating the aqueous ink, and an inkjet recording method. Further, according to an embodiment of the present invention, there can also be provided a production method of a titanium oxide particle dispersion liquid that can be used for producing an aqueous ink for inkjet recording, the aqueous ink having excellent ejection stability, and a production method of an aqueous ink using the titanium oxide particle dispersion liquid obtained by this production method.

[0152] Although the present invention has been described with reference to exemplary embodiments, it should be understood that the present invention is not limited to the disclosed exemplary embodiments. The scope of the appended claims is to be accorded the broadest interpretation so as to encompass all such modifications as well as equivalent structures and functions.

Claims

1. An aqueous ink for inkjet recording, comprising: titanium oxide particles; and a dispersant for the titanium oxide particles, characterized in that the titanium oxide particles comprise titanium oxide having at least a part of its surface coated with alumina and silica, the ratio of the alumina in the titanium oxide particles is 0.50 times or more and 1.00 times or less the ratio of the silica in the titanium oxide particles in terms of mass ratio, and the dispersant for the titanium oxide particles comprises a compound represented by the following general formula (1): In the general formula (1), R1, R2 and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, each R4 is independently an alkylene group having 2 to 4 carbon atoms, X is a single bond or an alkylene group having 1 to 6 carbon atoms, n is 6 to 24, a is 1 to 3, b is 0 to 2, and a + b = 3.

2. The aqueous ink according to claim 1, wherein the ratio of the titanium oxide in the titanium oxide particles is 90.0% by mass or more based on the total mass of the titanium oxide particles.

3. The aqueous ink according to claim 1, wherein the ratio of the titanium oxide in the titanium oxide particles is 98.5% by mass or less based on the total mass of the titanium oxide particles.

4. The aqueous ink according to claim 1, wherein the ratio of the alumina in the titanium oxide particles is 0.50% by mass or more and 4.00% by mass or less based on the total mass of the titanium oxide particles.

5. The aqueous ink according to claim 1, wherein the ratio of the silica in the titanium oxide particles is 1.00% by mass or more and 4.00% by mass or less based on the total mass of the titanium oxide particles.

6. The aqueous ink according to claim 1, wherein the volume-based 50% cumulative particle size of the titanium oxide particles is 200 nm or more and 400 nm or less.

7. The aqueous ink according to claim 1, wherein the amount of the titanium oxide particles contained in the aqueous ink is 1.00% by mass or more and 20.00% by mass or less based on the total mass of the ink.

8. The aqueous ink according to claim 1, wherein the amount of the compound represented by the general formula (1) contained in the aqueous ink is 0.002 times or more and 0.10 times or less the amount of the titanium oxide particles contained in the aqueous ink in terms of mass ratio.

9. The aqueous ink according to claim 1, wherein the amount of the compound represented by the general formula (1) covalently bonded to the surface of the titanium oxide particles is 0.001 times or less the amount of the titanium oxide particles contained in the aqueous ink in terms of mass ratio.

10. The aqueous ink according to claim 1, wherein the compound represented by the general formula (1) comprises a compound represented by the following formula (2): In the general formula (2), R1 and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and m is 8 to 24.

11. The aqueous ink according to claim 1, wherein the amount of the compound represented by the general formula (1) is 0.01% by mass or more and 1.00% by mass or less based on the total mass of the ink.

12. The aqueous ink according to claim 1, wherein the amount of the compound represented by the general formula (1) is 0.02% by mass or more and 0.50% by mass or less based on the total mass of the ink.

13. The aqueous ink according to claim 1, wherein the pH of the aqueous ink is 7.0 or more and 9.0 or less.

14. An ink cartridge comprising ink; and an ink storage portion for storing the ink, It is characterized in that wherein the ink contains the aqueous ink according to claim 1.

15. An inkjet recording method for recording an image on a recording medium by ejecting ink from an inkjet recording head, It is characterized in that wherein the ink contains the aqueous ink according to claim 1.

16. A titanium oxide particle dispersion for producing an aqueous ink for inkjet recording, comprising: titanium oxide particles; and a dispersant for the titanium oxide particles, characterized in that the titanium oxide particles contain titanium oxide having at least a part of its surface coated with alumina and silica, the dispersant for the titanium oxide particles contains a compound represented by the following general formula (1), and the ratio of alumina in the titanium oxide particles to silica in the titanium oxide particles is 0.50 times or more and 1.00 times or less by mass ratio: wherein in the general formula (1), R1, R2 and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, each R4 is independently an alkylene group having 2 to 4 carbon atoms, X is a single bond or an alkylene group having 1 to 6 carbon atoms, n is 6 to 24, a is 1 to 3, b is 0 to 2, and a + b = 3.

17. A method for producing a titanium oxide particle dispersion, which is used for producing an aqueous ink for inkjet recording, characterized in that, The method includes: providing titanium oxide particles containing titanium oxide having at least a part of its surface coated with alumina and silica; and dispersing the titanium oxide particles in a liquid medium with a compound represented by the following general formula (1) to disperse the titanium oxide particles, wherein the ratio of alumina in the titanium oxide particles to silica in the titanium oxide particles is 0.50 times or more and 1.00 times or less by mass ratio, wherein in the general formula (1), R1, R2 and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, each R4 is independently an alkylene group having 2 to 4 carbon atoms, X is a single bond or an alkylene group having 1 to 6 carbon atoms, n is 6 to 24, a is 1 to 3, b is 0 to 2, and a + b = 3.

18. A production method of an aqueous ink for inkjet recording, characterized in that, It includes: mixing the titanium oxide particle dispersion produced by the method according to claim 17 with other ink components.

19. An aqueous ink for inkjet recording, comprising: titanium oxide particles; and a dispersant for the titanium oxide particles, characterized in that the titanium oxide particles contain titanium oxide having at least a part of its surface coated with alumina and silica, the ratio of aluminum element in the titanium oxide particles to silicon element in the titanium oxide particles is 0.57 times or more and 1.13 times or less by mass ratio, which is obtained by inductively coupled plasma emission spectrometry, and the dispersant for the titanium oxide particles contains a compound represented by the following general formula (1): In general formula (1), R1, R2 and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, each R4 is independently an alkylene group having 2 to 4 carbon atoms, X is a single bond or an alkylene group having 1 to 6 carbon atoms, n is 6 to 24, a is 1 to 3, b is 0 to 2, and a + b = 3.

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