Method for producing a dispersion of titanium dioxide particles, method for producing ink, and inkjet recording method

Abstract

To provide a method for producing a titanium oxide particle dispersion that has excellent storage stability even when used in producing am aqueous ink for ink jet recording.SOLUTION: The present invention provides a method for producing a titanium oxide particle dispersion for use in producing an aqueous ink for ink jet recording. The method includes a dispersion step for dispersing titanium oxide particles comprising titanium oxide, whose surface is at least partly covered with silica, in a liquid medium at a pH of 7.5 or more and 12.5 or less with a compound represented by the general formula (1). (In the general formula (1), R1, R2, and R3 independently represent a hydrogen atom, or a C1-4 alkyl group. R4 independently represent a C2-4 alkylene group. X is a single bond, or a C1-6 alkylene group. n is 6 to 24, a is 1 to 3, and b is 0 to 2 with a+b=3).SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing a dispersion of titanium oxide particles, a method for producing an ink, and an inkjet recording method.

Background Art

[0002] In recent years, inkjet recording devices have been widely used when outputting advertisements and exhibits using recording media such as paper and resin films. For example, in order to express a clear color image even on a transparent recording medium, in addition to black and basic color inks (hereinafter, these may be collectively referred to as color inks), white ink is also used in combination. Specifically, a recording method is used in which white ink is previously applied to a portion including an area for recording an image on a transparent recording medium for undercoating, and color ink is applied thereon, or each ink is applied in the reverse order (so-called backprint).

[0003] As a coloring material for white ink, titanium oxide is widely used because it is low in cost and excellent in properties required for white ink such as whiteness and hiding power. On the other hand, as an aqueous ink for inkjet, since it is a metal oxide, it is necessary to stably disperse titanium oxide having a large specific gravity. Therefore, it is necessary to select an appropriate dispersant at the production stage of the dispersion used for producing the ink and to perform an appropriate dispersion treatment.

[0004] Methods for stably dispersing titanium dioxide have been investigated to date. For example, a method has been proposed for producing dried titanium dioxide by surface-treating titanium dioxide with silica, then further surface-treating it with a silane coupling agent, and drying it, thereby covalently bonding a portion of the silane coupling agent to the surface of the titanium dioxide particles (see Patent Document 1). In addition, an ink containing titanium dioxide surface-treated with alumina, a monovalent metal salt, and alumina fine particles has been proposed (see Patent Document 2). Furthermore, an ink containing titanium dioxide surface-treated with alumina and silica, then surface-treated with a silane coupling agent, a resin having anionic groups, a water-soluble organic solvent, and a basic compound has been proposed (see Patent Document 3). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Special Publication No. 2017-521348 [Patent Document 2] International Publication No. 2018 / 190848 [Patent Document 3] Japanese Patent Publication No. 2011-225867 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The present inventors investigated the storage stability of aqueous inks prepared using dried titanium dioxide as proposed in Patent Document 1, as well as the aqueous inks proposed in Patent Documents 2 and 3. As a result, it was found that the storage stability was insufficient for stably handling titanium dioxide with large particle sizes necessary to obtain whiteness, and there is room for improvement. Furthermore, in Patent Document 2, although the storage stability of the dispersion can be improved to some extent by using a dispersant with high affinity for titanium dioxide particles, it was found that the storage stability of the aqueous ink produced using the dispersion is insufficient.

[0007] Therefore, an object of the present invention is to provide a method for producing a dispersion of titanium dioxide particles that exhibits excellent storage stability even when used in the manufacture of aqueous inkjet inks. Another object of the present invention is to provide a method for producing an aqueous ink using the dispersion of titanium dioxide particles obtained by the above manufacturing method, and an inkjet recording method using the aqueous ink. [Means for solving the problem]

[0008] The above objective is achieved by the present invention as follows. That is, the method for producing a dispersion of titanium dioxide particles according to the present invention is a method for producing a dispersion of titanium dioxide particles for use in the production of aqueous inkjet ink, wherein the titanium dioxide particles are titanium dioxide in which at least a part of the surface thereof is coated with silica, and the method comprises a dispersion step of dispersing the titanium dioxide particles in a liquid medium with a compound represented by the following general formula (1) for dispersing the titanium dioxide particles, and the pH of the liquid medium in the dispersion step is 7.5 or more and 12.5 or less.

[0009] [ka]

[0010] (In general formula (1), R1, R2, and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. R4 is each 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.) [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a method for producing a dispersion of titanium dioxide particles that exhibits excellent storage stability even when used in the manufacture of aqueous inkjet inks. Furthermore, according to the present invention, it is possible to provide a method for producing an aqueous ink using the dispersion of titanium dioxide particles obtained by the above manufacturing method, and an inkjet recording method using the aqueous ink. [Brief explanation of the drawing]

[0012] [Figure 1] This figure schematically shows an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, where (a) is a perspective view of the main part of the inkjet recording apparatus and (b) is a perspective view of the head cartridge. [Modes for carrying out the invention]

[0013] The present invention will be described in more detail below with reference to preferred embodiments. In the present invention, when a compound is a salt, the salt exists in the ink dissociated into ions, but for convenience, it will be expressed as "contains salt." Titanium dioxide and titanium dioxide particles may be simply referred to as "pigment." A dispersion of titanium dioxide particles may be simply referred to as "pigment dispersion." Also, water-based ink for inkjet printers may be simply referred to as "ink." Unless otherwise specified, physical properties are values ​​at room temperature (25°C).

[0014] Inorganic oxides such as titanium dioxide react with water molecules that make up the aqueous medium in aqueous inks, generating hydroxyl groups (hereinafter sometimes referred to as "surface hydroxyl groups") on their surface. For this reason, in aqueous inkjet inks, it is common to use them after surface treatment with inorganic oxides such as alumina or silica to further improve the storage stability of the ink. The surface hydroxyl groups of titanium dioxide particles have properties specific to the inorganic oxide corresponding to the inorganic compound used for surface treatment, and the isoelectric point, which is an indicator of acid strength, differs depending on the type of inorganic compound. Therefore, although titanium dioxide itself is an inorganic oxide, the surface of titanium dioxide particles exhibits the properties of the inorganic oxide corresponding to the inorganic compound used for surface treatment, and the surface charge of titanium dioxide particles strongly depends on the pH of the aqueous medium, the type of surface treatment agent, and the amount of surface treatment agent used.

[0015] To stably disperse titanium dioxide particles in ink, it is necessary to select an appropriate dispersant and produce a dispersion of titanium dioxide particles with excellent storage stability using an appropriate manufacturing method. The inventors first investigated appropriate dispersants. As a result, it was found that the compound represented by general formula (1) is preferable as a dispersant. However, even when using an appropriate dispersant, it was found that depending on the manufacturing method of the dispersion, even if a certain level of storage stability is obtained in the dispersion state, when the ink is prepared, the storage stability level predicted from the storage stability of the dispersion may not be obtained. When the inventors investigated the cause of this, they found that the following phenomenon was occurring. That is, when dispersing titanium dioxide particles, condensation occurs between the molecules of the dispersant, forming condensates, or forming covalent bonds with the functional groups on the surface of the titanium dioxide particles, resulting in poor storage stability of the titanium dioxide particle dispersion and the ink using the dispersion.

[0016] Therefore, initially, the inventors of the present invention attempted to suppress the formation of covalent bonds between the condensate of the dispersant and the functional groups on the surface of titanium oxide particles by performing a dispersion treatment under acidic conditions with a pH of about 3 to 4 and then adding an alkaline compound such as potassium hydroxide to make it alkaline. However, salting out easily occurs in the dispersion or ink due to the salts generated by neutralization when adjusting the pH from acidic conditions to alkaline conditions. There is a concern that if the storage stability is improved, it will affect other inkjet suitability. To address such concerns, the dispersion of titanium oxide particles after pH adjustment can be purified by desalting, and then the compound represented by the general formula (1) is added again to adjust the pH to obtain a dispersion having desired performance. However, this method for producing the dispersion not only has a complicated process and requires re-addition of the dispersant, but also has problems in terms of cost.

[0017] As a result of further investigations by the inventors of the present invention, it has been found that by dispersing titanium oxide particles in a specific pH range using a specific compound, the storage stability can be improved without affecting other inkjet suitability as described above.

[0018] That is, the method for producing a dispersion of titanium oxide particles of the present invention has the following characteristics. First, titanium oxide particles, at least a part of the surface of which is coated with silica, are used. And a compound represented by the general formula (1) is used to disperse the titanium oxide particles. The process of the production method of the present invention includes a dispersion step of dispersing titanium oxide particles in a liquid medium with a compound represented by the general formula (1) for dispersing the titanium oxide particles, and the pH in the dispersion step is 7.5 or more and 12.5 or less. The inventors of the present invention speculate the mechanism by which storage stability can be obtained even when used in the production of ink as follows.

[0019] The titanium oxide particles are surface-treated with silica and are subjected to dispersion treatment with a compound represented by the general formula (1) under the condition that the pH is 7.5 or more and 12.5 or less, that is, under an alkaline condition. Under an alkaline condition, a large number of surface hydroxy groups derived from silica are formed due to the adhesion of water. And a part of the surface hydroxy groups derived from silica desorbs a hydrogen atom and becomes negatively charged. Therefore, even when a plurality of titanium oxide particles approach, since the surface of each titanium oxide particle is negatively charged, an electrostatic repulsive force is generated, and it is considered that the storage stability can be improved.

[0020] A part of the compound represented by the general formula (1) is known as a silane coupling agent. In the general formula (1), each OR1 independently represents a hydroxy group or an alkoxy group having 1 to 4 carbon atoms. A part of OR1 bonded to the silicon atom is hydrolyzed to form a silanol group, and a part of the silanol group can dissociate into ions. Such a reaction is particularly likely to occur under an alkaline condition. Therefore, a "weak affinity" is generated by the formation of a hydrogen bond between the surface hydroxy group of the titanium oxide particle and the silanol group of the compound represented by the general formula (1). Further, in a part between the surface hydroxy group derived from silica of the titanium oxide particle and the silanol group of the compound represented by the general formula (1), a covalently bonded state can be taken by a dehydration reaction. That is, the surface hydroxy group derived from silica of the titanium oxide particle and the silanol group in the compound represented by the general formula (1) generate an affinity by hydrogen bond and covalent bond, and desorption and adsorption are repeated to allow the compound represented by the general formula (1) to exist in the vicinity of the titanium oxide particle. When OR1 is an alkoxy group having more than 4 carbon atoms, it becomes difficult to hydrolyze to form a silanol group. As a result, the affinity with the surface hydroxy group of the titanium oxide particle cannot be obtained, and the titanium oxide particles cannot be stably dispersed.

[0021] Furthermore, compounds represented by general formula (1) are prone to intermolecular condensation in alkaline environments. However, under the conditions of the present invention, the formation of such condensates is effectively suppressed, and it was found that many of the compounds represented by general formula (1) exist in dispersions and inks without reacting or condensing with other molecules. One possible reason for this is the repulsive force due to steric hindrance caused by the structure of the compounds represented by general formula (1). In addition to the structure that can form a silanol group as described above, the compounds represented by general formula (1) have a structure having n alkylene oxides with 2 to 4 carbon atoms as repeating units via a linking group X (in general formula (1), (OR4)). n The structure has the following characteristics. n represents the number (average value) of repeating alkylene oxide groups, which is between 6 and 24. Hereafter, the above structure will also be referred to as an alkylene oxide chain. Because the alkylene oxide chain is hydrophilic, it extends appropriately in aqueous liquid media and exhibits repulsive force due to steric hindrance. It is presumed that this repulsive force due to steric hindrance not only stably disperses the titanium dioxide particles but also acts to suppress the formation of condensates. Furthermore, the hydroxyl groups on the surface of the titanium dioxide particles and the compound represented by general formula (1) have an appropriate affinity through hydrogen bonding, so they quickly orient themselves on the surface of the titanium dioxide particles and suppress the approach of the compounds represented by general formula (1) to each other. Therefore, it is presumed that the condensation reaction is suppressed.

[0022] <Method for producing a dispersion of titanium dioxide particles> The present invention provides a method for producing a dispersion of titanium dioxide particles, which is intended for use in the production of aqueous inkjet inks. The method includes a dispersion step in which titanium dioxide particles are contacted with a specific compound and dispersed in a liquid medium. The ink obtained using the titanium dioxide particle dispersion produced by the present invention is preferably white, as titanium dioxide is a white pigment. The present invention's method for producing a dispersion of titanium dioxide particles will now be described in detail.

[0023] (Colorants) The titanium dioxide particle dispersion produced by this invention contains titanium dioxide particles that have been surface-treated as a colorant (pigment). In other words, the titanium dioxide particle dispersion contains titanium dioxide particles in which at least a portion of the surface is coated with a specific inorganic oxide. This titanium dioxide particle dispersion is used in the manufacture of inkjet inks.

[0024] Titanium dioxide is a white pigment and exists in three crystalline forms: rutile, anatase, and brookite. Of these, rutile titanium dioxide is preferred. Industrial methods for producing titanium dioxide include the sulfuric acid method and the chlorine method, and the titanium dioxide used in this invention may be produced by either method.

[0025] The cumulative 50% particle diameter (D) of titanium dioxide particles, based on volume, is preferably 200 nm or more and 500 nm or less. In particular, the cumulative 50% particle diameter (D) of titanium dioxide particles, based on volume, is more preferably 200 nm or more and 400 nm or less. 50 ) is the diameter of the particle that, when integrated from the smallest particle diameter side, reaches 50% of the total volume of the measured particles in the particle diameter integration curve. 50 For example, the measurement can be performed under the following conditions: SetZero: 30 seconds, Number of measurements: 3, Measurement time: 180 seconds, Shape: Non-spherical, Refractive index: 2.60. A particle size analyzer using the dynamic light scattering method can be used as the particle size distribution analyzer. Of course, the measurement conditions are not limited to those mentioned above.

[0026] Titanium dioxide coated with silica is used. Surface treatment is expected to suppress photocatalytic activity and improve dispersibility. In addition to silica surface treatment, further surface treatment may be applied with oxides of inorganic compounds such as aluminum, zinc, and zirconium, i.e., alumina, zinc oxide, and zirconia. Surface treatment may also be applied with organic substances such as polyols. Among these, it is preferable to use titanium dioxide that has been further surface-treated with alumina. In this specification, "alumina" is a general term for aluminum oxides such as aluminum oxide. Also, in this specification, "silica" is a general term for silicon dioxide or substances composed of silicon dioxide. The majority of alumina and silica coating titanium dioxide exist in the form of silicon dioxide and aluminum oxide.

[0027] The proportion (mass%) of titanium dioxide in the titanium dioxide particles is preferably 85.00% by mass or more, based on the total mass of the titanium dioxide particles. Furthermore, the proportion (mass%) of titanium dioxide in the titanium dioxide particles is preferably 98.50% by mass or less, based on the total mass of the titanium dioxide particles. When titanium dioxide particles surface-treated with silica and alumina are used, the proportion (mass%) of alumina in the titanium dioxide particles is preferably 1.50 times or less by mass ratio to the proportion (mass%) of silica. In particular, the proportion (mass%) of alumina in the titanium dioxide particles is even more preferably 1.00 times or less by mass ratio to the proportion (mass%) of silica. The aforementioned mass ratio may be 0.00 times. In the pH range in which titanium dioxide particle dispersions and inks are normally used, the surface hydroxyl groups derived from alumina are positively charged, unlike the negatively charged surface hydroxyl groups derived from silica. Therefore, if the mass ratio exceeds 1.50, the proportion of alumina in the titanium oxide particles is too high compared to silica, which cancels out the negative charge from the surface hydroxyl groups derived from silica, and the electrostatic repulsive force of the titanium oxide particles cannot be obtained. As a result, sufficient storage stability may not be obtained. In particular, the storage stability of ink manufactured using a dispersion of titanium oxide particles may not be sufficient. This is because a dispersion of titanium oxide particles contains almost no components other than titanium oxide particles and the compound represented by general formula (1), whereas ink contains other components in addition to the dispersion.

[0028] Furthermore, the proportion (mass%) of silica in the titanium oxide particles is preferably 0.50% by mass or more and 9.00% by mass or less, based on the total mass of the titanium oxide particles. If the titanium oxide particles are titanium oxide in which at least a portion of their surface is further coated with alumina, the proportion (mass%) of alumina in the titanium oxide particles is preferably 9.00% by mass or less, based on the total mass of the titanium oxide particles. The proportion (mass%) of alumina in the titanium oxide particles may be 0.00% by mass, based on the total mass of the titanium oxide particles.

[0029] One method for measuring the proportion of alumina and silica in titanium oxide particles, i.e., the amount of alumina and silica coating, is quantitative analysis of aluminum and silicon elements by inductively coupled plasma (ICP) emission spectrometry. In this case, assuming that all atoms coating the surface are oxides, the obtained values ​​of aluminum and silicon can be calculated by converting them to their oxides, i.e., alumina and silica. The proportion of aluminum elements (mass%) in titanium oxide particles obtained by inductively coupled plasma emission spectrometry is preferably 1.71 times or less, and more preferably 1.13 times or less, in terms of mass ratio to the proportion of silicon elements (mass%). When these values ​​are converted to their oxides, i.e., alumina and silica, the proportion of alumina (mass%) in titanium oxide particles is 1.50 times or less and 1.00 times or less, respectively, in terms of mass ratio to the proportion of silica (mass%).

[0030] Titanium dioxide particles may be in a dry state or contained in a liquid medium such as water. To facilitate handling in the dispersion process, it is preferable to contain the titanium dioxide particles in a liquid medium. Furthermore, to ensure efficient dispersion, it is preferable to set the concentration of titanium dioxide particles in the liquid medium. Specifically, the content (mass%) of titanium dioxide particles in the liquid containing titanium dioxide particles is preferably 20.00% by mass or more and 60.00% by mass or less, based on the total mass of the liquid.

[0031] Other pigments besides titanium dioxide may be included, provided that the effects of the present invention are not impaired. In this case, when manufacturing the ink, an ink of a color other than white ink can be produced. The content (%) of other pigments in the dispersion of titanium dioxide particles is preferably 20.00% by mass or more and 60.00% by mass or less, from the viewpoint of ease of handling and other factors.

[0032] (Compounds represented by general formula (1)) In the method for producing a dispersion of titanium dioxide particles of the present invention, a compound represented by the following general formula (1) is used as a dispersant for dispersing the titanium dioxide particles. The content (mass%) of the compound represented by general formula (1) in the dispersion of titanium dioxide particles is preferably 0.01% by mass or more and 10.00% by mass or less, and more preferably 0.02% by mass or more and 5.00% by mass or less, based on the total mass of the dispersion of titanium dioxide particles.

[0033] [ka]

[0034] (In general formula (1), R1, R2, and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. R4 is each 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.)

[0035] In 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 alkyl groups having 1 to 4 carbon atoms include methyl, ethyl, n-propyl, i-propyl, and n-butyl groups. Among these, the methyl group is preferred from the viewpoint of ease of hydrolysis. If R1, R2, and R3 are each alkyl groups having more than 4 carbon atoms, it becomes difficult to hydrolyze them to form silanol groups, and affinity with titanium dioxide particles cannot be obtained. Therefore, the titanium dioxide particles cannot be stably dispersed, and storage stability cannot be obtained. a, which represents the number of R1O atoms, is 1 to 3, and b, which represents the number of R2 atoms, is 0 to 2, so a + b = 3. Among these, it is preferable that a is 3 and b is 0, that is, all three substituents on the silicon atom are R1O.

[0036] In general formula (1), each R4 is independently an alkylene group having 2 to 4 carbon atoms. In particular, it is preferable that R4 is an alkylene group having 2 to 4 carbon atoms. That is, it is preferable to have a repeating structure of alkylene oxide groups with the same number of carbon atoms. Examples of alkylene groups having 2 to 4 carbon atoms include ethylene, n-propylene, i-propylene, and n-butylene groups. In particular, the ethylene group is preferred. n, which represents the number (average value) of OR4, is between 6 and 24. If the number (average value) of the alkylene oxide groups is less than 6, the length of the alkylene oxide chain is too short, so sufficient repulsive force due to steric hindrance cannot be obtained, and storage stability cannot be obtained. If the number (average value) of the alkylene oxide groups is greater than 24, the length of the alkylene oxide chain is too long, so hydrophilicity increases and it becomes easier to release into the liquid medium. Therefore, affinity with the hydroxyl groups on the surface of the titanium dioxide particles cannot be obtained, and aggregation of the titanium dioxide particles cannot be suppressed. Therefore, storage stability cannot be obtained.

[0037] 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. Examples of alkylene groups having 1 to 6 carbon atoms include methylene, ethylene, n-propylene, i-propylene, n-butylene, n-pentylene, and n-hexylene groups. Among these, the n-propylene group is preferred. If X is an alkylene group having more than 6 carbon atoms, the hydrophobicity of the compound represented by general formula (1) becomes too high, making it impossible to stably disperse the titanium dioxide particles, and thus storage stability cannot be obtained.

[0038] The compound represented by general formula (1), which is used as a dispersant for titanium dioxide particles, is preferably the compound represented by general formula (2) below. The compound represented by general formula (2) has three OR1 groups bonded to the silicon atom, so in an aqueous medium, a portion of it can hydrolyze to form three hydroxyl groups bonded to the silicon atom, thereby increasing the portion that has affinity with titanium dioxide particles. Furthermore, the compound represented by general formula (2) below has a repeating structure of ethylene oxide groups. Therefore, the ethylene oxide chain can be appropriately extended in an aqueous liquid medium, and a repulsive force due to steric hindrance can be obtained. If the compound is included in general formula (1) but not in general formula (2), it does not significantly affect the storage stability in the state of a titanium dioxide particle dispersion, but it may affect the storage stability in inks containing various components.

[0039] [ka]

[0040] (In general formula (2), R1 and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. m is 8 to 24.)

[0041] The content (mass%) of the compound represented by general formula (1) in the dispersion of titanium dioxide particles is preferably 0.002 times or more and 0.10 times or less in mass ratio to the content (mass%) of titanium dioxide particles. If the mass ratio is less than 0.002 times, the effect of stably dispersing the titanium dioxide particles will be weakened, and sufficient storage stability may not be obtained. If the mass ratio is greater than 0.10 times, the proportion of the compound represented by general formula (1) will be too high, and intermolecular condensation (self-condensation) of the compound represented by general formula (1) will easily occur. As a result, the compound represented by general formula (1) will be consumed without acting as a dispersant, the effect of stably dispersing the titanium dioxide particles will be weak, and sufficient storage stability may not be obtained.

[0042] (Dispersion process) The present invention relates to a method for producing a dispersion of titanium dioxide particles, comprising the step of dispersing titanium dioxide particles in a liquid medium using a compound represented by general formula (1). The pH in the dispersion step must be between 7.5 and 12.5. In particular, it is preferable that the pH in the dispersion step be between 8.0 and 12.5. Furthermore, it is even more preferable that the pH in the dispersion step be between 9.0 and 11.5. In this specification, "pH in the dispersion step" preferably includes the time when dispersion is started, and more specifically, it is the time when dispersion is started. In particular, if preliminary dispersion is performed, it means the time when preliminary dispersion is completed and the main dispersion is performed. That is, even if the dispersion step (main dispersion) is started within the above range, it is possible that the pH may deviate from the above range due to fluctuations in pH during the process, but as long as the pH in the dispersion step stays within the above range, the effects of the present invention will be realized by the mechanism described above. However, considering ink production, it is preferable that the pH remains within the above range even after the time when dispersion is started. Not limited to the dispersion process, if the pH of the liquid containing titanium dioxide particles exceeds 12.5, dissolution of the silica coating the surface of the titanium dioxide particles or intermolecular condensation of the compound represented by general formula (1) may occur. To adjust the pH of the liquid medium to within the above range, pH adjusters and various additives may be added at any time.

[0043] In the dispersion process, titanium oxide particles are dispersed by a compound represented by general formula (1) as a dispersant, by applying shear force necessary to achieve the desired particle size distribution. Known dispersion methods such as media dispersion and media-less dispersion can be used in the dispersion process. Dispersers for media dispersion include paint shakers, bead mills, sand mills, ball mills, and roll mills. Dispersers for media-less dispersion include ultrasonic homogenizers and high-pressure homogenizers. The dispersion process may be carried out by combining two or more of the above-mentioned dispersers.

[0044] The titanium oxide particles used in the dispersion process are titanium oxide particles whose surfaces are coated with silica. As mentioned above, they may be further coated with other compounds in addition to silica. Specifically, inorganic oxides such as alumina, zinc oxide, and zirconia, and organic substances such as polyols can be used. The titanium oxide particles may be those that have been pre-coated with silica, or untreated titanium oxide that has been coated. In this case, an additional step of preparing the titanium oxide particles may be added. Methods of coating include wet treatment and dry treatment. Among these, wet treatment, which can provide a uniform surface treatment, is preferred. Specifically, a method of adding a surface treatment agent to a solution in which the raw material titanium oxide is dispersed can be used. Examples of surface treatment agents include sodium silicate. The surface treatment conditions can be general. Furthermore, the various properties of the titanium oxide particles may be adjusted by further coating those that have been pre-coated with silica.

[0045] The compound represented by general formula (1) used in the dispersion process is used to disperse titanium dioxide particles. The compound represented by general formula (1) may be a commercially available compound or it may be synthesized. For example, it can be synthesized by allylating and hydrosilylation of a starting material (such as a polyalkylene glycol monoalkyl ether).

[0046] The dispersion of titanium dioxide particles obtained in the dispersion process is stored for a certain period of time as needed before being used to prepare water-based inkjet inks. From this viewpoint, it is preferable to set the concentration of titanium dioxide particles in the dispersion. Specifically, the content (mass%) of titanium dioxide particles in the dispersion is preferably 20.00% by mass or more and 60.00% by mass, based on the total mass of the dispersion.

[0047] The temperature of the dispersion process can be set arbitrarily. Since the dispersion process is carried out in an aqueous liquid medium, it is preferable that the temperature be between 0°C and 100°C, and more preferably between 10°C and 40°C from the viewpoint of heat generation during the process and media reliability when using a media dispersion method. The duration of the dispersion process can be adjusted according to the equipment used, the concentration of the dispersion liquid, etc., and can be set arbitrarily as long as the pigment is not overdispersed. For example, when carrying out the dispersion process at 25°C using a paint shaker with 0.5 mm zirconia beads, it is preferable that the duration be between 10 hours and 20 hours.

[0048] Furthermore, a preliminary dispersion may be performed by mixing components containing titanium dioxide particles and wetting them in a liquid medium to facilitate dispersion. In this case, the preliminary dispersion step can be designated as the first dispersion step, and the main dispersion step as the second dispersion step. For the preliminary dispersion, the dispersion methods and apparatus described above as usable in the above steps can be used.

[0049] Patent documents 1 and 3 describe an operation in which the liquid component of a titanium dioxide particle dispersion is dried once. However, as a result of our investigation, we found that the storage stability of a titanium dioxide particle dispersion obtained by drying the liquid component of the titanium dioxide particle dispersion and then dispersing it again in a liquid medium decreases. This is thought to be because drying the liquid component of the titanium dioxide particle dispersion causes the compound represented by general formula (1) to form a covalent bond with the silica-derived surface hydroxyl groups of the titanium dioxide particles. In other words, the silica-derived surface hydroxyl groups that contributed to the dispersion of the titanium dioxide particles are consumed by the formation of a covalent bond by the compound represented by general formula (1). As a result, the negative charge on the surface of the titanium dioxide particles becomes insufficient, and sufficient electrostatic repulsion cannot be obtained. For this reason, it is preferable that the method for producing a titanium dioxide particle dispersion does not include a step of drying the liquid component of the titanium dioxide particle dispersion.

[0050] [Liquid medium] The dispersion step is carried out in a liquid medium, preferably an aqueous liquid medium. As the aqueous liquid medium, water alone or an aqueous medium using water as the main solvent in combination with protic or aprotic organic solvents can be used. The aqueous medium is a mixed solvent of water and an organic solvent. As the organic solvent, it is preferable to use one that can be miscible or dissolved in water in any proportion. In particular, it is preferable to use a homogeneous mixed solvent containing 50% by mass or more of water as the aqueous medium. As water, it is preferable to use deionized water or pure water.

[0051] Protic organic solvents are organic solvents that have hydrogen atoms bonded to oxygen or nitrogen (acidic hydrogen atoms). Aprotic organic solvents are organic solvents that do not have 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.

[0052] [pH adjuster] In the dispersion process, it is preferable to use a pH adjuster to control the state of the hydroxyl groups on the surface of the titanium dioxide particles. The pH adjuster may be an acidic compound or a basic compound. Among these, a basic compound is preferred because it can maintain the anionic state of the hydroxyl groups on the surface of the titanium dioxide particles. When the liquid medium is alkaline, some of the hydroxyl groups on the surface of the titanium dioxide particles are ionized, forming an electric double layer, and dispersion stability can be obtained due to electrostatic repulsion. Examples of basic compounds include ammonia, organic ammonium compounds, and alkali metal hydroxides such as potassium hydroxide and sodium hydroxide. Among these, potassium hydroxide is preferred considering that components contained in the manufactured dispersion are carried into the prepared ink. The content (mass%) of the pH adjuster in the titanium dioxide particle dispersion is preferably 0.60% by mass or less, more preferably 0.30% by mass or less, and preferably 0.0001% by mass or more, based on the total mass of the titanium dioxide particle dispersion. As mentioned above, the pH adjuster may be continuously added at any time to maintain the pH of the liquid medium within an appropriate range.

[0053] (purification process) A purification process may be carried out to remove impurities contained in the dispersion of titanium dioxide particles. The purification process can be carried out by known methods. Among these, an ultrafiltration process and a filtration process are preferred. The pore size of the material used in the filtration process (filter material) can be selected depending on the type of impurity to be removed. Therefore, in order to remove impurities of different sizes contained in the dispersion, it is preferable to carry out the ultrafiltration process and the filtration process in that order.

[0054] [Ultrafiltration process] The ultrafiltration process can be performed to remove impurities such as condensates of compounds represented by general formula (1) and polyvalent metal ions introduced from titanium dioxide particles. To remove the above impurities, it is preferable that the pore size of the filter element used in the ultrafiltration process is smaller than the average particle size of the titanium dioxide particles in the pigment dispersion. Hereinafter, this filter element will also be referred to as the first filter element. In particular, it is preferable that the pore size (nm) of the first filter element is between 10 nm and 200 nm. By setting the pore size of the first filter element within the above range, the above impurities that cause foaming can be effectively removed. By suppressing foaming, the pigment dispersion can be manufactured efficiently. If the pore size of the first filter element is outside the above range, even if filtration is performed, the above impurities may not be sufficiently removed from the titanium dioxide particle dispersion, and foaming may not be sufficiently suppressed.

[0055] Known membrane processing methods can be used for membrane filtration in the ultrafiltration process. Examples include filtration methods in which the processing liquid is flowed perpendicular to the membrane surface (total filtration method, dead-end method), and filtration methods in which the processing liquid is flowed parallel to the membrane surface (tangential flow method, cross-flow method). Among these, from the viewpoint of filtration efficiency, the filtration method in which the processing liquid is flowed parallel to the membrane surface is preferred. Known materials and shapes can be used for the membrane used as the first filtration element. For example, examples of membrane materials include metal oxides such as zirconia and titanium oxide, organic compounds such as polyethersulfonic acid and polyethylene resin, and glass. Examples of membrane shapes include flat membranes and hollow fiber membranes.

[0056] [Filtration process] The filtration process removes impurities larger than the average particle size of titanium dioxide particles, such as coarse titanium dioxide particles and other contaminants, that is, impurities larger than those removed in the ultrafiltration process. By performing this process, it is believed that impurities that affect the dispersion of titanium dioxide particles contained in the pigment dispersion can be removed, thereby further improving the dispersion stability of the pigment dispersion. The pore size of the filter material used in the second filtration process is preferably larger than the average particle size of titanium dioxide particles in the pigment dispersion. Hereinafter, this filter material will also be referred to as the second filter material. In particular, the pore size (nm) of the second filter material is preferably between 700 nm and 5,000 nm, and more preferably between 700 nm and 3,000 nm. If the pore size of the second filter material is less than 700 nm, the pore size is too small, which can easily lead to membrane clogging and a decrease in filtration efficiency. If the pore size of the second filter material is 5,000 nm or larger, the pore size becomes too large, which may prevent sufficient removal of coarse particles and contaminants.

[0057] As for the membrane filtration method in the filtration process, the same membrane treatment methods as those listed as usable in the ultrafiltration process can be used. Among these, a filtration method in which the treatment liquid is flowed perpendicular to the membrane surface is preferred from the viewpoint of yield. The material and shape of the membrane used as the filtration element can also be the same as those listed as usable in the ultrafiltration process. Among these, a glass filter is preferred from the viewpoint of filtration efficiency.

[0058] (Post-processing) The manufactured titanium dioxide particle dispersion is preferably used in the production of water-based inkjet ink after undergoing general post-processing methods, including the purification process described above. If only water is used as the liquid medium without using organic solvents, the obtained titanium dioxide particle dispersion can be used directly in ink preparation, or it can be washed and the titanium dioxide particle content adjusted to become the final titanium dioxide particle dispersion. If a liquid medium containing organic solvents is used, the organic solvents can be removed. Methods for removing organic solvents include, for example, using an evaporator to remove the organic solvents by reduced pressure or heating while adding water to obtain a water-based titanium dioxide particle dispersion. Furthermore, there are methods such as removing the organic solvents by ultrafiltration and then repeating the process of adding water. In particular, when a dispersion in which titanium dioxide particles are dispersed in a water-based liquid medium is obtained, neutralizing the silica-derived surface hydroxyl groups of the titanium dioxide particles generates a negative charge, and the dispersion state can be more stably maintained by the repulsive force due to electrostatic repulsion. Therefore, it is preferable to make the dispersion alkaline by adding the above-mentioned pH adjusters as needed.

[0059] Furthermore, to suppress the sedimentation of titanium dioxide particles, the pH of the prepared titanium dioxide particle dispersion may be lowered, that is, an acidic compound may be added. Examples of acidic compounds include weak acids such as acetic acid and carbonic acid, and strong acids such as hydrochloric acid, sulfuric acid, and methanesulfonic acid. In particular, it is preferable to add an acid whose acid dissociation constant (pKa) at 25°C is -0.1 or less. Specifically, examples include hydrochloric acid, sulfuric acid, and methanesulfonic acid. By adding the above acidic compounds, it is possible to efficiently adjust the pH of the titanium dioxide particle dispersion to the desired level with a small amount of solution, and the storage stability of the ink produced using this titanium dioxide particle dispersion can be further improved.

[0060] <Ink, ink manufacturing method> The present invention relates to an ink manufacturing method comprising the step of mixing a dispersion of titanium dioxide particles and other ink components. The dispersion of titanium dioxide particles used is the dispersion of titanium dioxide particles produced by the above manufacturing method. Other ink components include water, water-soluble organic solvents, resins, and other additives, as described later in the section on other additives. The ink manufacturing method can be carried out, for example, by adding the dispersion of titanium dioxide particles and other ink components to a suitable container and stirring. Conditions such as stirring speed, temperature, and time can be appropriately set according to the desired conditions. Other known manufacturing processes may also be combined. The components constituting the ink produced by the ink manufacturing method of the present invention, the physical properties of the ink, and the manufacturing method will be described in detail below.

[0061] (Colorants) The ink obtained by the manufacturing method of the present invention contains titanium dioxide particles as a colorant. The content (mass%) of titanium dioxide 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. Furthermore, the content (mass%) of titanium dioxide 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. In particular, the content (mass%) of titanium dioxide particles in the ink is especially preferably 1.00% by mass or more and 15.00% by mass or less, based on the total mass of the ink.

[0062] Titanium dioxide particles are produced using the method for producing a dispersion of titanium dioxide particles according to the present invention described above. Therefore, the titanium dioxide particles are dispersed in a liquid medium, and it is not necessary to add an additional dispersant to disperse the pigment in the ink, or the amount of dispersant added can be small. Furthermore, when a dispersant is added, it is preferable to use the compound represented by the general formula (1) described above. The content (mass%) of the compound represented by general formula (1) in the ink is preferably 0.01% by mass or more and 2.00% by mass or less, and more preferably 0.02% by mass or more and 1.70% by mass or less, based on the total mass of the ink.

[0063] The ink may contain pigments other than titanium dioxide, provided that the effects of the present invention are not impaired. In this case, the ink may be a color other than white. The content (%) of the other pigments in the ink is preferably 0.10% by mass or more and 5.00% by mass or less, and more preferably 0.10% by mass or more and 1.00% by mass or less, based on the total mass of the ink.

[0064] (resin) The ink may contain a resin. Examples of resins include acrylic resins, urethane resins, and urea resins. Among these, acrylic resins are preferred. The resin content (mass%) in the ink is preferably 1.00% to 25.00% by mass, and more preferably 3.00% to 15.00% by mass, based on the total mass of the ink. Among these, 5.00% to 15.00% by mass is particularly preferred.

[0065] Resins can be incorporated into inks to improve various properties of recorded images, such as scratch resistance and opacity. Examples of resin forms include block copolymers, random copolymers, graft copolymers, and combinations thereof. The resin may be a water-soluble resin that dissolves in an aqueous medium, or it may be resin particles dispersed in an aqueous medium. The resin particles do not need to contain colorants.

[0066] In this specification, "water-soluble resin" means that when the resin is neutralized with an alkali equivalent to its acid value, it exists in an aqueous medium without forming particles whose particle size can be measured by dynamic light scattering. Whether or not a resin is water-soluble can be determined according to the following method. First, prepare a liquid containing the resin (resin solids content: 10% by mass) neutralized with an alkali equivalent to its acid value (sodium hydroxide, potassium hydroxide, etc.). Next, prepare a sample solution by diluting the prepared liquid 10 times (by volume) with pure water. Then, when the particle size of the resin in the sample solution is measured by dynamic light scattering, if no particles with a particle size are measured, the resin can be determined to be water-soluble. The measurement conditions in this case can be, for example, SetZero: 30 seconds, Number of measurements: 3, Measurement time: 180 seconds. As a particle size distribution analyzer, a particle size analyzer using dynamic light scattering (for example, product name "UPA-EX150", manufactured by Nikkiso) can be used. Of course, the particle size distribution analyzer and measurement conditions used are not limited to those described above.

[0067] The acid value of the water-soluble resin is preferably 80 mg KOH / g or more and 250 mg KOH / g or less, and more preferably 100 mg KOH / g or more and 200 mg KOH / g or less. When resin particles are used, their acid value is preferably 0 mg KOH / g or more and 50 mg KOH / g or less. The weight-average molecular weight of the resin is preferably 1,000 or more and 30,000 or less, and more preferably 5,000 or more and 15,000 or less. The weight-average molecular weight of the resin is a polystyrene equivalent value measured by gel permeation chromatography (GPC).

[0068] (aqueous medium) The ink is an aqueous ink containing water as an aqueous medium. The ink may contain an aqueous medium which is water or a mixed solvent of water and a water-soluble organic solvent. It is preferable to use deionized water (ion-exchanged water) as the water. The water content (mass%) in the ink is preferably 50.00% by mass or more and 95.00% by mass or less, based on the total mass of the ink.

[0069] There are no particular restrictions on the water-soluble organic solvent, as long as it is water-soluble (preferably, soluble in water at 25°C in any proportion). Specifically, monohydric or polyhydric alcohols, alkylene glycols, glycol ethers, nitrogen-containing polar compounds, sulfur-containing polar compounds, etc., can be used. The content (mass%) of the water-soluble organic solvent in the ink is preferably 3.00% by mass or more and 50.00% by mass or less, and more preferably 10.00% by mass or more and 40.00% by mass or less, based on the total mass of the ink. If the content (mass%) of the water-soluble organic solvent is less than 3.00% by mass, the ink may solidify in the inkjet recording device, and sufficient adhesion resistance may not be obtained. If the content (mass%) of the water-soluble organic solvent exceeds 50.00% by mass, ink supply problems may occur.

[0070] (Other additives) In addition to the additives mentioned above, the ink may contain various other additives as needed, such as surfactants, pH adjusters, rust inhibitors, preservatives, fungicides, antioxidants, reduction inhibitors, evaporation accelerators, and chelating agents. Among these, it is preferable that the ink contains a surfactant. The surfactant content (mass%) in the ink is preferably 0.10% to 5.00% by mass, and more preferably 0.10% to 2.00% by mass, based on the total mass of the ink. Examples of surfactants include anionic surfactants, cationic surfactants, and nonionic surfactants. Among these, nonionic surfactants are preferred because they have low affinity for titanium dioxide particles and are effective even in small amounts, as they are used to adjust various physical properties of the ink.

[0071] (Ink properties) Since the ink is for use in an inkjet system, it is preferable to appropriately control its physical properties. The surface tension of the ink at 25°C is preferably 10 mN / m to 60 mN / m, and more preferably 20 mN / m to 40 mN / m. The surface tension of the ink can be adjusted by appropriately determining the type and amount of surfactant in the ink. Furthermore, the viscosity of the ink at 25°C is preferably 1.0 mPa·s to 10.0 mPa·s. The pH of the ink at 25°C is preferably 7.0 to 9.0. If the pH of the ink is within the above range, the generation of silanol groups by hydrolysis of the compound represented by general formula (1) will proceed, and the weak affinity between titanium dioxide particles and the compound represented by general formula (1) will be effectively exhibited. The pH of the ink can be measured with a general pH meter equipped with a glass electrode or the like.

[0072] <Inkjet recording method> The inkjet recording method of the present invention is a method of recording an image on a recording medium by ejecting the aqueous ink (white ink) of the present invention described above from an inkjet recording head. Methods for ejecting the ink include methods that impart mechanical energy to the ink and methods that impart thermal energy to the ink. In the present invention, it is particularly preferable to employ a method that imparts thermal energy to the ink to eject it. Aside from using the ink of the present invention, the steps of the inkjet recording method can be those of known origin. For example, when recording an image with white ink, a general inkjet recording method can be directly applied. Furthermore, when using white ink as a base treatment for color ink, the image can be recorded by applying color ink (black, cyan, magenta, yellow, etc.) so as to overlap at least a portion of the area where the white ink has been applied. It can also be used for back printing, where white ink is applied so as to overlap at least a portion of the area where the color ink has been applied. While there are no particular limitations on the recording medium, since the aqueous ink of the present invention can be used as white ink, it is preferable to use a transparent or colored recording medium. The recording medium may also be a poorly absorbent medium (non-absorbent medium) with low absorption of liquid media, such as a resin film.

[0073] Figure 1 is a schematic diagram showing an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, where (a) is a perspective view of the main part of the inkjet recording apparatus and (b) is a perspective view of the head cartridge. The inkjet recording apparatus is provided with a transport means (not shown) for transporting the recording medium 32 and a carriage shaft 34. A head cartridge 36 can be mounted on the carriage shaft 34. The head cartridge 36 comprises recording heads 38 and 40 and is configured to hold an ink cartridge 42. While the head cartridge 36 is transported along the carriage shaft 34 in the main scanning direction, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. Then, the recording medium 32 is transported in the sub-scanning direction by the transport means (not shown), and an image is recorded on the recording medium 32.

[0074] Multi-pass recording is preferred, in which ink is applied to a unit area of ​​the recording medium by dividing the process into multiple relative scans between the recording head and the recording medium. In particular, it is preferable to apply white ink and color ink to the unit area using different relative scans. This increases the time until each ink comes into contact, making it easier to suppress mixing. A unit area can be set as any area, such as one pixel or one band. [Examples]

[0075] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way by the following examples unless it exceeds the gist of the invention. Unless otherwise specified, amounts of components indicated in "parts" and "%" are based on mass.

[0076] <Preparation of Titanium Oxide> Commercially available titanium dioxide without surface treatment, and titanium dioxide particles prepared by surface treating untreated titanium dioxide were used. The cumulative 50% particle size (D) of the titanium dioxide particles was measured by volume. 50 The particle size was measured using a dynamic light scattering particle size analyzer (product name "Nanotrac WaveII-EX150", manufactured by Microtrac-Bell). The properties of titanium oxide particles are shown in Table 1.

[0077] (Measurement of the amount of alumina and silica coating) The proportion of alumina and silica in titanium oxide particles, i.e., the amount of alumina and silica coating, was measured as follows. The prepared titanium oxide particles were added to nitric acid to create a liquid sample, and quantitative analysis of aluminum and silicon elements was performed using an inductively coupled plasma (ICP) emission spectrometer. In this process, it was assumed that all atoms coating the surface were oxides, and the obtained values ​​of aluminum and silicon were converted to their oxides, i.e., alumina and silica, to calculate their mass ratios.

[0078] (Titanium dioxide particles 1-7) Titanium dioxide particles 1-7 were produced by surface treatment of titanium dioxide using a wet process. The wet surface treatment involves contacting untreated titanium dioxide with a surface treatment agent (such as sodium aluminate or sodium silicate). By appropriately adjusting the amount and ratio of the surface treatment agent, surface treatment was achieved in any desired ratio.

[0079] Specifically, 300 parts of rutile-type titanium dioxide (product name "TITANIX JR", manufactured by Teika), which had not undergone surface treatment, and 700 parts of pure water were mixed in a homogenizer. The mixture was then heated to 90°C while stirring, and potassium hydroxide (pH adjuster) was added to adjust the pH to 10.5. Next, sodium silicate was added, and dilute sulfuric acid (pH adjuster) was added over approximately 1 hour to adjust the pH to 5.0. The reaction was continued for approximately 1 hour. After that, sodium aluminate was added in small amounts at 90°C. During this process, dilute sulfuric acid was used in combination to maintain the pH between 6.0 and 8.0. After the addition of sodium aluminate, the reaction was continued for approximately 1 hour to obtain a dispersion. After cooling the dispersion to 25°C, it was purified by repeated sedimentation using a centrifuge and redispersion in deionized water, and then dried at 120°C to obtain titanium dioxide particles with at least one of alumina and silica surface treatments.

[0080] (Titanium dioxide particles 8) Commercially available rutile-type titanium dioxide particles (product name "TITANIX JR", manufactured by Teika, without surface treatment) were used as titanium dioxide particle 8. The properties of titanium dioxide particle 8 are also shown in Table 1. Some commercially available titanium dioxide particles contained inorganic oxides such as zinc oxide and zirconia, as well as organic compounds such as polyols, in addition to alumina and silica, but the proportion of these was at most about 1.00%. Therefore, for convenience, the proportion of titanium dioxide in the titanium dioxide particles, T (%) ("Titanium Dioxide T (%)" in Table 1), is shown as a whole.

[0081] [Table 1]

[0082] <Preparation of the compound represented by general formula (1)> The compound represented by general formula (1) was synthesized using the following procedure. The synthesis conditions and structures of the compound synthesized as represented by general formula (1), and the comparative compound, are shown in Tables 2 and 3, respectively. The compound represented by general formula (1) can be synthesized by allylation and hydrosilylation of the starting material (such as polyalkylene glycol monoalkyl ether).

[0083] (Compounds 1 to 13, comparative compound 14) In a three-necked flask equipped with a stirring bar and a nitrogen inlet tube, the raw materials, base, and solvent shown in Table 2 were placed and stirred at 25°C for 30 minutes. For "sodium hydride," a paraffin dispersion of 60% sodium hydride was used, with the amount of sodium hydride used being as shown in Table 2. The bromide listed in Table 2 was then added dropwise while stirring at 25°C, and stirring continued for 12 hours after the completion of the dropwise addition to obtain a solution containing the reaction products. Unreacted sodium hydride and the neutralized product (sodium bromide) were filtered off from the solution containing the reaction products, and THF was removed by reduced pressure to obtain a concentrate. The concentrate was dissolved in 500 parts of pure water, and this aqueous solution was extracted three times with 200 mL of hexane, followed by extraction with 200 mL of dichloromethane. The solvent containing the products was dried by adding magnesium sulfate, and concentrated under reduced pressure to obtain the allylated compounds (allylation step).

[0084] A dry, passivated round-bottom flask equipped with a stirring bar and an argon inlet tube was filled with the allylated starting materials and silane compounds shown in Table 2 and stirred at 85°C. Then, 0.54 parts of an aqueous solution of 65 mmol / L chloroplatinic acid monohydrate in isopropyl alcohol was added, and the mixture was heated at 85°C for 5 hours. After the reaction was complete, the mixture was allowed to cool to 25°C, and excess silane compounds were removed by reduced pressure. The residue was purified by column chromatography using silica gel that had been passivated with triethoxysilane as a support to obtain each compound (hydrosilylation step). For purification by column chromatography, an ethyl acetate / hexane / ethanol = 85 / 15 / 5 (by volume) eluent was used.

[0085] (Comparative compound 15) The starting materials and solvent shown in Table 2 were placed in a three-necked flask equipped with a stirring bar, reflux condenser, and argon inlet tube, and the starting materials were dissolved. The bromides listed in Table 2 were added dropwise with stirring over approximately 1 hour at 80°C, and refluxed for 30 minutes. After the reaction was complete, THF was removed by reduced pressure, and 300 parts of pure water and the base listed in Table 2 were added. The resulting liquid containing the reaction product was allowed to cool to 25°C and extracted twice with 200 mL of diethyl ether. The solvent containing the product was dried by adding magnesium sulfate and concentrated under reduced pressure to obtain the allylated compound. The hydrosilylation step was carried out in the same procedure as for compounds 1-13 and comparative compound 14 to obtain comparative compound 15.

[0086] (Comparative compound 16) The raw materials and solvent shown in Table 2 were placed in a three-necked flask equipped with a stirring bar and a nitrogen inlet tube, and the raw materials were dissolved. The mixture was stirred at 55°C for 3 hours, and after the reaction was complete, the solvent was removed by reduced pressure to obtain a concentrate. 100 parts of ethanol were added to the concentrate, and the mixture was filtered. Impurities were removed by washing the residue with ethanol. The liquid component was removed by reducing the pressure of the filtrate to obtain the allylated compound. The hydrosilylation step was carried out in the same procedure as for compounds 1-13 and comparative compound 14 to obtain comparative compound 16.

[0087] [Table 2]

[0088] [Table 3]

[0089] <Preparation of Pigment Dispersion> The pigment dispersion was prepared using the following procedure. The manufacturing conditions for the pigment dispersion are shown in Table 4.

[0090] (Pigment dispersions 1-37, 39-40, 42) 40.00 parts of titanium dioxide particles of the types shown in Table 4, a dispersant, a pH adjuster, and ion-exchanged water totaling 100.00 parts were mixed and pre-dispersed using a homogenizer. Potassium hydroxide and acetic acid were used as 1 mol / L aqueous solutions, and their amounts were adjusted as appropriate to achieve the pH of the liquid medium in the main dispersion shown in Table 4. The pH of the main dispersion was measured at the start of the dispersion using a pH meter (product name "Portable pH Meter D-74", manufactured by Horiba, Ltd.). Subsequently, 0.5 mm zirconia beads were dispersed in a paint shaker at 25°C for 12 hours (main dispersion). The zirconia beads were filtered off, and an appropriate amount of ion-exchanged water was added as needed to prepare each pigment dispersion with a titanium dioxide particle content of 40.00%.

[0091] (Pigment dispersion 38) 40.00 parts of titanium dioxide particles of the type shown in Table 4 and 60.00 parts of deionized water were mixed and pre-dispersed using a homogenizer. Then, a resin dispersant (product name "Floren G700", acid value: 60 mg KOH / g, manufactured by Kyoeisha Chemical Co., Ltd.) for dispersing the titanium dioxide particles was added to obtain a mixture. The pH of the liquid medium in the main dispersion was measured at the start of the main dispersion using the above apparatus. This mixture was dispersed for 12 hours using a paint shaker filled with 0.5 mm zirconia beads (main dispersion) to prepare a pigment dispersion 38 with a titanium dioxide particle content of 40.00%.

[0092] (Pigment dispersion 41) A pigment dispersion was prepared according to the method for preparing pigment 3k in Example 3 of Patent Document 1. Specifically, the pigment dispersion was prepared in the same manner as pigment dispersion 1, except that compound 4 was used instead of compound 5 as the dispersant. After removing moisture from the obtained pigment dispersion by drying with air at 35°C while stirring, it was dried in an oven at 105°C for 4 hours and 15 minutes to obtain titanium dioxide particle powder. The titanium dioxide particle powder was redispersed with an appropriate amount of deionized water to prepare pigment dispersion 41 with a titanium dioxide particle content of 40.00%.

[0093] (Pigment dispersion 43) A pigment dispersion was prepared in accordance with the preparation method for aqueous pigment dispersion A in Example 1 of Patent Document 3. Specifically, the pigment dispersion was prepared in the same manner as pigment dispersion 1, except that 1.2 parts of vinyltriethoxysilane (silane coupling agent) were used instead of compound 5. The obtained pigment dispersion was heated to dryness to obtain titanium dioxide particle powder surface-treated with the silane coupling agent. 40.00 parts of titanium dioxide particle powder, 3.20 parts of styrene-acrylic resin, and 56.80 parts of deionized 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. Subsequently, the dispersion treatment was performed again using the same procedure as for pigment dispersion 1 to prepare pigment dispersion 43 with a titanium dioxide particle content of 40.00% and a resin content of 3.20%. The pH of the liquid medium in this dispersion was 10.5, and the pH of pigment dispersion 43 was 8.7.

[0094] [Table 4]

[0095] <Preparation of a liquid containing alumina particles> The liquid containing alumina particles was prepared based on the preparation method for ink 3 in Example 1 of Patent Document 2. Specifically, an alumina particle dispersion was prepared containing 10% amphoteric alumina particles (product name "Dispal 23N4-80", dispersed particle size 90 nm, manufactured by Sasol). The pH of the alumina particle dispersion was adjusted to 4.0 with a strong acid (1 mol / L hydrochloric acid), the alumina particle dispersion was mixed uniformly using a propeller mixer, and then pulverized using a bead mill to obtain a liquid containing alumina particles (alumina particle content: 10%).

[0096] <Ink preparation> The components of the types and quantities shown in Tables 5 to 8 were mixed and stirred. Vinibran 2685 (product name) is the product name of an acrylic emulsion (acrylic resin particle content: 30%) manufactured by Nisshin Chemical Industry. Acetyleneol E60 (product name) is a nonionic surfactant manufactured by Kawaken Fine Chemical. The ion-exchanged water containing potassium hydroxide was used to adjust the pH of each ink to the values ​​listed in Tables 5 to 8, and the remaining amount was added so that the total of the components was 100.00%. After that, each ink was prepared by pressure filtration using a pore size 5.0 μm membrane filter (manufactured by Sartorius). The pH of each ink was measured using the above apparatus. The characteristics of each ink are summarized in the lower section of Tables 5 to 8.

[0097] [Table 5]

[0098] [Table 6]

[0099] [Table 7]

[0100] [Table 8]

[0101] <Rating> Each pigment dispersion and ink obtained above was evaluated for the following items. In this invention, "AA," "A," and "B" were defined as acceptable levels, and "C" as an unacceptable level in the evaluation criteria for each item below. The evaluation results are shown in Table 9.

[0102] (Storage stability of pigment dispersions) The particle size of titanium dioxide particles in the prepared pigment dispersion was measured (referred to as "particle size before storage"). Each ink was placed in a sealed container and stored at 70°C for 3 days. After returning the pigment dispersion to 25°C, the particle size of the titanium dioxide particles was measured again (referred to as "particle size after storage"). The particle size of the titanium dioxide particles was measured using the particle size analyzer by the dynamic light scattering method described above, and the cumulative 50% particle size (D 50 The particle size change rate was calculated based on the formula "Particle size change rate" (%) = 100 × ("Particle size after storage" - "Particle size before storage") / ("Particle size before storage"), and the storage stability of the pigment dispersion was evaluated according to the evaluation criteria shown below. AA: The particle size change rate was 10% or less. A: The particle size change rate was greater than 10% but less than or equal to 20%. B: The particle size change rate was greater than 20% but less than or equal to 30%. C: The particle size change rate exceeded 30%.

[0103] (Ink storage stability) The particle size of titanium dioxide particles in the prepared ink was measured (referred to as "particle size before storage"). Each ink was placed in a sealed container and stored at 70°C for 3 days. After the ink was returned to 25°C, the particle size of the titanium dioxide particles was measured again (referred to as "particle size after storage"). The percentage change in particle size was then calculated based on the formula "percentage change in particle size" (%) = 100 × ("particle size after storage" - "particle size before storage") / ("particle size before storage"), and the storage stability of the ink was evaluated according to the evaluation criteria shown below. AA: The particle size change rate was 10% or less. A: The particle size change rate was greater than 10% but less than or equal to 20%. B: The particle size change rate was greater than 20% but less than or equal to 30%. C: The particle size change rate exceeded 30%.

[0104] [Table 9]

[0105] <Foam suppression> For each pigment dispersion obtained above, the foaming suppression was further evaluated for each pigment dispersion obtained after the purification process (the pigment dispersion after the purification process).

[0106] (purification process) The purification process was carried out using the types of pigment dispersions, first filter members, and second filter members listed on the left side of Table 10. The ultrafiltration process was performed by flowing the treatment liquid parallel to the membrane surface. Specifically, water was added to maintain a constant pigment content in the pigment dispersion, and ultrafiltration was performed while circulating the treatment liquid. The ultrafiltration process was terminated when 10 times the volume of the pigment dispersion was filtered, and the circulated liquid was recovered. The filtration process was carried out by flowing the treatment liquid perpendicular to the membrane surface. Specifically, the process was terminated when the entire volume of the liquid before the filtration process was filtered, and the filtrate was obtained. If necessary, the amount of deionized water in the filtrate was reduced by vacuum to obtain each pigment dispersion after the purification process with a titanium dioxide content of 40.0%. In Table 10, "D of titanium dioxide particles" 50 The column labeled "(nm)" shows the average particle size of titanium dioxide particles in each pigment dispersion before the purification process, as measured using the method described above.

[0107] (Foam suppression) The foaming of each pigment dispersion after the purification process was evaluated as follows. Each pigment dispersion that passed through the second filter element in the filtration process was collected in a beaker. Using the end of the filtration process as a reference, the time it took for the bubbles on the liquid surface in the beaker to disappear at 25°C was measured, and the suppression of foaming was evaluated according to the evaluation criteria shown below. The evaluation results are shown in Table 10. A shorter time for bubbles to disappear means that foaming is suppressed, and that efficient production of pigment dispersions is possible without stopping the equipment due to foaming. A: The time required for the bubbles that had formed at the end of the filtration process to disappear was less than 5 minutes. B: The time required for the bubbles that had formed at the end of the filtration process to disappear exceeded 5 minutes.

[0108] [Table 10]

[0109] The evaluation results for foam suppression in Examples 35 and 36 were the same as in Example 32, "B," but Example 32 was superior.

[0110] This embodiment includes the following configurations and methods.

[0111] [Method 1] A method for producing a dispersion of titanium dioxide particles for use in the manufacture of water-based inkjet inks, The titanium oxide particles are titanium oxide in which at least a portion of their surface is coated with silica. The system includes a dispersion step of dispersing the titanium oxide particles in a liquid medium using a compound represented by the following general formula (1) for dispersing the titanium oxide particles, A method for producing a dispersion of titanium dioxide particles, characterized in that the pH of the liquid medium in the dispersion step is 7.5 or more and 12.5 or less.

[0112] [ka]

[0113] (In general formula (1), R1, R2, and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. R4 is each 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.)

[0114] [Method 2] A method for producing a dispersion of titanium oxide particles according to Method 1, wherein the content (mass%) of the compound represented by the general formula (1) in the dispersion of titanium oxide particles is 0.002 times or more and 0.10 times or less in mass ratio to the content (mass%) of the titanium oxide particles.

[0115] [Method 3] A method for producing a dispersion of titanium dioxide particles according to Method 1 or 2, wherein the proportion (mass%) of titanium dioxide in the titanium dioxide particles is 85.00% by mass or more, based on the total mass of the titanium dioxide particles.

[0116] [Method 4] A method for producing a dispersion of titanium oxide particles according to any one of methods 1 to 3, wherein the proportion (mass%) of titanium oxide in the titanium oxide particles is 98.50% by mass or less, based on the total mass of the titanium oxide particles.

[0117] [Method 5] A method for producing a dispersion of titanium oxide particles according to any one of methods 1 to 4, wherein the proportion (mass%) of silica in the titanium oxide particles is 0.50% by mass or more and 9.00% by mass or less, based on the total mass of the titanium oxide particles.

[0118] [Method 6] A method for producing a dispersion of titanium oxide particles according to any one of methods 1 to 5, wherein the titanium oxide particles are titanium oxide in which at least a portion of their surface is further coated with alumina, and the proportion (mass%) of alumina in the titanium oxide particles is 1.50 times or less in mass ratio to the proportion (mass%) of silica.

[0119] [Method 7] A method for producing a dispersion of titanium oxide particles according to Method 6, wherein the proportion (mass%) of alumina in the titanium oxide particles is 1.00 times or less in mass ratio to the proportion (mass%) of silica.

[0120] [Method 8] A method for producing a dispersion of titanium oxide particles according to method 6 or 7, wherein the proportion (mass%) of alumina in the titanium oxide particles is 9.00% by mass or less, based on the total mass of the titanium oxide particles.

[0121] [Method 9] A method for producing a dispersion of titanium dioxide particles according to any one of methods 1 to 8, wherein the pH of the liquid medium in the dispersion step is 9.0 or more and 11.5 or less.

[0122] [Method 10] A method for producing a dispersion of titanium oxide particles according to any one of methods 1 to 9, wherein the cumulative 50% particle diameter of the titanium oxide particles by volume is 200 nm or more and 400 nm or less.

[0123] [Method 11] A method for producing a dispersion of titanium dioxide particles according to any one of methods 1 to 10, wherein the compound represented by the general formula (1) is the compound represented by the general formula (2) below.

[0124] [ka]

[0125] (In general formula (2), R1 and R3 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. m is 8 to 24.)

[0126] [Method 12] A method for manufacturing water-based ink for inkjet printers, A method for producing aqueous ink, characterized by comprising the step of mixing a dispersion of titanium dioxide particles produced by the method for producing a dispersion of titanium dioxide particles described in any one of Methods 1 to 11 with other ink components.

[0127] [Method 13] An inkjet recording method that records an image on a recording medium by ejecting ink from an inkjet recording head, An inkjet recording method characterized in that the ink is an aqueous ink produced by the aqueous ink manufacturing method described in Method 12.

Claims

1. A method for producing a dispersion of titanium dioxide particles for use in the manufacture of water-based inkjet inks, The titanium oxide particles are titanium oxide in which at least a portion of their surface is coated with silica. The system includes a dispersion step of dispersing the titanium oxide particles in a liquid medium using a compound represented by the following general formula (1) for dispersing the titanium oxide particles, A method for producing a dispersion of titanium dioxide particles, characterized in that the pH of the liquid medium in the dispersion step is 7.5 or more and 12.5 or less. 【Chemistry 1】 (In general formula (1), R １ , R ２ , and R ３ Each of these is independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. ４ Each of these 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. A method for producing a dispersion of titanium oxide particles according to claim 1, wherein the content (mass%) of the compound represented by the general formula (1) in the dispersion of titanium oxide particles is 0.002 times or more and 0.10 times or less in mass ratio to the content (mass%) of the titanium oxide particles.

3. A method for producing a dispersion of titanium oxide particles according to claim 1, wherein the proportion (by mass) of titanium oxide in the titanium oxide particles is 85.00% by mass or more, based on the total mass of the titanium oxide particles.

4. A method for producing a dispersion of titanium oxide particles according to claim 1, wherein the proportion (mass%) of titanium oxide in the titanium oxide particles is 98.50% by mass or less, based on the total mass of the titanium oxide particles.

5. A method for producing a dispersion of titanium oxide particles according to claim 1, wherein the proportion (mass%) of silica in the titanium oxide particles is 0.50% by mass or more and 9.00% by mass or less, based on the total mass of the titanium oxide particles.

6. A method for producing a dispersion of titanium oxide particles according to claim 1, wherein the titanium oxide particles are titanium oxide in which at least a portion of their surface is further coated with alumina, and the proportion (mass%) of alumina in the titanium oxide particles is 1.50 times or less in mass ratio to the proportion (mass%) of silica.

7. A method for producing a dispersion of titanium oxide particles according to claim 6, wherein the proportion (mass%) of alumina in the titanium oxide particles is 1.00 times or less in mass ratio to the proportion (mass%) of silica.

8. A method for producing a dispersion of titanium oxide particles according to claim 6, wherein the proportion (mass%) of alumina in the titanium oxide particles is 9.00% by mass or less, based on the total mass of the titanium oxide particles.

9. A method for producing a dispersion of titanium dioxide particles according to claim 1, wherein the pH of the liquid medium in the dispersion step is 9.0 or more and 11.5 or less.

10. A method for producing a dispersion of titanium oxide particles according to claim 1, wherein the cumulative 50% particle diameter of the titanium oxide particles on a volume basis is 200 nm or more and 400 nm or less.

11. A method for producing a dispersion of titanium oxide particles according to claim 1, wherein the compound represented by the general formula (1) is the compound represented by the general formula (2) below. 【Chemistry 2】 (In general formula (2), R １ , and R ３ Each of these is independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. m is 8 to 24.

12. A method for manufacturing water-based ink for inkjet printers, A method for producing aqueous ink, characterized by comprising the step of mixing a dispersion of titanium dioxide particles produced by the method for producing a dispersion of titanium dioxide particles described in any one of claims 1 to 11 with other ink components.

13. An inkjet recording method that records an image on a recording medium by ejecting ink from an inkjet recording head, An inkjet recording method characterized in that the ink is an aqueous ink produced by the method for producing aqueous ink described in claim 12.

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