JP2023125909A - Inkjet recording method, and inkjet recording device - Google Patents

Abstract

To provide an inkjet recording method which can record a secondary color image excellent in a color development property using a white ink and a color ink without using a reaction liquid, while securing storage stability of the white ink.SOLUTION: An inkjet recording method records an image using a first ink and a second ink. The first ink contains titanium oxide particles that are titanium oxide with at least a part of its surface coated with alumina and silica, and a compound represented by general formula (1) as a dispersant of the titanium oxide particles, and the second ink contains a coloring material different from the titanium oxide particles. A content (ppm) of a monovalent cation in the first ink is 700 ppm or more and 1300 ppm or less, and a ratio (M2/M1) is 1.4 times or more and 5.0 times or less, where M2 is a monovalent cation content in the second ink, and M1 is a monovalent cation content in the first ink.SELECTED DRAWING: None

Description

The present invention relates to an inkjet recording method and an inkjet recording apparatus.

2. Description of the Related Art In recent years, inkjet recording devices have come to be widely used when outputting paper, resin films, and the like as recording media for advertisements and exhibits. For example, in order to express clear color images even on transparent recording media, white ink is used in addition to black or basic color ink (hereinafter sometimes referred to collectively as color ink). . Specifically, white ink is applied in advance to a portion of a transparent recording medium that includes an area where an image is to be recorded to perform a base treatment, and then color ink is applied from above, or each ink is applied in the reverse order ( A so-called back print) recording method is used.

Titanium oxide is widely used as a coloring material for white ink because it is low cost and has excellent properties required for white ink, such as whiteness and opacity. As water-based ink for inkjet use is a metal oxide, it is necessary to stably disperse titanium oxide, which has a high specific gravity, but increasing the content of titanium oxide in order to obtain whiteness makes it difficult to disperse stably. may be difficult. In addition, in order to record a secondary color image by combining white ink and color ink on a transparent recording medium with low liquid absorption such as a resin film, it is necessary to dry the ink that has been applied first before applying it again. It is necessary to apply one of the inks. If the other ink is applied before the previously applied ink has not dried sufficiently, the inks tend to mix and the image becomes cloudy. As a result, even though white ink is used to obtain a clear image, a secondary color image with excellent color development may not be obtained.

Until now, methods for stably dispersing titanium oxide have been studied. After surface treating titanium oxide with silica, the surface is further treated with a silane coupling agent and dried to produce a dry titanium dioxide product in which a portion of the silane coupling agent is covalently bonded to the surface of the titanium oxide particles. A method has been proposed (see Patent Document 1). Furthermore, an ink containing titanium oxide surface-treated with alumina, a monovalent metal salt, and alumina fine particles has been proposed (see Patent Document 2). Furthermore, in order to suppress mixing between inks, it has been proposed to use a reaction liquid for coagulating coloring materials (see Patent Document 3).

Special table 2017-521348 publication International Publication No. 2018/190848 International Publication No. 2018/144181

The present inventors investigated the storage stability of the aqueous ink prepared using dry titanium dioxide proposed in Patent Document 1 and the aqueous ink proposed in Patent Document 2. As a result, it was found that storage stability was insufficient to stably handle titanium oxide with a large particle size necessary to obtain whiteness, and there was room for improvement. Furthermore, in order to utilize the reaction liquid as in Patent Document 3, a mechanism for applying the reaction liquid to the recording medium is required, which leads to an increase in the size of the apparatus. Furthermore, the use of the reaction liquid may affect other image quality.

Therefore, an object of the present invention is to provide an inkjet jet that can record secondary color images with excellent color development using white ink and color ink without using a reaction liquid while ensuring the storage stability of the white ink. The goal is to provide a recording method. Another object of the present invention is to provide an inkjet recording apparatus for use in the inkjet recording method.

The above object is achieved by the present invention as follows. That is, the inkjet recording method according to the present invention is an inkjet recording method in which an image is recorded on a recording medium by ejecting ink from an inkjet recording head, and includes a step of applying a first ink to the recording medium, and a step of applying the first ink to the recording medium; a step of applying a second ink to the recording medium so as to overlap at least a portion of an area to which the first ink is applied, the first ink containing titanium oxide particles and a dispersant for the titanium oxide particles; The titanium oxide particles are titanium oxide whose surfaces are at least partially coated with alumina and silica, and the dispersant for the titanium oxide particles is represented by the following general formula (1). The content M ₁ (ppm) of monovalent cations in the first ink is 700 ppm or more and 1,300 ppm or less, and the second ink contains a coloring material different from the titanium oxide particles. The aqueous ink contains an aqueous ink in which the content M ₂ of monovalent cations in the second ink is a ratio (M ₂ /M ₁ ) to the content M ₁ of monovalent cations in the first ink, and 1. It is characterized by being 4 times or more and 5.0 times or less.

(In general formula (1), R ₁ , R ₂ , and R ₃ are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and R ₄ is 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.)

According to the present invention, an inkjet recording method can record a secondary color image with excellent color development using white ink and color ink without using a reaction liquid while ensuring the storage stability of the white ink. can be provided. Further, according to the present invention, it is possible to provide an inkjet recording apparatus for use in the inkjet recording method.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram schematically showing an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, in which (a) is a perspective view of the main part of the inkjet recording apparatus, and (b) is a perspective view of a head cartridge.

Hereinafter, the present invention will be further explained in detail by citing preferred embodiments. In the present invention, when a compound is a salt, the salt is dissociated into ions and exists in the ink, but for convenience, it is expressed as "containing a salt." Titanium oxide and titanium oxide particles are sometimes simply referred to as "pigments." Furthermore, water-based ink for inkjet is sometimes simply referred to as "ink." Physical property values are values at room temperature (25° C.) unless otherwise specified.

Titanium oxide reacts with water molecules constituting the aqueous medium in the aqueous ink to generate hydroxy groups (hereinafter sometimes referred to as "surface hydroxy groups") on its surface. In aqueous inkjet inks, titanium oxide (titanium oxide particles) is surface-treated with an inorganic oxide such as alumina or silica in order to improve the storage stability of the ink by utilizing the generated surface hydroxyl groups. It is generally used as The surface hydroxy groups of titanium oxide particles have properties unique 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 oxide itself is an inorganic oxide, the surface of titanium oxide particles exhibits the properties of an inorganic oxide corresponding to the inorganic compound used for surface treatment, and the surface charge of titanium oxide particles is It strongly depends on the pH of the surface treatment agent, the type of surface treatment agent, and the amount of surface treatment agent used.

The present inventors first studied how to improve the storage stability of white ink by changing the components contained in the ink. A dispersant is required to stably disperse titanium oxide. Although titanium oxide has a certain degree of dispersion stability due to electrostatic repulsion due to surface hydroxyl groups, it is difficult to maintain a stable dispersed state over a long period of time. Therefore, in general, the storage stability of the ink is improved by coating titanium oxide with silica or alumina, and the dispersion state is kept stable by combining a dispersant suitable for the surface condition of the titanium oxide particles. ing. Therefore, the present inventors conducted a study on an appropriate dispersant. As a result, it was found that a compound represented by the general formula (1) is preferable as a dispersant for titanium oxide.

Next, the present inventors conducted a study on recording secondary color images with excellent color development. As mentioned above, a secondary color image using white ink and color ink is recorded by applying one ink first and then applying the other ink. If the next ink is applied before the previously applied ink layer has sufficient physical strength, the inks will mix with each other, resulting in a decrease in color development. Therefore, the present inventors investigated the composition of each ink that can record images with excellent color development even when no reaction liquid is used. As a result, it was discovered that by adjusting the ratio of the monovalent cation content of the first ink and the second ink to a specific range, it is possible to record a secondary color image with excellent color development.

That is, the inkjet recording method of the present invention has the following characteristics. First, titanium oxide particles in which at least a portion of the surface of titanium oxide is coated with alumina and silica are used as the coloring material (pigment) of the first ink. As a dispersant for dispersing titanium oxide particles, a compound represented by the general formula (1) described below is used. The content (ppm) of monovalent cations in the first ink is 700 ppm or more and 1,300 ppm or less. Further, the coloring material of the second ink is a different coloring material from the titanium oxide particles of the first ink. The content of monovalent cations in the second ink is 1.4 times or more and 5.0 times or less relative to the content of monovalent cations in the first ink. Then, the first ink and the second ink described above are applied to the recording medium so as to overlap, thereby recording a secondary color image. The present inventors speculate as follows about the mechanism by which a secondary color image with excellent color development can be recorded with the above configuration while ensuring the storage stability of the first ink.

At least a portion of the surface of the titanium oxide particles in the first ink is coated with alumina and silica. Under alkaline conditions, which is the general liquid quality of aqueous inkjet inks, alumina and silica on the surface of titanium oxide particles form surface hydroxyl groups. Furthermore, the isoelectric point of silica in water at 25° C. (room temperature) is about 2.0, and surface hydroxyl groups derived from silica tend to be negatively charged. Since the surface hydroxyl groups derived from silica are negatively charged, even if titanium oxide particles approach each other, aggregation can be suppressed due to electrostatic repulsion. On the other hand, surface hydroxyl groups derived from alumina tend to be positively charged. However, by performing surface treatment with alumina and silica, it is possible to suppress the interaction between the titanium oxide particles and suppress the aggregation of the titanium oxide particles due to electrostatic repulsion. As a result, the storage stability of the first ink can be improved. Titanium oxide particles surface-treated with alumina alone cannot suppress agglomeration of the titanium oxide particles due to electrostatic repulsion, and storage stability of the first ink cannot be obtained. Furthermore, titanium oxide particles surface-treated with silica only have too strong an interaction between the titanium oxide particles, making it impossible to suppress aggregation of the titanium oxide particles, making it impossible to obtain storage stability of the first ink.

Further, the content of monovalent cations in the first ink is 700 ppm or more and 1,300 ppm or less. The term "monovalent cation" herein refers to an alkali metal ion, ammonium ion, or organic ammonium, and does not include protons (H ⁺ ). If the content of monovalent cations in the first ink is less than 700 ppm, the ionization of surface hydroxyl groups will not be sufficiently promoted because the cation concentration in the ink will be insufficient. As a result, electrostatic repulsion for stably dispersing the titanium oxide particles cannot be obtained, and storage stability of the first ink cannot be obtained. If the content of monovalent cations in the first ink exceeds 1,300 ppm, the cation concentration in the ink will be too high and the dispersion state of titanium oxide particles will become unstable due to salting out, and the first ink will Storage stability cannot be obtained.

Some of the compounds represented by general formula (1) are known as silane coupling agents. In general formula (1), OR ₁ each independently represents a hydroxy group or an alkoxy group having 1 or more and 4 or less carbon atoms. A portion of OR ₁ bonded to the silicon atom can be hydrolyzed to form a silanol group, and a portion of the silanol group can dissociate into ions. Such reactions are particularly likely to occur under alkaline conditions. Then, a hydrogen bond is formed between the silica-derived surface hydroxy group of the titanium oxide particle and the silanol group of the compound represented by the general formula (1). Further, a part of the silica-derived surface hydroxyl group of the titanium oxide particle and the silanol group of the compound represented by the general formula (1) can be covalently bonded by a dehydration reaction. In other words, the surface hydroxy group derived from silica of the titanium oxide particles and the silanol group in the compound represented by the general formula (1) exhibit "weak affinity" due to hydrogen bonds and covalent bonds. Therefore, by repeating desorption and adsorption, the compound represented by the general formula (1) can be made to exist in the vicinity of the titanium oxide particles. When OR ₁ is an alkoxy group having more than 4 carbon atoms, it becomes difficult to hydrolyze. As a result, affinity with the surface hydroxyl group of the titanium oxide particles cannot be obtained, and the titanium oxide particles cannot be stably dispersed, so that storage stability of the first ink cannot be obtained.

In addition to the structure capable of forming a silanol group as described above, the compound represented by the general formula (1) has an alkylene oxide having 2 to 4 carbon atoms as a repeating unit via X as a linking group. It has a structure having n pieces ((OR ₄ ) _n in general formula (1)). n represents the number (average value) of alkylene oxide groups that are repeating units, and is from 6 to 24. Hereinafter, the above structure will also be referred to as an alkylene oxide chain. Since the alkylene oxide chain has hydrophilicity, it stretches appropriately in an aqueous medium and exhibits repulsive force due to steric hindrance. Therefore, when the compound represented by the general formula (1) exists near the titanium oxide particles, the titanium oxide particles can be stably dispersed. In other words, the storage stability of the first ink can be improved.

Even if the first ink is dispersed with a resin or the like, a certain degree of dispersion stability can be obtained, but it is difficult to maintain a stable dispersed state over a long period of time. Further, when forming a secondary color image, aggregation is not sufficiently promoted even by the monovalent cation of the second ink. As a result, the image becomes cloudy and a secondary color image with excellent color development cannot be obtained.

The content M ₂ of monovalent cations in the second ink is a ratio (M ₂ /M ₁ ) to the content M ₁ of monovalent cations in the first ink, which is 1.4 times or more and 5.0 times or less. be. By containing a large amount of monovalent cations in the second ink, the titanium oxide particles in the first ink aggregate and thicken due to salting out. As a result, mixing of each ink dot can be suppressed, and a secondary color image with excellent color development can be recorded. If the value of M ₂ /M ₁ is less than 1.4 times, when each ink dot comes into contact, the concentration of monovalent cations will not be high enough, so aggregation of titanium oxide particles will not be promoted, and each ink will Mixing of dots cannot be suppressed. As a result, a secondary color image with excellent color development cannot be obtained. If the value of M ₂ /M ₁ is more than 5.0 times, the aggregation of titanium oxide particles is too strong, resulting in haze when recording a secondary color image, resulting in poor color development. The next color image cannot be obtained.

<Inkjet recording method and inkjet recording device>
The inkjet recording method (hereinafter also simply referred to as "recording method") of the present invention is a method of recording an image on a recording medium by ejecting ink from an inkjet recording head. The recording method of the present invention includes a step of applying a first ink to a recording medium, and a step of applying a second ink to the recording medium so as to overlap at least a portion of an area to which the first ink is applied. Either ink may be applied to the recording medium first.

Further, the inkjet recording apparatus (hereinafter also simply referred to as "recording apparatus") of the present invention is a device used to eject ink from an inkjet type recording head to record an image on a recording medium. The recording apparatus of the present invention includes means for applying the first ink to the recording medium, and means for applying the second ink to the recording medium so as to overlap at least a portion of the area to which the first ink is applied. Either ink may be applied to the recording medium first.

If necessary, the recorded image may be heated. Examples of methods for ejecting ink include a method of applying mechanical energy to the ink and a method of applying thermal energy to the ink. In the present invention, it is particularly preferable to adopt a method of ejecting ink by applying thermal energy to the ink.

FIG. 1 is a diagram schematically showing an example of an inkjet recording device used in the inkjet recording method of the present invention, in which (a) is a perspective view of the main parts of the inkjet recording device, and (b) is a perspective view of a head cartridge. It is. The inkjet recording apparatus is provided with a conveying means (not shown) for conveying 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 includes recording heads 38 and 40, and is configured such that an ink cartridge 42 is set therein. While the head cartridge 36 is transported 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, an image is recorded on the recording medium 32 by conveying the recording medium 32 in the sub-scanning direction by a conveying means (not shown).

Multi-pass printing is preferable, in which ink is applied to a unit area of a printing medium by dividing the printing head and the printing medium into multiple relative scans. In particular, it is preferable to apply the first ink and the second ink to the unit area using different relative scans. This lengthens the time it takes for the inks to come into contact with each other, making it easier to suppress mixing, thereby further improving the color development of the secondary color image.

<First ink>
The first ink used in the recording method of the present invention is an ink containing titanium oxide particles and a dispersant for the titanium oxide particles. Each component used in the first ink will be described in detail below.

(color material)
The first ink contains, as a coloring material (pigment), titanium oxide particles that are surface-treated with alumina and silica. Since the titanium oxide particles are white pigments, the first ink is white ink. The content (mass%) of titanium oxide particles in the first ink is preferably 0.1% by mass or more and 20.0% by mass or less, and 1.0% by mass or more, based on the total mass of the first ink. It is more preferably 15.0% by mass or less.

Titanium oxide is a white pigment, and exists in three crystal forms: rutile type, anatase type, and brookite type. Among these, rutile-type titanium oxide is preferred. Industrial methods for producing titanium oxide include a sulfuric acid method and a chlorine method, and the titanium oxide used in the present invention may be produced by either method.

The volume-based cumulative 50% particle diameter (hereinafter also referred to as average particle diameter) of the titanium oxide particles is preferably 200 nm or more and 500 nm or less, and more preferably 200 nm or more and 400 nm or less. The volume-based cumulative 50% particle diameter (D ₅₀ ) of titanium oxide particles is the particle diameter that is 50% cumulative from the small particle diameter side, based on the total volume of the measured particles in the particle diameter integration curve. It is. The _D50 of the titanium oxide particles can be measured, for example, under the following conditions: Set Zero: 30 seconds, number of measurements: 3 times, measurement time: 180 seconds, shape: spherical, refractive index: 1.59. As the particle size distribution measuring device, a particle size analyzer using a dynamic light scattering method can be used. Of course, the measurement conditions are not limited to the above.

Titanium oxide that has been surface-treated with alumina and silica is used. Surface treatment is expected to suppress photocatalytic activity and improve dispersibility. In this specification, "alumina" is a general term for aluminum oxides such as aluminum oxide. Furthermore, in this specification, "silica" is a general term for silicon oxides such as silicon dioxide. Most of the alumina and silica coating titanium oxide are present in the form of aluminum oxide and silicon dioxide.

The proportion (mass%) of titanium oxide in the titanium oxide particles is preferably 90.0% by mass or more, and preferably 98.5% by mass or less, based on the total mass of the titanium oxide particles. The proportion of alumina (mass %) in the titanium oxide particles is preferably 0.50 times or more and 1.00 times or less relative to the proportion of silica (mass %). If the mass ratio is less than 0.50 times, there will be too little alumina to silica, and the interaction between titanium oxide particles will become strong, and the first ink may not have sufficient storage stability. . If the mass ratio is more than 1.00 times, there will be too much alumina, and most of the surface hydroxyl groups on the surface of the titanium oxide particles will be positively charged. As a result, since there are not enough negatively charged surface hydroxyl groups, sufficient electrostatic repulsion may not be obtained, and the first ink may not have sufficient storage stability. Further, the proportion of silica (mass%) in the titanium oxide particles is preferably 1.0% by mass or more, and from the viewpoint of reactivity with the silane compound described later, it is preferably 4.0% by mass or less. preferable. Further, the proportion of alumina in the titanium oxide particles (mass%) is preferably 0.5% by mass or more and 4.0% by mass or less.

Examples of surface treatment methods for titanium oxide include wet treatment and dry treatment. For example, it is possible to perform surface treatment by dispersing titanium oxide in a liquid medium and then reacting it with a surface treatment agent such as sodium aluminate or sodium silicate, and by changing the ratio of these surface treatment agents as appropriate. It can also be adjusted to the characteristics of In addition to alumina and silica, oxides of inorganic compounds such as zinc and zirconium, and organic substances such as polyols can be used for surface treatment, as long as the effects of the present invention are not impaired.

(Compound represented by general formula (1))
The first ink contains a compound represented by the following general formula (1) as a dispersant for titanium oxide particles. The content (mass%) of the compound represented by general formula (1) in the ink is preferably 0.01% by mass or more and 1.0% by mass or less, and 0.02% by mass or less, based on the total mass of the ink. It is more preferable that the amount is not less than 0.5% by mass and not more than 0.5% by mass.

(In general formula (1), R ₁ , R ₂ and R ₃ are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and R ₄ is 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 satisfies a+b=3.)

In general formula (1), R ₁ , R ₂ and R ₃ 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 group, ethyl group, n-propyl group, i-propyl group, and n-butyl group. Among these, a methyl group is preferred from the viewpoint of ease of hydrolysis. If R ₁ , R ₂ , and R ₃ are each an alkyl group having more than 4 carbon atoms, it becomes difficult to hydrolyze to form a silanol group, and affinity with titanium oxide particles cannot be obtained. Therefore, the titanium oxide particles cannot be stably dispersed, and the storage stability of the first ink cannot be obtained. a representing the number of R ₁ O is 1 to 3, b representing the number of R ₂ is 0 to 2, and 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 R ₁ O.

In the general formula (1), each R ₄ independently represents an alkylene group having 2 to 4 carbon atoms. Examples of the alkylene group having 2 to 4 carbon atoms include ethylene group, n-propylene group, i-propylene group, and n-butylene group. Among these, ethylene group is preferred. n, which represents the number of _OR4s (average value), is from 6 to 24. When the number (average value) of the alkylene oxide groups is less than 6, the length of the alkylene oxide chain is too short, so that sufficient repulsive force due to steric hindrance cannot be obtained, and the storage stability of the first ink cannot be obtained. do not have. When the number (average value) of the alkylene oxide groups is more than 24, the alkylene oxide chains are too long, resulting in increased hydrophilicity and easy release into the liquid medium. Therefore, affinity with the surface hydroxyl groups of titanium oxide particles cannot be obtained, making it impossible to suppress aggregation of titanium oxide particles. Therefore, storage stability of the first ink cannot be obtained.

In the 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 OR ₄ are directly bonded. Examples of the alkylene group having 1 to 6 carbon atoms include methylene group, ethylene group, n-propylene group, i-propylene group, n-butylene group, n-pentylene group, and n-hexylene group. Among them, n-propylene group is preferred. When X is an alkylene group having more than 6 carbon atoms, the hydrophobicity of the compound represented by the general formula (1) becomes too high, and storage stability of the first ink cannot be obtained.

The compound represented by the general formula (1) which is a dispersant is preferably a compound represented by the following general formula (2). Since the compound represented by the general formula (2) has three _OR1s bonded to the silicon atom, a portion thereof is hydrolyzed in an aqueous medium to form three hydroxy groups bonded to the silicon atom. It is possible to increase the portion having affinity with titanium oxide particles. Further, the compound represented by the following general formula (2) has a repeating structure of ethylene oxide groups. Therefore, the ethylene oxide group can be appropriately extended in an aqueous liquid medium, and a repulsive force due to steric hindrance can be obtained. In the case of a compound that is not included in general formula (2), it will not have much effect on storage stability if it is in the form of a dispersion of titanium oxide particles, but it may have an effect on storage stability in ink containing various components. It may be harmful.

(In general formula (2), R ₁ and R ₃ are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. m is 8 to 24.)

The content (mass%) of the compound represented by the general formula (1) in the first ink is a mass ratio of 0.002 times to 0.10 times the content (mass%) of titanium oxide particles. It is preferable that there be. When the mass ratio is less than 0.002 times, the effect of stably dispersing the titanium oxide particles becomes weak, and the first ink may not have sufficient storage stability. It is believed that if the mass ratio is more than 0.10 times, the proportion of the compound represented by general formula (1) becomes too high, resulting in intermolecular condensation of the compound represented by general formula (1). It will be done. Therefore, the compound represented by the general formula (1) is consumed, and the effect of stably dispersing the titanium oxide particles is weak, and the first ink may not have sufficient storage stability.

(monovalent cation)
The first ink contains monovalent cations. Examples of monovalent cations include alkali metal ions, ammonium ions, and organic ammonium. Among these, it is preferable to contain potassium ions. The first ink may contain a plurality of monovalent cations. The content of monovalent cations in this case is the total amount of monovalent cations in the first ink. The content (ppm) of monovalent cations in the first ink is 700 ppm or more and 1,300 ppm or less, based on the total mass of the ink. In addition, when the first ink contains a cation having a valence of two or more, the content is preferably 10 ppm or less based on the total mass of the ink. The first ink does not need to contain a divalent or higher cation. The content of monovalent cations in the first ink can be measured, for example, by quantifying the content of various monovalent cations using ICP emission spectrometry or ion chromatography.

In order to make the first ink contain monovalent cations, for example, a compound that generates monovalent cations through ionic dissociation may be added to the ink. Examples of such compounds include alkali metal salts and ammonium salts. Examples of alkali metal salts include alkali metal salts of halogens such as fluorine, chlorine, and bromine; alkali metal salts of inorganic acids such as carbonic acid, sulfuric acid, phosphoric acid, nitric acid, and boric acid; alkali metal hydroxides; acetic acid, Examples include alkali metal salts of organic acids such as benzoic acid. Examples of ammonium salts include ammonium salts of halogens such as fluorine, chlorine, and bromine; ammonium salts of inorganic acids such as carbonic acid, sulfuric acid, phosphoric acid, nitric acid, and boric acid; ammonium salts of organic acids such as acetic acid and benzoic acid; Examples include.

(resin)
The first ink can contain a resin. Examples of the resin include acrylic resin, urethane resin, and urea resin. Among these, acrylic resins are preferred. The content (mass%) of the resin in the first ink is preferably 1.0% by mass or more and 25.0% by mass or less, and 3.0% by mass or more and 15.0% by mass, based on the total mass of the ink. % or less is more preferable. Among these, it is preferably 5.0% by mass or more and 15.0% by mass or less.

The resin can be included in the ink for purposes of improving various properties of recorded images, such as scratch resistance and hiding properties. Examples of the form of the resin include block copolymers, random copolymers, graft copolymers, and combinations thereof. Further, the resin may be a soluble resin that can be dissolved in a liquid medium, or may be resin particles that are dispersed in a liquid medium. The resin particles do not need to contain coloring material.

In this specification, "the resin is water-soluble" means that when the resin is neutralized with an alkali equivalent to the acid value, it does not form particles whose particle size can be measured by dynamic light scattering. means existing in a liquid medium. Whether or not a resin is water-soluble can be determined according to the method shown below. First, a liquid (resin solid content: 10% by mass) containing a resin neutralized with an alkali equivalent to an acid value (sodium hydroxide, potassium hydroxide, etc.) is prepared. Next, the prepared liquid is diluted 10 times (by volume) with pure water to prepare a sample solution. Then, when the particle size of the resin in the sample solution is measured by dynamic light scattering and no particles having the same particle size are measured, it can be determined that the resin is soluble. The measurement conditions at this time can be, for example, Set Zero: 30 seconds, number of measurements: 3 times, and measurement time: 180 seconds. As the particle size distribution measuring device, a particle size analyzer using a dynamic light scattering method (for example, trade name "UPA-EX150", manufactured by Nikkiso Co., Ltd.) or the like can be used. Of course, the particle size distribution measuring device and measurement conditions used are not limited to those described above.

The acrylic resin preferably has a hydrophilic unit and a hydrophobic unit as constituent units. Among these, resins having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one of a monomer having an aromatic ring and a (meth)acrylic acid ester are preferred. Particularly preferred is a resin having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one monomer of styrene and α-methylstyrene. Since these resins tend to interact with pigments, they can be suitably used as resin dispersants for dispersing pigments.

The hydrophilic unit is a unit having a hydrophilic group such as an anionic group. The hydrophilic unit can be formed, for example, by polymerizing a hydrophilic monomer having a hydrophilic group. Specific examples of hydrophilic monomers having a hydrophilic group include acidic monomers having a carboxylic acid group such as (meth)acrylic acid, itaconic acid, maleic acid, fumaric acid, and anhydrides of these acidic monomers. Examples include anionic monomers such as monomers and salts. Examples of the cation constituting the salt of the acidic monomer include ions such as lithium, sodium, potassium, ammonium, and organic ammonium. The first ink may contain a monovalent cation by using a monovalent cation obtained by ionically dissociating a salt of an acidic monomer.

A hydrophobic unit is a unit that does not have a hydrophilic group such as an anionic group. The hydrophobic unit can be formed, for example, by polymerizing a hydrophobic monomer that does not have a hydrophilic group such as an anionic group. Specific examples of hydrophobic monomers include monomers with aromatic rings such as styrene, α-methylstyrene, and benzyl (meth)acrylate; methyl (meth)acrylate, butyl (meth)acrylate, and ) (meth)acrylic acid ester monomers such as 2-ethylhexyl acrylate.

The acid value of the 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. 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 polystyrene equivalent value measured by gel permeation chromatography (GPC).

(aqueous medium)
The first ink is an aqueous ink containing water as an aqueous medium. The ink can contain an aqueous medium that is water or a mixed solvent of water and a water-soluble organic solvent. As water, it is preferable to use deionized water (ion-exchanged water). The water content (mass%) in the ink is preferably 50.0% by mass or more and 95.0% by mass or less, based on the total mass of the ink.

The water-soluble organic solvent is not particularly limited as long as it is water-soluble (preferably one that dissolves 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 3.0% by mass or more and 50.0% by mass or less, and 10.0% by mass or more and 40.0% by mass, based on the total mass of the ink. It is more preferable that it is less than % by mass. If the content (mass %) of the water-soluble organic solvent is less than 3.0 mass %, the ink will stick within the inkjet recording device, and sufficient sticking resistance may not be obtained. If the content (mass %) of the water-soluble organic solvent is more than 50.0 mass %, the viscosity of the ink becomes too high, resulting in decreased fluidity and ink supply failure.

(Other additives)
In addition to the above-mentioned additives, the first ink may optionally contain a surfactant, a pH adjuster, a rust preventive agent, a preservative, an anti-mold agent, an antioxidant, an anti-reduction agent, an evaporation accelerator, and Various additives such as chelating agents can be included. Among these, it is preferable that the first ink contains a surfactant. The content (% by mass) of the surfactant in the first ink is preferably 0.1% by mass or more and 5.0% by mass or less, and 0.1% by mass or more and 2.0% by mass or less, based on the total mass of the ink. More preferably, it is 0% by mass or less. Examples of the surfactant include anionic surfactants, cationic surfactants, and nonionic surfactants. Among these, nonionic surfactants are preferred because they have a low affinity with titanium oxide particles and are effective in small amounts because they are used to adjust various physical properties of ink.

(Physical properties of first ink)
Since the first ink is an ink applied to an inkjet system, it is preferable to appropriately control its physical properties. The surface tension of the first ink at 25° C. is preferably 10 mN/m or more and 60 mN/m or less, and more preferably 20 mN/m or more and 40 mN/m or less. The surface tension of the first ink can be adjusted by appropriately determining the type and content of the surfactant in the ink. Further, the viscosity of the first ink at 25° C. is preferably 1.0 mPa·s or more and 10.0 mPa·s or less. The pH of the first ink at 25° C. is preferably 7.0 or more and 9.0 or less. If the pH of the first ink is within the above range, the generation of silanol groups by hydrolysis of the compound represented by general formula (1) will proceed, so that titanium oxide particles and the compound represented by general formula (1) will be produced. The weak affinity of is effectively exerted. The pH of the first ink can be measured with a general pH meter equipped with a glass electrode or the like.

<Second ink>
The second ink contains a coloring material different from the titanium oxide particles. Therefore, the second ink is an ink (color ink) exhibiting colors such as black, cyan, magenta, and yellow. Each component used in the second ink will be described in detail below.

(color material)
The second ink contains a coloring material (hereinafter also referred to as "color pigment" or simply "pigment" to distinguish it from white) other than titanium oxide particles. The pigment content (mass%) in the second ink is preferably 0.1% by mass or more and 15.0% by mass or less, and 1.0% by mass or more and 10.0% by mass, based on the total mass of the ink. It is more preferable that it is the following.

Specific examples of pigments include inorganic pigments such as carbon black; organic pigments such as azo, phthalocyanine, quinacridone, isoindolinone, imidazolone, diketopyrrolopyrrole, and dioxazine. Among them, carbon black and organic pigments are preferably used.

Examples of pigment dispersion methods include resin-dispersed pigments using a resin as a dispersant, and self-dispersing pigments in which a hydrophilic group is bonded to the particle surface of the pigment. Other examples include resin-bonded pigments in which an organic group containing a resin is chemically bonded to the surface of pigment particles, and microcapsule pigments in which the surface of pigment particles is coated with a resin or the like. Among these, it is preferable to use a resin-dispersed pigment in which a resin as a dispersant is physically adsorbed onto the pigment particle surface, rather than a resin-bonded pigment or a microcapsule pigment.

As a self-dispersing pigment, one in which an anionic group such as a carboxylic acid group, a sulfonic acid group, or a phosphonic acid group is bonded to the particle surface of the pigment directly or via another atomic group (-R-) is used. be able to. The anionic group may be either an acid type or a salt type, and when it is a salt type, it may be either partially or completely dissociated. When the anionic group is a salt type, examples of the cation serving as a counter ion include an alkali metal cation, ammonium, and organic ammonium. Specific examples of other atomic groups (-R-) include linear or branched alkylene groups having 1 to 12 carbon atoms; arylene groups such as phenylene groups and naphthylene groups; carbonyl groups; imino groups; amide groups; sulfonyl groups. Groups; ester groups; ether groups, etc. can be mentioned. Furthermore, a combination of these groups may be used.

(monovalent cation)
The second ink contains monovalent cations. Examples of monovalent cations include alkali metal ions, ammonium ions, and organic ammonium. Among these, it is preferable to contain potassium ions. The second ink may contain a plurality of monovalent cations. In this case, the content of monovalent cations is the total amount of monovalent cations in the second ink. The content (ppm) of monovalent cations in the second ink is preferably 980 ppm or more and 6,500 ppm or less based on the total mass of the ink. If the content of monovalent cations in the second ink is larger than the above range, storage stability of the second ink may not be obtained due to salting out. In addition, when the second ink contains a cation having a valence of two or more, the content thereof is preferably 10 ppm or less based on the total mass of the ink. The second ink does not need to contain divalent or higher cations. The content of monovalent cations in the second ink can be measured, for example, by quantifying the content of various monovalent cations using ICP emission spectrometry or ion chromatography.

In order to make the second ink contain monovalent cations, for example, a compound that generates monovalent cations through ionic dissociation may be added to the ink. Examples of such compounds include alkali metal salts and ammonium salts. Examples of alkali metal salts include alkali metal salts of halogens such as fluorine, chlorine, and bromine; alkali metal salts of inorganic acids such as carbonic acid, sulfuric acid, phosphoric acid, nitric acid, and boric acid; alkali metal hydroxides; acetic acid, Examples include alkali metal salts of organic acids such as benzoic acid. Examples of ammonium salts include ammonium salts of halogens such as fluorine, chlorine, and bromine; ammonium salts of inorganic acids such as carbonic acid, sulfuric acid, phosphoric acid, nitric acid, and boric acid; ammonium salts of organic acids such as acetic acid and benzoic acid; Examples include.

The content M ₂ of monovalent cations in the second ink is a ratio (M ₂ /M ₁ ) to the content M ₁ of monovalent cations in the first ink, which is 1.4 times or more and 5.0 times or less. be. Among these, the ratio is preferably 3.0 times or less. By setting the value of M ₂ /M ₁ within the above range, agglomeration of titanium oxide particles can be appropriately controlled and a secondary color image with excellent color development can be recorded.

(Resin dispersant)
The second ink can contain a resin (resin dispersant) for dispersing the pigment. That is, the coloring material of the second ink can be a resin-dispersed pigment. The content (mass%) of the resin dispersant in the second ink is preferably 1.0% by mass or more and 25.0% by mass or less, and 3.0% by mass or more, based on the total mass of the second ink. It is more preferably 15.0% by mass or less. Among these, the content is particularly preferably 5.0% by mass or more and 15.0% by mass or less. When the dispersion method of the pigment is self-dispersion, it is not necessary to contain a resin dispersant.

As the resin dispersant, it is preferable to use one that can disperse the pigment in an aqueous medium by the action of an anionic group. As the resin dispersant, it is preferable to use a water-soluble resin. Examples include acrylic resins, urethane resins, urea resins, polysaccharides, and polypeptides. Among these, acrylic resins and urethane resins are preferred, and acrylic resins composed of units derived from (meth)acrylic acid or (meth)acrylic acid esters are more preferred.

The acrylic resin preferably has a hydrophilic unit and a hydrophobic unit as constituent units. Among these, resins having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one of a monomer having an aromatic ring and a (meth)acrylic acid ester are preferred. Particularly preferred is a resin having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one monomer of styrene and α-methylstyrene. Since these resins tend to interact with pigments, they can be suitably used as resin dispersants for dispersing pigments.

The hydrophilic unit is a unit having a hydrophilic group such as an anionic group. The hydrophilic unit can be formed, for example, by polymerizing a hydrophilic monomer having a hydrophilic group. Specific examples of hydrophilic monomers having a hydrophilic group include acidic monomers having a carboxylic acid group such as (meth)acrylic acid, itaconic acid, maleic acid, fumaric acid, and anhydrides of these acidic monomers. Examples include anionic monomers such as monomers and salts. Examples of the cation constituting the salt of the acidic monomer include ions such as lithium, sodium, potassium, ammonium, and organic ammonium. The second ink may contain a monovalent cation by using a monovalent cation obtained by ionically dissociating a salt of an acidic monomer.

A hydrophobic unit is a unit that does not have a hydrophilic group such as an anionic group. The hydrophobic unit can be formed, for example, by polymerizing a hydrophobic monomer that does not have a hydrophilic group such as an anionic group. Specific examples of hydrophobic monomers include monomers with aromatic rings such as styrene, α-methylstyrene, and benzyl (meth)acrylate; methyl (meth)acrylate, butyl (meth)acrylate, and ) (meth)acrylic acid ester monomers such as 2-ethylhexyl acrylate.

The acid value of the resin used as the resin dispersant is preferably 0 mgKOH/g or more and 250 mgKOH/g or less. The weight average molecular weight of the resin dispersant 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 dispersant is a polystyrene equivalent value measured by gel permeation chromatography (GPC).

(Other resins)
The second ink can contain a resin different from the resin dispersant (other resin). The content (mass%) of other resins in the second ink is preferably 0.1% by mass or more and 5.0% by mass based on the total mass of the second ink. Other resins can be included in the second ink for purposes of improving various properties of recorded images. Examples of the form of the resin include block copolymers, random copolymers, graft copolymers, and combinations thereof. Further, the resin may be a soluble resin that can be dissolved in a liquid medium, or may be resin particles that are dispersed in a liquid medium. The resin particles do not need to contain coloring material.

The acid value of the other resins is preferably 0 mgKOH/g or more and 250 mgKOH/g or less. The weight average molecular weight of the other resins 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 weights of other resins are polystyrene equivalent values measured by gel permeation chromatography (GPC).

(aqueous medium)
The second ink is an aqueous ink containing water as an aqueous medium. The second ink can contain an aqueous medium that is water or a mixed solvent of water and a water-soluble organic solvent. As water, it is preferable to use deionized water (ion-exchanged water). The content (mass%) of water in the second ink is preferably 50.0% by mass or more and 95.0% by mass or less, based on the total mass of the ink.

The water-soluble organic solvent is not particularly limited as long as it is water-soluble (preferably one that dissolves 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 second ink is preferably 3.0% by mass or more and 50.0% by mass or less, and 10.0% by mass, based on the total mass of the second ink. More preferably, the content is 40.0% by mass or less. If the content (mass %) of the water-soluble organic solvent is less than 3.0 mass %, the ink will stick within the inkjet recording device, and sufficient sticking resistance may not be obtained. If the content (mass %) of the water-soluble organic solvent is more than 50.0 mass %, the viscosity of the ink becomes too high, resulting in decreased fluidity and ink supply failure.

(Other additives)
In addition to the above-mentioned additives, the second ink may optionally contain a surfactant, a pH adjuster, a rust preventive agent, a preservative, a fungicide, an antioxidant, a reduction inhibitor, an evaporation accelerator, and Various additives such as chelating agents can be included. Among these, it is preferable that the second ink contains a surfactant. The content (mass%) of the surfactant in the second ink is preferably 0.1% by mass or more and 5.0% by mass or less, and 0.1% by mass or more, based on the total mass of the second ink. More preferably, it is 2.0% by mass or less. Examples of the surfactant include anionic surfactants, cationic surfactants, and nonionic surfactants. Among these, nonionic surfactants are preferred because they have a low affinity with titanium oxide particles and are effective in small amounts because they are used to adjust various physical properties of ink.

(Physical properties of second ink)
Since the second ink is an ink applied to an inkjet system, it is preferable to appropriately control its physical properties. The surface tension of the second ink at 25° C. is preferably 10 mN/m or more and 60 mN/m or less, and more preferably 20 mN/m or more and 40 mN/m or less. The surface tension of the second ink can be adjusted by appropriately determining the type and content of the surfactant in the second ink. Further, the viscosity of the second ink at 25° C. is preferably 1.0 mPa·s or more and 10.0 mPa·s or less. The pH of the second ink at 25° C. is preferably 7.0 or more and 11.0 or less, more preferably 7.0 or more and 9.0 or less. The pH of the second ink can be measured with a general pH meter equipped with a glass electrode or the like.

Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples in any way unless it exceeds the gist thereof. Regarding component amounts, "parts" and "%" are based on mass unless otherwise specified. Further, a dispersion of titanium oxide particles and a dispersion of color pigments will be referred to as a "white pigment dispersion" and a "color pigment dispersion", respectively.

<Preparation of titanium oxide>
Commercially available titanium oxide without surface treatment and titanium oxide particles prepared by surface treating untreated titanium oxide were prepared. The volume-based cumulative 50% particle diameter (D ₅₀ ) of the titanium oxide particles was measured using a dynamic light scattering particle size analyzer (trade name "Nanotrac Wave II-EX150", manufactured by Microtrac Bell). Table 1 shows the properties of the titanium oxide particles. In Table 1, TITANIX:JR and JR-600A are trade names of rutile-type titanium oxide manufactured by Teika.

(Measurement of coating amount of alumina and silica)
The proportion of alumina and silica in the titanium oxide particles, that is, the amount of coverage of alumina and silica, was measured as follows. Using a liquid prepared by adding titanium oxide particles to nitric acid as a sample, quantitative analysis of aluminum and silicon elements was performed using an inductively coupled plasma (ICP) emission spectrometer. At this time, assuming that all the atoms covering the surface were oxides, the obtained values of aluminum and silicon were converted to their oxides, that is, alumina and silica, to calculate the mass ratio.

(Titanium oxide particles 1 to 8)
Titanium oxide particles 1 to 8 were produced by surface treatment of titanium oxide by a wet method. In surface treatment using the wet method, untreated titanium oxide is brought into contact with a surface treatment agent (sodium aluminate, sodium silicate, etc.), and by adjusting the amount and ratio of the surface treatment agent used, arbitrary Surface treatment was applied to the ratio.

Specifically, 300 parts of rutile-type titanium oxide (trade name "TITANIX JR", manufactured by Teika), which has not been subjected to surface treatment, and 700 parts of pure water were mixed using a homogenizer. Then, the temperature was raised 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 about 1 hour to adjust the pH to 5.0. The reaction continued for about 1 hour. Thereafter, sodium aluminate was added little by little at 90°C. At this time, in order to maintain the pH, 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 about 1 hour to obtain a dispersion. After cooling the dispersion liquid to 25°C, it is purified by repeating sedimentation using a centrifuge and redispersion in ion-exchanged water, and then dried at 120°C, whereby at least one of alumina and silica is surface-treated. Each treated titanium oxide particle was obtained.

(Titanium oxide particles 9)
Commercially available rutile-type titanium oxide particles (trade name "TITANIX JR-600A", made by Teika, surface treated with alumina) were used as the titanium oxide particles 9. Table 1 also shows the characteristics of the acid value titanium particles 9.

(Titanium oxide particles 10)
Commercially available rutile-type titanium oxide particles (trade name "TITANIX JR", manufactured by Teika, without surface treatment) were used as the titanium oxide particles 10. Table 1 also shows the characteristics of the titanium oxide particles 10.

<Preparation of compound represented by general formula (1)>
A compound represented by general formula (1) was synthesized by the following procedure. The synthesis conditions and structures of the compound synthesized as the compound 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 allylating and hydrosilylating a raw material (polyalkylene glycol monoalkyl ether, etc.).

(Compounds 1 to 10, comparative compound 11)
The raw materials, base, 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 mixture was stirred at 25° C. for 30 minutes. As "sodium hydride", a paraffin dispersion of 60% sodium hydride was used, and the dispersion was used so that the amount of sodium hydride used was as shown in Table 2. Then, the bromide shown in Table 2 was added dropwise while stirring at 25° C., and stirring was continued for 12 hours after the completion of the addition to obtain a solution containing the reactant. After filtering off unreacted sodium hydride and neutralized product (sodium bromide) from the solution containing the reactants, tetrahydrofuran (THF) was removed under 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, and then extracted 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).

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

(Comparative compound 12)
The raw materials and solvent shown in Table 2 were placed in a three-necked flask equipped with a stirring bar, a reflux condenser, and an argon inlet tube, and the raw materials were dissolved. The bromides shown in Table 2 were added dropwise at 80° C. over about 1 hour with stirring, and the mixture was refluxed for 30 minutes. After the reaction was completed, THF was removed under reduced pressure, and 300 parts of pure water and the base shown in Table 2 were added. The resulting liquid containing the reaction product was allowed to cool to room temperature 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 manner as for Compounds 1 to 10 and Comparative Compound 11 to obtain Comparative Compound 12.

(Comparative compound 13)
The raw materials and solvent shown in Table 2 were placed in a three-necked flask equipped with a stirrer and a nitrogen introduction tube, and the raw materials were dissolved. The mixture was stirred at 55° C. for 3 hours, and after the reaction was completed, the solvent was removed under reduced pressure to obtain a concentrate. 100 parts of ethanol was added to the concentrate, filtered, and the residue was washed with ethanol to remove impurities. The liquid component was removed by reducing the pressure of the filtrate, and an allylated compound was obtained. The hydrosilylation step was carried out in the same manner as for Compounds 1 to 10 and Comparative Compound 11 to obtain Comparative Compound 13.

(Comparative compound 14)
Vinyltriethoxysilane (trade name "KBE-1003", manufactured by Shin-Etsu Chemical Co., Ltd.) was used as Comparative Compound 14. This compound is a silane coupling agent having a structure not included in general formula (1).

<Preparation of white pigment dispersion>
A white pigment dispersion was produced by the following procedure. The conditions for producing the pigment dispersion are shown in Tables 4 and 5.

(White pigment dispersion 1 to 32)
40.0% of the titanium oxide particles shown in Table 4, the compound represented by general formula (1), and ion-exchanged water for a total of 100.0% of the components were mixed, and preliminary dispersion was performed using a homogenizer. . Thereafter, dispersion treatment (main dispersion) was performed using 0.5 mm zirconia beads at 25° C. in a paint shaker for 12 hours. The zirconia beads were filtered and an appropriate amount of ion-exchanged water was added as needed to prepare white pigment dispersions 1 to 32 containing 40.0% titanium oxide particles.

(White pigment dispersion liquid 33)
40.0% of titanium oxide particles of the types shown in Table 4 and 60.0% of ion-exchanged water were mixed and predispersed using a homogenizer. Thereafter, a resin dispersant for dispersing titanium oxide particles (trade name: "Floren G700", acid value: 60 mgKOH/g, manufactured by Kyoeisha Chemical Co., Ltd.) was added to obtain a mixture. This mixture was subjected to a dispersion treatment (main dispersion) for 12 hours using a paint shaker filled with 0.5 mm zirconia beads to prepare a white pigment dispersion 33 having a content of titanium oxide particles of 40.0%.

(White pigment dispersion liquid 34)
A white pigment dispersion was prepared according to the method for preparing pigment 3k in Example 3 of Patent Document 1. Specifically, a white pigment dispersion liquid was prepared in the same manner as white pigment dispersion liquid 1, except that titanium oxide particles 4 were used instead of titanium oxide particles 1, and compound 3 was used instead of compound 4 as a dispersant. was prepared. The obtained white pigment dispersion was dried by blowing air at 35° C. while stirring to remove moisture, and then dried in an oven at 105° C. for 4 hours and 15 minutes to obtain a powder of titanium oxide particles. The titanium oxide particle powder was redispersed with an appropriate amount of ion-exchanged water to prepare a white pigment dispersion 34 containing 40.0% titanium oxide particles.

<Preparation of color pigment dispersion>
(Color pigment dispersion 1 to 4)
Pigments of the type and amount shown in Table 6, dispersant, and ion-exchanged water for a total of 100.0% of the components were mixed, and preliminary dispersion was performed using a homogenizer. As the dispersant, a copolymer having a pigment affinity group (trade name "DISPERBYK-199", manufactured by BYK Chemie Japan) was used. Thereafter, dispersion treatment (main dispersion) was performed using 0.5 mm zirconia beads at 25° C. in a paint shaker for 12 hours. The zirconia beads were filtered and an appropriate amount of ion-exchanged water was added as needed to prepare color pigment dispersions 1 to 4.

(Color pigment dispersion 5)
As the color pigment dispersion 5, ion-exchanged water was mixed with a commercially available pigment dispersion (trade name "Cab-O-Jet 400", manufactured by Cabot, pigment content 15.0%) to adjust the pigment content. 14.0% was used. The above pigment dispersion liquid contains a self-dispersing pigment in which a phosphonic acid group is bonded to the particle surface of a pigment (carbon black) via another atomic group.

<Preparation of liquid containing alumina particles>
The liquid containing alumina particles was prepared with reference to the method for preparing ink 3 in Example 1 of Patent Document 2. Specifically, an alumina particle dispersion containing 10% of amphoteric alumina particles (trade name "Dispal 23N4-80", dispersed particle diameter 90 nm, manufactured by Sasol) was prepared. 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 with a propeller mixer until it became uniform, and the alumina particles were pulverized using a bead mill. A liquid (alumina particle content: 10%) was obtained.

<Preparation of first ink and second ink>
The types and amounts of each component shown in the upper row of Tables 7 to 9 were mixed and stirred. Viniblan 2685 (trade name) is the trade name of an acrylic emulsion (acrylic resin particle content: 30%) manufactured by Nissin Chemical Industry. Acetylenol E60 (trade name) is a nonionic surfactant manufactured by Kawaken Fine Chemicals. The liquid containing potassium hydroxide is ion-exchanged water to which potassium hydroxide has been added so that the content of monovalent cations shown in the lower part of Tables 7 to 9 is obtained, and the total content of the components is 100.0%. The remaining amount was added to. Thereafter, pressure filtration was performed using a membrane filter (manufactured by Sartorius) with a pore size of 5.0 μm to prepare each ink. The pH of each ink was measured using a pH meter (trade name "Portable pH Meter D-74", manufactured by Horiba, Ltd.). The pH of the first ink was within the range of 7.0 to 9.0, and the pH of the second ink was within the range of 7.0 to 11.0.

(Calculation of monovalent cation content in ink)
The content of monovalent cations in the ink was measured by ICP emission spectrometry. Specifically, each prepared ink was diluted with ion-exchanged water so that the pigment content was 0.04%, and an ICP emission spectrometer (trade name "SPS5100 ICP-OES", manufactured by SII Nano Technology) was used. was used to quantify the content of various monovalent cations. Further, among the monovalent cations, the content of ammonium ions was measured using an ion chromatograph (trade name "DX-320", manufactured by Dionex). Then, the content (ppm) of monovalent cations in the ink was calculated from the quantified content of monovalent cations. The lower rows of Tables 7 to 9 summarize the characteristics of each ink as well as the content of monovalent cations.

<Evaluation>
Using the inks obtained above in the combinations shown on the left side of Table 10, the following items were evaluated. In the present invention, "A" and "B" were defined as acceptable levels, and "C" was defined as unacceptable levels based on the evaluation criteria for each item below. The evaluation results are shown in Table 10.

(Ink storage stability)
The volume-based cumulative 50% particle diameter of the titanium oxide particles in the prepared ink was measured (referred to as "particle diameter 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 volume-based cumulative 50% particle diameter of the titanium oxide particles was measured again (referred to as "particle diameter after storage"). The volume-based cumulative 50% particle diameter of the titanium oxide particles was measured using the above measurement conditions and apparatus. Then, the particle diameter change rate is determined based on the formula: "Particle diameter change rate" (%) = 100 x ("Particle diameter after storage" - "Particle diameter before storage") / ("Particle diameter before storage") was calculated, and the storage stability of the ink was evaluated according to the evaluation criteria shown below.
A: The particle diameter change rate was 20% or less.
B: The particle diameter change rate was more than 20% and less than 30%.
C: The particle diameter change rate exceeded 30%.

(color development)
Each ink prepared above was filled into an ink cartridge, and the cartridge was set in an ink jet recording apparatus (trade name: "PIXUS PRO 10-S", manufactured by Canon) equipped with a recording head that ejects ink by the action of thermal energy. This apparatus performs printing in a serial manner, and a plurality of ejection port arrays are arranged along a sub-scanning direction (a printing medium conveyance direction) orthogonal to a main scanning direction of a printing head. Ink cartridges for the first ink and the second ink were set in the apparatus so that their ejection port rows were located adjacent to each other. In this example, an image recorded under the condition that four ink droplets of 3.8 ng are applied to a unit area (one pixel) of 1/600 inch x 1/600 inch is recorded when the recording duty is 100%. Define. Under these conditions, the following evaluation images were printed by multi-pass printing (applying ink to a unit area of the printing medium is performed by dividing the printing head and the printing medium into multiple relative scans).

As recording condition 1, an evaluation image was recorded under conditions in which the first ink and the second ink were applied overlappingly in different recording passes. The first ink was set to be ejected from one-half of the ejection ports on the upstream side in the sub-scanning direction. Further, the second ink was set to be ejected from one-half of the ejection ports on the downstream side in the sub-scanning direction. Then, a solid image with a color ink recording duty of 100% was recorded so as to overlap the solid image with a white ink recording duty of 100%. The obtained recorded matter was dried in an oven at 120° C. for 10 minutes to obtain an image for evaluation.

As recording condition 2, an evaluation image was recorded under conditions in which the first ink and the second ink were applied overlappingly in the same recording pass. Then, a solid image with a color ink recording duty of 100% was recorded so as to overlap the solid image with a white ink recording duty of 100%. The obtained recorded matter was dried in an oven at 120° C. for 10 minutes to obtain an evaluation image with a total recording duty of 200%.

Regarding the secondary color image in the evaluation image obtained above, L ^* , a ^* , and b ^* was measured. Then, saturation C ^* is calculated based on the formula C ^* = {(L ^* ) ² + (a ^* ) ² + (b ^* ) ² } ^1/2 , and the color development of the image is evaluated according to the evaluation criteria shown below. The gender was evaluated. The higher the value of image saturation C ^* , the better the color development of the image.
A: The value of chroma C ^* of the image was 45 or more.
B: The chroma C ^* value of the image was 35 or more and less than 45.
C: The value of chroma C ^* of the image was less than 35.

Claims (6)

An inkjet recording method that records an image on a recording medium by ejecting ink from an inkjet recording head,
a step of applying a first ink to the recording medium, and a step of applying a second ink to the recording medium so as to overlap at least a part of the area to which the first ink is applied,
The first ink is an aqueous ink containing titanium oxide particles and a dispersant for the titanium oxide particles,
The titanium oxide particles are titanium oxide whose surfaces are at least partially coated with alumina and silica,
The dispersant for the titanium oxide particles is a compound represented by the following general formula (1),
The content M ₁ (ppm) of monovalent cations in the first ink is 700 ppm or more and 1,300 ppm or less,
The second ink is an aqueous ink containing a coloring material different from the titanium oxide particles,
The ratio (M ₂ /M ₁ ) of the content M ₂ of monovalent cations in the second ink to the content M ₁ of monovalent cations in the first ink is 1.4 times or more and 5.0 times An inkjet recording method characterized by the following.

(In general formula (1), R ₁ , R ₂ , and R ₃ are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. _{R 4} is each independently a hydrogen atom or an alkyl group having 2 to 4 carbon atoms. It is an alkylene group. 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; ) The inkjet recording according to claim 1, wherein the proportion (mass %) of the alumina in the titanium oxide particles is 0.50 times or more and 1.00 times or less relative to the proportion (mass %) of the silica. Method. The content (mass%) of the compound represented by the general formula (1) in the first ink is 0.002 times or more 0.10 times the content (mass%) of the titanium oxide particles. The inkjet recording method according to claim 1 or 2, wherein the inkjet recording method is less than or equal to twice as much. The content M ₂ of monovalent cations in the second ink is 3.0 times or less as a ratio (M ₂ /M ₁ ) to the content M ₁ of monovalent cations in the first ink. 4. The inkjet recording method according to any one of 1 to 3. Applying the ink to the unit area of the recording medium is performed by dividing the recording head and the recording medium into multiple relative scans, and applying the first ink and the second ink to the unit area. 5. The inkjet recording method according to claim 1, wherein the steps are performed using different relative scans. An inkjet recording device used to record an image on a recording medium by ejecting ink from an inkjet recording head, the inkjet recording device comprising:
comprising means for applying a first ink to the recording medium, and means for applying a second ink to the recording medium so as to overlap at least a part of the area to which the first ink is applied;
The first ink is an aqueous ink containing titanium oxide particles and a dispersant for the titanium oxide particles,
The titanium oxide particles are titanium oxide whose surfaces are at least partially coated with alumina and silica,
The dispersant for the titanium oxide particles is a compound represented by the following general formula (1),
The content M ₁ (ppm) of monovalent cations in the first ink is 700 ppm or more and 1,300 ppm or less,
The second ink is an aqueous ink containing a coloring material different from the titanium oxide particles,
The ratio (M ₂ /M ₁ ) of the content M ₂ of monovalent cations in the second ink to the content M ₁ of monovalent cations in the first ink is 1.4 times or more and 5.0 times An inkjet recording device characterized by the following.

(In general formula (1), R ₁ , R ₂ , and R ₃ are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. _{R 4} is each independently a hydrogen atom or an alkyl group having 2 to 4 carbon atoms. It is an alkylene group. 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; )

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