JP2023066996A - Aqueous ink, ink cartridge, and inkjet recording method - Google Patents

Abstract

To provide an aqueous ink for inkjet recording or the like, comprising titanium oxide and excelling in sedimentation recoverability and image uniformity.SOLUTION: An aqueous ink for inkjet recording comprises: titanium oxide particles whose surfaces are at least partially coated with alumina, with its zeta potential of more than 0 mV; a resin dispersant comprising a unit derived from a monomer comprising an aromatic ring and a unit derived from a monomer comprising a carboxylic acid group; and a surfactant comprising an aromatic ring.SELECTED DRAWING: None

Description

The present invention relates to an aqueous ink, an ink cartridge, and an inkjet recording method.

2. Description of the Related Art In recent years, inkjet recording apparatuses have come to be widely used when outputting images using paper, resin films, etc. as recording media for advertisements and exhibits. For example, in order to express clear color images even on a transparent recording medium, white ink is used in addition to black and basic color inks (hereinafter these may be collectively referred to as color inks). . 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, and then a color ink is applied thereon, or each ink is applied in reverse order ( A so-called backprint recording method is used.

Titanium oxide is used as a coloring material for white ink because it is low in cost and has excellent properties such as whiteness and opacity required for white ink. On the other hand, a dispersant is required to stably disperse titanium oxide in water-based ink. However, since titanium oxide, which is a metal oxide, has a higher specific gravity than coloring materials used in inks of other colors, it tends to settle even when a dispersant is used. Even if titanium oxide settles once, sedimentation recoverability, which is a characteristic of re-dispersing titanium oxide, is becoming important. Also, when recording on a non-absorbing medium that does not absorb the liquid component in the ink, such as a resin film (hereinafter sometimes referred to as "non-absorbing medium"), the recorded image is even. Uniformity (image uniformity) has come to be emphasized.

Until now, techniques for improving various performances such as sedimentation recovery properties of titanium oxide have been studied, depending on ink components. An ink containing titanium oxide, resin, and surfactant has been proposed (see Patent Document 1).

JP 2019-183063 A

The present inventors have studied various properties of the water-based ink proposed in Patent Document 1. As a result, it was found that, in the current situation where inks containing titanium oxide at a high concentration are required in order to obtain excellent whiteness and opacity, the sedimentation recovery properties are insufficient and there is room for improvement. It was also found that there is room for improvement in the uniformity of images recorded on non-absorbent media as well.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an aqueous inkjet ink containing titanium oxide, which is excellent in sedimentation recovery and image uniformity, an ink cartridge using the aqueous ink, and an inkjet recording method. .

The above object is solved by the present invention described below. That is, the water-based ink according to the present invention is a water-based ink for inkjet containing titanium oxide particles, a resin dispersant for dispersing the titanium oxide particles, and a surfactant, wherein the titanium oxide particles are Titanium oxide having a surface at least partly coated with alumina and having a zeta potential of more than 0 mV, and the resin dispersant contains units derived from a monomer having an aromatic ring and a monomer having a carboxylic acid group. It is a water-soluble resin containing a unit derived from, and the surfactant is a surfactant having an aromatic ring.

According to the present invention, it is possible to provide an aqueous ink for inkjet containing titanium oxide, which is excellent in sedimentation recovery and image uniformity, an ink cartridge using the aqueous ink, and an inkjet recording method.

1 is a cross-sectional view schematically showing one embodiment of an ink cartridge of the present invention; FIG. 1 is a perspective view showing an embodiment of an inkjet recording apparatus that can be used in the inkjet recording method of the present invention; FIG. 1 is a schematic diagram showing an embodiment of a heating section of an inkjet recording apparatus that can be used in the inkjet recording method of the present invention; FIG.

The present invention will be further described in detail below with reference to preferred embodiments. In the present invention, when the compound is a salt, the salt is dissociated into ions in the ink, but for convenience, it is expressed as "containing the salt." Titanium oxide and titanium oxide particles are sometimes simply referred to as "pigment". A "unit" of a resin refers to the smallest repeating unit that constitutes the resin, and means a structure formed by (co)polymerization of one monomer. In addition, water-based ink for inkjet is sometimes simply referred to as "ink". Physical properties are values at room temperature (25° C.) unless otherwise specified. "(Meth)acrylic acid" and "(meth)acrylate" shall mean "acrylic acid, methacrylic acid" and "acrylate, methacrylate", respectively.

Titanium oxide that has not been subjected to surface treatment 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 water-based ink for inkjet, titanium oxide (titanium oxide particles) surface-treated with an inorganic oxide such as alumina or silica is used in order to improve the storage stability of the ink. 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 index of strength as an acid, differs depending on the type of inorganic compound. Therefore, although titanium oxide itself is an inorganic oxide, the surface of the titanium oxide particles exhibits the properties of an inorganic oxide corresponding to the inorganic compound used for the surface treatment, and the surface charge of the titanium oxide particles is similar to that of an aqueous medium. It strongly depends on the pH of the surface treatment agent, the type of surface treatment agent, and the amount of the surface treatment agent used.

The inventors of the present invention first examined the factors that reduce sedimentation recovery and image uniformity when titanium oxide particles are dispersed in a resin as in Patent Document 1. Under alkaline conditions, which are common liquid properties of water-based inks for inkjet, the resin dispersant has a positive electrostatic charge on the surface hydroxy groups of the titanium oxide particles and a negative charge on the anionic groups in the resin dispersant. It is considered that the titanium oxide particles are adsorbed due to a strong interaction. However, the adsorption due to the electrostatic interaction between the titanium oxide particles and the resin dispersant is in a state of equilibrium between adsorption and desorption, so a part of the resin is desorbed from the titanium oxide particles. Therefore, when the electrostatic interaction between the titanium oxide particles and the resin dispersant is insufficient, the resin dispersant tends to detach, and the dispersed state of the titanium oxide becomes unstable. As a result, it was found that the sedimentation recovery was lowered.

Accordingly, the present inventors conducted studies to strengthen the electrostatic adsorption between the titanium oxide particles and the resin dispersant in order to improve the sedimentation recoverability of the ink containing the titanium oxide particles. First, the inventors considered making many of the surface hydroxyl groups of the titanium oxide particles positively charged. As a result, by using titanium oxide particles whose zeta potential is within a specific range and at least a part of the surface of which has been surface-treated with alumina, the adsorption of the titanium oxide particles and the resin dispersant is enhanced. I have learned that it is possible.

Next, the present inventors examined conditions of a resin dispersant necessary for dispersing titanium oxide particles. Resin dispersants commonly used to disperse carbon black and organic pigments are composed of a unit having an anionic group and a unit having no anionic group, the former being hydrophilic and the latter being hydrophobic. function as a part. Many general-purpose resins for dispersing carbon black and organic pigments have units derived from monomers having aromatic rings as hydrophobic portions. Further, carbon black and organic pigments are essentially hydrophobic, and many of them have aromatic rings such as benzene rings in their molecular structures. The aromatic ring of the resin dispersant and the aromatic ring of carbon black or organic pigment are strongly adsorbed by π-π interaction. Carbon black and organic pigments can be dispersed in the ink by the electrostatic repulsive force of the anionic group of the resin dispersant.

Consider the case where the above resin dispersant for dispersing carbon black or organic pigment is used as a dispersant for titanium oxide particles, which is an inorganic pigment. A unit having an anionic group acts on titanium oxide particles as an adsorption site due to electrostatic interaction. The titanium oxide particles can be dispersed in the ink due to the electrostatic repulsive force of the remaining anionic group-containing units in the resin that have not contributed to adsorption to the titanium oxide particles. On the other hand, since titanium oxide particles are hydrophilic due to their surface hydroxy groups, the aromatic ring-containing unit of the resin hardly contributes to the adsorption to the surface of the titanium oxide particles.

Next, a suitable material was examined to improve image uniformity. When recording an image on a non-absorbing medium, it is considered important to form a strong film in which the titanium oxide particles and the resin dispersant are integrated. Focusing on the fact that the aromatic ring is difficult to be adsorbed to the titanium oxide particles when using the above resin, we conducted a study and found that combining a surfactant with the same aromatic ring can improve image uniformity. I found a connection.

That is, the ink of the present invention has the following characteristics. First, titanium oxide particles, which are titanium oxide having at least a portion of the surface coated with alumina, are used. Furthermore, a resin dispersant having a specific structure and a surfactant having a specific structure are used. The present inventors presume the mechanism by which the sedimentation recovery property and image uniformity are improved by the above configuration as follows.

At least part of the surface of the titanium oxide particles is coated with alumina. Under alkaline conditions, which are typical liquid properties of water-based inks for inkjets, the alumina on the surface of the titanium oxide particles forms surface hydroxy groups. Furthermore, the isoelectric point of alumina in water at 25° C. (normal temperature) is about 9.0, and surface hydroxy groups derived from alumina tend to be positively charged. Therefore, the negatively charged resin derived from the carboxylic acid group can be adsorbed to the titanium oxide particles by electrostatic interaction via the surface hydroxyl group derived from alumina. Furthermore, the carboxylic acid groups of the resin dispersant that do not contribute to the adsorption of the titanium oxide particles are negatively charged, and therefore generate electrostatic repulsion between the titanium oxide particles, resulting in excellent dispersion stability.

The zeta potential of titanium oxide particles is greater than 0 mV. This zeta potential is the zeta potential measured for a sample in which titanium oxide particles are dispersed in water adjusted to pH 8.0. In order for the resin dispersant having an anionic group to be sufficiently adsorbed to the titanium oxide particles, the surface hydroxy groups need to have many positive charges at around pH 8.0, which is the general liquidity of water-based inks for inkjet. be. Therefore, by setting the zeta potential within the above range, the resin dispersant can be adsorbed to the titanium oxide particles, and the dispersion stability of the titanium oxide particles is improved. As a result, sedimentation recovery is improved. When the zeta potential is 0 mV or less, most of the surface hydroxyl groups of the titanium oxide particles are negatively charged, and the resin cannot be adsorbed. As a result, the dispersed state of the titanium oxide particles cannot be stably maintained, and sedimentation recovery properties cannot be obtained.

The resin dispersant is a water-soluble resin containing a unit derived from a monomer having an aromatic ring and a unit derived from a monomer having a carboxylic acid group. When the ink is ejected and applied to a non-absorbent medium, the liquid component evaporates, bringing the titanium oxide particles closer together. As a result, it becomes possible to form a uniform image. On the other hand, as described above, since titanium oxide particles are hydrophilic, units having an aromatic ring are less likely to be adsorbed to titanium oxide particles. Therefore, when the titanium oxide particles settle in the ink, the distance between the units having aromatic rings between the resins becomes close, and the aromatic rings are strongly adsorbed to each other due to π-π interaction. As a result, aggregation of titanium oxide particles is promoted, and sedimentation recovery is not obtained.

The ink contains a surfactant having an aromatic ring in addition to the above resin dispersant. Even if the titanium oxide particles settle in the ink, the unit having the aromatic ring of the resin dispersant can strongly adsorb to the aromatic ring of the surfactant through π-π interaction. Therefore, the surfactant can prevent the distance between the titanium oxide particles from approaching more than a certain amount, thereby suppressing the aggregation of the titanium oxide particles. As a result, sedimentation recovery can be improved. Furthermore, even in the ink applied to the non-absorbing medium, the titanium oxide particles, the resin dispersant, and the surfactant can be united to form a uniform image. Therefore, image uniformity can be improved.

<Aqueous ink>
The ink of the present invention is a water-based inkjet ink containing titanium oxide particles coated with alumina, a resin dispersant having a specific structure, and a surfactant having a specific structure. This ink is preferably a white ink because titanium oxide is a white pigment. The components constituting the ink of the present invention, physical properties of the ink, etc. will be described in detail below.

(colorant)
The ink contains, as a colorant (pigment), titanium oxide particles obtained by surface-treating titanium oxide with alumina. The content (% by mass) of titanium oxide particles in the ink is preferably 0.1% by mass or more and 20.0% by mass or less, and 5.0% by mass or more and 15.0% by mass, based on the total mass of the ink. % or less.

Titanium oxide is a white pigment and has three crystal forms: rutile, anatase, and brookite. Among them, rutile-type titanium oxide is preferable. Industrial production methods for titanium oxide include the sulfuric acid method and the 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 the average particle diameter) of the titanium oxide particles is preferably 200 nm or more and 500 nm or less, more preferably 200 nm or more and 400 nm or less. The volume-based cumulative 50% particle diameter (D ₅₀ ) of titanium oxide is the diameter of particles that are integrated from the small particle diameter side to 50% of the total volume of the measured particles in the particle diameter accumulation curve. be. The _D50 of titanium oxide can be measured, for example, under the conditions of SetZero: 30 seconds, Number of measurements: 3, Measurement time: 180 seconds, Shape: Spherical, Refractive index: 1.59. As a 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 those described above.

The zeta potential of titanium oxide particles is greater than 0 mV. Above all, it is preferably 5 mV or more, and preferably 25 mV or less. The zeta potential is an index indicating the charged state of the surface of titanium oxide particles, and can be measured with a zeta potential meter using an electrophoretic light scattering method. By setting the zeta potential of the titanium oxide particles to more than 0 mV, the amount of positive charges on the surface of the titanium oxide particles is greater than the amount of negative charges, thereby improving the adsorptive power of the titanium oxide and the resin having an anionic group. The zeta potential of titanium oxide particles is the zeta potential measured at 25° C. for a sample in which titanium oxide particles are dispersed in water adjusted to pH 8.0. A pH adjuster such as an alkali metal hydroxide or ammonia can be used to adjust the pH. Examples of alkali metal hydroxides include potassium hydroxide, sodium hydroxide, and lithium hydroxide. Among them, potassium hydroxide is preferred.

Titanium oxide that has been surface-treated with alumina is used. Surface treatment is expected to suppress photocatalytic activity and improve dispersibility. As used herein, "alumina" is a generic term for oxides of aluminum, such as aluminum oxide. The zeta potential of titanium oxide not subjected to surface treatment is about -25 mV, and the zeta potential tends to increase as the coating amount of alumina is increased. Also, the titanium oxide particles may be surface-treated with other inorganic oxides such as silica and zirconium oxide. As used herein, "silica" is a generic term for silicon dioxide or substances composed of silicon dioxide. Most of the alumina and silica coating titanium oxide is present in the form of aluminum oxide and silicon dioxide. When the surface is treated with silica, the isoelectric point of silica is on the acidic side unlike alumina, so the surface hydroxy groups derived from silica are negatively charged, that is, the zeta potential becomes small. Therefore, by adjusting the coating amount of various inorganic oxides such as alumina, the zeta potential of the titanium oxide particles can be adjusted within a range of more than 0 mV. The same applies to surface treatment with inorganic oxides such as zinc oxide and zirconium oxide. Further, the titanium oxide particles may be coated with an organic substance such as polyol as long as the effects of the present invention are not impaired.

The proportion (% by mass) of alumina in the titanium oxide particles is preferably 2.0% by mass or more and preferably 6.0% by mass or less based on the total mass of the titanium oxide particles. If the ratio is less than 2.0% by mass, the ratio of the alumina is too small, and the resin dispersant cannot be sufficiently adsorbed in the ink, and the titanium oxide particles may not be able to maintain a stable dispersed state. . As a result, sufficient sedimentation recovery may not be obtained.

Examples of surface treatment methods for titanium oxide include wet treatment and dry treatment. For example, after dispersing titanium oxide in a liquid medium, the surface can be treated by reacting it with a surface treatment agent such as sodium aluminate or sodium silicate. It is also possible to adjust to the desired characteristics as follows.

The ink may contain pigments other than titanium oxide as long as the effects of the present invention are not impaired. In this case, it is also possible to use a color ink other than white ink. The content (% by mass) of other pigments in the ink is preferably 0.1% by mass or more and 5.0% by mass or less, based on the total mass of the ink, and 0.1% by mass or more and 1.0% by mass. % or less.

(resin dispersant)
The ink contains a water-soluble resin containing a unit derived from a monomer having an aromatic ring and a unit derived from a monomer having a carboxylic acid group. This water-soluble resin is a resin dispersant for dispersing titanium oxide particles. Forms of the resin include block copolymers, random copolymers, graft copolymers, combinations thereof, and the like.

As used herein, "the resin is water-soluble" means that when the resin is neutralized with an alkali equivalent to its acid value, it does not form particles whose particle size can be measured by a dynamic light scattering method. It means that it exists in a liquid medium. Whether or not the resin is water-soluble can be determined according to the method described below. First, a liquid (resin solid content: 10% by mass) containing a resin neutralized with an acid value equivalent alkali (sodium hydroxide, potassium hydroxide, etc.) is prepared. Next, the prepared liquid is diluted 10-fold (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 the dynamic light scattering method, if particles having the particle size are not measured, it can be determined that the resin is water-soluble. The measurement conditions at this time can be, for example, SetZero: 30 seconds, the number of measurements: 3, and the measurement time: 180 seconds. As the particle size distribution analyzer, a particle size analyzer using dynamic light scattering (for example, trade name “UPA-EX150” manufactured by Nikkiso Co., Ltd.) can be used. Of course, the particle size distribution measuring device and measurement conditions to be used are not limited to those described above.

The resin dispersant contains a unit derived from a monomer having an aromatic ring and a unit derived from a monomer having a carboxylic acid group. Examples of the monomer that becomes a unit derived from a monomer having an aromatic ring by polymerization include styrene, α-methylstyrene, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate and the like. Examples of the monomer that becomes a unit derived from a monomer having a carboxylic acid group by polymerization include acrylic acid, methacrylic acid, and maleic acid, and these may be anhydrides or salts. Examples of cations constituting salts include ions such as lithium, sodium, potassium, ammonium and organic ammonium.

The ratio (% by mass) of units derived from monomers having aromatic rings in the resin dispersant is preferably 20.0% by mass or more and 70.0% by mass or less based on the total mass of the resin dispersant. If the proportion is less than 20.0% by mass, the number of aromatic rings in the resin dispersant is too small, and the number of aromatic rings capable of forming π-π interactions with the aromatic rings of the surfactant is small, resulting in a uniform image. may not be formed. As a result, image uniformity may not be sufficiently obtained. If the ratio is more than 70.0% by mass, the number of aromatic rings in the resin dispersant becomes too large, and the resin dispersant shrinks due to the π-π interaction within the molecular chain of the resin dispersant. . As a result, the number of sites that interact with the titanium oxide particles decreases, the titanium oxide particles cannot be maintained in a stable dispersed state, and there are cases where sufficient sedimentation recovery properties cannot be obtained.

The acid value of the resin dispersant is preferably 100 mgKOH/g or more and 320 mgKOH/g or less, more preferably 170 mgKOH/g or more and 220 mgKOH/g or less. When the acid value is less than 100 mgKOH/g, the amount of negative charge due to the anionic group of the resin dispersant is small, and the adsorption to the titanium oxide particles is insufficient, and the electrostatic repulsive force between the titanium oxide particles is reduced. become weak. As a result, the titanium oxide particles cannot be maintained in a stable dispersed state, and sufficient sedimentation recovery may not be obtained. If the acid value is more than 320 mgKOH/g, the hydrophilicity of the resin dispersant is too high, and it may become difficult to interact with the surfactant having an aromatic ring. As a result, a uniform image cannot be formed, and sufficient uniformity of the image may not be obtained. The acid value of the resin can be measured by colloidal titration using a potential difference.

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 20,000 or less. The weight average molecular weight (Mw) is a polystyrene-equivalent value obtained by gel permeation chromatography (GPC).

The content (% by mass) of the resin dispersant in the ink is preferably 0.1% by mass or more and 5.0% by mass or less based on the total mass of the ink. The content (% by mass) of the resin dispersant in the ink is preferably 0.02 to 0.20 times the content (% by mass) of the titanium oxide particles. If the content is less than 0.02 times, the resin dispersant cannot be sufficiently adsorbed to the titanium oxide particles, and the electrostatic repulsive force between the titanium oxide particles cannot be obtained, so that the titanium oxide particles are stabilized. A stable state of dispersion cannot be maintained. As a result, sufficient sedimentation recovery may not be obtained. If the content is more than 0.20 times, the amount of the resin in the ink is too large, resulting in the presence of free resin dispersants that do not interact with the titanium oxide particles. As a result, the resin dispersant aggregates due to mutual interaction between the resin dispersants, and the titanium oxide particles cannot maintain a stable dispersed state. As a result, sufficient sedimentation recovery may not be obtained.

Since the dispersion stability of titanium oxide particles dispersed with a resin dispersant is likely to be improved, the amount of negative charges introduced in the resin dispersant having a carboxylic acid group is 5 times the ratio of the amount of positive charges introduced in the titanium oxide particles. It is preferable to be 350 times or less. Among them, it is more preferable to be 10 times or more and 250 times or less. The amount of positive charge introduced into the titanium oxide particles can be calculated from the amount of acetic acid adsorbed after dispersing the titanium oxide particles in an acetic acid n-dibutyl ether solution.

For example, when the titanium oxide particles surface-treated with alumina (trade name “JR-600A” manufactured by Tayca), which are the titanium oxide particles used in the examples described later, are measured by the above method, the amount of positive charge introduced is 25 μmol. /g. That is, when "JR-600A" is used, the introduction amount of the negative charge of the carboxylic acid group of the resin dispersant is preferably 125 μmol/g or more and 8750 μmol/g or less. The introduction amount of the negative charge of the carboxylic acid group of the resin dispersant can be calculated from the acid value of the resin dispersant. Since the resin dispersant 1 used in Examples described later has an acid value of 155 mgKOH/g, the amount of negative charge introduced into the carboxylic acid group is 2768 μmol/g. In order to set the amount of negative charge introduced into the resin dispersant to a predetermined value, the acid value of the resin dispersant, the amount of the resin dispersant added, and the amount of neutralization of the anionic groups may be appropriately adjusted. If the amount of negative charge introduced into the carboxylic acid group of the resin dispersant is outside the above range, the titanium oxide particles cannot be maintained in a stable dispersed state, and the sedimentation recovery property may not be sufficiently obtained. .

(Surfactant having an aromatic ring)
The ink contains a surfactant having an aromatic ring. Examples of surfactants having an aromatic ring include nonionic surfactants such as polyoxyethylene alkylphenyl ether and polyoxyethylene styrenated phenyl ether; Anionic surfactants such as condensates and alkali metal salt-type compounds thereof can be mentioned. Among them, nonionic surfactants are preferable because they have low affinity with titanium oxide particles and bring about effects on various physical properties of the ink even in a small amount. The content (% by mass) of the surfactant having an aromatic ring in the ink is preferably 0.01% by mass or more and 2.00% by mass or less, and is preferably 0.03% by mass or more, based on the total mass of the ink. It is more preferably 1.00% by mass or less. When the content is less than 0.01% by mass, the amount of the surfactant having an aromatic ring is too small, and when the titanium oxide particles settle in the ink, aggregation of the titanium oxide particles can be suppressed. Sometimes you can't. In addition, the number of aromatic rings capable of forming π-π interactions with the aromatic rings of the resin dispersant is small, and a uniform image may not be formed in some cases. As a result, sufficient sedimentation recovery and image uniformity may not be obtained. If the content is more than 2.00% by mass, the amount of the surfactant having an aromatic ring is too large, and when applied to a non-absorbing medium, the effect of aggregating titanium oxide particles by the resin dispersant is weak. Become. As a result, image uniformity may not be sufficiently obtained.

(Other resins)
The ink can contain a resin (another resin) different from the resin dispersant. The content (% by mass) of other resins in the ink is preferably 0.1% by mass or more and 5.0% by mass or less based on the total mass of the ink.

Other resins can be contained in the ink for purposes such as (i) stabilizing the dispersed state of the pigment, that is, as a resin aid for the pigment, and (ii) improving various characteristics of the recorded image. Forms of the resin include block copolymers, random copolymers, graft copolymers, combinations thereof, and the like. Further, the other resin may be a water-soluble resin that is soluble in the liquid medium, or may be resin particles that are dispersed in the liquid medium. Among them, resin particles are preferable. By including resin particles in the ink, an effect of suppressing sedimentation of titanium oxide particles can be expected. Resin particles do not need to contain a coloring material.

The content (% by mass) of the resin particles in the ink is preferably 1.0% by mass or more and 50.0% by mass or less, and is 3.0% by mass or more and 15.0% by mass or less, based on the total mass of the ink. is more preferable. Moreover, the content (% by mass) of the resin particles is preferably 0.3 times or more and 1.5 times or less as a mass ratio with respect to the content (% by mass) of the titanium oxide particles.

In the present invention, when the other resin is a resin particle, when the resin is neutralized with an alkali equivalent to the acid value, it forms particles whose particle size can be measured by the above dynamic light scattering method. It is assumed that The reason why the particle size is measured using the neutralized resin is to confirm that particles are formed even when the resin is sufficiently neutralized to make it more difficult to form particles. Even under such conditions, the resin having the shape of particles exists in the form of particles even in the water-based ink.

As the material of the resin particles, known resins can be suitably used. Specifically, resin particles composed of various materials such as olefin-based, polystyrene-based, urethane-based, and acrylic-based materials can be used. The weight average molecular weight (Mw) of the resin particles is preferably in the range of 1,000 or more and 2,000,000 or less. The average particle diameter of the resin particles is preferably 10 nm or more and 1,000 nm or less, more preferably 100 nm or more and 500 nm or less.

Resin particles tend to have fewer anionic groups than water-soluble resins. Therefore, in order to accurately grasp the properties of the resin particles, it is preferable to use the "acid group introduction amount (μmol/g)" instead of the "acid value (mgKOH/g)". The amount of acid groups introduced into the resin particles is preferably 250 μmol/g or less, more preferably 120 μmol/g or more and 250 μmol/g or less.

(aqueous medium)
The ink is an aqueous ink containing water as the aqueous medium. The ink can contain water or an aqueous medium that is 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 (% by 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 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, and the like can be used. The content (% by 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. % by mass or less is more preferable. If the content (% by mass) of the water-soluble organic solvent is less than 3.0% by mass, the ink may adhere in the inkjet recording apparatus, and sufficient adhesion resistance may not be obtained. If the content (% by mass) of the water-soluble organic solvent exceeds 50.0% by mass, ink supply failure may occur.

(Other additives)
In addition to the above additives, if necessary, the ink may contain surfactants having no aromatic ring (other surfactants), pH adjusters, rust inhibitors, antiseptics, antifungal agents, antioxidants. , anti-reduction agents, evaporation enhancers, and chelating agents. Among others, the ink preferably contains other surfactants. The content (% by mass) of other surfactants in the ink is preferably 0.1% by mass or more and 5.0% by mass or less, based on the total mass of the ink, and is preferably 0.1% by mass or more. It is more preferably 0% by mass or less. Other surfactants include anionic surfactants, cationic surfactants, nonionic surfactants, and the like. Among these, nonionic surfactants that have low affinity with titanium oxide particles and are effective even in small amounts are preferred because they are used to adjust various physical properties of the ink.

(Physical properties of ink)
Since the ink is applied to the 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 or more and 60 mN/m or less, more preferably 20 mN/m or more and 40 mN/m or less. The surface tension of the ink can be adjusted by appropriately determining the type and content of the surfactant in the ink. Also, the viscosity of the ink at 25° C. is preferably 1.0 mPa·s or more and 10.0 mPa·s or less. The pH of the ink at 25° C. is preferably 7.0 or more and 9.0 or less, more preferably 7.5 or more and 9.0 or less, and particularly 7.5 or more and 8.5 or less. preferable. The pH of the ink can be measured with a general pH meter equipped with a glass electrode or the like.

<Ink cartridge>
The ink cartridge of the present invention includes ink and an ink containing portion that contains the ink. The ink contained in this ink container is the water-based ink of the present invention described above. FIG. 1 is a cross-sectional view schematically showing one embodiment of the ink cartridge of the present invention. As shown in FIG. 1, the bottom surface of the ink cartridge is provided with an ink supply port 12 for supplying ink to the recording head. The interior of the ink cartridge serves as an ink containing portion for containing ink. The ink storage section is composed of an ink storage chamber 14 and an absorber storage chamber 16 which communicate with each other through a communication port 18 . Also, the absorber storage chamber 16 communicates with the ink supply port 12 . The ink containing chamber 14 contains liquid ink 20, and the absorber containing chamber 16 contains absorbers 22 and 24 that hold the ink in an impregnated state. The ink storage section may have a configuration in which the absorber retains the total amount of ink stored without having an ink storage chamber for storing liquid ink. Further, the ink containing portion may have a form in which the ink is contained in a liquid state without having an absorber. Further, the ink cartridge may be configured to have an ink containing portion and a recording head.

<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 above-described water-based ink of the present invention from an inkjet recording head. Methods for ejecting ink include a method in which mechanical energy is applied to ink and a method in which thermal energy is applied to ink. In the present invention, it is particularly preferable to adopt a method of applying thermal energy to the ink to eject the ink. Other than using the ink of the present invention, the steps of the ink jet recording method may be known. For example, when recording an image with white ink, a general inkjet recording method can be applied as it is. When white ink is used as a base treatment for color ink, color ink (black, cyan, magenta, yellow ink, etc.) is applied so as to overlap at least a part of the area to which white ink is applied, and the image is printed. Record it.

An inkjet recording apparatus will be specifically described with an example. FIG. 2 is a perspective view showing an example of an ink jet recording apparatus (hereinafter sometimes simply referred to as a recording apparatus) according to one embodiment of the present invention. This recording apparatus 1 can be suitably used for the inkjet recording method of one embodiment of the present invention. The printing apparatus 1 is a so-called serial scanning type printing apparatus that prints an image by scanning a printing head (not shown) in a scanning direction X that is perpendicular to the conveying direction Y of the printing medium P. FIG.

The outline of the configuration of the recording apparatus 1 and the operation during recording will be described with reference to FIG. First, the recording medium P is conveyed in the Y direction (conveying direction) from the spool 6 holding the recording medium P by a conveying roller driven by a conveying motor (not shown) through a gear. On the other hand, at a predetermined transport position, a carriage motor (not shown) reciprocates the carriage unit 2 along a guide shaft 8 extending in the X direction (scanning direction) in the figure. In the course of this movement, the nozzles of a recording head (not shown) that can be mounted on the carriage unit 2 are caused to perform ejection operations at timings based on the position signals obtained by the encoder 7, and correspond to the arrangement range of the nozzles. records a fixed band (width) image.

The recording apparatus 1 includes a heater 25 as a heating unit that heats the ink applied to the recording medium P after the recording operation is completed and heats the recording medium P (see FIG. 3). In the printing apparatus 1, a heater 25 supported by a frame (not shown) is arranged at a position downstream of a position where the carriage unit 2 reciprocates in the scanning direction X in the transporting direction Y of the printing medium P. The heater 25 is covered with a heater cover 26 , and the heater cover 26 has a function of efficiently irradiating the recording medium P with heat from the heater 25 and a function of protecting the heater 25 . Specific examples of the heater 25 include sheathed heaters and halogen heaters. After being applied to the recording surface of the recording medium P, the ink ejected from the recording head (not shown) is dried and fixed by the heat from the heater 25 and the heated recording medium P. After that, the recording medium P is wound up to form a roll-shaped take-up medium 6 .

By heating the recording medium to which the ink has been applied (heating step), the fixation of the image on the recording medium can be promoted. The heating process promotes evaporation of the liquid component in the ink applied to the recording medium, so that the uniformity of the recorded image can be improved.

The heating means may be arranged vertically above the recording head (not shown) and may be configured to heat the recording medium to which ink is applied from above (surface). Alternatively, the heating means may be provided below the platen 4 in the vertical direction, and may be configured to heat the recording medium to which ink has been applied from below (back surface). In addition to heating, means for promoting fixation such as air blowing can also be used.

The recording medium P may be a paper-based recording medium (absorbing medium) having permeability, or may be a non-absorbing medium. Examples of the absorbing medium include a recording medium having no coat layer such as plain paper, and a recording medium having a coat layer such as glossy paper, art paper, and matte paper. Among them, it is preferable to use a non-absorbing medium. Examples of non-absorbent media that can be used include art paper, coated paper, polyvinyl chloride film, polypropylene film, PET film, tarpaulin, and polyester cloth.

EXAMPLES The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited by the following examples as long as the gist thereof is not exceeded. "Parts" and "%" regarding component amounts are based on mass unless otherwise specified. Further, the resin dispersant and the dispersion of titanium oxide particles are referred to as "resin" and "pigment dispersion", respectively.

<Zeta potential measurement>
The zeta potential of titanium oxide particles was measured by the following method. Specifically, the pigment dispersion was diluted with ion-exchanged water so that the pigment content was 0.01%. Then, the zeta potential of the titanium oxide particles was measured using a zeta potential/particle size measuring system (trade name “ELS-Z2”, manufactured by Otsuka Electronics). The measurement range was pH 3 to 11, and the pH was adjusted using 0.1 mol/L hydrochloric acid aqueous solution and 0.1 mol/L sodium hydroxide aqueous solution. In this example, the zeta potential of titanium oxide particles was measured using a pigment dispersion, but the zeta potential can also be measured in the same manner as described above using titanium oxide particles separated from ink by a conventional method. can be done.

<Preparation of titanium oxide particles>
Commercially available titanium oxide particles surface-treated in advance and titanium oxide particles prepared by surface-treating untreated titanium oxide were used. The volume-based cumulative 50% particle diameter (D ₅₀ ) of the titanium oxide particles and the particle diameter of the pigment are measured using a particle size analyzer (trade name “Nanotrac WaveII-EX150”, manufactured by Microtrac Bell) using a dynamic light scattering method. Measured using Table 1 shows the properties of the titanium oxide particles. In Table 1, TITANIX: JR-600A, JR-301, JR-805, JR-405, and JR are trade names of rutile-type titanium oxide manufactured by Tayka. Typaque CR-90 is a trade name of rutile type titanium oxide manufactured by Ishihara Sangyo Co., Ltd.

(Measurement of alumina coating amount)
The proportion of alumina in the titanium oxide particles, that is, the coating amount of alumina was measured as follows. Quantitative analysis of the aluminum element was carried out by an inductively coupled plasma (ICP) emission spectrometer using the prepared liquid in which titanium oxide particles were added to nitric acid as a sample. At this time, it was assumed that all the atoms covering the surface were oxides, and the mass ratio was calculated by converting the obtained value of aluminum to that oxide, that is, alumina.

(Titanium oxide particles 6 to 8, 10)
The surface treatment of titanium oxide was performed by a wet method to produce each titanium oxide particle. Surface treatment by the wet method involves contacting untreated titanium oxide with a surface treatment agent (sodium aluminate, sodium silicate, etc.). Surface treatment was applied to the ratio.

Specifically, 300 parts of unsurface-treated rutile-type titanium oxide (trade name “TITANIX JR”, manufactured by Tayka) and 700 parts of pure water were mixed with 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 the pH was adjusted to 5.0 by adding dilute sulfuric acid (pH adjuster) over about 1 hour. The reaction was allowed to continue for about 1 hour. After that, sodium aluminate was added in small portions at 90°C. At this time, in order to maintain the pH, dilute sulfuric acid was used together to maintain the pH at 6.0 or more and 8.0 or less. After adding the sodium aluminate, the reaction was continued for about 1 hour to obtain a dispersion. After cooling the dispersion to 25 ° C., it is purified by repeating sedimentation with a centrifugal separator and re-dispersion in ion-exchanged water, and dried at 120 ° C. to surface-treat with alumina and / or silica. Each titanium oxide particle was obtained.

(Titanium oxide particles 1 to 5, 9)
Commercially available titanium oxide particles (including those previously surface-treated with alumina or silica) were used as titanium oxide particles 1 to 5 and 9. Table 1 also shows the characteristics of each titanium oxide particle.

<Analysis of Resin>
(Weight average molecular weight)
The weight average molecular weight (Mw) of the resin was measured as a polystyrene-equivalent value measured by gel permeation chromatography (GPC). Specifically, the resin was dissolved in a solvent (tetrahydrofuran) at a temperature of 25° C. over 24 hours. The resulting solution was filtered through a membrane filter to obtain a sample solution. The sample solution was adjusted so that the concentration of solvent-soluble components was about 0.3%. Using this sample solution, the weight average molecular weight of the resin was measured under the following conditions.
Apparatus: A molecular weight measuring apparatus (trade name “Waters 2695 Separations Module”, manufactured by Waters) can be used.
・ Column: "KF-806M" (trade name, manufactured by Showa Denko) connected in series (4 columns)
・Eluent: tetrahydrofuran ・Flow rate: 1.0 mL/min
・Reagent injection volume: 0.100 mL
・Oven temperature: 40℃
・ Detector: RI detector (trade name “2414detector”, manufactured by Waters)

In calculating the weight average molecular weight, a molecular weight calibration curve prepared using a standard polystyrene resin was used. The resin used is TSK standard polystyrene: F-850, F-450, F-288, F-128, F-80, F-40, F-20, F-10, F-4, F-2, F -1, A-5000, A-2500, A-1000, and A-500 (these are trade names, manufactured by Tosoh).

(acid number)
The acid value of the resin was measured by a titration method based on JIS K-0070. 0.5 to 2.0 g of resin was precisely weighed and used as a sample to be measured. A sample was placed in a 50.0 mL beaker, and 25.0 mL of a mixed solution of tetrahydrofuran and ethanol (volume ratio=2:1) was added to dissolve the sample. A 0.1 mol/L ethanol solution of potassium hydroxide was used as a titrant, and titration was performed by potentiometric titration, and the amount of the titrant used was S (mL). A blank containing no sample was also titrated in the same manner, and the amount of ethanol solution of potassium hydroxide used was defined as B (mL). As a measuring device, an automatic titrator (trade name “COM-2500”, manufactured by Hiranuma Sangyo) was used. From the obtained S and B, the acid value was calculated by the following formula. f is the factor (potency) of the ethanol solution of potassium hydroxide, and M (g) is the precisely weighed value of the sample.
Acid value [mgKOH/g] = (SB) x f x 5.61/M

<Synthesis of Resin>
Using each monomer (unit: part) shown in Table 2, each resin was synthesized by a conventional method. The weight average molecular weight was adjusted by adjusting the heating temperature and time during polymerization. 10.0 parts of each resin is neutralized with potassium hydroxide having an equimolar acid value, and an appropriate amount of ion-exchanged water is added to prepare a liquid containing each resin having a resin content of 20.0%. bottom. Table 2 summarizes the acid value and weight average molecular weight of the resin measured by the above method.

The abbreviations of the monomers in Table 2 are AA: acrylic acid, MAA: methacrylic acid, nBA: n-butyl acrylate, EA: ethyl acrylate, PE-200: polyethylene glycol monomethacrylate (n = 4.5) ( Trade name “Blenmar PE-200” manufactured by NOF Corporation), BEMA: behenyl methacrylate, St: styrene, BzMA: benzyl methacrylate.

<Synthesis of resin particles>
(Resin particle 1)
0.2 parts of potassium persulfate and 82.6 parts of ion-exchanged water were placed in a four-necked flask equipped with a stirrer, a reflux condenser, and a nitrogen gas inlet tube, and nitrogen gas was introduced. Further, 15.7 parts of ethyl methacrylate, 1.2 parts of methacrylic acid, and 0.3 parts of a surfactant (trade name “Aquaron KH-05” manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) were mixed to obtain a mixture. The resulting mixture was added dropwise to a four-necked flask over 1 hour while stirring, and then reacted at a temperature of 80° C. for 2 hours. Then, after cooling the contents to room temperature, potassium hydroxide in an equimolar amount to the acid value of the resin and an appropriate amount of ion-exchanged water are added, and the resin particle content is 25.0%, and the volume-based cumulative 50 An aqueous dispersion of resin particles 1 having a % particle diameter of 160 nm was prepared. The amount of acid groups introduced into the resin particles was 200 μmol/g.

(Resin particles 2)
Commercially available resin particles (trade name “Elitel KT-0507” manufactured by Unitika) were used as the resin particles 2 . The content of the resin particles 2 in the aqueous dispersion of the resin particles 2 was adjusted to 25.0% by concentrating the liquid component.

<Preparation of pigment dispersion>
A pigment dispersion was prepared by the following procedure. Table 3 shows the conditions for producing the pigment dispersion. 40.0 parts of the titanium oxide particles shown in Table 3, a resin-containing liquid, and ion-exchanged water with a total component content of 100.0 parts were mixed and pre-dispersed using a homogenizer. Thereafter, dispersion treatment (main dispersion) was performed using 0.5 mm zirconia beads at 25° C. with a paint shaker for 12 hours. The zirconia beads were filtered off, and an appropriate amount of ion-exchanged water was added as necessary to prepare each pigment dispersion having a titanium oxide particle content of 40.0%. Table 3 summarizes the content T (%) of the titanium oxide particles in the pigment dispersion, the content B (%) of the resin that is the resin dispersant, and the value of B/T (times).

<Ink preparation>
The types and amounts of each component shown in the upper part of Tables 4 to 7 were mixed and stirred. PEG1000 is polyethylene glycol with a number average molecular weight of 1000. Surfactant 1 is Noigen EA-177 (trade name, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), which is a nonionic surfactant having an aromatic ring. Surfactant 2 is Neoperex G-15 (trade name, manufactured by Kao Corporation), an anionic surfactant having an aromatic ring (16% active ingredient). Surfactants 3 and 4 are acetylenol E100 and acetylenol E60 (trade names, manufactured by Kawaken Fine Chemicals), respectively, and are nonionic surfactants having no aromatic ring. Surfactant 5 is NIKKOL BL-9EX (trade name, manufactured by Nikko Chemicals), which is a nonionic surfactant having no aromatic ring. Proxel GXL(S) (trade name, manufactured by LONZA) is a preservative. Thereafter, pressure filtration was performed with a membrane filter (manufactured by Sartorius) having a pore size of 5.0 μm to prepare each ink. The properties of each ink are collectively shown at the bottom of Tables 4 to 7. The content of the surfactant having an aromatic ring is referred to as "the content of the specific surfactant". When the pH of each prepared ink was measured with a pH meter (trade name “Portable pH Meter D-74” manufactured by Horiba Ltd.), all were within the range of 7.5 to 8.5.

<Evaluation>
The following items were evaluated using each ink obtained above. In the present invention, in the evaluation criteria for each item shown below, "A" and "B" were defined as acceptable levels, and "C" was defined as an unacceptable level. Table 8 shows the evaluation results. The ink of Comparative Example 5 could not stably disperse the titanium oxide particles, so the following evaluation was not performed. Therefore, "-" is indicated in the evaluation result column.

(Sedimentation recovery)
8.0 parts of the ink obtained above was placed in a 10 mL polytetrafluoroethylene centrifugal tube. About 0.2 mL of ink was sampled from the liquid surface of the centrifugation tube, diluted 1000 times (by volume) with ion-exchanged water, and then the absorbance at 550 nm was measured (absorbance before sedimentation recovery). A spectrophotometer (trade name “U-3011”, manufactured by Hitachi High-Tech Science) was used to measure the absorbance. Centrifugation was performed at 700 rpm for 120 minutes using a centrifuge (trade name “HIMAC CR22E”, manufactured by Hitachi Koki). Then, the centrifugal tube was set in a shaker (trade name “Lab Shaker Wide SR-5”, manufactured by AS One) and stirred at 180 rpm for 300 seconds. After that, about 0.2 mL of ink was sampled from the liquid surface of the centrifugation tube, diluted 1000 times (by volume) with deionized water, and the absorbance at 550 nm was measured (absorbance after sedimentation recovery). "Sedimentation recovery rate" = 100 x (absorbance after sedimentation recovery) / (absorbance before sedimentation recovery) The sedimentation recovery rate was calculated based on the formula, and the sedimentation recovery rate of the ink was evaluated according to the evaluation criteria shown below. evaluated. It means that the larger the sedimentation recovery rate, the greater the sedimentation recovery effect when the sedimentation recovery operation of the ink jet recording apparatus is performed.
A: Sedimentation recovery rate was 80% or more.
B: Sedimentation recovery rate was 70% or more and less than 80%.
C: Sedimentation recovery rate was less than 70%.

(Image uniformity)
Each of the inks obtained above was filled in an ink cartridge, and mounted in an ink jet recording apparatus that ejects ink from a recording head by the action of thermal energy. This inkjet printing apparatus is a product name "image PROGRAF PRO-2000" (manufactured by Canon), in which a heater for heating the printing medium from the back surface is incorporated downstream of the printing head in the printing medium conveying direction. In this embodiment, the print duty of a solid image printed under the condition of applying 8 ink droplets of 4 ng per droplet to a unit area of 1/600 inch×1/600 inch is defined as 100%. Using the above-described inkjet recording apparatus, a 9 cm × 15 cm solid image with a recording duty of 200% is recorded in an environment of 25 ° C. and a relative humidity of 50% on a recording medium (trade name “Ultra-transparent PET film GIY-0306” , manufactured by LINTEC). Then, the image was fixed on the recording medium by setting the heating condition of the recording medium to which the ink was applied to 80°C. After the image was dried at 25°C for 24 hours, unevenness in the center of the solid image was visually checked, and the state of pigment coating was observed under a microscope to evaluate the uniformity of the image according to the evaluation criteria shown below. bottom. Example 29 was evaluated in the same manner without performing the heating step.
A: No unevenness was observed visually, and no areas not coated with the pigment were observed under a microscope.
B: No unevenness was observed visually, but spots where no pigment was adhered were observed under a microscope.
C: Unevenness was visually observed, and areas where no pigment was adhered were also observed under a microscope.

Claims (9)

An aqueous ink for inkjet containing titanium oxide particles, a resin dispersant for dispersing the titanium oxide particles, and a surfactant,
wherein the titanium oxide particles are titanium oxide having at least a portion of the surface coated with alumina, and the zeta potential thereof is greater than 0 mV;
The resin dispersant is a water-soluble resin containing a unit derived from a monomer having an aromatic ring and a unit derived from a monomer having a carboxylic acid group,
A water-based ink, wherein the surfactant is a surfactant having an aromatic ring. 2. The water-based ink according to claim 1, wherein the proportion (mass %) of said alumina in said titanium oxide particles is 2.0 mass % or more based on the total mass of said titanium oxide particles. 3. The water-based ink according to claim 1, wherein the resin has an acid value of 100 mgKOH/g or more and 320 mgKOH/g or less. The ratio (% by mass) of units derived from a monomer having an aromatic ring in the resin dispersant is 20.0% by mass or more and 70.0% by mass or less based on the total mass of the resin dispersant. 4. The water-based ink according to any one of 1 to 3. 2. The content (mass %) of the resin dispersant in the water-based ink is 0.02 times or more and 0.20 times or less as a mass ratio with respect to the content (mass %) of the titanium oxide particles. 5. The water-based ink according to any one of items 1 to 4. 6. The content (% by mass) of the surfactant in the water-based ink is 0.01% by mass or more and 2.00% by mass or less based on the total mass of the ink. The water-based ink described in . An ink cartridge comprising ink and an ink containing portion containing the ink,
An ink cartridge, wherein the ink is the water-based ink according to any one of claims 1 to 6. An inkjet recording method comprising a step of ejecting ink from an inkjet recording head and applying it to a recording medium to record an image,
An inkjet recording method, wherein the ink is the water-based ink according to any one of claims 1 to 6. 9. The inkjet recording method according to claim 8, further comprising a heating step of heating the recording medium to which the ink has been applied.

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