Inkjet recording method, inkjet recording device and set of aqueous ink and aqueous reaction liquid

Abstract

To provide an inkjet recording method and the like capable of recording an image having excellent ink storage stability, excellent concealability and suppressed occurrence of unevenness.SOLUTION: There is provided an inkjet recording method for recording an image using an aqueous ink and an aqueous reaction liquid containing a reactant which reacts with the aqueous ink. The aqueous ink contains first titanium oxide particles and second titanium oxide particles which are dispersed by the action of an anionic group. The 50% cumulative particle diameter of the first titanium oxide particles on a volume basis is larger than the 50% cumulative particle diameter of the second titanium oxide particles on a volume basis, the isoelectric point of the second titanium oxide particles is higher than that of the first titanium oxide particles and higher than the pH of the aqueous reaction liquid.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an inkjet recording method, an inkjet recording apparatus, and a set of an aqueous ink and an aqueous reaction liquid. [Background technology]

[0002] In recent years, inkjet recording devices have come to be widely used when outputting advertisements or exhibits using recording media such as paper or resin film. For example, in order to express clear color images even on transparent recording media, white ink is used in addition to black and basic color inks (hereinafter, these may be collectively referred to as color inks). Specifically, a recording method is used in which white ink is applied in advance to a location including an area where an image is to be recorded on a transparent recording medium to perform base treatment, and color inks are applied on top of that, or each ink is applied in the reverse order (so-called back printing). It is also known that a reaction liquid containing a reactant that reacts with the ink is used when recording on such recording media.

[0003] 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 concealment. When an ink containing titanium oxide is used as an inkjet ink, it is necessary to apply a large amount of titanium oxide particles to a recording medium in order to ensure the whiteness and concealment of the image. In order to apply a large amount of titanium oxide particles to a recording medium, it is necessary to increase the amount of white ink applied or to increase the content of titanium oxide particles in the white ink. In the former case, the amount of liquid components such as water-soluble organic solvents and water in the ink increases with an increase in the amount of ink applied, making it difficult to dry and fix the recorded image. In addition, the specific gravity of titanium oxide is larger than that of components such as coloring materials used in inks other than white. Therefore, in the latter case, the ink ejection stability and the ink flow path may be blocked. In addition, it is possible to use titanium oxide particles with a larger particle size in order to improve the whiteness and concealment of the image, but the titanium oxide particles are more likely to settle. On the other hand, if titanium oxide with a small particle size is used to suppress the settling, the whiteness of the image cannot be obtained.

[0004] Inks capable of recording images with excellent hiding power have been investigated up to now. For example, an ink composition containing titanium oxide particles, hollow particles, and an aggregating agent has been proposed (see Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2016-137482 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present inventors prepared an ink using the ink composition proposed in Patent Document 1 and investigated various properties. As a result, although the hiding power was good and image unevenness was suppressed to some extent, the storage stability of the ink was insufficient.

[0007] Therefore, an object of the present invention is to provide an inkjet recording method capable of recording images in which the ink has excellent storage stability and hiding power and in which the occurrence of unevenness is suppressed. Another object of the present invention is to provide an inkjet recording apparatus and a set of an aqueous ink and an aqueous reaction liquid for use in the inkjet recording method. [Means for solving the problem]

[0008] That is, according to the present invention, there is provided an inkjet recording method for recording an image using an aqueous reaction liquid containing an aqueous ink and a reactant that reacts with the aqueous ink, the inkjet recording method comprising a reaction liquid application step of applying the aqueous reaction liquid to the recording medium, and an ink application step of applying the aqueous ink so as to overlap at least a portion of the area of ​​the recording medium to which the aqueous reaction liquid is applied, the aqueous ink containing titanium oxide particles, the titanium oxide particles including first titanium oxide particles and second titanium oxide particles, the volume-based cumulative 50% particle diameter of the first titanium oxide particles being larger than the volume-based cumulative 50% particle diameter of the second titanium oxide particles, the isoelectric point of the second titanium oxide particles being higher than the isoelectric point of the first titanium oxide particles and being equal to or higher than the pH of the aqueous reaction liquid. Effect of the Invention

[0009] According to the present invention, it is possible to provide an inkjet recording method capable of recording an image in which the ink storage stability and hiding power are excellent and the occurrence of unevenness is suppressed. Another object of the present invention is to provide an inkjet recording apparatus used in the inkjet recording method, and a set of an aqueous ink and an aqueous reaction liquid. [Brief description of the drawings]

[0010] [Figure 1] 1 is a perspective view illustrating an embodiment of an inkjet recording apparatus of the present invention. [Diagram 2]1 is a side view diagrammatically illustrating an embodiment of an inkjet recording apparatus of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The present invention will be described in more detail below with reference to preferred embodiments. In the present invention, when the compound is a salt, the salt is present in the ink and reaction liquid in the form of dissociated ions, but for convenience, it is expressed as "containing a salt." In addition, the aqueous ink and aqueous reaction liquid for inkjet printing may be simply referred to as "ink" and "reaction liquid." Physical property values ​​are values ​​at room temperature (25°C) unless otherwise specified.

[0012] Inorganic oxides such as titanium oxide react with water molecules constituting the aqueous medium in the aqueous ink to generate hydroxyl groups (hereinafter, sometimes referred to as "surface hydroxyl groups") on the surface. For this reason, in the case of aqueous inkjet inks, in order to utilize the generated surface hydroxyl groups while further improving the storage stability of the ink, the ink is generally used in a state where the ink is surface-treated with inorganic oxides such as alumina or silica. The surface hydroxyl groups of titanium oxide particles have properties specific to the inorganic oxide corresponding to the inorganic compound used in the surface treatment, and the isoelectric point, which is an indicator of the strength of the acid, differs depending on the type of inorganic compound. Therefore, although titanium oxide itself is an inorganic oxide, the surface of titanium oxide particles shows the properties of an inorganic oxide corresponding to the inorganic compound used in the surface treatment, and the surface charge of titanium oxide particles is strongly dependent on the pH of the aqueous medium, the type of surface treatment agent, and the amount of the surface treatment agent used.

[0013] The present inventors have investigated the cause of the insufficient storage stability of the ink prepared with reference to the description in Patent Document 1. This ink is designed so that the titanium oxide particles do not easily aggregate in the ink state, but when applied to a recording medium, the concentration of the aggregating agent increases as the liquid components evaporate and permeate into the recording medium, and the titanium oxide particles aggregate. When this ink is stored for a long period of time, the concentration of the aggregating agent gradually increases as the ink evaporates, and the titanium oxide particles aggregate in the same way as when the ink is applied to a recording medium, which is thought to be why the storage stability was insufficient.

[0014] Therefore, the present inventors have considered using a reaction liquid containing a reactant that reacts with the ink, separately from the ink. However, even if the ink composition of Patent Document 1 is omitted and an image is recorded using a reaction liquid separately from the ink, it has been found that the concealing property is insufficient and image unevenness cannot be suppressed in some cases. The present inventors believe that the image unevenness is caused by the surface hydroxyl groups of titanium oxide. As described above, titanium oxide has surface hydroxyl groups. Although the isoelectric point can be changed by inorganic oxide present on the surface, some of the surface hydroxyl groups are negatively charged by desorption of protons. Titanium oxide particles with a large particle size have many such surface hydroxyl groups, so even if the negative charge is eliminated by a reactant, aggregation is difficult to proceed. As a result, it is believed that the image unevenness has occurred.

[0015] In order to record an image with excellent hiding power and suppressed occurrence of unevenness, the present inventors further investigated the composition of the reaction liquid and the ink so that the particles are stably dispersed in the ink while being rapidly aggregated on the recording medium by the action of the reaction liquid. As a result, they found that the above problem can be solved by using two types of titanium oxide particles and by making the isoelectric points of the particles and the pH of the reaction liquid satisfy a specific relationship.

[0016] That is, the inkjet recording method of the present invention has the following characteristics. First, the aqueous ink contains first titanium oxide particles and second titanium oxide particles. The volume-based cumulative 50% particle diameter of the first titanium oxide particles is larger than the volume-based cumulative 50% particle diameter of the second titanium oxide particles. The isoelectric point of the second titanium oxide particles is higher than the isoelectric point of the first titanium oxide particles and is equal to or higher than the pH of the aqueous reaction liquid. The aqueous reaction liquid contains a reactant that reacts with the aqueous ink. An image is recorded using the above aqueous ink and aqueous reaction liquid. The present inventors speculate as follows about the mechanism by which the above configuration enables recording of an image that is excellent in hiding power and suppresses the occurrence of unevenness while improving the storage stability of the ink.

[0017] The first ink contains first titanium oxide particles and second titanium oxide particles that are dispersed by the action of anionic groups. The volume-based cumulative 50% particle diameter of the first titanium oxide particles must be larger than the volume-based cumulative 50% particle diameter of the second titanium oxide particles. In addition, the second titanium oxide particles, which have a smaller particle diameter than the first titanium oxide particles, must have a higher isoelectric point than the first titanium oxide particles.

[0018] When the ink contacts the reaction liquid on the recording medium, the titanium oxide particles are more likely to be destabilized by the reactant due to their small particle diameter. That is, the surface hydroxyl groups of the titanium oxide particles, which are easily negatively charged due to their high isoelectric point, are easily neutralized, and the dispersion state of the titanium oxide particles is easily destabilized. Furthermore, by using a reaction liquid having a pH lower than the isoelectric point of the titanium oxide particles, the above action can be efficiently exerted. As a result, the aggregation of the titanium oxide particles proceeds quickly. Furthermore, it is considered that a part of the surface hydroxyl groups of the titanium oxide particles is positively charged and adsorbed to the surfaces of the unreacted first and second titanium oxide particles. As a result, even when the first titanium oxide particles are aggregated by the action of the reactant, they are quickly thickened and aggregated due to the influence of the titanium oxide particles present on the surface.

[0019] That is, by adopting the above-mentioned configuration, when the ink contacts the reaction liquid on the recording medium, the second titanium oxide particles first proceed to aggregate, and are adsorbed to the surface of the first titanium oxide particles, promoting the aggregation of the first titanium oxide particles. As a result, a uniform film is formed, and the occurrence of image unevenness can be suppressed. In addition, the film formed on the recording medium is in a state in which the second titanium oxide particles fill the gaps between the first titanium oxide particles, and light scattering is increased. As a result, the concealment of the image can also be improved. It is considered that when the pH of the aqueous reaction liquid is higher than the isoelectric point of the second titanium oxide particles, the second titanium oxide particles are less likely to be positively charged, and therefore are less likely to play the role of supporting the aggregation of the first titanium oxide particles described above. As a result, the occurrence of image unevenness cannot be suppressed.

[0020] If the particle size of the second titanium oxide particles is equal to or larger than that of the first titanium oxide particles, the amount of components with large particle sizes will be too large, and if the content is increased in an attempt to improve the concealment of the image, the storage stability of the ink will not be obtained. In addition, even if only the first titanium oxide particles, that is, only titanium oxide particles with small particle sizes, are used, the concealment of the image cannot be improved. In addition, if the isoelectric point of the first titanium oxide particles is higher than that of the second titanium oxide particles, the surface hydroxyl groups of the first titanium oxide particles are more susceptible to the action of the reactant than the surface hydroxyl groups of the second titanium oxide particles. Therefore, it is considered that the second titanium oxide particles are less likely to play a role in supporting the aggregation of the first titanium oxide particles. As a result, the occurrence of image unevenness cannot be suppressed.

[0021] <Inkjet recording method, inkjet recording apparatus, and set of aqueous ink and aqueous reaction liquid> The inkjet recording method of the present invention (hereinafter, also simply referred to as "recording method") is a method for ejecting ink from an inkjet recording head to record an image on a recording medium. The recording method of the present invention has a reaction liquid applying step of applying a reaction liquid to the recording medium, and an ink applying step of applying ink to the recording medium so as to overlap at least a part of the area to which the reaction liquid is applied. Either the reaction liquid or the ink may be applied to the recording medium first.

[0022] The recording method of the present invention may further include additional known processes. For example, when white ink is used as a base treatment for color inks, an image may be recorded by applying color inks (inks such as black, cyan, magenta, and yellow) so as to overlap at least a part of the area where the white ink is applied.

[0023] Furthermore, it can also be used for back printing, in which white ink is applied so as to overlap at least a part of the area where color inks are applied. The recording medium is not particularly limited, but since the water-based ink of the present invention can be used as a white ink, it is preferable to use a transparent or colored recording medium. In addition, the recording medium may be a poorly absorbing medium (non-absorbing medium) that has low absorbency of the liquid medium, such as a resin film.

[0024] The inkjet recording apparatus (hereinafter also simply referred to as "recording apparatus") of the present invention is an apparatus used for ejecting ink from an inkjet recording head to record an image on a recording medium. The recording apparatus of the present invention includes a means for applying a reaction liquid to the recording medium, and a means for applying ink to the recording medium so as to overlap at least a portion of the area to which the reaction liquid is applied. Either the reaction liquid or the ink may be applied to the recording medium first. In the inkjet recording method and inkjet recording apparatus of the present invention, it is not necessary to cure the image by irradiation with active energy rays or the like.

[0025] The set of aqueous ink and aqueous reaction liquid of the present invention is a set used in an inkjet recording method in which an aqueous ink and an aqueous reaction liquid containing a reactant that reacts with the aqueous ink are ejected from a recording head to record an image on a recording medium. The set is preferably used in the above-mentioned recording method. The form of the set includes a set of multiple ink cartridges each containing multiple inks (reaction liquids) independently, and an ink cartridge formed integrally by combining multiple ink containing parts each containing multiple inks (reaction liquids). The set of the present invention is not limited to the above form and may be in any form as long as it is configured so that the inks and reaction liquids can be used in combination.

[0026] The inkjet recording method and inkjet recording apparatus (hereinafter, also simply referred to as "recording method and recording apparatus") of the present invention will be described in detail below.

[0027] FIG. 1 is a perspective view showing an embodiment of an inkjet recording apparatus of the present invention. FIG. 2 is a side view showing an embodiment of an inkjet recording apparatus of the present invention. As shown in FIGS. 1 and 2, the recording apparatus of this embodiment includes an inkjet recording head 22 that ejects ink. The recording head 22 is a recording head that ejects ink by the action of thermal energy. A recording head that ejects ink by the action of thermal energy applies thermal energy to the ink by applying an electric pulse to an electrothermal conversion element, and ejects the ink from the ejection port. Here, a recording head that ejects ink by the action of thermal energy is exemplified, but a recording head that ejects ink by the action of mechanical energy may be adopted. The recording head may include a mechanism (temperature control mechanism) that heats the water-based ink ejected from the recording head. When the temperature control mechanism is included, the temperature of the ink ejected from the recording head is preferably 35° C. or higher and 70° C. or lower.

[0028] [Heating process] The recording method of the present invention may include a step of heating (heat treatment) the recording medium to which the ink (and the reaction liquid) has been applied. By heating the recording medium to which the ink has been applied, it is possible to promote drying and increase the strength of the image. Examples of the means for heating the recording medium include known heating means such as a heater, air blowing means using air blowing such as a dryer, and heating means such as a combination of these. Examples of the heating means include the above heating means, air blowing means, and a combination of these. Examples of the heat treatment method include a method of applying heat from the opposite side (back side) of the recording surface (ink application surface) of the recording medium using a heater, a method of applying warm or hot air to the recording surface of the recording medium, and a method of heating from the recording surface or back side using an infrared heater. A combination of these methods may also be used.

[0029] In order to enhance the abrasion resistance of the image, the heating temperature of the recording medium to which the ink has been applied is preferably 50° C. or more and 90° C. or less. The heating temperature of the recording medium to which the ink has been applied may be read by a sensor incorporated in a position corresponding to the heating means of the recording device, or may be determined from the relationship between the amount of heat and the temperature of the recording medium, which is determined according to the type of ink and recording medium.

[0030] In the recording device shown in FIGS. 1 and 2, a heater 25 supported by a frame (not shown) is disposed downstream in the sub-scanning direction A from the position where the recording head 22 reciprocates in the main scanning direction B. The recording medium 1 to which ink has been applied is heated by the heater 25. Examples of the heater 25 include a sheath heater and a halogen heater. The heater 25 is covered by a heater cover 26. The heater cover 26 is a member for efficiently irradiating the heat generated by the heater 25 to the recording medium 1. Furthermore, the heater cover 26 is also a member for protecting the heater 25. The recording medium 1 to which ink has been applied ejected from the recording head 22 is wound by a take-up spool 27 to form a roll-shaped wound medium 24.

[0031] It is preferable to apply ink to a unit area of ​​a recording medium by a multi-pass method in which the ink is applied in a plurality of relative scans between the recording head and the recording medium. In particular, it is preferable to apply the reaction liquid and the ink to the unit area by different relative scans. Here, the unit area can be set as any area such as one pixel or one band.

[0032] <Reaction solution> The recording method of the present invention includes a reaction liquid application step of applying an aqueous reaction liquid containing a reactant that reacts with the aqueous ink to a recording medium. Each component of the reaction liquid will be described in detail below.

[0033] (Reactant) The reaction liquid reacts with the ink when it comes into contact with the ink, and aggregates the components in the ink (components having anionic groups, such as resins, surfactants, and self-dispersing pigments), and contains a reactant. The presence of the reactant can destabilize the state of the components having anionic groups in the ink when the ink and the reactant come into contact with each other on the recording medium, and promote the aggregation of the ink. Examples of the reactant include cationic components such as polyvalent metal ions and cationic resins, and organic acids. The reactant may be used alone or in combination of two or more.

[0034] Examples of polyvalent metal ions constituting polyvalent metal salts include Ca. 2+ , Cu 2+ , Ni 2+ , Mg 2+ , Sr 2+ , B.A. 2+ , and Zn 2+ Divalent metal ions such as Fe 3+ , Cr 3+ , Y 3+ , and Al 3+ In order to add a polyvalent metal ion to the reaction solution, a water-soluble polyvalent metal salt (which may be a hydrate) formed by combining a polyvalent metal ion with an anion can be used. Examples of the anion include Cl. - , Br -, I - , ClO - , ClO 2 - , ClO 3 - , ClO 4 - , NO 2 - , NO 3 - , S.O. 4 2- , CO 3 2- , HCO 3 - , P.O. 4 3- , H.P.O. 4 2- , and H 2 PO 4 - Inorganic anions such as HCOO - , (COO - ) 2 , COOH(COO - ), C.H. 3 COO - , C.H. 3 CH(OH)COO - , C 2 H 4 (COO - ) 2 , C 6 H 5 COO - , C 6 H 4 (COO - ) 2 , and C.H. 3 SO 3 - and other organic anions. When a polyvalent metal ion is used as a reactant, the content (mass%) of the reaction solution calculated as a polyvalent metal salt is preferably 1.0 mass% or more and 20.0 mass% or less based on the total mass of the reaction solution. In this specification, when the polyvalent metal salt is a hydrate, the "content (mass%) of the polyvalent metal salt" in the reaction solution means the "content (mass%) of the anhydride of the polyvalent metal salt" excluding water as the hydrate.

[0035] The reaction liquid containing an organic acid has a buffering ability in the acidic range (less than pH 7.0, preferably pH 2.0 to 5.0), and thus efficiently converts the anionic groups of the components present in the ink into an acid form and causes them to aggregate. Examples of the organic acid include monocarboxylic acids and salts thereof, such as formic acid, acetic acid, propionic acid, butyric acid, benzoic acid, glycolic acid, lactic acid, salicylic acid, pyrrole carboxylic acid, furan carboxylic acid, picolinic acid, nicotinic acid, thiophene carboxylic acid, levulinic acid, and coumaric acid; dicarboxylic acids and salts and hydrogen salts thereof, such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, itaconic acid, sebacic acid, phthalic acid, malic acid, and tartaric acid; tricarboxylic acids and salts and hydrogen salts thereof, such as citric acid and trimellitic acid; and tetracarboxylic acids and salts and hydrogen salts thereof, such as pyromellitic acid. When an organic acid is used as the reactant, the content (mass %) of the organic acid in the reaction liquid is preferably 1.0 mass % or more and 50.0 mass % or less based on the total mass of the reaction liquid.

[0036] Examples of cationic resins include resins having a structure of primary to tertiary amines and resins having a structure of quaternary ammonium salts. Specific examples include resins having a structure of vinylamine, allylamine, vinylimidazole, vinylpyridine, dimethylaminoethyl methacrylate, ethyleneimine, guanidine, diallyldimethylammonium chloride, and alkylamine-epichlorohydrin condensate. In order to increase the solubility in the reaction solution, the cationic resin may be used in combination with an acidic compound or the cationic resin may be subjected to a quaternary treatment. When a cationic resin is used as a reactant, the content (mass%) of the cationic resin in the reaction solution is preferably 0.1 mass% or more and 10.0 mass% or less based on the total mass of the reaction solution.

[0037] (aqueous medium) The reaction liquid is an aqueous reaction liquid containing at least water as an aqueous medium. The aqueous medium used in the reaction liquid can be the same as the aqueous medium that can be contained in the ink described later. The aqueous medium used in the reaction liquid can contain the water-soluble organic solvent that can be contained in the ink described later. The content (mass%) of the water-soluble organic solvent in the reaction liquid is preferably 1.0 mass% or more and 45.0 mass% or less based on the total mass of the reaction liquid. The water-soluble organic solvent preferably contains a specific water-soluble hydrocarbon compound described later. The content (mass%) of the water-soluble hydrocarbon compound in the reaction liquid is preferably 1.0 mass% or more and 20.0 mass% or less based on the total mass of the reaction liquid. In addition, the content (mass%) of water in the reaction liquid is preferably 50.0 mass% or more and 95.0 mass% or less based on the total mass of the reaction liquid.

[0038] (Other Ingredients) The reaction liquid may contain various other components as necessary, including the same components as those that can be contained in the ink, which will be described later.

[0039] (Properties of reaction solution) The reaction liquid is an aqueous reaction liquid applied to the inkjet method. Therefore, from the viewpoint of reliability, it is preferable to appropriately control the physical property value. Specifically, the surface tension of the reaction liquid at 25°C is preferably 20 mN / m or more and 60 mN / m or less. In addition, the viscosity of the reaction liquid at 25°C is preferably 1.0 mPa·s or more and 10.0 mPa·s or less. The pH of the reaction liquid at 25°C is preferably 3.0 or more and 8.0 or less, and more preferably 3.0 or more and 6.5 or less. In addition, the pH of the reaction liquid is equal to or less than the isoelectric point of the titanium dioxide particles. By setting the pH of the reaction liquid as described above, the aggregation of the titanium dioxide particles can be further promoted and the concealment of the image can be further improved, as described above. The pH of the reaction liquid can be measured with a general pH meter equipped with a glass electrode or the like.

[0040] <Ink> The ink used in the recording method of the present invention is an aqueous inkjet ink containing first titanium oxide particles and second titanium oxide particles dispersed by the action of anionic groups. Each component of the ink will be described in detail below.

[0041] (First titanium oxide particles) The ink contains first titanium oxide particles as a coloring material (pigment). The content (mass%) of the first titanium oxide particles in the ink is preferably 0.10% by mass or more and 20.00% by mass or less, based on the total mass of the ink. The content (mass%) of the first titanium oxide particles in the ink is more preferably 1.00% by mass or more and 20.00% by mass or less, based on the total mass of the ink. In particular, the content (mass%) of the first titanium oxide particles in the ink is particularly preferably 1.00% by mass or more and 15.00% by mass or less, based on the total mass of the ink.

[0042] Titanium oxide having a certain particle size or more is white and can be used as a white pigment. Titanium oxide has three crystal forms: rutile, anatase, and brookite. Among them, rutile titanium oxide is preferred. Titanium oxide can be industrially produced by the sulfuric acid method and the chlorine method, and the titanium oxide used in the present invention can be produced by either method.

[0043] The volume-based cumulative 50% particle diameter (hereinafter sometimes referred to as the average particle diameter) of the first titanium oxide particles is preferably 200 nm or more and 500 nm or less. In particular, the volume-based cumulative 50% particle diameter of the first titanium oxide particles is more preferably 200 nm or more and 400 nm or less. When the average particle diameter is within the above range, it is easy to maintain a balance between the hiding power of the image and the storage stability of the ink. The volume-based cumulative 50% particle diameter (D 50 ) is the diameter of the particle that is 50% of the total volume of the measured particles in the particle size accumulation curve, calculated from the small particle size side. 50can be measured under the following conditions: SetZero: 30 seconds, number of measurements: 3, measurement time: 180 seconds, shape: aspheric, refractive index: 2.60. A particle size analyzer using a dynamic light scattering method can be used as the particle size distribution measuring device. Of course, the measurement conditions are not limited to those described above.

[0044] It is preferable to use the first titanium oxide particles that have been surface-treated with silica. That is, it is preferable that at least a part of the surface of the first titanium oxide particles is coated with silica. The surface treatment is expected to suppress photocatalytic activity and improve dispersibility. The surface hydroxyl group derived from silica is likely to be negatively charged, and is more likely to interact more efficiently with the first titanium oxide particles that are positively charged due to the influence of the reactant. As a result, the concealment of the image in particular can be further improved. The surface may be further treated with alumina. In this specification, "alumina" is a general term for oxides of aluminum such as aluminum oxide. In this specification, "silica" is a general term for silicon dioxide or a substance composed of silicon dioxide. Most of the alumina and silica that coat the titanium oxide are present in the form of silicon dioxide and aluminum oxide.

[0045] Surface treatment methods for titanium oxide include wet treatment and dry treatment. For example, titanium oxide can be dispersed in a liquid medium and then reacted with a surface treatment agent such as sodium aluminate or sodium silicate to perform surface treatment, and the desired characteristics can be achieved by appropriately changing the ratio of these surface treatment agents. In addition to alumina and silica, inorganic oxides such as zinc oxide and zirconia, and organic substances such as polyols can also be used for surface treatment, as long as the effects of the present invention are not impaired.

[0046] The ink may contain other pigments other than titanium oxide as long as the effect of the present invention is not impaired. The content (mass%) of other pigments in the ink is preferably 0.10 mass% or more and 5.00 mass% or less, and more preferably 0.10 mass% or more and 1.00 mass% or less, based on the total mass of the ink. In order to be used as a white ink, it is preferable that the ink does not substantially contain other pigments other than titanium oxide.

[0047] (Titanium dioxide particles) The ink contains titanium oxide particles in addition to titanium oxide particles. The titanium oxide particles are required to have a smaller average particle size than the titanium oxide particles, are not required to be white, and are not required to be used as a coloring material. The content (mass%) of the titanium oxide particles in the ink is preferably 0.01% by mass or more and 10.00% by mass or less, and more preferably 0.05% by mass or more and 5.00% by mass or less, based on the total mass of the ink.

[0048] The second titanium oxide particles can be obtained by the same method as the first titanium oxide particles. The second titanium oxide particles are also preferably rutile type titanium oxide.

[0049] The volume-based cumulative 50% particle diameter of the second titanium oxide particles is smaller than the volume-based cumulative 50% particle diameter of the first titanium oxide particles. The volume-based cumulative 50% particle diameter of the second titanium oxide particles is preferably 50 nm or less, more preferably 15 nm or less. By using second titanium oxide particles having a particle diameter within the above range, storage stability can be further improved. In addition, the volume-based cumulative 50% particle diameter of the second titanium oxide particles is preferably 10 nm or more.

[0050] The second titanium oxide particles are preferably titanium oxide whose surfaces are not coated with silica, and may not be substantially coated with other compounds.

[0051] The content (mass%) of the titanium oxide particles in the aqueous ink is preferably 0.01 to 0.10 times the content (mass%) of the titanium oxide particles. If the mass ratio is less than 0.01, the effect of promoting the aggregation of the titanium oxide particles does not work sufficiently, and the concealing property of the image may not be sufficiently obtained and the occurrence of image unevenness may not be sufficiently suppressed. If the mass ratio is more than 0.10, the amount of the titanium oxide particles is too large, and the titanium oxide particles are likely to aggregate with each other. As a result, the amount of the titanium oxide particles that promote the aggregation of the titanium oxide particles is insufficient, and the concealing property of the image may not be sufficiently obtained and the occurrence of image unevenness may not be sufficiently suppressed. In addition, the storage stability of the ink may not be sufficiently obtained due to the aggregation of the titanium oxide particles.

[0052] (resin) The ink may contain a resin. Examples of the resin include acrylic resin, urethane resin, and urea resin. Of these, acrylic resin is preferred. The content (mass%) of the resin in the ink is preferably 1.00% by mass or more and 25.00% by mass or less, and more preferably 3.00% by mass or more and 15.00% by mass or less, based on the total mass of the ink. Of these, the content is particularly preferably 5.00% by mass or more and 15.00% by mass or less.

[0053] The resin can be contained in the ink for the purpose of improving various properties of the recorded image, such as abrasion resistance and hiding power. Examples of the form of the resin include block copolymers, random copolymers, graft copolymers, and combinations thereof. The resin may be a water-soluble resin that can be dissolved in an aqueous medium, or may be resin particles that are dispersed in an aqueous medium. The resin particles do not need to encapsulate a coloring material.

[0054] In this specification, "a resin is water-soluble" means that when the resin is neutralized with an alkali equivalent to the acid value, the resin is present in an aqueous medium in a state in which it does not form particles whose particle size can be measured by a dynamic light scattering method. Whether or not a resin is water-soluble can be determined according to the following method. First, a liquid (resin solid content: 10 mass%) containing a resin neutralized with an alkali (sodium hydroxide, potassium hydroxide, etc.) equivalent to the acid value is prepared. Next, the prepared liquid is diluted 10 times (volume basis) with pure water to prepare a sample solution. Then, when the particle size of the resin in the sample solution is measured by a dynamic light scattering method, if particles having a particle size are not measured, the resin can be determined to be water-soluble. The measurement conditions at this time can be, for example, SetZero: 30 seconds, number of measurements: 3 times, and measurement time: 180 seconds. As a particle size distribution measurement device, a particle size analyzer using a dynamic light scattering method (for example, the product name "UPA-EX150", manufactured by Nikkiso) can be used. Of course, the particle size distribution measurement device and measurement conditions used are not limited to those described above.

[0055] The acid value of the water-soluble resin is preferably 100 mgKOH / g or more and 250 mgKOH / g or less, and the weight average molecular weight of the water-soluble resin is preferably 3,000 or more and 15,000 or less.

[0056] The acid value of the resin constituting the resin particles is preferably 5 mgKOH / g or more and 100 mgKOH / g or less. The weight average molecular weight of the resin constituting the resin particles is preferably 1,000 or more and 3,000,000 or less, and more preferably 100,000 or more and 3,000,000 or less. The volume-based cumulative 50% particle diameter (D 50) is preferably 50 nm or more and 500 nm or less. The volume-based cumulative 50% particle diameter of the resin particles is the diameter of the particles that is 50% of the total volume of the measured particles when integrated from the small particle diameter side in a particle diameter integration curve. The volume-based cumulative 50% particle diameter of the resin particles can be measured using the dynamic light scattering particle size analyzer and measurement conditions described above. The glass transition temperature of the resin particles is preferably 40° C. or more and 120° C. or less, and more preferably 50° C. or more and 100° C. or less. The glass transition temperature (° C.) of the resin particles can be measured using a differential scanning calorimeter (DSC). The resin particles do not need to contain a coloring material.

[0057] (aqueous medium) The ink is an aqueous ink containing water as an aqueous medium. The ink may contain water or an aqueous medium that is a mixed solvent of water and a water-soluble organic solvent. As the water, it is preferable to use deionized water (ion-exchanged water). The content (mass %) of water in the ink is preferably 50.00 mass % or more and 95.00 mass % or less based on the total mass of the ink.

[0058] The water-soluble organic solvent is not particularly limited as long as it is water-soluble (preferably, dissolves in water at any ratio at 25°C). Specifically, monohydric or polyhydric alcohols, alkylene glycols, glycol ethers, nitrogen-containing polar compounds, sulfur-containing polar compounds, etc. can be used. The content (mass%) of the water-soluble organic solvent in the ink is preferably 3.00% by mass or more and 50.00% by mass or less, and more preferably 10.00% by mass or more and 40.00% by mass or less, based on the total mass of the ink. If the content (mass%) of the water-soluble organic solvent is less than 3.00% by mass, the ink may be stuck in 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 50.00% by mass, poor ink supply may occur.

[0059] (Other additives) In addition to the above additives, the ink may contain various additives such as surfactants, pH adjusters, rust inhibitors, preservatives, antifungal agents, antioxidants, reduction inhibitors, evaporation promoters, and chelating agents, as necessary. Of these, it is preferable for the ink to contain a surfactant. The content (mass%) of the surfactant in the ink is preferably 0.10% by mass or more and 5.00% by mass or less, and more preferably 0.10% by mass or more and 2.00% by mass or less, based on the total mass of the ink. Examples of surfactants include anionic surfactants, cationic surfactants, and nonionic surfactants. Of these, nonionic surfactants are preferable because they have low affinity with the first titanium oxide particles and the second titanium oxide particles and are effective in small amounts, since they are used to adjust various physical properties of the ink.

[0060] (Ink properties) Since the ink is applied to the inkjet method, it is preferable to appropriately control the physical properties of the ink. The surface tension of the 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 ink can be adjusted by appropriately determining the type and content of the surfactant in the ink. In addition, 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. The pH of the ink can be measured with a general pH meter equipped with a glass electrode or the like. EXAMPLES

[0061] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples in any way as long as it does not deviate from the gist of the invention. "Parts" and "%" used to describe the amounts of components are based on mass unless otherwise specified. In addition, a dispersion of titanium oxide particles is referred to as a "pigment dispersion."

[0062] <Method of measuring physical properties> (Cumulative 50% particle size based on volume of titanium oxide particles) The pigment dispersion was diluted with ion-exchanged water to prepare a sample with a pigment content of about 0.01%. The average particle diameter D of the pigment was measured for this sample using the particle size distribution meter described above under the following measurement conditions. 50 (Cumulative 50% particle size on a volume basis) was measured.

[0063] [Measurement conditions]: SetZero:30s Number of measurements: 3 Measurement time: 120 seconds ·Shape: Non-spherical Refractive index: 2.6 ·Density: 4.0

[0064] (Confirmation of surface treatment of titanium oxide particles) Using a liquid in which titanium oxide particles were added to nitric acid as a sample, quantitative analysis of aluminum and silicon elements was performed using an inductively coupled plasma (ICP) emission spectrometer. In this case, it was assumed that all atoms covering the surface were in the form of oxides, and the obtained values ​​of aluminum and silicon were converted to the oxides, i.e., alumina and silica, to calculate the mass ratio. In other words, if values ​​for aluminum and silicon were obtained, it means that the surface had been treated with alumina or silica.

[0065] (Isoelectric point of titanium oxide particles) The isoelectric point 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, using a zeta potential / particle size measurement system (product name "ELS-Z2", manufactured by Otsuka Electronics), the pH at which the charge was 0 was determined as the isoelectric point. The measurement range was pH 3 to 11, and measurements were performed while adjusting the pH using a 0.1 mol / L hydrochloric acid solution and a 0.1 mol / L sodium hydroxide solution. In this example, the isoelectric point of titanium oxide particles was measured using the pigment dispersion, but the isoelectric point can also be measured in the same manner as above using titanium oxide particles separated from ink by a conventional method.

[0066] <Preparation of reaction solution> The components shown in Table 1 were mixed and thoroughly stirred, and then pressure filtered with a cellulose acetate filter (manufactured by Advantec) with a pore size of 3.0 μm to prepare each reaction solution. "PAA-HCL-05" is the trade name of a liquid containing allylamine hydrochloride polymer (primary amine, weight average molecular weight 5,000, cationic resin content 40.0%) manufactured by Nittobo Medical. "Capstone FS3100" is the trade name of a fluorine-based nonionic surfactant manufactured by Chemours. "Proxel GXL (S)" is the trade name of a preservative manufactured by Arch Chemicals. The pH of each reaction solution was measured using a pH meter (trade name "Portable pH Meter D-74", manufactured by Horiba, Ltd.). In addition, when preparing reaction solution 3, ion-exchanged water to which an appropriate amount of hydrochloric acid had been added was added in the amount shown in Table 1 instead of ion-exchanged water, and the pH was adjusted to 3.0.

[0067] [Table 1]

[0068] <Preparation of titanium oxide particles> The following titanium oxide particles were prepared. The properties of the titanium oxide particles are shown in Table 2. In Table 2, TITANIX: JR-800, JR-403, and JR-405, and fine titanium oxide MT-500B are product names of rutile-type titanium oxide manufactured by Teika. TYPAQUE PF-690 is the product name of rutile-type titanium oxide manufactured by Ishihara Sangyo Kaisha. TITONE: R-21, STR-100N, and STR-100W(G) are the product names of rutile-type titanium oxide manufactured by Sakai Chemical Industry Co., Ltd. TAINOC A-6 is the product name of anatase-type titanium oxide sol manufactured by Taki Chemical Industry Co., Ltd. Table 2 shows the average particle diameter D measured under the conditions described above. 50 and isoelectric point are shown.

[0069] [Table 2]

[0070] <Preparation of pigment dispersion> (Pigment dispersions 1-7, 9-11) 40.00 parts of titanium oxide particles of the type shown in Table 3, dispersants of the type and amount (parts) shown in Table 4, and ion-exchanged water with a total of 100.00 parts of components were mixed and pre-dispersed using a homogenizer. Then, dispersion treatment was performed for 12 hours at 25°C using 0.5 mm zirconia beads with a paint shaker. The zirconia beads were filtered off, and an appropriate amount of ion-exchanged water was added as necessary to prepare each pigment dispersion liquid with a titanium oxide particle content of 40.00%. 3-(methoxy(polyoxyethylene)9-12)propyltrimethoxysilane was used as dispersant 1. In addition, a styrene acrylic resin synthesized by a known method was used as dispersant 2. The weight average molecular weight of this styrene acrylic resin was 18,000 and the acid value was 110 mgKOH / g.

[0071] (Pigment Dispersion 8) Pigment dispersion liquid 8 was prepared by concentrating a liquid in which anatase-type titanium oxide manufactured by Taki Chemical Industry Co., Ltd. shown in Table 2 was dispersed with amine, so that the titanium oxide particle content was 40.0%.

[0072] [Table 3]

[0073] <Ink Preparation> The components (unit: %) shown in Tables 4 and 5 were mixed and thoroughly stirred, and then pressure filtered through a cellulose acetate filter (Advantec) with a pore size of 3.0 μm to prepare each ink. "Acetylenol E60" is the trade name of a surfactant manufactured by Kawaken Fine Chemicals. "Proxel GXL(S)" is the trade name of a preservative manufactured by Arch Chemicals.

[0074] [Table 4]

[0075] [Table 5]

[0076] <Evaluation> The prepared reaction liquid and ink were filled in a cartridge, and the cartridge was set in an inkjet recording device (product name "imagePROGRAF PRO-2000", manufactured by Canon) equipped with a recording head that ejects ink by thermal energy. A heating device for drying the recording medium to which the reaction liquid and ink were applied was incorporated in this recording device at a position downstream of the recording head in the conveying direction of the recording medium. The surface temperature of the recording medium was set to 80°C by heating with the heating device. In this example, an image recorded under the condition that one drop of 4.0 ng of ink is applied to a unit area of ​​1 / 1200 inch x 1 / 1200 inch is defined as having a recording duty of 100%. As the recording medium, "Ultra-transparent PET film GIY-0306" (manufactured by LINTEC, non-absorbent medium) was used. The recording environment was a temperature of 25°C and a relative humidity of 50%. The following items were evaluated for each of the inks obtained above. The image was recorded by applying the ink to the unit area of ​​the recording medium in eight separate relative scans of the recording head and the recording medium. In the present invention, in the evaluation criteria for each of the following items, "A" and "B" were considered to be acceptable levels, and "C" was considered to be an unacceptable level. The evaluation results are shown on the right side of Table 6.

[0077] (Concealment) The opacity of the image was measured using a spectrophotometer (product name "CM-2600d, manufactured by Konica Minolta) using the SCI method including specular reflection light. Evaluation was performed using an evaluation method conforming to ISO2471:2008. ISO2471:2008 is a standard for testing the opacity of paper and paperboard, and in this example, the opacity of the image was evaluated in accordance with this standard.

[0078] In ISO2471:2008, the reflectance is measured by backing the recording medium to be tested with a white plate and a black plate. ∞ ) and the reflectance when a black board is placed on the back (R 0 ) ratio ((R 0 / R ∞ ) × 100) is evaluated as opacity, or hiding rate.

[0079] In this embodiment, in accordance with this method, a white barium sulfate plate is placed on the back of the recording medium to obtain a reflectance Y 1 In addition, instead of using a black board as a backing, a sufficient thickness of air layer was placed on the back of the film, and the reflectance (luminous reflectance) Y 2 This is to measure only the reflectance derived from the image, without being affected by the background (film) or back surface of the image to be evaluated for hiding power. Using these reflectances, the hiding power of the image to be measured was calculated from the following formula (1), and the hiding power of the image was evaluated according to the evaluation criteria shown below. The reflectance measurement conditions were as follows: light source: D50 light source, measurement range: 3 mmφ, reflectance: SCI method that measures the visual reflectance of total reflected light including specular reflection component. Opacity=(Y 2 / Y 1 ) × 100 ···(1) A: The opacity was between 55% and 100%. B: The opacity was 50% or more and less than 55%. C: The opacity was less than 50%.

[0080] (Suppression of image unevenness) Using the above inkjet recording device, a 10 cm x 10 cm solid image with a recording duty of 200% was recorded on a recording medium. The center of the recorded solid image was visually inspected, and the suppression of image unevenness was evaluated according to the following evaluation criteria. A: There were no unevenness in the solid image. B: Some unevenness was observed in the solid image. C: The ink was repelled from the solid image, and there were areas where ink was not present.

[0081] (Storage stability) The inks were stored at 40°C for 30 days, and the average particle diameters were measured before and after storage to evaluate the rate of change. A dynamic light scattering particle size measuring device (product name "Nanotrac", manufactured by Microtrac) was used to measure the average particle diameter, and the average particle diameter D50 value calculated from the 50% cumulative value of the volume average particle diameter was taken as the average particle diameter. The suppression of image unevenness for each ink was evaluated according to the evaluation criteria shown below, based on the value of the rate of change in average particle diameter = (average particle diameter after storage / average particle diameter before storage) x 100 (%). A: The rate of change in average particle size was 5.0% or less. B: The rate of change in average particle diameter was more than 5.0% and 10.0% or less. C: The rate of change in average particle size exceeded 10.0%.

[0082] [Table 6]

[0083] The disclosure of this embodiment includes the following methods and configurations.

[0084] (Method 1) An inkjet recording method for recording an image using an aqueous reaction liquid containing an aqueous ink and a reactant that reacts with the aqueous ink, comprising: a reaction liquid applying step of applying the aqueous reaction liquid to the recording medium; an ink applying step of applying the aqueous ink so as to overlap at least a portion of an area of ​​the recording medium to which the aqueous reaction liquid is applied, the aqueous ink contains titanium oxide particles dispersed by the action of anionic groups; the titanium oxide particles include first titanium oxide particles and second titanium oxide particles, the first titanium oxide particles have a volume-based cumulative 50% particle diameter greater than the volume-based cumulative 50% particle diameter of the second titanium oxide particles; 13. An ink-jet recording method, wherein the second titanium oxide particles have an isoelectric point higher than that of the first titanium oxide particles and is equal to or higher than the pH of the aqueous reaction liquid.

[0085] (Method 2) The inkjet recording method according to Method 1, wherein the first titanium oxide particles have a volume-based cumulative 50% particle diameter of 200 nm or more, and the second titanium oxide particles have a volume-based cumulative 50% particle diameter of 15 nm or less.

[0086] (Method 3) the first titanium oxide particles are titanium oxide having at least a portion of their surfaces covered with silica, 3. The ink-jet recording method according to claim 1, wherein the second titanium oxide particles are titanium oxide the surfaces of which are not coated with silica.

[0087] (Method 4) The inkjet recording method according to any one of Methods 1 to 3, wherein the content (mass%) of the second titanium oxide particles in the aqueous ink is 0.01 times or more and 0.10 times or less, based on the mass, of the content (mass%) of the first titanium oxide particles.

[0088] (Method 5) 5. The inkjet recording method according to any one of Methods 1 to 4, wherein the pH of the aqueous reaction liquid is equal to or lower than the isoelectric point of the first titanium oxide particles.

[0089] (Configuration 1) An inkjet recording apparatus used to record an image using an aqueous reaction liquid containing an aqueous ink and a reactant that reacts with the aqueous ink, comprising: a reaction liquid applying means for applying the aqueous reaction liquid to the recording medium; an ink applying means for applying the water-based ink so as to overlap at least a portion of an area of ​​the recording medium to which the water-based reaction liquid is applied, the aqueous ink contains titanium oxide particles dispersed by the action of anionic groups; the titanium oxide particles include first titanium oxide particles and second titanium oxide particles, the first titanium oxide particles have a volume-based cumulative 50% particle diameter greater than the volume-based cumulative 50% particle diameter of the second titanium oxide particles; 13. An ink-jet recording apparatus, wherein the second titanium oxide particles have an isoelectric point higher than that of the first titanium oxide particles and is equal to or higher than the pH of the aqueous reaction liquid.

[0090] (Configuration 2) A set of an aqueous ink and an aqueous reaction liquid used in an inkjet recording method for recording an image using an aqueous ink and an aqueous reaction liquid containing a reactant that reacts with the aqueous ink, comprising: the inkjet recording method includes a reaction liquid applying step of applying the aqueous reaction liquid to the recording medium; an ink applying step of applying the aqueous ink so as to overlap at least a portion of an area of ​​the recording medium to which the aqueous reaction liquid is applied, the aqueous ink contains titanium oxide particles dispersed by the action of anionic groups; the titanium oxide particles include first titanium oxide particles and second titanium oxide particles, the first titanium oxide particles have a volume-based cumulative 50% particle diameter greater than the volume-based cumulative 50% particle diameter of the second titanium oxide particles; A set of an aqueous ink and an aqueous reaction liquid, characterized in that the isoelectric point of the second titanium oxide particles is higher than the isoelectric point of the first titanium oxide particles and is equal to or higher than the pH of the aqueous reaction liquid.

Claims

1. An inkjet recording method for recording an image using an aqueous reaction liquid containing an aqueous ink and a reactant that reacts with the aqueous ink, comprising: a reaction liquid applying step of applying the aqueous reaction liquid to the recording medium; an ink applying step of applying the aqueous ink so as to overlap at least a portion of an area of ​​the recording medium to which the aqueous reaction liquid is applied, the aqueous ink contains titanium oxide particles dispersed by the action of anionic groups; the titanium oxide particles include first titanium oxide particles and second titanium oxide particles, a volume-based cumulative 50% particle diameter of the first titanium oxide particles is larger than a volume-based cumulative 50% particle diameter of the second titanium oxide particles; an isoelectric point of the second titanium oxide particles being higher than that of the first titanium oxide particles and being equal to or higher than the pH of the aqueous reaction liquid;

2. 2. The inkjet recording method according to claim 1, wherein the first titanium oxide particles have a volume-based cumulative 50% particle diameter of 200 nm or more, and the second titanium oxide particles have a volume-based cumulative 50% particle diameter of 15 nm or less.

3. the first titanium oxide particles are titanium oxide having at least a portion of their surfaces covered with silica, 2. The ink-jet recording method according to claim 1, wherein the second titanium oxide particles are titanium oxide the surfaces of which are not coated with silica.

4. 2. The inkjet recording method according to claim 1, wherein the content (mass%) of the second titanium oxide particles in the aqueous ink is 0.01 to 0.10 times, in mass terms, the content (mass%) of the first titanium oxide particles.

5. 2. The ink jet recording method according to claim 1, wherein the pH of the aqueous reaction liquid is equal to or lower than the isoelectric point of the first titanium oxide particles.

6. An inkjet recording apparatus used to record an image using an aqueous reaction liquid containing an aqueous ink and a reactant that reacts with the aqueous ink, comprising: a reaction liquid applying means for applying the aqueous reaction liquid to the recording medium; an ink applying means for applying the water-based ink so as to overlap at least a portion of an area of ​​the recording medium to which the water-based reaction liquid is applied, the aqueous ink contains titanium oxide particles dispersed by the action of anionic groups; the titanium oxide particles include first titanium oxide particles and second titanium oxide particles, a volume-based cumulative 50% particle diameter of the first titanium oxide particles is larger than a volume-based cumulative 50% particle diameter of the second titanium oxide particles; an isoelectric point of the second titanium oxide particles being higher than that of the first titanium oxide particles and being equal to or higher than the pH of the aqueous reaction liquid;

7. A set of an aqueous ink and an aqueous reaction liquid used in an inkjet recording method for recording an image using an aqueous ink and an aqueous reaction liquid containing a reactant that reacts with the aqueous ink, comprising: the inkjet recording method includes a reaction liquid applying step of applying the aqueous reaction liquid to the recording medium; an ink applying step of applying the aqueous ink so as to overlap at least a portion of an area of ​​the recording medium to which the aqueous reaction liquid is applied, the aqueous ink contains titanium oxide particles dispersed by the action of anionic groups; the titanium oxide particles include first titanium oxide particles and second titanium oxide particles, a volume-based cumulative 50% particle diameter of the first titanium oxide particles is larger than a volume-based cumulative 50% particle diameter of the second titanium oxide particles; A set of an aqueous ink and an aqueous reaction liquid, characterized in that the isoelectric point of the second titanium oxide particles is higher than the isoelectric point of the first titanium oxide particles and is equal to or higher than the pH of the aqueous reaction liquid.

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