JP2024120880A - Inkjet recording method and inkjet recording apparatus - Google Patents

Abstract

To provide an inkjet recording method which enables recording of an image that is excellent in image clarity and color development property, and suppresses thin-film interference.SOLUTION: An inkjet recording method includes a first recording step of imparting first aqueous ink onto a recording medium, and a second recording step of imparting second aqueous ink onto the recording medium in this order. The first ink contains a pigment, first wax particles, a first silicone-based surface active agent, and an urethane resin. The second ink contains second wax particles and a second silicone-based surface active agent. Maximum absorbance in a wavelength region of 400 nm or more and 780 nm or less of a dilution liquid obtained by diluting the second ink 200 times with water on a mass basis is 1.0 or less, and the volume-based cumulative 50% particle size of the first wax particles is smaller than the volume-based cumulative 50% particle size of the pigment.SELECTED DRAWING: None

Description

The present invention relates to an inkjet recording method and an inkjet recording device used therein.

Inkjet recording is one of the image recording methods that has been developing remarkably in recent years because it can easily record high-quality images. In addition to the mainstream use of inkjet recording for recording documents, photographs, and other images at home, inkjet recording is rapidly becoming more and more popular in offices, commercial applications, and industrial applications. In particular, large-format inkjet recording devices suitable for printing photographs and posters are becoming more and more popular. Pigment inks that contain pigments as coloring materials are widely used as inks for inkjet recording methods to improve the fastness of the images recorded, such as gas resistance, light resistance, and water resistance.

Images recorded on glossy paper using pigment inks generally have poorer image clarity than images recorded on glossy paper using dye inks. Image clarity refers to a property that serves as an index of the sharpness of an image when projected onto an image; when image clarity is low, the image appears blurry, and when image clarity is high, the image appears sharp. In order to improve image clarity, it is effective to reduce the amount of pigment in the ink and reduce the step of the ink dots on the surface of the pigment layer that is formed. However, when an ink with a low pigment content is used, the color development of the image is reduced. In other words, it can be said that there is a trade-off between the color development and image clarity of an image, and there is a demand to provide an ink that can record images that have both image clarity and color development.

To meet the above demands, for example, an ink that uses a silicone-based surfactant and can improve the wettability of the recording medium, reduce the step of the ink dots, and improve the image clarity has been proposed (Patent Document 1). Also, a set of ink and post-treatment liquid that can record images with improved image clarity, each of which contains a water-soluble organic solvent whose solubility parameters are controlled to a predetermined relationship, has been proposed (Patent Document 2).

JP 2006-233083 A JP 2016-117872 A

The present inventors have studied the ink proposed in Patent Document 1. As a result, it was found that, although an image with good image clarity could be recorded, the color development was insufficient. The present inventors have also studied the set of ink and post-treatment liquid proposed in Patent Document 2. As a result, it was found that, when a drying process is not performed after recording, an image with good image clarity could be recorded, but the color development was insufficient. Furthermore, in order to record an image with excellent image clarity and color development, the present inventors have studied a set of an ink using a silicone surfactant and a post-treatment liquid, using an ink to which various additives have been added. As a result, it was found that the color tone of the pigment in the ink may be different in the area where the ink application area and the post-treatment liquid application area overlap. When the area where the ink application area and the post-treatment liquid application area overlap was observed in detail, it was found that the dots of the post-treatment liquid were connected to form a smooth layer with little surface unevenness. It was also found that so-called thin film interference occurs in this layer, and reflected light of a different color tone from the color tone of the pigment is recognized.

Therefore, an object of the present invention is to provide an inkjet recording method capable of recording images that have excellent image clarity and color development, and in which thin-film interference is suppressed. Another object of the present invention is to provide an inkjet recording device for use in this inkjet recording method.

That is, according to the present invention, there is provided an inkjet recording method having a first recording step of applying a water-based first ink to a recording medium, and a second recording step of applying a water-based second ink to the recording medium so as to overlap at least a part of the area of the recording medium to which the first ink has been applied, the first ink containing a pigment, a first wax particle, a first silicone-based surfactant, and a urethane resin, the second ink containing a second wax particle and a second silicone-based surfactant, the second ink being diluted 200 times by mass with water to obtain a diluted liquid, the maximum absorbance in the wavelength range of 400 nm to 780 nm being 1.0 or less, and the volume-based cumulative 50% particle diameter of the first wax particles being smaller than the volume-based cumulative 50% particle diameter of the pigment.

The present invention provides an inkjet recording method capable of recording images with excellent image clarity and color development, and with suppressed thin film interference. The present invention also provides an inkjet recording device for use in this inkjet recording method.

1A and 1B are diagrams illustrating an ink jet recording apparatus according to an embodiment of the present invention, in which FIG. 1A is a perspective view of a main portion of the ink jet recording apparatus, and FIG. 1B is a perspective view of a head cartridge.

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 in the form of dissociation into ions, but for convenience, it is expressed as "containing a salt." In addition, water-based ink for inkjet printing may be simply referred to as "ink." Physical property values are values at room temperature (25°C) unless otherwise specified. In the present invention, the "unit" constituting the resin means a repeating unit derived from one monomer.

The present inventors have investigated the use of inks containing various surfactants and additives in order to record images that are excellent in image clarity and color development and in which thin-film interference is suppressed. As a result, they have found that by satisfying the following requirements (i) to (iv), it is possible to record images that are excellent in image clarity and color development and in which thin-film interference is suppressed, and have arrived at the present invention.
(i) An inkjet recording method comprising a first recording step of applying a first water-based ink to a recording medium, and a second recording step of applying a second water-based ink to the recording medium so as to overlap at least a portion of the area of the recording medium to which the first ink has been applied.
(ii) The first ink contains a pigment, first wax particles, a first silicone-based surfactant, and a urethane resin, and the second ink contains second wax particles and a second silicone-based surfactant.
(iii) The maximum absorbance of a diluted liquid obtained by diluting the second ink 200 times by mass with water is 1.0 or less in the wavelength region of 400 nm or more and 780 nm or less.
(iv) The volume-based cumulative 50% particle size of the first wax particles is smaller than the volume-based cumulative 50% particle size of the pigment.

The inventors speculate as follows about the reason why the image clarity of the recorded image is improved by satisfying the above requirements. Among various surfactants, silicone-based surfactants are the ones that are most likely to reduce surface energy. When a first ink and a second ink each containing a silicone-based surfactant are used, the first ink wets and spreads, reducing the height of the dots, but the second ink applied later is more likely to be repelled by the surface of the layer (pigment layer) formed with the first ink. It is believed that the second ink is more likely to be repelled because the second ink is applied with the silicone-based surfactant localized and oriented on the surface of the layer formed with the first ink.

The inventors have investigated the use of various additives to eliminate the localized orientation of the silicone surfactant on the surface of the layer formed with the first ink. As a result, they have found that when a first ink containing first wax particles and a urethane resin is used, the second ink is not repelled by the surface of the layer formed with the first ink and spreads sufficiently, improving the clarity of the image.

The majority of the surface of the wax particles is occupied by highly hydrophobic hydrocarbons. For this reason, most of the silicone-based surfactant is adsorbed to the first wax particles, suppressing the localized orientation of the silicone-based surfactant on the surface of the layer formed with the first ink. Furthermore, it is presumed that the presence of urethane resin, a component that tends to increase surface energy, effectively suppresses the localized orientation of the silicone-based surfactant on the surface of the layer formed with the first ink, improving image clarity.

The present inventors speculate as follows as to why the color development of the recorded image is improved by satisfying the above requirements. When the cumulative 50% particle diameter ( _D50 ) of the first wax particles in the first ink is smaller than the cumulative 50% particle diameter ( _D50 ) of the pigment in the first ink, the color development of the image is improved. It is speculated that the first wax particles having a smaller particle diameter effectively penetrate into the voids in the aggregates of the pigment contained in the layer formed by the first ink, filling the voids, thereby improving the color development of the image. Furthermore, when the maximum absorbance of the diluted liquid obtained by diluting the second ink 200 times by mass with water is 1.0 or less in the wavelength range of 400 nm to 780 nm, it is considered that the color development of the layer formed by the first ink is unlikely to be blocked. The second ink satisfying the above characteristics is "very light-colored" or "colorless and transparent".

Furthermore, the present inventors speculate as follows as to why thin film interference is suppressed by satisfying the above requirements. As described above, by using the first ink containing a silicone surfactant, first wax particles, and urethane resin, a uniform layer is formed by the second ink, which is more likely to wet and spread. For this reason, although image clarity and color development are improved, thin film interference is more likely to occur, and it is thought that reflected light of a different color tone from the color tone of the pigment in the first ink is more likely to be recognized. Therefore, the present inventors have considered adding various additives to the second ink. As a result, it has been found that thin film interference is suppressed by using the second ink containing second wax particles. When the second ink containing the second wax is used, a layer is formed in which aggregates generated by hydrophobic interactions of the highly hydrophobic second wax particles are appropriately distributed. It is speculated that this breaks down the optically uniform state and suppresses thin film interference. To suppress thin film interference, the second ink must be applied so that it overlaps at least a portion of the area of the recording medium where the first ink has been applied; that is, the order in which the inks are applied must be such that the first ink is applied first, followed by the second ink.

<Inkjet recording method and inkjet recording apparatus>
The inkjet recording method of the present invention includes a first recording step of applying a first aqueous ink to a recording medium, and a second recording step of applying a second aqueous ink to the recording medium so as to overlap at least a part of the area of the recording medium to which the first ink has been applied. The first ink contains a pigment, first wax particles, a first silicone-based surfactant, and a urethane resin, and the second ink contains second wax particles and a second silicone-based surfactant. The maximum absorbance of a diluted liquid obtained by diluting the second ink 200 times by mass with water is 1.0 or less in a wavelength range of 400 nm to 780 nm. The volume-based cumulative 50% particle diameter of the first wax particles is smaller than the volume-based cumulative 50% particle diameter of the pigment.

The inkjet recording apparatus of the present invention is an apparatus suitable for use in the inkjet recording method described above. That is, the inkjet recording apparatus is an apparatus for use in an inkjet recording method having a first recording step of applying a first aqueous ink to a recording medium, and a second recording step of applying a second aqueous ink to the recording medium so as to overlap at least a portion of the area of the recording medium to which the first ink has been applied. The first ink contains a pigment, a first wax particle, a first silicone-based surfactant, and a urethane resin, and the second ink contains a second wax particle and a second silicone-based surfactant. The maximum absorbance of the diluted liquid obtained by diluting the second ink 200 times by mass with water is 1.0 or less in the wavelength range of 400 nm to 780 nm. The volume-based cumulative 50% particle diameter of the first wax particles is smaller than the volume-based cumulative 50% particle diameter of the pigment.

Figure 1 is a diagram showing an embodiment of the inkjet recording device of the present invention, where (a) is a perspective view of the main part of the inkjet recording device, and (b) is a perspective view of the head cartridge. The inkjet recording device is provided with a conveying means (not shown) for conveying a recording medium 32, and a carriage shaft 34. A head cartridge 36 can be mounted on the carriage shaft 34. The head cartridge 36 is equipped with recording heads 38 and 40, and is configured so that an ink cartridge 42 can be set. While the head cartridge 36 is conveyed in the main scanning direction along the carriage shaft 34, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. Then, an image is recorded on the recording medium 32 by conveying the recording medium 32 in the sub-scanning direction by a conveying means (not shown). It is preferable to adopt a serial method in which an image is recorded by reciprocating scanning of the recording head. In the present invention, there is no need to perform post-processing such as drying the recording medium to which the ink has been applied by heating or blowing air, or curing the ink by applying active energy rays.

Methods for ejecting water-based ink from a recording head include a method of applying mechanical energy to the ink and a method of applying thermal energy to the ink. Of these, it is preferable to adopt a method of ejecting ink by applying thermal energy to the ink.

In the inkjet recording method of the present invention, the first ink is applied to the recording medium, and then the second ink is applied to the recording medium. In addition, in the relative scanning of the recording head and the recording medium in the inkjet method, it is preferable to apply the first ink and the second ink to a unit area of the recording medium in different relative scanning (different passes). If the first ink and the second ink are applied to the recording medium in the same relative scanning (same pass) of the recording head and the recording medium, the second ink may be applied before the first ink has sufficiently penetrated and spread into the recording medium. For this reason, the first ink and the second ink may easily mix together, and the effect of improving image clarity may be reduced. The unit area can be set as any area, such as "one pixel" corresponding to the resolution of the recording head, an area obtained by dividing one pixel into any number of parts, or "one band" corresponding to an area that can be printed in one main scan of the recording head.

As the recording medium, it is preferable to use a recording medium having ink absorbency. Examples of recording media having ink absorbency include recording media based on paper, such as glossy paper, art paper, and plain paper. Of these, it is preferable that the recording medium is glossy paper. When glossy paper is used as the recording medium, it is particularly required to record an image with excellent image clarity. By recording an image on glossy paper using the inkjet recording method of the present invention, it is possible to record an image with a significant improvement in image clarity.

(Water-based ink)
In the inkjet recording method of the present invention, a first water-based ink containing a pigment, first wax particles, a first silicone-based surfactant, and a urethane resin, and a second water-based ink containing second wax particles and a second silicone-based surfactant are used. Hereinafter, when simply referring to "ink", it means either the "first ink" or the "second ink".

[Pigment]
The first ink contains a pigment as a coloring material. The content (mass %) of the pigment in the first ink is preferably 0.50 mass % or more and 15.00 mass % or less, and more preferably 0.50 mass % or more and 10.00 mass % or less, based on the total mass of the first ink.

The volume-based cumulative 50% particle diameter of the pigment in the first ink must be larger than the volume-based cumulative 50% particle diameter of the first wax particles. The volume-based cumulative 50% particle diameter of the pigment is preferably 50 nm or more and 250 nm or less. The volume-based cumulative 50% particle diameter of the pigment and the first wax particles can be measured by dynamic light scattering. The measurement conditions can be set, for example, as follows: SetZero: 30 seconds, number of measurements: 3 times, measurement time: 180 seconds. As a particle size distribution measurement device, a particle size analyzer using dynamic light scattering (for example, 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.

The second ink does not substantially contain a pigment or contains a small amount of pigment. When the second ink contains a small amount of pigment, the pigment content in the second ink is preferably an amount that does not cause the color tone of the recorded image to differ from the color tone of the pigment in the first ink. Specifically, the pigment content in the second ink is an amount such that the maximum absorbance of a diluted liquid obtained by diluting the second ink 200 times with water by mass is 1.0 or less in the wavelength range of 400 nm to 780 nm. The pigment content (mass %) in the second ink may be 0.00 mass % based on the total mass of the second ink, and is preferably 0.50 mass % or less, and more preferably 0.20 mass % or less.

Specific examples of pigments include inorganic pigments such as carbon black and titanium oxide; and organic pigments such as azo, phthalocyanine, quinacridone, isoindolinone, imidazolone, diketopyrrolopyrrole, and dioxazine.

Examples of pigment dispersion methods include resin-dispersed pigments that use a resin as a dispersant, and self-dispersed pigments in which hydrophilic groups are bonded to the pigment particle surface. Other examples include resin-bonded pigments in which organic groups containing a resin are chemically bonded to the pigment particle surface, and microcapsule pigments in which the pigment particle surface is coated with a resin or the like. Of these, it is preferable to use resin-dispersed pigments that use a resin as a dispersant.

As a resin dispersant for dispersing the pigment in an aqueous medium, one that can disperse the pigment in an aqueous medium by the action of a carboxylic acid group is preferred. The pigment content (mass %) in the first ink is preferably 0.30 to 10.00 times the mass ratio of the resin dispersant content (mass %).

The resin dispersant is preferably a resin having hydrophilic and hydrophobic units as constituent units. The resin dispersant is preferably an acrylic resin having at least an acrylic component such as a unit derived from (meth)acrylic acid or a unit derived from (meth)acrylic ester, and more preferably a water-soluble acrylic resin. Of these, a water-soluble acrylic resin in the form of a random copolymer is preferred. In the following description, "(meth)acrylic" means "acrylic" and "methacrylic", and "(meth)acrylate" means "acrylate" and "methacrylate".

The hydrophilic unit is a unit having a hydrophilic group such as an acid group or a hydroxy group. The hydrophilic unit can be formed, for example, by polymerizing a monomer having a hydrophilic group. Specific examples of monomers having a hydrophilic group include acidic monomers having a carboxylic acid group such as (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid; acidic monomers having a sulfonic acid group such as styrene sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, and 3-sulfopropyl (meth)acrylate; anionic monomers such as anhydrides and salts of these acidic monomers; monomers having a hydroxy group such as 2-hydroxyethyl (meth)acrylate and 3-hydroxypropyl (meth)acrylate; monomers having an ethylene oxide group such as methoxy (mono-, di-, tri-, and poly)ethylene glycol (meth)acrylate; and the like. Examples of cations constituting the salt of anionic monomers include ions of lithium, sodium, potassium, ammonium, and organic ammonium. The acid value of the resin used as the resin dispersant is preferably 50 mgKOH/g or more and 200 mgKOH/g or less.

As the self-dispersing pigment, it is preferable to use one in which a carboxylic acid group or the like is bonded to the particle surface of the pigment directly or via another atomic group (-R-). The carboxylic acid group may be either an acid type or a salt type, and if it is a salt type, it may be either a partially dissociated state or a completely dissociated state. When the carboxylic acid group is a salt type, examples of the cation that serves as the counter ion include an alkali metal cation, ammonium, and organic ammonium. Specific examples of the other atomic group (-R-) include a linear or branched alkylene group having 1 to 12 carbon atoms; an arylene group such as a phenylene group or a naphthylene group; a carbonyl group; an imino group; an amide group; a sulfonyl group; an ester group; and an ether group. A group that is a combination of these groups may also be used.

The maximum absorbance of the diluted liquid obtained by diluting the second ink 200 times by mass with water in the wavelength range of 400 nm to 780 nm is 1.0 or less, preferably 0.1 or less. If the maximum absorbance of the diluted liquid in the above wavelength range exceeds 1.0, the layer formed by the second ink will conceal the layer formed by the first ink, and the color development of the image will decrease. The maximum absorbance of the diluted liquid obtained by diluting the second ink 200 times by mass with water in the wavelength range of 400 nm to 780 nm is preferably adjusted as follows. First, when the ink contains a pigment, it can be adjusted by the type and content of the pigment contained in the ink. To adjust the maximum absorbance lower depending on the type of pigment, it is recommended to use a pigment with low saturation or high brightness, and to adjust the maximum absorbance lower depending on the pigment content, it is recommended to reduce the content.

[Wax particles]
The first ink contains first wax particles. The second ink contains second wax particles. The first wax particles and the second wax particles may be the same type or different types. Hereinafter, when simply referring to "wax particles", it means both "first wax particles" and "second wax particles".

The volume-based cumulative 50% particle diameter of the first wax particles in the first ink must be smaller than the volume-based cumulative 50% particle diameter of the pigment. The volume-based cumulative 50% particle diameter of the wax particles is preferably 30 nm or more and 200 nm or less.

Wax particles are particles formed from wax. The wax may be a composition containing components other than wax, or it may be wax itself. The wax particles may be dispersed using a dispersant such as a surfactant or resin.

In the narrow sense, wax is an ester of a water-insoluble higher monohydric or dihydric alcohol and a fatty acid, and includes animal waxes and vegetable waxes, but does not include oils and fats. In the broad sense, wax includes high melting point fats, mineral waxes, petroleum waxes, and blends and modified products of various waxes. Any wax in the broad sense can be used in the ink of the present invention without any particular restrictions. Wax in the broad sense can be classified into natural waxes, synthetic waxes, blends of these (blended waxes), and modified products of these (modified waxes).

Natural waxes include animal waxes such as beeswax, spermaceti, and wool wax (lanolin); vegetable waxes such as wood wax, carnauba wax, sugarcane wax, palm wax, candelilla wax, and rice wax; mineral waxes such as montan wax; and petroleum waxes such as paraffin wax, microcrystalline wax, and petrolatum. Synthetic waxes include hydrocarbon waxes such as Fischer-Tropsch wax and polyolefin wax (e.g., polyethylene wax, polypropylene wax). The blended wax is a mixture of the above-mentioned various waxes. The modified wax is a wax obtained by subjecting the above-mentioned various waxes to a modification process such as oxidation, hydrogenation, alcohol modification, acrylic modification, and urethane modification. Among these, the material forming the wax particles preferably contains polyolefin waxes such as polyethylene and polypropylene, and the wax particles are more preferably substantially formed of polyolefin wax.

The material constituting the first wax particles is preferably polyethylene oxide having an acid value of 20 mgKOH/g or less. If the acid value of the polyethylene oxide exceeds 20 mgKOH/g, the hydrophobicity of the first wax particles decreases, and the ability to adsorb the surfactant may be weakened. As a result, it becomes difficult to suppress the localized orientation of the first silicone-based surfactant on the surface of the layer formed with the first ink, making the second ink more likely to be repelled, and the effect of improving image clarity may be somewhat reduced.

The content (mass %) of the first wax particles in the first ink is preferably 0.05% by mass or more and 5.00% by mass or less, and more preferably 0.05% by mass or more and 1.00% by mass or less, based on the total mass of the ink. The content (mass %) of the second wax particles in the second ink is preferably 0.05% by mass or more and 5.00% by mass or less, and more preferably 0.05% by mass or more and 1.00% by mass or less, based on the total mass of the ink.

[Silicone-based surfactants]
The first ink contains a first silicone-based surfactant. The second ink contains a second silicone-based surfactant. The first silicone-based surfactant and the second silicone-based surfactant may be the same type or different types. Hereinafter, when simply referring to a "silicone-based surfactant", this means both a "first silicone-based surfactant" and a "second silicone-based surfactant".

The first silicone surfactant and the second silicone surfactant are preferably compounds represented by any one of the following general formulas (1) to (3). In the following general formulas (1) to (3), (C ₂ H ₄ O) represents an ethylene oxide unit, and (C ₃ H ₆ O) represents a propylene oxide unit. The ethylene oxide unit and the propylene oxide unit may be arranged in either a random state or a block state. "The units are arranged in a random state" means that the units are arranged irregularly. On the other hand, "the units are arranged in a block state" means that the blocks formed by assembling the units are arranged regularly. The number of carbon atoms of the alkyl group and the alkylene group in the general formulas (1) to (3) is preferably about 1 to 6.

（前記一般式（１）中、Ｒ_１は、アルキレン基を表し、Ｒ_２は、水素原子又はアルキル基を表し、ｍ及びｎは、相互に独立に１以上の整数を表し、ａ及びｂは、相互に独立に０以上の整数を表す）

(In the general formula (1), _R1 represents an alkylene group, _R2 represents a hydrogen atom or an alkyl group, m and n each independently represent an integer of 1 or more, and a and b each independently represent an integer of 0 or more.)

（前記一般式（２）中、Ｒ_３は、水素原子又はアルキル基を表し、Ｒ_４は、アルキレン基を表し、ｐは、１以上の整数を表し、ｃ及びｄは、相互に独立に０以上の整数を表す）

(In the general formula (2), _R3 represents a hydrogen atom or an alkyl group, _R4 represents an alkylene group, p represents an integer of 1 or more, and c and d each independently represent an integer of 0 or more.)

（前記一般式（３）中、Ｒ_５は、水素原子又はアルキル基を表し、Ｒ_６は、アルキレン基を表し、ｑ及びｒは、相互に独立に１以上の整数を表し、ｅ及びｆは、相互に独立に０以上の整数を表す）

(In the general formula (3), _R5 represents a hydrogen atom or an alkyl group, _R6 represents an alkylene group, q and r each independently represent an integer of 1 or more, and e and f each independently represent an integer of 0 or more.)

It is more preferable that the first silicone-based surfactant and the second silicone-based surfactant are each a compound represented by general formula (1). The silicone-based surfactant represented by general formula (1) has a stronger adsorption force to wax particles than the silicone-based surfactants represented by general formula (2) and general formula (3). Therefore, by using the first silicone-based surfactant represented by general formula (1) or the second silicone-based surfactant represented by general formula (1), the effect of improving image clarity can be further enhanced.

The content (mass %) of the first silicone-based surfactant in the first ink is preferably 0.20 times or more and 8.00 times or less, and more preferably 0.30 times or more and 6.00 times or less, in terms of mass ratio to the content (mass %) of the first wax particles. If the mass ratio is less than 0.20 times, the dots of the first ink may not wet and spread sufficiently, and the effect of improving image clarity may be reduced. On the other hand, if the mass ratio is more than 8.00 times, it may be somewhat difficult for the first wax particles to sufficiently capture the first silicone-based surfactant, and the second ink may be more likely to be repelled, and the effect of improving image clarity may be somewhat reduced.

The content (mass %) of the first silicone-based surfactant in the first ink is preferably 0.05% by mass or more and 5.00% by mass or less, and more preferably 0.10% by mass or more and 1.00% by mass or less, based on the total mass of the ink. The content (mass %) of the second silicone-based surfactant in the second ink is preferably 0.05% by mass or more and 5.00% by mass or less, and more preferably 0.10% by mass or more and 1.00% by mass or less, based on the total mass of the ink.

[Urethane resin]
The first ink contains a urethane resin, which is synthesized using, for example, polyisocyanate and a component that reacts with it (such as a polyol or a polyamine), and a crosslinking agent or a chain extender are also used as necessary.

[Polyisocyanate]
Polyisocyanates are compounds having two or more isocyanate groups in the molecule. Examples of polyisocyanates include aliphatic and aromatic polyisocyanates. Examples of the aliphatic polyisocyanate include polyisocyanates having a chain structure such as tetramethylene diisocyanate, hexamethylene diisocyanate, dodecamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, 2-methylpentane-1,5-diisocyanate, and 3-methylpentane-1,5-diisocyanate; and polyisocyanates having a cyclic structure such as isophorone diisocyanate, hydrogenated xylylene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 1,4-cyclohexane diisocyanate, methylcyclohexylene diisocyanate, and 1,3-bis(isocyanatemethyl)cyclohexane.

Examples of aromatic polyisocyanates include tolylene diisocyanate, 2,2'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-dibenzyl diisocyanate, 1,5-naphthylene diisocyanate, xylylene diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, α,α,α',α'-tetramethylxylylene diisocyanate, etc.

[Polyol]
A polyol can be used as a component that becomes a unit constituting a urethane resin by reacting with the polyisocyanate. A polyol is a compound having two or more hydroxyl groups in the molecule. Examples of the polyol include polyols that do not have an acid group, such as polyether polyols, polyester polyols, and polycarbonate polyols; polyols that have an acid group; and the like.

Examples of polyols that do not have an acid group include the following. Examples of polyether polyols include addition polymers of alkylene oxides and polyols; glycols such as (poly)alkylene glycol; and the like. Examples of polyester polyols include acid esters, etc. Examples of polycarbonate polyols include alkanediol-based polycarbonate diols, etc. The number average molecular weight of the polyol that does not have an acid group is preferably 450 or more and 4,000 or less.

Examples of polyols having acid groups include polyols containing acid groups such as carboxylic acid groups, sulfonic acid groups, and phosphonic acid groups in their molecules. In particular, it is preferable to use a urethane resin synthesized by further using a polyol having an acid group such as dimethylolpropionic acid or dimethylolbutanoic acid in addition to a polyol having no acid group, and it is even more preferable to use a water-soluble urethane resin. The acid group may be in the form of a salt. Examples of cations constituting the salt include ions of lithium, sodium, potassium, ammonium, and organic ammonium. When a water-soluble urethane resin has an acid group, the acid group is usually neutralized with a neutralizing agent such as a hydroxide of an alkali metal (lithium, sodium, potassium, etc.) or ammonia water to become water-soluble.

[Polyamine]
Examples of polyamines include monoamines having multiple hydroxy groups, such as dimethylolethylamine, diethanolmethylamine, dipropanolethylamine, and dibutanolmethylamine; bifunctional polyamines, such as ethylenediamine, propylenediamine, hexylenediamine, isophoronediamine, xylylenediamine, diphenylmethanediamine, hydrogenated diphenylmethanediamine, and hydrazine; and trifunctional or higher polyamines, such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, polyamidepolyamine, and polyethylenepolyimine. For convenience, compounds having multiple hydroxy groups and one "amino group, imino group" are also listed as "polyamines".

[Cross-linking agents, chain extenders]
The urethane resin may have a unit derived from a crosslinking agent or a chain extender. The crosslinking agent is usually used when synthesizing a prepolymer. The chain extender is usually used when extending the chain of a prepolymer that has been synthesized in advance. The crosslinking agent or the chain extender can be appropriately selected from water, the above-mentioned polyisocyanate, polyol, polyamine, etc., depending on the purpose of crosslinking or chain extension. As the chain extender, one that can crosslink the urethane resin can also be used.

The urethane resin is preferably a water-soluble urethane resin. If the urethane resin in the first ink is a water-insoluble urethane resin (urethane resin particles), the majority of the urethane resin particles tend to be uniformly distributed around the pigment in the first ink applied to the recording medium. On the other hand, if the urethane resin in the first ink is a water-soluble urethane resin, the water-soluble urethane resin tends to be present in a state of being adsorbed on the surface layers of the pigment and wax particles, so that the characteristics of the water-soluble urethane resin are strongly expressed on the surface of the layer formed by the first ink. This makes it easier to increase the surface energy of the image, making it harder for the second ink to be repelled, and further enhancing the effect of improving image clarity.

Whether a resin is a water-soluble resin or a resin particle (water-insoluble resin) can be determined according to the following method. First, prepare a liquid (resin content: about 10 mass%) containing a resin neutralized with a base (sodium hydroxide, potassium hydroxide, etc.) in an amount equivalent to or greater than the acid value. Next, dilute the prepared liquid with pure water to prepare a sample with a resin content of about 1 mass%. Then, measure the particle size of the resin in the sample by dynamic light scattering. If particles with a particle size are not measured, the resin is determined to be a water-soluble resin, and if particles with a particle size are measured, the resin is determined to be a resin particle. The measurement conditions at this time can be set, for example, as follows: SetZero: 30 seconds, number of measurements: 3 times, measurement time: 180 seconds. As a particle size distribution measurement device, a particle size analyzer using a dynamic light scattering method (for example, 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.

The content (mass %) of urethane resin in the first ink is preferably 0.10 mass % or more and 5.00 mass % or less, based on the total mass of the ink, and more preferably 0.10 mass % or more and 1.00 mass % or less. The second ink may contain urethane resin, but it is preferable that the second ink does not contain urethane resin.

[Organic amine]
The first ink preferably further contains an organic amine. By using a first ink that further contains an organic amine, the image clarity of the printed image can be further improved.

Organic amines include triethanolamine, tripropanolamine, triethylamine, 3-[N,N-bis(hydroxyethyl)amino]-2-hydroxypropanesulfonic acid, 2-hydroxy-3-[4-(2-hydroxyethyl)-1-piveladinyl]propanesulfonic acid, N,N-bis(2-hydroxyethyl)glycine, and N-tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid.

Among these, the organic amine is preferably an alkanolamine. In the case of an organic amine that does not contain a hydroxyl group, the hydrophilicity is low, and therefore the cohesive force of the pigment and the first wax particles tends to be high. As a result, voids may easily form in the image, and the effect of improving color development may be reduced.

The content (mass %) of the organic amine in the first ink is preferably 0.05 times or more, more preferably 0.07 times or more, and preferably 1.00 times or less, in terms of mass ratio to the content (mass %) of the pigment. If the mass ratio is less than 0.05 times, the effect of alleviating the aggregation of the pigment may be somewhat weakened, and voids may easily occur in the layer formed with the first ink, which may reduce the effect of improving color development.

[Aqueous medium]
The ink is an aqueous ink containing an aqueous medium which is water or a mixed solvent of water and a water-soluble organic solvent. As the water, deionized water (ion-exchanged water) is preferably used. 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. As the water-soluble organic solvent, any of those usable in inkjet inks, such as alcohols, glycols, (poly)alkylene glycols, nitrogen-containing compounds, and sulfur-containing compounds, can be used. The content (mass%) of the water-soluble organic solvent in the ink is preferably 3.00 mass% or more and 50.00 mass% or less based on the total mass of the ink.

[Other ingredients]
In addition to the above-mentioned components, the ink may contain, as necessary, water-soluble organic compounds that are solid at 25° C., such as polyhydric alcohols, such as trimethylolpropane and trimethylolethane, and urea derivatives, such as urea and ethyleneurea. Furthermore, the ink may contain, as necessary, various additives, such as surfactants other than the above-mentioned silicone-based surfactants, pH adjusters, defoamers, rust inhibitors, preservatives, antifungal agents, antioxidants, reduction inhibitors, and chelating agents.

[Ink properties]
The ink is a water-based ink applied to an inkjet method. Therefore, from the viewpoint of reliability, it is preferable to appropriately control the physical property values. The viscosity of the ink at 25°C is preferably 1.0 mPa·s or more and 10.0 mPa·s or less, and more preferably 1.0 mPa·s or more and 5.0 mPa·s or less. The surface tension of the ink at 25°C is preferably 20 mN/m or more and 60 mN/m or less, and more preferably 25 mN/m or more and 45 mN/m or less. The pH of the ink at 25°C is preferably 7.0 or more and 10.0 or less.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way to the following examples as long as it does not deviate from the gist of the invention. The terms "parts" and "%" used to describe the amounts of components are by weight unless otherwise specified.

<Method of measuring physical properties>
(Cumulative 50% Particle Diameter ( _D50 ) of Volume Distribution of Pigment and Wax Particles)
The pigment and wax particle dispersions were diluted with pure water to measure the volume-based cumulative 50% particle size ( _D50 (nm)) using a particle size analyzer (product name "Microtrac WAVE", manufactured by Microtrac Bell) using the dynamic light scattering method. The measurement conditions were: Set Zero: 30 seconds, number of measurements: 3, measurement time: 120 seconds, shape: spherical, refractive index: 1.59.

(Acid value of wax particles)
A sample was prepared by dispersing or dissolving wax in water. Using an automatic potentiometric titrator, the sample prepared using methyl glycol chitosan 200/N titrant was subjected to potentiometric titration to calculate the number of anionic functional groups per 1 g of wax. The calculated number of anionic functional groups was converted into KOH mass to calculate the acid value (mgKOH/g) of the wax particles. The automatic potentiometric titrator used was "AT510" (manufactured by Kyoto Electronics Manufacturing Co., Ltd.).

<Preparation of Pigment Dispersion>
(Pigment Dispersion 1)
A mixture was obtained by mixing 12.0 parts of carbon black (trade name "Black Pearls 880", manufactured by Cabot), 24.0 parts of an aqueous solution of a resin dispersant, and 64.0 parts of ion-exchanged water. As the aqueous solution of the resin dispersant, an aqueous solution containing 20.00% of resin (solid content) obtained by neutralizing a styrene-acrylic acid copolymer having an acid value of 170 mg KOH/g and a weight average molecular weight of 8,000 with a 10.0% aqueous solution of sodium hydroxide was used. The resulting mixture was charged into a batch-type vertical sand mill (manufactured by Imex), filled with 85 parts of 0.3 mm zirconia beads, and dispersed for 3 hours while cooling with water. Thereafter, the mixture was centrifuged to remove non-dispersed matter including coarse particles. Furthermore, the mixture was filtered under pressure with a cellulose acetate filter (manufactured by Advantec) having a pore size of 3.0 μm to prepare a pigment dispersion 1 having a pigment content (solid content) of 15.00% and a resin content of 6.00%. The volume-based cumulative 50% particle size (D ₅₀ ) of the pigment in Pigment Dispersion 1 was 85 nm.

(Pigment Dispersion 2)
Pigment dispersion 2 was prepared in the same manner as in the case of pigment dispersion 1, except that C.I. Pigment Blue 15:3 (trade name "Hostaperm Blue B2G" manufactured by Clariant) was used instead of carbon black. The pigment content in pigment dispersion 2 was 15.00%, and the resin content was 6.00%. The volume-based cumulative 50% particle diameter ( _D50 ) of the pigment in pigment dispersion 2 was 120 nm.

(Pigment Dispersion 3)
Instead of carbon black, a solid solution of C.I. Pigment Red 202 and C.I. Pigment Violet 19 (product name "CROMOPHTAL JET MAGENTA 2BC", manufactured by Ciba Specialty Chemicals) was used. Except for this, Pigment Dispersion 3 was prepared in the same manner as in the case of Pigment Dispersion 1 described above. The pigment content in Pigment Dispersion 3 was 15.00%, and the resin content was 6.00%. The volume-based cumulative 50% particle diameter ( _D50 ) of the pigment in Pigment Dispersion 3 was 120 nm.

(Pigment Dispersion 4)
Pigment dispersion 4 was prepared in the same manner as in the case of pigment dispersion 1, except that C.I. Pigment Yellow 74 (trade name "Hansa yellow 5GXB", manufactured by Clariant) was used instead of carbon black. The pigment content in pigment dispersion 4 was 15.00%, and the resin content was 6.00%. The volume-based cumulative 50% particle diameter ( _D50 ) of the pigment in pigment dispersion 4 was 120 nm.

(Pigment Dispersion 5)
A solution obtained by dissolving 5.0 g of concentrated hydrochloric acid in 5.5 g of water was cooled to 5°C, and 1.6 g of 4-aminophthalic acid (treatment agent) was added in this state. The container containing this solution was placed in an ice bath, and while stirring to maintain the temperature of the solution at 10°C or less, a solution obtained by dissolving 1.8 g of sodium nitrite in 9.0 g of ion-exchanged water at 5°C was added. After stirring for 15 minutes, 6.0 g of carbon black (specific surface area 260 m ² /g) was added under stirring, and further stirred for 15 minutes to obtain a slurry. The obtained slurry was filtered with filter paper (product name "Standard filter paper No. 2", manufactured by Advantec), and the particles were thoroughly washed with water and dried in an oven at 110°C. After replacing the counter ions from sodium ions to potassium ions by ion exchange, an appropriate amount of ion-exchanged water was added to adjust the pigment content. In this way, pigment dispersion 5 was obtained, which contained a self-dispersing pigment having a pigment content of 15.00% and in which a phthalic acid group having a potassium counter ion was bonded to the surface of the carbon black particles. The volume-based cumulative 50% particle diameter ( _D50 ) of the pigment in pigment dispersion 5 was 65 nm.

<Production of polyethylene oxide>
(Polyethylene oxide 1-3)
An autoclave with an internal volume of 2.0 L equipped with a thermometer, a stirrer, a temperature regulator, a pressure gauge, a gas inlet pipe, and a gas exhaust pipe was prepared. 500 parts of polyethylene powder was placed in this autoclave and heated. After the internal temperature reached 140° C., the stirrer was set to 250 rpm for stirring, while 1.2 L/min of air and 0.18 L/min of oxygen were introduced into the molten material, and the internal pressure was maintained at 0.69 MPa to cause the reaction. The reaction was carried out for a predetermined reaction time to obtain oxidized polyethylenes 1 to 3. The acid value of the oxidized polyethylene can be adjusted by changing the reaction time. The reaction time and the acid value of the obtained oxidized polyethylene are shown in Table 1.

<Preparation of aqueous dispersion of wax particles>
(Aqueous Dispersion of Wax Particles 1)
A four-neck flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube was prepared. 225 g of ion-exchanged water and 50 g of polyethylene oxide 1 were placed in this four-neck flask, and the mixture was melted and stirred while maintaining the temperature at 90 to 95°C. 25 g of a liquid containing an ethylene-acrylic resin was added, and the mixture was dispersed for 15 minutes using an ultrasonic homogenizer while maintaining the temperature at 90 to 95°C, and then cooled to room temperature to obtain a dispersion. Ion-exchanged water was added to the obtained dispersion to adjust the solid content to 10.00%, and an aqueous dispersion of wax particles 1 was obtained. The volume-based cumulative 50% particle diameter (D ₅₀ ) of wax particles 1 in the obtained aqueous dispersion was 40 nm.

(Aqueous Dispersions of Wax Particles 2 to 14)
Aqueous dispersions of wax particles 2 to 14 were obtained in the same manner as for the aqueous dispersion of wax particles 1 described above, except that the type of wax shown in Table 2 was used instead of polyethylene oxide 1. The volume-based cumulative 50% particle diameters (D ₅₀ ) of the wax particles in the obtained aqueous dispersions are shown in Table 2. The volume-based cumulative 50% particle diameters (D ₅₀ ) of the wax particles were adjusted by changing the time of dispersion treatment using an ultrasonic homogenizer.

<Preparing silicone surfactant>
(Silicone-based surfactant 1)
The product name "KF6017" (Shin-Etsu Chemical) was used as "silicone surfactant 1." This "silicone surfactant 1" corresponds to a compound represented by general formula (1).

(Silicone-based surfactant 2)
The product name "BYK-348" (manufactured by BYK) was used as "silicone-based surfactant 2." This "silicone-based surfactant 2" corresponds to the compound represented by general formula (1).

(Silicone-based surfactant 3)
The product name "X-22-4952" (manufactured by Shin-Etsu Chemical Co., Ltd.) was used as "silicone-based surfactant 3." This "silicone-based surfactant 3" corresponds to the compound represented by general formula (2).

(Silicone-based surfactant 4)
The product name "FZ-2231" (manufactured by Dow Toray) was used as "silicone surfactant 4." This "silicone surfactant 4" corresponds to the compound represented by general formula (3).

(Silicone-based surfactant 5)
The product name "KF-353A" (manufactured by Shin-Etsu Chemical Co., Ltd.) was used as "silicone-based surfactant 5." This "silicone-based surfactant 5" corresponds to the compound represented by general formula (1).

<Preparing urethane resin>
(Liquid containing urethane resin 1)
A four-neck flask equipped with a stirrer, a thermometer, a nitrogen gas inlet tube, and a reflux tube was prepared. Into this four-neck flask, 41.70 parts of isophorone diisocyanate, 40.10 parts of polypropylene glycol (number average molecular weight 2,000), 13.20 parts of dimethylolpropionic acid, and 200.00 parts of methyl ethyl ketone were placed. After reacting for 6 hours at 80°C under a nitrogen gas atmosphere, 0.60 parts of ethylenediamine, 2.00 parts of methanol, 2.40 parts of dimethylolpropionic acid, and 100.00 parts of methyl ethyl ketone were added. The residual rate of isocyanate groups was confirmed by FT-IR, and the reaction was continued at 80°C until the desired residual rate was reached to obtain a reaction liquid. The obtained reaction liquid was cooled to 40°C, and then ion-exchanged water was added, and while stirring at high speed with a homomixer, an aqueous potassium hydroxide solution was added to obtain a liquid. Methyl ethyl ketone was distilled off from the resulting liquid by heating under reduced pressure, to obtain a liquid containing water-soluble urethane resin 1 with a urethane resin (solid content) content of 20.00%.

(Liquid containing urethane resin 2)
Commercially available polyether urethane resin particles (product name "Takelac W5661", manufactured by Mitsui Chemicals, solid content 35%), which is a water-insoluble urethane resin, were used as "liquid containing urethane resin 2".

<Preparation of Aqueous Solution of Acrylic Resin>
A four-neck flask equipped with a stirrer, a reflux condenser, and a nitrogen gas inlet tube was prepared. 200.00 parts of ethylene glycol monobutyl ether was placed in this four-neck flask, and the mixture was stirred under a nitrogen gas atmosphere and heated to 130°C. 65.00 parts of styrene, 20.00 parts of butyl acrylate, 15.00 parts of acrylic acid, and 4.00 parts of t-butyl peroxide were added dropwise over 3 hours. After aging for 2 hours, ethylene glycol monobutyl ether was distilled off under reduced pressure to obtain a resin. To the obtained resin, potassium hydroxide in an amount equimolar to the acid value and an appropriate amount of ion-exchanged water were added, and the resin was dissolved by heating to 80°C. As a result, an aqueous solution of an acrylic resin with an acrylic resin (solid content) content of 20.00% was obtained.

<Preparation of Dispersion of Acrylic Resin Particles>
In a flask equipped with a stirrer, a nitrogen inlet tube, a reflux condenser, and a thermometer, 18.00 parts of butyl methacrylate, 0.35 parts of methacrylic acid, 2.00 parts of a polymerization initiator, and 2.00 parts of n-hexadecane were placed and stirred for 30 minutes while introducing nitrogen gas. 2,2'-azobis(2-methylbutylnitrile) was used as the polymerization initiator. Next, 78.00 parts of a 6.00% aqueous solution of polyoxyethylene cetyl ether (trade name "NIKKOL BC-20", manufactured by Nikko Chemicals) was dropped and stirred for 30 minutes to obtain a mixture. After emulsifying the mixture by irradiating it with ultrasonic waves for 3 hours using an ultrasonic irradiation machine, a polymerization reaction was carried out at 80 ° C. for 4 hours under a nitrogen atmosphere. After cooling to 25 ° C. and filtering, an appropriate amount of pure water was added to obtain a dispersion of acrylic resin particles with a resin (solid content) content of 20.00%.

<Ink Preparation>
(First inks 1 to 41, second inks 1 to 29)
Each ink was prepared by mixing the components (unit: %) shown in the upper rows of Tables 3-1 to 3-4 and 4-1 to 4-3, thoroughly stirring, and then filtering under pressure using a polypropylene filter (manufactured by Advantec) with a pore size of 1.0 μm. The ink properties are shown in the lower rows of Tables 3-1 to 3-4 and 4-1 to 4-3. The pH of each ink was adjusted to within the range of 8.5 to 9.0 using a 0.25 mol/L aqueous sulfuric acid solution. Details of the product names in Tables 3-1 to 3-4 and 4-1 to 4-3 are shown below.
・Malquid 3002: Rosin modified maleic acid resin (manufactured by Arakawa Chemical Industries)
Acetylenol E100: nonionic acetylene glycol surfactant (manufactured by Kawaken Fine Chemicals)
FS-300: Fluorine-based surfactant (manufactured by Zonyl)
Surfynol DF-110D: Acetylene glycol surfactant (manufactured by Nissin Chemical Industry Co., Ltd.)
Elastron E-37: Resin containing isocyanate groups (manufactured by Daiichi Kogyo Seiyaku)
・Permarin UA00: Ether-based urethane resin (manufactured by Sanyo Chemical Industries, Ltd.)

(First ink 42)
An ink was prepared according to the composition in Table 3-4 with reference to Example 1 described in Patent Document 1 (JP-A-2006-233083), and designated as first ink 42.

(First ink 43)
An ink was prepared according to the composition in Table 3-4 with reference to Example 1 described in Patent Document 2 (JP 2016-117872 A). This ink was designated as first ink 43.

(First ink 44)
An ink was prepared according to the composition in Table 3-4 with reference to Example 1 described in JP-A-2010-221670, and designated as first ink 44.

<Evaluation>
Each ink obtained above was filled into an ink cartridge. An inkjet recording device (trade name "PIXUS PRO-10S", a modified machine manufactured by Canon) equipped with a recording head that ejects ink by thermal energy was prepared, and the first ink and the second ink were set to have the combinations shown in Table 6. In this inkjet recording device, an image recorded under the condition that 30 ng of ink droplets are applied to a unit area of 1/600 inch x 1/600 inch is defined as having a recording duty of 100%. In the present invention, in the evaluation criteria for each item below, "AA", "A", and "B" were defined as acceptable levels, and "C" was defined as an unacceptable level. The evaluation results are shown in Table 6. The first ink and the second ink were applied by either of the methods of "different passes" and "same pass" shown below. In Comparative Example 17, an image was recorded by applying the first ink and the second ink in the reverse order to that of Example 1. Specifically, in Table 6, "1 ^* " in the column for the first ink of Comparative Example 17 means that ink 1 described in Table 4-1 was used, and "1 ^** " in the column for the second ink means that ink 1 described in Table 3-1 was used.

(Method of applying ink)
[Different Path]
The first ink was set to be discharged from the nozzles corresponding to 1/2 of the nozzle rows on the upstream side in the sub-scanning direction. The second ink was set to be discharged from the nozzles corresponding to 1/2 of the nozzle rows on the downstream side in the sub-scanning direction. In order to keep the application time difference between the first ink and the second ink constant, the inks were discharged in only one direction of the main scan of the recording head for convenience to record an image. Specifically, the first ink was first discharged from the nozzle rows on the upstream side to apply it to the recording medium. Next, while the carriage carrying the recording head was returned to the home position, the recording medium was transported in the sub-scanning direction with a width corresponding to the area where the previous ink was applied. After that, the second ink was also applied to the recording medium in the same procedure as in the case of the first ink. The application time difference between the first ink and the second ink was set to 5 seconds.

[Same Path]
The first ink and the second ink were each ejected and applied to the recording medium in the same main scanning direction of the recording head, thereby recording an image.

(Imageability)
Using the above-mentioned inkjet recording device, the inks were applied to the recording medium in an overlapping manner so that the recording duty of the first ink was 100% and the recording duty of the second ink was 20%, and a solid image with a total recording duty of 120% was recorded. As the recording medium, a product name "Canon Photo Paper Gloss Gold GL-101" (manufactured by Canon) was used. The obtained image was naturally dried at room temperature for 24 hours. Thereafter, two fluorescent lamps arranged at 10 cm intervals were used as observation light sources, and the fluorescent lamps were projected onto the image from a position 2 m away. The shape of the fluorescent lamps projected onto the image was visually confirmed under conditions of an illumination angle of 45 degrees and an observation angle of 45 degrees, and the image clarity of the image was evaluated according to the evaluation criteria shown below.
AA: The boundary between the two projected fluorescent lights was clearly visible, and no blurring was observed at the edges.
A: The boundary between the two projected fluorescent lights was clear, but the edges were slightly blurred.
B: The boundary between the two projected fluorescent lights was clear, but the edges were blurred.
C: I couldn't see the boundary between the two projected fluorescent lights.

(Color development)
Using the above-mentioned inkjet recording device, the inks were applied to the recording medium in an overlapping manner so that the recording duty of the first ink was 130% and the recording duty of the second ink was 20%, and a solid image with a total recording duty of 150% was recorded. As the recording medium, "Canon Photo Paper Glossy Gold GL-101" (manufactured by Canon) was used. The obtained image was naturally dried at room temperature for 24 hours. Thereafter, a fluorescent spectrodensitometer (trade name "FD-7", manufactured by Konica Minolta) was used to measure the optical density of the image under conditions of a light source of D50 and a viewing angle of 2°, and the color development of the image was evaluated according to the evaluation criteria shown in Table 5. In addition, in Table 5, the evaluation criteria for "magenta" described in the ink color column are the criteria used when evaluating the color development of the first ink, ink 3, and the evaluation criteria for "photomagenta" are the criteria used when evaluating the color development of the first ink, ink 5.

(thin film interference suppression)
Using the above inkjet recording device, the recording duty of the first ink was changed in 10% increments between 0 and 140%, and the recording duty of the second ink was 20%, so that each ink was applied to the recording medium in an overlapping manner to record 15 types of solid images. As the recording medium, a product name "Canon Photo Paper Gloss Gold GL-101" (manufactured by Canon) was used. The obtained images were dried at 25°C for 24 hours, and then visually observed, and the thin film interference suppression was evaluated according to the following evaluation criteria.
A: There was no thin film interference in 15 types of images.
B: There was some inconspicuous thin film interference in 15 types of images, but no noticeable thin film interference.
C: Visible thin film interference was observed in 15 types of images.

Claims (10)

An inkjet recording method comprising: a first recording step of applying a first water-based ink to a recording medium; and a second recording step of applying a second water-based ink to the recording medium so as to overlap at least a portion of an area of the recording medium to which the first ink has been applied,
the first ink contains a pigment, first wax particles, a first silicone-based surfactant, and a urethane resin;
the second ink contains second wax particles and a second silicone-based surfactant;
a maximum absorbance of a diluted liquid obtained by diluting the second ink 200 times by mass with water is 1.0 or less in a wavelength range of 400 nm or more and 780 nm or less;
an ink jet recording method, wherein a cumulative 50% particle size on a volume basis of the first wax particles is smaller than a cumulative 50% particle size on a volume basis of the pigment; The inkjet recording method according to claim 1, wherein the first ink and the second ink are applied to a unit area of the recording medium in different relative scans during relative scanning between the recording head and the recording medium. The ink jet recording method according to claim 1 , wherein the first silicone surfactant and the second silicone surfactant are each represented by the following general formula (1):

(In the general formula (1), _R1 represents an alkylene group, _R2 represents a hydrogen atom or an alkyl group, m and n each independently represent an integer of 1 or more, and a and b each independently represent an integer of 0 or more.) The inkjet recording method according to any one of claims 1 to 3, wherein the content (mass%) of the first silicone-based surfactant in the first ink is 0.20 times or more and 8.00 times or less in mass ratio to the content (mass%) of the first wax particles. The inkjet recording method according to any one of claims 1 to 3, wherein the urethane resin is a water-soluble urethane resin. The inkjet recording method according to any one of claims 1 to 3, wherein the material forming the first wax particles is polyethylene oxide having an acid value of 20 mgKOH/g or less. The inkjet recording method according to any one of claims 1 to 3, wherein the first ink further contains an organic amine. The inkjet recording method according to claim 7, wherein the content (mass%) of the organic amine in the first ink is 0.05 times or more the mass ratio of the content (mass%) of the pigment. The inkjet recording method according to any one of claims 1 to 3, wherein the maximum absorbance of the diluted liquid in the wavelength range of 400 nm to 780 nm is 0.1 or less. An inkjet recording apparatus for use in an inkjet recording method having a first recording step of applying a first water-based ink to a recording medium, and a second recording step of applying a second water-based ink to the recording medium so as to overlap at least a portion of an area of the recording medium to which the first ink has been applied,
the first ink contains a pigment, first wax particles, a first silicone-based surfactant, and a urethane resin;
the second ink contains second wax particles and a second silicone-based surfactant;
a maximum absorbance of a diluted liquid obtained by diluting the second ink 200 times by mass with water is 1.0 or less in a wavelength range of 400 nm or more and 780 nm or less;
an ink jet recording apparatus, wherein a cumulative 50% particle size on a volume basis of the first wax particles is smaller than a cumulative 50% particle size on a volume basis of the pigment;

Images