JP7566598B2 - Inkjet recording method and inkjet recording apparatus - Google Patents

Description

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

Inks containing metal particles have been used to form electrical circuits by utilizing the conductivity of the metal particles used, but in recent years they have also come to be used in applications that express a metallic feel, such as Christmas cards. In particular, there is a need to give a metallic image a color tone, i.e., to record a "color metallic image." In order to record a color metallic image, an inkjet recording method has been proposed in which an ink containing silver particles and an ink containing a pigment are applied in sequence to a recording medium (see Patent Document 1). In addition, an inkjet recording method has been proposed in which an ink containing silver particles, a clear ink, and an ink containing a coloring material are applied in sequence to a recording medium (see Patent Document 2).

JP 2012-121279 A JP 2012-206481 A

The present inventors recorded a color metallic image using the inkjet recording method described in Patent Document 1, and examined the resulting image. As a result, they found that thin film interference occurs in the area where the silver layer formed of silver particles and the color material layer formed of the color material overlap, and that the image may have a different color from the color material. They also examined an image recorded by applying three types of ink to a recording medium in the order described in Patent Document 2. As a result, they found that thin film interference was unlikely to occur, but that when an image such as a thin line is recorded with ink containing a color material, the ink bleeds and the image quality is impaired. Note that ink bleed can occur in more than just thin lines, but since the deterioration of image quality due to bleed is particularly easy to recognize in images such as thin lines, in this invention, this problem is referred to as "fine line quality" for convenience.

Therefore, the object of the present invention is to provide an inkjet recording method and an inkjet recording device that can suppress thin-film interference and record color metallic images with good fine-line quality.

The above object can be achieved by the present invention, which is described below. That is, an inkjet recording method according to the present invention is an inkjet recording method for recording an image on a recording medium by ejecting ink from an inkjet recording head, comprising the steps of: applying a first ink to the recording medium; applying a second ink to the recording medium so as to overlap at least a portion of an area to which the first ink has been applied; and applying a third ink to the recording medium so as to overlap at least a portion of an area to which the first ink and the second ink have been applied, wherein the first ink is an ink containing silver particles, and the second ink contains a resin and an additive. and the third ink does not contain a colorant, and the additive is selected from the group consisting of (1) a compound having an ethylene oxide group, a hydroxy group, or an anionic group, and a siloxane structure, and (2) a compound having an ethylene oxide group, a hydroxy group, or an anionic group, and a perfluoroalkyl structure, and when the additive is a compound having an ethylene oxide group and the siloxane structure among (1), the ethylene oxide group is located in a side chain of the siloxane structure, and the third ink is an ink containing a colorant.

The present invention provides an inkjet recording method and inkjet recording device that can suppress thin-film interference and record color metallic images with good fine-line quality.

1A and 1B are diagrams illustrating an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, in which FIG. 1A is a perspective view of a main portion of the inkjet 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 dissociated ions, but for convenience, it is expressed as "containing a salt." Physical property values are values at room temperature (25°C) unless otherwise specified.

As described in Patent Document 1, a color metallic image can be recorded by sequentially applying an ink containing silver particles and an ink containing a coloring material to a recording medium. The color metallic image recorded in this way has a layer structure in which a silver layer having no saturation and a coloring material layer are formed in this order on the recording medium. The silver layer formed by silver particles exhibits a metallic luster, resulting in a glossy metallic image. In addition, light incident on the color metallic image passes through the coloring material layer, is reflected by the silver layer, and passes through the coloring material layer again to express the color tone of the coloring material contained in the ink. In other words, the metallic feel, i.e., glossiness, is expressed by the silver layer reflecting light, and color development is expressed by the light passing through the coloring material layer. In this way, an image having "color development" in this invention refers to an image in which the specularly reflected light from the image exhibits the color tone of the coloring material of the third ink, rather than colorless silver. Specularly reflected light is light that is reflected at an angle (reflection angle) that is the same as the angle (incidence angle) between the normal direction of the image and the incident direction of the light.

Thin film interference is a phenomenon in which light reflected from the surface of a film interferes with light reflected from a lower layer of the film, causing light of specific wavelengths to constructively or destructively interfere with each other, resulting in the reflected light appearing colored. The constructive and destructive wavelengths vary depending on the optical path difference between the light reflected from the surface of the film and the light reflected from the lower layer. The optical path difference depends on the thickness of the film and the refractive index of the material that forms the film. If the surface of the film is very uneven, interference light corresponding to various optical path differences is generated, and the colors of these interference lights mix together and are whitened, so the coloring of the reflected light tends to be reduced. Conversely, if the surface of the film is smooth with few irregularities, light of specific wavelengths is significantly reflected, and the coloring of the reflected light is clearly noticeable.

In the inkjet recording method, ink droplets ejected from a recording head adhere to a recording medium while maintaining the shape of dots, and an image is recorded. When an image is recorded in this manner, the surface becomes uneven and irregular due to steps caused by overlapping dots and steps caused by the presence or absence of dots. It is believed that a phenomenon similar to that described above will occur when an ink containing a coloring material is applied to a silver layer that has already been formed on a recording medium with an ink containing silver, and therefore it is presumed that irregularities will also occur on the surface of the coloring material layer formed on the silver layer.

For example, when ink containing a yellow pigment is ejected from a recording head onto a silver layer formed on a recording medium, a colorant layer made of yellow pigment is formed on the silver layer. Because there are many irregularities on the surface of this colorant layer, it was expected that the reflected light would be whitened and become colorless, and that the regular reflected light from the color metallic image would pass through the colorant layer made of yellow pigment, appearing to be colored yellow.

However, the inventors' investigations revealed that the specularly reflected light from the color metallic image recorded in the above manner has a different color tone from the color material. When the color metallic image in which this phenomenon occurred was observed in detail, it was found that the dots of the ink containing the color material were connected on the silver layer to form a color material layer with a smooth surface with few irregularities. It was found that in the area where this color material layer with a smooth surface was formed, thin film interference occurred, and reflected light was recognized to have a different color tone from the color material.

The reason why the surface of the colorant layer formed on the silver layer has few irregularities is because silver has a high surface energy. When ink containing colorant is applied on top of the silver layer, the dots quickly blend into the silver layer and spread widely, easily connecting with adjacent dots. This forms a colorant layer with a smooth surface with few irregularities, which is thought to result in thin film interference.

The inventors have investigated the provision of a resin layer between the silver layer and the color material layer in order to suppress the occurrence of thin film interference in color metallic images. By interposing a resin layer, the silver layer and the color material layer are not in direct contact with each other. This makes it difficult for the dots of ink containing the color material applied to the silver layer to wet and spread, forming a color material layer with a surface with many irregularities, and it has been found that thin film interference is suppressed.

As described above, if a color metallic image is recorded by applying a first ink containing silver particles, a second ink containing a resin but no coloring material, and a third ink containing a coloring material to a recording medium in this order, the presence of the resin layer can suppress thin film interference. However, as a result of the inventors' investigations, it was found that when the second ink containing a resin is used to record a color metallic image, another problem occurs. Specifically, it was found that when the third ink is applied on top of the resin layer to record an image such as a fine line, ink bleeding occurs and image quality is impaired. Such a decrease in fine line quality is a problem that occurs specifically when a third ink containing a resin but no coloring material is used to record a color metallic image.

The silver particles used in inkjet inks have a particle size of several tens to several hundreds of nanometers, taking into consideration ejection stability. The silver layer formed on the recording medium by fusing silver particles with such particle sizes is not a uniform layer in which the silver particles are completely fused, but is fused while maintaining the particle shape to a certain extent, so there are pores in the silver layer. If the third ink is applied on the silver layer without applying the second ink, the liquid components of the third ink will penetrate through the pores in the silver layer to the recording medium, so that the ink bleeding is unlikely to cause a decrease in fine line quality, but thin film interference will occur. In contrast, thin film interference can be suppressed by applying the third ink after applying the second ink containing resin on the silver layer. However, in this case, the resin of the second ink will enter the pores of the silver layer and clog the pores, making it difficult for the third ink to penetrate into the recording medium, and the third ink will overflow on the silver layer, causing ink bleeding and decreasing fine line quality.

The present inventors have investigated the composition of the second ink in light of the cause of the above-mentioned deterioration in fine line quality caused by the second ink for forming the resin layer. As a result, they have found that it is sufficient to add a specified additive to the second ink. This additive is (1) a compound having an ethylene oxide group, a hydroxy group, or an anionic group, and a siloxane structure, or (2) a compound having an ethylene oxide group, a hydroxy group, or an anionic group, and a perfluoroalkyl structure. Hereinafter, these compounds may be referred to as "specified additives."

By using the second ink containing a specific additive, and the first and third inks, thin film interference is suppressed and color metallic images with good fine line quality can be recorded. The above additives have hydrophilic groups (ethylene oxide groups, hydroxyl groups, anionic groups) and hydrophobic groups (siloxane structure, perfluoroalkyl structure) in their molecules, and have the property of being easily oriented to the interface. After the dots of the second ink are attached to the silver layer, the specific additives are oriented near the surface of the dots. Compared to the hydrocarbon structure, which is the hydrophobic group in the hydrocarbon surfactant, which is a general-purpose additive for inkjet water-based inks, the siloxane structure and perfluoroalkyl structure have weak intermolecular forces. For this reason, the surface energy of the dots of the second ink in which the specific additives are oriented near the surface is significantly reduced, and the dots of the second ink that are subsequently attached are difficult to connect, and the ink is fixed with a certain distance between the multiple dots. In the resin layer formed in this way, there are gaps corresponding to the distance between dots, and the liquid components of the third ink applied later can penetrate smoothly through these gaps, making it less likely for the ink to overflow and bleeding to occur, and preventing a decrease in fine line quality.

It is known that thin film interference is a phenomenon that is likely to occur when a resin layer is present, even if a silver layer is not present. For example, if an ink containing a coloring material is applied to a recording medium, and then an ink containing a resin but no coloring material is applied, an image is formed with a resin layer on the surface, resulting in thin film interference. On the other hand, thin film interference does not occur if the image has a coloring material layer on the surface. In contrast, when a silver layer is present, thin film interference occurs, unlike the previous case, even if the image has a coloring material layer on the surface. This is because a silver layer with high surface energy is present between the coloring material layer and the recording medium.

<Inkjet recording method and inkjet recording apparatus>
The inkjet recording method of the present invention is a method for ejecting ink from an inkjet recording head to record an image on a recording medium. Specifically, the method includes a step of applying a first ink containing silver particles to the recording medium, a step of applying a second ink containing a resin and a predetermined additive and not containing a coloring material to the recording medium, and a step of applying a third ink containing a coloring material to the recording medium. The inkjet recording apparatus of the present invention is used to carry out the above inkjet recording method, and includes a means for applying the first ink, the second ink, and the third ink to the recording medium in this order to record an image on the recording medium. The first ink, the second ink, and the third ink are applied to the recording medium such that at least a portion of the areas to which they are applied overlap each other.

Methods of ejecting ink from an inkjet recording head include a method of applying mechanical energy to the ink and a method of applying thermal energy to the ink. In the present invention, it is preferable to adopt a method of ejecting ink by applying thermal energy to the ink. In the recording method of the present invention, there is no need to carry out a step of applying a reaction liquid that reacts with each ink, or a step of irradiating with active energy rays such as ultraviolet rays or electron beams. In addition, it is preferable to apply each ink directly to a recording medium (preferably a recording medium having permeability).

Figure 1 is a diagram showing an example of an inkjet recording device used in the inkjet recording method 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 a 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 thereon. 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 the conveying means (not shown).

It is preferable that the time difference between the step of applying the first ink to the recording medium and the step of applying the second ink to the recording medium is 1 second or more. This time difference can be said to be the time from when the first ink is applied to the recording medium until the second ink applied on top of it comes into contact with the first ink. If the time difference is less than 1 second, the time difference between the application of the first ink and the second ink in layers is short, so that the ink containing the resin is applied in a state in which the silver particles are not fused. Then, the resin gets between the multiple silver particles, making it difficult for the silver particles to fuse. In the silver layer formed in this way, there are silver particles that are not fused to other silver particles and have a particle diameter of several tens to several hundreds of nm, and therefore have the property of generating surface plasmon resonance. For this reason, when light hits the image, the reflectance of light from the silver layer is low, and a high level of glossiness may not be obtained sufficiently. If the time difference is too long, the productivity when recording an image may decrease, so it is preferable that it is 600 seconds or less. It is more preferable that the time difference is 3 seconds or more and 300 seconds or less, and especially preferable that it is 3 seconds or more and 90 seconds or less.

This describes a method for setting the time difference between the process of applying a first ink to a recording medium and the process of applying a second ink to the recording medium within a predetermined range, i.e., for applying the first ink and the second ink to the recording medium with a predetermined time difference. For example, when performing one-way recording using a serial method in which an image is recorded while moving the recording head in the main scanning direction, the following methods (1) to (3) can be used. A unit area can be set as any area, such as one pixel or one band.

(1) A recording head having rows of ejection openings for the first ink and the second ink arranged in a direction perpendicular to the main scanning direction is used. After the first ink is applied to a unit area of the recording medium, the second ink is applied to the unit area without transporting the recording medium.

(2) A print head is used that has nozzle rows for the first ink and the second ink arranged in a direction perpendicular to the main scanning direction. The first ink uses nozzles that are part of the nozzle row on the upstream side in the sub-scanning direction, and the second ink uses nozzles that are part of the nozzle row on the downstream side in the sub-scanning direction. Then, the first ink and the second ink are applied to a unit area on the print medium while the print medium is transported.

(3) A printhead is used that has an array of nozzles for the first ink on the upstream side in the sub-scanning direction and an array of nozzles for the second ink on the downstream side. The first ink uses nozzles that are part of the upstream nozzle array, and the second ink uses nozzles that are part of the downstream nozzle array. Then, while transporting the print medium, the first ink and the second ink are applied with a time difference of at least the time it takes to transport the print medium.

The above description has been given with an example of one-way recording. Of course, in the present invention, even when performing bidirectional recording, any method may be used as long as it is possible to apply two types of ink to the recording medium with a predetermined time difference. The time difference between the process of applying the second ink to the recording medium and the process of applying the third ink to the recording medium is not particularly limited, but it is preferable that it is, for example, 0.1 seconds or more and 600 seconds or less.

In the case of a clear ink containing a resin, applying the ink to a unit area of a recording medium all at once is more advantageous in terms of image clarity, because a resin layer with a flatter surface is formed when the ink is applied to the unit area of the recording medium in a single application, rather than in multiple applications. However, in the present invention, it is preferable to apply the second ink to the unit area of the recording medium in multiple applications. When applying the ink to a unit area of the recording medium in multiple applications, the number of main scans of the recording head (number of recording passes) can be adjusted. In the present invention, the third ink is applied following the second ink. When the second ink is applied to a unit area of the recording medium at once, the second ink is applied to the adjacent unit area before the previously applied dot of the second ink dries and a stable film is formed. For this reason, the previous dot is likely to come into contact with the subsequent dot before the surface energy of the previous dot is sufficiently reduced. Then, the effect of the specified additive to make it difficult to connect multiple dots is not fully exerted, making it difficult to form a resin layer with gaps, making it easier for bleeding due to ink overflow to occur, and the degree to which the deterioration of fine line quality is suppressed may be reduced. It is more preferable that the application of the third ink to the unit area of the recording medium is performed in four or more separate steps. From the viewpoint of productivity, it is preferable that the application of the third ink to the unit area of the recording medium is performed in twelve or less separate steps.

After the first ink, the second ink, and the third ink are applied to the recording medium to record a color metallic image, a process of laminating the laminate material may be performed. By laminating, the gloss, color development, ozone resistance, and other fastness of the color metallic image can be maintained at a high level for a long period of time. Examples of the laminate material include sheet- or film-shaped resins. Examples of the resin include polyesters such as polyethylene terephthalate, polyolefins such as polyethylene and polypropylene, polystyrene, vinyl chloride, and polyamides. It is also possible to use a laminate material in which an adhesive layer is provided on one side of a sheet or film formed of resin. The thickness of the laminate material is preferably 10 μm or more and 300 μm or less. The lamination process may be performed by a laminator incorporated in the recording device, or may be performed by a device separate from the recording device, and may be performed by applying heat or pressure.

<First Ink>
The first ink is an ink containing silver particles, and is preferably an aqueous ink containing at least water as an aqueous medium. The first ink does not need to be an active energy ray curable ink, and therefore does not need to contain a monomer having a polymerizable group. The components constituting the first ink are described below.

(Silver particles)
The coloring material of the first ink is silver particles. The silver particles are composed of silver atoms. The silver particles may be composed of other metal atoms, oxygen atoms, sulfur atoms, carbon atoms, and the like in addition to silver atoms. However, the ratio (%) of silver atoms in the silver particles is preferably 50.0% by mass or more and 100.0% by mass or less. The content (% by mass) of silver particles in the first ink is preferably 2.00% by mass or more and 15.00% by mass or less, and more preferably 2.00% by mass or more and 8.00% by mass or less, based on the total mass of the ink. The first ink may or may not further contain a coloring material other than the silver particles (described as "other coloring material"). The content (% by mass) of the other coloring material is preferably 0.00 times or more and 5.00 times or less, and more preferably 0.00 times or more and 3.00 times or less, in terms of the mass ratio to the content (% by mass) of the silver particles. The mass ratio is more preferably 0.00 times or more and 0.10 times or less.

Examples of methods for producing silver particles include a method in which silver lumps are pulverized in a pulverizer such as a ball mill or a jet mill (pulverization method), and a method in which silver ions or silver complexes are reduced and agglomerated with a reducing agent (reduction method). In the present invention, it is preferable to use silver particles produced by the reduction method from the viewpoints of ease of controlling the particle size of the silver particles and dispersion stability of the silver particles.

It is preferable to use silver particles dispersed using a dispersant such as a surfactant or resin, and resin is more preferable as the dispersant. The content (mass %) of the dispersant 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.

As a dispersant for silver particles, various surfactants such as anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants can be used. Examples of anionic surfactants include fatty acid salts, alkyl sulfate ester salts, alkylaryl sulfonates, alkyl diaryl ether disulfonates, dialkyl sulfosuccinates, alkyl phosphates, naphthalene sulfonate-formaldehyde condensates, polyoxyethylene alkyl ether sulfates, polyoxyethylene alkyl phosphate ester salts, and glycerol borate fatty acid esters. Examples of nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethylene polyoxypropylene block copolymers, sorbitan fatty acid esters, glycerin fatty acid esters, polyoxyethylene fatty acid esters, polyoxyethylene alkylamines, fluorine-based compounds, and silicone-based compounds. Examples of cationic surfactants include alkylamine salts, quaternary ammonium salts, alkylpyridinium salts, and alkylimidazolium salts. Examples of amphoteric surfactants include alkylamine oxides and phosphatidylcholines. In particular, it is preferable to use at least one surfactant selected from the group consisting of anionic surfactants and nonionic surfactants as the dispersant. As the anionic surfactant, it is preferable to use polyoxyethylene alkyl ether sulfate. As the nonionic surfactant, it is preferable to use polyoxyethylene alkyl ether. When a surfactant is used as the dispersant, the content (mass%) of the dispersant in the first ink is preferably 0.02 to 1.00 times the mass ratio of the content (mass%) of the silver particles.

In addition, as a dispersant for silver particles, a resin having a unit with an anionic group and a unit without an anionic group can be used. Examples of the resin skeleton include vinyl resin, ester resin, amino resin, acrylic resin, epoxy resin, urethane resin, ether resin, amide resin, phenol resin, silicone resin, and fluorine resin. When a resin is used as a dispersant, the content (mass%) of the dispersant in the first ink is preferably 0.05 to 1.00 times the mass% of the silver particle content (mass%).

The volume-based cumulative 50% particle diameter (d50 _S ) of the silver particles is preferably 100 nm or less, and more preferably 50 nm or less. The volume-based cumulative 50% particle diameter refers to the diameter of a particle 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. When the cumulative 50% particle diameter is small, the ratio of silver atoms present on the surface of the silver particles to the number of silver atoms per unit mass is high. When the ratio of silver atoms that are easily mobile in the silver particles increases, silver atoms present on the surface of a silver particle tend to form metal bonds with silver atoms present on the surface of the surrounding silver particles, and the silver particles tend to fuse together. Therefore, when d50 _S is 50 nm or less, the gloss tends to improve. The cumulative 50% particle diameter is preferably 1 nm or more, and more preferably 10 nm or more.

The cumulative 50% particle diameter based on volume of silver particles can be measured as follows, using the first ink or the dispersion of silver particles diluted with water as a sample. After applying the sample to a silicon substrate, the water is removed to prepare a sample. Using the obtained sample, 3,000 or more silver particles are observed with a scanning electron microscope (SEM), a transmission electron microscope (TEM), or the like, and the image is processed to calculate the particle diameter as defined above. In the examples described below, after observing the silver particles, the particle diameter is calculated using image analysis and measurement software (product name "WinROOF2015", manufactured by Mitani Shoji). The particle diameter of silver particles can also be measured by dynamic light scattering for the ink or dispersion. However, since the measured value is easily affected by aggregation and the like and is prone to fluctuate, it is preferable to sufficiently dilute with water when measuring by dynamic light scattering.

[Amount of silver particles applied]
The amount (g/ ^m2 ) of silver particles in the first ink applied per unit area of the recording medium is preferably 0.30 g/m2 or ^more and 1.00 g/m2 or ^less . The amount of silver particles applied can be adjusted by the content of silver particles in the first ink, the amount of the first ink applied, etc.

(Surfactant)
The first ink preferably contains a surfactant other than the surfactant that can be used as a dispersant for silver particles, and the content (mass %) of the surfactant other than the surfactant used as a dispersant for silver particles in the first ink is preferably 0.10 mass % or more and 2.00 mass % or less based on the total mass of the ink.

Examples of surfactants include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants. Among these, nonionic surfactants such as ethylene oxide adducts of acetylene glycol and polyoxyethylene alkyl ethers are preferred.

(Polyhydric alcohols)
The first ink preferably contains a polyhydric alcohol. Polyhydric alcohols are compounds in which one or more carbon atoms constituting a saturated hydrocarbon chain are substituted with a plurality of hydroxyl groups. Examples of polyhydric alcohols include dihydric alcohols having 3 to 7 carbon atoms, such as 1,2-propanediol, 1,2-butanediol, 2,3-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,2-hexanediol, 1,2,6-hexanetriol, and 1,2-heptanediol; polyhydric alcohols, such as glycerin and trimethylolpropane; and sugar alcohols, such as xylitol, D-glucose, and sorbitol. Of these, trihydric to hexahydric polyhydric alcohols or sugar alcohols are preferred, and pentahydric or hexahydric polyhydric alcohols or sugar alcohols are more preferred. The content (mass %) of polyhydric alcohols in the first ink is preferably from 1.00 mass % to 20.00 mass % based on the total mass of the ink, and more preferably from 3.00 mass % to 10.00 mass %.

(Aqueous medium)
The first ink preferably contains water or an aqueous medium which is a mixed solvent of water and a water-soluble organic solvent. The first ink is preferably an ink (aqueous ink) containing water as an aqueous medium. As the water, deionized water or ion-exchanged water is preferably used. The content (mass%) of water in the first ink is preferably 50.00 mass% or more and 95.00 mass% or less based on the total mass of the ink. The water-soluble organic solvent is not particularly limited as long as it is water-soluble, and any of alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing polar solvents, and sulfur-containing polar solvents can be used. The content (mass%) of the water-soluble organic solvent in the first ink is preferably 3.00 mass% or more and 50.00 mass% or less based on the total mass of the ink. When applied to an inkjet recording method, if the content of the water-soluble organic solvent is out of the above range, reliability such as sticking resistance and ejection stability may be slightly reduced.

(Other ingredients)
In addition to the above components, the first ink may contain a water-soluble organic compound that is solid at 25° C., such as urea or a derivative thereof. In addition to the above components, the first ink may contain various additives, such as an antifoaming agent, a pH adjuster, an antirust agent, an antiseptic, an antifungal agent, an antioxidant, an antireducing agent, and a chelating agent, as necessary.

(Physical Properties of First Ink)
The viscosity of the first ink at 25° C. is preferably from 1 mPa·s to 6 mPa·s, and more preferably from 1 mPa·s to 4 mPa·s. The surface tension of the first ink at 25° C. is preferably from 10 mN/m to 60 mN/m, and more preferably from 20 mN/m to 50 mN/m, and particularly preferably from 25 mN/m to 40 mN/m.

<Second Ink>
The second ink is an ink that contains a resin and a specific additive, but does not contain a coloring material, and is preferably an aqueous ink that contains at least water as an aqueous medium. The second ink does not need to be an active energy ray curable ink, and therefore does not need to contain a monomer having a polymerizable group. The components that make up the second ink are described below.

(resin)
The second ink contains a resin. The resin may be either a water-soluble resin or resin particles. In this specification, the term "water-soluble resin" refers to a resin that exists in a liquid medium in a state in which it does not form particles whose particle diameter can be measured when the resin is neutralized with a base whose acid value is 1.0 times (molar ratio) or more. In addition, the term "resin particles" refers to a resin that exists in a liquid medium in a state in which it forms particles whose particle diameter can be measured when the resin is neutralized with a base whose acid value is 1.0 times (molar ratio) or more. The particle diameter of the resin is measured by dynamic light scattering.

Whether a resin is a water-soluble resin or a resin particle (water-dispersible 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 10 times (volume basis) with pure water to prepare a sample. 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 units constituting the resin can be analyzed by nuclear magnetic resonance spectroscopy ( ¹ H-NMR), pyrolysis gas chromatography, etc. The form of the acrylic resin (random copolymer, block copolymer) can be analyzed by pyrolysis gas chromatography with multivariate analysis (EGA-GC/MS), etc.

Examples of resins include acrylic resins, urethane resins, and olefin resins, with acrylic resins and urethane resins being preferred, and acrylic resins being even more preferred. Examples of olefin resins include polyethylene and polypropylene. The water-soluble resins and resin particles that can be contained in the second ink can have the same configuration, except that their state of existence in the ink differs mainly due to their dissolution state. The water-soluble resin exists in a dissolved state in the ink, and the resin particles exist in a dispersed state in the ink, i.e., as a resin emulsion.

The following provides detailed explanations of resins (water-soluble resins, resin particles) for acrylic resins and urethane resins. "(Meth)acrylic acid" refers to "acrylic acid, methacrylic acid," and "(meth)acrylate" refers to "acrylate, methacrylate."

[Constituent units of acrylic resin]
The acrylic resin is a resin obtained by (co)polymerizing acrylic monomers such as (meth)acrylic acid, (meth)acrylic ester, etc. Preferably, a resin composed of a unit having an acid group and a unit not having an acid group obtained by copolymerizing a monomer having an acid group and a monomer not having an acid group is used.

Examples of monomers having an acid group that become units having an acid group by polymerization include monomers having a carboxylic acid group such as (meth)acrylic acid, maleic acid, itaconic acid, and fumaric acid; monomers having a sulfonic acid group such as styrenesulfonic acid; monomers having a phosphonic acid group such as 2-ethyl phosphonate (meth)acrylic acid; and anhydrides and salts of these monomers. Examples of salts include alkali metal salts such as lithium, sodium, and potassium, ammonium salts, and organic ammonium salts. As the monomer having an acid group, a monomer having a carboxylic acid group is preferable, and (meth)acrylic acid is more preferable.

Examples of monomers that do not have an acid group and that become units that do not have an acid group through polymerization include monomers that have a hydroxy group, such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 3-methyl-5-hydroxypentyl (meth)acrylate; monomers that have an aromatic group, such as styrene, α-methylstyrene, and benzyl (meth)acrylate; and alkyl (meth)acrylates, such as methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. As monomers that do not have an acid group, monomers that have an aromatic group and alkyl (meth)acrylates are preferred, and monomers that have an aromatic group are even more preferred.

[Constituent units of urethane resins]
The urethane resin is a resin synthesized by using at least a polyisocyanate and a component that reacts with it (a polyol or a polyamine), and if necessary, a crosslinking agent or a chain extender. Preferably, a urethane resin obtained by polymerizing a polyisocyanate, a polyol that does not have an acid group, and a polyol that has an acid group is used.

Polyisocyanates are compounds that have two or more isocyanate groups in their molecular structure. Examples of polyisocyanates include aliphatic polyisocyanates and aromatic polyisocyanates.

Examples of aliphatic polyisocyanates include polyisocyanates having a chain structure such as tetramethylene diisocyanate and hexamethylene diisocyanate; and polyisocyanates having a cyclic structure such as isophorone diisocyanate and hydrogenated xylylene diisocyanate. Examples of aromatic polyisocyanates include tolylene diisocyanate and 1,5-naphthylene diisocyanate. Of these, it is preferable that the polyisocyanate is isophorone diisocyanate.

A polyol is a compound that has two or more hydroxyl groups in its molecular structure. Examples of polyols include polyether polyols, polyester polyols, polycarbonate polyols, and other polyols that do not have acid groups; and polyols that have acid groups. Also, polyamines are compounds that have two or more "amino groups, imino groups" in their molecular structure.

Examples of polyether polyols include addition polymers of alkylene oxides and polyols; glycols such as (poly)alkylene glycols; and the like. Examples of polyester polyols include acid esters; and examples of polycarbonate polyols include alkanediol-based polycarbonate diols. The number average molecular weight of the polyol that does not have an acid group is preferably 400 or more and 4,500 or less.

Examples of polyols having an acid group include those having an acid group such as a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, or a phosphonic acid group. The acid group may form a salt, and examples of the salt include alkali metal salts such as lithium, sodium, and potassium, ammonium salts, and organic ammonium salts. As monomers having an acid group, polyols having a carboxylic acid group such as dimethylol acetic acid, dimethylol propionic acid, dimethylol butanoic acid, and dimethylol butanoic acid are preferred, and dimethylol propionic acid and dimethylol butanoic acid are more preferred.

Examples of polyamines include monoamines with multiple hydroxyl groups, such as dimethylolethylamine and diethanolmethylamine; bifunctional polyamines, such as ethylenediamine and propylenediamine; and trifunctional or higher polyamines, such as diethylenetriamine and triethylenetetramine. For convenience, compounds with multiple hydroxyl groups and one "amino group or imino group" are also listed as "polyamines."

When synthesizing urethane resins, crosslinking agents and chain extenders can be used. Crosslinking agents are usually used when synthesizing prepolymers, and chain extenders are used when carrying out a chain extension reaction on prepolymers that have already been synthesized. Basically, crosslinking agents and chain extenders can be appropriately selected from water, polyisocyanates, polyols, polyamines, etc., depending on the purpose, such as crosslinking or chain extension.

[Physical properties of resin]
The acid value of the acrylic resin is preferably 0 mgKOH/g or more and 300 mgKOH/g or less, more preferably 100 mgKOH/g or more and 300 mgKOH/g or less. The acid value of the urethane resin is preferably 45 mgKOH/g or more and 70 mgKOH/g or less. The acid value of the resin can be measured by potentiometric titration. The weight average molecular weight of the acrylic resin is preferably 1,000 or more and 30,000 or less, more preferably 3,000 or more and 15,000 or less. The weight average molecular weight of the urethane resin is preferably 8,000 or more and 22,000 or less. The weight average molecular weight of the resin can be measured as a polystyrene equivalent value by gel permeation chromatography.

[Resin content]
The content (mass%) of the resin in the second ink is preferably 0.10% by mass or more and 20.00% by mass or less, and more preferably 1.00% by mass or more and 10.00% by mass or less, based on the total mass of the ink. When a water-soluble resin is used, the content (mass%) of the water-soluble resin in the second ink is preferably 0.10% by mass or more and 10.00% by mass or less, and more preferably 0.10% by mass or more and 5.00% by mass or less, based on the total mass of the ink. When resin particles are used, the content (mass%) of the resin particles in the second ink is preferably 0.10% by mass or more and 10.00% by mass or less, and more preferably 0.10% by mass or more and 5.00% by mass or less, based on the total mass of the ink. The content (mass%) of the resin in the second ink is preferably the same as or greater than the content (mass%) of the resin in the third ink.

[Form of Water-Soluble Resin]
The water-soluble resin is preferably an acrylic random copolymer containing a unit having an acid group and a unit not having an acid group.

[Morphology and physical properties of resin particles]
The resin particles are preferably made of an acrylic resin, since they can suppress thin film interference at a higher level. In particular, the resin particles are preferably made of a core containing a unit having no acid group and a shell containing a unit having an acid group. In the resin particles having a core-shell structure, the resins constituting the core and the shell have different refractive indices. When light is applied to a resin layer containing resin particles having a core-shell structure, the light is reflected by the resin constituting the shell and the resin constituting the core, respectively, and interference light corresponding to a plurality of optical path differences is generated. The colors of these interference lights are mixed and the light is more likely to be whitened, so that thin film interference can be suppressed at a higher level. In addition, when the third ink is applied on the resin layer, the water of the third ink is drawn in by the hydrophilicity of the shell containing the unit having an acid group, so that the liquid components can easily penetrate, and a better fine line quality can be obtained. In addition, the core containing the unit having no acid group contributes to maintaining the shape of the resin particles after the second ink containing the resin particles is attached to the silver layer, and a resin layer having voids derived from the resin particles can be formed. The liquid component of the third ink applied later can smoothly permeate through these voids, resulting in superior fine line quality.

The volume-based cumulative 50% particle diameter (d50) of the resin particles is preferably 50 nm or more and 300 nm or less. The volume-based cumulative 50% particle diameter refers to the diameter of the particle that is 50% of the total volume of the measured particles when integrated from the small particle diameter side in the particle diameter integration curve.

[Combined use of water-soluble resin and resin particles]
When the second ink, which does not contain resin particles but contains a water-soluble resin, is applied onto the silver layer, a resin layer is formed by the water-soluble resin. When the third ink is applied onto this resin layer, a part of the water-soluble resin constituting the resin layer dissolves again in the liquid component of the third ink, forming a color material layer. In this case, the interface between the resin layer and the color material layer has a mixed part and a clearly separated part, and unevenness exists at the interface. The light transmitted through the color material layer is scattered by the unevenness of the interface, and interference light corresponding to various optical path differences is generated, and the colors of the interference light are mixed and whitened, so that the degree to which thin film interference is suppressed is high. However, since the liquid component of the third ink is difficult to penetrate into the resin layer formed by the water-soluble resin, the degree to which the deterioration of fine line quality is suppressed may be low.

On the other hand, when the second ink, which does not contain water-soluble resin but contains resin particles, is applied on the silver layer, a resin layer is formed by the resin particles. When the third ink is applied on this resin layer, the liquid components of the third ink quickly penetrate through the gaps between the resin particles in the resin layer, resulting in better fine line quality. However, when the third ink is applied to the resin layer formed by the resin particles, the liquid components quickly penetrate as described above, and more color material remains on the resin layer to form a color material layer. In this case, the interface between the resin layer and the color material layer is likely to be clearly separated. When light is incident on an image with such a layer structure, the degree of suppression of thin film interference may be reduced when a situation occurs in which the light reflected from each layer reinforces each other.

By using the second ink containing both the water-soluble resin and the resin particles, it is possible to achieve a high level of both the suppression of thin film interference by the water-soluble resin and the expression of excellent fine line quality by the resin particles. When the water-soluble resin and the resin particles are used, the content (mass%) of the resin particles in the second ink is preferably 0.30 times or more and 0.70 times or less in mass ratio to the content (mass%) of the resin. If the mass ratio is less than 0.30 times, the resin particles in the resin forming the resin layer are too few, and the voids in the resin layer are likely to be small, so that bleeding due to overflow of the ink is likely to occur, and the degree to which the deterioration of the fine line quality is suppressed may be low. On the other hand, if the mass ratio is more than 0.70 times, the resin particles in the resin forming the resin layer are too many, and the voids in the resin layer are likely to be large, so that the interface between the resin layer and the color material layer is likely to be clearly separated, and the degree to which the thin film interference is suppressed may be low.

[Amount of resin applied]
The amount of resin of the second ink applied per unit area of the recording medium (g/m ² ) is preferably 0.01 g/m ² or more and 1.00 g/m ² or less. The amount of resin applied can be adjusted by the content of the resin in the second ink, the amount of the second ink applied, etc. The amount of resin of the second ink applied per unit area of the recording medium (g/m ² ) is preferably 0.15 times or more and 2.00 times or less in ratio to the amount of silver particles applied in the first ink (g/m ² ). If the ratio is less than 0.15 times, the amount of resin of the second ink is too small relative to the silver particles, so that the third ink applied to the resin layer thereafter is likely to directly contact the silver layer, and the degree of suppression of thin film interference may be reduced. On the other hand, if the ratio exceeds 2.00, the amount of resin in the second ink is too large relative to the silver particles, so that not only the pores in the silver layer but also the voids in the resin layer are likely to become clogged, and the degree to which the deterioration of fine line quality is suppressed may be reduced.

(Additives)
The second ink contains a predetermined additive. The additive is (1) a compound having an ethylene oxide group, a hydroxy group, or an anionic group, and a siloxane structure, or (2) a compound having an ethylene oxide group, a hydroxy group, or an anionic group, and a perfluoroalkyl structure. Hereinafter, (1) may be referred to as a "siloxane compound," and (2) may be referred to as a "perfluoroalkyl compound." The content (mass %) of the predetermined additive 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.

A siloxane compound is a compound having an ethylene oxide group, a hydroxyl group, or an anionic group, and a siloxane structure (a repeating unit of Si-O). The ethylene oxide group, the anionic group, and the hydroxyl group are hydrophilic groups, and the siloxane structure is a hydrophobic group. Siloxane compounds are classified into side chain type, single end type, double end type, ABN type, etc., according to the bonding position of the hydrophilic group to the siloxane structure, and it is preferable to use a side chain type siloxane compound.

A compound having a perfluoroalkyl group is a compound having an ethylene oxide group, a hydroxyl group, or an anionic group, and a perfluoroalkyl structure. The ethylene oxide group, the anionic group, and the hydroxyl group are hydrophilic groups, and the perfluoroalkyl structure is a hydrophobic group. Perfluoroalkyl compounds include those in which the perfluoroalkyl group structure is linear and those in which the perfluoroalkyl group structure is branched.

As the additive, it is preferable to use a siloxane compound because it can provide better fine line quality. Compared with a perfluoroalkyl structure containing a carbon-carbon bond, a siloxane structure containing a carbon-silicon bond has a flexible three-dimensional structure in which the hydrophobic group is more easily oriented to the interface. Therefore, when a siloxane compound is used, the additive is more likely to be oriented to the dot surface more quickly after the dots of the second ink are attached to the silver layer, and better fine line quality is obtained. In particular, it is preferable to use a siloxane compound having a siloxane structure repeat number of 20 or more. The siloxane structure, which is the hydrophobic group of the siloxane compound, interacts hydrophobically with the resin in the second ink, effectively suppressing the penetration of the siloxane compound into the recording medium and efficiently oriented to the dot surface. If the number of repeats of the siloxane structure is small, the hydrophobic interaction is weakened, and the amount of siloxane compound that can be oriented to the dot surface is reduced, and the degree to which the deterioration of fine line quality is suppressed may be reduced. It is preferable that the number of repeats of the siloxane structure in the siloxane compound is 100 or less.

In addition, the specified additive is preferably one having an ethylene oxide group. As mentioned above, by utilizing the hydrophobic interaction between the hydrophobic group of the additive and the resin, the penetration of the additive can be suppressed and the additive can be oriented on the dot surface. On the other hand, if the hydrophobic group of the additive is made longer to generate a strong hydrophobic interaction, the additive becomes too hydrophobic as a whole and is prone to agglomeration in the ink. For this reason, it is preferable to use an additive having an ethylene oxide group, which makes it easier to adjust the hydrophilicity by changing the number of repetitions compared to hydroxyl groups or anionic groups.

Furthermore, it is preferable that the HLB value of a given additive determined by the Griffin method is 8 or more and 16 or less. The HLB value determined by the Griffin method can be calculated from the formula "HLB value = 20 x formula weight of ethylene oxide group of additive / molecular weight of additive". The HLB value determined by the Griffin method is a physical property value that indicates the degree of hydrophilicity or lipophilicity of an additive (compound), and takes a value from 0 to 20. The smaller the HLB value, the higher the lipophilicity, and the larger the HLB value, the higher the hydrophilicity. If the HLB value is less than 8, the additive is too hydrophobic, so the additive is easily aggregated and the additive is difficult to orient on the dot surface, and the degree to which the deterioration of the fine line quality is suppressed may be low. On the other hand, if the HLB value is more than 16, the additive is too hydrophilic, so the additive is also difficult to orient on the dot surface, and the degree to which the deterioration of the fine line quality is suppressed may be low. The HLB value determined by the Griffin method is a physical property value that applies to a compound that does not have an ionic group (a non-ionic compound).

[Amount of additives added]
The amount (g/m ² ) of the predetermined additive of the second ink per unit area of the recording medium is preferably 0.01 g/m ² or more and 1.00 g/m ² or less. The amount of the predetermined additive can be adjusted by the content of the predetermined additive in the second ink, the amount of the second ink applied, etc. The amount (g/m ² ) of the additive of the second ink per unit area of the recording medium is preferably 0.05 times or more in terms of the ratio to the amount (g/m ² ) of the silver particles of the first ink. If the ratio is less than 0.05 times, the amount of additive oriented on the dot surface of the second ink is too small relative to the silver particles, so that the surface energy of the dots does not decrease sufficiently, and the degree to which the deterioration of the fine line quality is suppressed may be low. The ratio is preferably 7.00 times or less, more preferably 1.00 times or less, and particularly preferably 0.50 times or less.

(Aqueous medium)
The second ink preferably contains water or an aqueous medium which is a mixed medium of water and a water-soluble organic solvent. The second ink is preferably an ink (aqueous ink) containing water as an aqueous medium. As the water, deionized water or ion-exchanged water is preferably used. The content (mass%) of water in the second ink is preferably 50.00 mass% or more and 95.00 mass% or less based on the total mass of the ink. The water-soluble organic solvent is not particularly limited as long as it is water-soluble, and any of alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing polar solvents, and sulfur-containing polar solvents can be used. The content (mass%) of the water-soluble organic solvent in the second ink is preferably 3.00 mass% or more and 50.00 mass% or less based on the total mass of the ink. When applied to an inkjet recording method, if the content of the water-soluble organic solvent is out of the above range, reliability such as sticking resistance and ejection stability may be slightly reduced.

(Other ingredients)
In addition to the above components, the second ink may contain water-soluble organic compounds that are solid at 25° C., such as urea or its derivatives, trimethylolpropane, and trimethylolethane. In addition to the above components, the second ink may contain various other components, such as surfactants, defoamers, pH adjusters, rust inhibitors, preservatives, antifungal agents, antioxidants, reduction inhibitors, and chelating agents, as necessary.

(Physical properties of second ink)
The viscosity of the second ink at 25° C. is preferably from 1 mPa·s to 6 mPa·s, and more preferably from 1 mPa·s to 4 mPa·s. The surface tension of the second ink at 25° C. is preferably from 10 mN/m to 60 mN/m, and more preferably from 20 mN/m to 50 mN/m, and particularly preferably from 25 mN/m to 40 mN/m.

<Third Ink>
The third ink is an ink containing a coloring material, and is preferably an aqueous ink containing at least water as an aqueous medium. The third ink does not need to be an active energy ray curable ink, and therefore does not need to contain a monomer having a polymerizable group. The components constituting the third ink are described below.

(Coloring material)
The coloring material may be a dye or a pigment. The content (mass %) of the coloring material in the third ink is preferably 0.05% by mass to 15.00% by mass, and more preferably 0.10% by mass to 10.00% by mass, based on the total mass of the ink.

The dye is preferably an anionic dye. The dye is preferably a compound having at least one selected from the group consisting of an azo skeleton, a phthalocyanine skeleton, an anthrapyridone skeleton, and a xanthene skeleton. In particular, a dye that is stable in solution in the ink and has the property of easily agglomerating on a recording medium is preferred, since this allows both the storage stability of the ink and the color development of the color metallic image to be compatible. Such properties can be adjusted by the skeleton of the dye and the number of anionic groups.

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. Pigment dispersion methods include resin-dispersed pigments that use resin as a dispersant, and self-dispersed pigments in which hydrophilic groups are bonded to the pigment particle surface. In addition, resin-bonded pigments in which resin is chemically bonded to the pigment particle surface, and microencapsulated pigments in which the pigment particle surface is coated with resin or the like can be used.

From the viewpoint of the gloss of the color metallic image, it is preferable to use a resin-dispersed pigment using a dye or a resin dispersant as the color material. Also, from the viewpoint of the color development of the color metallic image, it is preferable to use a pigment as the color material. In order to record a color metallic image with color development, it is important that the interface between the silver layer and the color material layer does not mix with each other on the recording medium and exists separately from each other. In the case of dye ink, it is difficult to form a color material layer separated from the silver layer. This is because dyes that do not have a particle shape are difficult to remain on the silver layer and sink into the silver layer or the recording medium. On the other hand, in the case of pigment ink, it is easy to form a color material layer separated from the silver layer. This is because pigments have a particle shape and tend to remain on the silver layer.

The volume-based cumulative 50% particle diameter (d50) of the pigment is preferably 10 nm or more and 300 nm or less. The volume-based cumulative 50% particle diameter refers to the diameter of the particle 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. When the mass of the particles is constant, the total surface area is large when the particle diameter is small, and the total surface area is small when the particle diameter is large. If the d50 is less than 10 nm, the total surface area of the particles is large and the hydrophobic interaction between the pigment particles tends to be strong, and the third ink may not be able to achieve a high level of storage stability. On the other hand, if the d50 is more than 300 nm, the pigment particle diameter is large, and therefore a high level of color development and glossiness may not be achieved sufficiently.

(Aqueous medium)
The third ink preferably contains water or an aqueous medium which is a mixed medium of water and a water-soluble organic solvent. The third ink is preferably an ink (aqueous ink) containing water as an aqueous medium. As the water, deionized water or ion-exchanged water is preferably used. The content (mass%) of water in the third ink is preferably 50.00 mass% or more and 95.00 mass% or less based on the total mass of the ink. The water-soluble organic solvent is not particularly limited as long as it is water-soluble, and any of alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing polar solvents, and sulfur-containing polar solvents can be used. The content (mass%) of the water-soluble organic solvent in the third ink is preferably 3.00 mass% or more and 50.00 mass% or less based on the total mass of the ink. When applied to an inkjet recording method, if the content of the water-soluble organic solvent is out of the above range, reliability such as sticking resistance and ejection stability may be slightly reduced.

(Other ingredients)
In addition to the above components, the third ink may contain water-soluble organic compounds that are solid at 25° C., such as urea or a derivative thereof, trimethylolpropane, and trimethylolethane. In addition to the above components, the third ink may contain various additives, such as surfactants, resins, defoamers, pH adjusters, rust inhibitors, preservatives, antifungal agents, antioxidants, anti-reducing agents, and chelating agents, as necessary.

The third ink may contain a resin. The resin can be added to the ink for the purposes of (i) stabilizing the dispersion state of the pigment, i.e., as a resin dispersant or its auxiliary, and (ii) improving various properties of the image to be recorded. When the third ink contains a resin, the content (mass %) of the resin in the third ink is preferably 1.20 times or less in mass ratio to the content (mass %) of the coloring material in the third ink. If the mass ratio is more than 1.20 times, not only does the coloring efficiency of the coloring material layer constituting the image tend to decrease, but light scattering by the resin may occur, and a high level of glossiness may not be obtained sufficiently. The mass ratio is preferably 0.10 times or more.

(Physical properties of the third ink)
The viscosity of the third ink at 25° C. is preferably from 1 mPa·s to 6 mPa·s, and more preferably from 1 mPa·s to 4 mPa·s. The surface tension of the third ink at 25° C. is preferably from 10 mN/m to 60 mN/m, and more preferably from 20 mN/m to 50 mN/m, and particularly preferably from 25 mN/m to 40 mN/m.

<Recording media>
Any recording medium may be used, but it is preferable to use a recording medium having permeability, such as plain paper or a recording medium having an ink receiving layer (glossy paper or art paper). In particular, it is preferable to use a recording medium having an ink receiving layer, such as glossy paper, because the recorded image has an excellent metallic feel. Recording media such as glossy paper used in inkjet recording methods usually have an ink receiving layer containing halide ions such as chloride ions. Chloride ions are contained in cationic compounds such as polydiallyldimethylammonium chloride and polyaluminum chloride.

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. Note that "parts" and "%" used to describe amounts of components are by weight unless otherwise specified.

<Preparation of Silver Particle Dispersion>
With reference to the description of Example 2 of JP-A 2010-507727, a dispersion of silver particles was prepared in which the silver particle content was 10.0% and the resin content was 3.0%. The volume-based cumulative 50% particle diameter of the silver particles was 42 nm. The volume-based cumulative 50% particle diameter of the silver particles was measured by the following procedure. First, the dispersion diluted about 2,000 times (by mass) with ion-exchanged water was applied to a substrate formed of a silicon material, and the water was removed by drying to prepare a sample. Next, using the obtained sample, 3,000 or more silver particles were observed with a scanning electron microscope, and image analysis and measurement software (product name "WinROOF2015", manufactured by Mitani Shoji) was used to process the images and calculate the particle diameter.

<Synthesis of Siloxane Compound>
(Method of Analyzing Siloxane Compounds)
The properties of each of the synthesized siloxane compounds having structures represented by general formulas (1) and (2) were determined from the weight average molecular weight calculated by the method described below.

The weight average molecular weight of the siloxane compound was determined by gel permeation chromatography in terms of polystyrene. The siloxane compound was added to tetrahydrofuran, dissolved, and filtered through a membrane filter to prepare a sample. The sample was adjusted so that the content of the siloxane compound was about 0.1%. The weight average molecular weight of the siloxane compound was measured for this sample under the following conditions.
Measurement device: Trade name "Waters 2695 Separations Module", manufactured by Waters RI (refractive index) detector: Trade name "2414 detector", manufactured by Waters Column: Trade name "Shodex KF-806M" 4-column series, manufactured by Showa Denko Eluent: Tetrahydrofuran Flow rate: 1.0 mL/min
Oven temperature: 40°C
Sample injection volume: 100 μL
Detector: RI (Refractive Index).

When calculating the weight-average molecular weight of the siloxane compound, a molecular weight calibration curve was used that was created using 17 types of standard polystyrene samples (product names "PS-1" and "PS-2", manufactured by Polymer Laboratories, molecular weights: 7,500,000, 2,560,000, 841,700, 377,400, 320,000, 210,500, 148,000, 96,000, 59,500, 50,400, 28,500, 20,650, 10,850, 5,460, 2,930, 1,300, and 580).

(Siloxane Compounds 1, 3 to 12)
Siloxane compounds 1, 3 to 12 each having a structure (side chain type) represented by the following general formula (1) were synthesized by a conventional method. Table 1 shows the properties of siloxane compounds 1, 3 to 12.

(Siloxane Compound 2)
Siloxane compound 2 having a structure (single-end type) represented by the following general formula (2) was synthesized by a conventional method. The properties of siloxane compound 2 are shown in Table 1.

<Preparation of First Ink>
The components (unit: %) shown in the upper row of Table 2 were mixed and thoroughly stirred, and then pressure filtered through a cellulose acetate filter (manufactured by Advantec) with a pore size of 0.8 μm to obtain a first ink. Acetylenol E100 is a nonionic surfactant manufactured by Kawaken Fine Chemicals.

<Preparation of water-soluble resin>
(Water-soluble resin 1)
100.0 parts of ethylene glycol monobutyl ether was placed in a four-neck flask equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux tube, and then nitrogen gas was introduced into the flask and the temperature was raised to 110 ° C. while stirring. A mixture of monomers (40.0 parts of styrene, 40.0 parts of methyl methacrylate, and 20.0 parts of acrylic acid) and ethylene glycol monobutyl ether in which 1.3 parts of a polymerization initiator (t-butyl peroxide) was dissolved were dropped into the flask over 3 hours. After aging for 2 hours, the ethylene glycol monobutyl ether was removed under reduced pressure to obtain water-soluble resin 1 as a solid. The obtained water-soluble resin 1 was neutralized with potassium hydroxide having an acid value of 1.0 times (molar ratio), and an appropriate amount of ion-exchanged water was added and dissolved at 80 ° C. to obtain a liquid containing water-soluble resin 1 with a resin content of 10.0%.

(Water-soluble resin 2)
The types of monomers were changed to 40.0 parts of benzyl methacrylate, 40.0 parts of n-butyl acrylate, and 20.0 parts of p-styrenesulfonic acid. A liquid containing water-soluble resin 2 with a resin content of 8.0% was obtained in the same manner as in the case of water-soluble resin 1.

(Water-soluble resin 3)
A "fluorene resin" was synthesized with reference to the description in Patent Document 2. This resin is a water-soluble urethane resin with a weight average molecular weight of 3,300. The obtained resin was neutralized with potassium hydroxide in an amount 1.0 times (molar ratio) its acid value, and an appropriate amount of ion-exchanged water was added and dissolved at 80° C. to obtain a liquid containing water-soluble resin 3 with a resin content of 10.0%.

<Preparation of resin particles>
(Resin particles 1 to 6)
2.0 parts of n-hexadecane, 1.0 part of a polymerization initiator (2,2'-azobis-(2-methylbutyronitrile)), and the core monomer shown in Table 3 (unit: part) were mixed and stirred for 30 minutes to obtain a mixture. The abbreviations of the monomers are St: styrene, nBA: n-butyl acrylate, and AA: acrylic acid. The obtained mixture was dropped into 229.5 parts of water in which 0.27 parts of sodium dodecyl sulfate had been dissolved, and then stirred for 30 minutes to obtain a core monomer mixture. Then, using an ultrasonic homogenizer (trade name "S-150D Digital Sonifier", manufactured by Branson), ultrasonic waves were irradiated to the core monomer mixture under the conditions of output: 400 W, frequency: 20 kHz, and 3 hours to disperse the components. Thereafter, the mixture was polymerized at 80°C for 4 hours under a nitrogen atmosphere to synthesize a resin, and a dispersion containing an acrylic resin that becomes the core of the resin particles was obtained.

Next, 200.0 parts of ion-exchanged water, 0.1 parts of potassium persulfate, 8.0 parts of sodium dodecyl sulfate, and the monomers for the shell part shown in Table 3 (units: parts) were emulsified to obtain an emulsion of the monomers for the shell part. The abbreviations for the monomers are MMA: methyl methacrylate, nBA: n-butyl acrylate, AA: acrylic acid, and StSA: styrene sulfonic acid. 0.1 parts of potassium persulfate and 600.0 parts of ion-exchanged water were added to 240.0 parts of the dispersion containing the resin that will become the core part obtained earlier, and the temperature was raised to 75°C under a nitrogen atmosphere. 350.0 parts of the emulsion of the monomers for the shell part were dropped into this dispersion over 3 hours. The temperature was then raised to 85°C, and the mixture was stirred for 2 hours to polymerize, synthesizing an acrylic resin that will become the shell part of the resin particles. After that, it was cooled to 25°C, and an appropriate amount of ion-exchanged water and an aqueous potassium hydroxide solution were added to obtain a liquid containing each resin particle with a pH of 8.5 and a resin content of 10.0%. Resin particles 1 to 6 were all resin particles composed of a core and a shell.

(Resin Particles 7)
A 300 mL four-neck flask equipped with a stirring seal, a stirring rod, a reflux condenser, a septum rubber, and a nitrogen inlet tube was charged with 9.0 parts of styrene, 1.5 parts of acrylic acid, 0.1 parts of sodium dodecyl sulfate, and 100.0 parts of distilled water, and mixed. The flask was placed in a thermostatic bath at 70 ° C., and nitrogen gas was introduced into the flask while stirring the contents at 300 rpm, and nitrogen substitution was performed for 1 hour. Thereafter, polymerization was started by injecting potassium persulfate dissolved in 100.0 parts of distilled water into the flask using a syringe. The end of polymerization was confirmed by monitoring the molecular weight by gel permeation chromatography. After purification by ultrafiltration, an appropriate amount of ion-exchanged water and an aqueous potassium hydroxide solution were added to obtain a liquid containing resin particles 7 with a pH of 8.5 and a resin content of 10.0%. Resin particles 7 were single-layer resin particles composed of acrylic resin.

(Resin Particles 8)
A suitable amount of ion-exchanged water was added to a commercially available aqueous dispersion containing resin particles made of urethane resin (product name "Takelac W-6061", manufactured by Mitsui Chemicals) to adjust the concentration, thereby obtaining a liquid containing resin particles 8.

<Preparation of second ink>
The components (unit: %) shown in the upper rows of Tables 4 and 5 were mixed and thoroughly stirred, and then pressure filtered through a cellulose acetate filter (manufactured by Advantec) with a pore size of 0.8 μm to obtain a second ink. Acetylenol E100 is a nonionic surfactant manufactured by Kawaken Fine Chemicals. The perfluoroalkyl compounds 1 to 3 shown below were used.
Perfluoroalkyl compound 1 (product name "Zonyl FS-3100", manufactured by DuPont, perfluoroalkyl ethylene oxide adduct)
Perfluoroalkyl compound 2 (product name "Zonyl FSO-100", manufactured by DuPont, perfluoroalkyl ethylene oxide adduct)
Perfluoroalkyl compound 3 (product name "Megafac F-410", manufactured by DIC, perfluoroalkyl carboxylate)

<Preparation of Pigment Dispersion>
(Pigment Dispersion 1)
A mixture was obtained by mixing 24.0 parts of C.I. Pigment Yellow 74, 48.0 parts of an aqueous solution of a resin dispersant, and 28.0 parts of ion-exchanged water. As the aqueous solution of the resin dispersant, a styrene-acrylic acid copolymer (trade name "Joncryl 680", manufactured by BASF) was neutralized with potassium hydroxide having an acid value of 0.85 times (molar ratio), and an appropriate amount of ion-exchanged water was added to obtain an aqueous solution having a water-soluble resin content of 20.0%. The obtained mixture and 85 parts of zirconia beads having a diameter of 0.3 mm were placed in a batch-type vertical sand mill (manufactured by Imex) and dispersed for 3 hours while cooling with water. Then, the mixture was centrifuged to remove coarse particles. The mixture was filtered under pressure with a cellulose acetate filter (manufactured by Advantec) having a pore size of 3.0 μm to prepare pigment dispersion 1 having a pigment content of 20.0% and a resin content of 8.0%.

(Pigment Dispersion 2)
Pigment dispersion 2 having a pigment content of 20.0% and a resin content of 8.0% was prepared in the same manner as in Pigment Dispersion 1, except that the pigment was changed to C.I. Pigment Blue 15:3.

(Pigment Dispersion 3)
Pigment dispersion 3 having a pigment content of 20.0% and a resin content of 8.0% was prepared in the same manner as in Pigment dispersion 1, except that the pigment was changed to carbon black.

<Preparation of third ink>
The components (unit: %) shown in Table 6 were mixed and thoroughly stirred, and then pressure filtered through a cellulose acetate filter (manufactured by Advantec) with a pore size of 0.8 μm to obtain ink No. 3. Acetylenol E100 is the trade name of a nonionic surfactant manufactured by Kawaken Fine Chemicals.

<Evaluation>
Each ink prepared above was filled into an ink cartridge, and the ink cartridge was set in an inkjet recording device (PIXUS PRO 10-S, manufactured by Canon) equipped with a recording head that ejects ink by the action of thermal energy. In this recording head, the ejection port arrays for the first ink, the second ink, and the third ink are arranged along a direction perpendicular to the main scanning direction. In this example, an image recorded under the condition that eight ink droplets of about 3.8 ng are applied to a unit area (one pixel) of 1/600 inch x 1/600 inch is defined as having a recording duty of 100%.

The first ink was set to be ejected from nozzles that make up 1/3 of the nozzle row on the upstream side in the sub-scanning direction. The second ink was set to be ejected from nozzles that make up 1/3 of the nozzle row in the center in the sub-scanning direction. The third ink was set to be ejected from nozzles that make up 1/3 of the nozzle row on the downstream side in the sub-scanning direction.

In order to keep the time difference between the application of the first, second, and third inks constant, for convenience, each ink was ejected in only one direction of the main scan of the print head to record an image. The first ink was ejected from the upstream nozzle row and applied to the print medium. Thereafter, while the carriage carrying the print head was returned to its home position, the print medium was transported in the sub-scanning direction for a width equivalent to the area where the previous ink had been applied. The second and third inks were also applied to the print medium in the same manner as the first ink. The time difference between the application of the first and second inks was set to 10 seconds.

The left side of Table 7 shows the type of each ink, the application order, the printing duty, and the number of main scans (number of printing passes) when applying the second ink to a unit area. Also shown are the amount A _S of silver applied in the first ink, the amount A _R of resin applied in the second ink, the amount A _A of additive applied in the second ink, the value of A _A /A _S , and the value of A _R /A _S. Glossy paper (product name "Canon Photo Paper Glossy Pro [Platinum Grade] PT-201", manufactured by Canon) was used as the printing medium. The ink receiving layer of this printing medium contains chloride ions.

In the present invention, in the evaluation criteria for each item below, 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 7.

(thin film interference suppression)
The first ink and the second ink were applied in overlapping fashion over the entire surface of an A4-sized recording medium under the conditions shown on the left side of Table 7, and the third ink was then applied in overlapping fashion at a predetermined recording duty to record a pattern including six types of 2 cm x 2 cm solid images. In addition, a pattern including six types of 2 cm x 2 cm solid images was recorded using only the third ink at a predetermined recording duty. In each of the above cases, the recording duty of the third ink was set to six types: 50%, 60%, 70%, 80%, 90%, and 100%. The solid images recorded using the three types of ink (six types of recording duty) were measured for hue angle h in the L ^* a ^* b ^* display system defined by the CIE (Commission Internationale de l'Eclairage). The solid images recorded using only the third ink (six types of recording duty) were also similarly measured for hue angle h. The hue angle h was measured using an integrating sphere type spectrophotometer (product name "CM-2600d", manufactured by Konica Minolta) under the conditions of SCI mode (conditions including specular reflection light), measurement diameter: 3 mm, viewing angle: 2°, and light source: D50.

The difference in hue angle was calculated for combinations of solid images with the same recording duty, and the difference Δh between the maximum and minimum values of the difference in hue angle for the six types of solid images was calculated, and the suppression of thin film interference was evaluated according to the evaluation criteria shown below.
A: Δh was less than 10.
B: Δh was 10 or more and less than 20.
C: Δh was 20 or more.

(Fine wire quality)
The first ink and the second ink were applied to the recording medium under the conditions shown on the left side of Table 7, and the character "電" was recorded using the third ink under the conditions of font: MS Mincho, font size: 24 points. The part of the image including "電" was saved as image data using an optical microscope (product name "ZEISS SteREO Discovery V8", manufactured by ZEISS, magnification: 8 times, with scale). This image data was read using image processing software (product name "Adobe Photoshop CC", manufactured by Adobe Systems), and the line width of the vertical line of the fourth stroke of "電" was measured using the ruler tool of the software. The line width of the vertical line of the actual image was calculated from the scale of the image data, and the thin line quality was evaluated from the average value of the three points according to the evaluation criteria shown below. For Reference Examples 3 and 4 in which the third ink was not used, the thin line quality was not evaluated.
A: The line width was less than 400 μm.
B: The line width was 400 μm or more and less than 500 μm.
C: The line width was 500 μm or more.

The color metallic image recorded in Example 1 was laminated using a laminator (product name "Arctic Titan 165", manufactured by Japan GBC). A polyethylene terephthalate film with an adhesive layer (product name "G-030EV50", manufactured by Lintec, thickness 50 μm) was used as the laminating material. By laminating the color metallic image, it was possible to maintain high levels of gloss, color development, and ozone resistance without compromising them.

Claims (20)

An inkjet recording method for recording an image on a recording medium by ejecting ink from an inkjet recording head, comprising:
a step of applying a first ink to the recording medium, a step of applying a second ink to the recording medium so as to overlap at least a portion of the area to which the first ink has been applied, and a step of applying a third ink to the recording medium so as to overlap at least a portion of the area to which the first ink and the second ink have been applied,
the first ink is an ink containing silver particles,
the second ink is an ink containing a resin and an additive, but not containing a coloring material, the additive being selected from the group consisting of (1) a compound having an ethylene oxide group, a hydroxy group, or an anionic group, and a siloxane structure, and (2) a compound having an ethylene oxide group, a hydroxy group, or an anionic group, and a perfluoroalkyl structure, and when the additive is a compound among (1) having an ethylene oxide group and the siloxane structure, the ethylene oxide group is located in a side chain of the siloxane structure,
The ink-jet recording method according to claim 1, wherein the third ink contains a coloring material. The inkjet recording method according to claim 1, wherein the additive has an HLB value determined by the Griffin method of 8 to 16. The inkjet recording method according to claim 1 or 2, wherein the additive contains a compound having a siloxane structure, and the number of repetitions of the siloxane structure in the compound having a siloxane structure is 20 or more. The inkjet recording method according to claim 3, wherein the number of repetitions of the siloxane structure is 100 or less. The inkjet recording method according to any one of claims 1 to 4, wherein the resin of the second ink contains a water-soluble resin and resin particles. The inkjet recording method according to claim 5, wherein the resin particles are composed of a core containing a unit having no acid group and a shell containing a unit having an acid group. The inkjet recording method according to any one of claims 1 to 6, wherein the content (mass %) of the silver particles in the first ink is 2.00 mass % or more and 15.00 mass % or less based on the total mass of the ink. The inkjet recording method according to any one of claims 1 to 7, wherein the content (mass %) of the resin in the second ink is 0.10 mass % or more and 20.00 mass % or less based on the total mass of the ink. The inkjet recording method according to any one of claims 5, 6, and 8, wherein the content (mass%) of the resin particles in the second ink is 0.30 times or more and 0.70 times or less in mass ratio to the content (mass%) of the resin. The inkjet recording method according to any one of claims 1 to 9, wherein the content (mass %) of the additive in the second ink is 0.05 mass % or more and 5.00 mass % or less based on the total mass of the ink. The inkjet recording method according to any one of claims 1 to 10, wherein the content (mass %) of the coloring material in the third ink is 0.05 mass % or more and 15.00 mass % or less based on the total mass of the ink. 12. The inkjet recording method according to claim 1, wherein the amount (g/ ^m2 ) of the additive in the second ink per unit area of the recording medium is 0.05 or more times the amount (g/ ^m2 ) of the silver particles in the first ink. 13. The inkjet recording method according to claim 12, wherein the amount (g/ ^m2 ) of the additive in the second ink per unit area of the recording medium is 7.00 times or less relative to the amount (g/ ^m2 ) of the silver particles in the first ink. 14. The inkjet recording method according to claim 1, wherein the ratio of the amount (g/ ^m2 ) of the resin of the second ink per unit area of the recording medium to the amount (g/ ^m2 ) of the silver particles of the first ink is 0.15 to 2.00 times. The inkjet recording method according to any one of claims 1 to 14, wherein the time difference between the step of applying the first ink to the recording medium and the step of applying the second ink to the recording medium is 1 second or more and 600 seconds or less. The inkjet recording method according to any one of claims 1 to 15, wherein the time difference between the step of applying the second ink to the recording medium and the step of applying the third ink to the recording medium is 0.1 seconds or more and 600 seconds or less. The inkjet recording method according to any one of claims 1 to 16, wherein the second ink is applied to a unit area of the recording medium in multiple separate applications. The inkjet recording method according to any one of claims 1 to 17, further comprising a step of laminating a laminate material on the image after recording the image on the recording medium by a step of applying the first ink to the recording medium, a step of applying the second ink to the recording medium so as to overlap at least a portion of the area to which the first ink has been applied, and a step of applying the third ink to the recording medium so as to overlap at least a portion of the area to which the first ink and the second ink have been applied. The inkjet recording method according to any one of claims 1 to 18, wherein the recording medium is a recording medium having an ink receiving layer. An inkjet recording apparatus used to record an image on a recording medium by ejecting ink from an inkjet recording head,
a means for applying a first ink to the recording medium, a means for applying a second ink to the recording medium so as to overlap at least a portion of an area to which the first ink has been applied, and a means for applying a third ink to the recording medium so as to overlap at least a portion of an area to which the first ink and the second ink have been applied,
the first ink is an ink containing silver particles,
the second ink is an ink containing a resin and an additive, but not containing a coloring material, the additive being selected from the group consisting of (1) a compound containing an ethylene oxide group, a hydroxyl group, or an anionic group, and having a siloxane structure, and (2) a compound having a perfluoroalkyl structure , and when the additive is a compound among (1) having the ethylene oxide group and the siloxane structure, the ethylene oxide group is located in a side chain of the siloxane structure,
2. An ink jet recording apparatus, wherein the third ink is an ink containing a coloring material.

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