JP7118713B2 - Inkjet recording method and inkjet recording apparatus - Google Patents

Description

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

Inks containing metal particles have been used to form electric circuits by utilizing the characteristics of the metal particles used, but in recent years, they have also come to be used for applications such as Christmas cards that express a metallic effect. is coming. In particular, in order to enhance the decorativeness of color images, there is a demand for recording color images with a metallic feel (hereinafter referred to as “color metallic images”). In order to record a color metallic image, an ink set of an oil-based ink containing an aluminum pigment and an oil-based ink containing a dye has been proposed (see Patent Document 1). In addition, an inkjet recording method is proposed in which a processing agent containing inorganic particles is applied to a recording medium, and an aqueous ink containing silver particles and an aqueous ink containing a pigment are sequentially applied to the areas to which the processing agent has been applied. (See Patent Document 2). Furthermore, an inkjet recording method is described in which ink containing silver particles and ink containing dye are applied in this order (see Patent Document 3).

JP 2016-14141 A JP 2015-193126 A JP 2015-193127 A

Since the oil-based ink described in Patent Document 1 requires countermeasures against the odor of the volatilized organic solvent, the present inventors assumed the use of water-based ink instead of oil-based ink. In addition, in order to obtain an image with excellent transparency, an image was printed in the same manner as the inkjet recording method described in Patent Document 2, except that the water-based ink containing pigment was changed to the water-based ink containing dye. was recorded. As a result, it was found that the color developing property of the image could not be obtained. Furthermore, when an image was recorded by a method similar to the ink jet recording method described in Patent Document 3, it was found that the image could not develop color.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an ink jet recording method capable of producing images with excellent color development properties. Another object of the present invention is to provide an inkjet recording apparatus using the inkjet recording method.

The above objects are achieved by the present invention described below. That is, the present invention includes a recording step of applying a first ink to a recording medium, a recording step of applying a second ink to the recording medium so as to overlap at least a part of the region to which the first ink is applied, wherein the first ink is an aqueous ink containing silver particles, the second ink is an aqueous ink containing an anionic dye, and the recording medium comprises a polyvalent metal It has an ink-receiving layer containing at least one cationic compound selected from the group consisting of salts and salts of cationic resins, and the polyvalent metal salt consists of (i) iron, aluminum, and zirconium. Salts of at least one polyvalent metal ion selected from the group and at least one anion selected from the group consisting of acetate ions, chloride ions, and sulfate ions, and (ii) polychloride aluminum, and the salt of the cationic resin is a nitrate of a resin having a primary to quaternary amine structure, and a sulfate of a resin having a primary to quaternary amine structure. and the content of the cationic compound per unit area of the ink-receiving layer (g/m ² ) is 0.20 g/m ² or more. The present invention relates to an inkjet recording method for printing.

The present invention also provides an inkjet recording apparatus comprising means for applying a second ink to the recording medium after applying the first ink to the recording medium, wherein the first ink is an aqueous ink containing silver particles. ink, the second ink is an aqueous ink containing an anionic dye, and the recording medium contains at least one cationic ink selected from the group consisting of polyvalent metal salts and salts of cationic resins. It has an ink-receiving layer containing a compound, and the polyvalent metal salt comprises (i) ions of at least one polyvalent metal selected from the group consisting of iron, aluminum, and zirconium, acetate ions, and chlorides. At least one selected from the group consisting of a salt with at least one anion selected from the group consisting of ions and sulfate ions, and (ii) polyaluminum chloride, and a salt of the cationic resin. is at least one selected from the group consisting of nitrates of resins having primary to quaternary amine structures and sulfate salts of resins having primary to quaternary amine structures, and The content (g/m ² ) of the cationic compound in (1) is 0.20 g/m ² or more.

According to the present invention, it is possible to provide an inkjet recording method and an inkjet recording apparatus which are excellent in image color development.

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

Hereinafter, embodiments of the present invention will be described in detail. Various physical property values are values at a temperature of 25° C. unless otherwise specified. In the present invention, the term "color-developing image" refers to an image having the color tone of the dye used instead of silver.

Generally, water-based inks for inkjet use particles having a particle size of several nanometers to several hundreds of nanometers in consideration of ejection stability. not. Silver particles are composed of silver atoms. Compared to silver particles with a large particle size, silver particles with a small particle size have a higher ratio of silver atoms existing on the surface of the silver particles to the total number of silver atoms. Compared to the silver atoms inside the silver particles, which are difficult to move due to metal bonds with the surrounding silver atoms, the silver atoms existing on the surface of the silver particles are more mobile, so they are present on the surface of the nearby silver particles. It fuses by metallic bonding with existing silver atoms. In this way, silver particles with a small particle size are fused with nearby silver particles, so that the image develops glossiness.

After the ink containing silver particles is applied to the recording medium, the ink containing the dye is applied to the recording medium so as to overlap at least a part of the area where the ink containing the silver particles is applied. It was found that although the color development property of the image was improved, the color development property of the image was not obtained.

When this image was analyzed, it was found that the silver layer formed on the recording medium had pores of several nanometers in size although the silver particles were partially fused to each other. It was also found that the dye in the ink permeates into the recording medium through the pores of the silver layer, thereby making it impossible to obtain the color developability of the image.

Accordingly, the present inventors have studied the configuration of the inkjet recording method for obtaining the color developability of the image. The inkjet recording method of the present invention includes a recording step of applying a first ink to a recording medium, and a recording step of applying a second ink to the recording medium so as to overlap at least a part of the region to which the first ink is applied. and The first ink is an aqueous ink containing silver particles and the second ink is an aqueous ink containing an anionic dye. Further, the recording medium has an ink-receiving layer containing at least one cationic compound selected from the group consisting of polyvalent metal salts and salts of cationic resins, and the cation per unit area of the ink-receiving layer is The content (g/m ² ) of the active compound is 0.20 g/m ² or more. The mechanism by which the ink-jet recording method having such a configuration can obtain the color developability of the image will be described in detail.

When the first ink adheres to the recording medium and the liquid component in the first ink evaporates, the silver particles come close to each other and are fused together. As a result, although pores are present, a silver layer is formed in which silver particles are fused together.

When the second ink is applied here, the cationic compound in the ink receiving layer dissolves in the liquid component in the second ink. Then, the cationic compound oozes out to the surface side of the silver layer through the pores of the silver layer. In this process, the cationic compound captures or saltes out the components that disperse the silver particles, and aggregates the silver particles, so that the pores in the silver layer are reduced and a dense silver layer is formed. . In this case, even if the dye is not agglomerated as will be described later, the dye tends to remain in the silver layer, and the image can be colored.

In addition, the cationic compound that has oozed out to the surface side of the silver layer reacts with the anionic group of the dye, or precipitates the dissolved dye by salting out. As a result, the dye agglomerates and becomes difficult to pass through the pores of the silver layer, so that the dye tends to remain in the silver layer, thereby obtaining a color developing property of the image.

The ink-receiving layer of the recording medium may contain either a polyvalent metal salt or a cationic resin salt.

Furthermore, the content (g/m ² ) of these cationic compounds per unit area of the ink-receiving layer is 0.20 g/m ² or more. Some conventional recording media having an ink-receiving layer for general inkjet contain cationic substances such as polydiallyldimethylammonium chloride and polyaluminum chloride. However, its content is much smaller than the range of 0.20 g/m ² or more specified in the present invention. An important point of the ink jet recording method of the present invention is that the amount of the cationic compound contained in the ink receiving layer of the recording medium is as large as 0.20 g/m ² or more. When the content is less than 0.20 g/m ² , the content of the cationic compound per unit area of the ink-receiving layer is small, so silver particles or dyes are less likely to agglomerate. As a result, it becomes difficult for the dye in the second ink to stay on the recording medium, so that the color development of the image cannot be obtained.

<Inkjet recording method>
The inkjet recording method of the present invention comprises a recording step of applying a first ink to a recording medium, and a recording step of applying a second ink to the recording medium so as to overlap at least a part of the area to which the first ink is applied. It is an inkjet recording method having.

In the recording step, it is preferable to record an image by ejecting ink from an inkjet recording head. Methods for ejecting ink include a method in which mechanical energy is applied to ink and a method in which thermal energy is applied to ink. In the present invention, it is preferable to adopt a method of applying thermal energy to the ink to eject the ink. In addition, in the inkjet recording method of the present invention, it is not necessary to irradiate an active energy ray.

FIG. 1 is a diagram schematically showing an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, (a) is a perspective view of the main part of the inkjet recording apparatus, and (b) is a perspective view of a head cartridge. is. The inkjet recording apparatus is provided with transport means (not shown) for transporting the recording medium 32 and a carriage shaft 34 . A head cartridge 36 can be mounted on the carriage shaft 34 . The head cartridge 36 has recording heads 38 and 40, and is constructed so that an ink cartridge 42 is set therein. While the head cartridge 36 is conveyed along the carriage shaft 34 in the main scanning direction, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32 . An image is recorded on the recording medium 32 by conveying the recording medium 32 in the sub-scanning direction by conveying means (not shown).

<First ink>
The first ink is an aqueous ink containing silver particles. The components constituting the first ink will be described below.

(silver particles)
Silver particles are composed of silver atoms. The silver particles may contain other metal atoms, oxygen atoms, sulfur atoms, carbon atoms, etc. in addition to silver atoms. It is preferably 0% by mass or more.

Methods for producing silver particles include, for example, a method of pulverizing silver lumps with a pulverizer such as a ball mill or a jet mill (pulverization method), and a method of reducing and aggregating silver ions or silver complexes with a reducing agent (reduction method). etc. In the present invention, it is preferable to produce silver particles by a reduction method from the viewpoints of ease of controlling the particle size of silver particles and dispersion stability of silver particles.

[Volume-Based Cumulative 50% Particle Size ( _D50 ) of Silver Particles]
The volume-based cumulative 50% particle size of the silver particles is the diameter of the particles at 50% when integrated from the small particle size side based on the total volume of the measured silver particles in the particle size accumulation curve. be. The volume-based cumulative 50% particle size (nm) of the silver particles is preferably 200 nm or less, more preferably 150 nm or less. The reason is as follows.

When the _D50 is 150 nm or less, the particle size of the silver particles becomes smaller, so that the ratio of silver atoms existing on the surface of the silver particles to the total number of silver atoms becomes larger. That is, by increasing the ratio of silver atoms that are easily mobile in the silver particles, it becomes easier to fuse with silver atoms present on the surfaces of nearby silver particles due to metallic bonds. This further improves the glossiness of the image. _D50 is a value obtained by observing the cross section of the obtained image with a scanning electron microscope (SEM), a transmission electron microscope (TEM), or the like. More preferably, _D50 is 1 nm or more.

[Method of Dispersing Silver Particles]
Methods for dispersing the silver particles include a surfactant dispersion type using a surfactant as a dispersant and a resin dispersion type using a resin as a dispersant. Of course, in the first ink, it is also possible to use silver particles with different dispersion methods.

An anionic surfactant, a nonionic surfactant, a cationic surfactant, an amphoteric surfactant and the like can be used as a surfactant used as a dispersant in the surfactant dispersion type. Examples of anionic surfactants include fatty acid salts, alkyl sulfates, alkylarylsulfonates, alkyldiaryletherdisulfonates, dialkylsulfosuccinates, alkylphosphates, naphthalenesulfonate formalin condensates, polyoxyethylene alkyl Ether sulfates, polyoxyethylene alkyl phosphate salts, glycerol borate fatty acid esters, and the like. Nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethyleneoxypropylene block copolymers, sorbitan fatty acid esters, glycerin fatty acid esters, polyoxyethylene fatty acid esters, polyoxyethylene alkylamines, fluorine compounds, silicone compounds, etc. is mentioned. Cationic surfactants include alkylamine salts, quaternary ammonium salts, alkylpyridinium salts, alkylimidazolium salts and the like. Amphoteric surfactants include alkylamine oxides, phosphatidylcholines and the like.

Among them, it is preferable to use at least one surfactant selected from anionic surfactants and nonionic surfactants. The anionic surfactant is preferably a polyoxyethylene alkyl ether sulfate and the nonionic surfactant is preferably a polyoxyethylene alkyl ether.

It is preferable that the resin used as a dispersing agent in the resin dispersion type has both a hydrophilic portion and a hydrophobic portion. Examples of resins include polyvinyl resins, polyester resins, amino resins, acrylic resins, epoxy resins, polyurethane resins, polyether resins, polyamide resins, unsaturated polyester resins, phenol resins, and silicone resins. , fluorine-based polymer compounds, and the like.

The polystyrene equivalent weight average molecular weight (Mw) obtained by gel permeation chromatography (GPC) of the resin is preferably 1,000 or more and 100,000 or less, and is preferably 3,000 or more and 50,000 or less. More preferred.

The content (% by mass) of the dispersant in the first ink is preferably 1.0 times or less as a mass ratio with respect to the content (% by mass) of the silver particles. If the mass ratio exceeds 1.0 times, the amount of the dispersing agent is too large relative to the silver particles, making it difficult for the silver particles to come close to each other, making it difficult for the silver particles to fuse together. As a result, sufficient glossiness of the image may not be obtained.

When the dispersant is a surfactant, the mass ratio is more preferably 0.02 times or more. When the dispersant is a resin, the mass ratio is more preferably 0.05 times or more. When the dispersant is a surfactant, if the mass ratio is less than 0.02 times, the amount of the dispersant is too small relative to the silver particles in the first ink, so the silver particles are not stable in the first ink. difficult to disperse. As a result, sufficient ink ejection stability may not be obtained. When the dispersant is a resin, if the mass ratio is less than 0.05 times, sufficient ink ejection stability may not be obtained for the same reason as described above.

The content (% by mass) of silver particles in the first ink is preferably 2.0% by mass or more and 15.0% by mass or less based on the total mass of the first ink. If the content is less than 2.0% by mass, the amount of silver particles is too small, making it difficult for the silver particles to come close to each other. This makes it difficult for the silver particles to fuse together, so that the image may not have sufficient glossiness. If the content exceeds 15.0% by mass, the amount of silver particles is too large, which may increase the viscosity of the ink and may not provide sufficient ink ejection stability. More preferably, the content (% by mass) of the silver particles in the first ink is 2.0% by mass or more and 8.0% by mass or less based on the total mass of the first ink.

(Surfactant)
The first ink preferably contains a surfactant in addition to the surfactant that can be used as a dispersing agent for silver particles. Surfactants that can be used include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants. Among them, the surfactant is preferably a nonionic surfactant. Examples of nonionic surfactants include ethylene oxide adducts of acetylene glycol. Among them, the nonionic surfactant preferably has an HLB value of 10 or more according to the Griffin method. If the HLB value is less than 10, the hydrophobicity is high, and it becomes difficult to dissolve in the first ink. Here, the HLB value according to the Griffin method is obtained from the formula weight of the ethylene oxide group of the surfactant and the molecular weight of the surfactant, HLB value = 20 × (formula weight of the ethylene oxide group of the surfactant) / (surfactant (molecular weight of). The HLB value indicates the degree of hydrophilicity or lipophilicity of a surfactant (compound) in a range of 0 to 20. A lower HLB value indicates a higher lipophilicity (hydrophobicity) of the compound. On the other hand, the higher the HLB value, the higher the hydrophilicity of the compound.

The content (% by mass) of the surfactant used as a dispersing agent for the silver particles in the first ink is preferably 0.1% by mass or more and 3.0% by mass or less. The content (% by mass) of the surfactant other than the surfactant used as the dispersing agent for the silver particles in the first ink is preferably 0.1% by mass or more and 2.0% by mass or less.

(aqueous medium)
The first ink contains water or an aqueous medium that is a mixed solvent of water and a water-soluble organic solvent. It is preferable to use deionized water (ion-exchanged water) as water. The water-soluble organic solvent is not particularly limited, and alcohols, glycols, glycol ethers, nitrogen-containing compounds, and the like, which can be used for inkjet inks, can all be used. Also, one or more of these water-soluble organic solvents can be contained in the ink.

The water content (% by mass) in the first ink is preferably 50.0% by mass or more and 95.0% by mass or less based on the total mass of the ink. Also, the content (% by mass) of the water-soluble organic solvent in the first ink is preferably 3.0% by mass or more and 50.0% by mass or less based on the total mass of the first ink. If the content of the water-soluble organic solvent is less than 3.0% by mass, reliability such as adhesion resistance may not be sufficiently obtained when the first ink is used in an inkjet recording apparatus. Moreover, when the content of the water-soluble organic solvent exceeds 50.0% by mass, the viscosity of the ink increases, which may cause ink supply failure.

(other ingredients)
In addition to the above components, the first ink may contain water-soluble organic compounds that are solid at a temperature of 25° C., such as urea and its derivatives, trimethylolpropane, and trimethylolethane. In addition, the first ink may contain various additives such as pH adjusters, antifoaming agents, rust inhibitors, preservatives, anti-mold agents, antioxidants, anti-reduction agents, and chelating agents, if necessary. may be included.

(Physical properties of the first ink)
The viscosity (mPa·s) of the first ink at a temperature of 25° C. is preferably from 1 mPa·s to 5 mPa·s, more preferably from 1 mPa·s to 3 mPa·s. In addition, the surface tension (mN/m) of the ink at a temperature of 25° C. is preferably 10 mN/m or more and 60 mN/m or less, more preferably 20 mN/m or more and 60 mN/m or less, and 30 mN/m or more. It is more preferably 40 mN/m or less. The surface tension of the ink can be adjusted by appropriately determining the type and content of the surfactant in the ink.

<Second ink>
The second ink is an aqueous ink containing a dye. The components constituting the second ink will be described below.

(dye)
The dye is not particularly limited as long as it is an anionic dye that can be used as a coloring material for general inkjet inks. Among them, 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. Furthermore, from the viewpoint of ink reliability, the dye is preferably a dye that dissolves in the ink, and from the viewpoint of the color development of an image, is a dye that easily aggregates on a recording medium. The balance between the solubility in the ink and the cohesiveness on the recording medium can be adjusted by adjusting the type of skeleton and the number of anionic groups of the dye.

The content (% by mass) of the dye in the second ink is preferably 1.0% by mass or more and 10.0% by mass or less, more preferably 2.0% by mass or more and 8.0% by mass or less. preferable.

(aqueous medium)
The second ink is an aqueous ink containing water as an aqueous medium. The aqueous medium can further contain a water-soluble organic solvent. It is preferable to use deionized water (ion-exchanged water) as water. The water-soluble organic solvent is not particularly limited, and alcohols, glycols, alkylene glycols, polyethylene glycols, nitrogen-containing compounds, and sulfur-containing compounds that can be used in inkjet inks can be used. Either can be used. Also, one or more of these water-soluble organic solvents can be contained in the ink.

The content (% by mass) of water in the second ink is preferably 50.0% by mass or more and 95.0% by mass or less based on the total mass of the ink. The content (% by mass) of the water-soluble organic solvent in the second ink is preferably 3.0% by mass or more and 50.0% by mass or less based on the total mass of the second ink. If the content of the water-soluble organic solvent is less than 3.0% by mass, reliability such as adhesion resistance may not be obtained when the ink is used in an inkjet recording apparatus. Moreover, when the content of the water-soluble organic solvent exceeds 50.0% by mass, there are cases where ink supply failure occurs.

(other ingredients)
The second ink may contain water-soluble organic compounds that are solid at room temperature (25° C.), such as urea and its derivatives, trimethylolpropane, and trimethylolethane, in addition to the above components. In addition, the second ink may contain surfactants, resins, pH adjusters, antifoaming agents, rust inhibitors, antiseptic agents, anti-mold agents, antioxidants, anti-reduction agents, chelating agents, etc., as necessary. may contain various additives.

(Physical properties of the second ink)
The viscosity (mPa·s) of the second ink at a temperature of 25° C. is preferably from 1 mPa·s to 5 mPa·s, more preferably from 1 mPa·s to 3 mPa·s. Further, the surface tension (mN/m) of the second ink at a temperature of 25° C. is preferably 10 mN/m or more and 60 mN/m or less, more preferably 20 mN/m or more and 60 mN/m or less, and 30 mN/m. It is more preferable to be m or more and 40 mN/m or less. The surface tension of the second ink can be adjusted by appropriately determining the type and content of the surfactant in the second ink.

<Recording medium>
The recording medium has an ink-receiving layer. The ink-receiving layer contains at least one cationic compound selected from the group consisting of polyvalent metal salts and salts of cationic resins. An ink-receiving layer is usually provided on the substrate.

(Base material)
Paper having a resin layer is preferably used as the substrate. The resin layer may be provided on one side of the paper, or may be provided on both sides of the paper. Paper having a resin layer includes, for example, paper having a resin layer containing a thermoplastic resin. Examples of thermoplastic resins include acrylic resins, acrylic silicone resins, polyolefin resins, and styrene-butadiene copolymers. Among them, the thermoplastic resin is preferably a polyolefin resin. Polyolefin resins include polyethylene, polypropylene, polyisobutylene, and the like. Among them, the polyolefin resin is preferably polyethylene. Polyethylene is preferably low density polyethylene (LDPE) or high density polyethylene (HDPE). The resin layer may contain a white pigment, a fluorescent whitening agent, ultramarine blue, etc., in order to adjust the opacity, whiteness, hue, and the like. Among them, it is preferable to contain a white pigment in order to improve the opacity. Titanium oxide etc. are mentioned as a white pigment. The content (% by mass) of the white pigment in the resin layer is preferably 25.0% by mass or less. Examples of paper include paper made from wood pulp as a main raw material, and optionally synthetic pulp such as polypropylene and synthetic fibers such as nylon and polyester. Examples of wood pulp include bleached hardwood kraft pulp (LBKP) and bleached softwood kraft pulp (NBKP). The thickness (μm) of the substrate is preferably 50 μm or more and 400 μm or less.

(Ink receiving layer)
The ink-receiving layer may be a single layer or a multi-layer of two or more layers. Also, the ink-receiving layer may be provided on only one side of the substrate, or may be provided on both sides.

The ink-receiving layer preferably contains chloride ions. Chloride ions may be contained in the ink-receiving layer as counter ions for polyvalent metal salts. When chloride ions are contained, the following mechanism promotes fusion between silver particles, forming a denser silver layer. When the ink-receiving layer contains chloride ions, the chloride ions react with the ionized silver (silver ions) on the surface of the silver particles to form silver chloride. Using the silver chloride as a nucleus, the fusion between silver particles is promoted to form a denser silver layer. This makes it easier for the dye to remain in the silver layer, improving the color development of the image.

However, when an image recorded on an ink-receiving layer containing chloride ions is exposed to light or gases such as nitrogen oxides, the silver layer gradually deteriorates and the glossiness of the image decreases. all right. This is because the reaction between chloride ions and silver ions forms more silver chloride than necessary, and the silver layer becomes white due to the silver chloride.

Therefore, the ink-receiving layer preferably contains chloride ions, a chloride ion scavenger, and at least one selected from the group consisting of organic antioxidants. Here, the chloride ion scavenger is capable of suppressing the formation of silver chloride due to the reaction between chloride ions and silver ions. When the ink-receiving layer contains a scavenger for chloride ions, the scavenger adheres to the surface of the silver particles, making it difficult for silver ions to react with chloride ions. This makes it difficult for silver chloride to be formed more than necessary, suppresses whitening of the silver layer due to silver chloride, and suppresses deterioration of image glossiness. Further, when the ink-receiving layer contains an organic antioxidant, ionization (reduction) of silver particles can be suppressed. This makes it difficult for silver chloride to be formed more than necessary, suppresses whitening of the silver layer due to silver chloride, and suppresses deterioration of image glossiness.

[Chloride ion scavenger]
Preferably, the chloride ion scavenger is 1,2,3-benzotriazole or a derivative thereof. Derivatives of 1,2,3-benzotriazole include 1-(methoxymethyl)-1H-benzotriazole, 1-(hydroxymethyl)-1H-benzotriazole, and the like.

The content (g/m ² ) of the chloride ion scavenger per unit area of the ink-receiving layer is preferably 0.02 g/m ² or more and 0.15 g/m ² or less. When the content is less than 0.02 g/m ² , the amount of the scavenger for chloride ions is small, so that the scavenger is less likely to adhere to the surface of the silver particles, and silver ions tend to react with chloride ions. As a result, silver chloride is likely to be formed more than necessary, and silver chloride promotes whitening of the silver layer, which may fail to sufficiently suppress the deterioration of the glossiness of the image. When the content exceeds 0.15 g/m ² , the amount of the scavenger for chloride ions is large, so the scavenger tends to adhere to the surface of the silver particles, and silver ions are less likely to react with chloride ions. This makes it difficult for silver chloride to be formed more than necessary, suppresses whitening of the silver layer due to silver chloride, and suppresses deterioration of image glossiness. However, due to the scavenger adhering to the surface of the silver particles, it is difficult for the silver particles to fuse together, making it difficult to form a dense silver layer. In addition, the trapping agent attached to the surface of the silver particles tends to create gaps between the silver particles, so the dye penetrates through the gaps between the silver particles, resulting in sufficient color development of the image. may not be obtained.

[Organic antioxidant]
The organic antioxidant is preferably ascorbic acid or a salt thereof. Examples of cations that form salts of ascorbic acid include ions of alkali metals such as sodium and potassium, and ions of alkaline earth metals such as magnesium and calcium.

The content (g/m ² ) of the organic antioxidant per unit area of the ink-receiving layer is preferably 0.05 g/m ² or more and 0.25 g/m ² or less. When the content is less than 0.05 g/m ² , it is difficult to suppress the ionization of the silver particles, so silver chloride is likely to be formed more than necessary, and the decrease in glossiness of the image may not be sufficiently suppressed. . If the content exceeds 0.25 g/m ² , the organic antioxidant tends to be present near the silver particles, making it difficult for the silver particles to fuse together, making it difficult to form a dense silver layer and resulting in an image. may not be sufficiently glossy. Further, if the amount of the organic antioxidant is large, the recording medium will have a yellowish tinge, which may result in insufficient color development of the image.

[Inorganic particles]
The ink-receiving layer preferably contains inorganic particles. The average primary particle size of the inorganic particles is preferably 150 nm or less, more preferably 1 nm or more and 100 nm or less, and even more preferably 3 nm or more and 30 nm or less. The average primary particle size of the inorganic particles is the number average particle size of the diameter of a circle having an area equal to the projected area of the primary particles of the inorganic particles when observed with an electron microscope. At this time, the measurement is performed at 100 points or more.

The inorganic particles are preferably used in a coating liquid for forming the ink-receiving layer (hereinafter referred to as "first coating liquid") in a state of being dispersed by a dispersant. The secondary average particle diameter of the inorganic particles in a dispersed state is preferably 0.1 nm or more and 500 nm or less, more preferably 1 nm or more and 300 nm or less, and even more preferably 10 nm or more and 250 nm or less. The secondary average particle size of inorganic particles in a dispersed state can be measured by a dynamic light scattering method.

The content (% by mass) of the inorganic particles in the ink-receiving layer is preferably 50.0% by mass or more and 98.0% by mass or less, preferably 70.0% by mass or more and 96% by mass, based on the total mass of the ink-receiving layer. 0% by mass or less is more preferable.

The coating amount (g/m ² ) of the inorganic particles applied when forming the ink-receiving layer is preferably 8 g/m ² or more and 45 g/m ² or less. By setting the thickness within the above range, the film thickness of the ink-receiving layer is likely to be preferable.

Inorganic particles include alumina hydrate, alumina, silica, colloidal silica, titanium dioxide, and the like. One or more of these inorganic particles can be used as necessary. Among these inorganic particles, it is preferable to use alumina hydrate, alumina, and silica, which can form a porous structure with high ink absorption.

Boehmite-type alumina hydrate or amorphous alumina hydrate is preferable as the alumina hydrate. As the alumina, gas-phase alumina is preferable. Examples of vapor phase alumina include γ-alumina, α-alumina, δ-alumina, θ-alumina, χ-alumina and the like. Among them, it is preferable to use γ-alumina as the vapor-phase method alumina from the viewpoint of the optical density of the image and the ink absorbability.

Alumina hydrate and alumina are preferably mixed in the first coating liquid as an aqueous dispersion, and an acid is preferably used as the dispersant. As the acid, it is preferable to use a compound represented by R--SO ₃ H since it has the effect of suppressing bleeding of the image. In the formula, R represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or an alkenyl group having 1 to 4 carbon atoms. R may be substituted with an oxo group, a halogen atom, an alkoxy group, and an acyl group.

The content of the acid is preferably 1.0% by mass or more and 2.0% by mass or less, based on the total content of alumina hydrate and alumina, and 1.3% by mass or more and 1.6% by mass. % or less.

Methods for producing silica used for the ink-receiving layer are broadly classified into wet methods and dry methods (vapor phase methods). As a wet method, a method is known in which hydrous silica is obtained by producing active silica by acid decomposition of silicate, appropriately polymerizing it, and coagulating and sedimenting it. On the other hand, as a dry method (gas phase method), there is a method by high-temperature gas phase hydrolysis of silicon halide (flame hydrolysis method), and silica sand and coke are heat-reduced and vaporized by an arc in an electric furnace. is known to obtain anhydrous silica by air oxidation (arc method). Silica obtained by a dry method (vapor phase method) (hereinafter also referred to as “vapor phase silica”) is preferably used. Vapor-phase method silica has a particularly large specific surface area, so that it has a particularly high ink absorbability, and since it has a low refractive index, it can impart transparency to the ink-receiving layer. Thereby, good color developability can be obtained.

Alumina hydrate, alumina, and silica may be used in combination. Specifically, there is a method of mixing and dispersing at least two kinds selected from alumina hydrate, alumina, and silica in a powder state to form a dispersion liquid. In the present invention, both alumina hydrate and fumed silica are preferably used as the inorganic particles.

[binder]
The ink receiving layer preferably contains a binder. A binder means a material capable of binding inorganic particles and forming a film.

From the viewpoint of ink absorption, the content of the binder in the ink-receiving layer is preferably 0.50 times or less, more preferably 0.30 times or less, in terms of mass ratio to the content of the inorganic particles. . From the viewpoint of the binding property of the ink-receiving layer, the mass ratio is preferably 0.05 times or more, more preferably 0.08 times or more.

Binders include cationized polymers with cationic groups; cationized polymer surfaces with cationic surfactants; , a polymer having polyvinyl alcohol distributed on its surface; a polymer having cationic colloidal particles distributed on its surface by polymerizing monomers constituting the polymer in a suspension dispersion of cationic colloidal particles; Aqueous binders such as thermosetting synthetic resins such as melamine resins and urea resins; synthetic resins such as polymers and copolymers of acrylic acid esters such as polymethyl methacrylate and methacrylic acid esters. One or more of these binders may be used as necessary.

Among the above binders, it is preferable to use polyvinyl alcohol or its derivatives, which can form a transparent film. Derivatives of polyvinyl alcohol include cation-modified polyvinyl alcohol, anion-modified polyvinyl alcohol, silanol-modified polyvinyl alcohol, polyvinyl acetal, and the like.

When preparing the first coating liquid, it is preferable to use polyvinyl alcohol or a derivative thereof as an aqueous solution. At that time, the content of polyvinyl alcohol or a derivative thereof is preferably 3.0% by mass or more and 20.0% by mass or less based on the total mass of the first coating liquid.

[Crosslinking agent]
In order to increase the water resistance of the ink receiving layer, the ink receiving layer preferably contains a cross-linking agent. Examples of cross-linking agents include zirconium-based compounds, amide-based compounds, aluminum-based compounds, boric acid, borate salts, and the like. One or more of these cross-linking agents can be used as necessary. Among the cross-linking agents described above, boric acid and borates are preferably used particularly when polyvinyl alcohol or a derivative thereof is used as the binder. Polyvinyl alcohol and its derivatives are highly hydrophilic, and hydroxy groups possessed by polyvinyl alcohol and its derivatives react with water in the ink, resulting in swelling. As a result, the absorbability of the ink may decrease. Even if the ink-receiving layer contains polyvinyl alcohol or a derivative thereof, the ink-receiving layer containing a cross-linking agent reacts with the hydroxy group instead of the water in the ink, resulting in a decrease in ink absorption. can be suppressed.

The ink-receiving layer contains polyvinyl alcohol or a derivative thereof and a cross-linking agent, and the content of the cross-linking agent in the ink-receiving layer is 0.40 equivalent or more and 1.00 equivalent to the content of the polyvinyl alcohol or its derivative. It is preferably equal to or less than the equivalent. This further improves the color developability and glossiness of the image. Here, theoretically, the amount of the cross-linking agent that reacts with the hydroxy group of polyvinyl alcohol or its derivative is 1.00 equivalent. If the amount of the cross-linking agent is less than 0.40 equivalents, the amount of the cross-linking agent is too small for the polyvinyl alcohol or its derivative, resulting in swelling of the polyvinyl alcohol or its derivative. As a result, even if ink containing silver particles is applied, the water in the ink is less likely to be absorbed and the silver particles are less likely to fuse together, making it difficult to form a dense silver layer. Therefore, sufficient color development and glossiness of the image may not be obtained. If the amount of the cross-linking agent exceeds 1.00 equivalents, the amount of the cross-linking agent is more than necessary relative to the polyvinyl alcohol, and thus the liquid component in the ink is difficult to be absorbed. This makes it difficult for the silver particles to fuse together, making it difficult to form a dense silver layer. As a result, sufficient color developability and glossiness of the image may not be obtained.

Boric acid includes orthoboric acid (H ₃ BO ₃ ), metaboric acid, diboric acid, and the like. The borate is preferably a water-soluble salt of boric acid. Examples thereof include alkali metal salts of boric acid such as sodium boric acid and potassium boric acid; alkaline earth metal salts of boric acid such as magnesium boric acid and calcium boric acid; and ammonium boric acid. Among these, it is preferable to use orthoboric acid from the viewpoint of the effect of suppressing the generation of cracks and the stability of the first coating liquid over time.

The amount of the cross-linking agent used can be appropriately adjusted according to the production conditions and the like. The content of the cross-linking agent in the ink-receiving layer is preferably 1.0% by mass or more and 50.0% by mass or less, more preferably 5.0% by mass or more and 40.0% by mass or less, based on the content of the binder. preferable.

[Cationic compound]
The cationic compound is at least one selected from the group consisting of polyvalent metal salts and salts of cationic resins. The molecular weight of the cationic resin salt is preferably 1,000 or more and 100,000 or less. The amine value of the cationic resin salt is preferably 50 mgKOH/g or more, more preferably 300 mgKOH/g or more.

The polyvalent metal salt is selected from the group consisting of (i) ions of at least one polyvalent metal selected from the group consisting of iron, aluminum, and zirconium, and acetate ions, chloride ions, and sulfate ions. It is preferably at least one selected from the group consisting of at least one salt with an anion and (ii) polyaluminum chloride. A salt of at least one polyvalent metal ion selected from the group consisting of iron, aluminum, and zirconium and at least one anion selected from the group consisting of acetate, chloride, and sulfate ions Examples include zirconium acetate, aluminum chloride, aluminum sulfate, iron(II) sulfate, zirconium hydrochloride, and the like. Among them, the polyvalent metal salt is preferably at least one selected from the group consisting of aluminum chloride, aluminum sulfate, iron(II) sulfate, and polyaluminum chloride.

The salt of the cationic resin includes a nitrate of a resin having a primary to quaternary amine structure, a sulfate of a resin having a primary to quaternary amine structure, and a cationic resin having a group represented by the following general formula (1): It is preferably at least one selected from the group consisting of compounds.

(In general formula (1), R ₁ is a hydrogen atom or an alkyl group having 1 to 30 carbon atoms, and R ₂ , R ₃ , R ₄ and R ₅ each independently have 1 to 6 carbon atoms. are the following alkyl groups.)

Examples of nitrates of resins having primary to quaternary amine structures include polyallylamine nitrates. Sulfates of resins having a primary to quaternary amine structure include sulfates of dicyandiamide/diethylenetriamine resins. Cationic compounds having a group represented by general formula (1) are commercially available as "hindered amines" such as ADEKA STAB LA-63P and LA-68 (manufactured by ADEKA). Among them, the cationic resin salt is preferably at least one selected from the group consisting of polyallylamine nitrate and Adekastab LA-63P.

The cohesion of silver particles or dye is preferably 30% or more. When the sample solution is exposed to light, the light is absorbed by the substances in the solution, which weakens the light transmitted through the solution. Let I ₀ be the intensity of light applied to the solution, and I be the intensity of light transmitted through the solution. Transmittance T can be expressed as I/I ₀ and absorbance K can be expressed as −Log ₁₀ T. Therefore, the greater the amount of light absorbed (the smaller the value of the transmittance T), the greater the absorbance K, and the less the amount of light absorbed (the greater the value of the transmittance T), the smaller the absorbance K.

Let Ka be the absorbance of Liquid 1 containing 10.0% by mass of silver particles, and _Kb be the absorbance of Liquid 3 obtained by mixing Liquid 2 containing 1.0% by mass of _a cationic compound with Liquid 1. . Generally, a dispersant does not greatly affect absorbance, so if a dispersant is required to disperse the silver particles, the dispersant may also be used in the preparation of Liquid 1. Both Ka and _Kb are values at _a wavelength of 420 nm. When light passes through the first ink containing silver particles, the light is absorbed by the silver particles, so the value of _Ka increases. However, when light passes through the mixture of the cationic compound and the first ink containing the silver particles, the silver particles aggregate and precipitate due to the _cationic compound. value becomes smaller. Therefore, an index indicating how much the cationic compound in the first ink can aggregate silver particles (strength to aggregate silver particles (%)) is (K _a −K _b )/K _a ×100. can be represented. In addition, the absorbance K is measured using a spectrophotometer.

Examples of cationic compounds having a cohesive force of silver particles of 30% or more include zirconium hydrochloride (60%), iron (II) sulfate (96%), aluminum chloride (99%), aluminum sulfate (99%), and Adekastab LA. -63P (92%) and the like. The value in parentheses is the cohesion force measured using the silver particle dispersion liquid 1 prepared in the example described later. Among them, a cationic compound having a cohesive strength of silver particles of 60% or more is more preferable.

Similarly, the absorbance of liquid 4 containing 5.0% by mass of dye is _Kc , and the absorbance of liquid 5 obtained by mixing liquid 2 containing 1.0% by mass of cationic compound and liquid 4 is _Kd. and Both K _c and K _d are values at the maximum absorption wavelength λ _max of the dye (however, in the case of black dye, the values are at a wavelength of 500 nm). When light passes through the second ink containing the dye, the value of _Kc increases because the dye absorbs the light. However, when light passes through the mixture of the cationic compound and the second ink containing the dye, the dye aggregates and precipitates due to the _cationic compound. Become. Therefore, an index indicating how much the cationic compound in the second ink can agglutinate the dye (dye aggregating power (%)) is expressed as (K _c −K _d )/K _c ×100. can be done. In addition, the absorbance K is measured using a spectrophotometer.

Examples of cationic compounds having a dye cohesion of 30% or more include aluminum chloride (30%), zirconium acetate (55%), dicyandiamide/diethylenetriamine resin sulfate (65%), polyaluminum chloride (90%), polyallylamine nitrate (96%) and the like. The value in parentheses is the cohesion force measured using the cationic compound 2 prepared in Examples described later. Among them, a cationic compound having a dye cohesive strength of 60% or more is more preferable.

The content (g/m ² ) of the cationic compound per unit area of the ink-receiving layer is 0.20 g/m ² or more. More preferably, the content is 0.70 g/m ² or more. Moreover, the content is preferably 5.00 g/m ² or less. If the content exceeds 5.00 g/m ² , the absorbability of the ink will be low, and water in the ink will remain on the recording medium, making it difficult for the silver particles to fuse together. As a result, sufficient glossiness of the image may not be obtained.

[Other additives]
The ink-receiving layer may contain other additives than those mentioned so far. Specific examples include pH adjusters, thickeners, mold release agents, fluorescent whitening agents, ultraviolet absorbers, preservatives, antifungal agents, waterproofing agents, and curing agents.

[Physical properties of recording medium]
The paper surface pH of the recording medium is preferably 6.0 or less. The paper surface pH of the recording medium is the pH of the ink receiving layer. When the paper surface pH exceeds 6.0, since the amount of hydroxide ions is large, the hydroxide ions and the ionized silver (silver ions) on the surface of the silver particles react to form silver oxide (Ag ₂ O ) is formed. This silver oxide makes it difficult for silver particles to fuse together, making it difficult to form a dense silver layer. As a result, sufficient color developability and glossiness of the image may not be obtained.

The paper surface pH of the recording medium is determined according to JAPAN TAPPI No. 49-1 "Paper and paperboard-Surface pH test method-Part 1: Glass electrode method".

[Method for producing recording medium]
A method for manufacturing a recording medium preferably includes a step of preparing a first coating liquid and a step of applying the first coating liquid to a substrate. Furthermore, a step of applying a liquid (hereinafter referred to as a "second coating liquid") containing a cationic compound and at least one of a chloride ion scavenger and an organic antioxidant to the substrate. is preferred. A method for manufacturing the recording medium will be described below.

Examples of methods for forming the ink-receiving layer on the substrate include the following methods. First, a first coating liquid is prepared. Then, the substrate is coated with the first coating liquid, and the substrate coated with the first coating liquid is dried. Further, a second coating liquid is applied to the ink-receiving layer so that the ink-receiving layer contains a cationic compound, a chloride ion scavenger, and an organic antioxidant. Let the material dry. Thereby, a recording medium can be obtained. As a method for applying the first coating liquid and the second coating liquid, a curtain coater, a coater using an extrusion method, a coater using a slide hopper method, or the like can be used. The first coating liquid and the second coating liquid may be heated during coating. In addition, drying methods after coating include methods using hot air dryers such as straight tunnel dryers, arch dryers, air loop dryers, sine curve air float dryers, and dryers using infrared rays, heating dryers, microwaves, etc. and the like.

(Density of Formed Silver Layer)
The denseness of the silver layer formed on the recording medium is a value obtained by observing the cross section of the recording medium with a scanning electron microscope (SEM). The compactness is defined as 100% when the silver layer has no pores. Therefore, the smaller the denseness of the silver layer, the more pores the silver layer has.

When the ink-receiving layer of the recording medium contains zirconium hydrochloride, iron(II) sulfate, aluminum chloride, aluminum sulfate, or Adekastab LA-63P, the denseness of the silver layer formed on the recording medium is 75% or more. is preferably If the denseness of the silver layer is less than 75%, the dye in the second ink enters between the silver particles together with the water, and further penetrates into the recording medium, causing the dye to enter the silver layer. It may not be possible to retain the color, and the image may not exhibit sufficient color development. The denseness of the silver layer is more preferably 90% or more, and further preferably 96% or less.

When the ink-receiving layer of the recording medium contains aluminum chloride, zirconium acetate, dicyandiamide/diethylenetriamine resin sulfate, polyaluminum chloride, or polyallylamine nitrate, the density of the silver layer is 90% or less. preferable. When the denseness of the silver layer exceeds 90%, it becomes difficult for the cationic compound to enter the recording medium, so that the dye in the second ink is less likely to react with the cationic compound contained in the recording medium. As a result, the dye cannot be retained on the recording medium, and the image may not exhibit sufficient color development. The denseness of the silver layer is more preferably 75% or less, more preferably 50% or more.

EXAMPLES The present invention will be described in more detail below with reference to examples , comparative examples , and reference examples , but the present invention is not limited by the following examples as long as it does not exceed the scope of the invention. "Parts" and "%" regarding component amounts are based on mass unless otherwise specified.

<Production of Recording Medium>
(Preparation of base material 1)
80 parts hardwood bleached kraft pulp (LBKP) with a Canadian standard freeness of 450 mLCSF, 20 parts softwood bleached kraft pulp (NBKP) with a Canadian standard freeness of 480 mLCSF, 0.60 parts cationic starch, 10 parts ground calcium carbonate , 15 parts of light calcium carbonate, 0.10 parts of alkyl ketene dimer, and 0.03 parts of cationic polyacrylamide are mixed, water is added so that the solid content of these is 3.0%, and paper stock got Next, the stock was made into paper by a fourdrinier paper machine, wet-pressed in three stages, and then dried by a multi-tube dryer. Then, it was impregnated with an oxidized starch aqueous solution and dried by a size press so that the solid content after drying was 1.0 g/m ² . Furthermore, machine calendered base paper having a basis weight of 170 g/m ² , a Stockigt sizing degree of 100 seconds, an air permeability of 50 seconds, a Bekk smoothness of 30 seconds, a Gurley stiffness of 11.0 mN, and a film thickness of 100 μm. made. Then, a resin composition of 70 parts of low-density polyethylene (LDPE), 20 parts of high-density polyethylene (HDPE), and 10 parts of titanium oxide was applied to one side of the base paper so that the dry coating amount was 25 g/m ² . coated. In addition, let this surface be the surface of a base material. Furthermore, a base material 1 was obtained by coating the other side of the base paper with low-density polyethylene.

(Production of base material 2)
20.0 parts of light calcium carbonate is added to a slurry of 100.0 parts of bleached hardwood kraft pulp, 2.0 parts of cationic starch and 0.3 parts of alkenylsuccinic anhydride-based neutral sizing agent are added, and sufficiently The raw materials for papermaking were obtained by mixing. Dry to 10% moisture using a fourdrinier multi-cylinder paper machine, apply 4 g/m ² of a 7% solution of oxidized starch to both sides using a size press, and dry to 7% moisture to achieve a basis weight of 110 g/m2. A base paper of m ² was made. A resin composition composed of 70 parts of high-density polyethylene and 20 parts of low-density polyethylene was applied to both the front and back surfaces of the base paper by melt extrusion so that the coating amount per side was 30 g/m ² . Material 2 was obtained.

<Preparation of coating solution for ink-receiving layer>
(Preparation of inorganic particle dispersion liquid 1)
40.0 g of alumina hydrate (DISPERAL HP14, manufactured by Sasol) and 0.6 g of methanesulfonic acid were added to 160.0 g of pure water. After that, the mixture was stirred for 30 minutes with a mixer to prepare an inorganic particle dispersion liquid 1 containing 20.0% of alumina hydrate as inorganic particles. The primary average particle size of the alumina hydrate in the inorganic particle dispersion was 130 nm.

(Preparation of inorganic particle dispersion liquid 2)
2 parts of acetic acid was added to 498 parts of ion-exchanged water. While stirring this acetic acid aqueous solution with a homomixer (TK homomixer MARK II 2.5 type, manufactured by Tokushu Kika Kogyo Co., Ltd.) under a rotation condition of 3000 rpm, 100 parts of alumina hydrate (DISPERAL HP14, manufactured by Sasol) is added little by little. added. After that, by stirring for 30 minutes, the alumina hydrate was dispersed with acetic acid to prepare an inorganic particle dispersion liquid 2 containing 23.0% of the alumina hydrate.

(Preparation of inorganic particle dispersion liquid 3)
To 333 parts of ion-exchanged water, 4 parts of a cationic polymer (Charol DC902P, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was added. While stirring this cationic polymer aqueous solution with a homomixer (TK homomixer MARK II 2.5 type, manufactured by Tokushu Kika Kogyo Co., Ltd.) at 3000 rpm, a small amount of 100 parts of gas-phase silica (AEROSIL 300, manufactured by EVONIK) was added. added one by one. After that, the resulting solution was diluted with ion-exchanged water and treated twice with a high-pressure homogenizer (Nanomizer, manufactured by Yoshida Kikai Kogyo Co., Ltd.) to micronize the vapor-phase silica so that the vapor-phase silica was 20.0%. An inorganic particle dispersion liquid 3 was prepared.

(Preparation of first coating liquid 1)
Polyvinyl alcohol (PVA235, manufactured by Kuraray, degree of polymerization: 3500, degree of saponification: 88%) was dissolved in ion-exchanged water to obtain an aqueous solution of polyvinyl alcohol with a solid content of 8.0%. Then, an aqueous polyvinyl alcohol solution was mixed with the inorganic particle dispersion liquid 1 so that the content of polyvinyl alcohol was 10.0% with respect to the content of the inorganic particles. Furthermore, orthoboric acid as a cross-linking agent was dissolved in ion-exchanged water to obtain an aqueous boric acid solution containing 3.0% orthoboric acid. A first coating liquid 1 was obtained by mixing an aqueous boric acid solution so that the content of boric acid was 1.0% with respect to the content of inorganic particles.

(Preparation of first coating liquids 2 to 6)
Polyvinyl alcohol (PVA235 manufactured by Kuraray, degree of polymerization: 3500, degree of saponification: 88%) was dissolved in deionized water to obtain a polyvinyl alcohol aqueous solution containing 8.0% of polyvinyl alcohol. Furthermore, orthoboric acid as a cross-linking agent was dissolved in ion-exchanged water to obtain an aqueous boric acid solution containing 3.0% orthoboric acid. Using these aqueous solutions, first coating solutions 2 to 6 were prepared so that the contents (parts) shown in Table 1 were obtained.

Here, a method for calculating the content (equivalent) of the cross-linking agent with respect to polyvinyl alcohol or a derivative thereof will be described using the first coating liquid 2 . Since orthoboric acid is used as a cross-linking agent and the maximum coordination number of boron is 4, boron can react with 4 hydroxyl groups of polyvinyl alcohol. In other words, in order for orthoboric acid to react with polyvinyl alcohol without excess or deficiency, the formula of the number of moles of hydroxyl groups possessed by polyvinyl alcohol x content (equivalent) of orthoboric acid with respect to polyvinyl alcohol = 4 x the number of moles of orthoboric acid was established. There must be. Therefore, the content (equivalent) of orthoboric acid relative to polyvinyl alcohol can be calculated from 4×moles of orthoboric acid/moles of hydroxy groups possessed by polyvinyl alcohol.

The basic structure of polyvinyl alcohol is —(CH ₂ —CH(OH)) _m —(CH ₂ —CH(OCOCH ₃ )) _n —. The degree of saponification (%) of polyvinyl alcohol is m/(m+n)×100 and indicates the ratio of CH ₂ —CH(OH) in polyvinyl alcohol. The degree of saponification of polyvinyl alcohol used in the first coating liquid 2 is 88%, the molecular weight (g/mol) of CH ₂ —CH(OH) is 44.00, and CH ₂ —CH(OCOCH ₃ ) has a molecular weight (g/mol) of 86.00. As a result, the number of moles of hydroxyl groups possessed by polyvinyl alcohol is 11.00/{(44.00×88/100)+86.00×(100−88)/100}×88/100. Since the molecular weight (g/mol) of orthoboric acid is 61.83, the number of moles of orthoboric acid is 1.01/61.83. From these values, the content (equivalent) of orthoboric acid relative to polyvinyl alcohol can be calculated.

The prepared first coating liquid was applied to the base material so that the dry coating amount (g/m ² ) was 25 g/m ² and dried with hot air at a temperature of 90°C to obtain an ink-receiving layer. rice field. Next, a second coating solution shown in Tables 2 to 4 was prepared so that the ink-receiving layer contained at least one of a cationic compound, a chloride ion scavenger, and an organic antioxidant. did. Then, the second coating liquid was applied to the ink-receiving layer and dried with hot air at a temperature of 90° C. to obtain a recording medium. Tables 5 to 7 show combinations of substrates, first coating liquids, and second coating liquids. The chloride ion content (g/m ² ) per unit area of the ink receiving layer of recording medium 1 was 0.00 g/m ² , and the chloride ion content per unit area of the ink receiving layer of recording media 2 to 58 was The content of compound ions (g/m ² ) was 0.20 g/m ² . Furthermore, the paper surface pH of the recording medium 18 was 6.0, and the paper surface pH of the recording medium 19 was 7.0. The paper surface pH of recording media 1 to 17 and 20 to 58 was 4.2. The paper surface pH of the recording medium is determined according to JAPAN TAPPI No. 49-1 "Paper and paperboard-Surface pH test method-Part 1: Glass electrode method", the value was measured with an electrode installation time of 30 seconds.

<Preparation of silver particle dispersion>
According to the preparation method described in Example 2 of WO 2008/049519, a silver particle dispersion 1 (silver particle content: 20.0%, resin content: 2.0%) was prepared. Obtained. Further, according to the preparation method described in Example 2-2 of JP-A-2004-285106, silver particle dispersion liquid 2 (silver particle content: 20.0%, surfactant content: 2 .0%) was obtained. Silver particle dispersions 3 and 4 were prepared in the same manner as silver particle dispersion 2, but the _D50 value was changed by changing the stirring speed during the preparation. Silver particle dispersion liquids 5, 6, 7 and 8 were prepared in the same manner as silver particle dispersion liquid 1, but the content of the dispersant was changed. The _D50 of the silver particles was determined by observing the cross section of the obtained image with a scanning electron microscope (SEM) and is listed in Table 7. Furthermore, the content of the dispersant in the dispersion is also listed in Table 7.

<Preparation of first ink>
After mixing each component shown in Table 8 and sufficiently stirring, pressure filtration was performed with a filter having a pore size of 1.2 μm to obtain a first ink. Acetylenol E100 is a nonionic surfactant manufactured by Kawaken Fine Chemicals. Acetylenol E100 has an HLB value of 13 determined by the Griffin method. Compound 1 was obtained from C.I. I. Acid Blue 9.

<Preparation of second ink>
(Preparation of dye)
The following compounds were prepared. Although the structural formula of compound 3 is shown in the free acid form, it was used as the potassium salt.
Compound 1: C.I. I. acid blue 9
Compound 2: C.I. I. acid red 249
Compound 3: A potassium salt of a compound represented by the following formula (2) as a free acid form synthesized according to the synthesis method described in JP-A-2016-108545

(second ink)
After mixing each component shown in Table 9 and sufficiently stirring, pressure filtration was performed with a filter having a pore size of 1.2 μm to obtain a second ink. Acetylenol E100 is a nonionic surfactant manufactured by Kawaken Fine Chemicals.

<Evaluation>
The first and second inks shown in Tables 10 and 11 were filled in ink cartridges, respectively, and set in an inkjet recording apparatus (PIXUS MG3630, manufactured by Canon Inc.) equipped with a recording head that ejects ink using thermal energy. In this embodiment, the first ink is assumed to have a print duty of 100% for an image printed under the condition that two ink droplets of about 11.2 ng are applied to a unit area of 1/600 inch×1/600 inch. Define. The second ink is defined as having a print duty of 100% for an image printed under the condition that two ink droplets of about 5.7 ng are applied to a unit area of 1/600 inch×1/600 inch. Using the inkjet recording apparatus, the first ink was applied to the recording media shown in Tables 10 and 11 at a recording duty of 100%. After that, the second ink was applied at a recording duty of 100% so that at least a part of the area to which the first ink was applied overlapped. In the present invention, according to the following evaluation criteria, AAA, AA, A or B was defined as an acceptable level, and C was defined as an unacceptable level. Evaluation results are described in Tables 10 and 11.

(chromogenic)
Using an integrating sphere type colorimeter CM-2600d (manufactured by Konica Minolta) in SCI mode, measurements were made as follows to evaluate color development. a ₀ ^* and b ₀ ^* of an image recorded at a recording duty of 100% using only the first ink containing silver particles, and each ink using an ink containing silver particles and an ink containing a dye at a recording duty of 100%. The a ₁ ^* and b ₁ ^* of the images recorded in % were measured. Using these values, ΔE _ab was calculated from the formula: color difference ΔE _ab ={(a ₁ ^* −a ^* ₀ ) ² +(b ₁ ^* −b ^* ₀ ) ² } ^1/2 . a ^* and b ^* are the values of the L ^* a ^* b ^* system defined by the CIE (International Commission on Illumination). When ΔE _ab is 2.0 or more, an image is obtained in which the color tone of the dye used is recognizable instead of silver. When the value of ΔE _ab is large, an image is obtained in which the color tone of the dye used is well recognized.

AAA: ΔE _ab was 12.0 or more AA: ΔE _ab was 10.0 or more and less than 12.0 A: ΔE _ab was 6.0 or more and less than 10.0 B: ΔE _ab was 2.0 or more was less than 6.0 C: ΔE _ab was less than 2.0.

(glossiness)
For the obtained image, using a goniospectrophotometer (GSP-2, manufactured by Murakami Shiki), sharpness = L / w (L is the highest lightness measured at the light receiving part of the spectrophotometer As the lightness value, w is the width of the light-receiving angle at two points representing the half value of L (L/2). If the definition value is 0.2 or more, when the image is visually observed, the brightness changes depending on the viewing angle, and thus the image can be recognized as having glossiness.
AA: The definition was 0.6 or more A: The definition was 0.4 or more and less than 0.6 B: The definition was 0.2 or more and less than 0.4.

(Change in gloss over time)
A solid image with a recording duty of 100% was recorded with the first ink. This solid image was placed in an environment with a temperature of 25° C. and a relative humidity of 50% for one day to dry, and the 60° specular gloss was measured using a gloss meter (VG7000, manufactured by Nippon Denshoku Industries) (before the test). It is assumed that the glossiness is a). After that, the solid image was placed in a gas corrosion tester (manufactured by Suga Test Instruments) with a temperature of 25 ° C. and a relative humidity of 80% for 36 hours, and a mixed gas (0.90 ppm NO ₂ gas, 0.05 ppm of _SO2 gas _, and 0.15 ppm O3 gas). After that, the 60-degree specular glossiness of the solid image was measured again (referred to as post-test glossiness b). Then, based on the formula of b/a×100 (%), the rate of decrease in glossiness was calculated, and change in glossiness over time was evaluated according to the following criteria. It means that the lower the rate of decrease in glossiness is, the more suppressed the decrease in glossiness is of the image.
A: The rate of decrease in glossiness was less than 50%
B: The reduction rate of glossiness was 50% or more.

Claims (17)

a recording step of applying the first ink to the recording medium;
a recording step of applying a second ink to the recording medium so as to overlap at least a part of the area to which the first ink is applied,
the first ink is an aqueous ink containing silver particles;
the second ink is an aqueous ink containing an anionic dye;
The recording medium has an ink-receiving layer containing at least one cationic compound selected from the group consisting of polyvalent metal salts and salts of cationic resins,
The polyvalent metal salt is (i) selected from the group consisting of ions of at least one polyvalent metal selected from the group consisting of iron, aluminum, and zirconium, and acetate ions, chloride ions, and sulfate ions. and (ii) polyaluminum chloride, at least one selected from the group consisting of
The salt of the cationic resin is at least one selected from the group consisting of a nitrate of a resin having a primary to quaternary amine structure and a sulfate of a resin having a primary to quaternary amine structure,
An inkjet recording method, wherein the content (g/m ² ) of the cationic compound per unit area of the ink receiving layer is 0.20 g/m ² or more. The polyvalent metal salt is at least one selected from the group consisting of aluminum chloride, aluminum sulfate, iron (II) sulfate, and polyaluminum chloride ,
2. The ink jet recording method according to claim 1, wherein the cationic resin salt is polyallylamine nitrate. The ink jet recording method according to claim 1, wherein the cationic compound is polyallylamine nitrate. The inkjet recording method according to any one of claims 1 to 3 , wherein the content of the cationic compound per unit area (g/ ^m2 ) of the ink receiving layer is 5.00 g/ ^m2 or less. The inkjet recording method according to any one of claims 1 to 4 , wherein the volume-based cumulative 50% particle size (nm) of the silver particles is 150 nm or less. The inkjet recording method according to any one of claims 1 to 5 , wherein the volume-based cumulative 50% particle size (nm) of the silver particles is 1 nm or more. 7. The ink-receiving layer according to any one of claims 1 to 6 , wherein the ink-receiving layer contains chloride ions, a chloride ion scavenger, and at least one selected from the group consisting of organic antioxidants. The ink jet recording method described. 7. The inkjet recording method according to any one of claims 1 to 6, wherein the ink receiving layer contains chloride ions and a chloride ion scavenger. 9. The method according to claim 7 , wherein the chloride ion scavenger is 1,2,3- benzotriazole or a derivative thereof, and the organic antioxidant is ascorbic acid or a salt thereof. The ink jet recording method described. 10. The inkjet recording method according to any one of Claims 7 to 9, wherein the ink-receiving layer satisfies at least one of the following conditions (1) and (2).
Condition (1) The content (g/m ² ) of the chloride ion scavenger per unit area of the ink receiving layer is 0.02 g/m ² or more and 0.15 g/m ² or less.
Condition (2) The content (g/m ² ) of the organic antioxidant per unit area of the ink receiving layer is 0.05 g/m ² or more and 0.25 g/m ² or less. The ink-receiving layer contains polyvinyl alcohol or a derivative thereof and a cross-linking agent,
11. The method according to any one of claims 1 to 10 , wherein the content of the cross-linking agent in the ink-receiving layer is 0.40 equivalent or more and 1.00 equivalent or less with respect to the content of the polyvinyl alcohol or derivative thereof. The ink jet recording method described. 12. The inkjet recording method according to claim 11 , wherein the cross-linking agent is boric acid or a salt thereof. The inkjet recording method according to any one of claims 1 to 12 , wherein the recording medium has a paper surface pH of 6.0 or less. 14. The inkjet recording method of any one of claims 1 to 13 , wherein the content of the cationic compound per unit area (g/ ^m2 ) of the ink receiving layer is 0.70 g/ ^m2 or more. The content (% by mass) of the silver particles in the first ink is 2.0% by mass or more and 15.0% by mass based on the total mass of the ink. Inkjet recording method. 16. The method according to any one of claims 1 to 15 , wherein the content (% by mass) of the anionic dye in the second ink is 1.0% by mass or more and 10.0% by mass based on the total mass of the ink. The ink jet recording method described. An inkjet recording apparatus comprising means for applying a second ink to a recording medium after applying a first ink to the recording medium,
the first ink is an aqueous ink containing silver particles;
the second ink is an aqueous ink containing an anionic dye;
The recording medium has an ink-receiving layer containing at least one cationic compound selected from the group consisting of polyvalent metal salts and salts of cationic resins,
The polyvalent metal salt is (i) selected from the group consisting of ions of at least one polyvalent metal selected from the group consisting of iron, aluminum, and zirconium, and acetate ions, chloride ions, and sulfate ions. and (ii) polyaluminum chloride, at least one selected from the group consisting of
The salt of the cationic resin is at least one selected from the group consisting of a nitrate of a resin having a primary to quaternary amine structure and a sulfate of a resin having a primary to quaternary amine structure,
An inkjet recording apparatus, wherein the content (g/m ² ) of the cationic compound per unit area of the ink receiving layer is 0.20 g/m ² or more.

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