Water-based ink, ink cartridge, and ink-jet recording method

Abstract

To provide a water-based ink capable of recording an image having excellent glossiness not only at beginning after recording, but also after storage.SOLUTION: There is provided a water-based ink containing silver particles, a first reductant, and a second reductant, in which the first reductant is a compound having oxidation reduction potential R(mV) of R<0, the second reductant has oxidation potential R(mV) of R≥0, and is at least one kind selected from a group consisting of a tertiary amine, and a polyvalent alcohol having a plurality of hydroxyl groups in one of structures divided into two based on one of two neighboring carbon atoms of the longest carbon chain in a molecule.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-based ink, an ink cartridge, and an inkjet recording method. [Background technology]

[0002] 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 such as Christmas cards to express a metallic look. In such applications, there is a demand for recording images with a metallic look (hereinafter sometimes referred to as "metallic images") to enhance the decorativeness of the image. Inks containing aluminum pigments have been proposed to record metallic images (see Patent Document 1). Furthermore, water-based inks containing silver particles have been proposed (see Patent Documents 2 and 3). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-064053 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-241242 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-140635 Summary of the Invention [Problem to be solved by the invention]

[0004] The present inventors have investigated conventional inks containing metal particles as a coloring material. As a result, they have found that in order to achieve the glossiness required for metallic images, it is necessary to use silver particles as a coloring material. That is, they have found that the glossiness of images recorded with aqueous inks containing aluminum as a coloring material, as described in Patent Document 1, is inferior to that of images recorded with silver particles.

[0005] Furthermore, metallic images with good glossiness can be recorded using inks that use silver particles as a coloring material, as described in Patent Documents 2 and 3. However, it has been found that when such images are stored, the glossiness (initial glossiness) that was present when the images were first recorded gradually deteriorates.

[0006] Therefore, an object of the present invention is to provide an aqueous ink capable of recording images that have excellent gloss not only immediately after recording but also after storage, an ink cartridge using the aqueous ink, and an inkjet recording method. [Means for solving the problem]

[0007] The above object is achieved by the present invention as described below. That is, the aqueous ink according to the present invention is an aqueous ink for inkjet use containing silver particles, a first reducing agent, and a second reducing agent, wherein the first reducing agent is selected from the group consisting of ascorbic acid, sodium ascorbate, N-methyl-p-aminophenol sulfate , hydroquinone, and 1-phenyl-3-pyrazolidone the second reducing agent is at least one selected from the group consisting of triethanolamine, triethylamine, sorbitol, 1,2-propanediol, 1,2-pentanediol, 1,2-hexanediol, 1,2,6-hexanetriol, glycerin, and trimethylolpropane; the content (mmol / g) of the first reducing agent in the aqueous ink is 0.1 times or more in terms of molar ratio to the content (mmol / g) of the silver particles; and the content (mmol / g) of the second reducing agent in the aqueous ink is 0.1 times or more in terms of molar ratio to the content (mmol / g) of the first reducing agent. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an aqueous ink capable of recording images that have excellent gloss not only immediately after recording but also after storage, an ink cartridge using the aqueous ink, and an inkjet recording method. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view schematically illustrating an embodiment of an ink cartridge of the present invention. [Figure 2] 1A and 1B are diagrams schematically 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 the main part of the inkjet recording apparatus, and FIG. 1B is a perspective view of a head cartridge. DETAILED DESCRIPTION OF THE INVENTION

[0010] 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 as dissociated ions, but for convenience, it will be expressed as "containing a salt." Furthermore, aqueous inks may be simply referred to as "ink." Physical property values ​​are values ​​at room temperature (25°C) unless otherwise specified.

[0011] Images with good initial gloss can be recorded using aqueous inks containing silver particles instead of aluminum as a coloring material. However, as a result of the inventors' investigations, it was found that images recorded using aqueous inks containing silver particles may lose gloss when stored for a long period of time. Analysis of an image with good gloss immediately after recording and an image whose gloss had decreased after storage revealed that silver chloride crystals had formed in the silver layer that constituted the image, and that the crystals had grown larger in the image after storage. It is believed that the formation and crystal growth of silver chloride occur as follows.

[0012] Recording media used for recording images generally contain chloride ions (Cl - ) are included. For example, recording media without an ink-receiving layer, such as plain paper, contain chloride ions derived from pulp bleaching agents. Recording media with an ink-receiving layer contain cationic compounds such as resins whose counter ions are chloride ions. When aqueous ink containing silver particles is applied to such a recording medium, the hydrogen atoms that make up the water molecules in the ink are converted to δ+ Since it is polarized to height, This water causes chloride ions to seep out near the surface of the recording medium. The chloride ions react with the silver attached to the recording medium to produce silver chloride. Silver chloride is poorly soluble in water, so it forms crystals. After that, even while the image is being stored, water molecules continue to be supplied by the recording medium's moisture absorption. As a result, the same reaction as above occurs, and silver chloride crystals grow, which is thought to be why the gloss of the image decreases when it is stored.

[0013] During the reaction between silver and chloride ions to produce silver chloride, silver atoms are oxidized. Therefore, the inventors decided to add a first reducing agent to the ink containing silver particles. Even if silver chloride is produced when the ink is applied to a recording medium, the first reducing agent reduces the silver chloride to silver, thereby improving the initial glossiness of the image. The first reducing agent used is a compound whose oxidation-reduction potential R1 (mV) is less than 0.

[0014] Here, the redox potential (mV) of a substance is measured by placing a reference electrode (platinum electrode) and a comparison electrode in a solution containing the substance. The redox potential (mV) is an index that indicates the ease with which electrons are transferred to the electrode, and the smaller the redox potential (mV), the easier it is for electrons to be transferred to the electrode, meaning that the reducing power of the substance to be measured is greater. In the present invention, a silver-silver chloride electrode is used as the comparison electrode. On the surface of the silver-silver chloride electrode, Ag+Cl - ⇔AgCl+e - A reversible oxidation-reduction reaction occurs. Compounds with a small oxidation-reduction potential R1 (mV) measured using a silver-silver chloride electrode, especially compounds that satisfy R1<0, tend to release electrons, and the released electrons move to the electrode and form AgCl+e on the surface of the electrode. - →Ag+Cl - This can cause the reaction:

[0015] In other words, when a compound with an oxidation-reduction potential R1 (mV) of R1<0 is used as the first reducing agent, the reaction of reducing silver chloride to silver can be efficiently carried out, thereby improving the initial glossiness of the image.

[0016] However, when the first reducing agent reduces silver chloride, it is oxidized. The oxide of the first reducing agent thus produced adheres to the unreduced silver chloride crystals, acting as a steric hindrance and preventing further reduction. In other words, the loss of gloss during image storage cannot be suppressed by the first reducing agent alone.

[0017] Therefore, the inventors decided to add a second reducing agent to the ink containing silver particles in addition to the first reducing agent. To ensure that the first reducing agent continues to function by reducing the oxides of the first reducing agent attached to the silver chloride crystals with the second reducing agent, the following two conditions must be met: The reducing power of the second reducing agent must be weaker than that of the first reducing agent, and the second reducing agent must be present at an appropriate distance from the silver chloride crystals. A specific compound with an oxidation-reduction potential R2 (mV) of R2≧0 is used as the second reducing agent that satisfies these conditions. The oxidation-reduction potential of the second reducing agent and the type of compound are described below.

[0018] The second reducing agent is a compound with a lower reducing power than the first reducing agent, i.e., a compound with an oxidation-reduction potential R2 (mV) of R2 ≧ 0. When a compound with an oxidation-reduction potential R2 (mV) of R2 ≧ 0 is used in combination with the first reducing agent, the first reducing agent, which has a higher reducing power, acts preferentially on silver chloride. Therefore, the second reducing agent does not act directly on silver chloride, but is preferentially used in a reaction to reduce the oxide of the first reducing agent.

[0019] Furthermore, while satisfying the above-mentioned reducing power relationship, the second reducing agent must be present at an appropriate distance from the silver chloride crystals. If the distance between the second reducing agent and the silver chloride crystals is too short, the first reducing agent will be prevented from approaching the silver chloride and acting on it. If the distance between the second reducing agent and the silver chloride crystals is too long, it will be difficult to reduce the oxide of the first reducing agent attached to the silver chloride crystals. From this perspective, a specific compound must be used as the second reducing agent. This specific compound is at least one selected from the group consisting of (1) tertiary amines and (2) polyhydric alcohols having multiple hydroxy groups on one side of a structure obtained by bisecting the longest carbon chain in the molecule along either of the two adjacent carbon atoms. Each compound is described below.

[0020] (1) Tertiary amines As can be seen from their widespread use as protective agents for silver particles, organic amines have the property of easily adsorbing to silver, a property that is affected by steric hindrance. Tertiary amines have greater steric hindrance than primary and secondary amines, and therefore have a high affinity for silver particles, but maintain a certain distance from the silver chloride crystals. This allows the first reducing agent to approach the silver chloride crystals more easily and efficiently reduce the oxides of the first reducing agent attached to the silver chloride crystals, thereby preventing the loss of gloss after image storage. On the other hand, primary and secondary amines have less steric hindrance, making them more easily approachable to silver and more likely to preferentially attach to the silver chloride crystals. This makes it difficult for the first reducing agent to act, preventing the loss of gloss after image storage.

[0021] (2) Certain polyhydric alcohols Although the hydroxyl groups of alcohols have an orientation property toward silver, their affinity is weaker than that of amines. Therefore, it is necessary to use alcohol compounds with a structure in which two or more hydroxyl groups can act to orient toward silver when considering the three-dimensional atomic arrangement. To achieve this, it is necessary to use polyhydric alcohols with multiple hydroxyl groups on one side of the structure obtained by dividing the longest carbon chain in the molecule into two halves based on either of the two adjacent carbon atoms. Compounds that do not meet this condition, such as compounds with one hydroxyl group at each end of a linear hydrocarbon chain (ethylene glycol, 1,6-hexanediol) or monohydric alcohols, cannot suppress the loss of gloss after image storage. This is thought to be because, when considering the three-dimensional atomic arrangement, there is only one hydroxyl group that effectively acts to orient toward silver, resulting in insufficient orientation toward silver and the inability to maintain an appropriate distance from the silver chloride crystals. As a result, the oxide of the first reducing agent cannot be reduced, and the loss of gloss after image storage cannot be suppressed.

[0022] <Water-based ink> The ink of the present invention is an aqueous ink containing silver particles, a first reducing agent, and a second reducing agent. This aqueous ink is suitable for use in inkjet printing. However, the ink of the present invention does not need to be actinic radiation curable, and therefore does not need to contain a monomer having a polymerizable group. The components constituting the aqueous ink are described below. The content of each component, expressed in mmol / g, is the number of millimoles of the component per gram of ink.

[0023] (silver particles) The silver particles are composed of silver atoms. In addition to silver atoms, the silver particles may also contain other metal atoms, oxygen atoms, sulfur atoms, carbon atoms, etc. However, the proportion (%) of silver atoms in the silver particles is preferably 50.0 mass% or more. The volume-based cumulative 50% particle diameter of the silver particles, measured by dynamic light scattering, is preferably 10 nm or more and 150 nm or less.

[0024] The content (mass %) of silver particles in the aqueous ink is preferably 2.0% to 15.0% by mass, and more preferably 2.0% to 8.0% by mass, based on the total mass of the ink. If the content is less than 2.0% by mass, the amount of silver particles is too small, and the initial gloss of the image may not be sufficient. If the content is more than 15.0% by mass, the ink ejection stability may be slightly reduced when applied to an ejection method using thermal energy. This is because the silver particles adhering to the heater of the recording head increase the thermal energy imparted to the ink, making it difficult to stabilize the foaming. The content (mmol / g) of silver particles in the aqueous ink is preferably 15.0 mmol / g to 150.0 mmol / g, and more preferably 15.0 mmol / g to 80.0 mmol / g.

[0025] Examples of methods for producing silver particles include 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 agglomerating silver ions or silver complexes with a general-purpose reducing agent (reduction method). In the present invention, from the viewpoints of ease of particle size control and dispersion stability of the silver particles, it is preferable to use silver particles produced by a reduction method.

[0026] The silver particles are preferably dispersed using a dispersant such as a surfactant or resin. The content (mass%) of the dispersant in the aqueous ink is preferably 0.1% by mass or more and 5.0% by mass or less, based on the total mass of the ink. Furthermore, the mass ratio of the dispersant content (mass%) in the aqueous ink to the content (mass%) of silver particles is preferably 0.2 to 1.5 times. If the mass ratio is less than 0.2, the amount of dispersant relative to the silver particles is too small, and the silver particles attached to the recording medium are not sufficiently covered with the dispersant, resulting in a large amount of exposed surface of the silver particles. This facilitates the reaction between silver and chloride ions, which easily produces silver chloride, and may result in insufficient suppression of gloss loss after image storage. If the mass ratio is more than 1.5, the amount of dispersant relative to the silver particles is too large, and the silver particles attached to the recording medium are excessively covered with the dispersant, resulting in a small amount of exposed surface of the silver particles. This makes it difficult for the silver particles to aggregate after the ink is applied to the recording medium, which may result in insufficient initial gloss of the image.

[0027] Various surfactants can be used as dispersants for silver particles, including anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants. Examples of anionic surfactants include fatty acid salts, alkyl sulfate ester salts, alkylaryl sulfonates, alkyl diaryl ether disulfonates, dialkyl sulfosuccinates, alkyl phosphates, naphthalene sulfonate-formalin 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 oxypropylene 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. Among them, 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. Furthermore, as the nonionic surfactant, it is preferable to use polyoxyethylene alkyl ether.

[0028] Furthermore, a resin having a unit with an anionic group and a unit without an anionic group can be used as a dispersant for silver particles. Examples of the resin skeleton include vinyl resins, ester resins, amino resins, acrylic resins, epoxy resins, urethane resins, ether resins, amide resins, phenol resins, silicone resins, and fluorine resins.

[0029] (reducing agent) The ink contains a first reducing agent and a second reducing agent. The first reducing agent is a compound whose redox potential R1 (mV) is R1<0. The second reducing agent has a redox potential R2 (mV) of R2≧0 and is at least one selected from the group consisting of tertiary amines and specific polyhydric alcohols. The reducing agent is a substance that has the ability to reduce other substances and is oxidized by releasing electrons.

[0030] The redox potential (mV) of each reducing agent is measured using a silver-silver chloride electrode at a pH of 9. The reason for selecting this electrode is not related to the fact that the ink colorant is silver, but rather because it is easy to measure and provides highly accurate measurements. Since the redox potential depends on pH, the pH is close to that of general aqueous inks, and the measured values ​​are stable. Therefore, in the present invention, the redox potential (mV) value at pH 9 is used.

[0031] Specifically, the substance to be measured is dissolved in a borate standard solution as a pH standard solution so that the content is 0.1 mass %, and the pH of the liquid is adjusted to 9 as necessary, and the oxidation-reduction potential is measured. If the pH of the liquid is less than 9, the pH can be increased using a base such as potassium hydroxide (which may be an aqueous solution), and if the pH of the liquid is greater than 9, the pH can be decreased using an acid such as sulfuric acid.

[0032] [First reducing agent] The first reducing agent is a compound whose redox potential R1 (mV) is less than zero, i.e., R1<0. Any substance may be used as the first reducing agent as long as it satisfies the definition, and examples thereof include sugars, phenols, and pyrazolidones having a lactone structure. These first reducing agents can efficiently reduce silver chloride to silver.

[0033] Examples of sugars having a lactone structure include ascorbic acid. Examples of phenols include N-methyl-p-aminophenol sulfate (trade name "Metol") and hydroquinone. Examples of pyrazolidones include 1-phenyl-3-pyrazolidone (trade name "Phenidone"), 1-phenyl-4-methyl-3-pyrazolidone, 1-phenyl-4-ethyl-3-pyrazolidone, and 1-phenyl-4,4-dimethyl-3-pyrazolidone. The acid group in compounds such as ascorbic acid may form a salt. Examples of cations that form salts include alkali metal ions such as lithium, sodium, and potassium.

[0034] Among these, the first reducing agent is preferably a sugar having a lactone structure, and more preferably ascorbic acid. Among the first reducing agents, ascorbic acid is easily reduced by the second reducing agent, and its function as the first reducing agent is easily restored, so that it can continue to efficiently reduce silver chloride. This makes it possible to more effectively prevent the deterioration of gloss after storage of the image.

[0035] The content (mmol / g) of the first reducing agent in the aqueous ink is preferably 0.1 mmol / g or more and 50.0 mmol / g or less, and more preferably 1.0 mmol / g or more and 30.0 mmol / g or less. Furthermore, the content (mmol / g) of the first reducing agent in the aqueous ink is preferably 0.1 or more times the molar ratio of the silver particle content (mmol / g). If the molar ratio is less than 0.1, the amount of the first reducing agent relative to the silver particles will be too small, which may result in a decrease in the initial gloss of the image, and may not be sufficient to prevent the decrease in gloss after storage of the image. The molar ratio is preferably 0.9 or less.

[0036] [Second reducing agent] The second reducing agent has a redox potential R2 (mV) greater than that of the first reducing agent, i.e., R2≧0, and is at least one selected from the group consisting of tertiary amines and specific polyhydric alcohols. Polyhydric alcohols are compounds having multiple hydroxy groups on one side of a structure obtained by bisecting the longest carbon chain in the molecule at either of the positions between two adjacent carbon atoms.

[0037] Tertiary amines include trialkanolamines such as triethanolamine and tripropanolamine; and trialkylamines such as triethylamine, tributylamine, and trihexylamine. Among these, alkanolamines are preferred. Alkanolamines not only have the property of easily adsorbing to silver, but also have orientation toward silver due to the hydroxyl groups present in their molecular structure. Therefore, alkanolamines can be present in a particularly suitable position for efficiently reducing the oxide of the first reducing agent, thereby more effectively suppressing the loss of gloss after image storage. Among alkanolamines, triethanolamine is particularly preferred.

[0038] Polyhydric alcohols are compounds that have multiple hydroxy groups on one side of the structure obtained by bisecting the longest carbon chain in the molecule based on either the distance between two adjacent carbon atoms. The "longest carbon chain in the molecule" conforms to the IUPAC nomenclature definition that the chain with the most carbon atoms constituting the chain is the "main chain." Polyhydric alcohols that satisfy the definition in the present invention have a structure in which, when taking into account the three-dimensional atomic arrangement, two or more hydroxy groups can act to orient toward silver.

[0039] 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, 1,2-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. Among these, sugar alcohols are preferred, with those having three or more hydroxy groups being even more preferred. These sugar alcohols can be oriented toward silver due to the presence of multiple hydroxy groups in sterically favorable positions. Therefore, these sugar alcohols can be positioned particularly well for efficient reduction of the oxide of the first reducing agent, thereby more effectively suppressing the loss of gloss after image storage. Among sugar alcohols, sorbitol is particularly preferred.

[0040] As the second reducing agent, alkanolamines are preferred, and among them, triethanolamine is particularly preferred.

[0041] The content (mmol / g) of the second reducing agent in the aqueous ink is preferably 0.1 mmol / g or more and 200.0 mmol / g or less, and more preferably 1.0 mmol / g or more and 150.0 mmol / g or less. Furthermore, the content (mmol / g) of the second reducing agent in the aqueous ink is preferably 0.1 or more times the molar ratio of the content (mmol / g) of the first reducing agent in the aqueous ink. If the molar ratio is less than 0.1, the amount of the second reducing agent relative to the first reducing agent is low, making it difficult to reduce the oxide of the first reducing agent, and may make it difficult to sufficiently suppress the decrease in glossiness after image storage. The molar ratio is preferably 20.0 or less, and more preferably 10.0 or less.

[0042] In order to efficiently exert the action of the second reducing agent, when primary or secondary amines are contained in the ink, it is preferable that the content thereof is not too high. Specifically, the content (mass %) of primary or secondary amines in the aqueous ink is preferably 0.1 mass % or less based on the total mass of the ink.

[0043] (aqueous medium) The ink is an aqueous ink containing at least water as an aqueous medium. The ink can contain water or an aqueous medium that is a mixed solvent of water and a water-soluble organic solvent. Deionized water or ion-exchanged water is preferably used as the water. The water content (mass %) in the aqueous ink is preferably 50.0 mass % or more and 95.0 mass % or less, based on the total mass of the ink.

[0044] The water-soluble organic solvent is not particularly limited as long as it is water-soluble, and any of monohydric alcohols, other polyhydric alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing polar solvents, sulfur-containing polar solvents, etc. can be used. While the term "water-soluble organic solvent" typically refers to a liquid, in the present invention, it also includes those that are solid at 25°C. The content (mass %) of the water-soluble organic solvent in the ink is preferably 3.0% to 50.0% by mass based on the total mass of the ink. This content includes the content of a specific compound used as a second reducing agent, if such a compound is used. When applied to an inkjet recording method, if the content of the water-soluble organic solvent is outside the above-mentioned range, reliability such as sticking resistance and ejection stability may be slightly reduced.

[0045] (surfactant) The ink preferably contains a surfactant other than the surfactant used as a dispersant for silver particles. The content (mass %) of the surfactant other than the surfactant used as a dispersant for silver particles in the aqueous ink is preferably 0.1 mass % or more and 2.0 mass % or less, based on the total mass of the ink.

[0046] Examples of surfactants include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants. Of these, nonionic surfactants such as ethylene oxide adducts of acetylene glycol and polyoxyethylene alkyl ethers are preferred.

[0047] The HLB value of the nonionic surfactant determined by the Griffin method is preferably 10 or greater. If the HLB value is less than 10, the surfactant may be highly hydrophobic and may have difficulty remaining dissolved in aqueous ink. The HLB value determined by the Griffin method is calculated from the formula weight of the ethylene oxide group of the surfactant and the molecular weight of the surfactant according to the formula: HLB value = 20 × (formula weight of the ethylene oxide group of the surfactant) / (molecular weight of the surfactant). The HLB value determined by the Griffin method is a physical property value that represents the degree of hydrophilicity or lipophilicity of the nonionic surfactant on a scale ranging from 0 to 20; a smaller HLB value indicates higher lipophilicity, and a larger HLB value indicates higher hydrophilicity.

[0048] (Other additives) In addition to the above components, the ink may contain various additives, such as antifoaming agents, pH adjusters, viscosity adjusters, rust inhibitors, preservatives, antifungal agents, other resins, and antioxidants, as needed. Among these, it is preferable for the ink to contain an antioxidant. As mentioned above, the ink contains a first reducing agent with high reducing power. Because compounds with high reducing power are inevitably prone to oxidation, even a very small amount of oxygen mixed into the ink can easily cause oxidation of the first reducing agent. Adding an antioxidant to the ink can suppress oxidation of the first reducing agent. Commonly used antioxidants can be used, such as sodium sulfite, 4-methoxyphenol, taurine, folic acid, mercaptopyridine, and p-toluenesulfinic acid. Of these, sodium sulfite is preferred. The content (mass %) of the antioxidant in the aqueous ink is preferably 0.1% to 2.0% by mass, based on the total mass of the ink. Some general-purpose antioxidants, such as sodium sulfite, have a redox potential of zero or higher and act as reducing agents. However, these so-called "antioxidants" do not exhibit the above-mentioned secondary reducing agent behavior due to their structure or functional groups.

[0049] (Ink properties) The viscosity (mPa·s) of the ink at 25°C is preferably 1 mPa·s or more and 6 mPa·s or less, and more preferably 1 mPa·s or more and 4 mPa·s or less. The surface tension (mN / m) of the ink at 25°C is preferably 10 mN / m or more and 60 mN / m or less, more preferably 20 mN / m or more and 50 mN / m or less, and even more preferably 25 mN / m or more and 40 mN / m or less. The surface tension of the ink can be adjusted by the type and content of the surfactant. The pH of the ink at 25°C is preferably 7 or more and 10 or less; within this range, the action of each reducing agent is efficiently exerted.

[0050] <Ink cartridges> The ink cartridge of the present invention includes ink and an ink storage section that stores the ink. The ink stored in the ink storage section is the ink of the present invention described above. FIG. 1 is a cross-sectional view schematically illustrating one embodiment of the ink cartridge of the present invention. As shown in FIG. 1, the bottom of the ink cartridge is provided with an ink supply port 12 for supplying ink to the recording head. The interior of the ink cartridge serves as an ink storage section for storing ink. The ink storage section is composed of an ink storage chamber 14 and an absorber storage chamber 16, which are connected via a communication port 18. The absorber storage chamber 16 is also connected to the ink supply port 12. The ink storage chamber 14 stores liquid ink 20, and the absorber storage chamber 16 contains absorbers 22 and 24 that retain the ink in an impregnated state. The ink storage section may not have an ink storage chamber that stores liquid ink, but may instead use an absorber to hold all of the ink stored therein. Alternatively, the ink storage section may not have an absorber and may store all of the ink in a liquid state. Furthermore, the ink cartridge may be configured to have an ink storage section and a recording head.

[0051] <Inkjet recording method> The inkjet recording method of the present invention is a method of recording an image on a recording medium by ejecting the ink of the present invention described above from an inkjet recording head. Methods for ejecting the ink 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 that the recording head ejects the ink by the action of thermal energy. Other than using the ink of the present invention, the steps of the inkjet recording method may be known.

[0052] FIG. 2 is a schematic diagram illustrating an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, where (a) is a perspective view of the main components of the inkjet recording apparatus, and (b) is a perspective view of a head cartridge. The inkjet recording apparatus is provided with a transport means (not shown) for transporting 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 includes recording heads 38 and 40, and is configured to accommodate an ink cartridge 42. While the head cartridge 36 is transported 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. An image is then recorded on the recording medium 32 by transporting the recording medium 32 in the sub-scanning direction by a transport means (not shown). Any recording medium may be used for recording using the ink of the present invention, but it is preferable to use a paper-based recording medium that has 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 it provides an excellent metallic feel to the recorded image. Recording media such as glossy paper used in inkjet recording methods usually have an ink-receiving layer containing chloride ions. Chloride ions are contained in cationic compounds such as polydiallyldimethylammonium chloride and polyaluminum chloride. Issues such as a decrease in glossiness (initial) of images recorded with ink containing silver particles and a decrease in glossiness after storage of the images arise due to these chloride ions. [Example]

[0053] Below are examples 、 Comparative Example , and reference examplesThe present invention will be described in further detail using the following examples, but the present invention is not limited to these examples in any way as long as it does not depart from the gist of the present invention. Note that "parts" and "%" used to describe component amounts are by mass unless otherwise specified. The volume-based cumulative 50% particle size of the silver particles was measured on a volume basis using a particle size analyzer (trade name "UPA-EX150", manufactured by Nikkiso) using the dynamic light scattering method.

[0054] <Preparation of Metal Particle Dispersion> (Dispersion 1, 3~6) Dispersions 1, 3 to 6 were prepared with reference to Example 2-2 of JP 2004-285106 A. Specifically, silver nanocolloid (80°C) was prepared by adding sodium citrate (reducing agent) to an aqueous solution containing silver nitrate and a dispersant to reduce the silver ions. A mixture (molar ratio 4.0:0.1) of sodium dodecanesulfonate, an anionic surfactant, and octaethylene glycol mono-n-dodecyl ether, a nonionic surfactant, was used as the dispersant. The molar ratio of silver nitrate to the dispersant was then appropriately adjusted to adjust the silver particle and dispersant contents to the values ​​shown in Table 1. The dispersion was then cooled to 15°C to adjust the volume-based cumulative 50% particle size of the silver particles to approximately 20 nm, thereby preparing Dispersions 1, 3 to 6. The oxidation-reduction potential of sodium citrate, measured under the same conditions as described below, was 216 mV.

[0055] (Dispersion 2) Dispersion 2 was prepared with reference to Example 2 of JP-A 2010-507727. Specifically, a mixture of aqueous sodium hydroxide solution and aqueous acrylic resin solution (trade name "Disperbyk190" manufactured by BYK-Chemie) was added to an aqueous silver nitrate solution under stirring to prepare a silver oxide nanosol. The amount of aqueous acrylic resin solution used was adjusted so that the contents of silver particles and dispersant were as shown in Table 1. An aqueous formaldehyde solution (reducing agent) was added to the solution so that the molar ratio of silver ions to reducing agent was 1.0:10.0, followed by heating at 60°C for 30 minutes and cooling. Subsequently, unnecessary substances were removed by ultrafiltration, and the concentration of the solids was adjusted to prepare Dispersion 2, in which the volume-based cumulative 50% particle size of silver particles was approximately 80 nm.

[0056] [Table 1]

[0057] (Dispersion 7) Water-resistant aluminum pigment dispersion A was prepared according to the description in Example 1 of Patent Document 1 and concentrated to an aluminum particle content of 20.0%, yielding dispersion 7. Dispersion 7 contains aluminum particles dispersed in an aqueous liquid medium using an acrylic resin having units derived from maleic acid.

[0058] <Oxidation-reduction potential> The oxidation-reduction potential (mV) of the components used in preparing the ink was measured at 25°C and pH 9 using a pH meter (trade name "Portable pH Meter D-74" manufactured by Horiba, Ltd.). An ORP electrode (trade name "Waterproof Platinum Composite ORP Electrode 9300-10D" manufactured by Horiba, Ltd.) with a 3.33 mol / L potassium chloride aqueous solution as the internal solution was used. This electrode is composed of a platinum electrode as the reference electrode and a silver-silver chloride electrode as the comparison electrode. The substance to be measured was dissolved in a borate standard solution (pH standard solution) to a concentration of 0.1% by mass. The pH of the solution was adjusted to 9 as necessary, and the oxidation-reduction potential was measured. A potassium hydroxide aqueous solution or sulfuric acid was used for pH adjustment. The measured oxidation-reduction potentials are shown in Table 2.

[0059] [Table 2]

[0060] <Ink Preparation> (Example 1 22, Reference Example 23, Examples 24-25, Reference Example 26, Examples 27- 35, Comparative Examples 1 to 10) The components shown in the upper rows of Tables 3 to 5 were mixed and thoroughly stirred, and then the pH of the ink was adjusted to 8 using a 1 mol / L aqueous potassium hydroxide solution (the content is expressed including the content in ion-exchanged water). The pH of the ink was measured using a pH meter (trade name "Portable pH Meter D-74", manufactured by Horiba, Ltd.). The ink was then pressure-filtered through a filter with a pore size of 1.2 μm to obtain the ink of Examples 1 to 5. 22, Reference Example 23, Examples 24-25, Reference Example 26, Examples 27- Inks were prepared according to the methods described above, including ink No. 35 and comparative examples 1 to 10. The ink properties are shown in the lower part of Tables 3 to 5.

[0061] Details of the various nonionic surfactants shown by trade name in Tables 3 to 5 and their HLB values ​​calculated by the Griffin method are as follows: Olfine E1010 (manufactured by Nissin Chemical Industry Co., Ltd.), ethylene oxide adduct of acetylene glycol, HLB value 13 NIKKOL BL-9EX (manufactured by Nikko Chemicals), polyoxyethylene lauryl ether, HLB value 14 Acetylenol E100 (Kawaken Fine Chemicals), ethylene oxide adduct of acetylene glycol, HLB value 13

[0062] [Table 3]

[0063] [Table 4]

[0064] [Table 5]

[0065] (Comparative Example 11) The ink of Comparative Example 11 was prepared according to the description in Example 5 of Patent Document 3. An oil-based dispersion was prepared by mixing 1.00 g of silver nanoparticles prepared according to the description in the document, 1.00 g of a naphthenic nonaqueous solvent (product name "AF Solvent No. 4" manufactured by Nippon Oil Corporation), and 0.13 g of an emulsifier (product name "Hexaglin 5-O" manufactured by Nikko Chemicals Corporation). Separately, 1.05 g of ethylene glycol, 0.05 g of ascorbic acid, and 3.38 g of ion-exchanged water were mixed and added to the total amount of the oil-based dispersion prepared above. The ink of Comparative Example 11 was prepared by thoroughly stirring the mixture. The silver particle content (P) in the ink was 15.1%, the emulsifier (dispersant) content (D) was 1.9%, and the D / P ratio was 0.1. The content S of silver particles in the ink was 140.0 mmol / g, the content R1 of the first reducing agent was 4.0 mmol / g, and the value of R1 / S was 0.03.

[0066] <Evaluation> Each ink prepared above was filled into an ink cartridge and set in an inkjet recording device (trade name "PIXUS MG3630", manufactured by Canon) equipped with a recording head that ejects ink using thermal energy. In this example, an image recorded under conditions in which two ink droplets of approximately 11.2 ng were deposited per unit area of ​​1 / 600 inch x 1 / 600 inch was defined as having a recording duty of 100%. Using the inkjet recording device, a solid image was recorded at a recording medium (glossy paper, trade name "Canon Photo Paper Glossy Pro [Platinum Grade]", manufactured by Canon) with a recording duty of 100%. The resulting solid image was evaluated as follows. In the present invention, the following evaluation criteria for each item were used: AA, A, and B were considered acceptable levels, and C was considered unacceptable. The evaluation results are shown in Table 6.

[0067] (Glossiness (initial)) The image obtained above was left in an environment at room temperature and normal pressure (temperature 25°C, relative humidity 50%) for 1 hour, and then the 20° specular gloss of the solid image was measured using a gloss meter (product name "Surface Reflection Analyzer RA-532H", manufactured by Canon). From the obtained gloss (referred to as "Gloss 1"), the gloss (initial) was evaluated according to the following evaluation criteria. A: Glossiness 1 was 650 or higher B: Glossiness 1 was 350 or more but less than 650 C: Glossiness 1 was less than 350.

[0068] (Prevents gloss loss) The image obtained above was left in an environment with a temperature of 30°C and a relative humidity of 80% for one week, and then the 20° specular gloss was measured in the same manner as in the evaluation of "glossiness (initial)." The conditions of 30°C and 80% relative humidity were an accelerated test designed to simulate long-term storage of the image. The glossiness retention rate (%) was calculated from the obtained glossiness (referred to as "glossiness 2") and the previously measured "glossiness 1": {(glossiness 2) / (glossiness 1)} × 100. The glossiness reduction was evaluated from the obtained retention rate according to the following evaluation criteria. A higher retention rate indicates better suppression of glossiness reduction even after storage of the image. AA: Survival rate was over 70% A: The survival rate was between 60% and 70%. B: The remaining rate was between 50% and 60%. C: The survival rate was less than 50%.

[0069] [Table 6]

Claims

1. 1. A water-based inkjet ink containing silver particles, a first reducing agent, and a second reducing agent, the first reducing agent is at least one selected from the group consisting of ascorbic acid, sodium ascorbate, N-methyl-p-aminophenol sulfate, hydroquinone, and 1-phenyl-3-pyrazolidone; the second reducing agent is at least one selected from the group consisting of triethanolamine, triethylamine, sorbitol, 1,2-propanediol, 1,2-pentanediol, 1,2-hexanediol, 1,2,6-hexanetriol, glycerin, and trimethylolpropane; the content (mmol / g) of the first reducing agent in the aqueous ink is 0.1 times or more, in terms of molar ratio, relative to the content (mmol / g) of the silver particles; The aqueous ink is characterized in that the content (mmol / g) of the second reducing agent in the aqueous ink is 0.1 times or more, in terms of molar ratio, relative to the content (mmol / g) of the first reducing agent.

2. 2. The aqueous ink of claim 1, wherein the first reducing agent is ascorbic acid.

3. 3. The aqueous ink according to claim 1, wherein the second reducing agent is triethanolamine.

4. 2. The aqueous ink of claim 1, wherein the first reducing agent is ascorbic acid and the second reducing agent is triethanolamine.

5. 5. The aqueous ink according to claim 1, wherein the content (mass %) of the silver particles in the aqueous ink is 2.0 mass % or more and 15.0 mass % or less based on the total mass of the ink.

6. 6. The aqueous ink according to claim 1, wherein the content (mmol / g) of the first reducing agent in the aqueous ink is 0.1 mmol / g or more and 50.0 mmol / g or less.

7. 7. The aqueous ink according to claim 1, wherein the content (mmol / g) of the second reducing agent in the aqueous ink is 0.1 mmol / g or more and 200.0 mmol / g or less.

8. 8. The aqueous ink according to claim 1, wherein the content (mmol / g) of the first reducing agent in the aqueous ink is 0.9 or less times the content (mmol / g) of the silver particles in terms of molar ratio.

9. 9. The aqueous ink according to claim 1, wherein the content (mmol / g) of the second reducing agent in the aqueous ink is 20.0 times or less, in terms of molar ratio, relative to the content (mmol / g) of the first reducing agent.

10. 10. The aqueous ink according to claim 1, wherein the content (mmol / g) of the second reducing agent in the aqueous ink is 10.0 times or less in terms of molar ratio relative to the content (mmol / g) of the first reducing agent.

11. An ink cartridge comprising ink and an ink storage section for storing the ink, 11. An ink cartridge, wherein the ink is the aqueous ink according to claim 1.

12. An inkjet recording method for recording an image on a recording medium by ejecting ink from an inkjet recording head, An ink-jet recording method, wherein the ink is the aqueous ink according to any one of claims 1 to 11.

13. 13. The inkjet recording method according to claim 12, wherein the recording medium comprises an ink-receiving layer.

14. The ink jet recording method according to claim 13, wherein the ink receiving layer contains chloride ions.

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