Aqueous ink, ink cartridge and ink jet recording method

Abstract

To provide an aqueous ink for thermal inkjet printing which has excellent ink ejection stability and can record a high-quality image or the like.SOLUTION: There is provided an aqueous ink used for an inkjet recording method in which an image is recorded by imparting ink ejected from a recording head to a recording medium by action of thermal energy. The aqueous ink contains a first pigment, a second pigment and a polyoxyethylene alkyl ether, the first pigment is carbon black having an anionic group bonded to its particle surface directly or via another atomic group (-R-), the second pigment is carbon black dispersed by the action of a resin having an anionic group and the HLB value of the polyoxyethylene alkyl ether by the Griffin method is 16.0 or more.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] In recent years, there has been a demand for inkjet recording devices capable of recording high-quality images on various recording media. For example, in the commercial printing field, inkjet recording devices capable of recording high-quality business documents with high optical density on plain paper and poster images with high image clarity on glossy paper are widely used. There is also a demand for inkjet recording devices capable of recording on recording media other than paper, such as pongee cloth (cloth).

[0003] Among the recording methods used in these inkjet recording devices, the thermal inkjet recording method is a method in which thermal energy is used to foam the ink and eject the ink onto a recording medium, and it is capable of high-speed, high-density recording. Therefore, it is suitable for recording high-quality documents with high optical density and high-definition glossy images with excellent image clarity, and is widely adopted. As a coloring material for ink used in outputting business documents, so-called self-dispersing carbon black, in which atomic groups containing hydrophilic groups are bonded to the particle surface of carbon black, is widely used because of its high coagulation property and the ability to record high-quality images with high optical density.

[0004] On the other hand, when recording on glossy paper, effects such as high fixation and high image clarity can be achieved, so-called resin-dispersed carbon black, in which carbon black is dispersed with a resin dispersant, is often used as a coloring material for recording on such paper. Furthermore, in order to obtain a well-balanced and good image for both business documents and glossy images, inks that use both self-dispersed carbon black and resin-dispersed carbon black as coloring materials have been proposed (see Patent Documents 1 and 2). Here, "image clarity" refers to the sharpness of an image when it is projected onto the surface of an image; when image clarity is low, the image appears blurry, and when image clarity is high, the image appears sharp. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2007-063493 A [Patent Document 2] JP 2000-239589 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present inventors have investigated aqueous inks capable of recording high-quality images with excellent image clarity on a variety of recording media, such as plain paper and glossy paper, using a thermal inkjet system. As a result, they have found that inks using a combination of self-dispersed carbon black and resin-dispersed carbon black, as described in Patent Documents 1 and 2, gradually decrease in optical density when continuously recorded, causing unevenness in the image.

[0007] Therefore, an object of the present inventors is to provide an aqueous ink for thermal inkjet printing that has excellent ink ejection stability and is capable of recording high-quality images, an ink cartridge using the aqueous ink, and an inkjet recording method. [Means for solving the problem]

[0008] The above object is achieved by the present invention, which is described below. That is, the aqueous ink according to the present invention is an aqueous ink used in an inkjet recording method in which an image is recorded by applying ink ejected from a recording head to a recording medium by the action of thermal energy, and is characterized in that it contains a first pigment, a second pigment, and a polyoxyethylene alkyl ether, the first pigment is carbon black having an anionic group bonded to its particle surface directly or via another atomic group (-R-), the second pigment is carbon black dispersed by the action of a resin having an anionic group, and the polyoxyethylene alkyl ether has an HLB value of 16.0 or more according to the Griffin method. Effect of the Invention

[0009] According to the present invention, it is possible to provide a water-based ink for thermal inkjet recording that is excellent in ink ejection stability and capable of recording high-quality images, an ink cartridge using the water-based ink, and an inkjet recording method. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view illustrating an embodiment of an ink cartridge of the present invention. [Diagram 2] 1A and 1B are diagrams illustrating an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, in which (a) is a perspective view of the main part of the inkjet recording apparatus, and (b) is a perspective view of a head cartridge. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0012] The present inventors first investigated the occurrence of unevenness in images when inks using both self-dispersed carbon black and resin-dispersed carbon black as described in Patent Documents 1 and 2 are ejected from a recording head by the action of thermal energy to continuously record images. As a result, it was found that a large amount of kogation was attached to the heater of the recording head. When ink is ejected by the action of thermal energy, the ink is instantaneously heated, which destabilizes the dispersion state of the carbon black. In particular, resin-dispersed carbon black is more likely to be destabilized because the resin for dispersing it is detached from the surface of the carbon black particles. As a result, aggregates of carbon black and resin whose dispersion state has become unstable are deposited on the heater as kogation, which is thought to be the cause of ejection failure during continuous recording.

[0013] The present inventors have studied suppressing ejection defects by increasing the dispersion stability of carbon black. Specifically, they have studied the addition of additives that improve the dispersion stability. As a result, they have found that ejection defects can be improved by adding polyoxyethylene alkyl ether. Polyoxyethylene alkyl ether has a structure in which a hydrophobic portion (alkyl chain) and a hydrophilic portion (repeated ethylene oxide portion) are connected in a linear chain. Therefore, it is considered that it can be effectively adsorbed to the hydrophobic portion of carbon black without causing physical steric hindrance. And, it is considered that the dispersion stability of carbon black can be improved by steric hindrance due to the hydrophilic portion being moderately elongated in the aqueous medium. As a result, it is considered that the aggregation of carbon black is suppressed and the ejection stability is improved.

[0014] Next, the present inventors performed recording on various recording media using the ink containing polyoxyethylene alkylene ether, and examined the image quality. As a result, bleeding of the image occurred on recording media with high ink permeability. Therefore, the present inventors further examined the addition of a material to the ink that not only increases the dispersion stability but also suppresses bleeding of the image. As a result, they found that the ink ejection stability and image quality can be improved by using an aqueous ink containing self-dispersing carbon black, resin-dispersed carbon black, and a specific polyoxyethylene alkyl ether.

[0015] That is, the ink of the present invention has the following characteristics. First, as the first pigment, carbon black having an anionic group bonded to its particle surface directly or via another atomic group (-R-) is contained, and as the second pigment, carbon black dispersed by the action of a resin having an anionic group is contained. Furthermore, polyoxyethylene alkyl ether is contained. The HLB value of the polyoxyethylene alkyl ether according to the Griffin method is 16 or more. Moreover, the ink is an aqueous ink used in an inkjet recording method in which ink is ejected from a recording head by the action of thermal energy and applied to a recording medium to record an image. The present inventors speculate as follows about the mechanism by which the above-mentioned configuration improves the ejection stability of the ink and the image quality.

[0016] When ink is continuously ejected from a recording head that ejects ink by the action of thermal energy (hereinafter also referred to as a thermal type recording head), heat is applied to the ink near the ejection port. At the same time, the water in the ink evaporates, and the concentration of the components in the ink (the cationic components brought into the ink by the counter ions of the resin having the anionic groups of the first pigment and the anionic groups that contribute to the dispersion of the second pigment) increases. In such a situation, as described above, carbon black and resins tend to aggregate. In particular, as the concentration of the cationic components in the resin increases, the electrostatic repulsion effect of the anionic groups is impaired, causing aggregation of the resins themselves. As a result, there is a shortage of resin to disperse the second pigment, and the dispersion stability of the second pigment decreases significantly.

[0017] As described above, polyoxyethylene alkyl ether has a structure in which a hydrophobic portion (alkyl chain) and a hydrophilic portion (repeated ethylene oxide portion) are linked in a linear chain. Therefore, it is considered that it can be effectively adsorbed to the hydrophobic portions of the first pigment, the second pigment, and the resin without causing physical steric hindrance. It is considered that the hydrophilic portion is appropriately elongated in the aqueous medium, and the dispersion stability of the carbon black can be improved due to steric hindrance. However, even when polyoxyethylene alkyl ether is added to the ink, the occurrence of image unevenness could not be suppressed on a recording medium with high permeability.

[0018] Next, consider the situation when the ink is applied to a recording medium. When the ink to which polyoxyethylene alkyl ether has been added is applied to a recording medium, the liquid components in the ink penetrate into the recording medium or evaporate. At that time, a part of the resin that contributes to the dispersion of the second pigment is adsorbed to the surface of the first pigment, inhibiting the aggregation of the first pigment. Furthermore, the addition of polyoxyethylene alkyl ether improves the stability of the resin, so the dispersion stability of the first pigment is less likely to decrease. It is believed that this is the result of the bleeding of the image.

[0019] Therefore, polyoxyethylene alkyl ethers having an HLB value of 16.0 or more according to the Griffin method are used. Polyoxyethylene alkyl ethers having an HLB value of 16.0 or more have relatively high water solubility, and therefore tend to take a linearly extended form in the ink, rather than a form in which the hydrophobic portion is curled inward. In addition, since the molecular size is smaller than that of resins, it is considered that polyoxyethylene alkyl ethers are more likely to selectively adsorb to the surface of the first pigment than resins. As described above, polyoxyethylene alkyl ethers have the effect of increasing dispersion stability by steric hindrance through moderate extension of the hydrophilic portion, but this effect is weaker than that of resins. And since they do not have ionic groups, they cannot exert the effect of increasing dispersion stability by electrostatic repulsion. Therefore, the dispersion stability of the first pigment after being applied to the recording medium tends to decrease to a certain extent. As a result, image unevenness is suppressed, and excellent image quality can be obtained even on recording media with high permeability. If the HLB value of polyoxyethylene alkyl ethers determined by the Griffin method is less than 16, the water solubility is not so high, and therefore adsorption to the surface of the first pigment is insufficient, and image unevenness cannot be suppressed. As a result, good image quality is not achieved.

[0020] <Water-based ink> The ink of the present invention contains a first pigment, a second pigment, and a polyoxyethylene alkyl ether having an HLB value of 16 or more according to the Griffin method. This ink is a water-based ink used in an inkjet recording method in which an image is recorded by applying the ink ejected from a recording head to a recording medium by the action of thermal energy. Each component constituting the ink of the present invention will be described in detail below.

[0021] (Colorant) The ink contains carbon black as a coloring material (pigment). The first pigment is carbon black having an anionic group bonded to its particle surface directly or via another atomic group (-R-). The second pigment is carbon black dispersed by the action of a resin having an anionic group. The total content (mass %) of the coloring materials including the first pigment and the second pigment in the ink is preferably 0.5% by mass or more and 15.0% by mass or less, and more preferably 1.0% by mass or more and 10.0% by mass or less, based on the total mass of the ink.

[0022] Any type of carbon black can be used as long as it is usable in inks, specifically, furnace black, gas black, lamp black, acetylene black, channel black, etc.

[0023] The DBP oil absorption (mL / 100g) of the self-dispersed carbon black, which is the first pigment, is preferably 120mL / 100g or more, and the DBP oil absorption (mL / 100g) of the resin-dispersed carbon black, which is the second pigment, is preferably 60mL / 100g or less. If the DBP oil absorption of the first pigment is less than 120mL / 100g, scattering on the surface of the recorded image increases, and the optical density of the image may not be sufficient. If the DBP oil absorption of the second pigment is more than 60mL / 100g, scattering on the surface of the recorded image decreases, and the image clarity may not be sufficient. The DBP oil absorption (mL / 100g) of the first pigment is preferably 500mL / 100g or less, and the DBP oil absorption (mL / 100g) of the second pigment is preferably 10mL / 100g or more.

[0024] The DBP oil absorption of carbon black can be measured by a method conforming to JIS K6221 or ASTM D 2414. These methods involve dropping dibutyl phthalate onto 100 g of carbon black while stirring, and measuring the amount of dibutyl phthalate dropped at the point when the torque reaches a maximum.

[0025] [First pigment] The first pigment is a self-dispersing pigment in which a hydrophilic group is bonded to the surface of the pigment particles. As the self-dispersing pigment, a pigment in which an anionic group such as a carboxylic acid group, a sulfonic acid group, or a phosphonic acid group is bonded directly or via another atomic group (-R-) to the surface of the pigment particles can be used. Among them, the anionic group is preferably a carboxylic acid group. If the anionic group is other than a carboxylic acid group, the kogation that adheres to the heater of the recording head tends to become strong, and the ink ejection stability may not be sufficient. In addition, the dispersion stability of the first pigment is less likely to decrease, and the character quality and optical density of the image quality may not be sufficient.

[0026] The anionic group may be either an acid type or a salt type, and in the case of the salt type, it may be either a partially dissociated state or a completely dissociated state. When the anionic group is a salt type, examples of the cation serving as the counter ion include an alkali metal cation, ammonium, and organic ammonium. Examples of the organic ammonium include alkylamines such as methylamine and ethylamine, and alkanolamines such as monoethanolamine, diethanolamine, and triethanolamine. Specific examples of the other atomic group (-R-) include a linear or branched alkylene group having 1 to 12 carbon atoms; an arylene group such as a phenylene group or a naphthylene group; a carbonyl group; an imino group; an amide group; a sulfonyl group; an ester group; and an ether group. The group may also be a combination of these groups.

[0027] The compound used to bond the organic group having an anionic group to the surface of carbon black particles is not particularly limited as long as it can be bonded to the surface of carbon black particles. Examples of the compound include p-aminobenzoic acid, 4-aminophthalic acid, 4-hydrazinobenzoic acid, 4-hydrazinophthalic acid, 4-aminobenzenesulfonic acid (sulfanilic acid), and 4-aminophenylphosphonic acid.

[0028] [Second pigment] The second pigment is a resin-dispersed pigment in which a resin is used as a dispersant, that is, the resin as a dispersant is physically adsorbed on the surface of the pigment particles. As a resin (resin dispersant) having an anionic group for dispersing the second pigment, it is preferable to use a resin as described below, especially a water-soluble resin. The content (mass%) of the second pigment in the ink is preferably 0.1 to 10.0 times, more preferably 0.5 to 5.0 times, in terms of mass ratio to the content (mass%) of the resin dispersant.

[0029] (resin) The ink may contain a resin. The content (mass%) of the resin including the resin dispersant in the ink is preferably 0.1% by mass or more and 20.0% by mass or less, more preferably 0.5% by mass or more and 15.0% by mass or less, based on the total mass of the ink. The resin may be added to the ink (i) to stabilize the dispersion state of the pigment, that is, as a resin dispersant or its auxiliary. Also, (ii) to improve various properties of the recorded image. Examples of the form of the resin include block copolymer, random copolymer, graft copolymer, and combinations thereof. The resin may be a water-soluble resin that can be dissolved in an aqueous medium, or may be resin particles that are dispersed in an aqueous medium. The resin may be used alone or in combination of two or more types.

[0030] [Resin composition] Examples of the resin include acrylic resins, urethane resins, olefin resins, etc. Among them, acrylic resins and urethane resins are preferred, and acrylic resins composed of units derived from (meth)acrylic acid or (meth)acrylate are more preferred.

[0031] The acrylic resin preferably has a hydrophilic unit and a hydrophobic unit as constituent units. Among them, a resin having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one monomer selected from the group consisting of a monomer having an aromatic ring and a (meth)acrylic acid ester monomer is preferred. In particular, a resin having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one monomer selected from the group consisting of styrene and α-methylstyrene is preferred. These resins are likely to interact with pigments, and therefore can be suitably used as a resin dispersant for dispersing pigments.

[0032] The hydrophilic unit is a unit having a hydrophilic group such as an anionic group. The hydrophilic unit can be formed, for example, by polymerizing a hydrophilic monomer having a hydrophilic group. Specific examples of the hydrophilic monomer having a hydrophilic group include acidic monomers having a carboxylic acid group such as (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid, and anionic monomers such as anhydrides and salts of these acidic monomers. Examples of cations constituting the salts of acidic monomers include ions of lithium, sodium, potassium, ammonium, and organic ammonium. The hydrophobic unit is a unit not having a hydrophilic group such as an anionic group. The hydrophobic unit can be formed, for example, by polymerizing a hydrophobic monomer not having a hydrophilic group such as an anionic group. Specific examples of the hydrophobic monomer include monomers having an aromatic ring such as styrene, α-methylstyrene, and benzyl (meth)acrylate; and (meth)acrylic acid ester monomers such as methyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.

[0033] The urethane resin can be obtained by reacting, for example, polyisocyanate with a polyol. In addition, a chain extender may be further reacted with the urethane resin. Examples of the olefin resin include polyethylene and polypropylene.

[0034] [Resin properties] In this specification, "a resin is water-soluble" means that when the resin is neutralized with an alkali equivalent to the acid value, the resin is present in an aqueous medium in a state in which it does not form particles whose particle size can be measured by a dynamic light scattering method. Whether or not a resin is water-soluble can be determined according to the following method. First, a liquid (resin solid content: 10 mass%) containing a resin neutralized with an alkali (sodium hydroxide, potassium hydroxide, etc.) equivalent to the acid value is prepared. Next, the prepared liquid is diluted 10 times (volume basis) with pure water to prepare a sample solution. Then, when the particle size of the resin in the sample solution is measured by a dynamic light scattering method, if no particles having a particle size are measured, the resin can be determined to be water-soluble. The measurement conditions at this time can be, for example, SetZero: 30 seconds, number of measurements: 3 times, and measurement time: 180 seconds. In addition, a particle size analyzer using a dynamic light scattering method (for example, the product name "UPA-EX150", manufactured by Nikkiso) can be used as a particle size distribution measuring device. Of course, the particle size distribution measuring device and the measurement conditions are not limited to those described above.

[0035] The acid value of the water-soluble resin is preferably 50 mgKOH / g or more and 250 mgKOH / g or less, and the weight average molecular weight of the water-soluble resin is preferably 1,000 or more and 30,000 or less.

[0036] The acid value of the resin constituting the resin particles is preferably 5 mgKOH / g or more and 100 mgKOH / g or less. The weight average molecular weight of the resin constituting the resin particles is preferably 1,000 or more and 3,000,000 or less, more preferably 100,000 or more and 3,000,000 or less. The volume-based cumulative 50% particle diameter (D50) of the resin particles measured by dynamic light scattering is preferably 50 nm or more and 500 nm or less. The volume-based cumulative 50% particle diameter of the resin particles is the diameter of the particles that is 50% of the total volume of the measured particles when integrated from the small particle diameter side in the particle diameter integration curve. The volume-based cumulative 50% particle diameter of the resin particles can be measured using the dynamic light scattering particle size analyzer and measurement conditions described above.

[0037] The glass transition temperature of the resin particles is preferably 40° C. or more and 120° C. or less, and more preferably 50° C. or more and 100° C. or less. The glass transition temperature (° C.) of the resin particles can be measured using a differential scanning calorimeter (DSC). The resin particles do not need to contain a coloring material.

[0038] [Block copolymer] The ink preferably further contains, as a resin, a water-soluble block copolymer having an A block composed of a unit derived from a monomer having an aromatic ring and a B block composed of a unit derived from a monomer having an anionic group. This block copolymer has a clearly separated hydrophobic portion (A block) and a hydrophilic portion (B block), and therefore, compared to a resin having a random structure, it is more likely to interact with the hydrophobic portion of the first pigment, the second pigment, and the resin. As a result, the ejection stability can be further improved. This effect is exhibited whether this block copolymer is used as a resin dispersant for the second pigment or as an additive to the ink.

[0039] The units constituting the A block and the B block can be appropriately selected from those described in the above resin composition. In addition to the A block and the B block, this block copolymer may have one or more blocks (other blocks) composed of units derived from monomers (other monomers) other than the monomers constituting the A block and the B block. In addition, when other blocks are included, the order of arrangement of each block is not particularly limited. For example, in the case of a copolymer having three types of blocks, the order may be other block-A block-B block, A block-other block-B block, or A block-B block-other block.

[0040] Examples of other monomers include (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, polyethylene glycol (meth)acrylate, polypropylene glycol (meth)acrylate, and methoxypolyethylene glycol (meth)acrylate; (meth)acrylic acid amides such as (meth)acrylamide and (meth)acryloylmorpholine; and nitrogen-containing vinyl compounds such as N-vinylacetamide.

[0041] (Polyoxyethylene alkyl ether) The ink contains polyoxyethylene alkyl ether having an HLB value of 16 or more according to the Griffin method. Polyoxyethylene alkyl ether is a type of surfactant, and has a structure represented by, for example, formula (1). The content (mass %) of polyoxyethylene alkyl ether in the ink is preferably 0.10 mass % or more and 2.0 mass % or less, based on the total mass of the ink, and more preferably 0.20 mass % or more and 1.0 mass % or less. One type of polyoxyethylene alkyl ether may be used alone, or two or more types may be used in combination. RO-(CH2-CH2-O) n -H···(1) (In formula (1), R represents an alkyl group, and n represents a natural number.)

[0042] In the polyoxyethylene alkyl ether, the carbon number of R (alkyl group) in formula (1), which is a hydrophobic group, is preferably in a range that exhibits surface activity. More specifically, the carbon number of R (alkyl group) is preferably 12 or more and 22 or less. Examples of R (alkyl group) in formula (1) include lauryl group (12), cetyl group (16), stearyl group (18), oleyl group (18), and behenyl group (22) (the number in parentheses is the carbon number of the alkyl group). Here, the oleyl group is an alkenyl group having one double bond, but for convenience, it is collectively described as polyoxyethylene alkyl ether in this specification. In addition, n (the number of moles of ethylene oxide group added) in formula (1) is determined from the structure of R and the HLB value. In particular, n is preferably 10 or more and 50 or less.

[0043] The HLB value according to the Griffin method can be calculated from the following formula (2). The HLB value calculated by the Griffin method is a physical property value that indicates the degree of hydrophilicity or lipophilicity of a surfactant, and takes a value between 0.0 and 20.0. The smaller the HLB value, the higher the lipophilicity, and the larger the HLB value, the higher the hydrophilicity. HLB value = 20 × (formula weight of hydrophilic group of surfactant / molecular weight of surfactant) (2)

[0044] (aqueous medium) The ink is an aqueous ink containing at least water as an aqueous medium. The ink may contain water or an aqueous medium that is a mixed solvent of water and a water-soluble organic solvent. As the water, deionized water or ion-exchanged water is preferably used. The content (mass%) of water 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. In addition, the content (mass%) of the water-soluble organic solvent in the aqueous ink is preferably 2.0 mass% or more and 40.0 mass% or less based on the total mass of the ink. As the water-soluble organic solvent, any of those usable for inkjet inks, such as alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing solvents, and sulfur-containing solvents, can be used. The water-soluble organic solvent may be used alone or in combination of two or more.

[0045] (Other Ingredients) In addition to the above-mentioned components, the ink may contain, as necessary, water-soluble organic compounds that are solid at 25° C., such as polyhydric alcohols, such as trimethylolpropane and trimethylolethane, and urea derivatives, such as urea and ethyleneurea. Furthermore, the ink may contain, as necessary, various additives, such as surfactants other than polyoxyethylene alkyl ethers, pH adjusters, defoamers, rust inhibitors, preservatives, antifungal agents, antioxidants, reduction inhibitors, and chelating agents.

[0046] (Ink properties) The ink of the present invention is an aqueous ink applied to the inkjet method. Therefore, from the viewpoint of reliability, it is preferable to appropriately control the physical property values. The viscosity of the ink at 25°C is preferably 1.0 mPa·s to 10.0 mPa·s, more preferably 1.0 mPa·s to 5.0 mPa·s, and particularly preferably 1.0 mPa·s to 3.0 mPa·s. The surface tension of the ink at 25°C is preferably 10 mN / m to 60 mN / m, more preferably 20 mN / m to 60 mN / m, and particularly preferably 30 mN / m to 50 mN / m. The pH of the ink at 25°C is preferably 5.0 to 10.0, and more preferably 7.0 to 9.5.

[0047] <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 water-based ink of the present invention described above. FIG. 1 is a cross-sectional view that shows a schematic diagram of an embodiment of the ink cartridge of the present invention. As shown in FIG. 1, an ink supply port 12 for supplying ink to a recording head is provided on the bottom surface of the ink cartridge. The inside of the ink cartridge is 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 communicated with each other via a communication port 18. The absorber storage chamber 16 is also communicated with the ink supply port 12. The ink storage chamber 14 stores liquid ink 20, and the absorber storage chamber 16 stores absorbers 22 and 24 that hold the ink in an impregnated state. The ink storage section may not have an ink storage chamber that stores liquid ink, and may be in a form in which the entire amount of ink stored is held by the absorber. The ink storage section may also be in a form in which the entire amount of ink stored is stored in a liquid state, without having an absorber. Furthermore, the ink cartridge may be configured to have an ink container and a recording head.

[0048] <Inkjet recording method> The inkjet recording method of the present invention is a method in which the above-described water-based ink of the present invention is ejected from a recording head (thermal recording head) by the action of thermal energy to record an image on a recording medium. Other than using the ink of the present invention, the steps of the inkjet recording method may be known.

[0049] FIG. 2 is a diagram showing an example of an inkjet recording device used in the inkjet recording method of the present invention, in which (a) is a perspective view of the main part of the inkjet recording device, and (b) is a perspective view of a head cartridge. The inkjet recording device is provided with a conveying means (not shown) for conveying a recording medium 32, and a carriage shaft 34. A head cartridge 36 can be mounted on the carriage shaft 34. The head cartridge 36 includes recording heads 38 and 40, and is configured so that an ink cartridge 42 is set thereon. While the head cartridge 36 is conveyed in the main scanning direction along the carriage shaft 34, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. Then, an image is recorded on the recording medium 32 by conveying the recording medium 32 in the sub-scanning direction by a conveying means (not shown).

[0050] Any recording medium may be used as the recording medium 32. For example, a recording medium having ink absorbency (permeability) such as a recording medium without a coating layer, such as plain paper, uncoated paper, or synthetic paper, may be used. Also, a recording medium having no permeability, such as a film or sheet made of a resin material, such as polyvinyl chloride (PVC) or polyethylene terephthalate (PET), may be used. In addition, a recording medium other than paper, such as cloth, may be used. EXAMPLES

[0051] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples without departing from the gist of the present invention. Note that "parts" and "%" used for the amounts of components are based on mass unless otherwise specified.

[0052] <Method of measuring physical properties> (Acid value of resin) The acid value of the resin was measured by neutralization titration using an automatic potentiometric titrator (product name "AT510", manufactured by Kyoto Electronics Manufacturing Co., Ltd.) A 0.5 mol / L potassium hydroxide ethanol solution was used as the titration reagent.

[0053] <Resin synthesis> (Resin 1, 2) A four-neck flask equipped with a stirrer, a thermometer, a nitrogen gas inlet tube, and a reflux tube was prepared, and the inside was replaced with nitrogen. Into this flask, 100.0 parts of methyl ethyl ketone, BzMA (unit: parts) in the amount shown in Table 1, RAFT agent (cumyl dithiobenzoate 0.5 parts), and polymerization initiator (azobisisobutyronitrile 0.1 parts) were placed. After replacing with nitrogen at 25°C for 30 minutes, the temperature was raised to 75°C to start the reaction. The aqueous solution was extracted at regular intervals, and the dry solid content in the aqueous solution was measured to calculate the polymerization rate. After confirming that the polymerization rate was 95% or more, the remaining monomers (unit: parts) shown in Table 1 were added in sequence and further reacted. After confirming that the final polymerization rate was 98%, 50.0 parts of methyl ethyl ketone was added, and the flask was cooled to stop the reaction. The reaction liquid was added to a large amount of methanol, and the resulting precipitate was vacuum dried at 40°C for 24 hours. The obtained dried product was dissolved in tetrahydrofuran, and an aqueous solution of potassium hydroxide was added to the aqueous solution so that the neutralization rate of the anionic groups in the block copolymer was 80% on a molar basis. An appropriate amount of water was further added and stirred, and then the tetrahydrofuran was removed under reduced pressure. An appropriate amount of water was added to obtain liquids containing resins 1 and 2, each with a resin (solid content) content of 10.0%. Resins 1 and 2 are water-soluble block copolymers having an A block composed of units derived from a monomer having an aromatic ring and a B block composed of units derived from a monomer having an anionic group.

[0054] (Resin 3, 4) A four-neck flask equipped with a stirrer, a thermometer, a nitrogen gas inlet tube, and a reflux tube was prepared, and the inside was replaced with nitrogen. 100.0 parts of tetrahydrofuran, the monomers (units: parts) shown in Table 1, and a polymerization initiator (azobisisobutyronitrile 0.1 parts) were placed in this flask, and the mixture was heated to a temperature of 70°C while stirring to polymerize. An appropriate amount of water was further added and stirred, and then the pressure was reduced to remove tetrahydrofuran, and an appropriate amount of water was added to obtain liquids containing resins 3 and 4, each of which had a resin (solid content) content of 10.0%. Resins 3 and 4 are water-soluble resins with a random structure.

[0055] The abbreviations of the monomers in Table 1 are: St: styrene, BzMA: benzyl methacrylate, nBA: n-butyl acrylate, AA: acrylic acid, and MAA: methacrylic acid.

[0056] [Table 1]

[0057] (Resin 5) A four-neck flask equipped with a stirrer, a thermometer, a nitrogen gas inlet tube, and a reflux tube was prepared. 32.9 parts of isophorone diisocyanate, 51.9 parts of polypropylene glycol (number average molecular weight 2,000), 15.7 parts of dimethylolpropionic acid, and 300.0 parts of methyl ethyl ketone were placed in this four-neck flask. Then, the mixture was reacted at 80°C for 6 hours under a nitrogen gas atmosphere. Next, 0.9 parts of ethylenediamine was added, and the reaction was carried out at 80°C until the product reached a predetermined weight average molecular weight to obtain a reaction liquid. After the obtained reaction liquid was cooled to 40°C, ion-exchanged water was added, and an aqueous potassium hydroxide solution was added while stirring at high speed with a homomixer to obtain a liquid. Methyl ethyl ketone was distilled off from the liquid obtained by heating and reducing the pressure, and a liquid containing resin 5 with a urethane resin (solid content) content of 10.0% was obtained. The acid value of the obtained resin 5 was 65 mgKOH / g.

[0058] <Preparation of pigment dispersion> (Pigment Dispersion 1, 4, 5, 6, 8) A liquid obtained by dissolving 5.0 g of concentrated hydrochloric acid in 5.5 g of water was cooled to 5°C and 1.5 g of 4-aminophthalic acid was added. Next, the container containing this liquid was placed in an ice bath and stirred to keep the liquid at 10°C or lower. An aqueous solution of 2.2 g of sodium nitrite dissolved in 9 g of water at 5°C was added to this liquid. After stirring for 15 minutes, 6.0 g of carbon black shown in Table 2 was added under stirring. Then, the mixture was stirred for another 15 minutes. Next, the obtained slurry was filtered with filter paper (product name "Standard Filter Paper No. 2", Advantec), and the particles were thoroughly washed with water and dried in an oven at 110°C. After replacing the counter ions from sodium ions to potassium ions by ion exchange, water was added appropriately to obtain each pigment dispersion with a pigment content of 10.0%.

[0059] (Pigment dispersion 2) A container containing a mixture of 15.0 g of carbon black with a DBP oil absorption of 140 mL / 100 g and 4.0 g of sulfanilic acid was placed in a water bath at 70°C. An aqueous solution of 3.4 g of sodium nitrite dissolved in 9.0 g of water was added to the container under stirring. Hydrochloric acid was added to adjust the pH of the liquid to 2, and the mixture was stirred at 70°C for 1 hour. The resulting slurry was filtered with filter paper (product name "Standard Filter Paper No. 2", Advantec), and the particles were thoroughly washed with water and dried in an oven at a temperature of 110°C. The counter ions were replaced from sodium ions to potassium ions by ion exchange, and then water was added appropriately to obtain pigment dispersion 2 with a pigment content of 10.0%.

[0060] (Pigment dispersion 3) 20.0 g of carbon black with a DBP oil absorption of 140 mL / 100 g, 7.5 g of ((4-aminobenzoylamino)-methane-1,1-diyl)bisphosphonic acid sodium salt, 4.5 g of nitric acid, and 200 g of water were mixed. After 30 minutes, an aqueous solution of 4.9 g of sodium nitrite dissolved in a small amount of water was slowly added. The temperature of the mixture reached 60°C by this mixing, and the mixture was reacted in this state for 1 hour. Thereafter, the pH of the mixture was adjusted to 10 using an aqueous sodium hydroxide solution. After 30 minutes, 20 g of water was added and ultrafiltered. After replacing the counter ion from sodium ion to potassium ion by ion exchange, an appropriate amount of water was added to obtain pigment dispersion 3 with a pigment content of 10.0%.

[0061] (Pigment Dispersion 7) Pigment dispersion 7 having a pigment content of 10.0% was obtained in the same manner as in Pigment dispersion 2, except that carbon black having a DBP oil absorption of 50 mL / 100 g was used.

[0062] (Dispersion 9~17) 10.0 parts of carbon black with DBP oil absorption shown in Table 2, 30.0 parts of liquid containing the resin shown in Table 2, and 60.0 parts of ion-exchanged water were mixed and dispersed for 2 hours using a batch-type vertical sand mill (manufactured by Imex). After removing coarse particles by centrifugation, the mixture was filtered under pressure using a microfilter (manufactured by Fujifilm) with a pore size of 1.2 μm. Appropriate water was added to obtain pigment dispersions 9 to 17, each with a pigment content of 10.0% and a resin (resin dispersant) content of 3.0%.

[0063] [Table 2]

[0064] <Preparation of surfactant> The structures and HLB values ​​of polyoxyethylene alkyl ethers added as surfactants when preparing the ink are shown in Table 3. The HLB values ​​are values ​​calculated by the Griffin method described above. The numbers in parentheses next to polyoxyethylene are the number of moles of ethylene oxide groups added. In addition, in Table 3, Acetylenol E100 and Acetylenol E300 are the trade names of hydrocarbon surfactants manufactured by Kawaken Fine Chemicals. Surfynol 465 is the trade name of a hydrocarbon surfactant manufactured by Nissin Chemical Industry.

[0065] [Table 3]

[0066] <Ink Preparation> Each of the inks was prepared by mixing the components (unit: %) shown in Tables 4 to 7, thoroughly stirring the mixture, and then filtering the mixture under pressure using a microfilter (manufactured by Fujifilm) having a pore size of 3.0 μm.

[0067] [Table 4]

[0068] [Table 5]

[0069] [Table 6]

[0070] [Table 7]

[0071] <Evaluation> The prepared inks were used to carry out the following evaluations. In the present invention, for the evaluation criteria of each evaluation item shown below, "A" and "B" were considered to be acceptable levels, and "C" was considered to be unacceptable. The evaluation results are shown in Table 5.

[0072] (Discharge stability) Each prepared ink was filled into an ink cartridge and set in an inkjet recording device (trade name "PIXUS PRO-10", manufactured by Canon). In this example, an image recorded under the condition of applying 35 ng of ink to a unit area of ​​1 / 600 inch x 1 / 600 inch was defined as having a recording duty of 100%. The recording environment was a temperature of 25°C and a relative humidity of 55%. Using the above inkjet recording device, 10,000 solid images were continuously recorded on the entire surface of A4-sized plain paper (trade name "CS-064", manufactured by Canon) with a recording duty of 10%. The solid pattern recorded on the 10,000th sheet was visually confirmed, and the ink ejection stability was evaluated according to the evaluation criteria shown below. A: The entire solid image was recorded evenly. B: Unevenness was observed only at the edges of the solid image. C: Unevenness was observed throughout the solid image.

[0073] (character quality) Under the same conditions as those for the above-mentioned ejection stability evaluation, a line formed with a width of 10 dots of ink droplets was recorded on a pongee cloth (product name "Saidex500", manufactured by Seiren) cut to A4 size. The recording conditions were temperature: 25°C, relative humidity: 10%. One day after recording, the raggedness value of the line was measured using an image evaluation device (product name "Personal IAS", manufactured by QEA), and the character quality was evaluated according to the following evaluation criteria. A smaller raggedness value indicates less bleeding of the ink, and therefore indicates better character quality. A: The roughness score was less than 5. B: The roughness score was 5 or more and less than 10. C: The roughness score was 10 or more.

[0074] (optical density) Under the same conditions as the above-mentioned ejection stability evaluation, a solid image was printed on A4-sized plain paper (product name "CS-064", manufactured by Canon) with a print duty of 50%. One day after printing, the optical density was measured using a spectrophotometer (product name "FD-7", manufactured by Konica Minolta), and the optical density was evaluated according to the following evaluation criteria. A: The optical density was 1.2 or more. B: The optical density was 1.1 or more and less than 1.2. C: The optical density was less than 1.1.

[0075] (Image clarity) Under the same conditions as those for the ejection stability evaluation described above, a solid image was recorded on A4-size glossy paper (product name "Canon Photo Paper Glossy Gold GL-101", manufactured by Canon) with a recording duty of 50%. One day after recording, two fluorescent lights placed at intervals of 5 cm and 10 cm were used as observation light sources, and the fluorescent lights were projected onto the image from a distance of 2 m. The shape of the fluorescent lights projected onto the image was visually confirmed under conditions of an illumination angle of 45 degrees and an observation angle of 45 degrees, and the image clarity was evaluated according to the evaluation criteria shown below. A: Two fluorescent lights spaced 5cm apart were projected onto the image. B: The edges of the two projected fluorescent lights spaced 5 cm apart were blurred, but the boundary between them was easily distinguishable. C: The boundary between the two projected fluorescent lights spaced 5 cm apart was not visible.

[0076] [Table 8]

[0077] The disclosure of this embodiment includes the following methods and configurations.

[0078] (Configuration 1) A water-based ink for use in an inkjet recording method in which an image is recorded by applying ink ejected from a recording head to a recording medium by the action of thermal energy, The ink composition includes a first pigment, a second pigment, and a polyoxyethylene alkyl ether, the first pigment is carbon black having an anionic group bonded to its particle surface directly or via another atomic group (-R-), the second pigment is carbon black dispersed by the action of a resin having an anionic group; The aqueous ink is characterized in that the polyoxyethylene alkyl ether has an HLB value of 16.0 or more according to the Griffin method.

[0079] (Configuration 2) The aqueous ink according to configuration 1, wherein the anionic group of the first pigment is a carboxylic acid group.

[0080] (Configuration 3) The aqueous ink according to configuration 1 or 2, wherein the DBP oil absorption (mL / 100 g) of the first pigment is 120 mL / 100 g or more, and the DBP oil absorption (mL / 100 g) of the second pigment is 60 mL / 100 g or less.

[0081] (Configuration 4) The aqueous ink according to any one of Configurations 1 to 3, further comprising a water-soluble block copolymer having an A block including a unit derived from a monomer having an aromatic ring, and a B block including a unit derived from a monomer having an anionic group.

[0082] (Configuration 5) An ink cartridge including an ink and an ink storage section for storing the ink, 5. An ink cartridge, wherein the ink is the aqueous ink described in any one of configurations 1 to 4.

[0083] (Method 1) An inkjet recording method in which ink is ejected from a recording head by the action of thermal energy onto a recording medium to record an image, 5. An ink-jet recording method, wherein the ink is the aqueous ink described in any one of configurations 1 to 4.

Claims

1. A water-based ink for use in an inkjet recording method in which ink is ejected from a recording head by the action of thermal energy and applied to a recording medium to record an image, Contains a first pigment, a second pigment, and a polyoxyethylene alkyl ether; the first pigment is carbon black having an anionic group bonded to its particle surface directly or via another atomic group (-R-), the second pigment is carbon black dispersed by the action of a resin having an anionic group; The aqueous ink is characterized in that the polyoxyethylene alkyl ether has an HLB value, as determined by the Griffin method, of 16.0 or more.

2. The aqueous ink of claim 1 , wherein the anionic group of the first pigment is a carboxylic acid group.

3. 2. The aqueous ink according to claim 1, wherein the first pigment has a DBP oil absorption (mL / 100 g) of 120 mL / 100 g or more, and the second pigment has a DBP oil absorption (mL / 100 g) of 60 mL / 100 g or less.

4. The aqueous ink according to claim 1, further comprising a water-soluble block copolymer having an A block comprising a unit derived from a monomer having an aromatic ring, and a B block comprising a unit derived from a monomer having an anionic group.

5. An ink cartridge including an ink and an ink storage section for storing the ink, 5. An ink cartridge, wherein the ink is a water-based ink as claimed in claim 1.

6. An inkjet recording method for recording an image by applying ink ejected from a recording head to a recording medium by the action of thermal energy, comprising:

5. An ink-jet recording method, wherein the ink is a water-based ink according to claim 1.

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