Inkjet recording method and inkjet recording apparatus

Abstract

To provide an inkjet recording device which can express a wide color gamut, and enables recording of an image having high fluorescence intensity.SOLUTION: An inkjet recording method using an inkjet recording device having a recording head having discharge ports for discharging first ink and second ink includes a step of imparting the first ink and the second ink to a recording medium, and recording an image so that a region where the first ink is imparted and a region where the second ink is imparted at least partially overlap each other. The first ink contains pigment particles dispersed by action of an anionic group and an alkaline buffer agent, the second ink contains fluorescent particles dispersed by action of the anionic group, and a density ρ1 (g / cm3) of the pigment particles is larger than a density ρ2 (g / cm3) of the fluorescent particles.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an inkjet recording method and an inkjet recording apparatus. [Background technology]

[0002] In recent years, inkjet recording methods have been used to record images on recording media for purposes such as posters, and their frequency of use has increased dramatically. Such applications require a wide color gamut and high image robustness. To meet these requirements, for example, an ink containing resin microparticles dyed with a fluorescent dye has been proposed (Patent Document 1). Also proposed is an ink set that includes an ink containing a pigment and an ink containing dispersed particles that are a mixture of a fluorescent colorant and a resin (Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-063546 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-363455 Summary of the Invention [Problem to be solved by the invention]

[0004] The present inventors investigated inks containing fluorescent particles in order to record images with a wide color gamut and excellent durability. As a result, they found that the lightfastness of recorded images tends to decrease when inks containing fluorescent particles are used. Fluorescent colorants absorb short-wavelength light corresponding to the ultraviolet to visible light region and emit light with wavelengths longer than the absorbed light. Upon absorbing light, the colorant transitions to an excited state, which is highly energetic and unstable. Fluorescence refers to the emission of light that occurs when an excited state returns from a singlet state to a ground state. The singlet state is a highly unstable state, prone to oxidation-reduction reactions. Therefore, fluorescent colorants are prone to decomposition and have low lightfastness.

[0005] Next, the inventors added a pigment to an ink containing fluorescent particles and conducted an investigation. The results showed that using an ink containing both fluorescent particles and a pigment improved the lightfastness of the printed image. The improved lightfastness of the image is believed to be due to the addition of a pigment with excellent lightfastness. However, the inventors also found that the fluorescence intensity of the image decreased. The pigment present on the fluorescent particles likely scatters or absorbs the fluorescence emitted from the fluorescent particles, reducing the fluorescence intensity. Furthermore, because fluorescent particles and pigment are contained in the same ink, the content of the fluorescent particles and pigment is fixed, making it impossible to expand the reproducible color gamut. Furthermore, the inventors investigated an ink containing resin microparticles dyed with a fluorescent dye, as proposed in Patent Document 1. The results showed that although the fluorescence intensity of the image improved, the reproducible color gamut remained narrow.

[0006] The present inventors also investigated images recorded by applying two types of ink to a recording medium so that the inks at least partially overlap, using the ink set proposed in Patent Document 2. As a result, they found that although the representable color gamut was expanded, it was difficult to increase the fluorescence intensity of the image.

[0007] Therefore, an object of the present invention is to provide an inkjet recording method that can represent a wide color gamut and can record images with high fluorescent intensity, and another object of the present invention is to provide an inkjet recording apparatus used in this inkjet recording method. [Means for solving the problem]

[0008] That is, according to the present invention, there is provided an inkjet recording method comprising a step of applying the first ink and the second ink to a recording medium so that an area to which the first ink is applied and an area to which the second ink is applied at least partially overlap, using an inkjet recording apparatus equipped with a recording head having ejection ports for ejecting the first ink and the second ink, and recording an image, wherein the first ink contains pigment particles and an alkaline buffer, and the pigment particles are dispersed by the action of anionic groups, and the second ink contains fluorescent particles dispersed by the action of anionic groups, and the density of the pigment particles is ρ1 (g / cm 3 ) is the density ρ2 (g / cm 3 ) is greater than or equal to 1000 ppm. [Effects of the Invention]

[0009] The present invention provides an inkjet recording method capable of recording an image with a wide color gamut and high fluorescent intensity, and also provides an inkjet recording apparatus for use in the inkjet recording method. [Brief explanation of the drawings]

[0010] [Figure 1] 1A and 1B are diagrams schematically illustrating an example of an inkjet recording apparatus used in the inkjet 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

[0011] The present invention will be described in further 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 inkjet inks may be simply referred to as "ink." Unless otherwise specified, physical property values are values at room temperature (25°C) and normal pressure (1 atmosphere). A "unit" of a resin refers to a repeating unit derived from one monomer. Furthermore, when "(meth)acrylic acid" and "(meth)acrylate" are written, they mean "acrylic acid, methacrylic acid" and "acrylate, methacrylate," respectively.

[0012] The present inventors have conducted extensive research into inkjet recording methods capable of recording images with a wide color gamut and high fluorescent intensity, and have found that satisfying the following requirements (i) to (iv) makes it possible to record images with a wide color gamut and high fluorescent intensity. (i) An image is recorded by applying the first ink and the second ink to a recording medium so that an area to which the first ink is applied and an area to which the second ink is applied at least partially overlap each other. (ii) The first ink contains pigment particles and an alkaline buffer, and the pigment particles are dispersed by the action of anionic groups. (iii) The second ink contains fluorescent particles dispersed by the action of anionic groups. (iv) Density of pigment particles ρ1 (g / cm 3 ) is the density of the fluorescent particles ρ2 (g / cm 3 ) is greater than

[0013] Common recording media used in inkjet recording methods contain cationic components. For example, recording media without a coating layer, such as plain paper, contain fillers such as calcium carbonate. Furthermore, recording media with a coating layer, such as glossy paper or art paper, contain protons, cationic polymers, cationic pigments, and the like in the coating layer. Meanwhile, anionic colorants are typically used as ink colorants to enhance fixation to the recording medium through anion-cation interactions.

[0014] We will use an example in which the second ink and the first ink are applied to the recording medium in this order, with at least a partial overlap. In this case, the fluorescent particles in the second ink applied to the recording medium first react with the cationic components in the recording medium and aggregate. The first ink, containing pigment particles and an alkaline buffer, is then applied to the recording medium. The alkaline buffer, which has a buffering effect that continuously captures the cationic components of the aggregated fluorescent particles, captures the cationic components absorbed by the fluorescent particles and reduces the aggregation of the fluorescent particles. That is, when the first ink applied to the recording medium undergoes solid-liquid separation, the action of the alkaline buffer loosens the aggregation of both the pigment particles and the fluorescent particles, making them more mobile, and the fluorescent particles, which have a relatively low density, are thought to migrate above the pigment particles. As a result, the recorded image, i.e., the pigment layer formed, has a relatively high concentration of pigment particles in the lower layer and a relatively high concentration of fluorescent particles in the upper layer. This is thought to result in improved fluorescence intensity, as the fluorescence emitted from the fluorescent particles is less susceptible to scattering or absorption by the pigment. The above mechanism occurs due to the relationship between the densities of the pigment particles and the fluorescent particles, so even if the order in which the inks are applied is reversed, the same phenomenon as described above occurs, and the fluorescent intensity can be improved.

[0015] Alkaline buffers have a more compact molecular size than pigment particles or fluorescent particles. Therefore, unlike pigment particles or fluorescent particles, they penetrate into the recording medium after solid-liquid separation of the ink. Furthermore, the pigment particles present in the lower layer of the image are bound to the cationic components in the recording medium through anion-cation interactions, so the image's fixation to the recording medium is less likely to be impaired. Furthermore, because the first ink containing pigment particles and the second ink containing fluorescent particles are applied to the recording medium separately, it is easy to expand the color gamut that can be reproduced.

[0016] <Inkjet recording method and inkjet recording apparatus> The inkjet recording method of the present invention uses an inkjet recording device equipped with a recording head having ejection ports for ejecting the first ink and the second ink, respectively. The inkjet recording method of the present invention includes a step of applying the first ink and the second ink to a recording medium so that an area where the first ink is applied and an area where the second ink is applied overlap at least partially to record an image (hereinafter also referred to as a "recording step"). The inkjet recording device of the present invention is used in an inkjet recording method including a step of applying the first ink and the second ink to a recording medium so that an area where the first ink is applied and an area where the second ink is applied overlap at least partially to record an image. The inkjet recording device of the present invention also includes a recording head having ejection ports for ejecting the first ink and the second ink, respectively. The first ink contains pigment particles and an alkaline buffer, and the pigment particles are dispersed by the action of anionic groups, and the second ink contains fluorescent particles dispersed by the action of anionic groups. The density ρ1 (g / cm) of the pigment particles is ρ1 (g / cm). 3 ) is the density of the fluorescent particles ρ2 (g / cm 3 ) is greater than

[0017] An image with excellent lightfastness can be recorded by applying the first ink and the second ink to a recording medium so that the areas where the first ink is applied and the areas where the second ink is applied overlap at least partially. If the areas where the first ink is applied and the areas where the second ink is applied do not overlap, the first ink and the second ink will not be in contact with each other on the recording medium, and it will be impossible to form a pigment layer (image) having a lower layer containing many pigment particles and an upper layer containing many fluorescent particles. As a result, a pigment layer equivalent to an image recorded with ink containing only fluorescent particles as a colorant will be formed, resulting in insufficient lightfastness of the image.

[0018] To verify from an image whether the area where the first ink is applied and the area where the second ink is applied at least partially overlap, observation can be performed using an instrument such as an optical microscope. An example of an optical microscope that can be used is an "STM6 Measuring Microscope" (manufactured by Olympus). It is preferable to apply the first ink and the second ink to the recording medium so that dots of the first ink and dots of the second ink overlap in the unit area. The unit area can be set to any area, such as one square inch, one pixel, or one pixel. For example, it can be set in units of one pixel. Specifically, the unit area is preferably 1 / 1,200 inch × 1 / 1,200 inch or more and 1 / 150 inch × 1 / 150 inch or less, and more preferably 1 / 600 inch × 1 / 600 inch or more and 1 / 300 inch × 1 / 300 inch or less.

[0019] Methods for ejecting ink from an inkjet recording head include a method of applying mechanical energy to ink and a method of applying thermal energy to ink, and among these, it is particularly preferable to adopt a method of ejecting ink by applying thermal energy to ink.

[0020] FIG. 1 is a diagram schematically 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 is equipped with recording heads 38 and 40 and is configured to accommodate an ink cartridge 42. While the head cartridge 36 is transported along the carriage shaft 34 in the main scanning direction, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. An image is then recorded on the recording medium 32 by transporting the recording medium 32 in the sub-scanning direction by a transport means (not shown).

[0021] In the recording process, a serial recording head is preferably used, and the recording head is scanned four to sixteen times in the main scan direction to deposit the first and second inks onto a unit area of the recording medium. The main scan refers to the direction of reciprocal scanning of the recording head, and the recording medium is transported in a direction perpendicular to the main scan direction (sub-scan direction). If the number of main scans of the recording head used to deposit the first and second inks onto a unit area of the recording medium is less than four, the amount of ink deposited per main scan when recording a predetermined image will be relatively large. This increases the time it takes for the aggregation of both the pigment particles and the fluorescent particles to be alleviated. This can lead to an anion-cation interaction between the cations of the alkaline buffer that have captured the cationic components and the pigment particles and fluorescent particles dispersed by the action of the anionic groups, which can easily cause the pigment particles and fluorescent particles to aggregate during solid-liquid separation of the first ink. This can make it difficult to form a pigment layer separated into two layers: an upper layer containing a large amount of fluorescent particles and a lower layer containing a large amount of pigment particles, resulting in a slight decrease in the fluorescent intensity of the image. On the other hand, if the number of main scans of the recording head that deposits the first ink and the second ink onto a unit area of the recording medium exceeds 16, the amount of ink deposited per main scan when recording a predetermined image becomes relatively small. This shortens the time it takes for the pigment particles and fluorescent particles to become less aggregated. This makes it easier for the cationic components contained in the recording medium to aggregate the fluorescent particles and pigment particles, making it difficult to form a pigment layer separated into two layers: an upper layer containing a large amount of fluorescent particles and a lower layer containing a large amount of pigment particles, and may result in a slight decrease in the fluorescent intensity of the image.

[0022] (ink) The inkjet recording method of the present invention uses a first ink and a second ink. The first ink contains pigment particles and an alkaline buffer. The second ink contains fluorescent particles dispersed by the action of anionic groups. Hereinafter, when simply referring to "ink," it means either the "first ink" or the "second ink." The components constituting the inks used in the inkjet recording method of the present invention will be described in detail below.

[0023] [Pigment particles] The coloring material of the first ink is pigment particles in which the pigment is dispersed by the action of anionic groups. The content (mass %) of the pigment particles in the first ink is preferably 0.10 mass % or more and 10.00 mass % or less based on the total mass of the ink. The density ρ1 (g / cm 3 ) is the density ρ2 (g / cm) of the fluorescent particles contained in the second ink 3 ) is larger than the density of the pigment particles ρ1 (g / cm 3 ) is the density of the fluorescent particles ρ2 (g / cm 3 ) or less, it is thought that when the first ink undergoes solid-liquid separation, many fluorescent particles will be present below the pigment particles. For this reason, a pigment layer is formed that has a lower layer containing many fluorescent particles and an upper layer containing many pigment particles, and the fluorescence emitted by the fluorescent particles in the lower layer is scattered or absorbed by the pigment particles in the upper layer, resulting in insufficient fluorescence intensity. 3 ) is 1.45g / cm 3 More than 5.00g / cm 3 Preferably, it is 1.50 g / cm or less. 3 More than 2.00g / cm 3 It is preferable that the second ink does not contain pigment particles dispersed by the action of anionic groups.

[0024] When pigment particles or fluorescent particles are the measurement target, the density of pigment particles and fluorescent particles can be measured in accordance with JIS Z8807:2012. The density of pigment particles and fluorescent particles in ink can be measured using a density / specific gravity meter (for example, a portable density / specific gravity meter DA-130N (manufactured by Kyoto Electronics Manufacturing Co., Ltd.)). When measuring the density of pigment particles and fluorescent particles in ink, it is preferable to adjust the content (mass %) of pigment particles and fluorescent particles in the ink to 1.0 mass % or more and 10.0 mass % or less, respectively, based on the total mass of the ink, by concentrating or diluting with water.

[0025] Even when the ink contains multiple types of pigment particles or multiple types of fluorescent particles, the densities measured using the ink as described above can be considered to be the density ρ1 of the pigment particles and the density ρ2 of the fluorescent particles. In this case, particles that satisfy the relationship ρ1 > ρ2 will also exhibit the above-mentioned phenomenon, further improving the fluorescence intensity of the image. Although the above describes a method for measuring the density of pigment particles and fluorescent particles using ink, the density can also be measured using pigment particles or fluorescent particles appropriately separated from the ink.

[0026] The density of pigment particles can be adjusted, for example, as follows. Pigments have specific densities depending on their type. Density can also be controlled by a technique in which pigment particles are dispersed by the action of anionic groups, i.e., by the dispersion method. For example, in the case of a dispersion method using a resin dispersant, the density of pigment particles can be adjusted by controlling the resin composition, the ratio of pigment to resin dispersant, etc.

[0027] The first ink contains pigment particles formed by dispersing the pigment through the action of anionic groups. If the pigment particles are not dispersed through the action of anionic groups, electrostatic repulsion will not occur between the pigment particles and the fluorescent particles when the first ink undergoes solid-liquid separation, and the pigment particles will tend to adhere to each other. This makes it difficult to form a pigment layer having a lower layer containing a large amount of pigment particles and an upper layer containing a large amount of fluorescent particles, resulting in insufficient fluorescent intensity in the image.

[0028] The pigment in the first ink preferably satisfies the relationship of the following formula (1), and more preferably satisfies the relationship of the following formula (2). A λ2 / A λ1 ≦0.5 (1) A λ2 / A λ1 ≦0.4 (2) A λ1 : Absorbance of pigment particles at the maximum absorption wavelength λ1 (nm) of the pigment particles A λ2 : Absorbance of pigment particles at the maximum fluorescence wavelength λ2 (nm) of fluorescent particles

[0029] To improve the fluorescence intensity of the image, it is preferable to have a relatively large number of pigment particles in the lower layer of the pigment layer and a relatively large number of fluorescent particles in the upper layer of the pigment layer. In this case, the absorbance A of the pigment particles at the maximum fluorescence wavelength λ2 (nm) of the fluorescent particles in the second ink is λ2 is the absorbance A of the pigment particle at the maximum absorption wavelength λ1 (nm) of the pigment particle. λ1 It is preferable that the value of A is relatively small compared to the value of A. This makes it difficult for the fluorescence emitted by the fluorescent particles to be absorbed by the pigment particles, thereby further increasing the fluorescence intensity of the image. λ2 / A λ1 If the value of "A" exceeds 0.5, the fluorescence emitted by the fluorescent particles may be easily absorbed by the pigment particles, and the effect of improving the fluorescence intensity of the image may be slightly reduced. λ2 / A λ1 It is preferable that the value of " is 0.0 or greater.

[0030] Typically, the absorbance of an ink containing pigment particles can be referred to as the "absorbance of pigment particles." The absorption spectrum of an ink containing pigment particles can be measured using a spectrophotometer (e.g., "U-3900 / 3900H" (Hitachi High-Tech Science)). The fluorescence spectrum of fluorescent particles can be measured using a spectrofluorometer (e.g., "F-2700" (Hitachi High-Tech Science)). The maximum absorption wavelength λ1 (nm) of the pigment particles and the maximum fluorescence wavelength λ2 (nm) of the fluorescent particles are determined within the visible light range of 380 to 800 nm for each measured spectrum. The maximum absorption wavelength λ1 (nm) and the maximum fluorescence wavelength λ2 (nm) are the wavelengths at which the absorbance and fluorescence intensity of each spectrum, when differentiated once with respect to wavelength, are closest to zero and the absorbance and fluorescence intensity of each spectrum, when differentiated twice with respect to wavelength, are negative. For pigment particles such as carbon black that do not have a maximum absorption wavelength, the wavelength at which the absorbance reaches its maximum value in the range of 380 to 800 nm is defined as the "maximum absorption wavelength."

[0031] Even when the ink contains multiple types of pigment particles or multiple types of fluorescent particles, the maximum absorption wavelength measured using the ink as described above may be regarded as the maximum absorption wavelength λ1 (nm) of the pigment particles and the maximum fluorescence wavelength λ2 (nm) of the fluorescent particles. λ2 / A λ1 Particles whose "value" satisfies the above relationship will cause the phenomenon described above, and therefore the fluorescence intensity of the image can be further improved. Although the method for measuring the absorbance of pigment particles and fluorescent particles using ink has been described above, it is also possible to measure the absorbance using pigment particles or fluorescent particles appropriately separated from the ink. The absorbance of pigment particles and fluorescent particles mainly depends on the type of colorant, so the absorbance can be adjusted by appropriately selecting the type of colorant.

[0032] Any pigment may be used to form the pigment particles as long as it satisfies the density relationship described above. However, the pigment used in the first ink does not necessarily have to emit fluorescence; a non-fluorescent pigment is preferred. Specific examples of pigments include inorganic pigments such as carbon black and titanium oxide; and organic pigments such as quinacridone, perinone, perylene, dioxazine, azo, phthalocyanine, isoindolinone, imidazolone, and diketopyrrolopyrrole. Among these, at least one pigment selected from the group consisting of carbon black, quinacridone, perinone, perylene, dioxazine, azo, and phthalocyanine is preferred, with quinacridone being particularly preferred, as it can further improve fluorescence intensity. The content (mass %) of the pigment in the first ink is preferably 0.10% by mass or more and 10.00% by mass or less, based on the total mass of the ink.

[0033] The pigment is preferably composed of molecules having fused rings formed by condensing five or more single rings. Pigments composed of molecules having fused rings formed by condensing five or more single rings have many conjugated systems in which single and multiple bonds are connected within the molecule, resulting in many delocalized π electrons. This results in π electron interactions between pigment particles, resulting in strong interparticle bonding. Here, we will explain an example in which the first ink and the second ink are applied to a recording medium in order so that they overlap. In this case, the second ink is applied on top of pigment particles in the first ink that have aggregated on the recording medium. In the case of pigment particles composed of molecules having fused rings formed by condensing five or more single rings, the interparticle bonding is strong, so the aggregation is not easily alleviated even when the second ink is applied. Therefore, pigment particles aggregated in the lower layer are less likely to move to the upper layer when the second ink is applied. This facilitates the formation of a pigment layer having a lower layer containing many pigment particles and an upper layer containing many fluorescent particles, thereby further increasing the fluorescence intensity.

[0034] Examples of pigment skeletons composed of molecules having fused rings formed by condensing five or more monocyclic rings include quinacridone skeletons, dioxazine skeletons, and perylene skeletons. Examples of pigments having fused rings formed by condensing five or more monocyclic rings in their molecular structure include CI Pigment Violet 19, 23; CI Pigment Red 122, 149; and CI Pigment Orange 43. Carbon black is also a pigment composed of molecules having fused rings formed by condensing five or more monocyclic rings.

[0035] The pigment particles may be dispersed by the action of anionic groups, and the dispersion method is not particularly limited. For example, resin-dispersed pigments dispersed using a resin dispersant having anionic groups, and microencapsulated pigments in which at least a portion of the pigment particle surface is coated with a resin having anionic groups, can be used. Pigments dispersed using a surfactant having anionic groups can also be used. Furthermore, self-dispersed pigments in which functional groups containing anionic groups are bonded to the pigment particle surface, and pigments in which organic groups containing polymers having anionic groups are chemically bonded to the pigment particle surface (resin-bonded self-dispersed pigments) can also be used. Pigments dispersed using different dispersion methods can also be used in combination.

[0036] Self-dispersing pigments can be used in which anionic groups such as carboxylic acid groups, sulfonic acid groups, and phosphonic acid groups are bonded to the surface of pigment particles directly or via another atomic group (-R-). The anionic group can be either an acid type or a salt type. If the anionic group is a salt type, it can be either partially dissociated or completely dissociated. When the anionic group is a salt type, examples of the cation that serves as the counter ion include alkali metal cations, ammonium, and organic ammonium. Specific examples of the other atomic group (-R-) include linear or branched alkylene groups having 1 to 12 carbon atoms; arylene groups such as phenylene and naphthylene; carbonyl groups; imino groups; amide groups; sulfonyl groups; ester groups; and ether groups. Furthermore, combinations of these groups may also be used.

[0037] Resin dispersants used in aqueous inks typically contain a hydrophilic unit having an anionic group and a hydrophobic unit not having an anionic group. The hydrophilic unit is a unit that ensures affinity to aqueous media. The hydrophobic unit is a unit that adsorbs to the pigment particle surface through hydrophobic interaction. Examples of resin-dispersed pigments include those in which a resin dispersant is physically adsorbed to the pigment particle surface to disperse the pigment, and microencapsulated pigments in which the pigment particle surface is coated with a resin dispersant.

[0038] It is preferable to use a water-soluble resin as the resin dispersant. In this specification, "water-soluble resin" means a resin that dissolves in an aqueous medium and can exist in the aqueous medium in a state where it does not form particles having a particle size. If the resin dispersant is water-dispersible (water-insoluble), the storage stability of the ink may be somewhat reduced. The content (mass %) of the resin dispersant in the ink is preferably 0.10 mass % or more and 5.00 mass % or less, based on the total mass of the ink.

[0039] Whether or not a resin used as a resin dispersant is water-soluble can be determined according to the following method. First, a liquid (resin solids content: 10% by 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 (by volume) with pure water to prepare a sample solution. Then, when the particle size of the resin in the sample solution is measured by dynamic light scattering, if no particles having the particle size are measured, the resin can be determined to be water-soluble. The measurement conditions can be, for example, as follows:

[0040] [Measurement conditions] SetZero: 30 seconds Number of measurements: 3 Measurement time: 180 seconds

[0041] As the particle size distribution measuring device, a particle size analyzer using a dynamic light scattering method (for example, the product name "UPA-EX150" manufactured by Nikkiso Co., Ltd.) can be used. Of course, the particle size distribution measuring device and measurement conditions to be used are not limited to those described above.

[0042] A preferred pigment dispersion method is a resin-dispersed pigment dispersed with a resin dispersant containing anionic groups. This resin dispersant disperses the pigment by physically adsorbing its hydrophobic units to the pigment particle surface and hydrating its hydrophilic units. When solid-liquid separation occurs between the liquid component and the pigment particles or fluorescent particles on a recording medium to which the ink has been applied, the pigment particles slowly aggregate due to steric repulsion caused by the resin dispersant between the pigment particles and between the pigment particles and fluorescent particles. During this process, pigment particles with a relatively high density tend to exist below the fluorescent particles, forming a pigment layer with an upper layer containing a large number of fluorescent particles and a lower layer containing a large number of pigment particles, thereby further increasing the fluorescence intensity.

[0043] [Alkaline buffer] The first ink contains an alkaline buffer. The alkaline buffer is a component that has buffering properties in the alkaline range of pH 7.0 or higher. Using a first ink containing an alkaline buffer enables recording of images with high fluorescent intensity. The pKa (acid dissociation constant) of the alkaline buffer at 25°C is preferably 7.6 or higher and 10.4 or lower. The pKa value is a physical property that indicates the basicity of a compound. The higher the pKa value, the stronger the basicity of the compound and the higher its ability to capture cationic components. If the pKa of the alkaline buffer is less than 7.6, the ability to capture cationic components is somewhat low, which may result in insufficient capture of cationic components by fluorescent particles. This may make it difficult to form a pigment layer having a lower layer containing a large amount of pigment particles and an upper layer containing a large amount of fluorescent particles, resulting in a slight decrease in the effect of improving fluorescent intensity.

[0044] On the other hand, if the pKa of the alkaline buffer exceeds 10.4, the cationic component trapping ability is somewhat high, and the alkaline buffer that has trapped the cationic component tends to exist in a cationic state. As a result, anion-cation interactions may easily occur between the cationic alkaline buffer and the pigment particles and fluorescent particles dispersed therein due to the action of anionic groups. This may cause the pigment particles and fluorescent particles to easily aggregate, making it difficult to form a pigment layer having a lower layer containing a large amount of pigment particles and an upper layer containing a large amount of fluorescent particles, and may slightly reduce the effect of improving the fluorescence intensity.

[0045] The ionization equilibrium of the acid represented by HA is HA⇔H + +A - The equilibrium constant Ka is expressed as Ka=[H + ]×[A - The acid dissociation constant is the negative logarithm of this equilibrium constant, pKa = -log 10 The pKa is defined as Ka. In this specification, the pKa value at 25°C is used. The pKa can be calculated by neutralization titration using a pH measuring device (for example, a product name "798MPT Titrino" manufactured by Metrohm). When an alkaline buffer has multiple dissociation stages, it is sufficient that the pKa at at least one of the dissociation stages is within the above range. In particular, it is more preferable that the highest pKa value among the multiple dissociation stages is within the above range.

[0046] Examples of alkaline buffers include diethanolamine (8.9), diisopropanolamine (8.1), triethanolamine (7.8), triisopropanolamine (8.1), tributylamine (10.0), trihexylamine (10.5), N-cyclohexyl-3-aminopropanesulfonic acid (10.5), N-cyclohexyl-2-aminoethanesulfonic acid (9.3), and N,N-bis(2-hydroxyethyl)glycine (8.4). The parenthesized values following the above compound names indicate the pKa values at 25°C. The alkaline buffer is preferably an amine compound, and more preferably a tertiary amine compound. Compared to primary and secondary amine compounds, tertiary amine compounds exert stronger steric repulsion on pigment particles and fluorescent particles, making it difficult for protons captured from the pigment particles or fluorescent particles to return to the pigment particles or fluorescent particles. This makes it easier for a large number of pigment particles with a relatively high density to be present below the fluorescent particles, forming a pigment layer having an upper layer containing a large number of fluorescent particles and a lower layer containing a large number of pigment particles, thereby further increasing the fluorescence intensity.

[0047] The content (mass %) of the alkaline buffer in the first ink is preferably 0.01 mass % or more and 5.00 mass % or less, and more preferably 0.10 mass % or more and 3.00 mass % or less, based on the total mass of the ink.

[0048] [Fluorescent particles] The second ink contains fluorescent particles dispersed by the action of anionic groups. The density ρ1 (g / cm) of the pigment particles contained in the first ink 3 ) is the density ρ2 (g / cm) of the fluorescent particles contained in the second ink 3 ) is larger than the density of the fluorescent particles, ρ2 (g / cm 3 ) is 1.00g / cm 3 More than 1.50g / cm 3 Preferably, it is 1.00 g / cm or less. 3 More than 1.45g / cm 3 It is preferable that the first ink does not contain fluorescent particles.

[0049] The density of fluorescent particles can be measured using the method described above. The density of fluorescent particles can also be adjusted using a method similar to that of pigment particles. For example, if the fluorescent particles are resin particles (having anionic groups) dyed with a fluorescent colorant (fluorescent dye, fluorescent pigment), the inherent density of the fluorescent colorant can be taken into consideration. Furthermore, the density can be adjusted by the composition of the resin having anionic groups that forms the resin particles, the ratio of the fluorescent colorant to the resin, etc. Furthermore, if the fluorescent particles are fluorescent pigments dispersed by the action of anionic groups, the density can be adjusted using a method similar to that of pigment particles.

[0050] As used herein, "fluorescent particles" refer to particles that emit fluorescence when exposed to ultraviolet or visible excitation light. Whether or not a particle is a "fluorescent particle" that exhibits fluorescence can be determined, for example, according to the following method. A sample obtained by dispersing particles in a liquid capable of dispersing the particles is irradiated with ultraviolet light (ultraviolet light) of a long wavelength (approximately 315 to 400 nm) that is barely visible to the naked eye, using a black light or the like. If light of a color different from the ultraviolet light irradiated by the black light can be visually observed, the particle can be determined to be a "fluorescent particle" that exhibits fluorescence. A commercially available black light (for example, "SLUV-4" (manufactured by AS ONE)) can be used.

[0051] Examples of fluorescent particles include resin particles dyed with fluorescent dyes and fluorescent pigments such as CI Pigment Yellow 101. The fluorescent dye in resin particles dyed with fluorescent dyes can be analyzed, for example, according to the following procedure. Resin particles extracted from ink using standard methods are dissolved in an organic solvent such as chloroform to prepare a sample. The fluorescent dye is isolated from the prepared sample using HPLC (high-performance liquid chromatography). The isolated dye is then analyzed using common structural analysis techniques such as nuclear magnetic resonance (NMR) spectroscopy and matrix-assisted laser desorption / ionization mass spectrometry (MALDI-MS).

[0052] As the fluorescent dye, basic dyes, acid dyes, disperse dyes, oil-soluble dyes, etc. can be used. Among them, basic dyes are preferred. Examples of the dye skeleton include xanthene, azine, azole, thiazole, azo, diarylmethane, triarylmethane, acridine, coumarin, and methine. Among them, compounds having a skeleton such as xanthene or coumarin are preferred, and compounds having a xanthene skeleton are more preferred. Compounds having a xanthene skeleton generally have a density of 1.0 g / cm, although this varies slightly depending on the structure. 3 Therefore, a large number of pigment particles, which have a relatively high density, tend to be present below the fluorescent particles, forming a pigment layer with an upper layer containing many fluorescent particles and a lower layer containing many pigment particles, thereby further increasing the fluorescence intensity.

[0053] Basic dyes are fluorescent compounds that contain an amino or imino group (which may form a salt) in their molecular structure. Examples of compounds that contain an amino or imino group in their molecular structure include dyes whose names include "basic" in the Colour Index. The Colour Index is a database of colour materials compiled by the British Society of Dyes and Colourists and others.

[0054] Specific examples of fluorescent basic dyes, expressed by CI number or general name, include CI Basic Red 1, 1:1, 2, 4, 8, 11, 12, and 13; CI Basic Violet 1, 3, 10, 11, 11:1, and 14; Rhodamine 19 and 575; CI Basic Yellow 1, 2, 9, 13, 24, 37, 40, and 96; CI Basic Blue 7; CI Basic Green 1; and CI Fluorescent Brightener 363. Among these, CI Basic Red 1 and 1:1; CI Basic Violet 11 and 11:1; and CI Basic Yellow 40 are preferred due to their excellent color development.

[0055] Acid dyes are fluorescent compounds that contain an acidic group (which may form a salt), such as a carboxylic acid group or a sulfonic acid group, in their molecular structure. Examples of compounds containing an acidic group in their molecular structure include dyes whose names in the Color Index include "acid." Specific examples of fluorescent acid dyes, listed by CI number, include CI Acid Blue 9, CI Acid Yellow 7, CI Acid Yellow 23, CI Acid Red 52, CI Acid Red 87, CI Acid Red 92, and CI Acid Black 2.

[0056] Disperse dyes are fluorescent compounds that have low or no water solubility. Examples of "disperse dyes" include dyes whose names in the Color Index include "disperse." Examples of dye skeletons include azo, coumarin, and anthraquinone. Among these, compounds with a coumarin or anthraquinone skeleton are preferred, and compounds with a coumarin skeleton are even more preferred.

[0057] Specific examples of fluorescent disperse dyes, expressed by CI numbers, include CI Disperse Yellow 82 and 186, CI Disperse Red 58 and 60, and CI Disperse Orange 11. Among these, CI Disperse Yellow 82 is preferred due to its excellent color development.

[0058] Oil-soluble dyes are compounds that exhibit fluorescence and have low or no water solubility. Examples of oil-soluble dyes include dyes whose names in the Color Index include "solvent." Examples of dye skeletons include coumarin, xanthene, azo, aminoketone, and anthraquinone. Among these, compounds having a coumarin or xanthene skeleton are preferred, and compounds having a coumarin skeleton are even more preferred.

[0059] Specific examples of fluorescent oil-soluble dyes, expressed by CI numbers, include CI Solvent Yellow 7, 43, 44, 85, 98, 131, 160:1, 172, and 196; CI Solvent Red 43, 44, 45, 49, and 149; and CI Solvent Orange 5, 45, 63, and 115. Among these, CI Solvent Yellow 160:1 and 196 are preferred due to their excellent color development properties.

[0060] Basic dyes are preferred as fluorescent colorants. Since the alkaline buffer that has captured protons carries a positive charge, electrostatic repulsion occurs between the alkaline buffer and the basic dye. This makes it difficult for the protons captured from the pigment particles or fluorescent particles to return to the pigment particles or fluorescent particles. This tends to increase the number of pigment particles with a relatively high density below the fluorescent particles, forming a pigment layer with an upper layer containing many fluorescent particles and a lower layer containing many pigment particles, thereby further increasing the fluorescence intensity.

[0061] The content (mass %) of fluorescent particles in the second ink is preferably 0.10 mass % or more and 15.00 mass % or less, and more preferably 1.00 mass % or more and 10.00 mass % or less, based on the total mass of the ink.

[0062] The resin particles constituting the "resin particles dyed with a fluorescent dye" preferably have a core and a shell covering the core, that is, a core-shell structure. The core preferably contains an aromatic group-containing unit and a cyano group-containing unit. The shell preferably contains an aromatic group-containing unit and an anionic group-containing unit, and may further contain a unit derived from a crosslinking agent.

[0063] The monomer that becomes the aromatic group-containing unit by polymerization is preferably one having one polymerizable functional group such as an ethylenically unsaturated bond in the molecule. Among them, styrene and its derivatives are more preferred, and styrene and vinyltoluene are particularly preferred, because they have good reactivity during polymerization and the resulting resin particles have excellent stability.

[0064] The monomer that becomes the cyano group-containing unit by polymerization is preferably one having one polymerizable functional group such as an ethylenically unsaturated bond in the molecule. Among them, acrylonitrile and methacrylonitrile are particularly preferred because they have good reactivity during polymerization and the resulting resin particles have excellent stability.

[0065] The anionic group in the anionic group-containing unit preferably has one polymerizable functional group, such as an ethylenically unsaturated bond, in the molecule. Specific examples include carboxylic acid groups, phenolic hydroxy groups, and phosphate ester groups. Of these, carboxylic acid groups are preferred because they provide good stability of the resin particles in the ink. The anionic group may be either an acid type or a salt type, and if it is a salt type, it may be either partially dissociated or completely dissociated. When the anionic group is a salt type, examples of the cation that serves as the counter ion include alkali metal cations, ammonium, and organic ammonium.

[0066] The core and shell of the resin particle may each contain a unit other than the above-mentioned units, as long as the effects of the present invention are not impaired. The unit other than the above-mentioned units is preferably one having one polymerizable functional group in the molecule, and specific examples thereof include a unit derived from an ethylenically unsaturated monomer.

[0067] The volume-based cumulative 50% particle diameter (D50) of the resin particles is preferably 120 nm or less. If the volume-based cumulative 50% particle diameter (D50) of the resin particles exceeds 120 nm, light scattering by the resin particles is likely to occur, and the color development of the image may be slightly reduced. The volume-based cumulative 50% particle diameter (D50) of the resin particles is preferably 50 nm or more.

[0068] [Method for producing dyed resin particles] The resin particles can be produced by a conventionally known method such as emulsion polymerization, miniemulsion polymerization, seed polymerization, or phase inversion emulsification. Examples of methods for dyeing resin particles include a method of polymerizing a monomer mixture in which a fluorescent dye is dissolved to form resin particles; a method of adding a fluorescent dye to resin particles and heating the particles; and the like. Among these, the method of adding a fluorescent dye to resin particles and heating the particles is preferred because it can be used with a wider variety of fluorescent dyes.

[0069] [Method for verifying resin particles] The composition of the resin particles can be verified according to the following methods (i) to (iii). Below, we will explain the method of extracting resin particles from ink and analyzing and verifying them, but resin particles extracted from an aqueous dispersion or the like can also be analyzed and verified in the same way.

[0070] (i) Extraction of resin particles Density gradient centrifugation can be used to separate and extract resin particles from ink containing them. Among density gradient centrifugation methods, density gradient sedimentation velocity separates and extracts resin particles based on the difference in sedimentation coefficient of the components. Also, among density gradient centrifugation methods, density gradient sedimentation equilibrium separates and extracts resin particles based on the difference in density of the components.

[0071] (ii) Confirmation and separation of layer structure First, resin particles are stained and fixed with ruthenium tetroxide, then embedded in epoxy resin to stabilize them. Next, the resin particles embedded in epoxy resin are cut with an ultramicrotome, and the cross section is observed using a scanning transmission electron microscope (STEM). By observing the cross section cut through the center of gravity of the resin particle, the layer structure of the resin particle can be confirmed. Resin particles embedded in epoxy resin are used as the analytical sample, and the elements contained in the layers (core and shell) that make up the resin particles can be quantitatively analyzed using STEM-EDX coupled with energy dispersive X-ray spectroscopy (EDX).

[0072] (iii) Analysis of the units (monomers) that make up the resin of each layer The resin particles used as a sample for separating the resin into each layer may be in the form of a dispersion. Alternatively, the resin particles may be dried to form a film. The sample resin particles are dissolved in an organic solvent, and then the layers are separated using gel permeation chromatography (GPC), and the resins constituting each layer are separated. The separated resins are then subjected to elemental analysis using a combustion method. Separately, the separated resins are pretreated using an acid decomposition method (addition of hydrofluoric acid) or an alkali fusion method, and then the inorganic components are quantitatively analyzed using inductively coupled plasma atomic emission spectroscopy. By comparing the results of the elemental analysis and quantitative analysis of the inorganic components with the results of the quantitative elemental analysis using STEM-EDX obtained in (ii) above, the resin particle layers that comprised the separated resin can be determined.

[0073] The fractionated resins are analyzed by nuclear magnetic resonance (NMR) spectroscopy and matrix-assisted laser desorption / ionization mass spectrometry (MALDI-MS). This allows us to determine the types and proportions of the units (monomers) and crosslinking components that make up the resin. Furthermore, the monomers produced by depolymerization can be directly detected by analyzing the fractionated resins using pyrolysis gas chromatography.

[0074] [Siloxane compounds] The first ink preferably contains a nonionic siloxane compound. Using a first ink containing a nonionic siloxane compound can improve the fixation of secondary color images recorded together with the second ink. The siloxane compound refers to a compound having a siloxane structure (Si-O), preferably one to which a nonionic hydrophilic group such as an ethylene oxide group or a hydroxyl group is bonded. The weight-average molecular weight of the siloxane compound is preferably 500 to 5,000. Examples of such siloxane compounds include modified siloxane compounds, and commercially available silicone oils and silicone surfactants can be used. The content (mass %) of the nonionic siloxane compound in the first ink is preferably 0.10 to 3.00 mass %, more preferably 0.50 to 2.00 mass %, based on the total mass of the ink. In particular, when the first ink contains a nonionic siloxane compound and the fluorescent particles of the second ink are resin particles dyed with a basic dye, the fixation of the secondary color image can be particularly improved.

[0075] [Aqueous medium] The ink is preferably an aqueous ink containing at least water as the aqueous medium. The ink may further contain a water-soluble organic solvent as the aqueous medium. Deionized water or ion-exchanged water is preferably used as the water. The water content (mass %) in the ink is preferably 50.00 mass % or more and 95.00 mass % or less, based on the total mass of the ink. Furthermore, any water-soluble organic solvent commonly used in inks can be used. Examples include alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing compounds, and sulfur-containing compounds. The water-soluble organic solvent content (mass %) in the ink is preferably 3.00 mass % or more and 50.00 mass % or less, based on the total mass of the ink.

[0076] [Other additives] In addition to the above components, the ink may optionally contain water-soluble organic compounds that are solid at room temperature, such as polyhydric alcohols such as trimethylolpropane and trimethylolethane, and urea derivatives such as urea and ethyleneurea. Furthermore, the ink may optionally contain various additives, such as surfactants, pH adjusters, rust inhibitors, preservatives, antifungal agents, antioxidants, antireducing agents, evaporation accelerators, chelating agents, and other resins.

[0077] [Ink properties] Since the ink is applied to the inkjet method, it is preferable to appropriately control the physical properties of the ink. Specifically, the static surface tension of the ink at 25°C, measured by the plate method, is preferably 30 mN / m or more and 50 mN / m or less. The viscosity of the ink at 25°C is preferably 2.0 mPa·s or more and 10.0 mPa·s or less. The pH of the ink at 25°C is preferably 5.0 or more and 10.0 or less, and more preferably 7.0 or more and 9.5 or less. [Example]

[0078] 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 these examples as long as it does not depart from the gist of the invention. The "parts" and "%" used to describe component amounts are based on mass unless otherwise specified. The acid value of the resin was measured by potentiometric titration using a potassium hydroxide-methanol titrant. The weight-average molecular weight of the resin is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0079] <Preparation of pigment dispersion> (Pigment dispersion 1) 10.0 parts of pigment, 10.0 parts of a resin dispersant solution, and 80.0 parts of water were mixed and dispersed in a sand grinder for 1 hour. The mixture was then centrifuged to remove undispersed material, including coarse particles. CI Pigment Red 122 was used as the pigment. The resin dispersant solution was prepared by neutralizing a styrene-acrylic acid copolymer with an acid value of 115 mg KOH / g and a weight-average molecular weight of 10,000 with potassium hydroxide in an amount equal to the acid value, and adding an appropriate amount of ion-exchanged water. This solution had a resin content of 20.0%. The solution was pressure-filtered through a 3.0 μm pore-size microfilter (Fujifilm), and then an appropriate amount of ion-exchanged water was added to prepare Pigment Dispersion 1. The pigment content in Pigment Dispersion 1 was 10.0%, and the resin content was 2.0%.

[0080] (Pigment dispersion 2) Pigment Dispersion Liquid 2 was obtained in the same manner as Pigment Dispersion Liquid 1, except that the type of pigment was changed to CI Pigment Orange 43. The pigment content in Pigment Dispersion Liquid 2 was 10.0%, and the resin content was 2.0%.

[0081] (Pigment dispersion 3) Pigment Dispersion Liquid 3 was obtained in the same manner as Pigment Dispersion Liquid 1, except that the type of pigment was changed to CI Pigment Red 149. The pigment content in Pigment Dispersion Liquid 3 was 10.0%, and the resin content was 2.0%.

[0082] (Pigment dispersion 4) Pigment dispersion 4 was obtained in the same manner as pigment dispersion 1, except that the type of pigment was changed to carbon black (product name "Printex85", manufactured by Orion Engineered Carbons). The pigment content in pigment dispersion 4 was 10.0%, and the resin content was 2.0%.

[0083] (Pigment dispersion 5) Pigment Dispersion Liquid 5 was obtained in the same manner as Pigment Dispersion Liquid 1, except that the type of pigment was changed to CI Pigment Violet 23. The pigment content in Pigment Dispersion Liquid 5 was 10.0%, and the resin content was 2.0%.

[0084] (Pigment dispersion 6) Pigment Dispersion Liquid 6 was obtained in the same manner as Pigment Dispersion Liquid 1, except that the type of pigment was changed to CI Pigment Yellow 74. The pigment content in Pigment Dispersion Liquid 6 was 10.0%, and the resin content was 2.0%.

[0085] (Pigment dispersion 7) Pigment Dispersion Liquid 7 was obtained in the same manner as Pigment Dispersion Liquid 1, except that the type of pigment was changed to CI Pigment Green 7. The pigment content in Pigment Dispersion Liquid 7 was 10.0%, and the resin content was 2.0%.

[0086] (Pigment dispersion 8) Pigment Dispersion Liquid 8 was obtained in the same manner as Pigment Dispersion Liquid 1, except that the type of pigment was changed to CI Pigment Blue 15:3. The pigment content in Pigment Dispersion Liquid 8 was 10.0%, and the resin content was 2.0%.

[0087] (Pigment Dispersion 9) A solution of 50 g of concentrated hydrochloric acid in 55 g of water was cooled to 5°C, and 8.3 g of 4-amino-1,2-benzenedicarboxylic acid was added. The container containing this solution was placed in an ice bath and stirred to maintain the solution temperature below 10°C. A solution of 9.9 g of sodium nitrite dissolved in 20.0 g of 5°C water was added while stirring. After stirring for 15 minutes, CI Pigment Red 122 was added with stirring. The mixture was stirred for another 15 minutes to obtain a slurry. The resulting slurry was filtered through filter paper (trade name "Standard Filter Paper No. 2" manufactured by Advantec), and the particles were thoroughly washed with water and dried in an oven at 110°C. Subsequently, sodium ions were replaced with potassium ions using ion exchange, resulting in a self-dispersing pigment with -CH-(COOK) groups bonded to the pigment particle surface. An appropriate amount of water was added to adjust the pigment content, yielding Pigment Dispersion 9 with a pigment content of 10.0%.

[0088] (Pigment Dispersion 10) A polyamine-based cationic resin (product name "Catiofast PR8154", manufactured by BASF) was added to pure water and dissolved to obtain a resin aqueous solution. CI Pigment Red 122 was added to the obtained resin aqueous solution, and the mixture was thoroughly stirred using a stirrer to obtain a dispersion. The content of CI Pigment Red 122 (pigment) in the dispersion was 10.0%. Furthermore, the content of cationic resin in the dispersion was 8 parts per 100 parts of pigment. An appropriate amount of pure water was added to obtain pigment dispersion 10, which had a pigment content of 10.0% and a resin content of 8.0%.

[0089] (Pigment Dispersion 11) Pigment Dispersion 11 was obtained in the same manner as Pigment Dispersion 1, except that the type of pigment was changed to CI Pigment Yellow 155. The pigment content in Pigment Dispersion 11 was 10.0%, and the resin content was 2.0%.

[0090] (Pigment Dispersion 12) Pigment Dispersion 12 was obtained in the same manner as Pigment Dispersion 1, except that the type of pigment was changed to CI Pigment Red 150. The pigment content in Pigment Dispersion 12 was 10.0%, and the resin content was 2.0%.

[0091] Details of pigment dispersions 1 to 12 are shown in Table 1. In the items of Table 1, the "specific ring structure" was marked with "○" if the pigment was "a pigment composed of molecules having a fused ring in which five or more single rings are fused," and marked with "×" if the pigment was "a pigment composed of molecules that do not have a fused ring in which five or more single rings are fused in the molecular structure." The densities of the pigment particles and fluorescent particles described below were measured in accordance with JIS Z8807:2012.

[0092] TIFF0007725353000001.tif96170

[0093] <Preparation of fluorescent particle dispersion> (Fluorescent particle dispersion 1) A reaction vessel equipped with a stirrer was placed in a warm water bath. 1,178 parts of water was placed in the reaction vessel, and the internal temperature was maintained at 70°C. A solution containing 233 parts of styrene, 233 parts of acrylonitrile, and 18 parts of a reactive surfactant (product name "SR-10" manufactured by ADEKA) and a solution containing 1.9 parts of potassium persulfate and 659 parts of water were simultaneously added dropwise to the reaction vessel over a period of 60 minutes while stirring. After the addition was complete, the mixture was further stirred for 30 minutes to form core particles that would become the cores of the resin particles.

[0094] A first solution was prepared by mixing 16 parts of styrene, 12 parts of methacrylic acid, 32 parts of ethylene glycol dimethacrylate (crosslinking agent component), 20 parts of ethylene glycol diglycidyl ether (crosslinking agent component), and 2.5 parts of a reactive surfactant. The reactive surfactant used was "SR-10" (manufactured by ADEKA). A second solution was prepared by mixing 0.1 parts of potassium persulfate and 133 parts of water. The prepared first and second solutions were simultaneously added dropwise to a reaction vessel over 10 minutes while stirring. After the addition was complete, the mixture was further stirred at 80°C for 5 hours to form a shell portion, thereby preparing core-shell structured resin particles having a core portion and a shell portion.

[0095] After adjusting the pH to 8.5 by adding an 8 mol / L potassium hydroxide aqueous solution to the reaction vessel, a dye solution prepared by dissolving a fluorescent dye in water was added to the reaction vessel without adding a surfactant. The fluorescent dyes used were CI Basic Red 1 and CI Basic Violet 11, both fluorescent compounds with xanthene skeletons. The temperature was raised to 80°C and the mixture was stirred for 2 hours to dye the resin particles. The amount of dye was adjusted to 5 parts per 100 parts of the total of the monomers, crosslinker components, and dye that make up the resin particles. An 8 mol / L potassium hydroxide aqueous solution was added to the reaction vessel to adjust the pH to 8.5. Further dilution with water was performed to obtain fluorescent particle dispersion 1. The fluorescent particle content in fluorescent particle dispersion 1 was 20.0%, and the fluorescent particle density ρ2 was 1.00 g / cm. 3 It was.

[0096] (Fluorescent particle dispersion 2) Fluorescent particle dispersion liquid 2 was obtained in the same manner as fluorescent particle dispersion liquid 1, except that the type of fluorescent dye was changed to CI Disperse Yellow 82, a disperse dye that exhibits fluorescence. The fluorescent particle content in fluorescent particle dispersion liquid 2 was 20.0%, and the fluorescent particle density ρ2 was 1.00 g / cm 3 It was.

[0097] (Fluorescent particle dispersion 3) 20.0 parts of resin particles dyed 1:1 with CI Basic Red (trade name "DayGlo ZQ-15 Blaze Orange", manufactured by DayGlo), 20.0 parts of an aqueous solution of resin dispersant, and 60.0 parts of water were mixed and dispersed in a sand grinder for 1 hour. The mixture was then centrifuged to remove undispersed material, including coarse particles. The aqueous solution of resin dispersant was the same as that used in preparing Pigment Dispersion 1. After pressure filtration through a 3.0 μm pore size microfilter (manufactured by Fujifilm), an appropriate amount of ion-exchanged water was added to prepare Fluorescent Particle Dispersion 3. The fluorescent particle content in Fluorescent Particle Dispersion 3 was 24.0%. The fluorescent particle density ρ2 was 1.10 g / cm 3 It was.

[0098] (Fluorescent particle dispersion 4) Fluorescent particle dispersion 4 was obtained in the same manner as fluorescent particle dispersion 3, except that the resin particles were changed to those dyed with CI Basic Red 1:1 (trade name "DayGlo T-15 Blaze Orange", manufactured by DayGlo). The fluorescent particle content in fluorescent particle dispersion 4 was 24.0%. The fluorescent particle density ρ2 was 1.40 g / cm 3 It was.

[0099] <Preparation of dye aqueous solution> A fluorescent dye, CI Acid Red 52, was dissolved in water to obtain an aqueous solution of the fluorescent dye, with a dye content of 20.0%.

[0100] <Preparation of siloxane compound> A siloxane compound (weight average molecular weight 2,832) having a structure represented by the following formula (A) was used.

[0101] TIFF0007725353000002.tif47170

[0102] <Ink Preparation> (Ink 1-1~1-23) The components (unit: %) shown in Tables 2-1 to 2-3 were mixed, thoroughly stirred, and then pressure-filtered through a 3.0 μm pore-size microfilter (manufactured by Fujifilm) to prepare each ink. In Tables 2-1 to 2-3, the number next to polyethylene glycol is the number-average molecular weight, and "Acetylenol E100" is the trade name of a nonionic surfactant (acetylene glycol ethylene oxide adduct) manufactured by Kawaken Fine Chemicals. The numbers in parentheses next to the compound names in Tables 2-1 to 2-3 are the pKa values of each compound at 25°C. The pH of the prepared inks was measured using a pH meter (trade name "F-21" manufactured by Horiba, Ltd.). The pH of each ink was found to be within the range of 7.0 to 9.5.

[0103] TIFF0007725353000003.tif235170

[0104] TIFF0007725353000004.tif237170

[0105] TIFF0007725353000005.tif247170

[0106] (Ink 1-24) The components shown below were mixed to obtain a mixture. Pigment dispersion 11:140 parts Styrene-butadiene copolymer latex: 56 parts Ethylene glycol: 150 parts Diethylene glycol: 120 parts Anionic surfactant: 4 parts Preservatives: 2 parts Sodium hydroxide: 0.1 parts Dioctyl sodium sulfosuccinate: 0.1 parts Potassium nitrate: 1.8 parts

[0107] The styrene-butadiene copolymer latex used was "Nipol SX1105" (manufactured by Nippon Zeon, 45% solids). The anionic surfactant used was "Pelex OT-P" (manufactured by Kao). The preservative used was "Proxel" (manufactured by LONZA). Ion-exchanged water was added to the resulting mixture to make a total volume of 1,000 parts. After thorough stirring, the mixture was filtered twice using a Millipore filter with a pore size of 1 μm to obtain Ink 1-24. The pigment particle content in Ink 1-24 was 1.68%.

[0108] (Ink 1-25) A flask equipped with a condenser, thermometer, separatory funnel, and stirrer was placed in a water bath. 440 parts of water, 8.5 parts of sodium dodecylbenzenesulfonate, and 7 parts of a nonionic surfactant (trade name "Emulgen LS-110" manufactured by Kao Corporation) were added to the flask and heated to 80°C. After adding 2.1 parts of potassium persulfate, a mixture of 140 parts of acrylonitrile, 120 parts of styrene, and 13 parts of acrylic acid (monomer solution) was added dropwise to the flask over 3 hours and polymerized for 4 hours to obtain a polymer. To the resulting polymer, 200 parts of water, 0.3 parts of Rhodamine F3B, 2.0 parts of Rhodamine F4G, and 17.5 parts of a special polycarboxylic acid-type polymer surfactant (trade name "Demol EP" manufactured by Kao Corporation) were added with stirring at room temperature. Rhodamine F3B (trade name "Vanisol Red 560" manufactured by BASF Corporation) was used. Rhodamine F4G used was Vanisol Red 485 (BASF). After uniform mixing, the mixture was gradually heated and dyed at 85°C for 1 hour to obtain a particle dispersion containing bright pink fluorescent particles with an average particle diameter of 90 nm. Water was added to dilute the mixture to obtain fluorescent particle dispersion (A) with a solids concentration of 40%.

[0109] The components shown below were premixed and then subjected to a circulation dispersion treatment for 20 hours using a disk-type bead mill (trade name "KDL type", manufactured by Shinmaru Enterprises, using zirconia balls with a diameter of 0.3 mm) to obtain pigment dispersion (A). The average particle size of the pigment particles in the obtained pigment dispersion (A) was 95.0 nm. CI Pigment Red 122: 151 parts Polyoxyethylene (n=40) β-naphthyl ether: 57 parts Distilled water: 796 parts

[0110] Component (ii) shown below was added to component (i), and then the mixture was stirred for 30 minutes to obtain pigment dispersion (B). (i) Pigment dispersion (A): 34.0 parts (ii) Self-emulsifying anionic polyether polyurethane emulsion (trade name "Takelac W-5025", manufactured by Mitsui Chemicals, solid content: 30.0%, average particle size = 20.3 nm): 10.0 parts

[0111] Next, component (iv) shown below was added to component (iii), and the mixture was stirred for 30 minutes to obtain pigment dispersion (C). (iii) Pigment dispersion (B): 44.0 parts (iv) Fluorescent particle dispersion (A): 30.0 parts

[0112] A mixture of the following components (v) to (viii) was added to the resulting pigment dispersion (C) and stirred for 30 minutes. The mixture was filtered through a membrane filter with a pore size of 0.8 μm and then degassed under vacuum to obtain ink 1-25. The pigment particle content in ink 1-25 was 4.89%, and the fluorescent particle content was 8.33%. (v) Glycerin: 5.0 parts (vi) Diethylene glycol: 15.0 parts (vii) 1-amino-2-ethyl-2,3-propanediol: 2.0 parts (viii) Distilled water: 4.0 parts

[0113] (Ink 2-1 to 2-5) The components (unit: %) shown in Table 3 were mixed, thoroughly stirred, and then pressure-filtered through a 3.0 μm pore size microfilter (manufactured by Fujifilm) to prepare each ink. In Table 3, the number attached to polyethylene glycol is the number average molecular weight, and "Acetylenol E100" is the trade name of a nonionic surfactant (acetylene glycol ethylene oxide adduct) manufactured by Kawaken Fine Chemicals. The pH of the prepared inks was measured using a pH meter (trade name "F-21" manufactured by Horiba, Ltd.). The pH of each ink was found to be within the range of 7.0 to 9.5.

[0114] TIFF0007725353000006.tif94170

[0115] (Ink 2-6) 150 parts of polyester resin (glass transition temperature 65°C, acid value 40 mgKOH / g) and 50 parts of CI Solvent Red 49 were dissolved in 500 parts of tetrahydrofuran. 10 parts of dimethanolamine and 0.8 parts of sodium hydroxide were then added and dissolved to obtain a solution. The resulting solution was added to 1,000 parts of a 3% aqueous solution of a dispersant (trade name "Demol N", manufactured by Kao) while stirring under a nitrogen atmosphere. Tetrahydrofuran was distilled off under reduced pressure to obtain ink 2-6, which contains fluorescent particles formed from polyester resin and fluorescent dye. The density ρ2 of the fluorescent particles in ink 2-6 was 1.00 g / cm. 3 It was.

[0116] <Evaluation> An inkjet recording device (product name "PIXUS PRO 10-S," manufactured by Canon) equipped with a recording head having nozzle arrays for the first and second inks arranged in a direction perpendicular to the main scanning direction was prepared. In this example, a recording duty of 100% is defined as an image recorded under conditions in which eight droplets of 3.5 ng ± 10% ink are deposited in a unit area of 1 / 600 inch x 1 / 600 inch. Each prepared ink was filled into an ink cartridge, and the combination shown in Table 4 was installed in the inkjet recording device. The first ink and the second ink were deposited on the recording medium under the following conditions so that the area where the first ink was deposited overlapped with the area where the second ink was deposited, thereby recording a solid image of 2 cm x 2 cm. The number of main scans of the recording head to deposit the first ink and the second ink on the unit area of the recording medium was the number shown in Table 4. The ratios of the application amounts (printing duty, unit: %) of the first ink and the second ink were set to 20% and 80%, 40% and 60%, 50% and 50%, 60% and 40%, and 80% and 20%, respectively. Glossy paper (product name "Canon Photo Paper Glossy Pro [Platinum Grade] PT-201", manufactured by Canon) was used as the recording medium. In the present invention, the following evaluation criteria for each evaluation item were used: "AAA", "AAA - "," "AA," "A," and "B" were considered acceptable levels, and "C" was considered unacceptable. The evaluation results are shown in Table 4.

[0117] (fluorescence intensity) The total amount of application (printing duty) of the first ink and the second ink was set to 20%, 40%, 60%, 80%, and 100%, and images of gradation patterns including 25 solid images were recorded under the conditions of applying each ink at the above five application rate ratios. The recorded images were left in an environment of 25°C for one day. Using the M1 light source of a spectrophotometer (trade name "eXact", manufactured by X-Rite), the hue angle (H) and chroma (C) in the Lab color system were measured. * ), and lightness (L * The fluorescence intensity of the image was evaluated based on the measured hue angle (H) according to the following criteria: * ) is the saturation (C *However, if the maximum saturation does not reach 50, the saturation (C ) was calculated by extrapolating the data obtained from the color measurement of the gradation pattern. * The evaluation was based on the calculated value of lightness at a hue angle (H) of 50. Different evaluation criteria were used depending on the hue angle (H) because the preferred color tone perceived visually differs depending on the type of color.

[0118] [When the hue angle (H) is between 0° and 180°] AAA: Among the five types of images, there was an image with a maximum saturation of 60 or more and a brightness of 73 or more, or an image with a maximum saturation of 50 or more and a brightness of 85 or more. AAA - Among the five images, there was one with a maximum saturation of 50 or more but less than 60 and a brightness of 82 or more but less than 85. AA: Among the five types of images, there was an image with a maximum saturation of 50 or more but less than 60 and a brightness of 79 or more but less than 82. A: Among the five images, there was an image with a maximum saturation of 50 or more but less than 60 and a brightness of 76 or more but less than 79. B: Among the five types of images, there was an image with a maximum saturation of 50 or more but less than 60 and a brightness of 73 or more but less than 76. C: All five images had a maximum saturation of less than 50 or a brightness of less than 73.

[0119] [When the hue angle is 180° or more and less than 360°] AAA: Among the five types of images, there was an image with a maximum saturation of 60 or more and a brightness of 63 or more, or an image with a maximum saturation of 50 or more and a brightness of 75 or more. AAA - Among the five images, there was one with a maximum saturation of 50 or more but less than 60 and a brightness of 72 or more but less than 75. AA: Among the five types of images, there was an image with a maximum saturation of 50 or more but less than 60 and a brightness of 69 or more but less than 72. A: Among the five images, there was an image with a maximum saturation of 50 or more but less than 60 and a brightness of 66 or more but less than 69. B: Among the five types of images, there was an image with a maximum saturation of 50 or more but less than 60 and a brightness of 63 or more but less than 66. C: All five images had a maximum saturation of less than 50 or a brightness of less than 63.

[0120] (color gamut) The total amount of application of the first ink and the second ink (printing duty) was set to 100%, and under the condition that each ink was applied at the above five patterns of application rate ratio, images of gradation patterns including a total of five solid images were recorded. The recorded images were left in an environment of 25°C for one day. Using the M1 light source of a spectrophotometer (trade name "eXact", manufactured by X-Rite), the hue angle (H) and color (a) in the Lab color system were measured. * , b * ), and lightness (L * The hue angle (H) was measured in the first quadrant (a * ≧0, b * ≧0), second quadrant (a * ≦0, b * ≧0), 3rd quadrant (a * ≦0, b * ≦0), and the fourth quadrant (a * ≧0, b * ≦0) and calculated using the formula below. [Quadrant 1]:H°=tan -1 (b * / a * ) [Second quadrant]:H°=180°+tan -1 (b * / a * ) [3rd quadrant]:H°=180°+tan -1 (b * / a * ) [4th quadrant]:H°=360°+tan -1 (b * / a * )

[0121] A: The difference between the maximum and minimum hue angles was 20° or more. C: The difference between the maximum and minimum hue angles was less than 20°.

[0122] TIFF0007725353000007.tif255170

Claims

1. an inkjet recording method comprising a step of recording an image by using an inkjet recording apparatus equipped with a recording head having ejection ports for ejecting a first ink and a second ink, and applying the first ink and the second ink to a recording medium so that an area to which the first ink is applied and an area to which the second ink is applied at least partially overlap each other, the first ink contains pigment particles and an alkaline buffer, and the pigment particles are dispersed by the action of anionic groups; the second ink contains fluorescent particles dispersed by the action of anionic groups; The density ρ of the pigment particles １ (g / cm ３ ) is the density ρ of the fluorescent particles ２ (g / cm ３ ) is larger than the ink jet recording method.

2. 2. The inkjet recording method according to claim 1, wherein the pigment particles satisfy the relationship of the following formula (1): A λ２ / A λ１ ≦0.5 ・・・(1) A λ１ : the maximum absorption wavelength λ of the pigment particles １ The absorbance of the pigment particles at (nm) A λ２ : maximum fluorescence wavelength λ of the fluorescent particle ２ The absorbance of the pigment particles at (nm)

3. 3. The ink jet recording method according to claim 1, wherein the pigment is at least one selected from the group consisting of carbon black, quinacridone, perinone, perylene, dioxazine, azo, and phthalocyanine.

4. 4. The ink jet recording method according to claim 1, wherein the pigment is a quinacridone.

5. 5. The ink jet recording method according to claim 1, wherein the pigment is composed of a molecule having a fused ring in which five or more monocyclic rings are fused.

6. 6. The ink jet recording method according to claim 1, wherein the pigment particles are dispersed in a resin having an anionic group.

7. 7. The inkjet recording method according to claim 1, wherein the content (mass %) of the pigment particles in the first ink is from 0.10 mass % to 10.00 mass % based on the total mass of the ink.

8. The density ρ of the pigment particles １ (g / cm ３ ) is 1.45 g / cm ３ 5.00g / cm or more ３ The inkjet recording method according to any one of claims 1 to 7, wherein:

9. 9. The ink jet recording method according to claim 1, wherein the alkaline buffer has a pKa at 25[deg.] C. of 7.6 or more and 10.4 or less.

10. 10. The ink jet recording method according to claim 1, wherein the alkaline buffer is a tertiary amine compound.

11. 11. The inkjet recording method according to claim 1, wherein the content (mass %) of the alkaline buffer in the first ink is 0.10 mass % or more and 15.00 mass % or less, based on the total mass of the ink.

12. 12. The inkjet recording method according to claim 1, wherein the first ink further contains a nonionic siloxane compound.

13. 13. The ink jet recording method according to claim 1, wherein the fluorescent particles are resin particles dyed with a fluorescent coloring material.

14. The ink jet recording method according to claim 13, wherein the fluorescent coloring material is a basic dye.

15. 15. The ink jet recording method according to claim 13, wherein the fluorescent coloring material is a compound having a xanthene skeleton.

16. 16. The inkjet recording method according to claim 1, wherein the content (mass %) of the fluorescent particles in the second ink is 0.10 mass % or more and 15.00 mass % or less based on the total mass of the ink.

17. The density ρ of the fluorescent particles ２ (g / cm ３ ) is 1.00 g / cm ３ 1.50g / cm or more ３ The inkjet recording method according to any one of claims 1 to 16, wherein:

18. 18. The inkjet recording method according to claim 1, wherein the first ink and the second ink are applied to a unit area of the recording medium by performing main scanning of the recording head 4 to 16 times.

19. An inkjet recording apparatus used in an inkjet recording method, comprising: a recording head having ejection ports for ejecting a first ink and a second ink, and a step of applying the first ink and the second ink to a recording medium so that an area to which the first ink is applied and an area to which the second ink is applied at least partially overlap each other, the first ink contains pigment particles and an alkaline buffer, and the pigment particles are dispersed by the action of anionic groups; the second ink contains fluorescent particles dispersed by the action of anionic groups; The density ρ of the pigment particles １ (g / cm ３ ) is the density ρ of the fluorescent particles ２ (g / cm ３ ) is larger than the ink jet recording apparatus.

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