Ink set, inkjet recording method, and inkjet recording apparatus

Abstract

This invention provides an inkjet ink set capable of recording white images with suppressed film thickness unevenness. [Solution] An inkjet ink set comprising a combination of a white aqueous ink containing titanium dioxide and resin particles, and a reaction solution containing a reagent that reacts with the aqueous ink. The aqueous ink further contains a water-soluble resin containing units having acidic groups, the proportion (mol%) of units having acidic groups in the water-soluble resin is 60.0 mol% or more, and the units having acidic groups include units having multiple carboxylic acid groups, and the reagent contains a polyvalent metal salt.

Description

[Technical Field]

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

[0002] In recent years, the application of inkjet recording methods to commercial and industrial printing fields has been explored. In these fields, in addition to black, cyan, magenta, and yellow inks (hereinafter collectively referred to as "color inks"), white ink is sometimes used. White ink is used when recording white images on non-white recording media such as colored paper, or when recording white images as a base for color inks on recording media such as transparent or translucent films. White pigments such as titanium dioxide are used in white inks from the standpoint of material stability and cost. When recording white images, it is necessary to apply a larger amount of white ink compared to color inks in order to improve the opacity and whiteness of the image. For this reason, inkjet recording devices that use white ink are sometimes equipped with a drying mechanism.

[0003] When commercial and industrial printing is implemented using inkjet recording methods, a high level of abrasion resistance is required for the recorded images. Adding resin particles to the ink is useful for improving abrasion resistance, and the addition of resin particles to white ink is also being considered. The resin particles in the ink added to the recording medium melt due to the reduction of liquid components or drying, forming a resin film, which suppresses the peeling of the colorant and improves abrasion resistance.

[0004] When recording images on low-absorbency recording media such as printing paper or non-absorbency recording media such as resin films, ink can easily migrate within the recording media, leading to variations in the thickness of the recorded image (so-called film thickness unevenness). Furthermore, when recording images on absorbent recording media such as plain paper, localized variations in ink penetration occur due to the influence of the fibers constituting the recording media, also leading to film thickness unevenness. To suppress such film thickness unevenness in images, for example, a set comprising ink and a processing liquid containing a flocculant that aggregates the colorants in the ink, and a recording method for recording images using such a set have been proposed (Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-189897 [Patent Document 2] Japanese Patent Publication No. 2021-187095 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, when using a recording device equipped with a drying mechanism that promotes image drying by methods such as blowing air onto the recording medium, the white ink on the recording medium is easily moved by the blown airflow. For this reason, even when using the sets proposed in Patent Documents 1 and 2, it has been difficult to sufficiently suppress film thickness unevenness in the recorded image.

[0007] Therefore, an object of the present invention is to provide an inkjet ink set capable of recording white images with suppressed film thickness unevenness. Another object of the present invention is to provide an inkjet recording method and an inkjet recording apparatus using this ink set. [Means for solving the problem]

[0008] In other words, the present invention provides an inkjet ink set comprising a combination of a white aqueous ink containing titanium dioxide and resin particles, and a reaction solution containing a reagent that reacts with the aqueous ink, wherein the aqueous ink further contains a water-soluble resin containing units having acidic groups, the proportion (mol%) of the units having acidic groups in the water-soluble resin is 60.0 mol% or more, the units having acidic groups include units having a plurality of carboxylic acid groups, and the reagent contains a polyvalent metal salt. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide an inkjet ink set capable of recording white images with suppressed film thickness unevenness. Furthermore, according to the present invention, it is possible to provide an inkjet recording method and an inkjet recording apparatus using this ink set. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic perspective view showing one embodiment of the inkjet recording apparatus of the present invention. [Figure 2] This is a schematic side view showing one embodiment of the inkjet recording apparatus of the present invention. [Modes for carrying out the invention]

[0011] The present invention will be described in more detail below with reference to preferred embodiments. In the present invention, when a compound is a salt, the salt exists in the ink dissociated into ions, but for convenience, it will be expressed as "contains a salt." Also, the water-based ink for inkjet and the reaction solution may be simply referred to as "ink" and "reaction solution." Furthermore, titanium dioxide, which is the colorant for white ink, may be referred to as "pigment." Unless otherwise specified, the physical properties are values ​​at room temperature (25°C), normal humidity (50% relative humidity), and normal pressure (1 atm).

[0012] The inventors first investigated a method for suppressing uneven film thickness in images. Some inkjet recording devices using white ink are equipped with a drying mechanism that promotes image drying by methods such as blowing warm air onto the recording medium. In this case, the white ink on the recording medium is easily moved by the air blown by the drying mechanism, which can easily cause uneven film thickness in the image. To suppress uneven film thickness, it is thought that the pigment needs to settle before the recording medium, to which the ink and reaction solution have been applied, is transported to the drying mechanism.

[0013] The settling velocity V of particles in ink can be calculated using the following formula (A) (Stokes' formula). V={g(ρ S -ρ)d 2} / 18μ ···(A) d: Particle size g:Gravity acceleration ρ S : Particle density ρ: Density of the dispersed medium μ: Viscosity of the dispersion medium

[0014] According to Stokes' equation, particles with larger particle size and higher density tend to settle more easily. Therefore, in poorly permeable to non-absorbent recording media where ink penetration is difficult, using a white ink containing pigments with larger particle size that can form higher-density aggregates in combination with a reaction solution increases the sedimentation rate of the pigment within the dots. This suppresses uneven film thickness caused by ink movement. In absorbent recording media where ink penetration is easy, using a white ink containing pigments with larger particle size in combination with a reaction solution increases the size of the aggregates, suppressing penetration into the depth direction of the recording media. This suppresses uneven film thickness caused by localized variations in ink permeability due to the influence of the fibers constituting the recording media. Furthermore, regardless of the permeability of the recording media, increasing the sedimentation rate of the pigment makes it easier for particles to accumulate uniformly on the surface of the recording media, more effectively suppressing uneven film thickness.

[0015] Next, the inventors investigated combinations of pigments and reactants that could form high-density aggregates. It was found that increasing the crosslinking density between pigment particles is necessary to form high-density aggregates. Specifically, they found that by adding a water-soluble resin with a high density of carboxylic acid groups to the ink and using a highly cohesive polyvalent metal salt as a reactant, the settling rate of the pigment increased, and high-density aggregates were formed.

[0016] Furthermore, the inventors investigated the configuration of an ink set that can record images with suppressed film thickness unevenness by increasing the sedimentation rate of the pigment and forming high-density aggregates. As a result, they found that it is possible to record images with suppressed film thickness unevenness by satisfying the requirements (i) to (iv) shown below, leading to the present invention. (i) A white aqueous ink containing titanium dioxide and resin particles is used in combination with a reaction solution containing a reagent that reacts with the aqueous ink. (ii) The water-based ink contains a water-soluble resin that includes units having acidic groups. (iii) The proportion (mol%) of units having acidic groups in the water-soluble resin is 60.0 mol% or more, and the units having acidic groups include units having multiple carboxylic acid groups. (iv) The reactant contains a polyvalent metal salt.

[0017] By satisfying the above requirements, the inventors speculate as follows on the reason why it becomes possible to record an image with suppressed film thickness unevenness. At least a part of the acidic groups contained in the unit having an acidic group can be ionized according to the acid dissociation constant (pKa). When a polyvalent metal salt is added to the ink, the electric double layer becomes thinner, the electrostatic repulsion weakens, and the pigments aggregate. At this time, the anions generated by the ionic dissociation crosslink with the polyvalent metal ions derived from the polyvalent metal salt, and it is considered that the acidic groups that have not undergone ionic dissociation form hydrogen bonds between a plurality of acidic groups. In addition, since the water-soluble resin has a plurality of carboxylic acid groups, the density of the carboxylic acid groups in the unit is high. Usually, in the range of about 7.0 to 9.0, which is the general pH of aqueous ink for inkjet, at least a part of the carboxylic acid groups of the water-soluble resin is ionized to become carboxylate ions. The carboxylate ions react with the polyvalent metal ions to form stable carboxylate complexes and form hydrogen bonds between a plurality of acidic groups that have not undergone ionic dissociation. Therefore, when a polyvalent metal salt is added to an ink containing a water-soluble resin containing a unit having a plurality of carboxylic acid groups, the density of the carboxylic acid groups and carboxylate ions that contribute to the aggregation of the particles is high, so the crosslink density of the pigments increases. As a result, the density of the pigment aggregates increases and the sedimentation rate increases, so it is considered that the occurrence of film thickness unevenness due to the movement of the ink can be suppressed.

[0018] The evaporation rate Q of the aqueous liquid component in the ink evap can be calculated by the following formula (B). Q evap =k(Y w -Y a ) ···(B) k: Mass transfer coefficient [m / s] Y w : Saturated vapor amount [kg / m 3 (Calculated as the wet bulb temperature) Y a : Water vapor amount [kg / m 3 (Calculated from the temperature and relative humidity) Sherwood number (Sh = kL / D, k: mass transfer coefficient [m / s], L: representative length [m], D: diffusion coefficient [m2 / s]) Schmidt number (Sc=μ / (ρD), μ: viscosity coefficient [Pa s], ρ: density [kg / m 3 ], D: diffusion coefficient [m 2 / s]) Reynolds number (Re=ρνL / μ, ρ: density [kg / m 3 ], ν: wind speed [m / s], L: representative length [m], μ: viscosity coefficient [Pa s])

[0019] The values ​​in equation (B) are calculated from the Sherwood number, Schmidt number, and Reynolds number, and the obtained values ​​are substituted into equation (B). In this case, a recording duty cycle of 100% is defined as an image recorded under the condition that 8 ink droplets with a mass of 3.5 ng each are applied to a unit area of ​​1 / 600 inch × 1 / 600 inch. At a temperature of 25°C, relative humidity of 50%, and wind speed of 1 m / s, the evaporation rate of the liquid component when ink is applied to a 1 inch × 1 inch area so that the recording duty cycle is 100% is 1.046 × 10⁻⁶ -5 The value is g / s. In this case, the rate of decrease in film thickness over a 1 inch x 1 inch area is "1.1 x 10 -5 It can be calculated as "cm / s".

[0020] As described above, the settling rate of the particles (titanium dioxide) in the ink is calculated using the above formula (A). When the ink comes into contact with the reaction solution, the particle size of titanium dioxide, which was on the order of nanometers before aggregation, aggregates to the extent that it reaches the order of micrometers. Furthermore, as an empirical rule, it is known that the density of aggregates is proportional to 0.4 times the crosslinking density, so if the number of acidic groups in the water-soluble resin increases by, for example, 1.25 times, the density of the titanium dioxide aggregates that are involved in the aggregation of the water-soluble resin will increase by 1.1 times. From these, the settling rate of titanium dioxide whose particle size has become about 1 μm after aggregation can be determined as follows. That is, if the proportion of units with acidic groups in the water-soluble resin is 50.0 mol%, then 9.8 × 10 -6 This becomes "cm / s", and if the same proportion is 60.0 mol%, then it is "1.2 × 10 -5 This translates to "cm / s".

[0021] In other words, when the proportion of units containing acidic groups in the water-soluble resin is 60.0 mol% or more, the settling rate of titanium dioxide aggregates becomes greater than the evaporation rate of the liquid components in the ink. This suppresses the occurrence of film thickness variations caused by ink migration.

[0022] To form high-density aggregates, it is necessary to use a water-soluble resin in which the proportion of units with acidic groups is 60.0 mol% or more, and in which units with acidic groups contain units with multiple carboxylic acid groups. Conversely, if the proportion of units with acidic groups in the water-soluble resin is less than 60.0 mol%, or if the water-soluble resin does not contain units with multiple carboxylic acid groups, the density of the aggregates cannot be increased. As a result, the pigment does not settle quickly after the ink is applied to the recording medium, and uneven film thickness is likely to occur.

[0023] Polyvalent metal salts used as reactants have a lower molecular weight compared to cationic resins and organic acids, which are also used as reactants, making them easily mobile. Furthermore, polyvalent metal salts readily dissolve in water and readily generate cationic components that cause pigment aggregation. Therefore, when ink is added to a recording medium to which a reaction solution containing a polyvalent metal salt has been applied, the cationic components rapidly migrate throughout the ink droplet, causing the pigment to aggregate. Consequently, using polyvalent metal salts as reactants allows for faster aggregation of titanium dioxide compared to using cationic resins or organic acids as reactants.

[0024] <Ink Set> The present invention relates to an inkjet ink set comprising a combination of a white ink containing titanium dioxide and resin particles, and a reaction solution containing a reagent that reacts with the ink. The ink further contains a water-soluble resin containing units having acidic groups, wherein the proportion (mol%) of units having acidic groups in the water-soluble resin is 60.0 mol% or more, and the units having acidic groups include units having multiple carboxylic acid groups. The reagent contains a polyvalent metal salt.

[0025] Ink sets can take various forms, such as a set of multiple ink cartridges, each containing ink and a reaction solution independently. Alternatively, an ink set may consist of a single integrated ink cartridge formed by combining multiple ink storage units, each containing ink and a reaction solution. The ink set of the present invention is not limited to the above forms, but can take any form as long as it is configured to allow the use of ink and a reaction solution in combination.

[0026] (ink) The ink is a white inkjet ink containing titanium dioxide and resin particles. White inks include those that can record white images even if they do not appear white in their ink state. White refers to a lightness level (L). * ) and chromaticity (a * , b * ) are, respectively, 70 ≤ L * ≤100, -4.5 ≤a * ≤ 2, and -6 ≤ b * This means it is within the range of ≤2.5. The components that make up the ink will be explained in detail below.

[0027] [Titanium Oxide] The ink contains particulate titanium dioxide. The titanium dioxide content (by mass) in the ink is preferably 0.1% by mass or more and 45.0% by mass or less, and more preferably 5.0% by mass or more and 38.0% by mass or less, based on the total mass of the ink.

[0028] Titanium dioxide is typically dispersed in ink in the form of secondary aggregates (secondary particles) consisting of two or more primary particles. The average primary particle size D of the titanium dioxide contained in the ink is... P0 The average primary particle size (nm) of titanium dioxide is preferably 150 nm or less, more preferably 50 nm or less, and particularly preferably 30 nm or less. P0The (nm) is preferably 5 nm or greater. The primary particle size of titanium dioxide can be measured by observing the titanium dioxide particles using a scanning electron microscope. Average primary particle size of titanium dioxide D P0 (nm) can be calculated as the average value of the primary particle diameters of multiple particles (for example, 100 particles).

[0029] In this specification, "average particle diameter (nm)" means "cumulative 50% particle diameter (nm) in a volume-based particle size distribution." The cumulative 50% particle diameter (nm) (average particle diameter (nm)) in a volume-based particle size distribution is the diameter of the particle that reaches 50% of the total volume of measured particles in the particle size integration curve, starting from the smallest particle diameter. The average particle diameter (nm) can be measured using a particle size distribution analyzer that employs dynamic light scattering. For example, measurement conditions include SetZero: 30 seconds, number of measurements: 3, measurement time: 180 seconds, shape: non-spherical, etc. Furthermore, a particle size analyzer that employs dynamic light scattering (for example, product name "UPA-EX150," manufactured by Nikkiso) can be used as the particle size distribution analyzer. Of course, the particle size distribution analyzer and measurement conditions used are not limited to those described above.

[0030] Average particle size D of titanium dioxide P The (nm) is preferably 500 nm or less, and more preferably 400 nm or less. Also, the average particle size D of titanium dioxide. P (nm) is preferably 5 nm or greater.

[0031] Average particle diameter D P Using titanium dioxide with a particle size of 150 nm or less makes it possible to form a porous film with voids, allowing for the recording of images with excellent opacity. When titanium dioxide with a small average particle size is used, voids are formed between the titanium dioxide particles aggregated by the reaction solution. Subsequently, the temperature at which the resin particles melt (the glass transition temperature Tg (°C) or melting point T of the resin particles) is reached. MWhen the recording medium is heated to a temperature of (°C) or higher, the resin particles melt and the resulting resin penetrates into the voids by capillary force. At the same time, voids are formed where the resin particles were located. The penetration of the molten resin into the voids forms a binder, which is a mixture of titanium dioxide and resin. Average particle size D P Even when using titanium dioxide with a wavelength of 150 nm or less, if the aforementioned resin particles are not used in combination, the incident visible light is hardly scattered. However, when the resin particles are melted as described above, low refractive index air is present in the formed voids, and the refractive index of the voids is considered to be relatively lower than that of the binder. Under these conditions, the incident visible light is scattered due to the difference in refractive index between the binder and the voids. This is thought to result in the recording of an image with improved opacity.

[0032] The refractive index of titanium dioxide is preferably 2.1 or higher, and more preferably 2.5 to 2.8. The particle surface of titanium dioxide may be coated with inorganic oxides such as alumina, silica, zinc oxide, and zirconia; or organic substances such as polyols. Using titanium dioxide with a coated particle surface is expected to suppress photocatalytic activity and improve dispersibility. There are three crystalline forms of titanium dioxide: rutile, anatase, and brookite. Among these, it is preferable to use rutile titanium dioxide, which has low photocatalytic activity. Industrial methods for producing titanium dioxide include the sulfuric acid method and the chlorine method. Titanium dioxide produced by either method can be used.

[0033] [Resin particles] The ink contains resin particles. The resin particles are effective in improving the scratch resistance of the image. The resin particles do not need to contain colorants. The content (mass%) of resin particles in the ink is preferably 1.0% by mass or more and 50.0% by mass or less, and more preferably 2.5% by mass or more and 15.0% by mass or less, based on the total mass of the ink.

[0034] In this specification, "resin particles" refers to resins that exist in an insoluble state in the aqueous medium of the ink, and more specifically, resins that can exist in the aqueous medium in a state in which particles whose particle size can be measured by dynamic light scattering are formed. In contrast, "water-soluble resin" refers to resins that exist in a dissolved state in the aqueous medium of the ink.

[0035] Whether a resin is classified as "resin particles" or "water-soluble resin" can be determined by the following method. First, a liquid containing the resin to be evaluated is prepared and diluted with pure water to a resin content of approximately 1.0% to prepare a sample. Then, the particle size of the resin in the sample is measured by dynamic light scattering. If particles with a particle size are measured, the resin is determined to be "resin particles" (i.e., "water-dispersible resin"). On the other hand, if no particles with a particle size are measured, the resin is determined not to be "resin particles" (i.e., "water-soluble resin"). The measurement conditions in this case can be, for example, SetZero: 30 seconds, Number of measurements: 10 times, Measurement time: 120 seconds, Shape: Spherical, Refractive index: 1.5, Density: 1.0.

[0036] As a particle size distribution analyzer, a dynamic light scattering particle size analyzer (for example, product name "UPA-EX150," manufactured by Nikkiso) can be used. Of course, the particle size distribution analyzer and measurement conditions used are not limited to those mentioned above.

[0037] Cumulative 50% particle size D in volume-based particle size distribution of resin particles E (Average particle size D E The average particle diameter D of the resin particles is preferably between 10 nm and 1,000 nm, and more preferably between 100 nm and 500 nm. It is particularly preferably between 100 nm and 400 nm. E When the wavelength is between 100 nm and 400 nm, a porous film with voids can be formed. This enhances the opacity of the image due to scattering caused by the difference between the refractive index of the binder, which is composed of titanium dioxide and resin, and the refractive index of the voids.

[0038] The content of resin particles (mass%) in the ink is preferably 0.3 to 1.5 times the content of titanium dioxide (mass%), and more preferably 0.3 to 1.0 times. Furthermore, the content of resin particles (volume%) in the ink is preferably 1.3 to 5.0 times the content of titanium dioxide (volume%), and more preferably 2.0 to 4.0 times. By using the above mass or volume ratios, a porous film with voids can be formed. This enhances the opacity of the image due to scattering caused by the difference between the refractive index of the binder composed of titanium dioxide and resin and the refractive index of the voids. The volume-based content (volume%) for calculating the volume ratio can be simply determined by dividing the mass-based content (mass%) by the density (specific gravity) of the resin particles or titanium dioxide particles.

[0039] The resin forming the resin particles preferably has anionic groups. Since the anionic groups of the resin react with the reactant, when an ink containing resin particles formed from a resin with anionic groups is used, the resin particles also form aggregates. This can further increase the settling velocity of the aggregates.

[0040] Examples of resins that form resin particles include vinyl chloride resins, styrene resins, urethane resins, acrylic resins, and polyester resins. Among these, acrylic resins, polyester resins, and urethane resins are preferred from the viewpoint of ejection from the recording head. If resin particles formed from resins other than those mentioned above are used, ink ejection may become somewhat unstable, and as a result, uneven film thickness may occur.

[0041] In particular, the resin forming the resin particles is preferably an acrylic resin. The acrylic resin is preferably one that has hydrophilic units and hydrophobic units as constituent units. Among these, a resin having hydrophilic units derived from (meth)acrylic acid and hydrophobic units derived from at least one of a monomer having an aromatic ring and a (meth)acrylic acid ester monomer is preferred. In particular, a resin having hydrophilic units derived from (meth)acrylic acid and hydrophobic units derived from at least one of a monomer, styrene and α-methylstyrene, is preferred.

[0042] Hydrophilic units are units that have hydrophilic groups, such as anionic groups. Hydrophilic units can be formed, for example, by polymerizing hydrophilic monomers that have hydrophilic groups. Specific examples of hydrophilic monomers that have hydrophilic groups include acidic monomers having carboxylic acid groups, such as (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid, and anionic monomers such as anhydrides and salts of these acidic monomers. Cations that constitute salts of acidic monomers include ions such as lithium, sodium, potassium, ammonium, and organic ammonium. Hydrophobic units are units that do not have hydrophilic groups, such as anionic groups. Hydrophobic units can be formed, for example, by polymerizing hydrophobic monomers that do not have hydrophilic groups, such as anionic groups. Specific examples of hydrophobic monomers include monomers having aromatic rings such as styrene, α-methylstyrene, benzyl (meth)acrylate, and phenoxyethyl (meth)acrylate; and (meth)acrylic acid ester monomers such as methyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.

[0043] The glass transition temperature (Tg) of the resin particles is preferably between -20°C and 120°C, and more preferably between 40°C and 100°C. The glass transition temperature of the resin particles can be measured using a thermal analyzer such as a differential scanning calorimeter (DSC).

[0044] The resin forming the resin particles may be a non-crystalline or crystalline resin. Whether or not the resin forming the resin particles is a crystalline resin can be determined by measuring the degree of crystallinity of the resin using a differential scanning calorimeter. If no melting peak is observed using a differential scanning calorimeter, the resin can be determined to be an amorphous resin. On the other hand, if a melting peak is observed, the resin can be determined to be a crystalline resin. In the case of a crystalline resin, the heat of fusion can be determined from the peak area, and the degree of crystallinity can be calculated from the ratio of the determined heat of fusion to the heat of fusion of a perfect crystal with 100% crystallinity calculated by theoretical calculation.

[0045] The acid value of the resin forming the resin particles is preferably 5 mg KOH / g or more and 100 mg KOH / g or less. The weight-average molecular weight of the resin forming the resin particles is preferably 1,000 or more and 3,000,000 or less, and more preferably 100,000 or more and 3,000,000 or less. The resin particles do not need to contain a colorant.

[0046] [Water-soluble resin] The ink contains a water-soluble resin that includes units with acidic groups. The proportion (mol%) of units with acidic groups in this water-soluble resin is 60.0 mol% or more, and the units with acidic groups include units that have multiple carboxylic acid groups. Examples of resin forms include block copolymers, random copolymers, graft copolymers, and combinations thereof.

[0047] Water-soluble resins contain units with acidic groups, and these acidic units contain units with multiple carboxylic acid groups. By using water-soluble resins containing units with multiple carboxylic acid groups, the density of aggregates can be increased by increasing the crosslinking density between pigment particles, thereby suppressing the occurrence of uneven film thickness in images. Examples of monomers that become units with acidic groups through polymerization include monomers with one acidic group, such as (meth)acrylic acid, crotonic acid, and vinylsulfonic acid; monomers with multiple acidic groups, such as maleic acid, fumaric acid, itaconic acid, 2-sulfo(meth)acrylic acid, and 3-sulfo(meth)acrylic acid; anhydrides and salts of these acidic monomers; and so on. Examples of cations that constitute the salts include ions such as lithium, sodium, potassium, ammonium, and organic ammonium.

[0048] The water-soluble resin may be a copolymer that further contains units derived from other monomers other than those with acidic groups (other units), in addition to the units with acidic groups. Examples of other monomers that become other units through polymerization include aromatic ring monomers such as styrene, α-methylstyrene, benzyl (meth)acrylate, and phenoxyethyl (meth)acrylate; and (meth)acrylic acid ester monomers such as methyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.

[0049] The proportion (mol%) of units with acidic groups in the water-soluble resin is 60.0 mol% or more, preferably 70.0 mol% or more. If the proportion of units with acidic groups is less than 60.0 mol%, the crosslinking density of the aggregate cannot be increased. As a result, the settling velocity of the particles cannot be increased, and the occurrence of uneven film thickness cannot be suppressed. The proportion (mol%) of units with acidic groups in the water-soluble resin may be 100.0 mol%. The proportion (mol%) of units with acidic groups in the water-soluble resin can be calculated from the number of moles of carboxylic acid groups calculated from the acid value of the water-soluble resin and the ratio of the constituent units of the resin analyzed by NMR. The acid value of the water-soluble resin can be measured by colloidal titration using potential difference.

[0050] It is preferable that the units having multiple carboxylic acid groups include units derived from dicarboxylic acid compounds. In this case, it is preferable that the proportion (mol%) of units derived from dicarboxylic acid compounds in the units having multiple carboxylic acid groups is 7.5 mol% or more. When the proportion of units derived from dicarboxylic acid compounds is 7.5 mol% or more, the density of aggregates increases by increasing the crosslinking density between pigment particles, and the occurrence of uneven film thickness in the image can be more effectively suppressed.

[0051] The dicarboxylic acid compound is preferably maleic acid. Maleic acid has a molecular structure in which a carboxylic acid group is bonded to each of the adjacent carbon atoms. For this reason, water-soluble resins containing units derived from maleic acid tend to have a high density of carboxylic acid groups. The acid dissociation constant (pKa) of maleic acid at 25°C is pKa1 = 1.94 and pKa2 = 6.22. In the pH range of approximately 7.0 to 9.0, which is typical for water-based inkjet inks, some of the carboxylic acid groups contained in the units derived from maleic acid dissociate to form carboxylate ions. When ink in this state comes into contact with a reaction solution containing a polyvalent metal salt, the density of carboxylic acid groups and carboxylate ions is high, which increases the crosslinking density between pigment particles and the density of aggregates, thereby more effectively suppressing the occurrence of uneven film thickness in the image.

[0052] The acid value of the water-soluble resin is preferably 100 mg KOH / g or higher, and more preferably 1,000 mg KOH / g or lower. If the acid value of the water-soluble resin is less than 100 mg KOH / g, there are few acidic groups, and the reactivity when in contact with the reaction solution may be weak. As a result, the cohesiveness decreases, and aggregates containing pigment particles are not sufficiently formed, making it difficult to increase the sedimentation rate, and the effect of suppressing film thickness unevenness may be slightly reduced. On the other hand, if the acid value of the water-soluble resin is greater than 1,000 mg KOH / g, the hydrophilicity becomes too high, and the water-soluble resin may have difficulty approaching the pigment particles when it aggregates. As a result, the cohesiveness decreases, aggregates containing pigment particles are not sufficiently formed, making it difficult to increase the sedimentation rate, and the effect of suppressing film thickness unevenness may be slightly reduced. The acid value of the water-soluble resin can be measured by colloidal titration using potential difference. Furthermore, if the structure of the water-soluble resin is known, the number of moles of acidic groups can be determined from the number of moles of each unit contained in 1g of resin. The acid value can then be calculated by assuming that the number of moles of acidic groups is equal to the number of moles of potassium hydroxide (molecular weight 56.1) required to neutralize them.

[0053] The weight-average molecular weight of the water-soluble resin is preferably between 1,000 and 30,000, and more preferably between 5,000 and 20,000. The weight-average molecular weight (Mw) is a polystyrene-based value measured by gel permeation chromatography (GPC).

[0054] In the ink, the content (mass%) of water-soluble resin is preferably 0.03 to 0.20 times the mass ratio of the titanium dioxide content (mass%), and more preferably 0.05 to 0.10 times. If the above mass ratio is less than 0.03 times, there are few acidic groups in the water-soluble resin, and the crosslinking density between pigment particles may not be sufficiently high. As a result, aggregates containing pigment particles may not be sufficiently formed, making it difficult to increase the sedimentation rate, and the effect of suppressing film thickness unevenness may be slightly reduced. On the other hand, if the above mass ratio is greater than 0.20 times, the amount of water-soluble resin present in a free state in the ink tends to increase. As a result, steric hindrance by the water-soluble resin inhibits the aggregation of pigments, making it difficult to form aggregates containing pigment particles, making it difficult to increase the sedimentation rate, and the effect of suppressing film thickness unevenness may be slightly reduced.

[0055] It is preferable that titanium dioxide is dispersed in the ink by the aforementioned water-soluble resin. That is, it is preferable that the water-soluble resin is a resin dispersant for dispersing titanium dioxide. Compared to titanium dioxide dispersed without the aforementioned water-soluble resin, titanium dioxide dispersed by the aforementioned water-soluble resin aggregates rapidly upon contact with the reaction solution, and the crosslinking density between pigment particles and the density of aggregates tend to increase. Therefore, the occurrence of uneven film thickness in the image can be more effectively suppressed.

[0056] [Aqueous medium] The ink is an aqueous ink containing at least water as an aqueous medium. The ink may contain an aqueous medium which is water or a mixed solvent of water and a water-soluble organic solvent. Deionized water or ion-exchanged water is preferably used as the water. The water content (mass%) in the ink is preferably 50.0% by mass or more and 95.0% by mass or less based on the total mass of the ink. The water-soluble organic solvent content (mass%) in the ink is preferably 3.0% by mass or more and 50.0% by mass or less based on the total mass of the ink. As the water-soluble organic solvent, any of those usable in inkjet inks, such as alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing compounds, and sulfur-containing compounds, can be used.

[0057] [Other ingredients] The ink may further contain water-soluble organic compounds that are solid at 25°C, such as urea and its derivatives, trimethylolpropane, and trimethylolethane. The content (by mass) of the water-soluble organic compounds in the ink is preferably 0.1% by mass or more and 10.0% by mass or less, based on the total mass of the ink. In addition to the above components, the ink may also contain various other components as needed. Examples of other components include various additives such as surfactants, defoamers, pH adjusters, viscosity adjusters, rust inhibitors, preservatives, fungicides, antioxidants, and reduction inhibitors. However, it is preferable that the ink does not contain the reactants included in the reaction solution.

[0058] [Ink properties] The ink is an aqueous ink for use in inkjet systems. Therefore, from the viewpoint of reliability, it is preferable to appropriately control its physical properties. Specifically, the surface tension of the ink at 25°C is preferably 20 mN / m or more and 60 mN / m or less. The viscosity of the ink at 25°C is preferably 1.0 mPa·s or more and 10.0 mPa·s or less. The pH of the ink at 25°C is preferably 7.0 or more and 9.5 or less, and more preferably 8.0 or more and 9.5 or less.

[0059] (Reaction solution) The reaction solution reacts with the ink upon contact, causing the components in the ink to coagulate. It contains a reactant, which includes a polyvalent metal salt. The components of the reactant are described in detail below.

[0060] [Reactive agent] The reactant is a component that causes the components in the ink (components with acidic groups, such as water-soluble resins and resin particles) to aggregate. The reactant also contains polyvalent metal salts.

[0061] Polyvalent metal salts are compounds composed of two or more valent metal ions (polyvalent metal ions) and anions. In the reaction solution, polyvalent metal salts dissociate into polyvalent metal ions, which then aggregate acidic components in the ink. Examples of polyvalent metal ions include Ca 2+ Cu 2+ Ni 2+ Mg 2+ Sr 2+ Ba 2+ , and Zn 2+ Divalent metal ions such as Fe 3+ , Cr 3+ , Y 3+ , and Al 3+ Examples of trivalent metal ions include Cl. - , Br - , I - , - ClO2 - ClO3 - ClO4 - NO2 - NO3 - SO4 2- CO3 2- , HCO3 - , PO4 3- HPO4 2- , and H2PO4 - Inorganic anions such as HCOO - , (COO - )2, COOH(COO - ), CH3COO - CH3CH(OH)COO -, C2H4(COO - )2, C6H5COO - , C6H4(COO - )2, and CH3SO3 - Examples of organic anions include the following. In order to include polyvalent metal ions in the reaction solution, a water-soluble polyvalent metal salt (which may also be a hydrate) formed by the bonding of a polyvalent metal ion and an anion can be used.

[0062] Specific examples of polyvalent metal salts include calcium carbonate such as heavy calcium carbonate and light calcium carbonate, calcium nitrate, calcium chloride, calcium sulfate, magnesium sulfate, calcium hydroxide, magnesium chloride, magnesium carbonate, barium sulfate, barium chloride, zinc carbonate, zinc sulfide, aluminum silicate, calcium silicate, magnesium silicate, copper nitrate, calcium acetate, magnesium acetate, aluminum acetate, aluminum sulfate, calcium methanesulfonate, calcium lactate, magnesium lactate, calcium propionate, calcium acetate, calcium pantothenate, and calcium gluconate. Among these, magnesium sulfate is preferred because it allows for easy adjustment of the reaction rate.

[0063] The content (mass%) of the polyvalent metal salt in the reaction solution is preferably 1.0% by mass or more and 20.0% by mass or less, based on the total mass of the reaction solution. In this specification, when the polyvalent metal salt is a hydrate, the "content (mass%) of the polyvalent metal salt" in the reaction solution means the "content (mass%) of the anhydrous polyvalent metal salt," excluding water as the hydrate. Furthermore, the concentration (mol / L) of metal ions derived from the polyvalent metal salt in the reaction solution is preferably 0.4 mol / L or more, and more preferably 0.4 mol / L or more and 1.5 mol / L or less. If the concentration of metal ions is less than 0.4 mol / L, a large amount of reaction solution will need to be added to aggregate the components in the ink. As a result, the liquid component may increase, and it may take longer for the aggregates to settle, which may reduce the effect of suppressing film thickness unevenness. The concentration of metal ions in the reaction solution can be calculated from the formula weight of the polyvalent metal salt.

[0064] As a reactant, polyvalent metal salts, organic acids, cationic resins, etc., can be used. In particular, using a cationic resin as a reactant can increase the size of the aggregates. For this reason, it is preferable that the reactant further contains a cationic resin. That is, by using a cationic resin as a reactant together with a polyvalent metal salt, the effect of suppressing film thickness unevenness can be further enhanced.

[0065] Cationic resins have cationic moieties in their structure and aggregate pigments and other materials dispersed in ink through the action of anionic groups. Examples of cationic resins include resins having primary to tertiary amine structures and resins having quaternary ammonium salt structures. Specifically, examples include resins having vinylamine, allylamine, vinylimidazole, vinylpyridine, dimethylaminoethyl methacrylate, ethyleneimine, guanidine, diallyldimethylammonium chloride, and alkylamine-epichlorohydrin condensate structures. To improve solubility in the reaction solution, cationic resins can be used in combination with acidic compounds, or the cationic resins can be subjected to quaternization treatment. The content (mass%) of cationic resin in the reaction solution is preferably 0.03% by mass or more and 10.0% by mass or less, based on the total mass of the reaction solution.

[0066] The reaction solution containing organic acids has buffering capacity in the acidic range (pH less than 7.0, preferably pH 2.0 to 5.0), which efficiently converts the anionic groups of components present in the ink into acidic forms and aggregates them. Examples of organic acids include monocarboxylic acids and their salts such as formic acid, acetic acid, propionic acid, butyric acid, benzoic acid, glycolic acid, lactic acid, salicylic acid, pyrrole carboxylic acid, furanic acid, picolinic acid, nicotinic acid, thiophene carboxylic acid, levulinic acid, and coumaric acid; dicarboxylic acids and their salts or hydrogen salts such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, itaconic acid, sebacic acid, phthalic acid, malic acid, and tartaric acid; tricarboxylic acids and their salts or hydrogen salts such as citric acid and trimellitic acid; and tetracarboxylic acids and their salts or hydrogen salts such as pyromellitic acid. The content (by mass) of organic acids in the reaction solution is preferably 1.0% by mass or more and 50.0% by mass or less, based on the total mass of the reaction solution.

[0067] [Aqueous medium] The reaction solution is a reaction solution that contains at least water as an aqueous medium. Examples of aqueous mediums that can be used in the reaction solution include those similar to the aforementioned aqueous mediums that can be incorporated into ink.

[0068] [Other ingredients] The reaction solution may contain various other components as needed. Examples of other components include those similar to those mentioned above that can be included in the ink.

[0069] [Physical properties of the reaction solution] The reaction solution is intended for use in inkjet printing. Therefore, from the viewpoint of reliability, it is preferable to appropriately control its physical properties. Specifically, the surface tension of the reaction solution at 25°C is preferably 20 mN / m or more and 60 mN / m or less. The viscosity of the reaction solution at 25°C is preferably 1.0 mPa·s or more and 10.0 mPa·s or less. The pH of the reaction solution at 25°C is preferably 5.0 or more and 9.5 or less, and more preferably 6.0 or more and 9.0 or less.

[0070] <Inkjet recording method, inkjet recording device> The inkjet recording method of the present invention comprises the step of ejecting ink and a reaction solution from an inkjet recording head and applying them to a recording medium. The ink and reaction solution are a combination of aqueous ink and reaction solution included in the aforementioned ink set. More specifically, the inkjet recording method of the present invention preferably comprises a reaction solution application step of applying the reaction solution to the recording medium, and an ink application step of applying aqueous ink so as to overlap at least a portion of the area on the recording medium to which the reaction solution is applied. In particular, it is preferable to have the reaction solution application step before the ink application step, or to perform the ink application step and the reaction solution application step in parallel. It is not necessary to cure the image by irradiation with active energy rays or the like.

[0071] Furthermore, the inkjet recording apparatus of the present invention is a device suitably used in the above-described inkjet recording method, comprising ink and reaction solution, and an inkjet recording head that ejects the ink and reaction solution. The ink and reaction solution are a combination of aqueous ink and reaction solution included in the aforementioned ink set. The details of the inkjet recording method and inkjet recording apparatus of the present invention (hereinafter also simply referred to as "recording method" and "recording apparatus") will be described below.

[0072] (Inkjet recording device) Figure 1 is a schematic perspective view showing one embodiment of the inkjet recording apparatus of the present invention. Figure 2 is a schematic side view showing one embodiment of the inkjet recording apparatus of the present invention. As shown in Figures 1 and 2, the recording apparatus of this embodiment includes an inkjet recording head 1 that ejects liquid (ink and reaction solution). Examples of recording heads include recording heads that eject ink and reaction solution by the action of mechanical energy, and recording heads that eject ink and reaction solution by the action of thermal energy. Among these, recording heads that eject ink and reaction solution by the action of thermal energy are preferred. A recording head that ejects ink and reaction solution by the action of thermal energy is a thermal recording head that imparts thermal energy to the ink and reaction solution by applying an electric pulse to an electrothermal conversion element, and ejects the ink and reaction solution from the ejection port. It is preferable that the recording head is equipped with a mechanism (temperature control mechanism) that heats the liquid inside it to a predetermined temperature. When a temperature control mechanism is provided, it is preferable that the temperature of the liquid (ink and reaction solution) ejected from the recording head be 35°C or higher and 70°C or lower.

[0073] It is preferable to apply ink to a unit area of ​​the recording medium by multi-pass recording, which is performed by dividing the process into multiple relative scans between the recording head and the recording medium. When recording an image using both white ink and color ink, it is preferable to apply the white ink and the color ink to the unit area using different relative scans. This increases the time it takes for the inks to come into contact with each other, making it easier to suppress mixing. The unit area can be set to any area, such as one pixel or one band.

[0074] The amount of reaction solution applied per unit area of ​​the recording medium is preferably 0.10 to 0.80 times the amount of ink applied by mass, and more preferably 0.20 to 0.50 times. By applying the ink and reaction solution to the recording medium in the above mass ratio, high-density aggregates are formed, which increases the sedimentation rate and thus more effectively suppresses film thickness unevenness.

[0075] [Drying process] The recording method of the present invention preferably further includes a drying step in which air is blown onto a recording medium to which ink and reaction solution have been applied, thereby drying at least a portion of the liquid component on the recording medium. By performing such a drying step, high-quality images can be recorded even on non-absorbent recording media. The liquid component on the recording medium will gradually dry even without performing the drying step. However, performing the drying step promotes the drying of the liquid component, and allows for the recording of images in which the pigment is efficiently filled. The drying step may be performed in multiple stages. The drying step may also be performed simultaneously with the heating step, and the drying step can be performed using the same heating means as described later.

[0076] When blowing air onto the image for drying, the wind speed is preferably between 1 m / s and 100 m / s. If heating is not required, there is no need to adjust the air temperature; room temperature air is sufficient. If heating is required, the air temperature should be above room temperature, preferably between 30°C and 200°C, more preferably between 50°C and 150°C, and particularly preferably between 80°C and 120°C. The air temperature can be measured using, for example, a K-type thermocouple thermometer. A specific measuring instrument could be, for example, the "AD-5605H" (manufactured by A&D). If necessary, the airflow may be directed to the back surface of the recording medium, but it is preferable to direct the airflow to the front surface (recording surface) of the recording medium. The distance from the air-blowing device to the recording medium is preferably between 5 mm and 50 mm.

[0077] [Heating process] The recording method of the present invention preferably includes a heating step of heating (heat treatment) a recording medium to which ink and reaction solution have been applied. In the heating step, the recording medium to which ink and reaction solution have been applied is heated. This promotes the melting of resin particles in the ink, thereby forming a resin film, and enabling the recording of images with improved scratch resistance.

[0078] If the following requirements (i) to (iii) are met, the recording medium to which the ink and reaction solution have been applied shall have a glass transition temperature Tg (°C) or melting point T of the resin particles. M It is preferable to heat to a temperature of (°C) or higher. That is, heating temperature T H (°C) is the glass transition temperature Tg (°C) or melting point T of the resin particles. M The temperature can be appropriately set according to (°C). If the resin forming the resin particles is a crystalline resin, the glass transition temperature Tg (°C) and melting point Tg of the resin particles are also specified. M Since it has (°C), the melting point T M It is sufficient to heat it to a temperature above (°C). (i) Average primary particle size D of titanium dioxide P0 The (nm) value is 150 nm or less. (ii) Average particle size D of resin particles E The (nm) range is between 100 nm and 400 nm. (iii) The content of resin particles in the ink (volume %) is 1.3 times or more and 5.0 times or less in volume ratio to the content of titanium dioxide (volume %).

[0079] When air is blown onto a recording medium coated with ink and reaction solution, at least a portion of the liquid component on the recording medium is evaporated and dried, voids are formed between the aggregated pigment particles. Subsequently, the glass transition temperature Tg (°C) or melting point T of the resin particles is determined. M When heated to a temperature above (°C), the resin particles melt, and the molten resin penetrates into the voids by capillary force. Simultaneously, voids are formed where the resin particles were located. The penetration of the molten resin into the voids forms a binder, which is a mixture of pigment and molten resin, allowing an image containing voids to be fixed onto a recording medium. Since the refractive index of the voids is relatively lower than that of the binder, the incident visible light is scattered due to the difference in refractive index between the binder and the voids. This allows for the recording of images with even greater opacity.

[0080] Heating temperature T H The upper limit of (°C) is not particularly limited, but from the viewpoint of the heat resistance temperature of the recording medium, it is preferable to set it to 200°C or less.H (°C) represents the highest temperature reached on the surface of the recording medium during the heating process. H The phrase "(°C)" can be interpreted as "the set temperature of the heating method (°C)". H The temperature in degrees Celsius (°C) can be measured using, for example, a contact thermometer that brings a thermocouple or the like into contact with the surface of the recording medium, or a non-contact infrared thermometer. In the embodiment described later, a non-contact infrared thermometer digital radiation temperature sensor (product name "FT-H20", manufactured by Keyence) (not shown) was used to measure the surface temperature of the recording medium from a position 10 cm vertically above the surface of the recording medium. H (°C) is preferably 30°C or higher, more preferably 50°C or higher, and particularly preferably 80°C or higher.

[0081] Means for heating the recording medium include, for example, known heating means such as heaters, blowing means using blowers such as dryers, and means combining these. Means for heating include the above heating means, blowing means, and means combining these. Means for heating include, for example, applying heat from the side (back side) of the recording medium opposite the recording surface (ink application surface) with a heater, applying warm air or hot air to the recording surface of the recording medium, and heating from the recording surface or back side using an infrared heater. Multiple of these may also be combined. Furthermore, the recording medium may be heated by bringing a heated component into contact with the recording surface or back side. The duration of the heating process should be sufficient to melt the resin particles. For example, when heating the recording medium with a blowing means, the temperature of the air can be 80°C or higher and 120°C or lower.

[0082] In the recording apparatus shown in Figures 1 and 2, the main scanning direction B, in which the recording head 1 reciprocates, and the sub-scanning direction A, which is the transport direction of the recording medium P, are orthogonal to each other. In the sub-scanning direction A, a heater 25 supported by a frame (not shown) is positioned downstream of the position where the recording head 1 reciprocates. The recording medium P, to which the reaction liquid and ink have been applied, can be heated from its back surface by the heater 25. Specific examples of the heater 25 include sheath heaters and halogen heaters. The heater 25 is covered by a heater cover 26. The heater cover 26 is a component for efficiently irradiating the recording medium P with the heat generated from the heater 25. Furthermore, the heater cover 26 also serves as a component to protect the heater 25. The recording medium P, to which the ink ejected from the recording head 1 has been applied, is wound up by a take-up spool 27 to form a roll-shaped winding medium 24.

[0083] [Recording medium] The type of recording medium used to record the image is not particularly limited, and any recording medium may be used. In particular, since white ink capable of recording images such as white is used, it is preferable to use a recording medium other than so-called "white," such as transparent film, translucent film, and colored paper.

[0084] According to the inkjet recording method of the present invention, images with suppressed film thickness unevenness can be recorded on low-to-non-absorbent recording media. In this specification, "low-to-non-absorbent recording media" refers to recording media from the start of contact to 30 msec in the Bristow method. 1 / 2 Up to 10 mL / m² of water absorption capacity 2 The following recording media are used. Water absorption capacity is 0 mL / m³. 2This can also occur. The Bristow method is a widely used method for measuring the amount of liquid absorbed in a short time, and is also adopted by the Japan Paper & Pulp Technology Association (JAPAN TAPPI). Details of the test method are described in Standard No. 51, "Paper and Paperboard - Liquid Absorbency Test Method - Bristow Method," of the "JAPAN TAPPI Paper & Pulp Test Methods 2000 Edition." Recording media with an ink-receiving layer for inkjet recording (glossy paper, matte paper, etc.) and plain paper without a coating layer have a water absorption capacity of 10 mL / m². 2 It is a "highly absorbent recording medium" that exceeds [a certain limit].

[0085] Examples of low-absorption recording media include recording media without an ink-receiving layer and recording media with a thin ink-receiving layer. Examples of printing papers include art paper, fine coated paper, medium coated paper, fine lightweight coated paper, medium lightweight coated paper, lightly coated paper, and cast coated paper. Examples of non-absorbent recording media include recording media without an ink-receiving layer and recording media with a thin ink-receiving layer. Examples of non-absorbent recording media include plastic films and materials such as paper on which plastic is coated. Examples of plastics include polyvinyl chloride, polyethylene terephthalate, polycarbonate, polystyrene, polyurethane, polyethylene, and polypropylene. Other examples include glass, metal, and ceramics. Among these, plastic films and materials such as paper on which plastic is coated are preferred. In this specification, "recording media" refers to an object on which an image is recorded as a recording, not a transfer medium. [Examples]

[0086] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way by the following examples unless it exceeds the gist of the invention. Unless otherwise specified, amounts of components described in "parts" and "%" are based on mass. Hereinafter, "titanium dioxide dispersion" will also be referred to as "pigment dispersion".

[0087] <Measurement of Physical Properties> (Average Primary Particle Diameter, Average Particle Diameter) The average primary particle diameter (D P0 ) of titanium oxide was measured by the following method. First, using a scanning electron microscope (trade name "S-4700", manufactured by Hitachi High-Tech), the sample was photographed at a magnification of 100,000 times. Next, the diameters of the circles circumscribing 100 primary particles of the metal oxide were measured, and the average value was taken as the "average primary particle diameter (D P0 )". Also, the average particle diameter (D P ) of titanium oxide was measured using a particle size measuring device based on the dynamic light scattering method (trade name "UPA-EX150", manufactured by Nikkiso Co., Ltd.). The average particle diameter (D E ) of the resin particles (the cumulative 50% particle diameter in the volume-based particle size distribution) was also measured using the above particle size measuring device.

[0088] (Glass Transition Temperature, Melting Point of Resin Particles) The glass transition temperature Tg of the resin particles was measured using a differential scanning calorimeter (DSC). Specifically, 2 mg of the resin particles obtained by drying the liquid containing the resin particles at 60 °C was placed in an aluminum container and sealed to prepare a sample for measurement. For the prepared sample, using a differential scanning calorimeter (trade name "DSC-2500", manufactured by TA instruments), thermal analysis was performed according to the temperature program shown below. The glass transition temperature of the resin particles in this specification is defined as follows. That is, in the heating curve (horizontal axis: temperature, vertical axis: heat) of the following temperature program (3), the temperature at the intersection of the straight line extended to the high temperature side through two points in the curve on the low temperature side and the tangent line drawn at the point where the gradient of the stepped change part in the curve is maximum is determined. The temperature thus determined was taken as the "glass transition temperature Tg of the resin particles". [Temperature Program]: (1) Heating from 20 °C to 200 °C at 10 °C / min (2) Cooling from 200 °C to -50 °C at 5 °C / min (3) Heating from -50 °C to 200 °C at 10 °C / min

[0089] (Specific Gravity) The specific gravity of titanium dioxide and resin particles was measured using the Gay-Lussac type pycnometer method in accordance with JIS Z 8807.

[0090] <Preparation of Titanium Oxide> The titanium dioxide types 1-5 shown in Table 1 were prepared. The specific gravity of titanium dioxide types 1-5 was all 4.2, and the refractive index of all types was within the range of 2.5 to 2.8.

[0091] TIFF2026091245000001.tif55170

[0092] <Preparation of water-soluble resin> The following water-soluble resins 1 to 15 were prepared. The properties of the water-soluble resins are shown in Table 2. Each water-soluble resin was used in ink preparation after being converted to a sodium salt form as needed.

[0093] (Water-soluble resins 1, 5, and 9-15) The commercially available resin aqueous solutions shown in Table 2 were used as aqueous solutions for water-soluble resins 1, 5, and 9-15.

[0094] (Water-soluble resin 2) 61.8 parts acrylic acid and 20.0 parts toluene were placed in a flask and purged with nitrogen. Then, while heating and stirring at 90°C, 5.2 parts maleic acid and 0.36 parts initiator (di-tert-butyl peroxide) were added. The reaction was carried out at 160°C for 6 hours, during which the toluene was evaporated. After the reaction was complete, the contents were cooled and solidified to obtain a maleic acid-acrylic acid copolymer. Sodium hydroxide was added and dissolved in deionized water to obtain an aqueous solution of water-soluble resin 2 with a resin content of 40.0%.

[0095] (Water-soluble resin 3) An aqueous solution of water-soluble resin 3 with a resin content of 40.0% was obtained in the same manner as in the case of water-soluble resin 2 described above, except that 59.3 parts of acrylic acid and 7.7 parts of maleic acid were used.

[0096] (Water-soluble resin 4) An aqueous solution of water-soluble resin 4 with a resin content of 40.0% was obtained in the same manner as in the case of water-soluble resin 2 described above, except that 47.8 parts of acrylic acid and 19.2 parts of maleic acid were used.

[0097] (Water-soluble resin 6) 85 parts of pure water were placed in a 2.5 L stainless steel separable flask equipped with a thermometer, stirrer, and reflux condenser, and heated to boiling point while stirring to achieve reflux. 221.2 parts of itaconic acid, 73.7 parts of pure water, and 255.0 parts of 48% sodium hydroxide aqueous solution were mixed to prepare a 40% sodium itaconate aqueous solution. While stirring and maintaining reflux at boiling point, 262.5 parts of 80% acrylic acid aqueous solution, 49.7 parts of 40% sodium itaconate aqueous solution, 34.0 parts of 5% hydrogen peroxide solution, and 31.4 parts of 13% sodium persulfate aqueous solution were added dropwise into the flask from separate dropping nozzles. After the addition was complete, the mixture was heated for 50 minutes while maintaining reflux at boiling point to polymerize the monomers. An appropriate amount of deionized water was added to obtain an aqueous solution of water-soluble resin 6 with a resin content of 40.0%.

[0098] (Water-soluble resin 7) An aqueous solution of water-soluble resin 7 with a resin content of 40.0% was obtained in the same manner as in the case of water-soluble resin 6 described above, except that 282.0 parts of an 80% acrylic acid aqueous solution and 83.0 parts of a 40% sodium itaconate aqueous solution were used.

[0099] (Water-soluble resin 8) An aqueous solution of water-soluble resin 8 with a resin content of 40.0% was obtained in the same manner as in the case of water-soluble resin 6 described above, except that 153.0 parts of an 80% acrylic acid aqueous solution and 553.0 parts of a 40% sodium itaconate aqueous solution were used.

[0100] TIFF2026091245000002.tif194170

[0101] <Preparation of Pigment Dispersion> (Pigment dispersions 1-20, 22-38, and 40-44) Mix titanium oxide of the types and amounts shown in Tables 3-1 and 3-2, an aqueous solution of a water-soluble resin, and ion-exchanged water such that the total of the components is 100.0%, and perform preliminary dispersion using a homogenizer. After mixing 100 parts of 0.05 mm zirconia beads, perform dispersion treatment for 5 hours using a bead mill with the peripheral speed adjusted to obtain a desired particle size. Filter off the zirconia beads, and add an appropriate amount of ion-exchanged water as necessary to obtain Pigment Dispersions 1 to 20, 22 to 38, and 40 to 44. The content C P (%) of titanium oxide, the content C S (%) of the water-soluble resin, the value of C S / C P (times), and the average particle diameter D P (nm) of titanium oxide are shown in Tables 3-1 and 3-2.

[0102] (Pigment Dispersions 21 and 39) Mix 35.0 parts of titanium oxide of the types shown in Tables 3-1 and 3-2, 1.75 parts of triethanolamine, 63.25 parts of ion-exchanged water, and 100 parts of 0.05 mm zirconia beads, and perform dispersion treatment for 5 hours using a bead mill with the peripheral speed adjusted to obtain a desired particle size. Filter off the zirconia beads, and add an appropriate amount of ion-exchanged water as necessary to obtain Pigment Dispersions 21 and 39. The content C P (%) of titanium oxide, the content C S (%) of the water-soluble resin, the value of C S / C P (times), and the average particle diameter (D P ) of titanium oxide are shown in Tables 3-1 and 3-2.

[0103] TIFF2026091245000003.tif195170

[0104] TIFF2026091245000N004.tif195170

[0105] <Preparation of Resin Particles> Resin particles 1 to 7, as shown below, were prepared. Aqueous dispersions of resin particles 1 to 3 were prepared according to the methods described below. Styrene (St), acrylonitrile (AN), 2-ethylhexyl acrylate (2EHA), methacrylic acid (MAA), and methyl methacrylate (MMA) were used as monomers. 2-ethylhexyl-3-mercaptopropionate (trade name "EHMP", manufactured by SC Organic Chemicals) was used as a chain transfer agent. In addition, "Adekaria Soap SR-10" (manufactured by ADEKA) was used as a reactive surfactant. The specific gravity of the resin particles measured according to the above method was 1.0 for all of them.

[0106] (Resin particle 1) A reaction vessel equipped with a stirrer was placed in a hot water bath. 1.178 parts of water were added to the reaction vessel, and the internal temperature was maintained at 70°C. A monomer mixture was prepared by mixing 97.0 parts of St and 3.0 parts of a reactive surfactant. Furthermore, an aqueous polymerization initiator solution was prepared by mixing 1.9 mol% of a chain transfer agent, 1.9 parts of potassium persulfate, and 659 parts of water, based on the total amount of monomer. The monomer mixture and the aqueous polymerization initiator solution were added dropwise to the reaction vessel in parallel over 60 minutes. The reaction was allowed to proceed with continuous stirring for 30 minutes to produce resin particles 1. An appropriate amount of 8 mol / L potassium hydroxide aqueous solution was added to adjust the pH to 8.5, obtaining an aqueous dispersion of resin particles 1. Resin particle content (%), average particle size D of resin particles. E Table 4 shows the (nm) and glass transition temperature Tg (°C).

[0107] (Resin particles 2) St44.0 parts, AN19.0 parts, 2EHA28.0 parts, MAA6.0 parts, and reactive surfactant3.0 parts were used, but no chain transfer agent was used. Except for these points, the aqueous dispersion of resin particles 2 was obtained in the same manner as described above for the aqueous dispersion of resin particles 1. Resin particle content (%), average particle size D of resin particles. E Table 4 shows the (nm) and glass transition temperature Tg (°C).

[0108] (Resin particles 3) In a 500 mL separatory flask, 400 parts water, 12.0 parts St, 11.5 parts MMA, and 0.044 parts sodium styrene sulfonate were added. Nitrogen gas was bubbled in while stirring at 50 rpm, and the temperature was raised to 70°C. After stirring for 30 minutes, 0.8 parts of polymerization initiator (potassium peroxodisulfate, manufactured by Fujifilm Wako Pure Chemical Industries) dissolved in 20 parts water was added. The mixture was stirred at 70°C and 200 rpm for 8 hours, after which an appropriate amount of deionized water was added to obtain an aqueous dispersion of resin particles 3. Resin particle content (%), average particle size D of resin particles E Table 4 shows the (nm) and glass transition temperature Tg (°C).

[0109] (Resin particles 4-7) The commercially available aqueous dispersions of resin particles shown in Table 4 were used as aqueous dispersions of resin particles 4-7. Resin particle content (%), average particle size D of the resin particles. E Table 4 shows the (nm) and glass transition temperature Tg (°C).

[0110] TIFF2026091245000005.tif63170

[0111] <Ink preparation> Each component (in %) shown in the middle section of Tables 5-1 to 5-6 was mixed. Potassium hydroxide (shown as part of the amount of deionized water used in Tables 5-1 to 5-3) was added to adjust the pH to a range of 8 to 9, and then each ink was prepared by pressure filtration through a 5.0 μm pore size microfilter (manufactured by Fujifilm). In Tables 5-1 to 5-6, "Acetylenel E100" is the trade name of a nonionic surfactant (acetylene glycol ethylene oxide adduct) manufactured by Kawaken Fine Chemicals. The properties of the prepared inks are shown in the lower section of Tables 5-1 to 5-6.

[0112] TIFF2026091245000006.tif155170

[0113] TIFF2026091245000007.tif166170

[0114] TIFF2026091245000008.tif166170

[0115] TIFF2026091245000009.tif166170

[0116] TIFF2026091245000010.tif165170

[0117] TIFF2026091245000011.tif166170

[0118] <Preparation of reaction solution> After mixing each component (in %) shown in the upper part of Table 6, the reaction solutions were prepared by pressure filtration through a 5.0 μm pore size microfilter (manufactured by Fujifilm). As the aqueous solution of cationic resin, an aqueous solution containing poly(diallyldimethylammonium chloride) (product name "Unisense FPA100L" (manufactured by Senka, cationic resin content: 27.0%)) was used. In Table 6, "Acetylenel E100" is the product name of a nonionic surfactant (acetylene glycol ethylene oxide adduct) manufactured by Kawaken Fine Chemicals. The characteristics of the prepared reaction solutions are shown in the lower part of Table 6.

[0119] TIFF2026091245000012.tif88170

[0120] <Rating> Each ink and reaction solution obtained as described above was evaluated for the following items. In this invention, "AA," "A," and "B" were considered acceptable levels in the evaluation criteria shown below, and "C" was considered an unacceptable level. The evaluation results are shown in Tables 7-1, 7-2, and 8.

[0121] (Image recording) An inkjet recording device (product name "PIXUS PRO-10S", manufactured by Canon) equipped with a recording head that ejects liquid using thermal energy was prepared. In this example, the recording duty cycle of an image recorded using this inkjet recording device under the condition that 8 ink droplets with a mass of 3.5 ng each are applied to a unit area of ​​1 / 600 inch x 1 / 600 inch is defined as 100%. The types of inks and reaction solutions shown in Tables 7-1, 7-2, and 8 were filled into cartridges and set in the above inkjet recording device. As the recording medium, PET film (product name "LLRPCF1372", manufactured by Sakurai, 30 msec from the start of contact in the Bristow method) was used. 1 / 2 The amount of water absorbed up to this point is 0 mL / m². 2 More than 10mL / m 2 A sheet cut to A4 size (within the following range) was used. Using the recording device described above, the reaction liquid and ink were ejected from the recording head in this order and applied to the recording medium in layers to record a solid 50mm x 50mm image. The recording duty cycles of the reaction liquid and ink are shown in Tables 7-1, 7-2, and 8. Subsequently, a hot air dryer (product name "Multi Dryer HAS-17T", manufactured by Kansai Denki Co., Ltd.) was used to dry and heat the recording medium by applying hot air at the heating temperatures shown in Tables 7-1, 7-2, and 8 at a wind speed of 25 m / s, thereby fixing the image to the recording medium.

[0122] (Suppression of uneven film thickness) The cross-sections formed by cutting a recording medium containing images were observed using a scanning electron microscope (product name "S-4000", Hitachi), and the film thickness of 30 images was measured. The standard deviation of the 30 measured film thicknesses was calculated, and the suppression of unevenness in image film thickness was evaluated according to the evaluation criteria shown below. A smaller standard deviation indicates that the image film thickness is more uniform and unevenness is suppressed. AA: The standard deviation was less than 3.0. A: The standard deviation was between 3.0 and 3.5. B: The standard deviation was between 3.5 and 4.0. C: The standard deviation was 4.0 or greater.

[0123] (Concealing ability) The opacity of the recorded images was evaluated by measuring and calculating the opacity of the recorded images according to a method compliant with ISO 2471:2008. ISO 2471:2008 measures the reflectance by placing a white plate and a black plate on the back of the recording medium (paper) under test, and calculates the opacity from the following formula (C). Concealment rate (%) = (R0 / R ∞ ) × 100 ···(C) R0: Reflectance measured with a black board placed behind it. R ∞ Reflectance measured with a white board placed behind it.

[0124] In this example, the opacity of images recorded using opacity test paper (white and black boards, manufactured by TP Giken, with inspection certificate from the Japan Paint Inspection Association) was measured and calculated in accordance with this method. The opacity of the images was then evaluated according to the evaluation criteria shown below. AA: The concealment rate was 60% or higher. A: The concealment rate was between 50% and 60%. B: The concealment rate was between 40% and 50%. C: The concealment rate was less than 40%.

[0125] TIFF2026091245000013.tif235170

[0126] TIFF2026091245000014.tif235170

[0127] TIFF2026091245000015.tif255155

Claims

1. An inkjet ink set comprising a combination of a white aqueous ink containing titanium dioxide and resin particles, and a reaction solution containing a reagent that reacts with the aqueous ink, The aforementioned aqueous ink further contains a water-soluble resin that includes a unit having an acidic group, The proportion (mol%) of the units having the acidic group in the water-soluble resin is 60.0 mol% or more, and the units having the acidic group include units having multiple carboxylic acid groups. An ink set characterized in that the reactant contains a polyvalent metal salt.

2. The ink set according to claim 1, wherein the content (mass%) of the water-soluble resin in the aqueous ink is 0.03 times or more by mass ratio to the content (mass%) of the titanium dioxide.

3. The unit having the plurality of carboxylic acid groups includes a unit derived from a dicarboxylic acid compound. The ink set according to claim 1, wherein the proportion (mol%) of the unit derived from the dicarboxylic acid compound in the unit having the plurality of carboxylic acid groups is 7.5 mol% or more.

4. The ink set according to claim 3, wherein the dicarboxylic acid compound is maleic acid.

5. The ink set according to claim 1, wherein the water-soluble resin is a resin dispersant for dispersing the titanium dioxide.

6. The ink set according to claim 1, wherein the concentration (mol / L) of metal ions derived from the polyvalent metal salt in the reaction solution is 0.4 mol / L or more.

7. The ink set according to claim 1, wherein the reactant further comprises a cationic resin.

8. The ink set according to claim 1, wherein the polyvalent metal salt comprises magnesium sulfate.

9. The average primary particle diameter D of the titanium oxide Ｐ０ (nm) is 150 nm or less, Cumulative 50% particle size D in the volume-based particle size distribution of the aforementioned resin particles Ｅ (nm) is between 100 nm and 400 nm. The ink set according to claim 1, wherein the content (volume %) of the resin particles in the aqueous ink is 1.3 times or more and 5.0 times or less in volume ratio to the content (volume %) of the titanium dioxide.

10. An inkjet recording method comprising the step of ejecting ink and reaction solution from an inkjet recording head and applying them to a recording medium, An inkjet recording method characterized in that the ink and the reaction solution are a combination of aqueous ink and reaction solution included in the ink set described in any one of claims 1 to 9.

11. Furthermore, the inkjet recording method according to claim 10, further comprising a drying step of blowing air onto the recording medium to which the ink and the reaction solution have been applied, thereby drying at least a portion of the liquid components on the recording medium.

12. The inkjet recording method according to claim 10, wherein the amount of the reaction solution applied per unit area of ​​the recording medium is 0.10 times or more and 0.80 times or less by mass ratio to the amount of the aqueous ink applied.

13. Furthermore, the inkjet recording method according to claim 10, further comprising a heating step of heating the recording medium to which the aqueous ink and the reaction solution have been applied.

14. From the start of contact in the Bristow method of the aforementioned recording medium to 30 msec １／２ The amount of water absorbed up to 10 mL / m² is ２ The inkjet recording method according to claim 10, which is as follows:

15. An inkjet recording apparatus comprising ink and a reaction solution, and an inkjet recording head for ejecting the ink and the reaction solution, An inkjet recording apparatus characterized in that the ink and the reaction solution are a combination of aqueous ink and reaction solution included in the ink set according to any one of claims 1 to 9.

Images