Ink, ink set, inkjet recording method, and inkjet recording apparatus

Abstract

To improve the dispersibility of white pigment in inks using a white pigment composed of titanium dioxide. [Solution] The ink contains a white pigment composed of titanium dioxide, a resin binder, a surfactant with an HLB value of 10 or higher, and water. By incorporating a resin binder into this ink, the adhesion of white images to the recording medium can be improved. Furthermore, by incorporating a surfactant with an HLB value of 10 or higher into this ink, high dispersibility of the white pigment composed of titanium dioxide in the aqueous solvent is obtained, making it easier to maintain the dispersion state of the white pigment in the aqueous solvent over a long period of time.

Description

Technical Field

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

Background Art

[0002] Patent Document 1 discloses an inkjet ink for forming a white image on a recording medium. In an inkjet ink, high dispersibility of a pigment in an aqueous solvent is required to improve storage stability and redispersibility. Further, in an inkjet ink, high adhesion of an image to a recording medium and high rubbing resistance of the image formed on the recording medium are also required.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The inkjet ink according to Patent Document 1 contains a white inorganic pigment. Titanium oxide is well known as a white inorganic pigment. Titanium oxide can obtain high coloring power due to its high refractive index, but it is difficult to obtain high dispersibility in an aqueous solvent. Further, in an inkjet ink, aggregation of pigments tends to cause a decrease in adhesion of an image to a recording medium and rubbing resistance of the image formed on the recording medium.

[0005] In view of the above circumstances, an object of the present invention is to improve the dispersibility of a white pigment in an ink using a white pigment composed of titanium oxide.

Means for Solving the Problems

[0006] To achieve the above objective, an ink according to one embodiment of the present invention contains a white pigment composed of titanium dioxide, a resin binder, a surfactant with an HLB value of 10 or higher, and water. In the above ink, the content of the above resin binder may be 3% by mass or more and 15% by mass or less.

[0007] This ink incorporates a resin binder to improve the adhesion of white images to recording media. Furthermore, by incorporating a surfactant with an HLB value of 10 or higher, this ink achieves high dispersibility of the titanium dioxide-based white pigment in aqueous solvents, making it easier to maintain the dispersion state of the white pigment in aqueous solvents over long periods.

[0008] An ink set according to one embodiment of the present invention comprises a first ink and a second ink. The above-mentioned first ink contains a first white pigment composed of titanium dioxide, a first resin binder, a first surfactant having an HLB value of 10 or more and 11 or less, and water. The above-mentioned second ink contains a second white pigment composed of titanium dioxide, a second resin binder, a second surfactant with an HLB value of 13 or more and 16 or less, and water.

[0009] In this ink set, by incorporating a first surfactant and a second surfactant with an HLB value of 10 or higher into the first and second inks, high dispersibility of the white pigment composed of titanium dioxide in the first and second inks is achieved. Furthermore, because the HLB value of the surfactant in the first ink is lower than that of the second ink, the white pigment is more prone to aggregation than in the second ink. For this reason, the particle size of the pigment dispersion composed of white pigment tends to be larger in the first ink than in the second ink. In this ink set, a white image is formed on the recording medium by ejecting the first ink onto the recording medium followed by the second ink onto the recording medium. In other words, in the white image formed on the recording medium with this ink set, a second white layer formed by the second ink is laminated on top of a first white layer formed by the first ink. In the white image formed with this ink set, a high opacity is obtained due to the first white layer formed by the first ink, which has large pigment dispersion particle sizes. In addition, in the white image formed with this ink set, a high scratch resistance is obtained due to the second white layer formed by the second ink, which has small pigment dispersion particle sizes.

[0010] An inkjet recording method according to one embodiment of the present invention is a method for forming a white image on a recording medium. In the above inkjet recording method, a first ink containing a first white pigment made of titanium dioxide, a first resin binder, a first surfactant with an HLB value of 10 to 11, and water is ejected onto the recording medium, and then a second ink containing a second white pigment made of titanium dioxide, a second resin binder, a second surfactant with an HLB value of 13 to 16, and water is ejected onto the recording medium. The above recording medium may also be a non-absorbent recording medium.

[0011] An inkjet recording apparatus according to one embodiment of the present invention is an apparatus that performs a recording operation to form a white image on a recording medium. The above-mentioned inkjet recording device comprises a recording unit and a control unit. The recording unit includes a first recording head having a first nozzle for ejecting a first ink containing a first white pigment made of titanium dioxide, a first resin binder, a first surfactant having an HLB value of 10 to 11, and water onto the recording medium, and a second recording head having a second nozzle for ejecting a second ink containing a second white pigment made of titanium dioxide, a second resin binder, a second surfactant having an HLB value of 13 to 16, and water onto the recording medium. When the above-mentioned control unit performs the above-mentioned recording operation, it controls the recording unit so that the first recording head ejects the first ink onto the recording medium, and then the second recording head ejects the second ink onto the recording medium. The above recording medium may also be a non-absorbent recording medium.

[0012] The above-described inkjet recording apparatus may further include a circulation unit configured to circulate the first ink through the first channel and the second ink through the second channel, which are mutually independent first and second channels that do not pass through the first and second nozzles. The control unit may, when not performing the recording operation, control the circulation unit to circulate the first ink through the first flow path and the second ink through the second flow path. [Effects of the Invention]

[0013] As described above, the present invention makes it possible to improve the dispersibility of the white pigment in an ink using a white pigment composed of titanium dioxide. [Brief explanation of the drawing]

[0014] [Figure 1] This is a block diagram showing the schematic configuration of an inkjet recording device according to one embodiment of the present invention. [Modes for carrying out the invention]

[0015] Embodiments of the present invention will be described below.

[0016] [Ink composition] (Schematic configuration) The ink according to an embodiment of the present invention contains a white pigment a, a resin binder b, a surfactant c, and water. The ink according to this embodiment is an aqueous ink that is ejected from a recording head of an inkjet recording apparatus onto a recording medium to form a white image on the recording medium. The recording medium on which a white image is formed by the ink according to this embodiment is typically a non-absorbent recording medium with low ink absorbency.

[0017] Examples of the non-absorbent recording medium include foil paper, overhead projector (OHP) sheets, and plastic recording media. Examples of the plastic constituting the recording medium include polyester, polypropylene, polyethylene, polystyrene, and polyvinyl chloride. At least one surface of the recording medium may be subjected to a surface treatment such as corona discharge treatment, plasma treatment, or primer.

[0018] In the ink according to this embodiment, the white pigment a can be fixed to the recording medium by blending the resin binder b, and the dispersibility of the white pigment a in the aqueous solvent can be enhanced by blending a surfactant with a large HLB value. Hereinafter, the details of each component of the ink according to this embodiment will be described.

[0019] (White pigment a) In the ink according to this embodiment, fine particles of titanium oxide are used as the white pigment a. Thereby, in the ink according to this embodiment, a high-quality white image with high whiteness and opacity can be formed. The primary particle diameter of the fine particles of titanium oxide used as the white pigment a is preferably 100 nm or more and 500 nm or less, and more preferably 150 nm or more and 400 nm or less. Further, the fine particles of titanium oxide used as the white pigment a may be surface-treated with, for example, silica or alumina. In the ink according to this embodiment, the content of the white pigment a is preferably 5.0% by mass or more and 15.0% by mass or less.

[0020] (Resin binder b) In the ink according to this embodiment, a resin binder b is incorporated to improve the adhesion of the white image to the recording medium. The resin binder b is fine particles formed of resin. Examples of resins that form the resin binder b include (meth)acrylic resin, styrene resin, polyvinyl resin, polyester resin, amino resin, epoxy resin, urethane resin, polyether resin, polyamide resin, phenolic resin, silicone resin, fluororesin, and copolymers containing monomers of these resins, with urethane resin being preferred.

[0021] In the ink according to this embodiment, it is preferable that the resin binder b content be 3% by mass or more, from the viewpoint of obtaining high adhesion of the white image to the recording medium. Furthermore, in the ink according to this embodiment, it is preferable that the resin binder b content be 15% by mass or less, from the viewpoint of obtaining high ejection performance from the recording head.

[0022] (Surfactant c) The surfactant c incorporated into the ink according to this embodiment is a highly hydrophilic nonionic surfactant with an HLB value of 10 or higher. In the ink according to this embodiment, since the titanium dioxide fine particles constituting the white pigment a are hydrophilic, using a surfactant c with a high HLB value and high hydrophilicity provides high dispersibility of the white pigment a in the aqueous solvent. As a result, the dispersion state of the white pigment a in the aqueous solvent is more easily maintained over a long period of time in the ink according to this embodiment. Furthermore, because the aggregation of the white pigment a is moderately suppressed, high adhesion of the white image to the recording medium is obtained, as well as high scratch resistance of the white image formed on the recording medium.

[0023] Examples of nonionic surfactants that can be used as surfactant c in the ink according to this embodiment include acetylene glycol surfactants (surfactants containing acetylene glycol compounds), silicone surfactants (surfactants containing silicone compounds), and fluorine surfactants (surfactants containing fluororesins or fluorine-containing compounds). Examples of acetylene glycol surfactants include ethylene oxide adducts of acetylene glycol and propylene oxide adducts of acetylene glycol.

[0024] In the ink according to this embodiment, it is preferable that the surfactant c content be 0.05% by mass or more, from the viewpoint of obtaining high dispersibility of the white pigment a in the aqueous solvent, high adhesion of the white image to the recording medium, and high scratch resistance of the white image formed on the recording medium. Furthermore, in the ink according to this embodiment, it is preferable that the surfactant c content be 0.5% by mass or less, from the viewpoint of discharge performance.

[0025] (water) In the ink according to this embodiment, for example, ion-exchanged water, purified water, or distilled water can be used as water. In the ink according to this embodiment, from the viewpoint of drying properties and ejection reliability, it is preferable that the water content is 52% by mass or more and 75% by mass or less.

[0026] (Other ingredients) The ink according to this embodiment may contain components other than those mentioned above, as needed. For example, the ink according to this embodiment may contain a dispersant that enhances the dispersibility of the white pigment a in the solvent. Examples of dispersants include pigment dispersing resins and surfactants.

[0027] The pigment dispersion resin consists of fine resin particles that adsorb onto the surface of the white pigment a, forming a pigment dispersion together with the white pigment a and improving the dispersibility of the white pigment a in the solvent. Styrene-(meth)acrylic resin is preferred as the pigment coating resin. The styrene-(meth)acrylic resin has repeating units derived from at least one monomer among alkyl (meth)acrylate and (meth)acrylic acid, and styrene units. Examples of alkyl (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, and butyl (meth)acrylate. A copolymer of styrene, methyl methacrylate, methacrylic acid, and butyl acrylate is preferred as the styrene-(meth)acrylic resin.

[0028] The surfactant added as a dispersant is added separately from surfactant c and improves the dispersibility of white pigment a in the solvent by reducing the interfacial tension between white pigment a and the solvent. Examples of such surfactants include nonionic surfactants and anionic surfactants.

[0029] Furthermore, in addition to the dispersant, the ink according to this embodiment may contain various additives as needed, such as water-soluble organic solvents, dissolution stabilizers, drying inhibitors, antioxidants, viscosity modifiers, pH adjusters, neutralizing agents, and antifungal agents.

[0030] [Ink set configuration] An ink set according to one embodiment of the present invention comprises a first ink and a second ink. The first ink and the second ink are stored separately without being mixed and ejected separately by different recording heads. The first ink contains a first white pigment a composed of titanium dioxide, a first resin binder b, a first surfactant c having an HLB value of 10 or more and 11 or less, and water. The second ink contains a second white pigment a composed of titanium dioxide, a second resin binder b, a second surfactant c having an HLB value of 13 or more and 16 or less, and water.

[0031] In the ink set according to this embodiment, both the first ink and the second ink have the same configuration as the ink according to the above embodiment, so that high dispersibility of the white pigment a in aqueous solvent, high adhesion of the white image to the recording medium, and high scratch resistance of the white image formed on the recording medium are obtained. The first white pigment a of the first ink and the second white pigment a of the second ink may have the same configuration or different configurations. Also, the first resin binder b of the first ink and the second resin binder b of the second ink may have the same configuration or different configurations.

[0032] In the ink set according to this embodiment, the first surfactant c of the first ink has a lower HLB value than the second surfactant c of the second ink, meaning that surfactant c has lower hydrophilicity. Therefore, in the ink set according to this embodiment, aggregation of white pigment a is more likely to occur in the first ink than in the second ink. Consequently, in the ink set according to this embodiment, secondary particles and even higher-order particles (tertiary and above) are more likely to be formed in the first ink due to the aggregation of primary particles of white pigment a than in the second ink, and the particle size of the pigment dispersion (pigment particles) composed of white pigment a tends to be larger.

[0033] In the inkjet recording method using the ink set according to this embodiment, a white image is formed on the recording medium by ejecting the first ink onto the recording medium followed by ejecting the second ink onto the recording medium. In other words, in the white image formed on the recording medium with the ink set according to this embodiment, a second white layer formed by the second ink is laminated on top of a first white layer formed by the first ink. In the white image formed with the ink set according to this embodiment, a high opacity is obtained by the first white layer formed by the first ink, which has a large particle size of pigment dispersion. Furthermore, in the white image formed with the ink set according to this embodiment, a high scratch resistance is obtained by the second white layer formed by the second ink, which has a small particle size of pigment dispersion.

[0034] [Inkjet recording device] The inkjet recording apparatus 1 according to this embodiment is configured to implement an inkjet recording method using the ink set according to this embodiment. Figure 1 is a block diagram showing the schematic configuration of the inkjet recording apparatus 1. The inkjet recording apparatus 1 includes a recording unit 2, a circulation unit 3, and a control unit 4. The recording unit 2 has a first recording head 2a and a second recording head 2b. The circulation unit 3 has a first flow path 3a and a second flow path 3b.

[0035] The first recording head 2a has a first nozzle for ejecting first ink onto the recording medium. The second recording head 2b has a second nozzle for ejecting second ink onto the recording medium. When the inkjet recording device 1 performs a recording operation, the control unit 4 controls the recording unit 2 so that the first recording head 2a ejects first ink onto the recording medium, and then the second recording head 2b ejects second ink onto the recording medium.

[0036] In the circulation unit 3, both the first channel 3a and the second channel 3b are configured so that neither passes through the first nozzle of the first recording head 2a nor the second nozzle of the second recording head 2b. The circulation unit 3 is configured to circulate the first ink through the first channel 3a and the second ink through the second channel 3b. When the inkjet recording device 1 is not performing a recording operation, the control unit 4 controls the circulation unit to circulate the first ink through the first channel 3a and the second ink through the second channel 3b.

[0037] In the inkjet recording apparatus 1 according to this embodiment, the dispersion state of the white pigment a in the first ink and the second ink is easily maintained. Therefore, even without providing a separate stirring device for stirring the first ink and the second ink, the white pigment a can be redispersed in the first ink and the second ink simply by circulating the first ink and the second ink by the circulation unit 3.

[0038] The inkjet recording device 1 is not limited to the above configuration and can be modified as needed. For example, the inkjet recording device 1 does not need to have a circulation unit 3. In this case, it is preferable that the inkjet recording device 1 provides another configuration for redispersing the white pigment a in the first ink and the second ink.

[0039] [Examples and Comparative Examples] As examples and comparative examples of the present invention, ink preparation and evaluation, as well as ink set evaluation, were performed.

[0040] (Ink preparation) First, a pigment dispersion was prepared by dispersing white pigment a in water. 75g of pigment dispersion resin (SOLSPERSE® W100, manufactured by Lubrizol, non-volatile content: 40% by mass) was diluted with 425g of deionized water, and 500g of white pigment (JR-804, manufactured by Teika Co., Ltd.) was added. The mixture was then premixed in a homodisperser at a rotation speed of 5000 rpm for 1 hour. Subsequently, the pigment dispersion was obtained by dispersion treatment using a bead mill (manufactured by Nippon Coke Co., Ltd.). In the dispersion treatment, zirconia beads (0.2 mmφ) were packed into the vessel at a filling rate of 80% of the volume. The white pigment a and pigment dispersion resin contained in the pigment dispersion constitute the components of the pigment dispersion in each ink. The water contained in the pigment dispersion constitutes the water component in each ink.

[0041] Next, inks according to the examples and comparative examples were prepared. Pigment dispersion, resin binder b, surfactant c, water-soluble organic solvent d, and water were measured and placed in a beaker. The contents of the beaker were stirred at a rotation speed of 400 rpm using a stirrer (Shinto Kagaku Co., Ltd. "Three One Motor BL-600") to uniformly mix the contents of the beaker and obtain ink. The ink was filtered using a filter (pore size 5 μm) to remove foreign matter and coarse particles contained in the mixture.

[0042] In this example and comparative example, resin binders b1 to b3 shown in Table 1 were used as resin binder b, surfactants c1 to c16 shown in Table 2 were used as surfactant c, and water-soluble organic solvents d1 and d2 shown in Table 3 were used as water-soluble organic solvents d. Note that all products shown as resin binders b1 to b3 are emulsions in which resin binders b1 to b3, composed of urethane resin, are dispersed in water. The water contained in this emulsion constitutes the water component in each ink.

[0043] [Table 1]

[0044] [Table 2]

[0045] [Table 3]

[0046] (Evaluation method) The inks used in the examples and comparative examples were evaluated for their scratch resistance and adhesion to white images, as well as the dispersibility of white pigment a.

[0047] • Method for evaluating the scratch resistance of white images Each ink was applied to a non-absorbent recording medium (PET film "FE2001" manufactured by Futamura Chemical Co., Ltd.) using a bar coater (#02 grit), and then dried at 80°C for 2 minutes to form a white image with a thickness of 6 μm. The white image formed on the recording medium was rubbed once with a weight of 1000 g. The peeling rate, which is the percentage of the white image that peeled off in the entire area rubbed with the weight, was observed visually. The peeling rate of each white image was evaluated according to the following criteria A to D. In terms of abrasion resistance, white images with evaluations of A and B were considered acceptable, and white images with evaluations of C and D were considered unacceptable. A: 0% B: More than 0% and less than 5% C: More than 5% and less than 10% D: More than 10%

[0048] • Method for evaluating the adhesion of white images Each ink was applied to a non-absorbent recording medium (Futamura Chemical Co., Ltd. PET film "FE2001") using a bar coater (No. 02), and then dried at 80°C for 2 minutes to form a 6 μm thick white image. A 5x5 grid pattern was cut into the white image formed on the recording medium at 1 mm intervals using a cutter, and a tape peel test was performed using the cross-cut method. The number of peeled squares in the 25 grid-like white image squares after the tape peel test was counted. The number of peeled squares in each white image was evaluated according to the following criteria A to D. Regarding adhesion, white images with evaluations of A and B were considered acceptable, while white images with evaluations of C and D were considered unacceptable. A:0 pieces B: 1 or more and 4 or less C: 5 or more and 9 or less D: 10 or more

[0049] • Method for evaluating the dispersibility of white pigment a For each ink diluted 100 times, the particle size of the pigment dispersion composed of white pigment a was measured using a Zetasizer (Malvern Panalytical). Subsequently, the particle size of the pigment dispersion composed of white pigment a was measured again for the diluted ink after it had been left to stand for 4 weeks at 60°C. The percentage change in the particle size of the pigment dispersion before and after standing was then calculated for the diluted ink. The percentage change in the particle size of the pigment dispersion for each ink was evaluated according to the following criteria A to D. In terms of dispersibility, inks with evaluations of A and B were considered acceptable, while inks with evaluations of C and D were considered unacceptable. A: Less than 10% B: 10% or more but less than 15% C: 15% or more, less than 20% D: 20% or more

[0050] (Examples 1-15) In Examples 1 to 15, inks were prepared using the above method by varying the types and amounts of resin binder b and surfactant c. Table 4 shows the compositions of the inks in Examples 1 to 14. The type and amount of water-soluble organic solvent d were the same for all inks in Examples 1 to 14.

[0051] [Table 4]

[0052] The inks from Examples 1 to 15 were evaluated for their abrasion resistance and adhesion to white images, as well as the dispersibility of white pigment a. Table 5 shows the evaluation results for the abrasion resistance and adhesion to white images, and the dispersibility of white pigment a, for the inks from Examples 1 to 15. All of the inks from Examples 1 to 15 passed the evaluations for both the abrasion resistance and adhesion to white images, and the dispersibility of white pigment a.

[0053] [Table 5]

[0054] (Comparative Examples 1-6) In Comparative Examples 1 to 6, inks were prepared using the above method, but with different compositions of resin binder b and surfactant c compared to the above examples. Table 6 shows the compositions of the inks for Comparative Examples 1 to 6. In all of the inks for Comparative Examples 1 to 6, the type and amount of water-soluble organic solvent d were the same as those for the inks for Examples 1 to 14. The ink for Comparative Example 1 differs from the ink for the above examples in that it does not contain resin binder b. The inks for Comparative Examples 2 to 5 differ from the inks for the above examples in that they use surfactants c9 to c13 with an HLB value of less than 10.

[0055] [Table 6]

[0056] The inks related to Comparative Examples 1 to 6 were evaluated for their abrasion resistance and adhesion of white images, as well as the dispersibility of white pigment a. Table 7 shows the evaluation results for the abrasion resistance and adhesion of white images, and the dispersibility of white pigment a, for the inks related to Comparative Examples 1 to 6. The ink related to Comparative Example 1 failed in terms of abrasion resistance and adhesion of white images. This is thought to be because the ink related to Comparative Example 1, which does not contain resin binder b, had insufficient adhesion strength of the white image to the recording medium. The inks related to Comparative Examples 2 to 6 all failed in terms of abrasion resistance and adhesion of white images, as well as the dispersibility of white pigment a. This is thought to be because the inks related to Comparative Examples 2 to 6, which have low HLB values ​​of surfactant c, experienced excessive aggregation of white pigment a.

[0057] [Table 7]

[0058] (Ink set evaluation) Inks s1 to s7, which constitute the first and second inks of the ink set, were prepared by the method described above. Table 8 shows the compositions of inks s1 to s7. The type and amount of water-soluble organic solvent d were the same for all inks s1 to s7 as for the inks in Examples 1 to 14.

[0059] [Table 8]

[0060] Ink sets S1 to S10 were prepared by combining the first and second inks shown in Table 9. Table 9 shows the HLB values ​​of the first surfactant c used in the first ink and the second surfactant c used in the second ink for each of the ink sets S1 to S10.

[0061] [Table 9]

[0062] For ink sets S1 to S10, the scratch resistance, adhesion, and opacity of white images were evaluated.

[0063] • Method for evaluating the scratch resistance of white images For each ink set S1 to S10, the first and second inks were sequentially ejected onto the recording medium under conditions of 25°C and 50% RH to form a white image on the recording medium. An image forming apparatus (inkjet recording device with line head, test machine manufactured by Kyocera Document Solutions) was used to form the white image, and a non-absorbent recording medium (PET film "FE2001" manufactured by Futamura Chemical Co., Ltd.) was used as the recording medium. The white image formed on the recording medium was rubbed once with a weight of 1000g. The peeling rate (%), which is the percentage of the white image that was peeled off in the entire area rubbed with the weight, was observed visually. The peeling rate of each white image was evaluated according to the following criteria A to D. A: 0% B: More than 0% and less than 5% C: More than 5% and less than 10% D: More than 10%

[0064] • Method for evaluating the adhesion of white images For each ink set S1 to S10, the first and second inks were sequentially ejected onto the recording medium under conditions of 25°C and 50% RH to form a white image on the recording medium. An image forming apparatus (inkjet recording device with line head, test machine manufactured by Kyocera Document Solutions) was used to form the white image, and a non-absorbent recording medium (PET film "FE2001" manufactured by Futamura Chemical Co., Ltd.) was used as the recording medium. Cuts were made in a 5x5 grid pattern at 1mm intervals in the white image formed on the recording medium using a cutter, and a tape peel test was performed using the cross-cut method. The number of peeled squares out of the 25 grid-like white image squares after the tape peel test was counted. The number of peeled squares in each white image was evaluated according to the following criteria A to D. A:0 pieces B: 1 or more and 4 or less C: 5 or more and 9 or less D: 10 or more

[0065] • Method for evaluating the opacity of white images For each ink set S1 to S10, the first and second inks were sequentially ejected onto the recording medium under conditions of 25°C and 50% RH to form a 150mm x 200mm solid white image on the recording medium. An image forming apparatus (line head-equipped inkjet recording device, Kyocera Document Solutions test machine) was used to form the solid image, and a non-absorbent recording medium (Futamura Chemical Co., Ltd. PET film "FE2001") was used as the recording medium. After the recording medium with the solid image formed was heated for a certain period of time to confirm that the solid image had dried, the recording medium was placed on opacity test paper (black paper) as a base. For the recording medium on the opacity test paper, the K value k0 of the area where the solid image was not formed (base area) and the K value k1 of the area where the solid image was formed (image formation area) were measured using a portable reflectance densitometer RD-19 (Gretag Macbeth Corporation). For each ink set S1 to S10, the opacity was calculated from the obtained K values ​​k0 and k1 using the following formula. Concealment rate (%) = 100 × (1 - k1 / k0) The opacity of each solid image was evaluated according to the following criteria A to C. A: More than 85% B: More than 80% and less than 85% C: 80% or less

[0066] Table 10 shows the evaluation results for the scratch resistance, adhesion, and opacity of white images for ink sets S1 to S10. For ink sets S1 and S2, where the HLB value of the first surfactant c of the first ink is 10 or more and 11 or less, and the HLB value of the second surfactant c of the second ink is 13 or more and 16 or less, the evaluation for the scratch resistance, adhesion, and opacity of white images was A. On the other hand, for ink sets S3 to S10, which do not satisfy at least one of the conditions of the first surfactant c of the first ink being 10 or more and 11 or less, and the HLB value of the second surfactant c of the second ink being 13 or more and 16 or less, at least one evaluation for the scratch resistance, adhesion, and opacity of white images was B, C, or D.

[0067] [Table 10] [Explanation of Symbols]

[0068] 1… Inkjet recording device 2…Records Department 2a...First recording head 2b...Second recording head 3...Circulation section 3a...First channel 3b…Second flow path 4…Control Unit

Claims

1. A white pigment composed of titanium dioxide, Resin binder and Surfactants with an HLB value of 10 or higher, Water and, Ink containing [a specific ingredient / material].

2. The ink according to claim 1, The content of the aforementioned resin binder is 3% by mass or more and 15% by mass or less. ink.

3. A first ink containing a first white pigment composed of titanium dioxide, a first resin binder, a first surfactant having an HLB value of 10 or more and 11 or less, and water. A second ink containing a second white pigment composed of titanium dioxide, a second resin binder, a second surfactant with an HLB value of 13 or more and 16 or less, and water. An ink set including this ink.

4. An inkjet recording method for forming a white image on a recording medium, After discharging a first ink containing a first white pigment composed of titanium dioxide, a first resin binder, a first surfactant with an HLB value of 10 to 11, and water onto the recording medium, a second ink containing a second white pigment composed of titanium dioxide, a second resin binder, a second surfactant with an HLB value of 13 to 16, and water is dispensed onto the recording medium. Inkjet recording method.

5. The inkjet recording method according to claim 4, The recording medium is a non-absorbable recording medium. Inkjet recording method.

6. An inkjet recording device that performs a recording operation to form a white image on a recording medium, A recording unit comprising: a first recording head having a first nozzle for ejecting a first ink containing a first white pigment composed of titanium dioxide, a first resin binder, a first surfactant having an HLB value of 10 to 11, and water onto the recording medium; and a second recording head having a second nozzle for ejecting a second ink containing a second white pigment composed of titanium dioxide, a second resin binder, a second surfactant having an HLB value of 13 to 16, and water onto the recording medium; A control unit controls the recording unit so that, when the recording operation is performed, the first recording head ejects the first ink onto the recording medium, and then the second recording head ejects the second ink onto the recording medium. An inkjet recording device equipped with the following.

7. An inkjet recording apparatus according to claim 6, The recording medium is a non-absorbable recording medium. Inkjet recording device.

8. An inkjet recording apparatus according to claim 6 or 7, The device further comprises a circulation unit having mutually independent first and second flow paths that do not pass through the first and second nozzles, and configured to circulate the first ink through the first flow path and the second ink through the second flow path. The control unit controls the circulation unit to circulate the first ink through the first flow path and the second ink through the second flow path when the recording operation is not being performed. Inkjet recording device.

Images