Ink set and image forming method

Abstract

To provide, about an image formed with an ink, an ink set capable of suppressing deterioration of a color tone balance of an image generated by discoloration due to ultra-violet rays and hardly generating difference in color reproducibility regardless of a degree of uv-rays contained in light emitted from a light source used for observing an image and an image formation method.SOLUTION: The ink set includes four types of ink, yellow ink, magenta ink, cyan ink, and black ink, and each of the four types of ink contains water and a colorant. At least one type of ink contains an ultraviolet absorber, and when, with respect to light of wavelength of 385 nm, the absorbance of the yellow ink is Abs (Y), the absorbance of the magenta ink is Abs (M), the absorbance of the cyan ink is Abs (C), and the absorbance of the black ink is Abs (K), the ratio relative to Abs (K) of each of Abs (Y), Abs (M), and Abs (C) satisfies a predetermined condition.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an ink set and an image forming method. [Background technology]

[0002] The inkjet method, which enables digital printing without using a plate, is used in various printing fields because it allows images to be formed easily and inexpensively.

[0003] There are several types of inks used in the inkjet method, including aqueous inks composed of water and a small amount of organic solvent, non-aqueous inks that contain organic solvents but essentially no water, hot-melt inks that are printed by heating and melting ink that is solid at room temperature, and actinic radiation-curable inks that are cured by irradiation with actinic radiation after printing, and these inks are used according to the application. Of these, aqueous inks generally have little odor and are highly safe.

[0004] Furthermore, in full-color image formation technologies such as inkjet printing, there is a demand for printing technologies that improve color reproducibility, which indicates the size of the color gamut of the formed image. Generally, when forming full-color images with inkjet printing, images are formed based on subtractive color mixing, so images are formed using four basic colors: yellow, magenta, and cyan, which are the three primary colors, plus black. Therefore, there is a demand for technological developments that can improve color reproducibility for images formed using inks with these color tones, as well as for images formed by mixing these color tones.

[0005] For example, Patent Document 1 discloses an ink set containing black ink, cyan ink, magenta ink, and yellow ink, in which the ratio of the maximum absorbance of the cyan ink, magenta ink, and yellow ink to the absorbance of the black ink at the wavelength showing the maximum absorbance is 0.41 or more and 0.80 or less. Patent Document 1 states that this can improve color reproducibility in dark areas of an image. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-50843 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when an image formed using the ink set described in Patent Document 1 is exposed outdoors and absorbs ultraviolet light, the colorants in the inks that make up the image are decomposed by the ultraviolet light, which can cause the color density of the image to decrease over time. Furthermore, because the amount of ultraviolet light absorbed by each ink color tone differs, the degree of fading varies from color to color, which can lead to a loss of color balance within the image.

[0008] Therefore, the inventors tried adding an ultraviolet absorber to the ink and adjusting the amount added for each ink, but this still resulted in different degrees of fading for each color, which could lead to poor color balance within the image.

[0009] It is known that the appearance (visibility) of an image formed using ink or the like varies depending on the type of light source used during observation. In contrast, the ink set described in Patent Document 1 minimizes the difference in absorbance between each color, so it is expected that the visibility of each color will be equal regardless of the type of light source. However, the inventors' investigations revealed that even images produced using the ink set described in Patent Document 1 showed significant variations in visibility depending on the type of light source. Further investigation of the variation in visibility revealed a significant difference between the color tone measured using a D65 light source, which contains ultraviolet light, and the color tone measured using a C light source, which does not contain ultraviolet light. This suggests that the degree (level) of ultraviolet light contained in the light emitted from the light source significantly affects the measured color tone (color reproducibility), which in turn significantly affects the visibility of each color.

[0010] The present invention has been made in consideration of the above circumstances, and aims to provide an ink set and an image forming method that can suppress deterioration in the color balance of an image formed with inks caused by fading due to ultraviolet rays, and that are less likely to cause differences in color reproducibility regardless of the level of ultraviolet rays contained in the light emitted from a light source used to observe the image. [Means for solving the problem]

[0011] An ink set according to one embodiment of the present invention for solving the above-mentioned problems is an ink set comprising four types of ink: yellow ink, magenta ink, cyan ink, and black ink, wherein each of the four types of ink contains water and a colorant, and at least one of the four types of ink contains an ultraviolet absorber, and wherein, when the absorbance of the yellow ink is Abs(Y), the absorbance of the magenta ink is Abs(M), the absorbance of the cyan ink is Abs(C), and the absorbance of the black ink is Abs(K) for light having a wavelength of 385 nm, the four types of ink have R(Y,K), R(M,K), and R(C,K), which are expressed by formulas (1) to (3), satisfying conditions (a) and (b). R(Y, K) = Abs(Y) / Abs(K) (1) R(M, K) = Abs(M) / Abs(K) (2) R(C, K) = Abs(C) / Abs(K) (3) Condition (a) R(Y, K), R(M, K), and R(C, K) are all between 0.2 and 1.0. Condition (b) The difference between any absorbance ratios selected from R(Y, K), R(M, K), and R(C, K) is 0.5 or less.

[0012] Furthermore, an image forming method according to one embodiment of the present invention for solving the above-mentioned problems is an image forming method using four types of ink: yellow ink, magenta ink, cyan ink, and black ink, the image forming method comprising the steps of: applying any one of the yellow ink, the magenta ink, and the cyan ink to the surface of a recording medium; and applying any other of the yellow ink, the magenta ink, and the cyan ink to the surface of the recording medium, wherein each of the four types of ink contains water and a colorant, and at least one of the four types of ink contains an ultraviolet absorber, and wherein, for light having a wavelength of 385 nm, the absorbance of the yellow ink is Abs(Y), the absorbance of the magenta ink is Abs(M), the absorbance of the cyan ink is Abs(C), and the absorbance of the black ink is Abs(K), and the four types of ink satisfy conditions (a) and (b) in the formulas (1) to (3). R(Y, K) = Abs(Y) / Abs(K) (1) R(M, K) = Abs(M) / Abs(K) (2) R(C, K) = Abs(C) / Abs(K) (3) Condition (a) R(Y, K), R(M, K), and R(C, K) are all between 0.2 and 1.0. Condition (b) The difference between any absorbance ratios selected from R(Y, K), R(M, K), and R(C, K) is 0.5 or less. [Effects of the Invention]

[0013] The present invention provides an ink set and an image forming method that can suppress deterioration in the color balance of an image formed with inks caused by fading due to ultraviolet light, and that are less likely to cause differences in color reproducibility regardless of the level of ultraviolet light contained in the light emitted from a light source used to observe the image. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 shows the configuration of an image forming apparatus according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following embodiments.

[0016] 1. Ink set An ink set according to one embodiment of the present invention for solving the above-mentioned problems is an ink set comprising four types of ink: yellow ink, magenta ink, cyan ink, and black ink, wherein each of the four types of ink contains water and a colorant, and at least one of the four types of ink contains an ultraviolet absorber, and wherein, when the absorbance of the yellow ink is Abs(Y), the absorbance of the magenta ink is Abs(M), the absorbance of the cyan ink is Abs(C), and the absorbance of the black ink is Abs(K) for light having a wavelength of 385 nm, the four types of ink have R(Y,K), R(M,K), and R(C,K), which are expressed by formulas (1) to (3), satisfying conditions (a) and (b). R(Y, K) = Abs(Y) / Abs(K) (1) R(M, K) = Abs(M) / Abs(K) (2) R(C, K) = Abs(C) / Abs(K) (3) Condition (a) R(Y, K), R(M, K), and R(C, K) are all greater than or equal to 0.2 and less than or equal to 1.0. Condition (b) The difference between any absorbance ratios selected from R(Y, K), R(M, K), and R(C, K) is 0.5 or less.

[0017] As described above, even when an image is formed using inks that improve color reproducibility, as in Patent Document 1, the image does not have sufficient resistance to ultraviolet light, and the image absorbs ultraviolet light, causing photodecomposition of the colorant, resulting in fading over time. Since the absorbance of ultraviolet light varies depending on the color tone of each ink, the degree of fading due to ultraviolet light varies, which can lead to a poor balance of color tones within the image.

[0018] Therefore, the present inventors have considered adding an ultraviolet absorber to inks of fading colors to improve their resistance to ultraviolet light. However, even with this approach, it has sometimes been impossible to achieve a good color balance in images after storage. The present inventors' investigations into this issue have revealed that when ultraviolet light is absorbed by an ultraviolet absorber, the ultraviolet energy is converted into thermal energy within the image, and the generated heat may cause decomposition of the colorant within the image. As a result, fading due to thermal decomposition of the colorant can make it even more difficult to adjust the color balance within the image.

[0019] Furthermore, as mentioned above, the measured color tone (color reproducibility) can change significantly depending on the level (degree) of ultraviolet light contained in the light emitted from the light source.

[0020] In order to solve the above problems, the present inventors conducted extensive research and found that by adding an ultraviolet absorber to inks whose main color tones are yellow, magenta, cyan, and black, the degree of fading due to thermal decomposition can be made uniform for each color while adjusting the absorbance at a wavelength of 385 nm, which is included in the ultraviolet region, thereby achieving a good balance of color tones within an image. In other words, the present inventors found that even for inks that originally have low absorbance to ultraviolet light, adding an ultraviolet absorber to adjust the absorbance of each ink can achieve a good balance of color tones within an image.

[0021] Specifically, as described above, when the absorbance of yellow ink is Abs(Y), the absorbance of magenta ink is Abs(M), the absorbance of cyan ink is Abs(C), and the absorbance of black ink is Abs(K), R(Y, K), R(M, K), and R(C, K) represented by equations (1) to (3) were all adjusted to be 0.2 or greater and 1.0 or less, and the maximum difference between any two values ​​of R(Y, K), R(M, K), and R(C, K) was adjusted to be 0.5 or less.

[0022] R(Y, K) = Abs(Y) / Abs(K) (1) R(M, K) = Abs(M) / Abs(K) (2) R(C, K) = Abs(C) / Abs(K) (3)

[0023] In this specification, "yellow ink" refers to an ink that can form a yellow image when applied alone to a recording medium. "Magenta ink" refers to an ink that can form a magenta image when applied alone to a recording medium. "Cyan ink" refers to an ink that can form a cyan image when applied alone to a recording medium. "Black ink" refers to an ink that can form a black image when applied alone to a recording medium. These inks are so-called water-based inks that contain water as a liquid component, and the above ink set is a so-called water-based ink set. However, as long as the problem of the present invention is solved, the type and content of water and other liquid components in each ink are not particularly limited.

[0024] Furthermore, the inventors have found that images formed using these inks can reduce the difference in color reproducibility (hereinafter simply referred to as the difference in color reproducibility) caused by the degree (degree) of ultraviolet light contained in the light source used to observe the image. The reason why the difference in color reproducibility can be reduced by adjusting the absorbance of each ink is thought to be as follows.

[0025] When an image is viewed using a light source that includes ultraviolet light, if the absorbance of each ink to ultraviolet light differs, the absorbance of each ink to light on the short wavelength side of the visible light range, close to the ultraviolet range, may differ. Therefore, when an image is viewed using a light source that includes ultraviolet light, the intensity of reflected light differs for each ink, which may result in different color reproducibility. Therefore, by absorbing light with an ultraviolet wavelength of 385 nm and adjusting the absorbance of each ink to light of this wavelength to the same level, it is possible to make the absorbance of each ink to the short wavelength side light the same. By making the absorbance of each ink to the short wavelength side light the same, it is possible to improve color reproducibility regardless of the level of ultraviolet light contained in the light source.

[0026] The values ​​of R(Y, K), R(M, K), and R(C, K) are all 0.2 or greater and 1.0 or less, but preferably 0.5 or greater and 0.8 or less. By ensuring that R(Y, K), R(M, K), and R(C, K) are each 0.2 or greater, the difference between the maximum and minimum values ​​of R(Y, K), R(M, K), and R(C, K) can be reduced. This, as described below, is believed to reduce the difference in the degree of fading due to thermal decomposition of colorants caused by ultraviolet light absorption, thereby suppressing deterioration in color balance and reducing differences in color reproducibility depending on whether or not ultraviolet light is present in the light source. Furthermore, by ensuring that the values ​​are 1.0 or less, thermal decomposition of colorants in the image due to excessive ultraviolet light absorption can be suppressed, thereby suppressing image fading.

[0027] The absorbance of each ink was measured using a spectrophotometer (U-3300, manufactured by Hitachi, Ltd.). Specifically, each ink was diluted 4000 times with pure water and placed in a quartz glass cell with an optical path length of 10 mm. Scanning was performed at wavelengths from 340 nm to 800 nm in increments of 5 nm or less, and the absorbance at a wavelength of 385 nm in the resulting absorption spectrum was measured. Pure water was used as a reference.

[0028] Furthermore, by ensuring that the R(Y, K), R(M, K), and R(C, K) values ​​are within the above-mentioned ranges and that the difference between the maximum and minimum values ​​does not exceed 0.5, the difference in UV absorbance between the inks is reduced. This is believed to reduce the difference in the degree of discoloration due to thermal decomposition of the colorant caused by UV absorption, thereby suppressing deterioration in color balance. Furthermore, by reducing the difference in UV absorbance between the inks, the absorbance of each ink for light on the short wavelength side of the visible light range can be made approximately the same, thereby reducing differences in color reproducibility.

[0029] The difference between the maximum and minimum values ​​of R(Y, K), R(M, K), and R(C, K) does not exceed 0.5, but is preferably 0.1 to 0.5, and more preferably 0.1 to 0.3. When it is 0.3 or less, the difference in the degree of fading due to thermal decomposition of the colorant caused by absorption of ultraviolet light can be further reduced.

[0030] In this embodiment, at least one of the yellow ink, magenta ink, cyan ink, and black ink contains an ultraviolet absorber. This can suppress photodecomposition of the colorant after image formation. The content of the ultraviolet absorber in the ink containing the ultraviolet absorber is not particularly limited, but is preferably 0.1% by mass or more and 3.0% by mass or less, more preferably 0.1% by mass or more and 2.0% by mass or less, and even more preferably 0.5% by mass or more and 2.0% by mass or less, relative to the total mass of the ink containing the ultraviolet absorber. A content of 0.1% by mass or more can suppress photodecomposition of the colorant due to ultraviolet light, thereby further suppressing image fading. A content of 3.0% by mass or less can sufficiently prevent the color tone of the formed image from shifting toward yellow, which is the complementary color of the wavelength absorbed by the ultraviolet absorber, thereby improving color reproducibility. Furthermore, thermal decomposition of the colorant in the image due to excessive ultraviolet light absorption can be suppressed, thereby suppressing image fading.

[0031] There are no particular limitations on the method for adjusting R(Y, K), R(M, K), and R(C, K) to fall within the ranges defined by condition (a), but from the viewpoint of facilitating the color balance of an image, it is preferable to adjust the content of the ultraviolet absorber in the ink set for each ink. For example, the ultraviolet (wavelength 385 nm) absorbance of each ink is measured when none of the yellow ink, magenta ink, cyan ink, and black ink contains an ultraviolet absorber, and then the content of the ultraviolet absorber is adjusted so that R(Y, K), R(M, K), and R(C, K) fall within the desired ranges.

[0032] In an ink set, the order of the UV absorber content is preferably magenta ink, cyan ink, yellow ink, and black ink. The absorbance of the magenta colorant to UV rays is lower than that of the yellow colorant and cyan colorant. Therefore, by adding more UV absorbent to the magenta ink than to the other inks, the absorbance of the magenta ink can be increased, reducing the difference with the absorbance of the other inks, and improving the balance of the image color tone. From the same perspective, it is also preferable to adjust the UV absorber content of the other inks so that the UV absorber content in the ink set is in the above order. Details of the preferred range of the UV absorber content in each ink will be described later.

[0033] 1-1. Yellow ink As described above, yellow ink refers to ink that can form an image that exhibits a yellow color when applied alone to a recording medium.

[0034] 1-1-1.Water The yellow ink contains water. The type of water contained in the yellow ink is not particularly limited. The water content in the yellow ink is not particularly limited, but is preferably 50% by mass or more relative to the total mass of the yellow ink. By having a water content of 50% by mass or more, the ink can be made fluid and its ejection properties can be sufficiently improved. From the above viewpoints, the water content is preferably 50% by mass or more relative to the total mass of the liquid components in the ink.

[0035] 1-1-2. Coloring agents The yellow ink contains a colorant. The colorant contained in the yellow ink is not particularly limited and may be, for example, a dye or a pigment. From the viewpoint of improving the water resistance and light resistance of the formed image, the colorant is preferably a pigment.

[0036] Yellow ink usually contains a yellow colorant, but may contain a colorant having a color tone other than yellow to produce a yellow color. In this specification, "yellow colorant" refers to a yellow dye or a yellow pigment.

[0037] Examples of yellow dyes include CI Acid Yellow 7:1, 17, 19, 23, 25, 29, 38, 42, 49, 61, 72, 78, 110, 127, 135, 141, and 142 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0038] Examples of yellow pigments include CI Pigment Yellow 12, CI Pigment Yellow 13, 14, 15, 15:3, 17, 74, 93, 128, 94, 138, and 155 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0039] When the yellow ink contains a pigment, it preferably contains a pigment dispersant. The content of the pigment dispersant is not limited, but is preferably 0.1% by mass or more and 10.0% by mass or less relative to the total mass of the yellow ink. The content of the pigment dispersant is more preferably 0.5% by mass or more and 5.0% by mass or less.

[0040] Examples of pigment dispersants include DISPERBYK190, DISPERBYK2164, DISPERBYK168, DISPERBYK N22024 (all manufactured by BYK-Chemie), and the like.

[0041] The particle size of the yellow pigment particles is not particularly limited, but from the viewpoint of further improving color reproducibility, it is preferable that the volume-based median diameter is 50 nm or more and 200 nm or less. The particle size of the pigment can be determined, for example, by a commercially available particle size measuring device using a dynamic light scattering method or an electrophoresis method. From the viewpoint of simple and highly accurate measurement, measurement by the dynamic light scattering method is preferred.

[0042] The content of the colorant in the yellow ink is not particularly limited, but is preferably 0.5% by mass or more and 10% by mass or less relative to the total mass of the yellow ink. A content of 0.5% by mass or more can further improve color reproducibility, and a content of 10% by mass or less can further increase the dispersion stability of the colorant in the ink.

[0043] 1-1-3. UV absorbers In this embodiment, the yellow ink may contain an ultraviolet absorber.

[0044] The type of ultraviolet absorber is not particularly limited. Examples of ultraviolet absorbers include benzophenone-based ultraviolet absorbers, benzotriazole-based ultraviolet absorbers, benzoate-based ultraviolet absorbers, and triazine-based ultraviolet absorbers.

[0045] Commercially available examples of benzophenone-based UV absorbers include ADK STAB 1413 (manufactured by ADEKA CORPORATION, "ADK STAB" is a registered trademark of the company), SEESORB 100, 101, 101S, 102, 103, 106, and 107 (all manufactured by Shipro Chemical Co., Ltd., "SEESORB" is a registered trademark of the company), Sumisorb 130 (manufactured by Sumika Chemtex Co., Ltd., "Sumisorb" is a registered trademark of Sumitomo Chemical Co., Ltd.), and KEMISORB 10, 11, 11S, and 12 (all manufactured by Chemipro Chemical Co., Ltd., "KEMISORB" is a registered trademark of the company).

[0046] Commercially available examples of benzotriazole-based ultraviolet absorbers include Tinuvin 109, 171, 234, 326, 327, 329, 360, and 928 (all manufactured by BASF Japan Ltd., and "Tinuvin" is a registered trademark of BASF), SEESORB 701, 703, 704, 706, 707, and 709 (all manufactured by Shipro Chemical Co., Ltd.), Sumisorb 200, 250, 300, 340, and 350 (all manufactured by Sumika Chemtex Co., Ltd.), and KEMISORB 71, 73, 74, 79, and 279 (all manufactured by Chemipro Chemical Co., Ltd.).

[0047] Examples of benzoate-based ultraviolet absorbers include Tinuvin 120 (manufactured by BASF Japan), SEESORB 712 (manufactured by Shipro Chemical Co., Ltd.), Sumisorb 400 (manufactured by Sumika Chemtex Co., Ltd.), KEMISORB 112, 113, 113 (all manufactured by Chemipro Chemical Co., Ltd.), and the like.

[0048] Examples of commercially available triazine-based ultraviolet absorbers include Tinuvin 477-DW(N) (manufactured by BASF Japan Ltd.) and KEMISORB102 (manufactured by Chemipro Chemical Co., Ltd.).

[0049] In this embodiment, the yellow ink preferably contains an ultraviolet absorber contained in resin particles. By including an ultraviolet absorber in the resin particles, aggregation of the ultraviolet absorber in the ink can be sufficiently suppressed, even when an ultraviolet absorber with low water solubility is used. This can sufficiently improve the ejection stability of the ink. Furthermore, by including an ultraviolet absorber in the resin particles, the heat generated when ultraviolet rays are absorbed is less likely to reach the colorant, thereby suppressing fading due to thermal decomposition of the colorant. From the above perspective, it is preferable that the ultraviolet absorber be encapsulated inside the resin particles. The resin particles may also be used as a fixing resin.

[0050] The material of the resin particles is not particularly limited. Examples of the material of the resin particles include acrylic resin, styrene resin, styrene-acrylic resin, urethane resin, urethane-acrylic resin, etc. From the viewpoint of ejection stability and fixation, the material of the resin particles is preferably acrylic resin, styrene resin, or styrene-acrylic resin. These have high heat resistance to the extent that they do not impair fixation, so that the resin softens during ejection and is prevented from adhering to the ink nozzle, thereby improving ejection stability. The ultraviolet absorber may be contained in only one type of resin particle made of these resins, or in two or more types of resin particles.

[0051] From the viewpoint of improving dispersibility in ink, the material of the resin particles is preferably a hydrophilic resin. Examples of hydrophilic resins include acrylic resins, urethane resins, and urethane-acrylic resins. In this specification, "hydrophilic resin" refers to a resin having a solubility of 10 g or more in water at 25°C. The hydrophilic resin may also be a resin that has been hydrophilized by introducing a hydrophilic group during polymerization.

[0052] The particle size of the resin particles containing the ultraviolet absorber is not particularly limited, but is preferably 50 nm or more and 200 nm or less. The particle size of the resin particles can be determined, for example, by a commercially available particle size measuring device using a dynamic light scattering method or an electrophoresis method. From the viewpoint of simple and highly accurate measurement, measurement by the dynamic light scattering method is preferred.

[0053] The method for incorporating the ultraviolet absorber into the resin particles is not particularly limited. For example, by using an ultraviolet absorber polymerizable with a monomer constituting the resin, an emulsifier, and a polymerization initiator, the ultraviolet absorber undergoes a polymerization reaction with the monomer, and the ultraviolet absorber is incorporated into the resin particles.

[0054] The type of the emulsifier is not particularly limited, and the surfactants described below can be used.

[0055] The type of the polymerization initiator is not particularly limited, but any known oil-soluble or water-soluble polymerization initiator can be used.

[0056] Examples of the oil-soluble polymerization initiator include azo-based or diazo-based polymerization initiators and peroxide-based polymerization initiators.

[0057] Examples of the azo or diazo polymerization initiator include 2,2'-azobis-(2,4-dimethylvaleronitrile), 2,2'-azobisisobutyronitrile, 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis-4-methoxy-2,4-dimethylvaleronitrile, and azobisisobutyronitrile.

[0058] These include benzoyl peroxide, methyl ethyl ketone peroxide, diisopropyl peroxycarbonate, cumene hydroperoxide, t-butyl hydroperoxide, di-t-butyl peroxide, dicumyl peroxide, 2,4-dichlorobenzoyl peroxide, lauroyl peroxide, 2,2-bis-(4,4-t-butylperoxycyclohexyl)propane, and tris-(t-butylperoxy)triazine.

[0059] Examples of the water-soluble polymerization initiator include persulfates such as potassium persulfate and ammonium persulfate, azobisaminodipropane acetate, azobiscyanovaleric acid and its salts, and hydrogen peroxide.

[0060] Examples of the reactive ultraviolet absorber include 2(2'-hydroxy-5'-methacryloxyethylphenyl)-2H-benzotriazole (RUVA-93, manufactured by Otsuka Chemical Co., Ltd.) and DAINSORB T-31 (trade name, manufactured by Daiwa Kasei Co., Ltd.).

[0061] Alternatively, the ink may contain a commercially available product containing an ultraviolet absorber in its resin particles, such as Tinuvin 9945-DW(N), Tinuvin 400-DW(N), Tinuvin 477-DW(N), Tinuvin 479-DW(N), Tinuvin 123-DW(N), or Tinuvin 5333-DW(N) (all manufactured by BASF Japan).

[0062] The content of the resin particles is not particularly limited, but is preferably 0.5% by mass or more and 15% by mass or less relative to the total mass of the yellow ink. When the content is 0.5% by mass or more, the amount of resin particles containing an ultraviolet absorber can be increased, further improving the lightfastness of the image. When the content is 15% by mass or less, thickening of the ink can be suppressed, further improving the ejection stability of the ink. Furthermore, by being within the above range, it is possible to easily adjust the difference between the maximum and minimum values ​​of R(Y, K), R(M, K), and R(C, K) to a desired range, further reducing differences in color reproducibility and making it easier to match the degree of fading with other inks.

[0063] When the yellow ink contains an ultraviolet absorber, the content of the ultraviolet absorber relative to the total mass of the yellow ink is not particularly limited, but is preferably 0.1% by mass to 3.0% by mass, and more preferably 0.1% by mass to 1.0% by mass. By setting the content of the ultraviolet absorber within the above range, it is possible to easily adjust the difference between the maximum and minimum values ​​of R(Y,K), R(M,K), and R(C,K) to the desired range, thereby further reducing differences in color reproducibility and making it easier to match the degree of fading with other inks. The content of the ultraviolet absorber can be calculated from the area percentage of the component after identifying the component and obtaining a high-performance liquid chromatography (HPLC) calibration curve for that substance.

[0064] The content of the ultraviolet absorber contained in the resin particles is not particularly limited, but is preferably 1% by mass or more and 20% by mass or less as an active ingredient per resin particle. At 1% by mass or more, a larger amount of ultraviolet absorber can be contained in the yellow ink while suppressing aggregation of the ultraviolet absorber, thereby sufficiently suppressing fading due to photodecomposition of the coloring agent. At 20% by mass or less, precipitation of the ultraviolet absorber can be suppressed. The content of ultraviolet absorber per resin particle can be determined, for example, by melting multiple resin particles, determining the amount of ultraviolet absorber contained therein by high-performance liquid chromatography (HPLC), and dividing the amount by the number of resin particles measured with a particle counter.

[0065] 1-1-4. Organic solvents In this embodiment, the yellow ink may contain an organic solvent. The type of organic solvent is not particularly limited, but from the viewpoint of increasing compatibility with water, a water-soluble organic solvent is preferable. In this specification, "water-soluble organic solvent" refers to an organic solvent having a solubility of 5 g or more in water at 25°C. Examples of water-soluble organic solvents include alcohols, polyhydric alcohols, amines, amides, glycol ethers, and 1,2-alkanediols having 4 or more carbon atoms. These organic solvents may be contained alone or in combination of two or more.

[0066] Examples of the alcohols include methanol, ethanol, propanol, isopropanol, butanol, isobutanol, secondary butanol, and tertiary butanol.

[0067] Examples of the polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycols having 5 or more ethylene oxide groups, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycols having 4 or more propylene oxide groups, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, and thiodiglycol.

[0068] Examples of the amines include ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, and tetramethylpropylenediamine.

[0069] Examples of the amides include formamide, N,N-dimethylformamide, N,N-dimethylacetamide, and the like.

[0070] Examples of the glycol ethers include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, and tripropylene glycol monomethyl ether.

[0071] Examples of the 1,2-alkanediols having 4 or more carbon atoms include 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, and 1,2-heptanediol.

[0072] Among these, when the organic solvent is a polyhydric alcohol, bleeding during high-speed printing can be suitably suppressed. Preferred examples of polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol.

[0073] The content of the organic solvent in the yellow ink is preferably 5.0% by mass or more and 30% by mass or less, based on the total mass of the yellow ink. A content of 5.0% by mass or more can improve the wettability of the ink to a recording medium, while a content of 30% by mass or less can make it easier to adjust the ink viscosity so as to prevent a decrease in the ink ejection stability, and can prevent a decrease in the drying speed of the ink after it lands on a recording medium.

[0074] In this embodiment, the ratio of water and organic solvents in the liquid components of the ink is preferably 85% by mass or more, and more preferably 90% by mass or more.

[0075] 1-1-5.Other Depending on the purpose, the yellow ink may contain fixing resins, surfactants, pH adjusters, oil droplet fine particles, fluorescent brighteners, polysaccharides, viscosity adjusters, resistivity adjusters, film-forming agents, antioxidants, antifungal agents, anticorrosive agents, etc. Only one of these components may be contained, or two or more may be contained in combination.

[0076] The fixing resin is preferably a water-soluble resin or a water-dispersible resin from the viewpoint of enhancing compatibility with the ink. Examples of fixing resins include acrylic resins, urethane resins, urethane-acrylic resins, polyester resins, polyamine resins, polyvinyl alcohol, and water-dispersible latex resins.

[0077] These resins may have a crosslinkable group, examples of which include an amine group, a urethane bond, a urea bond, a polyol in which adjacent carbon molecules have hydroxyl groups, and a carboxyl group.

[0078] When the fixing resin has a crosslinkable group, the ink may contain a crosslinking agent. Examples of the crosslinking agent include carbodiimide compounds, isocyanate compounds, epoxy compounds, silyl compounds, hydrazine compounds, and oxazoline compounds.

[0079] Examples of the surfactant include anionic surfactants such as dialkyl sulfosuccinates, alkyl naphthalene sulfonates, and fatty acid salts; nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene-polyoxypropylene block copolymers; cationic surfactants such as alkylamine salts and quaternary ammonium salts; and silicone-based and fluorine-based surfactants.

[0080] Examples of the pH adjuster include known acids, bases, and buffers. Among these, ammonia, monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, methylethylamine, monoethanolamine, diethanolamine, triethanolamine, ethylenediamine, methylaminoethanol, and dimethylaminoethanol, as well as salts thereof, are preferred because they are less likely to inhibit the reaction between the crosslinked resin and the crosslinking agent.

[0081] 1-1-6.Physical Properties From the viewpoint of improving the ejection stability of the yellow ink from the nozzles of the inkjet head, the viscosity of the yellow ink is preferably 1 cP or more and less than 100 cP. From the viewpoint of further improving the ejection stability, the viscosity of the yellow ink is preferably 1 cP or more and 50 cP or less, and more preferably 1 cP or more and 15 cP or less.

[0082] From the viewpoint of improving ejection stability from the nozzles of the inkjet head, the surface tension of the yellow ink is preferably 20 mN / m or more and 50 mN / m or less. From the viewpoint of improving wettability to the substrate and forming higher-resolution images, the surface tension of the yellow ink is more preferably 20 mN / m or more and 35 mN / m or less. The surface tension of the yellow ink can be adjusted to fall within the above range by changing the types or amounts of the surfactant and organic solvent.

[0083] The absorbance Abs(Y) of the yellow ink for light with a wavelength of 385 nm is not particularly limited as long as it satisfies formula (1), but is preferably 0.5 to 0.9, and more preferably 0.6 to 0.8. A value of 0.5 or greater reduces the difference between the maximum and minimum values ​​of R(Y,K), R(M,K), and R(C,K), making it possible to further reduce differences in color reproducibility, while a value of 0.9 or less can further suppress thermal decomposition of the colorant due to excessive absorption of ultraviolet light.

[0084] In formula (1), the value of R(Y, K) is 0.2 or more and 1.0 or less, preferably 0.5 or more and 1.0 or less, and more preferably 0.7 or more and 1.0 or less. When the value of R(Y, K) is within the above range, the difference in color reproducibility can be further reduced and deterioration of color balance due to fading can be suppressed.

[0085] 1-1-7. Preparation of yellow ink The yellow ink can be prepared by mixing the above-mentioned components.

[0086] When a pigment is used as the colorant, a pigment dispersion containing the pigment and a pigment dispersant may be prepared in advance, and this may then be mixed with the remaining components to prepare the yellow ink. The pigment can be dispersed using, for example, a ball mill, a sand mill, an attritor, a roll mill, an agitator, a Henschel mixer, a colloid mill, an ultrasonic homogenizer, a pearl mill, a wet jet mill, or a paint shaker.

[0087] 1-2. Magenta ink As described above, the magenta ink is an ink that can form an image that exhibits a magenta color when applied alone to a recording medium.

[0088] 1-2-1.Water The magenta ink contains water. The type and amount of water contained in the magenta ink can be the same as those described for the yellow ink, so a detailed description will be omitted.

[0089] 1-2-2. Coloring agents The magenta ink contains a colorant. The colorant contained in the magenta ink is not particularly limited and may be, for example, a dye or a pigment. From the viewpoint of improving the water resistance and light resistance of the formed image, the colorant is preferably a pigment.

[0090] Magenta ink typically contains a magenta colorant, but may also contain a colorant having a hue other than magenta to produce a magenta color. In this specification, the term "magenta colorant" refers to a magenta dye or magenta pigment.

[0091] Examples of magenta dyes include CI Acid Red 8, 9, 14, 18, 26, 27, 35, 37, 51, 57, 82, 87, 92, 94, 111, 129, 131, 138, 186, 249, 254, 265, and 276 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0092] Examples of magenta pigments include CI Pigment Red 2, CI Pigment Red 3, 5, 6, 7, 15, 16, 48;1, 53;1, 57;1, 122, 123, 139, 144, 149, 150, 166, 177, 178, 184, 222, 238, and the like.

[0093] When the magenta ink contains a pigment, it preferably contains a pigment dispersant. The content of the pigment dispersant is not limited, but is preferably 0.1% by mass or more and 10.0% by mass or less relative to the total mass of the magenta ink. The content of the pigment dispersant is more preferably 0.5% by mass or more and 5.0% by mass or less.

[0094] Examples of pigment dispersants include DISPERBYK190, DISPERBYK2164, DISPERBYK168, DISPERBYK N22024 (all manufactured by BYK-Chemie), and the like.

[0095] The pigment can be dispersed using, for example, a ball mill, sand mill, attritor, roll mill, agitator, Henschel mixer, colloid mill, ultrasonic homogenizer, pearl mill, wet jet mill, paint shaker, or the like.

[0096] The particle size of the magenta pigment particles is not particularly limited, but from the viewpoint of further improving color reproducibility, it is preferable that the volume-based median diameter is 50 nm or more and 200 nm or less. The particle size of the pigment can be determined, for example, by a particle size measuring device using dynamic light scattering or electrophoresis. From the viewpoint of simple and highly accurate measurement, measurement by dynamic light scattering is preferred.

[0097] The content of the colorant in the magenta ink is not particularly limited, but is preferably 0.5% by mass or more and 10% by mass or less relative to the total mass of the magenta ink. A content of 0.5% by mass or more can further improve color reproducibility, and a content of 10% by mass or less can further increase the dispersion stability of the colorant in the ink.

[0098] 1-2-3. UV absorbers The magenta ink may contain an ultraviolet absorber. The type and content of the ultraviolet absorber contained in the magenta ink may be the same as those described for the yellow ink.

[0099] The ultraviolet absorber is preferably contained in resin particles. The type, particle size, and content of the ultraviolet absorber per resin particle can be the same as those described for the yellow ink.

[0100] When the magenta ink contains an ultraviolet absorber, the content of the ultraviolet absorber relative to the total mass of the magenta ink is not particularly limited, but is preferably from 0.1 to 3.0% by mass, more preferably from 1.0 to 3.0% by mass, and even more preferably from 1.2 to 2.5% by mass. The content of the ultraviolet absorber makes it easy to adjust the maximum difference between any two values ​​of R(Y,K), R(M,K), and R(C,K) to a desired range, thereby reducing differences in color reproducibility and making it easier to match the degree of fading with other inks.

[0101] 1-2-4. Organic solvents The magenta ink may contain an organic solvent. The type and amount of the organic solvent contained in the magenta ink can be the same as those described for the yellow ink, so a detailed description will be omitted.

[0102] 1-2-5.Other The other components contained in the magenta ink may be the same as the other components contained in the yellow ink, and may be appropriately contained depending on the purpose.

[0103] 1-2-6.Physical Properties The viscosity and surface tension of the magenta ink can be similar to those described for the yellow ink, and therefore a detailed description will be omitted.

[0104] The absorbance Abs(M) of the magenta ink for light with a wavelength of 385 nm is not particularly limited as long as it satisfies formula (1), but is preferably 0.3 to 0.8, and more preferably 0.3 to 0.7. A value of 0.3 or greater reduces the difference between the maximum and minimum values ​​of R(Y,K), R(M,K), and R(C,K), making it possible to further reduce differences in color reproducibility, while a value of 0.8 or less makes it possible to further suppress thermal decomposition of the colorant due to excessive absorption of ultraviolet light.

[0105] In formula (1), the value of R(M, K) is 0.2 or more and 1.0 or less, preferably 0.4 or more and 1.0 or less, and more preferably 0.4 or more and 0.9 or less. When the value of R(M, K) is within the above range, the difference in color reproducibility can be further reduced, and deterioration of color balance due to fading can be suppressed.

[0106] 1-2-7. Preparation of magenta ink The preparation of the magenta ink can be similar to that described for the yellow ink, and therefore a detailed description thereof will be omitted.

[0107] 1-3. Cyan ink As described above, the cyan ink is an ink that can form an image that exhibits a cyan color when applied alone to a recording medium.

[0108] 1-3-1.Water Cyan ink contains water. The type and amount of water contained in cyan ink can be the same as those described for yellow ink, so a detailed description will be omitted.

[0109] 1-3-2. Coloring agents The cyan ink contains a colorant. The colorant contained in the cyan ink is not particularly limited and may be, for example, a dye or a pigment. From the viewpoint of improving the water resistance and light resistance of the formed image, the colorant is preferably a pigment.

[0110] Cyan ink typically contains a cyan colorant, but may contain a colorant having a hue other than cyan to produce a cyan color. In this specification, the term "cyan colorant" refers to a cyan dye or a cyan pigment.

[0111] Examples of cyan dyes include CI Acid Blue 1, 7, 9, 15, 22, 23, 25, 40, 41, 43, 62, 78, 83, 90, 93, 103, 112, 113, and 158 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0112] Examples of cyan pigments include CI Pigment Blue 15, 15;2, 15;3, 15;4, 16, 60, 62, and 66 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0113] When the cyan ink contains a pigment, it preferably contains a pigment dispersant. The content of the pigment dispersant is not limited, but is preferably 0.1% by mass or more and 10.0% by mass or less relative to the total mass of the cyan ink. The content of the pigment dispersant is more preferably 0.5% by mass or more and 5.0% by mass or less.

[0114] Examples of pigment dispersants include DISPERBYK190, DISPERBYK2164, DISPERBYK168, DISPERBYK N22024 (all manufactured by BYK-Chemie), and the like.

[0115] The pigment can be dispersed using, for example, a ball mill, sand mill, attritor, roll mill, agitator, Henschel mixer, colloid mill, ultrasonic homogenizer, pearl mill, wet jet mill, paint shaker, or the like.

[0116] The particle size of the cyan pigment particles is not particularly limited, but from the viewpoint of further improving color reproducibility, the volume-based median diameter is preferably 50 nm or more and 200 nm or less. The particle size of the pigment can be determined, for example, by a particle size measuring device using dynamic light scattering or electrophoresis. From the viewpoint of simple and highly accurate measurement, measurement by dynamic light scattering is preferred.

[0117] The content of the colorant in the cyan ink is not particularly limited, but is preferably 0.5% by mass or more and 10% by mass or less relative to the total mass of the cyan ink. A content of 0.5% by mass or more can further improve color reproducibility, and a content of 10% by mass or less can further increase the dispersion stability of the colorant in the ink.

[0118] 1-3-3. UV absorbers The cyan ink may contain an ultraviolet absorber, the type and content of which may be the same as those described for the yellow ink.

[0119] The ultraviolet absorber is preferably contained in resin particles. The type, particle size, and content of the ultraviolet absorber per resin particle can be the same as those described for the yellow ink.

[0120] When the cyan ink contains an ultraviolet absorber, the content of the ultraviolet absorber relative to the total mass of the cyan ink is not particularly limited, but is preferably from 0.1% to 3.0% by mass, and more preferably from 0.5% to 2.0% by mass. By setting the content of the ultraviolet absorber within the above range, it becomes easier to adjust the maximum difference between any two values ​​of R(Y,K), R(M,K), and R(C,K) to a desired range, reducing differences in color reproducibility and making it easier to match the degree of fading with other inks.

[0121] 1-3-4. Organic solvents The cyan ink may contain an organic solvent. The type and content of the organic solvent contained in the cyan ink can be the same as those described for the yellow ink, and therefore a detailed description thereof will be omitted.

[0122] 1-3-5.Other The other components contained in the cyan ink may be the same as the other components contained in the yellow ink, and may be appropriately contained depending on the purpose.

[0123] 1-3-6.Physical Properties The viscosity and surface tension of the cyan ink can be similar to those described for the yellow ink, and therefore a detailed description will be omitted.

[0124] The absorbance Abs(C) of the cyan ink for light with a wavelength of 385 nm is not particularly limited as long as it satisfies formula (1), but is preferably 0.4 to 0.9, more preferably 0.4 to 0.7. A value of 0.4 or greater reduces the difference between the maximum and minimum values ​​of R(Y,K), R(M,K), and R(C,K), making it possible to further reduce differences in color reproducibility, while a value of 0.9 or less can further suppress thermal decomposition of the colorant due to excessive absorption of ultraviolet light.

[0125] In formula (1), the value of R(C, K) is 0.2 or more and 1.0 or less, preferably 0.4 or more and 1.0 or less, and more preferably 0.4 or more and 0.8 or less. When the value of R(C, K) is within the above range, the difference in color reproducibility can be further reduced, and the deterioration of color balance due to fading can be suppressed.

[0126] 1-3-7. Preparation of cyan ink The preparation of the magenta ink can be similar to that described for the yellow ink, and therefore a detailed description thereof will be omitted.

[0127] 1-4.Black ink As described above, the black ink is an ink that can form an image that exhibits a black color when applied alone to a recording medium.

[0128] 1-4-1.Water The black ink contains water, which can be the same as that described for the yellow ink, and therefore a detailed description thereof will be omitted.

[0129] 1-4-2. Coloring agents The black ink contains a colorant. The colorant contained in the black ink is not particularly limited and may be, for example, a dye or a pigment. From the viewpoint of improving the water resistance and light resistance of the formed image, the colorant is preferably a pigment.

[0130] Black ink usually contains a black colorant, but may contain a colorant having a color tone other than black to produce a black color. In this specification, the term "black colorant" refers to a black dye or black pigment.

[0131] Examples of black dyes include CI Acid Black 1, 2, 24, 26, 31, 52, 107, 109, 110, 119, and 154 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0132] Examples of black pigments include carbon black, CI Pigment Black 7, 26, 28, and the like.

[0133] The particle size of the black pigment particles is not particularly limited, but from the viewpoint of further improving color reproducibility, it is preferable that the volume-based median diameter is 50 nm or more and 200 nm or less. The particle size of the pigment can be determined, for example, by a particle size measuring device using dynamic light scattering or electrophoresis. From the viewpoint of simple and highly accurate measurement, measurement by dynamic light scattering is preferred.

[0134] The content of the colorant in the black ink is not particularly limited, but is preferably 0.5% by mass or more and 10% by mass or less relative to the total mass of the black ink. A content of 0.5% by mass or more can further improve color reproducibility, and a content of 10% by mass or less can further increase the dispersion stability of the colorant in the ink.

[0135] 1-4-3. UV absorbers The black ink may contain an ultraviolet absorber. The type and content of the ultraviolet absorber contained in the black ink may be the same as those described for the yellow ink.

[0136] The ultraviolet absorber is preferably contained in resin particles, which may be the same as those described for the yellow ink.

[0137] When the black ink contains an ultraviolet absorber, the content of the ultraviolet absorber relative to the total mass of the black ink is not particularly limited, but is preferably 0.1% to 3.0% by mass, more preferably 0.1% to 1.0% by mass, and even more preferably 0.1% to 0.5% by mass. By having the content within this range, it becomes easier to adjust the maximum difference between any two values ​​of R(Y,K), R(M,K), and R(C,K) to a desired range, thereby further reducing differences in color reproducibility and making it easier to match the degree of fading with other inks.

[0138] 1-4-4. Organic solvents The black ink may contain an organic solvent. The organic solvent contained in the cyan ink may be the same as that described for the yellow ink, and therefore a detailed description thereof will be omitted.

[0139] 1-4-5.Other The other components contained in the black ink may be the same as the other components contained in the yellow ink, and may be appropriately contained depending on the purpose.

[0140] 1-4-6.Physical Properties The viscosity and surface tension of the black ink can be the same as those described for the yellow ink, and therefore a detailed description will be omitted.

[0141] 1-4-7. Preparation of black ink The preparation of the magenta ink can be similar to that described for the yellow ink, and therefore a detailed description thereof will be omitted.

[0142] 2. Image forming method An image forming method according to one embodiment of the present invention relates to a method for forming an image using the ink set described above. The image forming method can be carried out in the same manner as a conventionally known image forming method, except that the ink set described above is used. Note that in this embodiment, a step of irradiating the ink with actinic rays is not carried out.

[0143] The image forming method in this embodiment includes a step of applying yellow ink onto a recording medium, a step of applying magenta ink onto the recording medium, a step of applying cyan ink onto the recording medium, and a step of applying black ink onto the recording medium.

[0144] 2-1. Addition process In this process, the above-mentioned yellow ink, magenta ink, cyan ink, and black ink are applied to the recording medium at positions corresponding to the image to be formed.

[0145] The method for applying the ink to the recording medium is not particularly limited, and the ink may be applied to the surface of the substrate using a roll coater, a spin coater, or the like, or may be applied to the surface of the substrate by methods such as spray coating, dipping, screen printing, gravure printing, offset printing, or the like, or the ink may be deposited on the surface of the substrate by an inkjet method. Of these, the inkjet method is preferred from the viewpoint of forming a finer recorded matter.

[0146] The ejection method from the inkjet head may be either an on-demand method or a continuous method. On-demand inkjet heads may be any of electromechanical conversion types such as single-cavity, double-cavity, bender, piston, shear-mode, and shared-wall types, and electrothermal conversion types such as thermal inkjet and bubble jet ("Bubble Jet" is a registered trademark of Canon Inc.).

[0147] The type of recording medium is not particularly limited. For example, the recording medium may be a highly water-absorbent paper substrate, a less water-absorbent substrate such as coated paper for gravure or offset printing, or a non-water-absorbent substrate such as a film or a plastic board (soft vinyl chloride, hard vinyl chloride, acrylic board, polyolefin-based, etc.).

[0148] The inkjet head may be either a scanning type or a line type, but is preferably a line type.

[0149] The order in which the inks are applied to the recording medium is not particularly limited.

[0150] After each ink is applied to the recording medium, the ink may be dried appropriately at the position where the ink is applied. The drying method is not particularly limited, and may be carried out using, for example, a known heater or infrared lamp.

[0151] 3. Image forming device FIG. 1 shows the configuration of an image forming apparatus 100 that can perform the image forming method described above.

[0152] The image forming apparatus 100 has a head carriage 110 having an inkjet head that ejects ink droplets to land on an area on a substrate. The image forming apparatus 100 may also have a dryer 120 that dries the ink applied to the recording medium.

[0153] The head carriage 110 is equipped with, for example, an inkjet head 111a that ejects yellow ink, an inkjet head 111b that ejects magenta ink, an inkjet head 111c that ejects cyan ink, and an inkjet head 111d that ejects black ink. The inkjet heads 111a-d have nozzles 112a-d that eject yellow, magenta, cyan, and black ink, respectively, and cause the ink to land on a recording medium 140 transported by a transport belt 130.

[0154] The dryer 120 may be a known heater, an irradiator that irradiates an infrared lamp, or the like. [Example]

[0155] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0156] 1. Preparation of ink set 1-1. Preparation of pigment dispersion 18.0 parts by mass of a yellow pigment (Pigment Yellow 155, manufactured by Tokyo Chemical Industry Co., Ltd.) was mixed with 5.4 parts by mass of a pigment dispersant (DISPERBYK190, manufactured by BYK Japan, graft copolymer, acid value 10 mgKOH / g, solids content 40% by mass), 20.0 parts by mass of ethylene glycol, and 56.6 parts by mass of ion-exchanged water. The mixture was then dispersed using a sand grinder filled with 50% by volume of zirconia beads with an average particle size of 0.5 mm to prepare a yellow pigment dispersion containing the pigment at 18.0% by mass.

[0157] A magenta pigment dispersion, a cyan pigment dispersion and a black pigment dispersion were obtained in the same manner as the yellow pigment dispersion, except that the type of pigment was changed to the following pigment. Magenta pigment: Pigment Red 122, manufactured by Tokyo Chemical Industry Co., Ltd. Cyan pigment: Pigment Blue 15:3, manufactured by Tokyo Chemical Industry Co., Ltd. Black pigment: Carbon black MA100, manufactured by Mitsubishi Chemical Corporation

[0158] 1-2. Preparation of inkjet ink To 27.8 parts by mass of the yellow pigment dispersion, 20.0 parts by mass of propylene glycol, 5.0 parts by mass of glycerin, and 0.5 parts by mass of a silicone surfactant (KF-351A, manufactured by Shin-Etsu Chemical Co., Ltd.) were added while stirring, and ion-exchanged water was further added to bring the total to 100 parts by mass to prepare an ink composition. The ink composition was filtered through a 0.8 μm filter to obtain a yellow ink.

[0159] A magenta ink was obtained in the same manner as the yellow ink, except that the yellow pigment dispersion was replaced with a magenta pigment dispersion and 1.2 parts by mass of an ultraviolet absorber (Tinuvin 477-DW(N), manufactured by BASF, solids content 40% by mass) was added. The ultraviolet absorber was hydroxyphenyltriazine incorporated into an acrylic polymer and dispersed in water, and the active ingredient was 20% by mass relative to the solids (i.e., 1.2 parts by mass of ultraviolet absorber was contained in 6.0 parts by mass of acrylic polymer). The mass of the ultraviolet absorber refers to the parts by mass of the active ingredient.

[0160] A cyan ink was obtained in the same manner as the yellow ink, except that the yellow pigment dispersion was changed to a cyan pigment dispersion.

[0161] A black ink was obtained in the same manner as the yellow ink, except that the yellow pigment dispersion was changed to a black pigment dispersion.

[0162] The resulting ink set consisting of a combination of yellow ink, magenta ink, cyan ink, and black ink was designated ink set 1.

[0163] Ink sets 2 to 10 were obtained in the same manner as ink set 1, except that the amount of ultraviolet absorber added to the yellow ink, magenta ink, cyan ink, and black ink was changed as shown in Table 1. The amount of ultraviolet absorber added shown in Table 1 represents parts by mass of the active ingredient.

[0164] 1-3. Measurement of absorbance Yellow, magenta, cyan, and black inks were diluted 4000-fold with pure water and placed in a quartz glass cell with a 10 mm optical path length. A spectrophotometer (U-3300, Hitachi, Ltd.) was then used to scan wavelengths from 340 nm to 800 nm at intervals of 5 nm or less, and the absorbance at a wavelength of 385 nm in the resulting absorption spectrum was measured. Pure water was used as a reference.

[0165] 1-4.Evaluation Two independently driven piezoelectric inkjet heads (Konica Minolta, Inc., 360 dpi, 6 pL ejection volume) were arranged with their nozzles staggered to create a 720 dpi x 720 dpi head module for each of the colors yellow, magenta, cyan, and black. The head modules were then mounted on a stage transporter with the nozzle rows perpendicular to the transport direction. The inkjet heads of each color head module were filled with the inks comprising Ink Sets 1 to 8, and an inkjet recording device was constructed to record solid images in a single pass on coated paper (OK Topcoat+ 73.3 gsm, Oji Paper Co., Ltd.) transported by the stage transporter.

[0166] Using the above inkjet recording device, the discharge rate was 10 mL / m 2Solid images of yellow, magenta, cyan and black were formed. Secondary color solid images were formed by ejecting magenta and cyan inks at a rate of 5 mL / m. 2 A blue solid image was formed. Similarly, a green solid image (yellow and cyan inks were ejected at a rate of 5 mL / m) was formed. 2 red solid image (magenta and yellow inks ejected at a rate of 5 mL / m) 2 (Formed by discharging at 1000 W / m²)

[0167] (Difference in color reproducibility) The yellow, magenta, cyan, red, blue, and green solid images obtained using each of the ink sets 1 to 10 were measured using a spectrophotometric densitometer (X-Rite 938, manufactured by X-Rite Corporation), and the hue L of each solid image was * a * b * The colorimetry was performed using a viewing angle of 2° and status I conditions, and the L*a*b* values ​​for the D65 and C illuminants were obtained. * a * b * The color difference △E D65-A was calculated from the following formula (A) and evaluated according to the following evaluation criteria, with A and B being considered acceptable. △E D65-A =[(L * D65 -L * A ) 2 +(a * D65 -a * A ) 2 +(b * D65 -b * A ) 2 ] 1 / 2 (A) A: Color difference △E D65-A is greater than or equal to 1.0 and less than 2.5. B: Color difference △E D65-A is between 2.5 and 3.5. C: Color difference △E D65-A is between 3.5 and 4.0. D: Color difference △E D65-A is 4.0 or higher.

[0168] (Color balance evaluation) Each monochrome solid image and each secondary color solid image formed by the above method was subjected to forced deterioration treatment by leaving it for 2000 hours under conditions of a black panel temperature of 63°C and humidity of 50% using a Sunshine Weather Meter (S80, manufactured by Suga Test Instruments Co., Ltd.).

[0169] Each color patch of the above-prepared forced aging treated sample was measured using a spectrophotometric densitometer (X-Rite938, manufactured by X-Rite Co., Ltd.). * b * A color reproduction space graph was created by connecting the coordinate points of each image expressed on the CIE chromaticity coordinates, and the color reproduction space graphs before and after the forced aging treatment were compared to evaluate the lightfastness according to the following criteria, with A and B being considered passing. A: There is almost no change in the balance of the color reproduction space for each color between the sample before and after the forced aging process. B: There is only a slight change in the balance of the color reproduction space of the sample after the forced degradation process compared to the color reproduction space of the sample before the forced degradation process. C: A change in the balance of the color reproduction space of the sample after the forced degradation process is observed compared to the color reproduction space of the sample before the forced degradation process. D: A significant change in the balance of the color reproduction space of the sample after the forced degradation process is observed compared to the color reproduction space of the sample before the forced degradation process.

[0170] For each of ink sets 1 to 10, Table 1 shows the mass of the ultraviolet absorber added to each ink constituting the ink set, the absorbance of each ink, and the evaluation results.

[0171] [Table 1]

[0172] Ink sets 1 to 5 were evaluated as having better color reproducibility and color balance than ink sets 6 to 10. By aligning the absorbance ratios of each ink in the ink set to the same level with respect to ultraviolet light, the absorbance at the short wavelength end of the visible light range was also made to the same level, which is thought to have reduced the color reproducibility differences. Furthermore, by aligning the degree of thermal decomposition of the colorant due to ultraviolet light absorption, it is thought to have suppressed the deterioration of color balance after ultraviolet light absorption. In particular, ink sets 3 to 5 were evaluated as having better color reproducibility and color balance because the difference between the maximum and minimum absorbance ratios was smaller.

[0173] In contrast, the difference between the maximum and minimum absorbance ratios was greater than 0.5 in ink sets 6 to 8, which is thought to have resulted in a decline in the evaluation of color reproducibility and color balance. In ink sets 8 and 9, the difference in absorbance ratio was 0.5 or less, but R(C, K) exceeded 1.0, which is thought to have resulted in excessive absorption of ultraviolet light and excessive thermal decomposition of the cyan colorant, resulting in a decline in color balance. [Industrial Applicability]

[0174] By using the ink set of the present invention, it is possible to suppress deterioration of color balance after storage and reduce the difference in color reproducibility depending on whether or not the light source used when observing an image contains ultraviolet light. Therefore, the present invention is useful, for example, in an image forming method using a water-based ink. [Explanation of symbols]

[0175] 100 Image forming device 110 Head carriage 120 Dryer 130 conveyor belt 140 Recording Media

Claims

1. An ink set including four inks: a yellow ink, a magenta ink, a cyan ink, and a black ink, Each of the four inks contains water and a colorant. At least one of the four inks contains an ultraviolet absorber, In all cases, when the absorbance of the yellow ink is Abs(Y), the absorbance of the magenta ink is Abs(M), the absorbance of the cyan ink is Abs(C), and the absorbance of the black ink is Abs(K) for light with a wavelength of 385 nm, The four types of inks are such that R(Y, K), R(M, K), and R(C, K) represented by formulas (1) to (3) satisfy conditions (a) and (b), Ink set. R(Y,K)=Abs(Y) / Abs(K) (1) R(M,K)=Abs(M) / Abs(K) (2) R(C,K)=Abs(C) / Abs(K) (3) Condition (a) R(Y, K), R(M, K), and R(C, K) are all equal to or greater than 0.2 and equal to or less than 1.

0. Condition (b) The difference between the maximum and minimum values ​​of R(Y, K), R(M, K), and R(C, K) is 0.5 or less.

2. The ink set according to claim 1 , wherein the four inks satisfy condition (c). Condition (c) R(Y, K), R(M, K), and R(C, K) are all equal to or greater than 0.5 and equal to or less than 0.

8.

3. The ink set according to claim 1 or 2, wherein the four inks satisfy condition (d). Condition (d): The difference between the maximum and minimum values ​​of the absorbance ratios R(Y, K), R(M, K), and R(C, K) is 0.3 or less.

4. 4. The ink set according to claim 1, wherein the yellow ink and the black ink satisfy condition (e). Condition (e) R(Y, K) is equal to or greater than 0.5 and equal to or less than 1.0

5. 5. The ink set according to claim 1, wherein the magenta ink and the black ink satisfy condition (f). Condition (f) R(M, K) is equal to or greater than 0.4 and equal to or less than 1.

0.

6. 6. The ink set according to claim 1, wherein the cyan ink and the black ink satisfy condition (g). Condition (g) R(C, K) is equal to or greater than 0.4 and equal to or less than 1.

0.

7. 7. The ink set according to claim 1, wherein the ink containing the ultraviolet absorber contains the ultraviolet absorber in an amount of 0.1% by mass or more and 3.0% by mass or less relative to the total mass of the ink.

8. 8. The ink set according to claim 1, wherein the ink containing an ultraviolet absorber includes the ultraviolet absorber contained in resin particles.

9. The ink set according to claim 8, wherein the ink containing an ultraviolet absorber comprises the ultraviolet absorber contained in at least one type of resin particle selected from the group consisting of acrylic resin particles, styrene resin particles, urethane resin particles, urethane-acrylic resin particles, and styrene-acrylic resin particles.

10. The ink set according to any one of claims 1 to 9, wherein the four types of inks are all inkjet inks.

11. An image forming method using four types of ink, namely, yellow ink, magenta ink, cyan ink, and black ink, applying any one of the yellow ink, the magenta ink, and the cyan ink to a surface of a recording medium; applying any other ink from among the yellow ink, the magenta ink, and the cyan ink to the surface of the recording medium; Each of the four inks contains water and a colorant. At least one of the four inks contains an ultraviolet absorber, In all cases, when the absorbance of the yellow ink is Abs(Y), the absorbance of the magenta ink is Abs(M), the absorbance of the cyan ink is Abs(C), and the absorbance of the black ink is Abs(K) for light with a wavelength of 385 nm, The four types of inks are such that R(Y, K), R(M, K), and R(C, K) represented by formulas (1) to (3) satisfy conditions (a) and (b), Image forming method. R(Y,K)=Abs(Y) / Abs(K) (1) R(M,K)=Abs(M) / Abs(K) (2) R(C,K)=Abs(C) / Abs(K) (3) Condition (a) R(Y, K), R(M, K), and R(C, K) are all equal to or greater than 0.2 and equal to or less than 1.

0. Condition (b) The difference between the maximum and minimum values ​​of R(Y, K), R(M, K), and R(C, K) does not exceed 0.

5.

12. The image forming method according to claim 11 , wherein the one type of ink and the other type of ink are both applied to the surface of the recording medium by an inkjet method.

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