Ink set

Abstract

The object of the present invention is to provide an inkjet recording ink set that can be applied to print heads requiring higher viscosity and that can maintain the gradation or color balance of the printed image even after time has passed since printing. [Solution] The present invention provides an inkjet recording ink set comprising a first ink containing a first coloring agent and a second ink containing a second coloring agent, characterized in that it satisfies the following requirements (1) to (3). (1) The amount of the first colorant in the first ink (by mass) is less than the amount of the second colorant in the second ink (by mass). (2) The first ink contains a polymer whose viscosity, measured at 25°C as a 20% by mass aqueous solution, is in the range of 1.8 to 20 mPa·s. (3) The ratio of the total solid content in the second ink to the total solid content in the first ink is within the range of 0.4 to 2.5 by mass.

Description

[Technical Field]

[0001] The present invention relates to an ink set for inkjet recording. More specifically, the present invention relates to an ink set for inkjet recording that can be applied to inkjet heads requiring higher viscosity and that can maintain the gradation or color balance of the printed image even after time has elapsed since printing. [Background technology]

[0002] In current inkjet recording methods, a method is employed to smoothly reproduce the gradation in the resulting printed image (hereinafter simply referred to as "image") by printing low-density areas with light-colored ink containing less colorant and high-density areas with dark-colored ink containing more colorant. When inkjet recording ink (hereinafter simply referred to as "ink") adheres to a recording medium, it forms dots, penetrates the recording medium, and spreads across the surface of the recording medium, with some overlap between adjacent dots. Furthermore, when the colorant penetrates deep into the recording medium, the optical density of the image often decreases.

[0003] Thus, as ink dots formed on a recording medium change over time, the dot diameter of each color changes, or the distribution of the colorant in the depth direction changes, resulting in color fluctuations. Furthermore, if the way the colorant spreads differs between dark and light inks over a long period of time, the gradation of the high-density areas formed by the dark ink and the low-density areas formed by the light ink will become disconnected in the image as time passes after printing.

[0004] In other words, if the balance between the density changes of low-density and high-density areas is disrupted, the tonal range of the image deteriorates over time, resulting in a problem where smooth gradations cannot be obtained. Furthermore, if the density changes at the same rate and to a similar extent for each color, the color fluctuations in the image are not noticeable, but if the balance between the densities of each color is disrupted, the color fluctuations become even more noticeable. If the amount of colorant in each ink differs significantly, even if the surface tension is equalized with surfactants, the permeability between the inks will not be uniform, resulting in a problem where the tonal range and color balance of the image cannot be maintained for a long period of time.

[0005] As a method to solve the above problem, Patent Document 1 proposes a technique for controlling the amount of ions in a dye ink by equalizing the surface tension between inks in an ink set having multiple inks of the same hue but different densities.

[0006] On the other hand, in recent years, in order to further improve printing speed, industrial inkjet heads (hereinafter simply referred to as "heads") with higher ink ejection speeds, such as ejection frequencies of 35 kHz or higher, have become commercially available. Industrial heads with higher ejection speeds generally require higher viscosity ink properties, such as 6 mPa·s or higher, to ensure ejection stability. Also, since heads generally have an upper limit on applicable viscosity, when using the same head across different ink sets, it is necessary to keep the viscosity range of each ink within a certain range.

[0007] Furthermore, if the amount of colorant differs significantly between inks in an ink set, the amount of colorant will also affect the viscosity of the ink. Therefore, careful consideration is needed to ensure that the spread of colorant between inks over time is consistent while matching the viscosity of the ink to the viscosity required by the print head. For example, one could adjust the viscosity by adjusting the amount of a relatively high-viscosity organic solvent such as glycerin. However, if the composition and amount of organic solvent vary significantly between inks in the ink set, it becomes difficult to achieve consistent penetration of colorant between different colored inks over time.

[0008] Furthermore, while Patent Document 2 describes a method for matching the viscosity between inks in an ink set by using a thickening agent, it does not describe a method for maintaining the gradation and color balance of the printed image over time. Thus, existing technologies have not provided an ink set that can be applied to print heads requiring high viscosity and that can maintain the gradation and color balance of the printed image even after time has passed since printing. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Patent No. 4182649 [Patent Document 2] Japanese Patent Publication No. 2011-202029 [Overview of the project] [Problems that the invention aims to solve]

[0010] This invention has been made in view of the above-mentioned problems and circumstances, and its objective is to provide an inkjet recording ink set that can be applied to print heads requiring higher viscosity and that can maintain the gradation or color balance of the printed image even after time has passed since printing. [Means for solving the problem]

[0011] In order to solve the above problems, the inventors, in the process of investigating the causes of the above problems, discovered that by including a specific polymer in the ink with a lower colorant content and maintaining a specific relationship between the total solid content of the two inks, an inkjet recording ink set can be obtained that can be applied to print heads requiring higher viscosity and that can maintain the gradation or color balance of the printed image even after time has passed since printing. This led to the present invention. In other words, the above problems according to the present invention are solved by the following means.

[0012] 1. An ink set for inkjet recording, comprising a first ink containing a first colorant and a second ink containing a second colorant, the ink set being characterized by satisfying the following requirements (1) to (3). (1) The content (% by mass) of the first colorant in the first ink is less than the content (% by mass) of the second colorant in the second ink. (2) The first ink contains a polymer having a viscosity, measured at 25 °C as an aqueous solution with a concentration of 20% by mass, within the range of 1.8 to 20 mPa·s. (3) The value of the ratio of the total solid content in the second ink to the total solid content in the first ink is within the range of 0.4 to 2.5 on a mass basis.

[0013] 2. The ink set according to claim 1, wherein the first colorant and the second colorant are of the same color system.

[0014] 3. The ink set according to claim 1 or 2, wherein the value of the ratio of the content (% by mass) of the second colorant in the second ink to the content (% by mass) of the first colorant in the first ink is 3.0 or more.

[0015] 4. The ink set according to any one of claims 1 to 3, wherein the first colorant and the second colorant are dyes.

[0016] 5. The ink set according to any one of claims 1 to 4, wherein the value of the ratio of the total solid content in the second ink to the total solid content in the first ink is within the range of 0.6 to 1.5 on a mass basis.

[0017] 6. The ink set according to any one of claims 1 to 5, wherein the value of the ratio of the viscosity of the second ink at 25 °C to the viscosity of the first ink at 25 °C is within the range of 0.7 to 1.3.

[0018] 7. Both the first ink and the second ink contain an organic solvent, and the difference between the content (mass %) of the organic solvent in the first ink and the content (mass %) of the organic solvent in the second ink is 10 mass % or less. The ink set according to any one of claims 1 to 6.

[0019] 8. Both the first ink and the second ink contain an organic solvent, and the content (mass %) of the organic solvent in the first ink and the content (mass %) of the organic solvent in the second ink are both 30 mass % or less. The ink set according to any one of claims 1 to 7.

[0020] 9. The ratio of the polymer to the solid content excluding the first colorant in the total solid content of the first ink is 50 mass % or more. The ink set according to any one of claims 1 to 8.

[0021] 10. The polymer has an anionic group. The ink set according to any one of claims 1 to 9.

[0022] 11. The degree of polymerization of the polymer is in the range of 2 to 10. The ink set according to any one of claims 1 to 10.

[0023] 12. The polymer contains a salt of polycarboxylic acid or a salt of naphthalenesulfonic acid formalin condensate. The ink set according to any one of claims 1 to 11.

[0024] 13. The viscosity of the first ink at 25°C and the viscosity of the second ink at 25°C are both 6 mPa·s or more. The ink set according to any one of claims 1 to 12.

Advantages of the Invention

[0025] The above-described means of the present invention can be applied to print heads requiring higher viscosity, and it is possible to provide an ink set for inkjet recording that can maintain the gradation or color balance of the printed image even after time has passed since printing. The mechanism by which the effects of the present invention manifest or operate is presumed to be as follows.

[0026] The present invention provides an ink set comprising a first ink and a second ink having a greater colorant content (mass%) than the first ink (requirement (1)), wherein the first ink contains a specific polymer (requirement (2)) and the ratio of the total solid content of the first ink to the second ink is within a specific range (requirement (3)).

[0027] Specifically, in requirement (2), a specific polymer is a polymer whose viscosity, measured at 25°C as a 20% by mass aqueous solution, is in the range of 1.8 to 20 mPa·s. In requirement (3), the ratio of the total solid content in the second ink to the total solid content in the first ink is in the range of 0.4 to 2.5 by mass. Hereinafter, a polymer whose viscosity, measured at 25°C as a 20% by mass aqueous solution, is in the range of 1.8 to 20 mPa·s will also be referred to as "polymer A".

[0028] As described above, in order to maintain the gradation or color balance of the printed image even after time has passed since printing, it is necessary to match the change in dot diameter over time and the penetration properties of the first ink and the second ink. In the present invention, in order to achieve this, the first ink contains a polymer having the above viscosity characteristics in requirement (2), and the ratio of the total solid content of the first ink and the second ink in requirement (3) is set to be within the above range.

[0029] In order to synchronize the dot diameter changes over time and the penetration properties of the first and second inks, it is important to synchronize not only the surface tension of each ink, but also the physical properties and amount of solid content, and the change in viscosity when water evaporates. In particular, with high-viscosity inks, the viscosity increase due to water evaporation is significant, and the influence of changes in solid content and viscosity is especially large, so it is necessary to synchronize the dot diameter changes over time and the penetration properties of the first and second inks.

[0030] The ink set of the present invention satisfies requirements (1) to (3), thereby matching not only the surface tension of the first ink and the second ink, but also the physical properties and amount of solid content, and the change in viscosity when water evaporates. As a result, the change in dot diameter over time and the penetration of the first ink and the second ink can be matched, and as a result, it is possible to maintain the gradation or color balance of the printed image even after time has passed since printing. [Modes for carrying out the invention]

[0031] The present invention is an inkjet recording ink set comprising a first ink containing a first coloring agent and a second ink containing a second coloring agent, characterized in that it satisfies the following requirements (1) to (3). (1) The content (by mass) of the first colorant in the first ink is less than the content (by mass) of the second colorant in the second ink. (2) The first ink contains a polymer whose viscosity, measured at 25°C as a 20% by mass aqueous solution, is in the range of 1.8 to 20 mPa·s. (3) The ratio of the total solid content in the second ink to the total solid content in the first ink is within the range of 0.4 to 2.5 by mass. This feature is a technical feature common to or corresponding to each of the embodiments described below.

[0032] In an embodiment of the present invention, when the first colorant and the second colorant are of the same color, the ink set of the present invention can more effectively exhibit the effect of maintaining the gradation of the printed image over time.

[0033] In embodiments of the present invention, from the viewpoint of more effectively exhibiting the effects of the present invention, it is preferable that the ratio of the content (mass%) of the second colorant in the second ink to the content (mass%) of the first colorant in the first ink is 3.0 or higher. Furthermore, it is preferable that the first colorant and the second colorant are dyes.

[0034] In embodiments of the present invention, from the viewpoint of more effectively exhibiting the effects of the present invention, it is preferable that the ratio of the total solid content in the second ink to the total solid content in the first ink is within the range of 0.6 to 1.5 by mass.

[0035] In embodiments of the present invention, from the viewpoint of more effectively exhibiting the effects of the present invention, it is preferable that the ratio of the viscosity of the second ink at 25°C to the viscosity of the first ink at 25°C is within the range of 0.7 to 1.3.

[0036] In embodiments of the present invention, from the viewpoint of better exhibiting the effects of the present invention, it is preferable that both the first ink and the second ink contain an organic solvent, and that the difference between the content of the organic solvent in the first ink (mass%) and the content of the organic solvent in the second ink (mass%) is 10% by mass or less. Furthermore, it is preferable that both the first ink and the second ink contain an organic solvent, and that the content of the organic solvent in the first ink (mass%) and the content of the organic solvent in the second ink (mass%) are both 30% by mass or less.

[0037] In embodiments of the present invention, from the viewpoint of more effectively exhibiting the effects of the present invention, it is preferable that the proportion of the polymer to the amount of solid content in the first ink excluding the first colorant is 50% by mass or more.

[0038] In embodiments of the present invention, from the viewpoint of more effectively exhibiting the effects of the present invention, it is preferable that the polymer has anionic groups. Furthermore, the degree of polymerization of the polymer may be in the range of 2 to 10. It is also preferable that the polymer contains a salt of a polycarboxylic acid or a salt of a naphthalene sulfonic acid formalin condensate.

[0039] In embodiments of the present invention, from the viewpoint of more effectively exhibiting the effects of the present invention, it is preferable that the viscosity of the first ink at 25°C and the viscosity of the second ink at 25°C are both 6 mPa·s or higher.

[0040] The present invention, its components, and embodiments and models for carrying out the present invention will be described in detail below. In this application, "~" is used to mean that the numerical values ​​before and after it are included as the lower limit and upper limit.

[0041] [Ink Set] The present invention is an inkjet recording ink set comprising a first ink containing a first coloring agent and a second ink containing a second coloring agent, characterized in that it satisfies the following requirements (1) to (3). (1) The content (by mass) of the first colorant in the first ink is less than the content (by mass) of the second colorant in the second ink. (2) The first ink contains a polymer (polymer A) whose viscosity, measured at 25°C as a 20% by mass aqueous solution, is in the range of 1.8 to 20 mPa·s. (3) The ratio of the total solid content in the second ink to the total solid content in the first ink is within the range of 0.4 to 2.5 by mass.

[0042] The following explains each of the requirements (1) to (3). (1) Requirements The ink set of the present invention includes a first ink containing a first colorant and a second ink containing a second colorant. In the requirement (1), it is defined that the content (% by mass) of the first colorant in the first ink is less than the content (% by mass) of the second colorant in the second ink.

[0043] Hereinafter, unless otherwise specified, the units of the content of the first colorant in the first ink and the content of the second colorant in the second ink are % by mass.

[0044] Regarding the relationship between the first colorant and the second colorant contained in the first ink and the second ink in the ink set of the present invention, other than the difference in content, there are no particular limitations.

[0045] The first colorant and the second colorant may be of the same color system or not. In this specification, when two colorants are of the same color system, it means that the values of the hue angles that each colorant can represent are not greatly different. The allowable range of the difference in hue angles between the two varies depending on the hue. The hue angle has a specific range defined for each hue. If the hue angles of the two colorants are within the range of the hue angle of the same hue, the two are regarded as being of the same color system.

[0046] Specifically, when the two colorants are of the same color system of cyan, it is preferable that the hue angles of the two colorants in the L * a * b * color coordinate system are both within the range of 215° to 255°. When the two colorants are of the same color system of magenta, it is preferable that the hue angles of the two colorants in the L * a * b * color coordinate system are both within the range of 320° to 360° or within the range of 0° to 20°. When the two colorants are of the same color system of yellow, it is preferable that the hue angles of the two colorants in the L * a * b * color coordinate system are within the range of 70° to 140°.

[0047] If the two colorants are the same shade of blue, then the L of those two colorants * a * b * It is preferable that the hue angles in the color system are both within the range of 270° to 285°. If the two colorants are the same color of red, then the L of those two colorants * a * b * It is preferable that the hue angles in the color system are all within the range of 20° to 35°.

[0048] When the first and second colorants are of the same color, the ink set of the present invention can more effectively maintain the gradation of the printed image over time after printing. When the first and second colorants are not of the same color, that is, when the hues of the first and second colorants are different, the ink set of the present invention can more effectively maintain the color balance of the printed image over time after printing.

[0049] In inkjet recording, multiple inks of different hues, such as cyan, magenta, and yellow inks, may be used in combination. In such cases, it is preferable to use an ink set consisting of a first ink and a second ink of the present invention for each hue. Specifically, it is preferable to use a combination of a cyan ink set in which both the first and second colorants are cyan in the first and second inks satisfying requirements (1) to (3), a magenta ink set in which both the first and second colorants are magenta in the first and second inks satisfying requirements (1) to (3), and a yellow ink set in which both the first and second colorants are yellow in the first and second inks satisfying requirements (1) to (3).

[0050] By using the above-mentioned cyan, magenta, and yellow ink sets together, the resulting printed image will retain both its gradation and color balance even after time has passed since printing.

[0051] With respect to requirement (1), it is preferable that the ratio of the content of the second colorant in the second ink to the content of the first colorant in the first ink is 3.0 or higher (hereinafter, this requirement is also referred to as "requirement (12)"), and more preferably 3.5 or higher. This reduces graininess and enables smooth gradation reproduction. The upper limit of the ratio of the content of the second colorant in the second ink to the content of the first colorant in the first ink is preferably around 10, and more preferably around 5, from the viewpoint of gradation reproduction.

[0052] The specific compositions of the first and second inks will be described later, but the content of the first colorant in the first ink satisfies requirement (1) in relation to the content of the second colorant in the second ink, and preferably satisfies requirement (12). For example, the content is preferably in the range of 0.5 to 10% by mass, and more preferably in the range of 1 to 7% by mass, relative to the total amount of the first ink. Furthermore, the content is preferably in the range of 2 to 50% by mass, and more preferably in the range of 5 to 30% by mass, relative to the total amount of solids in the first ink.

[0053] Furthermore, the content of the second colorant in the second ink is preferably in the range of 1.5 to 30% by mass, and more preferably in the range of 3 to 21% by mass, relative to the total amount of the second ink, in relation to the content of the first colorant in the first ink, while satisfying requirement (1), and preferably requirement (12).

[0054] Specific examples of the first and second colorants are described below. In the following explanation, where it is not necessary to distinguish between the first and second colorants, that is, where there are commonalities between the two, they are collectively referred to as "colorants."

[0055] (Coloring agent) The coloring agent is not particularly limited to pigments, dyes, or colorants, but dyes are more effective in this invention because they have higher diffusivity. Examples of dyes include acid dyes, direct dyes, basic dyes, reactive dyes, or food colorants.

[0056] The following are some representative dyes, but the present invention is not limited to these. <Direct dye> CI Direct Yellow 1, 4, 8, 11, 12, 24, 26, 27, 28, 33, 39, 44, 50, 58, 85, 86, 100, 110, 120, 132, 142, 144, CI Direct Red 1, 2, 4, 9, 11, 13, 17, 20, 23, 24, 28, 31, 33, 37, 39, 44, 47, 48, 51, 62, 63, 75, 79, 80, 81, 83, 89, 90, 94, 95, 99, 220, 224, 227, 243, CI Direct Blue 1, 2, 6, 8, 15, 22, 25, 71, 76, 78, 80, 86, 87, 90, 98, 106, 108, 120, 123, 163, 165, 192, 193, 194, 195, 196, 199, 200, 201, 202, 203, 207, 236, 237, CI Direct Black 2, 3, 7, 17, 19, 22, 32, 38, 51, 56, 62, 71, 74, 75, 77, 105, 108, 112, 117, 154,

[0057] <Acid dye> CI Acid Yellow 2, 3, 7, 17, 19, 23, 25, 29, 38, 42, 49, 59, 61, 72, 99, CI Acid Orange 56, 64 CI Acid Red 1, 8, 14, 18, 26, 32, 37, 42, 52, 57, 72, 74, 80, 87, 115, 119, 131, 133, 134, 143, 154, 186, 249, 254, 256, CI Acid Violet 11, 34, 75 CI Acid Blue 1, 7, 9, 29, 87, 126, 138, 171, 175, 183, 234, 236, 249, CI Acid Green 9, 12, 19, 27, 41 CI Acid Black 1, 2, 7, 24, 26, 48, 52, 58, 60, 94, 107, 109, 110, 119, 131, 155,

[0058] <Reactive dyes> CI Reactive Yellow 1, 2, 3, 13, 14, 15, 17, 37, 42, 76, 95, 168, 175, CI Reactive Red 2, 6, 11, 21, 22, 23, 24, 33, 45, 111, 112, 114, 180, 218, 226, 228, 235, CI Reactive Blue 7, 14, 15, 18, 19, 21, 25, 38, 49, 72, 77, 176, 203, 220, 230, 235, CI Reactive Orange 5, 12, 13, 35, 95 CI Reactive Brown 7, 11, 33, 37, 46 CI Reactive Green 8, 19, CI Reactive Violet 2, 4, 6, 8, 21, 22, 25, CI Reactive Black 5, 8, 31, 39

[0059] <Basic dyes> CI Basic Yellow 11, 14, 21, 32 CI Basic Red 1, 2, 9, 12, 13 CI Basic Violet 3, 7, 14 CI Basic Blue 3, 9, 24, 25

[0060] Other dyes that can be used as colorants according to the present invention include chelate dyes and azo dyes used in so-called silver dye bleaching photosensitive materials (for example, Cibachrome manufactured by Ciba-Geigy).

[0061] Chelate dyes are described, for example, in British Patent No. 1,077,484.

[0062] Regarding azo dyes for silver-dye bleaching photosensitive materials, see, for example, British Patent Nos. 1,039,458, 1,004,957, and 1,077,628, and U.S. Patent No. 2,612,448.

[0063] Pigments that can be used as colorants according to the present invention include conventionally known organic and inorganic pigments. Examples include azo pigments such as azo lakes, insoluble azo pigments, condensed azo pigments, and chelate azo pigments; polycyclic pigments such as phthalocyanine pigments, perylene and perylene pigments, anthraquinone pigments, quinacridone pigments, dioxandine pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments; dye lakes such as basic dye-type lakes and acid dye-type lakes; organic pigments such as nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments; and inorganic pigments such as carbon black.

[0064] (2) Requirements The requirement in (2) specifies that the first ink contains polymer A. In the ink set of the present invention, the above effects of the present invention can be achieved by containing polymer A in the first ink of the two inks that satisfy the requirement in (1), and by satisfying the requirement in (3).

[0065] [A] Polymer A is a compound whose viscosity, measured at 25°C as a 20% by mass aqueous solution, is in the range of 1.8 to 20 mPa·s. Hereinafter, the viscosity measured at 25°C as a 20% by mass aqueous solution may also be referred to as "20% aqueous solution viscosity." Here, polymer A is a compound for which the 20% aqueous solution viscosity can be measured, that is, a water-soluble compound that can dissolve in water at 25°C at a concentration of 20% by mass or more (the content of polymer A relative to the total volume of the aqueous solution is 20% by mass or more). In this specification, viscosity refers to the viscosity at 25°C unless otherwise specified. Viscosity can be measured, for example, using an E-type viscometer.

[0066] If a polymer not within the category of polymer A is used, for example, a polymer with a 20% aqueous solution viscosity of less than 1.8 mPa·s, the permeability of the first ink will be greater than that of the second ink, and the gradation or color balance over time cannot be maintained in the printed image. Also, if a polymer not within the category of polymer A is used, for example, a polymer with a 20% aqueous solution viscosity greater than 20 mPa·s, the permeability of the first ink will be less than that of the second ink, and the gradation or color balance over time cannot be maintained in the printed image. The viscosity of polymer A in a 20% aqueous solution is preferably in the range of 2 to 10 mPa·s, more preferably in the range of 2 to 4 mPa·s.

[0067] Generally, a polymer is a compound obtained by the polymerization of multiple monomers, and the monomer-derived units in a polymer are called polymerization units (hereinafter also referred to as constituent units). In this specification, a polymer means a compound with two or more constituent units, in other words, a degree of polymerization of 2 or more. That is, polymers include oligomers and polymers. As polymer A, depending on its constituent units, for example, a polymer with a degree of polymerization in the range of 2 to 10 may be used.

[0068] Furthermore, polymer A preferably has anionic groups from the viewpoint of maintaining gradation or color balance over time in printed images. The anionic groups are not particularly limited as long as they have a negative charge. In the present invention, from the viewpoint of dispersion stability, they are preferably phosphate groups, phosphonic acid groups, phosphinic acid groups, sulfate groups, sulfonic acid groups, sulfinic acid groups, or carboxylic acid groups, more preferably phosphate groups, sulfonic acid groups, or carboxylic acid groups, and even more preferably sulfonic acid groups or carboxylic acid groups. In addition, as a salt, some of the above anionic groups may be salts, or all of them may be salts.

[0069] Polymer A may be used alone or in combination of two or more types. Alternatively, it may be a mixture of two or more polymers such that the viscosity of a 20% aqueous solution is in the range of 1.8 to 20 mPa·s. In this invention, it is preferable to use a polymer as Polymer A that can achieve the above-mentioned viscosity of a 20% aqueous solution on its own.

[0070] Examples of polymer A include polycarboxylic acids and their salts, condensates of sulfonated aryl compounds and formaldehyde (hereinafter referred to as "sulfonated aryl compound formalin condensates") and their salts, polyalkylene glycols (alkylenes having 2 to 3 carbon atoms are preferred), polyethylene glycols, polypropylene glycols, and the like, all of which satisfy the above condition (2).

[0071] Polymer A is preferably at least one selected from polycarboxylic acids and their salts, sulfonated aryl compound formalin condensates and their salts, from the viewpoint of having anionic groups, etc., that satisfies the above (2). Examples of salts include alkali metal salts, alkaline earth metal salts, and quaternary ammonium salts, with alkali metal salts, particularly sodium salts, being preferred.

[0072] In sulfonated aryl compounds, preferred aryl compounds are polycyclic aromatic compounds, specifically compounds and derivatives thereof in which two or more aromatic rings (including heterocycles) share two or more carbon atoms to form a fused ring, and compounds and derivatives thereof in which two or more aromatic rings (including heterocycles) are bonded directly or via alkylene groups.

[0073] Examples of polycyclic aromatic compounds include naphthalene, anthracene, phenanthrene, pyrene, acenaphthene, indene, fluoranthene, chrysene, benzofluoranthene, benzopyrene, perylene, benzoperylene, fluorene, diphenyl, and dibenzofuran. Preferably, it is at least one selected from naphthalene, anthracene, phenanthrene, pyrene, acenaphthene, and fluorene, and more preferably naphthalene. These may have substituents. That is, as the sulfonated aryl compound, sulfonated naphthalene (naphthalenesulfonic acid), which may have substituents, is preferred.

[0074] Examples of naphthalene sulfonic acid formalin condensates and salts thereof, which may have substituents, include polymer A having at least one structural unit represented by the following general formula (I) (hereinafter also referred to as structural unit (I)).

[0075] [ka]

[0076] In formula (I), R is a substituent, and X represents the number of R atoms, which is 0, 1, or 2. M is independently a hydrogen atom, an alkali metal, or a quaternary ammonium group. Examples of R include a hydroxyl group, a carbon-1 to carbon-4 alkoxy group, a carbon-1 to carbon-4 alkyl group, etc. If there are multiple R atoms, they may be the same or different.

[0077] Furthermore, polymer A having the above-mentioned structural unit (I) may contain other structural units within a range that does not impair the properties of polymer A. Examples of other structural units include those formed from copolymerizable monomers such as alkyl maleic anhydride and polycarboxylic acid. In this case, it is preferable that the other structural units constitute 30% by mass or less of the total amount of polymer A.

[0078] In polymer A having structural unit (I), the degree of polymerization is preferably in the range of 2 to 10. In this case, a degree of polymerization of 2 to 10 means that the number of sulfonated naphthalene residues, which may have substituents, is 2 to 10. The weight-average molecular weight of the naphthalene sulfonic acid formalin condensate and its salt containing polymer A having structural unit (I) is preferably in the range of 400 to 3000, and more preferably in the range of 450 to 950. The weight-average molecular weight can be measured by gel permeation chromatography (GPC) using a polystyrene standard.

[0079] Furthermore, as polymer A having constituent unit (I), a salt of naphthalene sulfonic acid formalin condensate (hereinafter also referred to as "compound (II)") which may have substituents shown in the following general formula (II) is preferred.

[0080] [ka]

[0081] In formula (II), R has the same meaning as in formula (I), and M is independently an alkali metal or a quaternary ammonium group. x is an integer from 0 to 3, and n1 is an integer from 1 to 9.

[0082] Among the compounds (II), the sodium salt of the naphthalene sulfonic acid formalin condensate is particularly preferred, where M is sodium, x is 0, and n1 is 1.

[0083] Naphthalene sulfonic acid formalin condensate (salt) can be produced by commonly known methods, for example, by condensation polymerization using naphthalene sulfonic acid (salt) and an equivalent amount of formaldehyde, and other components as needed. Examples of other components include sulfonic acid (salt) such as β-methylnaphthalene, α-methylnaphthalene, acenaphthene, dibenzofuran, fluorene, phenanthrene, anthracene, fluoranthene, and pyrene.

[0084] As the sodium salt of naphthalene sulfonic acid formalin condensate, commercially available products such as Demol NL, Demol SS-L, Demol N, Demol MS, and Demol C (all trade names) manufactured by Kao Corporation can also be used.

[0085] Examples of polycarboxylic acids and their salts include polymer A, which has at least one structural unit represented by the following general formula (III) (hereinafter also referred to as structural unit (III)).

[0086] [ka]

[0087] In formula (III), R 1 ~R 3 Each of the following is independently a hydrogen atom or a substituent, and each of the following is independently a hydrogen atom, an alkali metal, or a quaternary ammonium group. 1 ~R 3 Examples of substituents shown include C1-C5 alkyl groups, C1-C5 alkoxy groups, -COOM (where M represents the same atom or group as above), -OH, etc.

[0088] Polymer A having the above-mentioned structural unit (III) may also contain other structural units as long as it does not impair the properties of polymer A. Examples of other structural units include those formed from copolymerizable monomers such as olefins, cyclic olefins, and aromatic vinyl compounds. When polymer A contains other structural units, the molar ratio of structural unit (III) to other structural units is preferably 10 / 0 to 1 / 10, and more preferably 10 / 0 to 8 / 2. In this case, the polymerization method may be either block copolymerization or random copolymerization.

[0089] Specifically, one or more of (meth)acrylic acid (acrylic acid or methacrylic acid), maleic acid, their derivatives, and their salts are preferred as the constituent unit (III). The ends of polymer A containing constituent unit (III) are not particularly limited, but examples include a hydrogen atom, -OH, a C1-C5 alkyl group, a C1-C5 alkoxy group, or SO3M (where M has the same meaning as M in formula (III)), and may be the same or different.

[0090] Examples of polycarboxylic acids and their salts in the present invention include (meth)acrylic acid homopolymers, (meth)acrylic acid-maleic acid copolymers, α-hydroxyacrylic acid homopolymers, C5 olefin-maleic acid copolymers, isobutylene-maleic acid copolymers, and salts thereof. Preferably, these are acrylic acid homopolymers, acrylic acid-maleic acid copolymers, isobutylene-maleic acid copolymers, and salts thereof.

[0091] The weight-average molecular weight of polycarboxylic acids and their salts is preferably between 1,000 and 10,000, and more preferably between 1,500 and 5,000, in order to ensure that the viscosity of a 20% aqueous solution is within the above range. The weight-average molecular weight can be measured by gel permeation chromatography (GPC) using a polystyrene standard, as described above.

[0092] As polycarboxylic acids and their salts, commercially available products such as polyacrylic acid or its salts, such as Aron T-50 (manufactured by Toagosei Co., Ltd.), Aron A-210 (manufactured by Toagosei Co., Ltd.), and Aron A-6001 (manufactured by Toagosei Co., Ltd.), can also be used.

[0093] The first ink contains a first colorant and polymer A as solid components. In addition to polymer A, the first ink may also contain various solid components such as water-soluble polymers other than polymer A, water-insoluble resin fine particles, and surfactants. The content of polymer A in the first ink is preferably 50% by mass or more, relative to the amount of solids obtained by subtracting the first colorant from the total solids. The upper limit of the content of polymer A relative to the amount of solids obtained by subtracting the first colorant from the total solids is 100% by mass, and the range of 80 to 100% by mass is more preferable. If the content of polymer A in the first ink is within the above range, the permeability of the colorant between inks can be standardized, and the gradation and color balance of the printed image can be sufficiently maintained over time.

[0094] (3) Requirements The ink set of the present invention is defined in requirement (3) as having a ratio of the total solid content in the second ink to the total solid content in the first ink, which is within the range of 0.4 to 2.5 by mass. The ratio of the total solid content in the second ink to the total solid content in the first ink is calculated as the total solid content (mass) in the second ink / the total solid content (mass) in the first ink, and is hereinafter simply referred to as the "ratio of total solid content". Solid content refers to components other than the solvent contained in the ink. Unless otherwise specified, the solid content of the ink is expressed in mass or mass%.

[0095] If the ratio of total solid content is less than 0.4, the penetration of the first ink will be less than that of the second ink, and the gradation or color balance cannot be maintained over time in the printed image. If the ratio of total solid content is greater than 2.5, the penetration of the first ink will be greater than that of the second ink, and the gradation or color balance cannot be maintained over time in the printed image. The solid content ratio is more preferably in the range of 0.6 to 1.5.

[0096] The first and second inks consist of a solid component and a solvent, the solvent typically consisting of water, or water and an organic solvent. The organic solvent is preferably hydrophilic.

[0097] The solid content in the first ink and the second ink can be in the range of 5 to 50% by mass, and preferably in the range of 15 to 25% by mass, while satisfying the requirements of (1) to (3) above.

[0098] The requirements (1) to (3) for the ink set of the present invention have been described above. The ink set of the present invention preferably satisfies at least one of the following requirements (4) to (7), and more preferably satisfies all of the requirements.

[0099] (4) Both the first ink and the second ink contain an organic solvent, and the difference between the organic solvent content (mass%) in the first ink and the organic solvent content (mass%) in the second ink is 10% by mass or less. (5) Both the first ink and the second ink contain an organic solvent, and the content of the organic solvent in the first ink (mass%) and the content of the organic solvent in the second ink (mass%) are both 30% by mass or less. (6) The ratio of the viscosity of the second ink at 25°C to the viscosity of the first ink at 25°C is within the range of 0.7 to 1.3. (7) The viscosity of the first ink at 25°C and the viscosity of the second ink at 25°C are both 6 mPa·s or higher.

[0100] Requirements of (4) and (5) As described above, the first ink and the second ink typically contain water, or a solvent consisting of water and an organic solvent. The solvent content in the first ink and the second ink is in the range of 100% by mass minus the solid content, and specifically can be in the range of 50 to 95% by mass, and preferably in the range of 75 to 85% by mass.

[0101] In the first and second inks, it is preferable to use a combination of water and an organic solvent as the solvent, from the viewpoint of injection properties and image quality. Requirements (4) and (5) specify the content of the organic solvent when the first and second inks contain an organic solvent.

[0102] Requirement (5) specifies that the upper limit of the organic solvent content in the first and second inks is 30% by mass. The upper limit of the organic solvent content in the first and second inks can be increased to about 80% by mass, but setting the upper limit to 30% by mass is preferable in that it suppresses changes in gradation or color balance in printed images over time.

[0103] The content of the organic solvent in the first ink and the second ink can be in the range of 5 to 30% by mass, and preferably in the range of 10 to 30% by mass.

[0104] Requirement (4) specifies that the difference between the content of the organic solvent in the first ink and the content of the organic solvent in the second ink must be within 10% by mass. The content of the organic solvent in the first ink and the content of the organic solvent in the second ink may be higher in either case, but a difference of 10% by mass or less makes it possible to more effectively suppress changes in gradation or color balance over time in the printed image. It is more preferable that the difference in the content of the organic solvent in the first ink and the second ink is within 5% by mass.

[0105] Examples of organic solvents used in the first and second inks include monohydric alcohols, polyhydric alcohols, polyhydric alcohol ethers, amines, amides, and nitrogen-containing heterocyclic compounds.

[0106] Examples of monohydric alcohols include methanol, ethanol, propanol, isopropanol, butanol, isobutanol, secondary butanol, tertiary butanol, pentanol, hexanol, cyclohexanol, and benzyl alcohol.

[0107] Examples of polyhydric alcohols include ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, 1,2-butanediol, 1,4-butanediol, 1,2-pentanediol, thiodiglycol, glycerin, and pentaerythritol.

[0108] Examples of polyhydric alcohol ethers include ethylene glycol monoethyl ether, ethylene glycol monophenyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol dimethyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, ethylene glycol monomethyl ether acetate, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, triethylene glycol dimethyl ether, and tripropylene glycol dimethyl ether.

[0109] Examples of amines include ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, and tetramethylpropylenediamine.

[0110] Examples of amides include formamide, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0111] Examples of nitrogen-containing heterocyclic compounds include 2-pyrrolidone, N-methyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, 2-oxazolidone, and 1,3-dimethyl-2-imidazolidinone.

[0112] In addition, the first and second inks may also be used as organic solvents other than those mentioned above, such as sulfoxides (e.g., dimethyl sulfoxide), sulfones (e.g., sulfolane), sulfonates (e.g., sodium 1-butanesulfonate), urea, acetonitrile, acetone, etc.

[0113] Requirements of (6) and (7) The requirements in (6) and (7) above specify the viscosity of the first ink and the second ink.

[0114] The requirement in (6) specifies that the ratio of the viscosity of the second ink to the viscosity of the first ink must be within the range of 0.7 to 1.3. The ratio of the viscosity of the second ink to the viscosity of the first ink is the value obtained by dividing the viscosity of the second ink by the viscosity of the first ink, and is hereinafter also referred to simply as the "viscosity ratio value".

[0115] When the viscosity ratio is within the above range, the penetration of the first and second inks is more easily equalized, making it easier to maintain tonal gradation or color balance over time in printed images. It is more preferable that the viscosity ratio is within the range of 0.9 to 1.1.

[0116] The requirement in (7) specifies that the viscosity of both the first ink and the second ink must be 6 mPa·s or higher. By setting the viscosity of the first ink and the second ink to 6 mPa·s or higher, it is possible to ensure ejection stability in industrial heads with increased ejection speeds, for example. From the viewpoint of ejection stability, the viscosity of the first ink and the second ink is preferably around 6 to 13 mPa·s, and more preferably around 6 to 10 mPa·s.

[0117] The essential requirements (1) to (3) and preferred requirements (4) to (7) for the ink set of the present invention have been described above. Below, the specific compositions of the first ink and the second ink constituting such an ink set of the present invention will be described. In the following description, the relationship between the content of the components of both inks, such as requirements (1), (3), and (4), will not be mentioned, but the composition is based on the premise that requirements (1) and (3) are essential requirements and that requirement (4) is a preferred requirement.

[0118] [First Ink] The first ink consists of a solid component and a solvent. In the first ink, the solid component contains the first colorant and polymer A as essential components. The specific types and amounts of the first colorant and polymer A in the solid component are as described above.

[0119] As described above, the first ink may further contain other solid components as solid matter, such as water-soluble polymers other than polymer A, water-insoluble resin fine particles, and surfactants, to the extent that it does not impair the effects of the present invention. However, it is preferable that it does not contain water-soluble polymers other than polymer A and water-insoluble resin fine particles. It is preferable to include a surfactant as another solid component in order to adjust the surface tension of the first ink.

[0120] Examples of surfactants preferably used in the first ink include anionic surfactants such as dialkyl sulfosuccinates, alkylnaphthalene sulfonates, and higher fatty acid salts; nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene-polyoxypropylene block copolymers; and cationic surfactants such as alkylamine salts and quaternary ammonium salts.

[0121] In the present invention, the use of nonionic surfactants such as acetylene glycols is preferred. Commercially available surfactants may also be used. Examples of commercially available acetylene glycols include the Orphine E series (manufactured by Nisshin Chemical Industry Co., Ltd.), such as Orphine E1010.

[0122] The surfactant content in the first ink is preferably in the range of approximately 0.01 to 1% by mass, and more preferably 0.02 to 0.2% by mass, relative to the total amount of solids.

[0123] A combination of water and an organic solvent is preferred as the solvent in the first ink. The specific type and content of the organic solvent are as described above. The water content is the remainder after subtracting the content of the organic solvent from the total solvent content.

[0124] [The Second Ink] The second ink consists of a solid component and a solvent. In the second ink, the solid component contains the second colorant as an essential component. The specific type and content of the second colorant in the solid component are as described above.

[0125] The second ink may further contain other solid components as solid content, such as a water-soluble polymer, water-insoluble resin fine particles, or surfactant, to the extent that it does not impair the effects of the present invention. Of these, the second ink preferably contains a water-soluble polymer as solid content, and preferably contains polymer A as the water-soluble polymer. When the second ink contains polymer A, the specific embodiment of polymer A is the same as that of polymer A in the first ink, including preferred embodiments.

[0126] Furthermore, if the second ink contains polymer A, the content of polymer A is preferably 50% by mass or more relative to the amount of solids obtained by subtracting the second colorant from the total solids, similar to the case of the first ink. The upper limit of the content of polymer A relative to the amount of solids obtained by subtracting the second colorant from the total solids is 100% by mass, and the content is more preferably in the range of 80 to 100% by mass.

[0127] If the second ink contains polymer A, it is preferable that, similar to the first ink, it does not contain water-soluble polymers other than polymer A or water-insoluble resin fine particles. Furthermore, it is preferable to include a surfactant as another solid component in order to adjust the surface tension of the second ink. The surfactant can be the same as that used in the first ink, including its type and content.

[0128] A combination of water and an organic solvent is preferred as the solvent in the second ink. The specific type and content of the organic solvent are as described above. The water content is the remainder after subtracting the content of the organic solvent from the total solvent content.

[0129] The first ink and the second ink can each be manufactured by mixing the components of the above composition.

[0130] The ink set of the present invention is an ink set for inkjet recording. When printing on a recording medium by ejecting the ink set according to the present invention, the inkjet head used may be either on-demand or continuous. Furthermore, any ejection method may be used, such as electromechanical conversion methods (e.g., single cavity type, double cavity type, bender type, piston type, shear mode type, shared wall type, etc.) or electrothermal conversion methods (e.g., thermal inkjet type, bubble jet® type, etc.).

[0131] As described above, the viscosity of the ink constituting the ink set in the present invention can be made high, so that ejection stability can be ensured even when used in an industrial head with a high ejection speed.

[0132] The recording media to which the ink set of the present invention can be applied are not particularly limited. Specifically, as recording media, plain paper, coated paper, swelling inkjet recording paper provided with an ink-receiving layer that absorbs and swells upon absorbing ink, porous inkjet recording paper having a porous ink-receiving layer, and a recording medium using a resin support such as polyethylene terephthalate film instead of a base paper can be used.

[0133] Another recording medium to which the ink set of the present invention can be applied is a fabric. The material of the fibers constituting the fabric is not particularly limited. Examples include natural fibers such as cellulose fibers (natural cotton), hemp, wool, or silk (hydrophilic fibers), and chemical fibers such as rayon, vinylon, nylon, acrylic, polyurethane, polyester, or acetate. The fabric may be made from these fibers in any form, such as woven fabric, nonwoven fabric, or knitted fabric. The fabric may also be a blended woven fabric or blended nonwoven fabric of two or more types of fibers. [Examples]

[0134] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0135] (Ink manufacturing) Inks 1-11 were manufactured by mixing the components shown in Table I. In Table I, blank spaces in the composition column indicate that the component is not present. Inks 1, 2, 4-6, 8, 9, and 11 are magenta inks, and inks 3, 7, and 10 are cyan inks. The components in Table I are as follows:

[0136] <Coloring agent> • RR24; CI Reactive Red 24 RB49; CI Reactive Blue 49

[0137] <Polymer A and other water-soluble polymers> • Demol N; sodium salt of β-naphthalene sulfonic acid formalin condensate (CAS number: 9084-06-4), manufactured by Kao Corporation, viscosity of 20% aqueous solution: 2 mPa·s • Sodium polyacrylate (MW1200); sodium salt of polyacrylic acid, weight-average molecular weight: 1200, manufactured by Sigma-Aldrich, viscosity of 20% aqueous solution: 5 mPa·s • Sodium polyacrylate (MW8000); sodium salt of polyacrylic acid, weight-average molecular weight: 8000, manufactured by Sigma-Aldrich, viscosity of 20% aqueous solution: 11 mPa·s • Polyethylene glycol 2000; Polyethylene glycol, weight-average molecular weight: 2000, manufactured by Fujifilm Wako Pure Chemical Industries, 20% aqueous solution viscosity: 2 mPa·s Sodium polyacrylate (MW25000); sodium salt of polyacrylic acid, weight-average molecular weight: 25000, manufactured by Sigma-Aldrich, viscosity of 20% aqueous solution: 25 mPa·s • ULV-1; Sodium alginate, manufactured by Kimika, 20% aqueous solution viscosity: 1000 mPa·s

[0138] <Surfactants> • Orphine E1010; Acetylene glycol-based nonionic surfactant (manufactured by Nisshin Chemical Industry Co., Ltd.)

[0139] In the above, the viscosity of 20% aqueous solutions of polymer A and other water-soluble polymers was measured using an E-type viscometer (TV-25, manufactured by Toki Sangyo Co., Ltd.) at 25°C and 20 rpm. The viscosity of the obtained ink was measured in the same manner as above, and the surface tension was measured using the following method, and the results are shown in Table I.

[0140] The surface tension (mN / m) of the ink was measured using a Wilhelmie surface tensimeter in an environment with a relative humidity of 55% and a temperature of 23°C.

[0141] Furthermore, Table I also shows the total solid content [mass%], the total content of organic solvents [mass%], and the proportion of polymer A in the solid content excluding colorants [mass%].

[0142] [Table 1]

[0143] [Ink Sets 1-9] The inks 1 to 11 obtained above were combined as shown in Table II to create ink sets 1 to 9 for the examples and comparative examples. In Table II, the "Ink Category" column indicates whether the ink corresponds to the first ink or the second ink.

[0144] Table II also indicates whether each ink set satisfies requirements (1) to (7), with "○" indicating satisfaction and "×" indicating non-satisfaction. Ink sets that satisfy all requirements (1) to (3) are the ink sets of the embodiment of the present invention (ink sets 1, 3 to 5, 8, 9), while the others are the ink sets of the comparative example (ink sets 2, 6, 7).

[0145] Ink sets 4 and 7 are ink sets in which the first colorant (cyan) and the second colorant (magenta) have different hues, while ink sets 1-3, 4, 6, and 8 are ink sets in which the first and second colorants are of the same color family (magenta). Ink set 9 is an ink set in which the first and second colorants are of the same color family (cyan).

[0146] 〔evaluation〕 The following printing tests were conducted on the above ink sets. For ink sets 1-3, 4, 6, 8, and 9, the transition between gradations was evaluated, and for ink sets 4 and 7, the balance of colors was evaluated. The results are shown in Table II.

[0147] <Printing Test> Using a Konica Minolta KM1024aSAE-Q inkjet head, the ink from the above ink set was ejected under the following conditions, and an image was printed on 100% cotton fabric (with cotton broadcloth, manufactured by Irozome Co., Ltd.) under the following printing conditions.

[0148] (Printing conditions) Driving frequency: 35kHz Droplet volume: 13pl Recording density (resolution): 720 x 720 dpi Printed image: Images with gradations were created by varying the dot density per unit area. Specifically, patch patterns were printed with the duty cycle changed in 10% increments from 100% to 10%. The ink coverage at 100% was 13 g / m². 2 The following was done: When the first and second inks were the same color, the dots were positioned so that the gradation transitions smoothly after 1 minute of image recording. When the first and second inks were not the same color, the dots were positioned so that the print coverage of the first and second inks was equal in each duty cycle.

[0149] (Conditions for leaving images unattended and evaluation criteria) After observing the image one minute after recording, the image was left in an environment of 25°C and 60% relative humidity for one week. Images were visually observed 3 hours, 24 hours, and 1 week after recording, and the gradation (continuity of tones) or color balance was evaluated according to the following criteria. In all evaluations, an A or B rating indicates that the image can be used without problems in practical applications.

[0150] (1) Criteria for evaluating the continuity of tonal gradations A: The transitions between tonal gradations were smooth throughout the entire process. B: Poor transitions in tonal gradation were observed when observed one week later. C: Poor transitions in tonal gradation were observed when observed 24 hours later. D: Poor transitions in tonal gradation were observed when observed 3 hours later.

[0151] (2) Evaluation criteria for the color balance between low-concentration and high-concentration areas A: The color balance was maintained from low density to high density throughout the entire time period. B: When observed one week later, the color balance differed between the low-density and high-density areas. C: When observed 24 hours later, the color balance differed between the low-concentration and high-concentration areas. D: When observed after 3 hours, the color balance differed between the low-concentration and high-concentration areas.

[0152] [Table 2]

[0153] [Ink set combinations] The above evaluation was performed using two ink sets: Ink Set 1, in which the first and second inks are of the same color (magenta), and Ink Set 9, in which the first and second inks are of the same color (cyan). Both the tonal continuity and color balance were evaluated. The results are shown in Table III. In this case, the printed images were formed using the same image data (patch pattern) as the printed images used in the evaluation of Ink Sets 1 to 9 above, by printing so that the images from Ink Set 1 and Ink Set 2 overlapped in the same location.

[0154] [Table 3]

[0155] This provides an inkjet recording ink set that can be applied to print heads requiring higher viscosity and that can maintain the gradation or color balance of the printed image even after time has passed since printing.

Claims

1. An inkjet recording ink set comprising a first ink containing a first coloring agent and a second ink containing a second coloring agent, characterized in that it satisfies the following requirements (1) to (3). (1) The content (by mass) of the first colorant in the first ink is less than the content (by mass) of the second colorant in the second ink. (2) The first ink contains a polymer whose viscosity, measured at 25°C as a 20% by mass aqueous solution, is in the range of 1.8 to 20 mPa·s. (3) The ratio of the total solid content in the second ink to the total solid content in the first ink is within the range of 0.4 to 2.5 by mass.

2. The ink set according to claim 1, characterized in that the first colorant and the second colorant are of the same color.

3. The ink set according to claim 1 or 2, characterized in that the ratio of the content (mass%) of the second colorant in the second ink to the content (mass%) of the first colorant in the first ink is 3.0 or more.

4. The ink set according to any one of claims 1 to 3, characterized in that the first coloring agent and the second coloring agent are dyes.

5. The ink set according to any one of claims 1 to 4, characterized in that the ratio of the total solid content in the second ink to the total solid content in the first ink is within the range of 0.6 to 1.5 by mass.

6. The ink set according to any one of claims 1 to 5, characterized in that the ratio of the viscosity of the second ink at 25°C to the viscosity of the first ink at 25°C is within the range of 0.7 to 1.

3.

7. The ink set according to any one of claims 1 to 6, characterized in that both the first ink and the second ink contain an organic solvent, and the difference between the content of the organic solvent in the first ink (mass%) and the content of the organic solvent in the second ink (mass%) is 10% by mass or less.

8. The ink set according to any one of claims 1 to 7, characterized in that both the first ink and the second ink contain an organic solvent, and the content (mass%) of the organic solvent in the first ink and the content (mass%) of the organic solvent in the second ink are both 30% by mass or less.

9. The ink set according to any one of claims 1 to 8, characterized in that the proportion of the polymer to the amount of solid content in the first ink excluding the first colorant from the total amount of solid content is 50% by mass or more.

10. The ink set according to any one of claims 1 to 9, characterized in that the polymer has anionic groups.

11. The ink set according to any one of claims 1 to 10, wherein the degree of polymerization of the polymer is in the range of 2 to 10.

12. The ink set according to any one of claims 1 to 11, characterized in that the polymer comprises a salt of a polycarboxylic acid or a salt of a naphthalene sulfonic acid formalin condensate.

13. The ink set according to any one of claims 1 to 12, characterized in that the viscosity of the first ink at 25°C and the viscosity of the second ink at 25°C are both 6 mPa·s or more.

Images