Ink set and printing method

Abstract

The objective is to provide an ink set and a method for manufacturing printed materials that can produce printed materials with excellent image density even when multiple types of ink are applied in layers. [Solution] An ink set comprising two types of inkjet inks with different thixotropy indices calculated from the formula described above, wherein each of the two types of inks contains a colorant, a dispersant, and an organic solvent, and the ratio (TB) / (TA) of the thixotropy index (TA) of the ink with the lower thixotropy index (A) to the thixotropy index (TB) of the ink with the higher thixotropy index (B) is 1.04 or more, and the ink (A) is applied to the printing medium, and then the ink (B) is applied so as to overlap at least a part of the area where the ink (A) has been applied. Thixotropy index = [23°C, shear rate 53.6 seconds] -1 Viscosity of inkjet ink measured at 23°C, shear rate 1000 seconds -1 [Viscosity of inkjet ink measured by...]

Description

[Technical Field]

[0001] This disclosure relates to an ink set and a method for manufacturing printed materials. [Background technology]

[0002] Inkjet printing records images on a printing medium placed opposite the nozzles by ejecting highly fluid ink as droplets from fine nozzles. Its low noise and high-speed printing capabilities have led to its rapid adoption in recent years. Ink used in such inkjet printing systems includes various types: water-based inks containing water as the main solvent, non-water-based inks containing non-water-based solvents, UV-curing inks (UV inks) containing high concentrations of polymerizable monomers, and hot-melt inks (solid inks) containing high concentrations of wax. While water is the main solvent in water-based inks, it is common to use water-soluble organic solvents in combination to prevent clogging of inkjet nozzles. Non-water-based inks can be further classified into solvent inks (solvent-based inks) whose main solvent is a volatile organic solvent, and oil-based inks (oil-based inks) whose main solvent is a low-volatility or non-volatile organic solvent. Solvent inks dry primarily on the printing medium through the evaporation of the organic solvent, while oil-based inks dry mainly through penetration into the printing medium.

[0003] Patent Document 1 describes that ink bleeding can be suppressed by using an ink containing charged particles mainly composed of a colorant, a substance having the opposite charge, and a medium. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-168368 [Overview of the project] [Problems that the invention aims to solve]

[0005] In inkjet printing, multiple types of ink are sometimes applied to the printing medium in layers to obtain the desired color tone. In this case, with water-based inks, non-water-based inks, and other inks containing organic solvents, the organic solvents in the previously applied ink can cause the subsequently applied ink to penetrate into the printing medium, resulting in insufficient image density. Furthermore, if the ink penetration into the printing medium is significant, bleed-through occurs. Moreover, if the amount of ink applied later is increased in an attempt to obtain sufficient image density, the likelihood of bleed-through increases even further.

[0006] One of the objectives of this disclosure is to provide an ink set and a method for manufacturing printed materials that can produce printed materials with excellent image density even when multiple types of inks are applied in layers. [Means for solving the problem]

[0007] One embodiment of the present disclosure relates to an ink set comprising two types of inkjet inks with different thixotropy indices calculated from the following formula, wherein each of the two types of inks comprises a colorant, a dispersant, and an organic solvent, and the ratio (TB) / (TA) of the thixotropy index (TA) of the ink with the lower thixotropy index (A) to the thixotropy index (TB) of the ink with the higher thixotropy index (B) is 1.04 or more, and the ink (A) is applied to the printing medium, and then the ink (B) is applied so as to overlap at least a part of the area where the ink (A) has been applied. Thixotropy index = [23°C, shear rate 53.6 seconds] -1 Viscosity of inkjet ink measured at 23°C, shear rate 1000 seconds -1 [Viscosity of inkjet ink measured by...]

[0008] Another embodiment of the present disclosure relates to a method for manufacturing a printed article using two types of inkjet inks with different thixotropy indices calculated from the following formula, wherein each of the two types of inks contains a colorant, a dispersant, and an organic solvent, and the ratio (TB) / (TA) of the thixotropy index (TA) of the ink with the lower thixotropy index (A) to the thixotropy index (TB) of the ink with the higher thixotropy index (B) is 1.04 or more, and the method for manufacturing a printed article involves applying ink (A) to a printing medium, and then applying ink (B) so as to overlap at least a portion of the area where ink (A) has been applied. Thixotropy index = [23°C, shear rate 53.6 seconds] -1 Viscosity of inkjet ink measured at 23°C, shear rate 1000 seconds -1 [Viscosity of inkjet ink measured by...]

[0009] Another embodiment of the present disclosure relates to an ink set comprising two types of oil-based inkjet inks with different thixotropy indices calculated from the following formula, wherein the ratio (Tβ) / (Tα) of the thixotropy index (Tα) of the oil-based ink (α) with the lower thixotropy index to the thixotropy index (Tβ) of the oil-based ink (β) with the higher thixotropy index is 1.04 or greater. Thixotropy index = [23°C, shear rate 53.6 seconds] -1 Viscosity of oil-based inkjet ink measured at 23°C, shear rate 1000 seconds -1 Viscosity of oil-based inkjet ink measured by [method / tool] [Effects of the Invention]

[0010] According to this disclosure, it is possible to provide an ink set and a method for manufacturing printed materials that can produce printed materials with excellent image density even when multiple types of ink are applied in layers. [Modes for carrying out the invention]

[0011] The following describes in detail some embodiments of the present invention, but this disclosure is not limited to these embodiments.

[0012] In this disclosure, inkjet ink will also be referred to as "ink." Oil-based inkjet ink will also be referred to as "oil-based ink." Acrylic resin refers collectively to resins obtained by polymerizing acrylic acid, methacrylic acid, and their derivatives, either individually or in combination. (Meth)acrylic acid refers collectively to acrylic acid and methacrylic acid, and (meth)acrylic acid ester refers collectively to acrylic acid ester and methacrylic acid ester.

[0013] One embodiment of this ink set is an ink set containing two types of inkjet inks with different thixotropy indices calculated from the following formula, wherein each of the two types of ink contains a colorant, a dispersant, and an organic solvent, and the ratio (TB) / (TA) of the thixotropy index (TA) of the ink with the lower thixotropy index (A) to the thixotropy index (TB) of the ink with the higher thixotropy index (B) is 1.04 or more, and after applying ink (A) to the printing medium, ink (B) is applied so as to overlap at least a part of the area where ink (A) has been applied, this is an ink set (hereinafter also referred to as "ink set (1)"). Thixotropy index = [23°C, shear rate 53.6 seconds] -1 Viscosity of inkjet ink measured at 23°C, shear rate 1000 seconds -1 [Viscosity of inkjet ink measured by...]

[0014] In the ink set (1), the ratio (TB) / (TA) of the thixotropy index (TA) of the ink (A) with the lower thixotropy index and the thixotropy index (TB) of the ink (B) with the higher thixotropy index is 1.04 or more. By applying the ink (A) with the lower thixotropy index first and then applying the ink (B) with the higher thixotropy index, the action of the ink (B) penetrating into the printing medium due to the organic solvent contained in the previously applied ink (A) can be reduced. As a result, the coloring material contained in the ink (B) remains sufficiently on the surface of the printing medium, and a printed matter with excellent image density can be obtained. In addition, since it is not necessary to apply the ink (B) excessively to increase the image density, bleeding of the ink can also be suppressed.

[0015] The ratio (TB) / (TA) of the thixotropy indices of the two inks is 1.04 or more. The upper limit value is not particularly limited. For example, from the perspective of balance with the ejection property of inkjet, it may be 2 or less, may be 1.5 or less, or may be 1.2 or less.

[0016] The value of the viscosity of the ink used for calculating the thixotropy index may be measured in any manner as long as the conditions of temperature and shear rate are satisfied. As an example, it can be measured using a rheometer "MCR 302" manufactured by Anton Paar.

[0017] The types of colorants contained in the ink set (1), including ink (A) and ink (B), the color tone of the ink, etc. are not particularly limited, and any number of colors of ink may be used. Generally, as combinations of inks that are often applied in layers, inks that have a light-dark color relationship with each other can be mentioned. As a specific example, ink (A) may be a dark black ink and the ink (B) may be a light black ink. Also, ink (A) and ink (B) may have the same hue, with ink (A) being a dark ink and ink (B) being a light ink. In the present disclosure, when two or more types of inks have the same hue, it means that the hue angle is within 45° on the Ostwald hue circle. The hues of ink (A) and ink (B) may be hues with a hue angle within 45° on the Ostwald hue circle, may be hues within 30°, or may be hues within 15°. The lightness and darkness of the ink can be adjusted by the content of the colorant, etc.

[0018] The ink set (1) may contain three or more types of inks. When the ink set (1) contains three or more types of inks, it is sufficient that the ratio of the thixotropy index is 1.04 or more in at least two types of inks, and it may contain one or more types of inks that do not satisfy the requirement that the ratio of the thixotropy index is 1.04 or more in relation to other inks. As an example of the combination of inks, for example, for one type of ink corresponding to ink (A) with a low thixotropy index, it may contain one or more types of inks corresponding to ink (B) with a high thixotropy index. For multiple types of inks corresponding to ink (A) with a low thixotropy index, it may contain one type of ink corresponding to ink (B) with a high thixotropy index. It may contain multiple sets of combinations of inks in which the ratio of the thixotropy index is 1.04 or more.

[0019] More specific configurations of the ink set (1) include, for example, one in which ink (A) is a dark black ink, ink (B) is a light black ink, and further includes one or more selected from the group consisting of cyan ink, magenta ink, and yellow ink having any thixotropy index; one or more in which inks (A1) and (B1) are dark black inks, and inks (AX) and (BX) are dark black inks; and one or more in which inks (A1) and (B1) are dark black inks, and inks (AX) and (BX) are dark black inks, and one or more in which inks have any thixotropy index.

[0020] The two types of inks included in ink set (1) may each contain a colorant, a surfactant, and an organic solvent, and may be, for example, water-based ink or oil-based ink. In particular, the effects of this disclosure are more pronounced when the ink is oil-based. Below, an example of an ink, an oil-based ink, will be described.

[0021] The colorants contained in the oil-based ink may be one type alone or two or more types in combination. The colorants are preferably non-oil soluble, and in particular, it is preferable that they have very low solubility in the non-aqueous solvent contained in the oil-based ink. As an indicator of the degree of non-oil solubility, for example, the amount of colorant that can be dissolved in 100g of the non-aqueous solvent contained in the oil-based ink is preferably 3g or less, more preferably 1g or less, and particularly preferably 0.5g or less, at a temperature of 23°C. When multiple types of colorants are used, the above dissolution amounts are the dissolution amounts of each individual colorant.

[0022] Various pigments can be used as colorants. Examples of pigments include organic pigments such as azo pigments, phthalocyanine pigments, polycyclic pigments, and underglaze lake pigments; and inorganic pigments such as carbon black and metal oxides. Examples of azo pigments include soluble azo lake pigments, insoluble azo pigments, and condensed azo pigments. Examples of phthalocyanine pigments include metallic phthalocyanine pigments and metal-free phthalocyanine pigments. Examples of polycyclic pigments include quinacridone pigments, perylene pigments, perinone pigments, isoindoline pigments, isoindolinone pigments, dioxazine pigments, thioindigo pigments, anthraquinone pigments, quinophthalone pigments, metal complex pigments, and diketopyrrolopyrrole (DPP). Examples of carbon black include furnace carbon black, lamp black, acetylene black, and channel black. Examples of metal oxides include titanium dioxide and zinc oxide.

[0023] The median diameter of the pigment in the ink is preferably 300 nm or less, and more preferably 200 nm or less, from the viewpoint of storage stability and ejection stability of the oil-based ink. The median diameter of the pigment in the ink may be in the range of 50 to 300 nm, or it may be 100 to 200 nm.

[0024] The amount of colorant in the oil-based ink is not particularly limited, but may be in the range of 1 to 30% by mass, for example, from the viewpoint of the surface density of the printed material and the viscosity of the ink.

[0025] Oil-based inks may contain extender pigments as needed. Extender pigments may be used individually or in combination of two or more. When oil-based inks contain extender pigments, the thixotropy index tends to increase. Examples of extender pigments include talc, diatomaceous earth, calcium carbonate, barium carbonate, barium sulfate, alumina white, silica, kaolin, mica, acid clay, activated clay, and bentonite. The median diameter of the extender pigment is not particularly limited, but may be in the range of 50 to 300 nm, for example.

[0026] If the oil-based ink contains an extender pigment, its content is adjusted as appropriate according to the desired thixotropy index, etc., but may be in the range of, for example, 0.5 to 15% by mass.

[0027] As mentioned above, the thixotropy index of oil-based inks can be adjusted by including extender pigments. However, as another method of adjusting the thixotropy index, oil-based inks may also contain thickeners. One type of thickener may be used alone, or two or more types may be used in combination. Examples of thickeners include the Esben series from Hojun Co., Ltd. When oil-based inks contain thickeners, the amount is appropriately adjusted according to the desired thixotropy index, etc., but may be in the range of, for example, 0.5 to 15% by mass.

[0028] The dispersant contained in oil-based ink may be used alone or in combination of two or more types. Examples of dispersants include polymer dispersants and surfactant-type dispersants.

[0029] Examples of commercially available polymer dispersants include, for example, the TEGO Disperse series from EVONIK ("TEGO Disperse 740W", "TEGO Disperse 750W", "TEGO Disperse 755W", "TEGO Disperse 757W", "TEGO Disperse 760W", etc.) and the Solsperse series from Lubrizol Japan Co., Ltd. ("Solsperse 11200", "Solsperse 13940", "Solsperse 16000", "Solsperse 17000", "Solsperse 18000", "Solsperse 19000", "Solsperse 20000", "Solsperse 22000", "Solsperse 24000", "Solsperse 27000", "Solsperse 28000", "Solsperse 32000", "Solsperse 38500", "Solsperse 39 Examples include "Luspers 41000", "Solperspers 41090", "Solperspers 43000", "Solperspers 44000", "Solperspers 46000", "Solperspers 71000", etc., the Joncryl series from BASF Japan Ltd. ("Joncryl 57", "Joncryl 60", "Joncryl 62", "Joncryl 63", "Joncryl 71", "Joncryl 501", etc.), "DISPERBYK-102", "DISPERBYK-185", "DISPERBYK-190", "DISPERBYK-193", "DISPERBYK-199", etc. from BIC Chemie Japan Co., Ltd., "Polyvinylpyrrolidone K-30", "Polyvinylpyrrolidone K-90" from Daiichi Kogyo Seiyaku Co., Ltd., and "Antaron V200" from Ashland Japan Co., Ltd.

[0030] Examples of commercially available surfactant-type dispersants include anionic surfactants such as the Demol series manufactured by Kao Corporation ("Demol P", "Demol EP", "Demol N", "Demol RN", "Demol NL", "Demol RNL", "Demol T-45", etc.) and nonionic surfactants such as the Emulgen series manufactured by Kao Corporation ("Emulgen A-60", "Emulgen A-90", "Emulgen A-500", "Emulgen B-40", "Emulgen L-40", "Emulgen 420", etc.).

[0031] The amount of dispersant in the oil-based ink is adjusted as appropriate depending on the type and amount of colorant, for example, but may be in the range of 1 to 20% by mass.

[0032] Oil-based inks contain a non-aqueous solvent as an organic solvent. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination. The non-aqueous solvent may be either a non-polar organic solvent or a polar organic solvent. The non-aqueous solvent may be a non-water-soluble organic solvent that does not mix uniformly with the same volume of water at 1 atmosphere and 20°C. Oil-based inks may also contain other organic solvents to the extent that they can form a single phase with the non-aqueous solvent.

[0033] Examples of nonpolar organic solvents include petroleum-based hydrocarbon solvents such as aliphatic hydrocarbon solvents, alicyclic hydrocarbon solvents, and aromatic hydrocarbon solvents. Examples of aliphatic hydrocarbon solvents and alicyclic hydrocarbon solvents include non-aqueous solvents such as paraffinic, isoparaffinic, and naphthenic solvents. These commercially available products include: ENEOS Corporation's "No. 0 Solvent L", "No. 0 Solvent M", "No. 0 Solvent H", "Cactus Normal Paraffin N-10", "Cactus Normal Paraffin N-11", "Cactus Normal Paraffin N-12D", "Cactus Normal Paraffin N-13", "Cactus Normal Paraffin N-14", "Cactus Normal Paraffin YHNP", "Cactus Normal Paraffin SHNP", "Isozol 300", "Isozol 400", "Tecclean N16", "Tecclean N20", "Tecclean N22", "AF Solvent No. 4", "AF Solvent No. 5", "AF Solvent No. 6", "AF Solvent No. 7", "Naphthezol 160", "Naphthezol 200", "Naphthezol 220"; ExxonMobil's "Isopar G", "Isopar H Examples include "BHT", "Isopar L", "Isopar M", "Exol D40", "Exol D60", "Exol D80", "Exol D110", and "Exol D130"; and "Moresco White P-60", "Moresco White P-70", "Moresco White P-80", "Moresco White P-100", "Moresco White P-120", "Moresco White P-150", "Moresco White P-200", "Moresco White P-260", and "Moresco White P-350P" manufactured by MORESCO Corporation. Examples of aromatic hydrocarbon solvents include "Solvesso 100", "Solvesso 150", "Solvesso 200", and "Solvesso 200ND" manufactured by ExxonMobil. The initial boiling point of the distillation of petroleum-based hydrocarbon solvents is preferably 100°C or higher, more preferably 150°C or higher, and particularly preferably 200°C or higher. The initial boiling point of the distillation can be measured according to JIS K0066 "Distillation Test Method for Chemical Products".

[0034] Preferred polar organic solvents include fatty acid ester solvents, higher alcohol solvents, and higher fatty acid solvents. For example, isononyl isononanoate, isodecyl isononanoate, isotridecyl isononanoate, methyl laurate, isopropyl laurate, hexyl laurate, isopropyl myristate, octyldodecyl myristate, isopropyl palmitate, hexyl palmitate, isooctyl palmitate, isostearyl palmitate, methyl oleate, ethyl oleate, isopropyl oleate, butyl oleate, hexyl oleate, methyl linoleate, ethyl linoleate, isobutyl linoleate, butyl stearate, hexyl stearate, isooctyl stearate, isopropyl isostearate, 2-octyldecyl pivalate, soybean oil fatty acid methyl Examples include fatty acid ester solvents with 13 or more carbon atoms per molecule, preferably 16 to 30, such as fatty acid esters, soybean oil fatty acid isobutyl esters, tall oil fatty acid methyl esters, and tall oil fatty acid isobutyl esters; higher alcohol solvents with 6 or more carbon atoms per molecule, preferably 12 to 20, such as isomyristyl alcohol, isopalmityl alcohol, isostearyl alcohol, oleyl alcohol, isoeicosyl alcohol, and decyltetradecanol; and higher fatty acid solvents with 12 or more carbon atoms per molecule, preferably 14 to 20, such as lauric acid, isomyristic acid, palmitic acid, isopalmitic acid, α-linolenic acid, linoleic acid, oleic acid, and isostearic acid. The boiling point of polar organic solvents such as fatty acid ester solvents, higher alcohol solvents, and higher fatty acid solvents is preferably 150°C or higher, more preferably 200°C or higher, and even more preferably 250°C or higher. Note that non-aqueous solvents with a boiling point of 250°C or higher also include non-aqueous solvents that do not exhibit a boiling point.

[0035] The content of non-aqueous solvents in oil-based inks is adjusted as appropriate according to the desired viscosity of the oil-based ink, but may be in the range of, for example, 20 to 95% by mass.

[0036] Oil-based ink may contain resin as needed. One type of resin may be used alone, or two or more types may be used in combination. The resin is preferably non-oil soluble, and in particular, it is preferable that it has low solubility in the non-aqueous solvent contained in the oil-based ink. As an indicator of the degree of non-oil solubility, for example, the amount of resin that can dissolve in 100g of the non-aqueous solvent contained in the oil-based ink is preferably 3g or less at 23°C. When multiple types of resin are used, the above dissolution amount is the dissolution amount of each individual resin. The type of resin is not particularly limited, and examples include acrylic resin, urethane resin, polyester resin, alkyd resin, silicone resin, etc.

[0037] Oil-based inks may contain various additives as needed. These additives may include nozzle clogging inhibitors, antioxidants, conductivity modifiers, viscosity modifiers, surface tension modifiers, oxygen absorbers, and the like.

[0038] The method for manufacturing oil-based ink is not particularly limited, but for example, it can be manufactured by blending and mixing a colorant, a dispersant, an organic solvent, and other optional components. Dispersion treatment may be performed using a bead mill during mixing.

[0039] The viscosity of the ink contained in ink set (1) is adjusted as appropriate from the standpoint of inkjet ejection performance, regardless of the type of ink, i.e., whether the ink is water-based or oil-based. The viscosity of the ink may be similar to that of general inkjet inks, for example, in the range of 9 to 13 mPa·s at 23°C. The method for measuring viscosity is as described above.

[0040] The method of inkjet printing using the ink set (1) of one embodiment is not particularly limited, and any method such as piezo, electrostatic, or thermal may be used. When using an inkjet recording device, it is preferable to eject ink from the inkjet head based on a digital signal and to adhere the ejected ink droplets to the printing medium.

[0041] The printing medium is not particularly limited, and printing papers such as plain paper, coated paper, and special paper, cloth, porous sheets, etc., and adhesive sheets provided with an adhesive layer on the back surface using these as a base material can be used. When the ink is an oil-based ink, printing papers such as plain paper and coated paper can preferably be used from the viewpoint of the permeability of the non-aqueous solvent.

[0042] Here, plain paper is paper on which an ink receiving layer, a film layer, etc. are not formed on normal paper. Examples of plain paper include high-quality paper, medium-quality paper, PPC paper, waste paper, recycled paper, etc. Plain paper is paper in which paper fibers with a thickness of several μm to several tens of μm form voids of several tens to several hundreds of μm, so it is paper through which a non-aqueous solvent easily penetrates.

[0043] Also, as the coated paper, inkjet coated papers such as matte paper, glossy paper, and semi-glossy paper, and so-called coated printing papers can preferably be used. Here, the coated printing paper is a printing paper that has conventionally been used in letterpress printing, offset printing, gravure printing, etc., and is a printing paper provided with a coating layer by a coating containing an inorganic pigment such as clay or calcium carbonate and a binder such as starch on the surface of high-quality paper or medium-quality paper. The coated printing paper is classified into micro-coated paper, high-quality lightweight coated paper, medium-quality lightweight coated paper, high-quality coated paper, medium-quality coated paper, art paper, cast coated paper, etc. depending on the coating amount and coating method of the coating.

[0044] A method for manufacturing a printed matter according to an embodiment is a method for manufacturing a printed matter using two types of inkjet inks having different thixotropy indices calculated from the following calculation formula. The two types of inks each contain a coloring material, a dispersant, and an organic solvent, and the ratio (TB) / (TA) of the thixotropy index (TA) of the ink (A) with a lower thixotropy index and the thixotropy index (TB) of the ink (B) with a higher thixotropy index is 1.04 or more. After applying the ink (A) to the printing medium, the ink (B) is applied so as to overlap at least a part of the application region of the ink (A). Thixotropy index = [23°C, shear rate 53.6 seconds -1Viscosity of inkjet ink measured at 23°C, shear rate 1000 seconds -1 [Viscosity of inkjet ink measured by...]

[0045] The method for manufacturing the printed material can be carried out, for example, using two types of inkjet inks, ink (A) and ink (B), which are included in the above-mentioned ink set.

[0046] In the method for manufacturing printed materials, ink (A) with a lower thixotropy index is applied to the printing medium, and then ink (B) with a higher thixotropy index is applied so as to overlap at least a portion of the area where ink (A) has been applied. The application of ink (A) and ink (B) may be performed consecutively, or a step of applying another ink may be inserted between the application of ink (A) and ink (B). The other ink may be one type or two or more types.

[0047] In a method for manufacturing printed materials, if there are multiple types of inks whose application areas overlap, it is sufficient that at least two of these inks have a relationship between ink (A) and ink (B). However, for obtaining higher-quality images, it is preferable that the relationship between ink (A) and ink (B) holds true for all inks whose application areas overlap. Note that "overlapping application areas" refers to areas where the application areas are intentionally overlapped to obtain a desired color tone, and does not apply to areas that overlap unintentionally due to ink bleeding or other reasons on the printing medium.

[0048] One embodiment of the ink set is an ink set containing two types of oil-based inkjet inks with different thixotropy indices calculated from the following formula, wherein the ratio (Tβ) / (Tα) of the thixotropy index (Tα) of the oil-based ink (α) with the lower thixotropy index to the thixotropy index (Tβ) of the oil-based ink (β) with the higher thixotropy index is 1.04 or higher, and this ink set (hereinafter also referred to as "ink set (2)") is used. Thixotropy index = [23°C, shear rate 53.6 seconds] -1Viscosity of oil-based inkjet ink measured at 23°C, shear rate 1000 seconds -1 Viscosity of oil-based inkjet ink measured by [method / tool]

[0049] Examples of oil-based ink (α) include the oil-based inks among the inks (A) included in ink set (1). Examples of oil-based ink (β) include the oil-based inks among the inks (B) included in ink set (1), etc.

[0050] The ratio (Tβ) / (Tα) of the thixotropy index (Tα) of oil-based ink (α) to the thixotropy index (Tβ) of oil-based ink (β) is 1.04 or greater. While there is no particular upper limit, for example, from the standpoint of balancing inkjet ejection performance, it may be 2 or less, 1.5 or less, or 1.2 or less, as described above. The method for measuring the viscosity of the ink used to calculate the thixotropy index is as described above.

[0051] The types of colorants contained in oil-based ink (α) and oil-based ink (β), as well as the ink's hue, are not particularly limited; any color ink is acceptable. For example, inks that are shades of each other may be used. Specifically, oil-based ink (α) may be a dark black ink, and oil-based ink (β) may be a light black ink. Alternatively, oil-based ink (α) and oil-based ink (β) may be of the same hue, with oil-based ink (α) being a dark ink and oil-based ink (β) being a light ink. The ink's shade can be adjusted by the amount of colorant contained.

[0052] Ink set (2) may contain three or more types of oil-based inks. If ink set (2) contains three or more types of oil-based inks, it is sufficient that the ratio of thixotropy indices of at least two of the oil-based inks is 1.04 or higher, and it may contain one or more types of oil-based inks that do not satisfy the requirement of having a ratio of thixotropy indices of 1.04 or higher in relation to the other oil-based inks. As an example of combinations of oil-based inks, for example, one type of oil-based ink corresponding to oil-based ink (α) may contain one or more types of oil-based inks corresponding to oil-based ink (β). Multiple types of inks corresponding to oil-based ink (α) may contain one type of oil-based ink corresponding to oil-based ink (β). Multiple combinations of oil-based inks that have a thixotropy index ratio of 1.04 or higher may be included.

[0053] More specific configurations of the ink set (2) include, for example, one in which oil-based ink (α) is a dark black ink, oil-based ink (β) is a light black ink, and further includes one or more selected from the group consisting of cyan ink, magenta ink, and yellow ink having any thixotropy index; one or more in which oil-based inks (α1) and (β1) are dark black inks, and oil-based inks (αX) and (βX) are dark black inks; and one or more in which oil-based inks (α1) and (β1) are dark black inks, and oil-based inks (αX) and (βX) are dark black inks, and one or more in which oil-based inks have any thixotropy index.

[0054] The viscosity of the oil-based ink contained in ink set (2) is adjusted as appropriate from the viewpoint of inkjet ejection performance, etc. The viscosity of the oil-based ink may be similar to that of general inkjet inks, for example, in the range of 9 to 13 mPa·s at 23°C. The method for measuring viscosity is as described above.

[0055] The ink set (2) can be used particularly favorably when two or more types of ink are applied in layers, for example, to adjust the color tone. Specifically, by applying oil-based ink (α) to the printing medium and then applying oil-based ink (β) so as to overlap at least a portion of the area where oil-based ink (α) has been applied, the effect of the organic solvent contained in the previously applied oil-based ink (α) causing the oil-based ink (β) to penetrate into the printing medium can be reduced. As a result, the colorants contained in the oil-based ink (β) remain sufficiently on the surface of the printing medium, and a printed material with excellent image density can be obtained. In addition, since it is not necessary to apply an excessive amount of oil-based ink (β) to increase image density, ink bleed-through can also be suppressed.

[0056] Some embodiments of this disclosure are shown below. <1> An ink set comprising two types of inkjet inks with different thixotropy indices calculated from the following formula, wherein each of the two inks contains a colorant, a dispersant, and an organic solvent, and the ratio (TB) / (TA) of the thixotropy index (TA) of the ink with the lower thixotropy index (A) to the thixotropy index (TB) of the ink with the higher thixotropy index (B) is 1.04 or higher, and the ink set is configured such that ink (A) is applied to the printing medium, and then ink (B) is applied so as to overlap at least a portion of the area where ink (A) has been applied. Thixotropy index = [23°C, shear rate 53.6 seconds] -1 Viscosity of inkjet ink measured at 23°C, shear rate 1000 seconds -1 [Viscosity of inkjet ink measured by...]

[0057] <2 The ratio (TB) / (TA) of the thixotropy index (TA) of ink (A) to the thixotropy index (TB) of ink (B) is 1.20 or less. <1> The ink set described above.

[0058] <3> The ink (A) is a dark black ink, and the ink (B) is a light black ink, <1> or <2> The oil-based inkjet ink described above.

[0059] <4> The ink (A) and the ink (B) are of the same hue, the ink (A) is a dark ink, and the ink (B) is a light ink, <1> or <2> The oil-based inkjet ink described above.

[0060] <5> The viscosity of ink (A) and the viscosity of ink (B) are each independently in the range of 9 to 13 mPa·s. <1> ~ <4> Oil-based inkjet ink as specified in one of the following lists.

[0061] <6> A method for manufacturing printed materials using two types of inkjet inks with different thixotropy indices calculated from the following formula, wherein each of the two inks contains a colorant, a dispersant, and an organic solvent, and the ratio (TB) / (TA) of the thixotropy index (TA) of the ink with the lower thixotropy index (A) to the thixotropy index (TB) of the ink with the higher thixotropy index (B) is 1.04 or more, and the method for manufacturing printed materials involves applying ink (A) to a printing medium, and then applying ink (B) so as to overlap at least a portion of the area where ink (A) has been applied. Thixotropy index = [23°C, shear rate 53.6 seconds] -1 Viscosity of inkjet ink measured at 23°C, shear rate 1000 seconds -1 [Viscosity of inkjet ink measured by...]

[0062] <7> An ink set containing two types of oil-based inkjet inks with different thixotropy indices calculated using the following formula, wherein the ratio (Tβ) / (Tα) of the thixotropy index (Tα) of the oil-based ink (α) with the lower thixotropy index to the thixotropy index (Tβ) of the oil-based ink (β) with the higher thixotropy index is 1.04 or greater. Thixotropy index = [23°C, shear rate 53.6 seconds] -1 Viscosity of oil-based inkjet ink measured at 23°C, shear rate 1000 seconds -1 Viscosity of oil-based inkjet ink measured by [method / tool]

[0063] <8> The oil-based ink (α) is a dark black ink, and the oil-based ink (β) is a light black ink, <7> The ink set described above.

[0064] <9> The oil-based ink (α) and the oil-based ink (β) are of the same hue, the oil-based ink (α) is a dark color ink, and the oil-based ink (β) is a light color ink, <7> The ink set described above. [Examples]

[0065] The present invention will be described in detail below with reference to examples. The present invention is not limited to the following examples. Throughout the following examples and comparative examples, common components are identical unless otherwise specified. Unless otherwise specified, "%" indicates "mass%".

[0066] [Production of pigment dispersions 1-12] Each component was blended in the proportions shown in Table 1, and the mixture was dispersed using a bead mill manufactured by Shinmaru Enterprises, Inc. ("DinoMill MULTI LAB") to obtain pigment dispersions 1 to 12.

[0067] The details of each component in Table 1 are as follows: Colorant 1: Carbon black, manufactured by Asahi Carbon Co., Ltd. "SB200" Colorant 2: Phthalocyanine blue pigment, DIC Corporation "FASTOGEN BLUE FA5380" Extender pigment 1: Silica, manufactured by Nippon Aerosil Co., Ltd. "AEROSIL R976" Extender pigment 2: Calcium carbonate, manufactured by Kondo Lime Industry Co., Ltd. Thickening agent: "Esben N400" manufactured by Hojun Co., Ltd. Dispersant 1: Ashland Japan Co., Ltd. "Antaron V200" Dispersant 2: Lubrizol Nippon Co., Ltd. "Solspers 18000" Organic solvent 1: "Exsol D-130" manufactured by ENEOS Corporation

[0068] [Table 1]

[0069] [Examples 1-8 and Comparative Examples 1 and 2] Pigment dispersions and organic solvents 2 and 3 were blended in the proportions shown in Tables 2 and 3 to prepare each ink and ink set. The viscosity of the ink used to calculate the thixotropy index was measured using an Anton Paar rheometer "MCR 302". The ink sets were loaded into a line-type inkjet printer (Riso Kagaku Corporation's "Orphis FW5230"), and a solid image of ink (A) was printed on one side of plain paper (Riso Kagaku Corporation's "Riso Paper Multi") at 30 pl. Then, a solid image of ink (B) was printed over the printed area of ​​ink (A) at 20 pl. This was repeated 100 times to obtain printed materials. Various evaluation tests were performed on the obtained printed materials using the following methods. The results are shown in Tables 2 and 3. In Comparative Example 1, a solid image of ink (B) was printed at 30 pl, and then a solid image of ink (A) was printed over the printed area of ​​ink (B) at 20 pl.

[0070] The details of each component in Tables 2 and 3 are as follows. Pigment dispersants 1-12: Pigment dispersants 1-12 obtained earlier Organic solvent 2: Octyldodecyl myristate manufactured by Higher Alcohol Industry Co., Ltd. Organic solvent 3: "AF Solvent No. 5" manufactured by ENEOS Corporation

[0071] [Evaluation of surface density of printed materials] After leaving the printed materials at room temperature for 24 hours, one sheet was randomly selected, and the surface OD value was measured using an optical densitometer (Macbeth Corporation "RD920") and evaluated according to the following criteria. A: OD value is 1.10 or higher B: OD value is less than 1.10

[0072] [Evaluation of bleed-through in printed materials] After leaving the printed materials at room temperature for 24 hours, one sheet was randomly selected, and the OD value on the reverse side was measured using an optical densitometer (Macbeth Corporation "RD920") and evaluated according to the following criteria. A: OD value less than 0.13 B: OD value is 0.13 or higher

[0073] [Ink Mist Review] One sheet was randomly selected from the printed materials, and the interface between the solid image and the paper was observed. The degree of mist staining for both ink (A) and ink (B) was visually checked and evaluated according to the following criteria. A: No contamination is observed around the image, or there is almost no contamination, so there are no practical problems. B: Contamination is visible around the edges of the image.

[0074] [Evaluation of conveyor roller contamination] After completing a continuous print run of 100 sheets, a blank sheet of paper was fed into the printer, and the presence or absence of ink on the blank paper was visually checked and evaluated according to the following criteria. A: There is no ink residue, or very little ink residue, so there are no practical problems. B: Ink stains are visible.

[0075] [Table 2]

[0076] [Table 3]

[0077] As shown in Tables 2 and 3, in the example where multiple inks were applied in layers, using two inkjet inks with a thixotropy index ratio of 1.04 or higher, and applying the ink with the lower thixotropy index (A) first, followed by the ink with the higher thixotropy index (B), resulted in printed materials with excellent image density. Furthermore, ink bleed-through, ink mist generation, and transport roller contamination were suppressed. On the other hand, in Comparative Example 1, where the thixotropy index ratio was less than 1.04 and the ink with the higher thixotropy index was applied first, and in Comparative Example 2, where the thixotropy index ratio was less than 1.04, the image density was low, and ink bleed-through also occurred.

Claims

1. An ink set comprising two types of inkjet inks with different thixotropy indices calculated from the following formula, wherein each of the two inks contains a colorant, a dispersant, and an organic solvent, and the ratio (TB) / (TA) of the thixotropy index (TA) of the ink with the lower thixotropy index (A) to the thixotropy index (TB) of the ink with the higher thixotropy index (B) is 1.04 or more, and the ink set is configured such that ink (A) is applied to the printing medium, and then ink (B) is applied so as to overlap at least a portion of the area where ink (A) has been applied. Thixotropy index = [23°C, shear rate 53.6 seconds] －１ [Viscosity of inkjet ink measured at 23°C, shear rate 1000 seconds] －１ Viscosity of inkjet ink measured by [method / tool]

2. The ink set according to claim 1, wherein the ratio (TB) / (TA) of the thixotropy index (TA) of ink (A) to the thixotropy index (TB) of ink (B) is 1.20 or less.

3. The ink set according to claim 1, wherein ink (A) is a dark black ink and ink (B) is a light black ink.

4. The ink set according to claim 1, wherein ink (A) and ink (B) are of the same hue, ink (A) is a dark ink, and ink (B) is a light ink.

5. The ink set according to claim 1, wherein the viscosity of ink (A) and the viscosity of ink (B) are each independently in the range of 9 to 13 mPa·s.

6. A method for manufacturing printed materials using two types of inkjet inks with different thixotropy indices calculated from the following formula, wherein each of the two types of inks contains a colorant, a dispersant, and an organic solvent, and the ratio (TB) / (TA) of the thixotropy index (TA) of the ink with the lower thixotropy index (A) to the thixotropy index (TB) of the ink with the higher thixotropy index (B) is 1.04 or more, and after applying ink (A) to a printing medium, ink (B) is applied so as to overlap at least a part of the area where ink (A) has been applied. Thixotropy index = [23°C, shear rate 53.6 seconds] －１ [Viscosity of inkjet ink measured at 23°C, shear rate 1000 seconds] －１ Viscosity of inkjet ink measured by [method / tool]

7. An ink set containing two types of oil-based inkjet inks with different thixotropy indices calculated from the following formula, wherein the ratio (Tβ) / (Tα) of the thixotropy index (Tα) of the oil-based ink (α) with the lower thixotropy index to the thixotropy index (Tβ) of the oil-based ink (β) with the higher thixotropy index is 1.04 or greater. Thixotropy index = [23°C, shear rate 53.6 seconds] －１ Viscosity of oil-based inkjet ink measured at [23°C, shear rate 1000 seconds] －１ Viscosity of oil-based inkjet ink measured by [method / tool]

8. The ink set according to claim 7, wherein the oil-based ink (α) is a dark black ink, and the oil-based ink (β) is a light black ink.

9. The ink set according to claim 7, wherein the oil-based ink (α) and the oil-based ink (β) are of the same hue, the oil-based ink (α) is a dark-colored ink, and the oil-based ink (β) is a light-colored ink.

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