Image formation methods

Abstract

To provide an image formation method which can form a high-quality image with good workability, in image formation by an inkjet method.SOLUTION: An image formation method includes a landing step of discharging droplets of inkjet ink from an inkjet head, and landing the droplets onto a base material, wherein the inkjet ink contains an aqueous solvent, a pigment, a fixing resin, and a temperature-responsive polymer, the temperature-responsive polymer has a lower limit critical consolute temperature to water, with respect to the total amount of the inkjet ink, a content of the pigment is within the range of 2 to 10 mass%, a content of the fixing resin is within the range of 1 to 10 mass%, a content of the temperature-responsive polymer is within the range of 0.01 to 1 mass%, and the landing step heats the droplets at the lower limit critical consolute temperature of ±10°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an image forming method, and more particularly to an image forming method using an inkjet method that is capable of forming high-quality images with good workability. [Background technology]

[0002] In image formation using the inkjet method, droplets of inkjet ink (hereinafter simply referred to as "ink") are ejected from an inkjet head (hereinafter simply referred to as "head") toward a substrate, and after landing on the substrate, the solvent is dried to form a coating (image). When an image is formed on a non-absorbent substrate made of PET (polyethylene terephthalate) or the like using this inkjet method, problems arise such as adjacent ink droplets being pulled together after landing on the substrate but before drying, causing streaks, or adjacent ink droplets of different colors mixing and causing bleeding.

[0003] One possible solution to this problem is to introduce a heated platen into the image forming apparatus, which instantly dries the solvent after the ink lands on the substrate and thickens it. However, this requires the platen to be heated to a high temperature to prevent the bleeding, which causes the meniscus surface of the head to dry out and the ink to stick around the head, resulting in ejection problems.

[0004] It is also disclosed that additives are added to the ink to give it thixotropy, and that as the ink circulates within the head, shear is applied, resulting in low viscosity; however, when the ink is ejected and lands on the substrate in a low shear state, the viscosity increases, making it difficult for the ink to be absorbed by the absorbent substrate, and allowing high-density images to be formed without heating the platen.

[0005] For example, Patent Document 1 describes an aqueous inkjet ink in which the ink viscosity at low shear is 1.5 times or more that at high shear by adding a water-soluble polymer compound to impart thixotropy to the ink. Patent Document 1 also describes that when the ink is circulated through a head and ejected, it has low viscosity at high shear, but when it lands, it becomes high viscosity due to low shear, preventing a decrease in concentration due to the absorbing substrate. However, Patent Document 1 does not describe an attempt to control ink viscosity increase by combining the thixotropy of the ink itself with drying conditions. It is assumed that a method of controlling the thixotropy of the ink itself, as in Patent Document 1, alone cannot achieve both good ejection performance and high-quality image formation.

[0006] Furthermore, Patent Document 2 describes an aqueous inkjet ink to which a water-soluble temperature-responsive polymer has been added. Patent Document 2 describes the effect of suppressing ink penetration into a paper substrate and increasing ink density. Patent Document 2 describes the addition of approximately 5% by mass of the temperature-responsive polymer to the entire ink, more than twice the amount of the colorant. It also describes that when heated, the resin contracts and covers the colorant, thereby fixing it. However, Patent Document 2 does not control the thixotropy so as to change the viscosity of the ink inside the head and at the time of impact. Therefore, it is assumed that the ink described in Patent Document 2 cannot achieve both good ejection performance and high-quality image formation. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-225859 [Patent Document 2] Japanese Patent Application Publication No. 11-236523 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in consideration of the above problems and circumstances, and the problem to be solved by the present invention is to provide an image forming method that is capable of forming high-quality images with good workability when forming images by an inkjet method. [Means for solving the problem]

[0009] In order to solve the above-mentioned problems, the present inventors investigated the causes of the above-mentioned problems and investigated the composition of an inkjet ink using a temperature-responsive polymer, using the viscosity of the inkjet ink at high shear during circulation through the head and the viscosity at low shear during impact as indicators. As a result, they discovered that by incorporating a temperature-responsive polymer and a fixing resin in combination, each in a predetermined content range, into an inkjet ink containing an aqueous solvent and a pigment, it is possible to obtain an ink that has a low viscosity during circulation through the head and a sufficiently high viscosity upon impact with slight temperature adjustment, thereby arriving at the present invention. By possessing the above-mentioned properties, the inkjet ink is capable of maintaining good ejection properties and forming high-quality images. That is, the above-mentioned problems of the present invention are solved by the following means.

[0010] 1. An image forming method having a landing step of ejecting inkjet ink droplets from an inkjet head and landing them on a substrate, the inkjet ink contains a water-based solvent, a pigment, a fixing resin, and a temperature-responsive polymer; the temperature-responsive polymer has a lower critical solution temperature with respect to water; the content of the pigment is within a range of 2 to 10% by mass, the content of the fixing resin is within a range of 1 to 10% by mass, and the content of the temperature-responsive polymer is within a range of 0.01 to 1% by mass, relative to the total amount of the inkjet ink; In the impact step, +5~ An image forming method characterized by heating in the range of 10°C.

[0011] 2. The image forming method according to 1 above, wherein in the impacting step, only the substrate and the inkjet ink are heated within a range of the lower critical solution temperature +5 to 10°C. 3The ink-jet ink has a viscosity of 15 mPa·s or less at 25°C at a shear rate of 1000 (1 / s), and a viscosity of 45 mPa·s or more at a shear rate of 1 (1 / s) at any temperature within a range of the lower critical solution temperature ±10°C. Or the above 2 The image forming method according to claim 1.

[0012] 4 The ink-jet ink has a viscosity of 100 mPa·s or more at a shear rate of 1 (1 / s) at any temperature within a range of the lower critical solution temperature ±10°C when the aqueous solvent is removed from the ink-jet ink so that the ink-jet ink has a mass of 80% of the initial mass. Any one of the above items 1 to 3 The image forming method according to claim 1.

[0013] 5 The temperature-responsive polymer has a lower critical solution temperature in water in the range of 30 to 60°C. 4 10. The image forming method according to claim 1, wherein the first and second inks are ink-receiving inks.

[0014] 6 The temperature-responsive polymer comprises a water-soluble cellulose resin. 5 10. The image forming method according to claim 1, wherein the first and second inks are ink-receiving inks.

[0015] 7 The content of the aqueous solvent in the total amount of the inkjet ink is in the range of 50 to 90% by mass. 6 10. The image forming method according to claim 1, wherein the first and second inks are ink-receiving inks.

[0016] 8 The fixing resin is selected from the group consisting of polyacrylic resin, polyurethane resin, and polyester resin. 7 10. The image forming method according to claim 1, wherein the first and second inks are ink-receiving inks.

[0017] 9 The ink-jet ink further contains a thixotropic agent. 8 10. The image forming method according to claim 1, wherein the first and second inks are ink-receiving inks.

[0018] 10 The thixotropic agent includes cellulose nanofibers or smectite clay minerals. 9 The image forming method according to claim 1.

[0019] 11 The content of the thixotropy-imparting agent is in the range of 0.01 to 1% by mass relative to the total amount of the inkjet ink. 9 or the above 10 The image forming method according to claim 1.

[0020] 12 The viscosity of the droplets when they land on the substrate or immediately after they land is 150 mPa·s or more. 11 10. The image forming method according to claim 1, wherein the first and second inks are ink-receiving inks.

[0021] 13 The step of depositing the droplets includes heating the droplets on the substrate within a range of 30 to 60°C. 12 10. The image forming method according to claim 1, wherein the first and second inks are ink-receiving inks.

[0022] 14 The immediately after landing is within 100 msec after the droplets land on the substrate. 12 or the above 13 The image forming method according to claim 1. [Effects of the Invention]

[0025] The above-described means of the present invention can provide an inkjet ink that can maintain good ejection properties while forming high-quality images. Furthermore, an image forming method can be provided that can form high-quality images with good workability when forming images by an inkjet method. The mechanism by which the effects of the present invention are manifested or acted upon is not clear, but is speculated as follows.

[0026] The inkjet ink of the present disclosure contains an aqueous solvent, a pigment, a fixing resin, and a temperature-responsive polymer, the temperature-responsive polymer having a lower critical solution temperature with water, and the content of the pigment is within the range of 2 to 10% by mass, the content of the fixing resin is within the range of 1 to 10% by mass, and the content of the temperature-responsive polymer is within the range of 0.01 to 1% by mass, relative to the total amount of the inkjet ink. Unless otherwise specified, in this specification, the lower critical solution temperature of the temperature-responsive polymer refers to the lower critical solution temperature with water.

[0027] By setting the ink composition within the above range, the viscosity at high shear can be kept low, for example, the ink viscosity (25°C) at a shear rate of 1000 (1 / s) can be kept at 15 mPa s or less, which allows the ink to circulate smoothly in the head and be ejected smoothly from the head.

[0028] By ensuring that the ink composition falls within the above range, the viscosity at the time of impact can be high; for example, the viscosity at a shear rate of 1 (1 / s) can be 45 mPa·s or higher at any temperature within a range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer. In other words, the maximum viscosity at a shear rate of 1 (1 / s) within a range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer can be 45 mPa·s or higher. If the viscosity at this temperature satisfies the above requirement, good pinning can be achieved at the time of impact without removing the aqueous solvent from the ink, and the resulting image can be of high quality. In other words, the ink of the present invention has the high viscosity required for good pinning within a range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer with water. This allows a large amount of aqueous solvent to remain in the ink from the time of ejection to the time of impact, thereby suppressing adhesion of ink solids to the area around the head, reducing ejection defects, and facilitating maintenance.

[0029] Furthermore, the combined use of a temperature-responsive polymer and a fixing resin can impart water resistance to the ink coating film obtained by image formation. This effect is thought to be achieved by the entanglement of the temperature-responsive polymer and the fixing resin during the image formation process. Furthermore, this entanglement improves the substrate fixation of the ink coating film obtained by image formation.

[0030] The image forming method of the present invention includes a landing step of ejecting droplets of inkjet ink from an inkjet head equipped with an ink circulation mechanism and landing the ink on a substrate, wherein the inkjet ink is circulated within the inkjet head so that the viscosity thereof is 15 mPa s or less, which is a viscosity that allows the ink to be smoothly circulated within the head and ejected from the head.

[0031] In the image forming method of the present invention, the ink droplet landing process is performed so that the mass loss rate of the ink droplets when they land on the substrate or immediately after landing is within 20% of the ink circulating within the head and the viscosity is 100 mPa·s or greater. When the mass loss rate and viscosity of the ink droplets when they land on the substrate or immediately after landing are within the above ranges, good pinning is possible and the resulting image can be of high quality. Furthermore, adhesion of ink solids to the area around the head is suppressed, which reduces ejection defects and facilitates maintenance. [Brief explanation of the drawings]

[0032] [Figure 1] FIG. 1 is a schematic diagram showing the main components of an inkjet image forming apparatus equipped with an example of an inkjet head that can be used in the image forming method of the present invention. [Figure 2] An enlarged cross-sectional view of the head chip of the inkjet head shown in FIG. [Figure 3] FIG. 2 is a plan view of the nozzle plate of the inkjet head shown in FIG. 1; [Figure 4] Graph used to determine crossover distortion (%) of the ink in Example 1 DETAILED DESCRIPTION OF THE INVENTION

[0033] The inkjet ink of the present disclosure is an inkjet ink containing an aqueous solvent, a pigment, a fixing resin, and a temperature-responsive polymer, the temperature-responsive polymer having a lower critical solution temperature with water, and characterized in that the content of the pigment is within the range of 2 to 10% by mass, the content of the fixing resin is within the range of 1 to 10% by mass, and the content of the temperature-responsive polymer is within the range of 0.01 to 1% by mass, relative to the total amount of the inkjet ink. This feature is a technical feature common to the following embodiments of the inkjet ink of the present disclosure.

[0034] In an embodiment of the ink of the present disclosure, from the viewpoint of maintaining good jetting properties and forming high-quality images at the same time, it is preferred that the viscosity at 25°C and a shear rate of 1000 (1 / s) is 15 mPa s or less, and that the viscosity at a shear rate of 1 (1 / s) at any temperature within a range of ±10°C from the lower critical solution temperature is 45 mPa s or more.

[0035] In an embodiment of the ink of the present disclosure, from the viewpoint of both maintaining good ejection properties and forming high-quality images, it is preferred that the ink has a viscosity of 100 mPa s or more at a shear rate of 1 (1 / s) at any temperature within a range of ±10°C from the lower critical solution temperature when the aqueous solvent is removed from the inkjet ink so that the ink has a mass of 80% of the initial mass (hereinafter, this state is also referred to as "ink drying rate 20%).

[0036] This allows the ink to maintain the viscosity required for bleeding resistance even at a drying rate of 20%, allowing a large amount of water-based solvent to remain in the ink from the time of ejection to the time of impact. This prevents ink solids from adhering to the area around the head, making ejection less likely to occur and facilitating maintenance.

[0037] In an embodiment of the ink of the present disclosure, from the viewpoint of maintaining good jetting properties and forming high-quality images at the same time, the lower critical solution temperature of the temperature-responsive polymer in water is preferably within a range of 30 to 60° C. Furthermore, the temperature-responsive polymer preferably contains a water-soluble cellulose resin.

[0038] In an embodiment of the ink of the present disclosure, from the viewpoint of maintaining good ejection properties and forming high-quality images at the same time, it is preferable that the content of the aqueous solvent is within the range of 50 to 90% by mass relative to the total amount of the inkjet ink.

[0039] In an embodiment of the ink of the present disclosure, from the viewpoint of exerting the effects of the present invention, it is preferable that the fixing resin contains at least one resin selected from the group consisting of polyacrylic resin, polyurethane resin, and polyester resin.

[0040] In an embodiment of the ink of the present disclosure, from the viewpoint of maintaining good ejection properties while simultaneously forming high-quality images, it is preferable that the inkjet ink further contains a thixotropy-imparting agent. From the same viewpoint, it is also preferable that the thixotropy-imparting agent contains cellulose nanofiber or a smectite clay mineral. From the same viewpoint, it is also preferable that the content of the thixotropy-imparting agent is in the range of 0.01 to 1% by mass of the total amount of the inkjet ink.

[0041] The image forming method of the present disclosure includes a landing step of ejecting inkjet ink droplets from an inkjet head equipped with an ink circulation mechanism and landing them on a substrate, wherein the inkjet ink is circulated within the inkjet head so that the viscosity of the inkjet ink is 15 mPa s or less, and the landing step is performed so that the mass loss rate of the inkjet ink droplets upon or immediately after landing on the substrate is 20% or less and the viscosity of the inkjet ink droplets is 100 mPa s or more. This feature is a technical feature common to the following embodiments of the image forming method of the present disclosure.

[0042] In an embodiment of the image forming method of the present disclosure, from the viewpoint of further exhibiting the effects of the present invention, it is preferable that the viscosity of the droplets when they land on the substrate or immediately after they land is 150 mPa·s or more.

[0043] In an embodiment of the image forming method of the present disclosure, from the viewpoint of further exhibiting the effects of the present invention, it is preferable that the landing step includes heating the droplets on the substrate at a temperature in the range of 30 to 60° C. Furthermore, it is preferable that the time immediately after landing is within 100 msec after the droplets land on the substrate.

[0044] In an embodiment of the image forming method of the present disclosure, when the substrate is a non-absorbent substrate, the effects of the present invention are more pronounced, which is preferable.

[0045] In an embodiment of the image forming method of the present disclosure, from the viewpoint of further exerting the effects of the present invention, it is preferable that the inkjet head comprises: a pressure chamber into which the inkjet ink is injected via an injection path; pressure generating means for generating pressure fluctuations in the pressure chamber; a nozzle that communicates with the pressure chamber and serves as a flow path for the inkjet ink that is ejected from the pressure chamber to the outside due to the pressure fluctuations in the pressure chamber; and two or more circulation paths that communicate with the pressure chamber and discharge the inkjet ink inward from the nozzle and return it to the injection path.

[0046] In an embodiment of the image forming method of the present disclosure, from the viewpoint of further exhibiting the effects of the present invention, it is preferable to use the inkjet ink of the present invention as the inkjet ink.

[0047] The present invention, its components, and embodiments for carrying out the present invention will be described in detail below. In this application, the symbol "to" is used to mean that the numerical values ​​before and after it are included as lower and upper limits.

[0048] [Inkjet ink overview] <Inkjet ink composition> The ink of the present disclosure is an inkjet ink containing a water-based solvent, a pigment, a fixing resin, and a temperature-responsive polymer, wherein the temperature-responsive polymer has a lower critical solution temperature with respect to water, and the content of the pigment is within a range of 2 to 10 mass %, the content of the fixing resin is within a range of 1 to 10 mass %, and the content of the temperature-responsive polymer is within a range of 0.01 to 1 mass %, relative to the total amount of the inkjet ink.

[0049] In this specification, the term "aqueous solvent" refers to water or a solvent containing water and an aqueous solvent. An aqueous solvent is a solvent that is compatible with water at room temperature.

[0050] In addition to the aqueous solvent, pigment, fixing resin, and temperature-responsive polymer, the ink of the present disclosure may contain any optional components that do not impair the effects of the present invention. Examples of optional components include a thixotropic agent, a pigment dispersant, and a surfactant. Each component of the ink of the present disclosure is described below.

[0051] (temperature-responsive polymer) The temperature-responsive polymer contained in the ink of the present disclosure is a polymer that has a lower critical solution temperature (hereinafter also referred to as "LCST") with respect to water. By having an LCST with respect to water, the temperature-responsive polymer has the property of being soluble in water below the LCST and insoluble in water above the LCST, e.g., by gelling. In other words, an aqueous solution of the temperature-responsive polymer has the property of undergoing a phase change with the LCST as the critical point. This property is reversible. The temperature-responsive polymer's property makes it possible to adjust the viscosity of the ink in the head and when it lands on the substrate to appropriate ranges, respectively, and thereby achieve both good ejection properties and high-quality image formation.

[0052] The LCST of a temperature-responsive polymer in water can be measured by, for example, measuring the phase transition temperature of an aqueous solution of the temperature-responsive polymer using a differential scanning calorimeter (DSC), or by measuring the change in light transmittance of the aqueous solution while gradually increasing (decreasing) the temperature of the aqueous solution.

[0053] The LCST of the temperature-responsive polymer is preferably in the range of 30 to 60°C, more preferably in the range of 40 to 50°C. If the LCST of the temperature-responsive polymer is 30°C or higher, the viscosity of the ink does not increase significantly in the temperature range in which the ink normally circulates within the head. Furthermore, if the LCST of the temperature-responsive polymer is 60°C or lower, the viscosity of the ink droplets can be increased to a desired level without increasing the temperature to a high level upon impact and without removing the aqueous solvent.

[0054] As described below, the aqueous solvent contained in the ink may consist of water, but may also contain an optional aqueous solvent other than water. When the aqueous solvent contains water and an aqueous solvent, the LCST of the temperature-responsive polymer relative to the aqueous solvent may differ from the LCST of the temperature-responsive polymer relative to water. The difference between the two is preferably within approximately 10°C. In other words, the LCST of the temperature-responsive polymer relative to the aqueous solvent is preferably within ±10°C of the LCST of the temperature-responsive polymer relative to water.

[0055] Specific examples of temperature-responsive polymers include water-soluble cellulose resins, poly(N-substituted (meth)acrylamides), poly(N-vinyl acylamides), poly((meth)acrylic acid alkyl esters), and other temperature-responsive polymers. Among these, water-soluble cellulose resins are preferred as the temperature-responsive polymers used in the ink of the present invention. In this specification, (meth)acrylamide is a general term for acrylamide and methacrylamide. Similarly, (meth)acrylic acid is a general term for acrylic acid and methacrylic acid.

[0056] Water-soluble cellulose resins are resins that have been given water solubility by substituting some of the hydrogen atoms of the numerous OH groups present in cellulose to eliminate hydrogen bonds. Examples of substituents that substitute the hydrogen atoms of the OH groups include alkyl groups, hydroxyalkyl groups, and carboxy groups. Preferred substituents are methyl groups, hydroxypropyl groups, and hydroxyethyl groups. One type of substituent may be used alone, or two or more types may be used in combination.

[0057] Specific examples of the water-soluble cellulose resin include methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, and carboxymethyl cellulose.

[0058] The substituent in the poly(N-substituted (meth)acrylamide) is preferably an alkyl group which may have an ether bond (-O-) between carbon atoms. The alkyl group may be cyclic, straight-chain or branched, or a combination thereof. The nitrogen atom may have one or two substituents, and when there are two substituents, they may be bonded to form a ring. The alkyl group preferably has 1 to 10 carbon atoms.

[0059] Specific examples of poly(N-substituted (meth)acrylamide) include poly(N-ethylacrylamide), poly(Nn-propyl(meth)acrylamide), poly(N-isopropyl(meth)acrylamide), poly(N-cyclopropyl(meth)acrylamide), and poly(N,N-diethyl(meth)acrylamide).

[0060] The poly(N-substituted (meth)acrylamide) does not have to be composed of a single monomer. That is, the poly(N-substituted (meth)acrylamide) may be a copolymer of N-substituted (meth)acrylamides having different substituents. Furthermore, the poly(N-substituted (meth)acrylamide) may be a copolymer of N-substituted (meth)acrylamide and at least one monomer selected from the monomers constituting the following polymers, for example, poly(N-vinyl acylamides) and poly((meth)acrylic acid alkyl esters).

[0061] Examples of poly(N-vinyl acyl amide) include poly(N-vinyl acetamide), poly(N-vinyl propionic acid amide), poly(N-vinyl butyric acid amide), and poly(N-vinyl isobutyric acid amide).

[0062] Examples of poly((meth)acrylic acid alkyl esters) include poly(n-butyl methacrylate), poly(dodecyl methacrylate), poly(n-hexyl methacrylate), poly(methyl methacrylate), and poly(n-octyl methacrylate).

[0063] Other temperature-responsive polymers include poly(N,N-acrylpyrrolidone), poly(N-acrylpiperidine), and their copolymers, alternating copolymers of polyisobutyl vinyl ether and maleic anhydride, poly(2-methyl-5-vinylpyridine), poly(vinyl alcohol), poly(N-vinylpyrrolidone), poly(N-vinylsuccinic acid), poly(vinyl sulfonic acid), poly(oxyethylene), poly(tetrahydrofuran), amylopectin, amylose, poly(vinyl acetate), poly(propylene glycol), polyvinyl methyl ether (PVME), and polyvinylmethyloxazolidinone.

[0064] In the present invention, as the temperature-responsive polymer, one of these may be used alone, or two or more of them may be used in combination.

[0065] The weight-average molecular weight of the temperature-responsive polymer is not particularly limited, but is preferably from 10,000 to 100,000, and more preferably from 20,000 to 80,000. If the weight-average molecular weight of the temperature-responsive polymer is 10,000 or more, the polymer will be entangled with the fixing resin, making it easier to impart water resistance to the ink coating (image). Furthermore, if the weight-average molecular weight is 100,000 or less, there is little risk of the ink becoming too viscous and deteriorating its ejection properties.

[0066] The weight average molecular weight of the temperature-responsive polymer is a weight average molecular weight measured by gel permeation chromatography (GPC) and calculated as polystyrene.

[0067] The temperature-responsive polymer may be a commercially available product, such as water-soluble cellulose resins manufactured by Shin-Etsu Chemical Co., Ltd., under the trade names Metolose SM-04 (methylcellulose resin, LCST: 55°C) and Metolose 60SH-03 (hydroxypropylmethylcellulose resin, LCST: 75°C).

[0068] Commercially available poly(N-substituted (meth)acrylamide) products include poly(N-isopropylacrylamide) manufactured by Sigma-Aldrich Corporation, such as poly(N-isopropylacrylamide (LCST: 35°C, weight-average molecular weight: 30,000).

[0069] The content of the temperature-responsive polymer in the ink of the present invention is within the range of 0.01 to 1% by mass of the total amount of ink. If the content of the temperature-responsive polymer is within this range, the effects of the present invention can be obtained by blending the temperature-responsive polymer. The content of the temperature-responsive polymer is preferably within the range of 0.05 to 0.8% by mass.

[0070] (fixing resin) The ink of the present invention contains a fixing resin. By containing a fixing resin in addition to a temperature-responsive polymer, the ink of the present invention can appropriately adjust the viscosity of the ink inside the head and when it lands on the substrate, and can impart water resistance to the ink coating film obtained by image formation. The fixing resin also functions as a binder for the pigment colorant, improving the adhesion of the coating film to the substrate, particularly a non-absorbent substrate, and improving the abrasion resistance of the coating film obtained using the ink. The fixing resin is preferably a water-insoluble resin. The water-insoluble resin as the fixing resin is preferably used in the form of fine particles dispersed in an aqueous solvent.

[0071] The water-insoluble resin microparticles are formed by dispersing the water-insoluble resin in an aqueous solvent as microparticles. The microparticles may be, for example, formed by forcibly emulsifying the water-insoluble resin using an emulsifier or the like and dispersing the microparticles in the aqueous solvent. Alternatively, the water-insoluble resin may be self-emulsified by introducing a hydrophilic functional group into the molecule, thereby forming stable microparticles in the aqueous solvent without using an emulsifier or dispersion stabilizer. The aqueous solvent in which the water-insoluble resin microparticles are dispersed can be the same aqueous solvent as described above, and typically, water or a water / alcohol mixed solvent is used. Hereinafter, the water-insoluble resin microparticles dispersed in the aqueous solvent will also be referred to as an aqueous dispersion.

[0072] In the present invention, the term "water-insoluble resin" refers to a resin that, when dried at 105° C. for 2 hours and then dissolved in 100 g of water at 25° C., dissolution amounts of the resin are 10 g or less, preferably 5 g or less, and more preferably 1 g or less. However, if the resin has salt-forming groups, the dissolution amount refers to the amount dissolved when the salt-forming groups of the resin are 100% neutralized with acetic acid or sodium hydroxide, depending on the type of resin.

[0073] The fixing resin according to the present invention is preferably a water-insoluble resin such as a polyester resin, a polyurethane resin, or a polyacrylic resin. As the fixing resin according to the present invention, one of these may be used alone, or two or more of them may be used in combination.

[0074] The fixing resin is contained in the range of 1 to 10% by mass relative to the total mass of the ink (100% by mass). If the fixing resin content is within this range, the effects of the present invention can be obtained by blending the fixing resin. The fixing resin content is preferably within the range of 2 to 8% by mass, and more preferably within the range of 3 to 6% by mass.

[0075] The content of the temperature-responsive polymer and the fixing resin in the ink is preferably such that the ratio (percentage) of the content of the temperature-responsive polymer to the total content of the temperature-responsive polymer and the fixing resin is within the range of 0.1 to 20% by mass. This ratio is more preferably 0.5 to 10% by mass. By having the ratio of the content of the temperature-responsive polymer to the total content of the temperature-responsive polymer and the fixing resin within the above range, it is easy to achieve both water resistance and good pinning properties in the coating film (image) obtained from the ink.

[0076] [Polyester resin] The polyester resin used as the fixing resin can be obtained by using a polyhydric alcohol component and a polycarboxylic acid component such as a polycarboxylic acid, a polycarboxylic acid anhydride, or a polycarboxylic acid ester.

[0077] The polyhydric alcohol component includes dihydric alcohols (diols), specifically alkylene glycols having 2 to 36 carbon atoms (ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butylene glycol, 1,6-hexanediol, etc.), alkylene ether glycols having 4 to 36 carbon atoms (diethylene glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, polybutylene glycol, etc.), alicyclic diols having 6 to 36 carbon atoms (diols), and alkylene ether glycols having 6 to 36 carbon atoms (diols). Examples of suitable alkylene oxides include alicyclic diols (1,4-cyclohexanedimethanol, hydrogenated bisphenol A, etc.), adducts of the above-mentioned alicyclic diols with alkylene oxides (ethylene oxide (hereinafter abbreviated as EO), propylene oxide (hereinafter abbreviated as PO), butylene oxide (hereinafter abbreviated as BO)) having 2 to 4 carbon atoms (number of moles added: 1 to 30), and adducts of bisphenols (bisphenol A, bisphenol F, bisphenol S, etc.) with alkylene oxides (EO, PO, BO, etc.) having 2 to 4 carbon atoms (number of moles added: 2 to 30). These may be used alone or in combination of two or more.

[0078] Examples of the polyvalent carboxylic acid component include dicarboxylic acids (dicarboxylic acids), specifically alkane dicarboxylic acids having 4 to 36 carbon atoms (succinic acid, apidic acid, sebacic acid, etc.), alkenyl succinic acids (dodecenyl succinic acid, etc.), alicyclic dicarboxylic acids having 4 to 36 carbon atoms (dimer acids (dimerized linoleic acid), etc.), alkene dicarboxylic acids having 4 to 36 carbon atoms (maleic acid, fumaric acid, citraconic acid, mesaconic acid, etc.), and aromatic dicarboxylic acids having 8 to 36 carbon atoms (phthalic acid, isophthalic acid, terephthalic acid or derivatives thereof, naphthalenedicarboxylic acid, etc.). These may be used alone or in combination of two or more.

[0079] The number average molecular weight of the polyester resin is preferably within a range of 1,000 to 50,000, and more preferably within a range of 2,000 to 20,000.

[0080] As the polyester resin, a commercially available product may be used. For example, a dispersion in which the polyester resin is dispersed in an aqueous solvent as an aqueous dispersion may be used as the commercially available product. The following examples of commercially available dispersions are given below. In the examples, the number average molecular weight of the polyester resin contained in the product (dispersion) is shown in parentheses. These may be used alone or in combination of two or more.

[0081] The following are all trade names: Elitel KA-5034 (manufactured by Unitika Ltd., number average molecular weight: 8500), Elitel KA-5071S (manufactured by Unitika Ltd., number average molecular weight: 8500), Elitel KA-1449 (manufactured by Unitika Ltd., number average molecular weight: 7000), Elitel KA-0134 (manufactured by Unitika Ltd., number average molecular weight: 8500), Elitel KA-3556 (manufactured by Unitika Ltd., number average molecular weight: 8000), Elitel KA-6137 (manufactured by Unitika Ltd., number average molecular weight: 5000), Elitel KZA-6034 (manufactured by Unitika Ltd., number average molecular weight: 6500), Examples include Elitel KT-8803 (manufactured by Unitika Ltd., number average molecular weight: 15,000), Elitel KT-8701 (manufactured by Unitika Ltd., number average molecular weight: 13,000), Elitel KT-9204 (manufactured by Unitika Ltd., number average molecular weight: 17,000), Elitel KT-8904 (manufactured by Unitika Ltd., number average molecular weight: 17,000), Elitel KT-0507 (manufactured by Unitika Ltd., number average molecular weight: 17,000), Elitel KT-9511 (manufactured by Unitika Ltd., number average molecular weight: 17,000), and Vylonal MD-2000 (manufactured by Toyobo Co., Ltd., number average molecular weight: 18,000).

[0082] [Polyurethane resin] The polyurethane resin used as the fixing resin may have a hydrophilic group. Examples of the hydrophilic group include a carboxyl group (-COOH) and its salts, a sulfonic acid group (-SO3H) and its salts, etc. Examples of the salts include alkali metal salts such as sodium salts and potassium salts, and amine salts. Of the hydrophilic groups, a carboxyl group or its salt is preferred.

[0083] The polyurethane resin is preferably an aqueous dispersion in which a self-emulsifying polyurethane having a water-soluble functional group in its molecule is dispersed in an aqueous solvent, or an aqueous dispersion in which a forced-emulsifying polyurethane emulsified under strong mechanical shear force in combination with a surfactant is dispersed in an aqueous solvent. The polyurethane resin in the aqueous dispersion can be obtained by reacting a polyol with an organic polyisocyanate and a hydrophilic group-containing compound.

[0084] Examples of polyols that can be used to prepare the aqueous dispersion of the polyurethane resin include polyester polyols, polyether polyols, polycarbonate polyols, and polyolefin polyols.

[0085] Examples of polyester polyols include low molecular weight polyols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2- and 1,3-propylene glycol, neopentyl glycol, 1,3- and 1,4-butanediol, 3-methylpentanediol, hexamethylene glycol, 1,8-octanediol, 2-methyl-1,3-propanediol, bisphenol A, hydrogenated bisphenol A, trimethylolpropane, and cyclohexanedimethanol; and condensates thereof with polycarboxylic acids such as succinic acid, glutaric acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, tetrahydrofuran acid, endomethinetetrahydrofuran acid, and hexahydrophthalic acid.

[0086] Examples of polyether polyols include polyethylene glycol, polypropylene glycol, polyethylene polytetramethylene glycol, polypropylene polytetramethylene glycol, and polytetramethylene glycol.

[0087] Examples of polycarbonate polyols include compounds obtained by reacting a carbonic acid derivative, such as diphenyl carbonate, dimethyl carbonate, or phosgene, with a diol. Examples of the diol include ethylene glycol, diethylene glycol, triethylene glycol, 1,2- and 1,3-propylene glycol, neopentyl glycol, 1,3- and 1,4-butanediol, 3-methylpentanediol, hexamethylene glycol, 1,8-octanediol, 2-methyl-1,3-propanediol, bisphenol A, hydrogenated bisphenol A, trimethylolpropane, and cyclohexanedimethanol.

[0088] Examples of organic polyisocyanates that can be used to prepare aqueous dispersions of polyurethane resins include aromatic isocyanates such as tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), polymeric MDI, xylylene diisocyanate (XDI), and tetramethylxylylene diisocyanate (TMXDI); aliphatic isocyanates such as hexamethylene diisocyanate (HMDI); and alicyclic isocyanates such as isophorone diisocyanate (IPDI) and 4,4'-dicyclohexylmethane diisocyanate (hydrogenated MDI, H12MDI). These may be used alone or in combination of two or more.

[0089] Examples of hydrophilic group-containing compounds that can be used to prepare the aqueous dispersion of polyurethane resin include carboxylic acid-containing compounds such as 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolbutyric acid, 2,2-dimethylolvaleric acid, and glycine, and derivatives thereof such as sodium salts, potassium salts, and amine salts; and sulfonic acid-containing compounds such as taurine (i.e., aminoethylsulfonic acid) and ethoxypolyethylene glycol sulfonic acid, and derivatives thereof such as sodium salts, potassium salts, and amine salts.

[0090] The polyurethane resin can be obtained by a known method, for example, by mixing the above-mentioned polyol, organic polyisocyanate, and hydrophilic group-containing compound and reacting them at 30 to 130°C for 30 minutes to 50 hours.

[0091] The polyurethane resin is polymerized by extending the chain with a chain extender to form a polyurethane resin having a hydrophilic group. The chain extender is preferably water and / or an amine compound. By using water or an amine compound as the chain extender, the chain can react with free isocyanate in a short time, efficiently extending the isocyanate-terminated prepolymer.

[0092] Examples of the amine compound as a chain extender include aliphatic polyamines such as ethylenediamine and triethylenediamine, aromatic polyamines such as metaxylenediamine and toluylenediamine, and polyhydrazino compounds such as hydrazine and adipic acid dihydrazide. The amine compound may contain, together with the polyamine, a monovalent amine such as dibutylamine or methyl ethyl ketoxime as a reaction terminator to the extent that the polymerization is not significantly inhibited.

[0093] In addition, in the synthesis of polyurethane resin, a solvent that is inert to isocyanate and can dissolve the urethane prepolymer may be used. Examples of such solvents include dioxane, methyl ethyl ketone, dimethylformamide, tetrahydrofuran, N-methyl-2-pyrrolidone, toluene, and propylene glycol monomethyl ether acetate. These hydrophilic organic solvents used in the reaction step are preferably finally removed.

[0094] In addition, in the synthesis of polyurethane resins, catalysts such as amine catalysts (e.g., triethylamine, N-ethylmorpholine, triethyldiamine, etc.), tin-based catalysts (e.g., dibutyltin dilaurate, dioctyltin dilaurate, tin octoate, etc.), and titanium-based catalysts (e.g., tetrabutyl titanate, etc.) may be added to promote the reaction.

[0095] The number-average molecular weight of the polyurethane resin is preferably increased as much as possible by introducing a branched structure or an internal crosslinked structure, and a number-average molecular weight of 50,000 to 10,000,000 is preferable. By keeping the molecular weight within this range, the polyurethane resin becomes less soluble in solvents, resulting in a coating film with excellent weather resistance and water resistance. In this specification, the number-average molecular weight (Mn) is a value measured by gel permeation chromatography (GPC). The number-average molecular weight (Mn) can be determined, for example, from a calibration curve prepared using a polystyrene standard sample using a Shimadzu Corporation "RID-6A" column (column: Tosoh Corporation "TSK-GEL" column, solvent: tetrahydrofuran (THF), column temperature: 40°C).

[0096] The polyurethane resin may be a commercially available product, such as a dispersion in which the polyurethane resin is dispersed in an aqueous solvent as an aqueous dispersion.

[0097] Examples of commercially available dispersions of the polyurethane resin include WBR-016U (manufactured by Taisei Fine Chemical Co., Ltd.), Superflex 620, Superflex 650, Superflex 500M, Superflex E-2000 (all manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., "Superflex" is a registered trademark of the company), Permarin UC-20 (manufactured by Sanyo Chemical Industries, Ltd., "Permarin" is a registered trademark of the company), Parasurf UP-22 (manufactured by Ohara Palladium Chemical Co., Ltd.), and Evaphanol HA-560 (manufactured by Nicca Chemical Co., Ltd.).

[0098] [Polyacrylic resin] Examples of polyacrylic resins used as fixing resins include (co)polymers of (meth)acrylic ester components, and copolymers of (meth)acrylic ester components and polymerizable components other than (meth)acrylic ester components, such as styrene components.

[0099] Examples of the (meth)acrylic acid ester component include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, cyclohexyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, cyclohexyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxypropyl (meth)acrylate, cyclohexyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate, 4 ... butyl (meth)acrylate, benzyl (meth)acrylate, glycidyl (meth)acrylate, (meth)acrylic acid, (di)ethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin di(meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, and acrylamide.

[0100] Examples of the styrene component include styrene, 4-methylstyrene, 4-hydroxystyrene, 4-acetoxystyrene, 4-acetylstyrene, and styrene sulfonic acid. These components may be used alone or in combination of two or more.

[0101] The number average molecular weight (Mn) of the polyacrylic resin is preferably 1000 to 50000, and more preferably 2000 to 20000. When the number average molecular weight (Mn) of the polyacrylic resin is 1000 or more, the cohesive force of the coating film is strong and the adhesion is improved, and when it is 50000 or less, the solubility in organic solvents is good and the particle size of the emulsion dispersion is promoted to be miniaturized.

[0102] The polyacrylic resin may be a commercially available product, such as a dispersion in which the polyacrylic resin is dispersed in an aqueous solvent as an aqueous dispersion.

[0103] Examples of commercially available dispersions of the above polyacrylic resins include Delpet 60N and 80N (manufactured by Asahi Kasei Corporation, "Delpet" is a registered trademark of the company), Dianal BR52, BR80, BR83, BR85, and BR88 (manufactured by Mitsubishi Chemical Corporation, "Dianal" is a registered trademark of the company), KT75 (manufactured by Denka Company Limited), or Vinyblanc 2680, 2682, 2684, and 2685 (manufactured by Nissin Chemical Industry Co., Ltd., "Vinyblanc" is a registered trademark of the company), and Mowinyl 6800D (manufactured by Japan Coating Resins Co., Ltd.).

[0104] Among these, it is preferable that the fixing resin contains an acid structure. The acid structure allows dispersion in an aqueous solvent without the addition of a surfactant, i.e., self-emulsification is possible, improving the water resistance of the coating film. Such self-emulsifying resins can be dispersed and stabilized in an aqueous solvent solely through the ionic nature of the molecules. Examples of acid structures include acid groups such as a carboxyl group (-COOH) and a sulfonic acid group (-SO3H). The acid structure may be present in a side chain or at the terminal of the resin.

[0105] It is preferable that the acid structures are partially or entirely neutralized. Neutralizing the acid structures can improve the water dispersibility of the resin. Examples of neutralizing agents for neutralizing the acid structures include organic amines, and it is preferable to use organic amines such as trimethylamine, triethylamine, tripropylamine, tributylamine, N-methyldiethanolamine, and triethanolamine.

[0106] (pigment) The pigment contained in the ink according to the present invention may be any conventionally known organic or inorganic pigment, including 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 quinophthaloni pigments, dye lakes such as basic dye lakes and acid dye lakes, organic pigments such as nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments, and inorganic pigments such as carbon black.

[0107] Specific examples of organic pigments that can be preferably used include the following pigments.

[0108] Examples of pigments for magenta or red include CI Pigment Red 2, CI Pigment Red 3, CI Pigment Red 5, CI Pigment Red 6, CI Pigment Red 7, CI Pigment Red 15, CI Pigment Red 16, CI Pigment Red 48:1, CI Pigment Red 53:1, CI Pigment Red 57:1, CI Pigment Red 122, CI Pigment Red 123, CI Pigment Red 139, CI Pigment Red 144, CI Pigment Red 149, CI Pigment Red 166, CI Pigment Red 177, CI Pigment Red 178, CI Pigment Red 202, CI Pigment Red 222, and CI Pigment Violet 19.

[0109] Examples of orange or yellow pigments include CI Pigment Orange 31, CI Pigment Orange 43, CI Pigment Yellow 12, CI Pigment Yellow 13, CI Pigment Yellow 14, CI Pigment Yellow 15, CI Pigment Yellow 15:3, CI Pigment Yellow 17, CI Pigment Yellow 74, CI Pigment Yellow 93, CI Pigment Yellow 128, CI Pigment Yellow 94, CI Pigment Yellow 138, and CI Pigment Yellow 155. In particular, CI Pigment Yellow 155 is preferred in terms of the balance between color tone and lightfastness.

[0110] Examples of pigments for green or cyan include CI Pigment Blue 15, CI Pigment Blue 15:2, CI Pigment Blue 15:3, CI Pigment Blue 16, CI Pigment Blue 60, and CI Pigment Green 7.

[0111] Examples of black pigments include CI Pigment Black 1, CI Pigment Black 6, and CI Pigment Black 7.

[0112] The pigment content in the ink of the present invention is within the range of 2 to 10% by mass. If the pigment content is within this range, the ink will function as an inkjet ink, and high-quality images can be formed while maintaining good ejection properties. The pigment content is preferably within the range of 3 to 7% by mass.

[0113] (pigment dispersant) The ink of the present invention optionally contains a pigment dispersant to disperse the pigment. The pigment dispersant is not particularly limited, but is preferably a polymer dispersant having an anionic group, and preferably has a number average molecular weight in the range of 5,000 to 200,000.

[0114] Examples of pigment dispersants include block copolymers and random copolymers having a structure derived from two or more monomers selected from styrene, styrene derivatives, vinylnaphthalene derivatives, acrylic acid, acrylic acid derivatives, maleic acid, maleic acid derivatives, itaconic acid, itaconic acid derivatives, fumaric acid, and fumaric acid derivatives, as well as salts thereof, polyoxyalkylenes, and polyoxyalkylene alkyl ethers.

[0115] The pigment dispersant preferably has an acryloyl group and an acidic group. The acidic group is preferably neutralized with a neutralizing base before addition. The neutralizing base is not particularly limited, but is preferably an organic base such as ammonia, monoethanolamine, diethanolamine, triethanolamine, or morpholine.

[0116] The amount of pigment dispersant added is preferably within a range of 10 to 100% by mass, more preferably within a range of 10 to 40% by mass, relative to the pigment.

[0117] It is particularly preferred that the pigment be in the form of a so-called capsule pigment, which is a pigment coated with the pigment dispersant. As a method for coating a pigment with a pigment dispersant, various known methods can be used, and preferred examples thereof include a phase inversion emulsification method, an acid precipitation method, and a method in which a pigment is dispersed in a polymerizable surfactant, a monomer is supplied thereto, and the monomer is coated while polymerizing.

[0118] A particularly preferred method is to dissolve the pigment dispersant in an organic solvent such as methyl ethyl ketone, partially or completely neutralize the acidic groups in the resin with a base, add the pigment and ion-exchanged water, disperse the pigment, remove the organic solvent, and add water as necessary to prepare the dispersion.

[0119] The average particle size of the pigment dispersed in the ink is preferably 50 nm or more and less than 200 nm. This improves the dispersion stability of the pigment and the storage stability of the ink. The particle size of the pigment can be measured using a commercially available particle size measuring device that uses dynamic light scattering, electrophoresis, or the like, but measurement using dynamic light scattering is simple and can accurately measure the particle size range.

[0120] The pigment can be used by dispersing it in a disperser together with a pigment dispersant and other additives required for various desired purposes.

[0121] As the dispersing machine, conventionally known ball mills, sand mills, line mills, high-pressure homogenizers, etc. can be used. Among them, dispersing the pigment using a sand mill is preferred because it results in a sharp particle size distribution. Furthermore, the material of the beads used for sand mill dispersion is not particularly limited, but zirconia or zircon is preferred from the viewpoint of preventing the generation of bead fragments and contamination of ionic components. Furthermore, the diameter of the beads is preferably within the range of 0.3 to 3 mm.

[0122] (aqueous solvent) The ink of the present invention contains an aqueous solvent. The aqueous solvent contains water as an essential solvent, and preferably optionally contains a known aqueous solvent for viscosity adjustment, etc. When the aqueous solvent contains an aqueous solvent, it is preferable to adjust the type and amount of the aqueous solvent to be combined with water so that the temperature-responsive polymer has a lower critical solution temperature with the aqueous solvent.

[0123] The water contained in the ink according to the present invention is not particularly limited, and may be ion-exchanged water, distilled water, or pure water.

[0124] The aqueous solvent contained in the ink is preferably an aqueous solvent that, when used in combination with water, does not significantly change the LCST of the temperature-responsive polymer in an aqueous solvent from the LCST of the temperature-responsive polymer in water. As described above, the difference between the LCST of the temperature-responsive polymer in an aqueous solvent and the LCST of the temperature-responsive polymer in water is preferably within approximately 10°C.

[0125] Examples of aqueous solvents include alcohols, polyhydric alcohols, amines, amides, glycol ethers, and 1,2-alkanediols having 4 or more carbon atoms.

[0126] Examples of alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, t-butanol, 3-methoxy-1-butanol, 3-methoxy-3-methylbutanol, 1-octanol, 2-octanol, n-nonyl alcohol, tridecyl alcohol, n-undecyl alcohol, stearyl alcohol, oleyl alcohol, and benzyl alcohol.

[0127] Examples of polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycols having 5 or more ethylene oxide groups, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycols having 4 or more propylene oxide groups, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, and thiodiglycol.

[0128] Examples of amines include ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, and tetramethylpropylenediamine.

[0129] Examples of amides include formamide, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0130] Examples of glycol ethers include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, and tripropylene glycol monomethyl ether.

[0131] Examples of 1,2-alkanediols having 4 or more carbon atoms include 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, and 1,2-heptanediol.

[0132] Particularly preferred aqueous solvents are polyhydric alcohols, which can effectively suppress bleeding during high-speed printing. Specifically, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol are preferred.

[0133] The ink may contain one or a combination of two or more selected from these aqueous solvents.

[0134] The water content in the ink of the present disclosure is preferably within a range of 20 to 80% by mass, more preferably within a range of 40 to 70% by mass, based on the total mass of the ink. The aqueous solvent content in the ink is preferably within a range of 10 to 60% by mass, based on the total mass of the ink. The ratio of water to aqueous solvent in the ink is preferably adjusted so that the LCST of the temperature-responsive polymer relative to the aqueous solvent is within a range of ±10°C of the LCST of the temperature-responsive polymer relative to water.

[0135] The ink of the present disclosure preferably contains substantially only an aqueous solvent as the solvent, i.e., does not contain a non-aqueous solvent. The content of the aqueous solvent in the ink is preferably in the range of 50 to 95% by mass, more preferably in the range of 50 to 90% by mass, as the total content of water and aqueous solvent.

[0136] (thixotropic agent) The ink of the present disclosure may optionally contain a thixotropy-imparting agent. The thixotropy-imparting agent is used for the purpose of further enhancing the effect of the ink of the present disclosure, which is to maintain good jetting properties and form high-quality images, by imparting thixotropy to the ink, i.e., low viscosity at high shear and high viscosity at low shear.

[0137] The thixotropy-imparting agent can be any material that can impart thixotropy to the ink without any particular limitation. The thixotropy-imparting agent is preferably in a particulate form (however, this particulate form includes fibrous forms), and more preferably has an aspect ratio of 20 or more.

[0138] When the thixotropy-imparting agent is in particulate form, its shape is preferably ellipsoidal, scaly, plate-like, needle-like, fibrous, or the like. The aspect ratio, which indicates the ratio of the major axis to the minor axis of the thixotropy-imparting agent, is preferably 20 or more. When the aspect ratio is 20 or more, thixotropy can be easily imparted by the ink. The major axis of the thixotropy-imparting agent is preferably 2 μm or less. If the major axis of the thixotropy-imparting agent exceeds 2 μm, inkjet ejection properties may be affected.

[0139] In this specification, the cross section for measuring the aspect ratio of a thixotropic agent is a cross section parallel to the length direction of the particle and cut in the thickness direction. The aspect ratio is a value calculated from the average particle long diameter and average particle short diameter of 50 particles obtained from the cross section. Here, when the particle shape is scaly or plate-like, the short diameter is the particle thickness, and the long diameter is the length of the long side of the cross section for measuring the aspect ratio of the particle, or the length of the long side or the maximum diameter when the particle is viewed in plan. When the particle shape is needle-like or fibrous, the long diameter is the length of the particle, and the short diameter is the long diameter of the cross section perpendicular to the length direction of the particle, or the maximum width when the particle is viewed in plan.

[0140] Examples of the thixotropic agent include polysaccharides, inorganic particles, etc. Specific examples of polysaccharides include cellulose, chitin, chitosan, xanthan gum, welan gum, succinoglycan, guar gum, locust bean gum and derivatives thereof, glycomannan, agar, carrageenan, etc.

[0141] The polysaccharide is preferably a natural polysaccharide having a weight-average molecular weight of several million, specifically, xanthan gum, guar gum, carrageenan, etc.

[0142] Furthermore, in the ink of the present disclosure, the thixotropy-imparting agent preferably used is a polysaccharide nanofiber obtained by extremely finely breaking down aggregates of polysaccharides, such as those from trees and the shells of crustaceans such as crabs and shrimps, using a conventionally known method such as oxidation treatment using a catalyst or mechanical treatment using a grinder, etc. The polysaccharide in the polysaccharide nanofiber is preferably at least one of cellulose, chitin, and chitosan, and more preferably cellulose.

[0143] In this specification, nanofibers refer to those having a width of approximately 1 to 100 nm and an aspect ratio of 100 or more. The length and width of nanofibers can be measured using, for example, an electron microscope. The width of a nanofiber may be measured, for example, as the width in a planar view, or as the diameter of a cross section perpendicular to the length direction of the nanofiber. In either case, the "width" of a nanofiber is the average of the maximum widths of 50 nanofibers. The "length" of a nanofiber is the average length of 50 nanofibers. The aspect ratio of a nanofiber is calculated by dividing the length by the width.

[0144] When polysaccharide nanofibers are used as the thixotropic agent in the ink according to the present disclosure, smaller nanofibers are preferred. The width of the nanofibers is preferably 1 to 50 nm, more preferably 1 to 5 nm. The length of the nanofibers is preferably 0.5 to 2 μm, more preferably 1 to 5 μm, but is not limited thereto. The aspect ratio of the nanofibers is more preferably in the range of 100 to 400, and even more preferably in the range of 100 to 300.

[0145] In polysaccharide aggregates, polysaccharides such as cellulose, chitin, and chitosan exist in a state where structural units called microfibrils are bound together. These microfibrils are 3 to 4 nm wide and several μm long (e.g., 2 to 5 μm), but they are difficult to unravel one by one. When polysaccharide aggregates are mechanically disrupted by most conventional methods, nanofibers with widths of approximately 20 to 50 nm are obtained. While such nanofibers may be used as polysaccharide nanofibers in the present invention, it is more preferable to use TEMPO-oxidized nanofibers that have been unraveled into smaller units, such as microfibril units, by TEMPO oxidation.

[0146] TEMPO oxidation is an oxidation reaction catalyzed by 2,2,6,6-tetramethyl-1-piperidine-oxy radical (TEMPO). By oxidizing polysaccharide aggregates in the presence of TEMPO, extremely fine nanofibers with widths of 3-4 nm and lengths of several μm (e.g., 2-5 μm), equivalent to microfibrils, can be obtained.

[0147] The cellulose nanofiber used as the thixotropic agent is obtained by converting cellulose into nanofibers. Examples of the form of nanofibered cellulose include powdered cellulose and microcrystalline cellulose.

[0148] Suitable cellulose nanofibers that can be used include Rheocrysta (registered trademark) manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., TEMPO-oxidized cellulose nanofiber, Cellenpia TC-01A, and Cellenpia TC-02X ("Cellenpia" is a registered trademark) manufactured by Nippon Paper Industries Co., Ltd., IMa-10002, BMa-10002, WMa-10002, AMa-10002, and FMa-10002 manufactured by Sugino Machine Ltd., ELEX-☆ and ELEX-S manufactured by Daio Paper Corporation, and Aurovisco manufactured by Oji Paper Co., Ltd.

[0149] As inorganic particles, particles of various natural or synthetic clay minerals are preferred. As clay minerals, smectite clay minerals are preferred. Smectite clay minerals are classified as layered silicate minerals or phyllosilicates of the bentonite group of minerals. Smectite clay minerals are classified into the montmorillonite subgroup and the saponite subgroup based on their layered structure. The montmorillonite subgroup includes montmorillonite, nontronite, and beidellite. The saponite subgroup includes hectorite, saponite, and sauconite.

[0150] Smectite clay minerals may be natural or synthetic. Smectite clay minerals are layered substances in which platelets are stacked, and when used as a thixotropic agent, they are usually used as delaminated plate-like particles. When the smectite clay mineral is synthetic, it has a smaller aspect ratio and a lower impurity content than natural clay minerals.

[0151] The plate-like particles of the smectite clay mineral preferably have a thickness in the range of 0.2 to 3.0 nm and a length in the range of 10 to 150 nm. The plate-like particles more preferably have a thickness in the range of 0.2 to 2.0 nm and a length in the range of 10 to 125 nm. The aspect ratio, which is the length of the plate-like particle divided by the thickness, is preferably 20 or more. The aspect ratio is more preferably in the range of 20 to 200.

[0152] The length and thickness of the plate-like particles can be measured, for example, using an electron microscope. The thickness of the plate-like particles is, for example, the average value of the thicknesses of 50 plate-like particles measured at a predetermined cross section. The "length" of the plate-like particles is the average value of the lengths of 50 plate-like particles measured as the maximum diameter when viewed from above. The aspect ratio of the plate-like particles is calculated by dividing the length by the thickness.

[0153] As a smectite clay mineral, for example, Laponite (manufactured by Big Chemie), a synthetic layered silicate, can be used. Laponite is a synthetic low-charge clay whose structure and chemical composition are similar to that of hectorite, a natural smectite clay mineral. The main particles of Laponite are disc-shaped with a maximum diameter of 30 nm and a thickness of 1 nm.

[0154] Commercially available smectite clay minerals may be used, such as Laponite RD (manufactured by BYK-Chemie) and Kunipia F and Kunipia G, which are purified bentonites manufactured by Kunimie Kogyo Co., Ltd.

[0155] Furthermore, as inorganic particles, nanofibers, such as alumina nanofibers (minor diameter 4 nm, major diameter 1400 nm) manufactured by Kawaken Fine Chemicals, may be used.

[0156] The content of the thixotropy-imparting agent in the ink of the present disclosure is preferably in the range of 0.01 to 1 mass % of the total amount of ink, and more preferably in the range of 0.08 to 0.5 mass %, from the viewpoint of imparting thixotropy to the ink and further enhancing the effects of the present invention.

[0157] In the ink of the present disclosure, one type of thixotropy-imparting agent may be used alone, or two or more types may be used in combination. In the ink of the present invention, the thixotropy-imparting agent is preferably composed of two or more materials. Furthermore, one of the two or more materials is preferably a smectite clay mineral. Preferred combinations of thixotropy-imparting agents include a combination of cellulose nanofibers and a smectite clay mineral, and a combination of xanthan gum and a smectite clay mineral. A combination of cellulose nanofibers and a smectite clay mineral is particularly preferred.

[0158] Even when used alone, cellulose nanofibers and smectite clay minerals are thought to be able to impart elastic properties to inks by forming a specific gel structure, for example, at an ink drying rate of 20%. This allows inks containing cellulose nanofibers or smectite clay minerals to easily achieve the properties (1-4) and (1-5) described below. Furthermore, using cellulose nanofibers and smectite clay minerals in combination further enhances the above-mentioned elastic properties, which is preferable.

[0159] The ratio of smectite clay minerals to other thixotropic agents can be selected depending on the ink viscosity and thixotropy, but the mass ratio of smectite clay minerals to other thixotropic agents can be adjusted within the range of 10:1 to 1:10. By combining them, the thixotropy of the ink is greatly improved compared to adding each agent alone, resulting in better image quality. The reason for the improved thixotropy is speculation, but it is thought that smectite clay minerals have an electric charge, and the smectite clay minerals and other thixotropic agents electrically associate to form a structure.

[0160] (surfactant) The ink may optionally contain a surfactant, which can improve the ejection stability of the ink and control the spread (dot diameter) of the ink droplets that land on the substrate.

[0161] The surfactant can be used without particular limitation as long as it does not impair the effects of the present invention. However, when an anionic compound is contained in other constituent components of the ink, the ionicity of the surfactant is preferably anionic, nonionic, or betaine type.

[0162] In the present invention, fluorine-based or silicone-based surfactants having a high static surface tension reducing ability, anionic surfactants such as dioctyl sulfosuccinate having a high dynamic surface tension reducing ability, and nonionic surfactants such as relatively low molecular weight polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, acetylene glycols, Pluronic (registered trademark) surfactants, and sorbitan derivatives are preferably used. It is also preferable to use a fluorine-based or silicone-based surfactant in combination with a surfactant having a high dynamic surface tension reducing ability.

[0163] By adding a silicone-based or fluorine-based surfactant as the surfactant, it is possible to further suppress ink mixing with substrates made of various hydrophobic resins, including PVC sheets, and substrates with slow absorption, such as printing paper, thereby obtaining high-quality printed images.

[0164] The silicone surfactant is preferably a polyether-modified polysiloxane compound, such as KF-351A and KF-642 manufactured by Shin-Etsu Chemical Co., Ltd., or BYK345, BYK347, or BYK348 manufactured by BYK-Chemie.

[0165] The fluorine-based surfactants mentioned above refer to surfactants in which part or all of the hydrogen atoms bonded to the carbon atoms of the hydrophobic groups of ordinary surfactants have been substituted with fluorine. Among these, surfactants having a perfluoroalkyl group in the molecule are preferred.

[0166] Some of the above fluorine-based surfactants are commercially available from Dainippon Ink and Chemicals, Inc. under the trade name Megafac F, from Asahi Glass Co., Ltd. under the trade name Surflon, from Minnesota Mining and Manufacturing Company, Inc. under the trade name Fluorad FC, from Imperial Chemical Industries, Inc. under the trade name Monflor, from E.I. duPont Nemelas and Company, Inc. under the trade name Zonyls, and from Falbewerke-Hoechst under the trade name Licowet VPF.

[0167] The content of the surfactant in the ink is not particularly limited, but is preferably in the range of 0.1 to 5.0% by mass.

[0168] (Other additives) In addition to the additives described above, various known additives such as viscosity modifiers, resistivity modifiers, film-forming agents, ultraviolet absorbers, antioxidants, anti-fading agents, anti-mold agents, and anti-rust agents may be appropriately selected and used in the ink of the present disclosure, as needed, within the scope that does not impair the effects of the present invention, in accordance with the purpose of improving ejection stability, compatibility with print heads and ink cartridges, storage stability, image storage stability, and other performances.

[0169] Specific examples include oil droplet fine particles such as liquid paraffin, dioctyl phthalate, tricresyl phosphate, and silicone oil; ultraviolet absorbers described in JP-A Nos. 57-74193, 57-87988, and 62-261476; anti-fading agents described in JP-A Nos. 57-74192, 57-87989, 60-72785, 61-146591, 1-95091, and 3-13376; and fluorescent brighteners described in JP-A Nos. 59-42993, 59-52689, 62-280069, 61-242871, and 4-219266.

[0170] The ink is prepared by mixing the above-mentioned components so as to obtain the above-mentioned contents. Preferably, the pigment is dispersed in a part of the aqueous solvent with a pigment dispersant, and then mixed with the other components. When a fixing resin is contained, the fixing resin is dispersed in a part of the aqueous solvent with a surfactant added as needed, and then mixed with the other components.

[0171] <Physical properties of inkjet ink> The inkjet ink of the present disclosure preferably satisfies the following conditions (1-1) and (1-2) regarding viscosity characteristics, and more preferably satisfies condition (1-3) in addition to these.

[0172] (1-1) The viscosity at a shear rate of 1000 (1 / s) at 25°C is 15 mPa·s or less. (1-2) The viscosity of the temperature-responsive polymer at a shear rate of 1 (1 / s) at any temperature within a range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer in water is 45 mPa s or more. In other words, the maximum viscosity of the temperature-responsive polymer at a shear rate of 1 (1 / s) within a range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer in water is 45 mPa s or more. (1-3) When the ink drying rate is 20%, the viscosity at a shear rate of 1 (1 / s) at any temperature within ±10°C of the lower critical solution temperature of the temperature-responsive polymer with water is 100 mPa s or more. In other words, when the ink drying rate is 20%, the maximum viscosity at a shear rate of 1 (1 / s) within ±10°C of the lower critical solution temperature of the temperature-responsive polymer with water is 100 mPa s or more.

[0173] In the above (1-1) to (1-3), the viscosity can be measured using a rotational viscometer. An example of a rotational viscometer is the MCR-102 manufactured by Anton Paar. In this specification, unless otherwise specified, the viscosity is measured at 25°C.

[0174] An ink drying rate of 20% refers to the state in which the aqueous solvent has been removed from the inkjet ink so that the mass is 80% of the initial mass. The ink drying rate can be calculated using the following formula (A) from the mass before and after drying when the ink is dried at a temperature condition of 60°C.

[0175] Formula (A) Ink drying rate [%] = (W BEFORE -W AFTER ) / W BEFORE ×100 However, in formula (A), W BEFORE represents the mass of the ink before drying (initial mass). AFTER indicates the mass of the ink after drying. Specifically, the ink is dried by dropping approximately 100 mL of ink onto a glass substrate, weighing it to determine the mass of the ink before drying, and then heating it to 60°C on a hot plate that allows for mass measurement.

[0176] To obtain the viscosity at an ink drying rate of 20%, the drying is stopped when the mass reaches 80% of the initial mass (mass before drying), and the viscosity is measured using the dried ink obtained.

[0177] With regard to (1-1) above, the viscosity of the ink of the present invention is more preferably 10 mPa s or less at a shear rate of 1000 (1 / s). There is no particular lower limit to the viscosity at a shear rate of 1000 (1 / s), but from the viewpoint of inkjet ejection properties, it is preferably about 5 mPa s.

[0178] With regard to (1-2) above, the viscosity of the ink of the present invention at a shear rate of 1 (1 / s) at any temperature within a range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer in water is preferably 70 mPa s or more, and more preferably 100 mPa s or more. There is no particular upper limit to the viscosity, but from the viewpoint of ease of head maintenance, it is preferably about 1000 mPa s.

[0179] In relation to (1-3) above, the ink of the present invention preferably has a viscosity of 100 mPa s or more, more preferably 150 mPa s or more, at a shear rate of 1 (1 / s) at any temperature within a range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer in water at an ink drying rate of 20%. There is no particular upper limit to the viscosity in this case, but from the viewpoint of ease of head maintenance, a viscosity of around 1000 mPa s is preferred.

[0180] Here, in the ink of the present disclosure, if the viscosity at a shear rate of 1 (1 / s) at any temperature within a range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer in water is at least 45 mPa s, then when the ink impacts the substrate, color mixing and other problems can be easily suppressed by simply making a slight temperature adjustment, without removing the aqueous solvent from the ink.

[0181] The viscosity of the ink increases as the ink drying rate increases. In inks that satisfy the above (1-3), by keeping the ink drying rate at a maximum of 20%, a viscosity of 100 mPa·s can be achieved at a shear rate of 1 (1 / s) at any temperature within a range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer with water. This makes it possible to suppress color mixing and other issues when the ink lands on the substrate, without excessively removing the water-based solvent from the ink, simply by slightly adjusting the temperature.

[0182] The inkjet ink of the present disclosure preferably further satisfies at least one of the following conditions (1-4) and (1-5), and more preferably satisfies both of them.

[0183] (1-4) When the ink drying rate is 20%, the loss tangent (tanδ) at 1% strain is less than 1 when the strain is changed at an angular frequency ω of 10 rad / s and an oscillation angle γ of 1 to 1000% at any temperature within ±10°C of the lower critical solution temperature of the temperature-responsive polymer with water. (1-5) When the ink drying rate is 20%, the crossover strain between the storage modulus and the loss modulus is 20% or more when the strain is changed at an angular frequency ω of 10 rad / s and an oscillation angle γ of 1 to 1000% at any temperature within ±10°C of the lower critical solution temperature of the temperature-responsive polymer with water.

[0184] The loss tangent (tanδ) in (1-4) and the storage modulus and loss modulus in (1-5) can be measured using a rheometer (viscoelasticity measuring device). An example of a rheometer is the MCR-102 manufactured by Anton Paar. In this specification, the loss tangent may also be simply referred to as "tanδ."

[0185] In this specification, tan δ, storage modulus, and loss modulus can be measured using a rheometer, specifically, an Anton Paar MCR-102, in oscillation mode at any temperature within the range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer in water, under measurement conditions of an angular frequency ω of 10 rad / s and an oscillation angle γ of 1 to 1000%. During measurement, the oscillation angle γ of the cone-plate of the rheometer is changed to strain the measurement sample. Here, since the measurement sample is strained in accordance with the oscillation angle γ, the oscillation angle γ of the cone-plate and the strain of the measurement sample have the same value.

[0186] In (1-4), the tan δ at 1% strain is used as an index in the above measurement. If tan δ is less than 1 at 1% strain under the measurement conditions, the ink will have more elastic properties, and compared with inks with similar viscosities, the pinning properties will be better and the image quality will be improved. In (1-4), tan δ at 1% strain is more preferably 0.8 or less, and even more preferably 0.6 or less.

[0187] In (1-5), the crossover strain between the storage modulus and the loss modulus when the strain is changed in the above measurement is used as an index. Specifically, the crossover strain is the strain value (%) at the intersection of a graph with the storage modulus (Pa) on the vertical axis and the strain (%) on the horizontal axis, both plotted logarithmically, and a graph with the loss modulus (Pa) on the vertical axis and the strain (%) on the horizontal axis, both plotted logarithmically. If the crossover strain (%) is 20% or more under the measurement conditions, the ink will have more elastic properties, and as described above, image quality will be further improved. The crossover strain is more preferably 30% or more, and even more preferably 40% or more.

[0188] In (1-4), when the same measurement is performed not when the ink drying rate is 20% but when the ink drying rate is 0%, i.e., when the ink of the present invention is used, the loss tangent (tanδ) at a strain of 1% is preferably less than 1, more preferably 0.8 or less, and even more preferably 0.6 or less.

[0189] Furthermore, in (1-5), when a similar measurement is performed not when the ink drying rate is 20% but when the ink drying rate is 0%, i.e., when the ink of the present invention is used, the crossover distortion is preferably 20% or more, more preferably 30% or more, and even more preferably 40% or more.

[0190] The ink of the present disclosure exhibits almost no difference in tan δ and crossover strain when the ink drying rate is 0% or 20%. In other words, even when the ink drying rate is 0%, the ink exhibits elastic behavior within a range of ±10°C of the lower critical solution temperature of the temperature-responsive polymer in water, making pinning easier and enabling the formation of high-quality images.

[0191] [Image formation method overview] The image forming method of the present disclosure is an image forming method having a landing step in which droplets of inkjet ink are ejected from an inkjet head equipped with an ink circulation mechanism and landed on a substrate, and has the following features. (2-1) The inkjet ink contains an aqueous solvent, a pigment, and a thixotropic agent, and is circulated within the inkjet head so that the viscosity is 15 mPa·s or less. (2-2) The droplet landing process is carried out so that the mass loss rate of the droplets when they land on the substrate or immediately after landing is within 20% from the inkjet ink and the viscosity is 100 mPa·s or more.

[0192] The image forming method of the present disclosure typically further includes a drying step of removing the aqueous solvent from the ink droplets on the substrate after the landing step. The drying step allows the desired image to be formed on the substrate. The image forming method of the present disclosure may also include a step of forming a primer layer on the surface of the substrate before forming a coating film with the ink (hereinafter also referred to as a "primer layer forming step"). In this case, the ink droplets are landed on the primer layer in the landing step.

[0193] (Primer layer formation process) The primer layer can be formed by applying a primer ink containing a resin and a solvent onto a substrate and drying it. Alternatively, the primer layer may be formed by applying a primer ink containing an active energy-polymerizable compound and a polymerization initiator and irradiating the coating with active energy.

[0194] The method for applying the primer ink is not particularly limited, and may be any method such as roll coating, spin coating, spray coating, dipping, screen printing, inkjet printing, gravure printing, offset printing, etc. Among these, when the surface roughness of the substrate needs to be precisely controlled, the screen printing method or inkjet method is preferred, and the inkjet method is particularly preferred.

[0195] After the primer ink is applied, the method for curing or drying the coating film is appropriately selected depending on the type of primer ink, and may be, for example, a method of heating or irradiating with active energy.

[0196] (Impact process) In the landing process, if the viscosity of the ink inside the head is 15 mPa s or less, the ink can be circulated in the head and ejected smoothly from the head. It is preferable that the ink is circulated so that the viscosity of the ink inside the head is 10 mPa s or less.

[0197] With regard to (2-2) above, the mass loss rate from the inkjet ink (hereinafter simply referred to as "mass loss rate") can be defined in the same way as the ink drying rate described above. If the mass loss rate and viscosity of the ink droplets when they land on the substrate or immediately after landing are within the above ranges, good pinning is possible and the resulting image can be of high quality. Furthermore, adhesion of ink solids to the area around the head is suppressed, which makes ejection failure less likely and facilitates maintenance.

[0198] Here, "immediately after impact" refers to, for example, within 100 msec after the ink droplet lands on the substrate. The above effect is fully achieved if the mass loss rate of the ink droplet within 100 msec after landing on the substrate is within 20% and the viscosity is 100 mPa·s or greater. Hereinafter, the time of impact on the substrate or immediately after impact is also referred to as "at the time of impact on the substrate." The viscosity of the ink droplet upon impact on the substrate is preferably 150 mPa·s or greater, more preferably 200 mPa·s or greater, and even more preferably 300 mPa·s or greater. The "immediately after impact" period is generally within 30 msec when the substrate is transported at a high speed (e.g., 100 m / min), and within 100 msec when the substrate is transported at a high speed (e.g., 50 m / min).

[0199] In the image forming method of the present disclosure, the ink or apparatus is appropriately selected to satisfy the above conditions (2-1) and (2-2). In the image forming method of the present disclosure, by using the ink of the present disclosure described above, conditions (2-1) and (2-2) can be achieved without any particular changes to the apparatus. That is, in the image forming method of the present disclosure, it is preferable to use the ink of the present disclosure described above. Note that when the ink of the present disclosure is used, condition (2-2) can be achieved, for example, within the LCST of the temperature-responsive polymer ±10°C. That is, when the ink of the present disclosure is used, it is preferable to heat the ink droplets on the substrate within the LCST of the temperature-responsive polymer ±10°C in the impact step to satisfy condition (2-2).

[0200] In the image forming method of the present disclosure, in order to satisfy condition (2-2), it is preferable to heat the ink droplets on the substrate to a temperature in the range of 30 to 60°C in the landing step. That is, the image forming method of the present invention may include a step of heating the ink droplets at 30 to 60°C as needed so that the state of the ink droplets upon landing on the substrate is such that the mass loss rate is 20% or less and the viscosity is 100 mPa s or more. This temperature is sufficiently low, so there is almost no risk of the ink solidifying around the head and causing ejection defects.

[0201] Heating may be carried out using a non-contact heating device such as a thermostatic oven or a hot air blower, or may be carried out using a contact heating device such as a hot plate or a heat roller.

[0202] The heating temperature can be obtained by measuring any one of (a) the ambient temperature such as the temperature inside the furnace or the hot air temperature when a non-contact heating device such as a thermostatic oven or a hot air blower is used, (b) the temperature of the contact heating part when a contact heating device such as a hot plate or a heat roller is used, or (c) the surface temperature of the ink droplets, and it is more preferable to measure (c) the surface temperature of the ink droplets.

[0203] (drying process) The drying step is a step of removing components other than solid components such as the aqueous solvent (hereinafter also referred to as "volatile components") from the ink droplets after the landing step, and forming a coating film that constitutes the desired image on the substrate, or on the primer layer if one is present. As described above, the ink droplets after the landing step have a mass loss rate of 20% or less. Since inkjet ink contains, for example, approximately 50 to 95% by mass of aqueous solvent, the drying step removes the remaining volatile components, including the aqueous solvent.

[0204] In the drying step, it is preferable to remove volatile components while drying under conditions such that, if the ink contains a fixing resin, the fixing resin does not completely fuse. The drying temperature is preferably within a range of, for example, 60 to 110°C. The drying time is preferably within a range of, for example, 5 to 60 seconds. The drying in the drying step can be carried out, for example, by the same method as the heating in the landing step.

[0205] (base material) The substrate that can be used in the present invention is not particularly limited, but is preferably a non-absorbent substrate. By using a non-absorbent substrate, the effect of the image forming method of the present invention becomes more pronounced. In the present invention, non-absorbent means non-absorbent to water.

[0206] Examples of non-water-absorbent substrates that can be used include known plastic films. Specific examples include polyester films such as polyethylene terephthalate, polyethylene films, polypropylene films, polyamide films such as nylon, and biodegradable films such as polystyrene films, polyvinyl chloride films, polycarbonate films, polyacrylonitrile films, and polylactic acid films. Furthermore, films coated with polyvinylidene chloride on one or both sides or films vapor-deposited with metal oxides can also be preferably used to impart gas barrier properties, moisture resistance, aroma retention, and the like. Both unstretched and stretched films can be preferably used as non-water-absorbent films.

[0207] In addition to these, examples of non-water-absorbent substrates include substrates made of inorganic compounds such as metals and glass.

[0208] It can also be suitably used as a packaging material for retort foods, in which a thermosetting resin is provided as a coating layer on a metal substrate. The packaging material for retort foods is made of a film in which a thermoplastic resin layer or an aluminum foil layer, such as polypropylene on the food side and polyester on the outside, are laminated together to block air, moisture, and light and seal the food inside.

[0209] In the present invention, the thickness of the substrate is preferably in the range of 10 to 120 μm, more preferably 12 to 60 μm.

[0210] (inkjet head) The image forming method of the present disclosure is applied to image formation using an inkjet head equipped with an ink circulation mechanism.

[0211] The inkjet head is not particularly limited as long as it has an ink circulation mechanism, and may be either an on-demand type or a continuous type. Examples of on-demand type inkjet heads include electro-mechanical conversion types such as single-cavity type, double-cavity type, bender type, piston type, shear mode type, and shared wall type, and electro-thermal conversion types such as thermal inkjet type and bubble jet type ("Bubble Jet" is a registered trademark of Canon Inc.).

[0212] Among the above inkjet heads, an inkjet head using a piezoelectric element as the electromechanical conversion element used in the electromechanical conversion method (also called a "piezo type inkjet head") is preferable.

[0213] The inkjet head may be either a scanning type or a line type, but is preferably a line type.

[0214] A line-type inkjet head is an inkjet head whose length is equal to or greater than the width of the printing range. As a line-type inkjet head, a single head whose length is equal to or greater than the width of the printing range may be used, or multiple heads may be combined to form a head whose length is equal to or greater than the width of the printing range.

[0215] Furthermore, a plurality of heads may be arranged in parallel so that the nozzles are arranged in a staggered pattern, thereby increasing the overall resolution of the heads.

[0216] The inkjet head used in the image forming method of the present disclosure preferably comprises a pressure chamber into which inkjet ink is injected via an injection path, a pressure generating means for generating pressure fluctuations in the pressure chamber, a nozzle communicating with the pressure chamber and serving as a flow path for the inkjet ink to be ejected from the pressure chamber to the outside due to the pressure fluctuations in the pressure chamber, and two or more circulation paths communicating with the pressure chamber and for discharging inkjet ink located inside the nozzle and returning it to the injection path. Such an inkjet head will be described below with reference to the drawings.

[0217] 1 is a schematic diagram of the essential parts of an inkjet image forming apparatus 100 equipped with an example of an inkjet head that can be used in the image forming method of the present invention, showing a partial cross section of the inkjet head 1. Note that the inkjet head etc. shown in FIG. 1 is merely an example, and the inkjet head etc. to which the image forming method of the present invention can be applied are not limited to this.

[0218] In the inkjet image forming apparatus 100, ink droplets are ejected from the inkjet head 1 onto a substrate 109 being transported in a fixed direction (sub-scanning direction) by a transport means 108, causing the droplets to land on the substrate, and an image is formed by drying the droplets. In a one-pass type inkjet image forming apparatus, the inkjet head 1 is fixedly disposed, and as the substrate 109 is transported, ink droplets are ejected from the nozzles 22 toward the substrate 109, causing the droplets to land on the substrate, thereby forming an image. In a scan type inkjet image forming apparatus, the inkjet head 1 is mounted on a carriage mechanism 107, and as the carriage mechanism 107 moves back and forth in the main scanning direction, ink droplets are ejected from the nozzles 22 toward the substrate 109, causing the ink droplets to land on the substrate, thereby forming an image. The transport means 108 and the carriage mechanism 107 are driven and controlled by a control unit 104.

[0219] 1, the inkjet image forming apparatus 100 has a non-contact or contact heater that adjusts the temperature of ink droplets when they land. The heater can also be used in the drying process.

[0220] 1 shows only one inkjet head 1, but generally, an inkjet image forming apparatus 100 is equipped with a plurality of inkjet heads 1 for inks of various colors, such as yellow (Y), magenta (M), cyan (C), and black (K). In the inkjet image forming apparatus 100, an ink tank 101 that stores ink and a common ink chamber 41 of the inkjet head 1 are connected by an ink transfer pipe 102 that serves as a transfer path and an ink return pipe 103 that serves as a recovery path.

[0221] A transfer pump 105a is provided in the ink transfer pipe 102, and is driven and controlled by the control unit 104 of the inkjet image forming apparatus 100. When the transfer pump 105a is driven, the ink in the ink tank 101 is transferred to the inkjet head 1 via the ink transfer pipe 102.

[0222] Furthermore, a transfer-side sub-tank 111a is provided midway along the ink transfer tube 102. The transfer-side sub-tank 111a is configured as a buffer space in which ink is temporarily stored before being transferred to the inkjet head 1. The pressure of the ink in the ink transfer tube 102 can be controlled via the transfer-side sub-tank 111a by a transfer pressure control pump 110a, which constitutes a pressure control means. The transfer pressure control pump 110a is controlled by a control unit 104a in the inkjet head 1.

[0223] A return pump 105b, which is driven and controlled by the control unit 104, is provided in the ink return pipe 103. When the return pump 105b is driven, the ink in the inkjet head 1 is returned to the ink tank 101 via the ink return pipe 103.

[0224] Furthermore, a return sub-tank 111b is provided midway along the ink return pipe 103. The return sub-tank 111b is configured as a buffer space in which ink returned from the inkjet head 1 is temporarily stored. The ink pressure in the ink return pipe 103 can be controlled via the return sub-tank 111b by a return pressure control pump 110b, which constitutes pressure control means. The return pressure control pump 110b is controlled by a control unit 104a in the inkjet head 1.

[0225] The pressure control means is not limited to being composed of the transfer pressure control pump 110a and the return pressure control pump 110b, but may be composed of either one of them. In this case, the return pressure control pump 110b serves as the first pressure control means, and the transfer pressure control pump 110a serves as the second pressure control means.

[0226] Although not particularly limited, it is preferable that the ink tank 101 be divided into an ink transport chamber 101b and an ink return chamber 101c by a partition plate 101a that does not reach the bottom of the tank. In this case, one end of the ink transport tube 102 is disposed within the ink transport chamber 101b, and one end of the ink return tube 103 is disposed within the ink return chamber 101c. The partition plate 101a is provided to sufficiently degas the ink so that air bubbles contained in the ink returned to the ink return chamber 101c do not flow back into the ink transport tube 102. Because air bubbles themselves have high buoyancy, the air bubbles are prevented from passing under the partition plate 101a and flowing into the ink transport chamber 101b. This configuration is preferable when circulating ink.

[0227] The inkjet head 1 is configured to have an ink manifold 4 that forms a common ink chamber 41, a wiring board 3 adhered to the ink manifold 4, a head chip 2 adhered to the underside of the wiring board 3, and a nozzle plate 21 adhered to the underside of the head chip 2.

[0228] The ink manifold 4 is made of a synthetic resin material or the like and has a horizontally long box shape with an opening 4a on the bottom surface. The opening 4a of this ink manifold 4 is closed by a wiring board 3 adhered to the bottom surface. The internal space of the ink manifold 4 becomes a common ink chamber 41 in which ink transferred from the ink tank 101 is stored. The wiring board 3 is, for example, a glass board. A wiring pattern (not shown) is formed on this wiring board 3 and is connected to a power circuit (not shown) via an FPC board.

[0229] An ink supply pipe 5a, which serves as a flow path for supplying ink into the common ink chamber 41, is connected to the common ink chamber 41. The ink supply pipe 5a is connected to the common ink chamber 41 on the side (upper side) farther from the wiring board 3. A connection part 7a is provided at the upper end of the ink supply pipe 5a. The connection part 7a is detachably connected to a connection part 106a on the inkjet image forming apparatus 100 side. The connection part 106a on the inkjet image forming apparatus 100 side is connected to the ink transport pipe 102. This enables the inkjet head 1 to transport ink from the ink tank 101 and supply ink to the common ink chamber 41.

[0230] In addition, an ink recovery tube 5b, which serves as a flow path for recovering ink from the common ink chamber 41, is connected to the common ink chamber 41. The ink recovery tube 5b is connected to the common ink chamber 41 on the side (upper side) farther from the wiring board 3. A connection part 7b is provided at the upper end of the ink recovery tube 5b. The connection part 7b is detachably connected to a connection part 106b on the inkjet image forming apparatus 100 side. The connection part 106b on the inkjet image forming apparatus 100 side is connected to an ink return tube 103. This enables the inkjet head 1 to recover ink from the common ink chamber 41 and return ink to the ink tank 101.

[0231] In the inkjet head 1, the flow path from the ink supply pipe 5a to the buffer space portion 6 midway through the ink recovery pipe 5b serves as the main flow path F1.

[0232] 2 is an enlarged cross-sectional view of the head chip 2 of the inkjet head 1. FIG.

[0233] The head chip 2 is formed with a plurality of ink channels (pressure chambers) 23 and a plurality of dummy channels (pseudo pressure chambers) 25. Each ink channel 23 and each dummy channel 25 is a through hole drilled from the upper surface to the lower surface of the head chip 2. The upper end of each ink channel 23 communicates with the common ink chamber 41 via an injection hole 31a opened in the wiring substrate 3. Each ink channel 23 is filled with ink flowing in from the injection hole 31a due to the potential energy of the ink in the ink tank 101 and the pressure controlled by the transfer pressure control pump 110a caused by the transfer pump 105a.

[0234] The lower end of each ink channel 23 communicates with the outside (downward) via the nozzle 22. In this inkjet head 1, the area inward of the nozzle 22 is within the ink channel 23. If a communication path exists between the ink channel 23 and the nozzle 22, the area inward of the nozzle 22 is within the communication path. Each dummy channel 25 has its upper end closed by the wiring substrate 3 and its lower end closed by the nozzle plate 21, forming a sealed air chamber. The ink channels 23 and dummy channels 25 are arranged in one direction (the direction of arrow X in FIG. 2) to form a channel row.

[0235] Both wall portions of each ink channel 23 (partition walls between the ink channel 23 and the dummy channel 25) are composed of a pair of piezoelectric elements (drive walls) 24, 24 that serve as pressure generating means. The piezoelectric elements 24, 24 undergo shear deformation when a voltage is applied from a power supply circuit (not shown) via the wiring pattern of the FPC board and the wiring board 3. The shear deformation of the piezoelectric elements 24, 24 that form both wall portions of the ink channel 23 causes pressure fluctuations in the ink channel 23 (reduced pressure due to expansion or increased pressure due to contraction). The pressure fluctuations (reduced pressure or increased pressure) in the ink channel 23 apply pressure to the area inward of the nozzle 22, i.e., to the ink inside the ink channel 23, and the ink is ejected through the nozzle 22.

[0236] Two piezoelectric elements 24 and 24 (a pair) are provided per ink channel 23, forming both wall portions of each ink channel 23. There is a gap between the piezoelectric element 24 that forms the wall portion of one ink channel 23 and the piezoelectric element 24 that forms the wall portion of the adjacent ink channel 23, and this gap is a dummy channel 25. Therefore, each ink channel 23 can be driven (reduced or increased pressure) independently.

[0237] An introduction path 425 is formed in the head chip 2. The introduction path 425 is provided at one end of a channel row formed by each ink channel 23 and each dummy channel 25, and is positioned outside the channel row. The introduction path 425 is a through hole drilled from the upper surface to the lower surface of the head chip 2, and has a cross-sectional opening area larger than the cross-sectional opening area of ​​one ink channel 23. The upper end of the introduction path 425 communicates with the common ink chamber 41 via an introduction hole 31c opened in the wiring substrate 3, and ink flows in from the introduction hole 31c due to the potential energy of the ink in the ink tank 101 and the pressure controlled by the transfer pressure control pump 110a resulting from the transfer pump 105a.

[0238] A flat nozzle plate 21 adhered to the underside of the head chip 2 has a plurality of nozzles 22 formed therein, each corresponding to one of the ink channels 23. The nozzles 22 are through-holes that connect the ink channels 23 to the outside. The ink in each ink channel 23 is given ejection pressure by the action of a piezoelectric element 24, and is ejected through the nozzles 22 toward the outside (downward) of the substrate. In other words, the nozzles 22 serve as flow paths for ink that is ejected from inside each ink channel 23 to the outside (downward). The underside of the nozzle plate 21 forms the ink ejection surface 1S.

[0239] The inkjet head 1 is equipped with a nozzle circulation mechanism that discharges ink injected into the ink channels 23 from the vicinity of the nozzles 22 and returns the ink to the injection path to the ink channels 23. Each ink channel 23 is connected to two individual ink circulation paths 26a, 26a.

[0240] The individual ink circulation paths 26a and 26a communicate with the ink channel 23 at both ends in the longitudinal direction of the cross section of the ink channel 23. Because air bubbles often remain near both ends of the ink channel 23, it is preferable to provide the individual ink circulation paths 26a and 26a at both ends in the longitudinal direction of the cross section of the ink channel 23. The individual ink circulation paths 26a and 26a may communicate with the ink channel 23 at any point on the ink channel 23. The number of individual ink circulation paths 26a per ink channel 23 may be increased or decreased, but two or more is preferred. Inks containing temperature-responsive polymers or fixing resins tend to exhibit thixotropy. However, if there are two or more circulation paths as described above, ink stagnation is less likely to occur even when such ink is used, and bubbles near the nozzles can be easily removed.

[0241] The individual ink circulation paths 26a and 26a are formed by a flow path forming groove 28 formed on the upper surface of the nozzle plate 21 with a starting end near the nozzle 22 and closed by the lower surface of the head chip 2.

[0242] Two introduction grooves 425a and 425a communicate with introduction path 425. The introduction grooves 425a and 425a communicate with introduction path 425 on both sides of introduction path 425. Note that introduction grooves 425a and 425a may communicate with introduction path 425 at any point on introduction path 425. Furthermore, the number of introduction grooves 425a and 425a per introduction path 425 may be increased or decreased.

[0243] Introduction grooves 425a and 425a are formed on the upper surface of nozzle plate 21 with their starting ends near introduction path 425, and are closed by the lower surface of head chip 2 to form a flow path.

[0244] A common ink circulation path 421 is formed on the lower surface of head chip 2. Common ink circulation path 421 is configured by abutting a groove formed on the lower surface of head chip 2 and a groove 422 formed on the upper surface of nozzle plate 21.

[0245] The common ink circulation path 421 is composed of multiple flow paths formed in the direction of the channel row (X direction). The individual ink circulation paths 26a, 26a, which are connected to the ink channels 23, merge by communicating with the common ink circulation path 421. A pressure difference between the inside of each ink channel 23 and the inside of the common ink circulation path 421 causes ink to flow from each ink channel 23 to the common ink circulation path 421. In addition, the introduction grooves 425a, 425a communicate with the common ink circulation path 421. A pressure difference between the inside of the introduction path 425 and the inside of the common ink circulation path 421 causes ink to flow from the introduction path 425 to the common ink circulation path 421. These flows then merge to cause ink to flow within the common ink circulation path 421.

[0246] The other end of the common ink circulation path 421 is connected to the lower end of a discharge channel 424 formed in the head chip 2. The discharge channel 424 is provided at the other end of the channel row formed by the ink channels 23 and the dummy channels 25, positioned outside the channel row. The ink flow rate in the discharge channel 424 is greater than the ink flow rate in the introduction path 425 by the amount of ink that has passed through the ink channels 23 and merged together, so the cross-sectional opening area of ​​the discharge channel 424 is made large so as not to increase the flow path resistance.

[0247] The ink discharged from the individual ink circulation path 26a, which is connected to the ink channel 23 serving as a pressure chamber, to the common ink circulation path 421 passes through the discharge channel 424, the ink discharge chamber 412, and the ink discharge pipe 5c, reaches the buffer space 6, and is then returned to the ink tank 101 via the above-mentioned path.The ink is then introduced again into the injection path to the ink channel 23.

[0248] 1, an ink discharge chamber 412 is provided in the ink manifold 4, located above the discharge channel 424. The ink discharge chamber 412 is provided adjacent to the common ink chamber 41 in the ink manifold 4. The ink discharge chamber 412 is separated from the common ink chamber 41 by a partition wall 45. The partition wall 45 can be formed integrally with the ink manifold 4.

[0249] In this way, a portion of the ink that has flowed into the ink channel 23 from the injection hole 31a (ink that is not ejected from the nozzle 22) passes from the individual ink circulation paths 26a, 26a through the common ink circulation path 421 to the discharge channel 424, and then passes through the discharge hole 31b formed in the wiring substrate 3 to reach the ink discharge chamber 412. In addition, the ink that has flowed into the introduction path 425 from the introduction hole 31c passes through the introduction grooves 425a, 425a and the common ink circulation path 421 to reach the discharge channel 424, and then passes through the discharge hole 31b to reach the ink discharge chamber 412.

[0250] An ink discharge pipe 5c, which forms a flow path for discharging ink from the ink discharge chamber 412, is connected to the ink discharge chamber 412 via a circulation path connecting portion 5d. The circulation path connecting portion 5d is located above the discharge channel 424, and is provided at the other end of the channel row formed by the ink channels 23 and the dummy channels 25, outside the channel row. The upper end of the ink discharge pipe 5c merges with the ink recovery pipe 5b. The ink recovery pipe 5b and the ink discharge pipe 5c are joined by being connected to the buffer space portion 6.

[0251] In the inkjet head 1, the flow path from the inlet path 425 and the individual ink circulation paths 26a, 26a via the common ink circulation path 421, the discharge channel 424, the discharge hole 31b, the ink discharge chamber 412, and the ink discharge pipe 5c to the buffer space 6 constitutes the circulation path 423. The circulation path 423 is in communication with the inlet path 425 and the ink channel 23, and discharges ink from the inlet path 425 and the ink channel 23 and merges with the ink recovery pipe 5b in the buffer space 6. However, the circulation path 423 does not need to be limited in any way as long as it discharges ink from the individual ink circulation paths 26a, 26a near the nozzle 22 and returns this ink to the injection path to the ink channel 23. The path from the inlet hole 31c and each injection hole 31a to the circulation path 423 constitutes the sub-flow path F2. [Example]

[0252] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these. In the examples, the terms "parts" and "%" are used, but unless otherwise specified, they represent "parts by mass" or "% by mass."

[0253] [Examples 1 to 13, Comparative Examples 1 to 5] Using the following materials, inkjet inks of the examples and comparative examples having the compositions shown in Table I or Table II were prepared.

[0254] (temperature-responsive polymer) Metrose SM-04 (product name, manufactured by Shin-Etsu Chemical Co., Ltd., methylcellulose resin, LCST in water: 55°C) Metrose 60SH-03 (product name, manufactured by Shin-Etsu Chemical Co., Ltd., hydroxypropyl methylcellulose resin, LCST in water: 75°C) Poly(N-isopropylacrylamide) (Sigma-Aldrich, LCST in water: 35°C, weight-average molecular weight: 30,000)

[0255] Magenta pigment: JM2120 (trade name, manufactured by DIC Corporation, compound name: PR202 / PV19) Pigment dispersant: JONCRYL 819 (trade name; manufactured by BASF, an acrylic dispersant having a carboxyl group neutralized with sodium hydroxide, acid value 75 mg KOH / g, solid content 20% by mass) Water; ion-exchanged water

[0256] (aqueous solvent) Ethylene glycol Propylene glycol Glycerin

[0257] (fixing resin) Vylonal MD-2000 (product name: Toyobo Co., Ltd., aqueous dispersion of polyester resin with a number average molecular weight of 18,000; solid content: 40% by mass) Movinyl 6800D (product name: Japan Coating Resin Co., Ltd., aqueous dispersion of polyacrylic resin; solid content 45% by mass) Evaphanol HA-560 (product name; manufactured by Nicca Chemical Co., Ltd., aqueous dispersion of polyurethane resin; solid content 35% by mass)

[0258] (surfactant) KF-351A (product name, manufactured by Shin-Etsu Chemical Co., Ltd., polyether-modified polysiloxane compound) Olfine E1010 (product name, manufactured by Nissin Chemical Industry Co., Ltd., acetylene glycol) (thixotropic agent) Cellenpia TC-01A (product name, manufactured by Nippon Paper Industries Co., Ltd., cellulose nanofiber, average width 3-4 nm, average length 0.8 μm, aspect ratio 200)

[0259] (Preparation of pigment dispersion) A mixture of 4 parts by weight of magenta pigment (JM2120), 1.6 parts by weight of pigment dispersant (Joncryl 819) (solids), and 30.5 parts by weight of an aqueous solvent consisting of ethylene glycol and ion-exchanged water in a 100:68 mass ratio was premixed and then dispersed using a sand grinder filled with 50% by volume of 0.5 mm zirconia beads to prepare a pigment dispersion with a pigment content of 18% by weight. The average particle size of the pigment particles in this pigment dispersion was 110 nm. The average particle size was measured using a Zetasizer 1000HS (manufactured by Marballoon Co., Ltd.).

[0260] (Preparation of Inkjet Ink) The pigment dispersion obtained above and the above components were mixed to obtain the composition shown in Table I or Table II, and the resulting mixture was filtered through a 1 μm filter to obtain an inkjet ink. In Table I or Table II, the content (mass %) of each component indicates the solid content of the component when a dispersion or solution is used as the component. For example, in Example 1, for Vylonal MD-2000, the content of the polyester resin itself, excluding the amount of water used as the dispersion medium, is 5.0 mass %. A blank space in the composition column of Table I or Table II indicates that the component is not contained.

[0261] (Evaluation of ink properties) The ink obtained above was evaluated for the following physical properties (A1) to (A8). The results are shown in Table I or Table II. Properties (A1), (A2), (A5), and (A6) were evaluated using the ink obtained above as is, i.e., when the ink drying rate was 0%.

[0262] (A1) Viscosity at a shear rate of 1000 (1 / s) (25°C) (A2) Viscosity at shear rate 1 (1 / s) (LCST+5℃) (A3) Viscosity (LCST+5℃) at a shear rate of 1 (1 / s) with an ink drying rate of 20% (A4) Viscosity (LCST+5℃) at a shear rate of 1 (1 / s) when the ink drying rate is 50% (A5) Loss tangent (tanδ) at 1% strain (LCST+5℃) (A6) Crossover strain between storage modulus and loss modulus (LCST+5℃) (A7) Loss tangent (tanδ) at 1% strain with an ink drying rate of 20% (LCST+5℃) (A8) Crossover strain between storage modulus and loss modulus at an ink drying rate of 20% (LCST+5°C)

[0263] In the above, LCST+5°C refers to the LCST+5°C of the temperature-responsive polymer contained in each ink relative to water. However, for Comparative Example 6, since no temperature-responsive polymer was contained, the measurement temperature was 60°C instead of LCST+5°C. In (A2), if the viscosity at LCST+5°C at a shear rate of 1 (1 / s) is 45 mPa·s or greater, the above condition (1-2) is satisfied. In (A3), if the viscosity at a shear rate of 1 (1 / s) at an ink drying rate of 20% is 100 mPa·s or greater, the above condition (1-3) is satisfied.

[0264] The above (A1) to (A8) were measured using an MCR-102 manufactured by Anton Paar. (A4) was measured only in Comparative Example 5. (A5) to (A8) were measured in the oscillation mode of the MCR102, with the strain being varied under measurement conditions of an angular frequency ω of 10 rad / s and an oscillation angle γ ranging from 1 to 1000%. The time settings were 300 measurement points, a measurement interval of 2 s, a shear rate of 1 (1 / s), and a temperature that was changed linearly from 10°C to LCST+5°C.

[0265] FIG. 4 shows a graph illustrating the relationship between the loss tangent (tanδ), storage modulus G' (logarithm), and loss modulus G" (logarithm) and strain (logarithm) for the ink of Example 1, measured under the above conditions with an ink drying rate of 20%. From this graph, the loss tangent (tanδ) at a strain of 1% (A5) and the crossover strain (A6) can be determined.

[0266] (Evaluation of ink printing characteristics) The ink obtained above was used in an inkjet image forming apparatus (an inkjet printer manufactured by Tritec Corporation) using a Konica Minolta KM1024iMHE head (having a mechanism for circulating ink within the head using two or more circulation paths) at a resolution of 720 x 720 dpi to evaluate the following print properties (B1), (B2), and (B4). Note that for (B3), an ink coating film was formed and evaluated using the following method. The results are shown in Table I or Table II.

[0267] (B1) Injection properties Solid and linear images were printed on a PET substrate, and the state of streaks in the solid areas and the state of linear formation were evaluated according to the following evaluation criteria.

[0268] (Evaluation criteria) ◎: No streaks in solid areas, and straight lines are printed neatly. ○: There are no streaks in the solid areas, but there is some unevenness in the straight lines. ×: There are white streaks in the solid areas and gaps in the straight lines.

[0269] (B2) Image quality (pinning) An image having white characters of 4 pt, 6 pt, and 8 pt in the solid area was printed on a PET substrate, and the quality of the characters was evaluated according to the following evaluation criteria.

[0270] (Evaluation criteria) ◎: All blank characters are printed clearly. 〇: Characters of 6pt or larger are printed clearly. ×: There is bleeding in characters 6pt or larger.

[0271] (B3) Adhesion to substrate Using a PET substrate, the substrate fixation of the ink coating film was evaluated by the following method. The ink coating film was obtained by applying the ink to the substrate with a wire bar #7 and drying at 100°C for 3 minutes. The resulting ink coating film (100 mm x 100 mm, 10 μm thick) was cut with a cutter to create 25 grids of 5 vertical x 5 horizontal, and Nichiban Cellotape (registered trademark) was applied and peeled off, and the adhesion state was evaluated according to the following evaluation criteria.

[0272] (Evaluation criteria) ◎: No peeling. ○: Peeling is within 5 squares. ×: Peeling is 6 squares or more.

[0273] (B4) Paint film water resistance A solid image was printed on a PET substrate. Water was dropped onto the resulting printed image (ink coating) using a dropper, and the surface of the printed image was rubbed with a cotton swab 10 times. The condition of the printed image was evaluated according to the following criteria:

[0274] (Evaluation criteria) ◎: No change in printed image. ◯: The printed image remains almost unchanged. The density becomes slightly lighter. ×: The printed image has changed and is distorted.

[0275] [Table 1]

[0276] [Table 2]

[0277] As can be seen from Tables I and II, in ink jet printing using the inks of the examples, the ink ejection from the head was good, and high quality printed images were obtained. [Industrial Applicability]

[0278] The inkjet ink of the present disclosure can maintain good ejection properties while forming high-quality images in image formation by the inkjet method. Furthermore, the image formation of the present disclosure can form high-quality images with good workability in image formation by the inkjet method. [Explanation of symbols]

[0279] 1: Inkjet head (end shooter type) 11: Inkjet head (MEMS type) 2: Head chip 21: Nozzle plate 22: Nozzle 23: Ink channel 24: Piezoelectric element 25: Dummy channel 26a: Individual ink circulation path 28: Flow path forming groove 3: Wiring board 31a: Injection hole 31b: Discharge hole 31c:Introduction hole 4: Ink manifold 41: Common ink chamber 412: Ink discharge chamber 421: Common ink circulation path 422: Groove 423: Circulation path 424: Discharge channel 425:Introduction path 425a: Introduction groove 45: Bulkhead 5a: Ink supply pipe 5b: Ink recovery pipe 5c: Ink discharge pipe 5d: Circulation route connection part 6: Buffer space 7a: Connection 7b: Connection F1: Main flow path F2: Sub-channel 100: Inkjet recording device 101: Ink tank 102: Ink transfer pipe 103: Ink return pipe 104: Control unit 104a: control unit 105a: Transfer pump 105b: Return pump 107: Carriage mechanism 108: Means of transport 109: Base material 110a: Transfer pressure control pump 110b: Return pressure control pump 111a: Transfer side sub-tank 111b: Return side subtank

Claims

1. 1. An image forming method including a landing step of ejecting inkjet ink droplets from an inkjet head and landing them on a substrate, the inkjet ink contains a water-based solvent, a pigment, a fixing resin, and a temperature-responsive polymer; the temperature-responsive polymer has a lower critical solution temperature with respect to water; the content of the pigment is within a range of 2 to 10% by mass, the content of the fixing resin is within a range of 1 to 10% by mass, and the content of the temperature-responsive polymer is within a range of 0.01 to 1% by mass, relative to the total amount of the inkjet ink; The image forming method is characterized in that the ink droplets are heated within a range of the lower critical solution temperature +5 to 10° C. in the impacting step.

2. The image forming method according to claim 1, wherein in the impact step, only the substrate and the inkjet ink are heated within a range of the lower critical solution temperature + 5 to 10°C.

3. 3. The image forming method according to claim 1, wherein the inkjet ink has a viscosity of 15 mPa s or less at a shear rate of 1,000 (1 / s) at 25°C and a viscosity of 45 mPa s or more at a shear rate of 1 (1 / s) at any temperature within a range of ±10°C from the lower critical solution temperature.

4. 4. The image forming method according to claim 1, wherein the inkjet ink has a viscosity of 100 mPa s or more at a shear rate of 1 (1 / s) at any temperature within a range of the lower critical solution temperature ±10°C when the aqueous solvent is removed from the inkjet ink so that the inkjet ink has a mass of 80% of its initial mass.

5. 5. The image forming method according to claim 1, wherein the temperature-responsive polymer has a lower critical solution temperature in water within a range of 30 to 60°C.

6. The image forming method according to claim 1 , wherein the temperature-responsive polymer comprises a water-soluble cellulose resin.

7. 7. The image forming method according to claim 1, wherein the content of the aqueous solvent is in the range of 50 to 90% by mass with respect to the total amount of the inkjet ink.

8. 8. The image forming method according to claim 1, wherein the fixing resin comprises at least one resin selected from the group consisting of a polyacrylic resin, a polyurethane resin, and a polyester resin.

9. The image forming method according to any one of claims 1 to 8, wherein the ink-jet ink further contains a thixotropic agent.

10. The image forming method according to claim 9 , wherein the thixotropy-imparting agent comprises cellulose nanofiber or a smectite clay mineral.

11. 11. The image forming method according to claim 9, wherein the content of the thixotropy-imparting agent is in the range of 0.01 to 1% by mass with respect to the total amount of the inkjet ink.

12. 12. The image forming method according to claim 1, wherein the viscosity of the droplets when they land on the substrate or immediately after they land on the substrate is 150 mPa·s or more.

13. 13. The image forming method according to claim 1, wherein the landing step includes heating the droplets on the substrate at a temperature in the range of 30 to 60°C.

14. 14. The image forming method according to claim 12, wherein the time immediately after the droplets land on the substrate is within 100 msec after the droplets land on the substrate.

Images