Water-based ink for inkjet printing

Abstract

To provide a water-based ink for inkjet printing having excellent storage stability and excellent substrate adhesiveness (tape peeling resistance and scratch resistance) of an ink coating film of an obtained printed matter when used for printing on a low-liquid absorbing printing substrate and an inkjet printing method using the water-based ink.SOLUTION: There are provided: a water-based ink for inkjet printing on a low-liquid absorbing printing substrate, wherein the water-based ink contains a pigment, a pigment dispersant, a fixing resin, a water-soluble organic solvent and water and the fixing resin is a vinyl-based resin containing a structural unit derived from a cyclic ether alkyl acrylate (a-1) having a cyclic ether structure in the molecule and a structural unit derived from one or more carboxy group-containing monomers (a-2) selected from the group consisting of acrylic acid and methacrylic acid; and an inkjet printing method using the water-based ink.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an aqueous ink for inkjet printing and an inkjet printing method using the aqueous ink.

Background Art

[0002] A printing method using an inkjet recording system is a method of discharging ink droplets from fine nozzles and directly attaching them to a printing substrate to obtain a printed matter or the like on which characters and images are recorded. This printing method has many advantages such as being easily full-color and inexpensive, being able to use various printing substrates such as plain paper, label paper, and resin films, and being non-contact with the printing substrate, so it has become very popular. In particular, from the viewpoints of the weather resistance and water resistance of printed matter, inks using pigments as colorants have become mainstream.

[0003] For example, Patent Document 1 discloses an A-B block copolymer containing polymer chains A2 and B2 for the purpose of providing an environmentally considerate binder component and emulsion capable of forming a dry film such as an image excellent in adhesion, abrasion resistance, and blocking resistance. The polymer chain A2 is a polymer block containing a structural unit derived from a methacrylate derived from a biological material, and the polymer chain B2 contains a structural unit derived from methacrylic acid and a structural unit derived from a methacrylate derived from a biological material, and at least a part of the carboxy groups is neutralized with an alkali. An emulsion containing a binder component and binder particles formed of the binder component is disclosed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, in the sign graphics printing market for printing on advertising billboards and the like, synthetic resins are mainly used as printing substrates from the viewpoint of the durability of printed materials by indoor and outdoor posting. On the other hand, as a specific problem of using synthetic resin as a printing substrate, improvement in the substrate adhesion of the ink coating film to the obtained printed material, particularly improvement in tape peel resistance and abrasion resistance, is required. If the tape peel resistance and abrasion resistance of the ink coating film are insufficient, when posting an advertising billboard or the like, the ink coating film may peel off from the synthetic resin, leading to a decrease in the cosmetic appearance of the printed material and the posting of inaccurate information. Therefore, when printing on synthetic resin as a printing substrate, strong improvement in the substrate adhesion of the ink coating film to the printed material is required. In particular, when printing on synthetic resin using water-based ink, the ink coating film formed by the water-based ink needs to adhere sufficiently to the synthetic resin. In addition, unlike paper printing substrates, synthetic resins have a hydrophobic and low liquid-absorbing printing substrate surface, so the wettability of the ink to the synthetic resin surface is poor. Therefore, in order to improve the wettability of the ink, a hydrophobic organic solvent may be blended into the ink. However, in the case of an ink containing such a hydrophobic organic solvent, the storage stability of the ink decreases due to the aggregation of the solid components (pigment and polymer) in the ink during ink storage, and it may be difficult to discharge ink droplets from fine inkjet nozzles. Therefore, improvement in the storage stability of the ink is required. However, when the binder component of Patent Document 1 was blended into a water-based ink and used for inkjet printing on synthetic resin as a low liquid-absorbing printing substrate, it was found that the storage stability of the water-based ink was not sufficient, and there was room for improvement in the substrate adhesion (particularly tape peel resistance) of the ink coating film of the obtained printed material. An object of the present invention is to provide a water-based ink for inkjet printing and an inkjet printing method using the water-based ink, which are excellent in storage stability and, when used for printing on a low liquid-absorbing printing substrate, are excellent in the substrate adhesion (tape peel resistance and abrasion resistance) of the ink coating film of the obtained printed material.

Means for Solving the Problems

[0006] The present inventors used the mode of using polyvinyl chloride resin widely used as a low liquid-absorbing printing substrate as a model case, and when screening (meth)acrylic monomers using the Hansen solubility parameter (HSP value), "the interaction radius R of polyvinyl chloride resin 0 The relative energy difference (RED) calculated as the ratio of the difference R a of the solubility parameters (HSP values) of polyvinyl chloride resin and monomer to is 1 or less", and found that a polymer containing a structural unit derived from an acrylic monomer satisfying this condition exhibits high abrasion resistance against polyvinyl chloride resin. Paying attention to the fact that all acrylic monomers satisfying this condition have a cyclic ether structure in the molecule, an aqueous ink for inkjet printing containing a pigment, a pigment dispersant, a fixing resin, and water, wherein the fixing resin has a cyclic ether structure in the molecule. By being a vinyl resin containing a structural unit derived from a cyclic ether alkyl acrylate (a-1) and a structural unit derived from one or more carboxy group-containing monomers (a-2) selected from the group consisting of acrylic acid and methacrylic acid, it was found that the above problems can be solved.

[0007] That is, the present invention provides the following [1] and [2]. [1] An aqueous ink for inkjet printing on a low liquid-absorbing printing substrate, The aqueous ink contains a pigment, a pigment dispersant, a fixing resin, a water-soluble organic solvent, and water, The fixing resin is a vinyl resin containing a structural unit derived from a cyclic ether alkyl acrylate (a-1) having a cyclic ether structure in the molecule and a structural unit derived from one or more carboxy group-containing monomers (a-2) selected from the group consisting of acrylic acid and methacrylic acid. An aqueous ink for inkjet printing. [2] An inkjet printing method of printing on a low liquid-absorbing printing substrate using the aqueous ink according to [1]. [Advantages of the Invention]

[0008] According to the present invention, there can be provided an aqueous ink for inkjet printing which is excellent in storage stability and, when used for printing on a low-absorbency printing substrate, is excellent in the substrate adhesion (tape peeling resistance and rubbing resistance) of the ink coating film of the obtained printed matter, and an inkjet printing method using the aqueous ink.

Mode for Carrying Out the Invention

[0009] [Aqueous Ink for Inkjet Printing] The aqueous ink for inkjet printing of the present invention (hereinafter also simply referred to as "aqueous ink" or "ink") is an aqueous ink for inkjet printing on a low-absorbency printing substrate, and the aqueous ink contains a pigment, a pigment dispersant, a fixing resin, a water-soluble organic solvent, and water. The fixing resin is a vinyl resin containing a structural unit derived from a cyclic ether alkyl acrylate (a-1) having a cyclic ether structure in the molecule and a structural unit derived from one or more carboxy group-containing monomers (a-2) selected from the group consisting of acrylic acid and methacrylic acid. In the present invention, "aqueous" means that water occupies the largest proportion by mass in the medium. Further, in the present invention, the "water-soluble organic solvent" refers to an organic solvent whose dissolution amount is 5 mL or more when 100 mL of water at 25 °C is dissolved with the organic solvent. Also, in the present specification, the "low-absorbency" of the low-absorbency printing substrate is a concept including low-absorbency and non-absorbency, and the water absorption amount of the printing substrate at a contact time of 100 msec between the printing substrate and pure water is 0 g / m 2 10 g / m or more 2 The following. The water absorption amount can be measured as the transfer amount at a contact time of 100 msec of pure water under the conditions of 23 °C and a relative humidity of 50% using an automatic scanning liquid absorption meter (for example, "KM500win" manufactured by Kumagai Riki Kogyo Co., Ltd.). Also, "printing" is a concept including printing and printing characters and images, and "printed matter" is a concept including printed matter and printed matter on which characters and images are recorded.

[0010] According to the present invention, it has excellent storage stability and, when used for printing on a low liquid-absorbing printing substrate, has excellent adhesion of the ink coating film of the obtained printed matter to the substrate (tape peeling resistance and rubbing resistance). Although the reason is not clear, it is considered as follows. The aqueous ink of the present invention contains, as a fixing resin, a vinyl resin containing a structural unit derived from a cyclic ether alkyl acrylate having a cyclic ether structure in the molecule and a structural unit derived from one or more carboxy group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid. Since the cyclic ether structure part of the structural unit derived from this cyclic ether alkyl acrylate easily interacts with the polar functional groups present on the surface of the synthetic resin used as the low liquid-absorbing printing substrate, it is considered that the wettability with respect to the synthetic resin used as the low liquid-absorbing printing substrate is improved and the adhesion of the ink coating film to the substrate is improved. Furthermore, depending on the type of synthetic resin constituting the printing substrate, dipole-dipole interaction and hydrogen bond interaction are exhibited between the polar functional groups present on the surface of the synthetic resin and the cyclic ether structure part of the structural unit derived from the cyclic ether alkyl acrylate, and also, since van der Waals interaction can be efficiently exhibited between the hydrophobic part of the synthetic resin and the main chain of the vinyl resin containing the structural unit derived from the cyclic ether alkyl acrylate, it is considered that the adhesion of the ink coating film to the synthetic resin used as the low liquid-absorbing printing substrate is further improved. In addition, since the vinyl resin contains a structural unit derived from a cyclic ether alkyl acrylate having a low glass transition temperature, the mobility when the aqueous ink applied to the synthetic resin as a low liquid-absorbing printing substrate dries is improved, and it contributes to increasing the degree of freedom of the conformation of the structural unit derived from the cyclic ether alkyl acrylate so that the structural unit derived from the cyclic ether alkyl acrylate can strongly exhibit the interaction with the synthetic resin, and strong interaction can be efficiently exhibited. Also, since it has the effect of relaxing the stress when the ink coating film peels off from the printing substrate, it is considered that the adhesion of the ink coating film to the synthetic resin used as the low liquid-absorbing printing substrate can be further improved. Furthermore, in addition to the structural unit derived from at least one carboxy group-containing monomer selected from the group consisting of acrylic acid and methacrylic acid, which can efficiently exhibit electrostatic repulsive force, the vinyl resin further contains a structural unit derived from cyclic ether alkyl acrylate. Different from the case of containing a structural unit derived from cyclic ether alkyl methacrylate, it is surprisingly found that a stable dispersion system can be obtained, which is considered to have the effect of improving the storage stability of the aqueous ink. Hereinafter, the storage stability of the aqueous ink is simply referred to as "ink storage stability", and the tape peel resistance and rubbing resistance of the ink coating film of the printed matter obtained when used for printing on a low liquid-absorbing printing substrate are simply referred to as "tape peel resistance" and "rubbing resistance", respectively.

[0011] <Pigment> The pigment used in the present invention may be either an inorganic pigment or an organic pigment, and a lake pigment or a fluorescent pigment can also be used. Further, if necessary, they can be used in combination with a extender pigment. Specific examples of the inorganic pigment include, for example, metal oxides such as carbon black, titanium oxide, iron oxide, red iron oxide, chromium oxide, and nacreous pearlescent pigments. Particularly in the case of black ink, carbon black is preferred. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Specific examples of the organic pigment include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and fluoranthene pigments. In the case of achromatic ink, achromatic pigments such as white, black, and gray can be used, and in the case of chromatic ink, chromatic pigments such as yellow, magenta, cyan, blue, red, orange, and green can be used. Specific examples of preferred organic pigments include one or more product numbers selected from C.I. Pigment Yellow, C.I. Pigment Red, C.I. Pigment Orange, C.I. Pigment Violet, C.I. Pigment Blue, and C.I. Pigment Green. Examples of extender pigments include silica, calcium carbonate, and talc. The above pigments can be used alone or in combination of two or more.

[0012] <Pigment dispersant> In the aqueous ink of the present invention, the pigment is dispersed in the medium by a pigment dispersant. Forms of the pigment in the aqueous ink of the present invention include forms dispersed by a resin (hereinafter also referred to as "pigment-dispersing resin") or a surfactant as the pigment dispersant. Among these, as the form of the pigment in the aqueous ink of the present invention, from the viewpoints of improving the storage stability of the ink and the tape peeling resistance and rubbing resistance, a form dispersed by a pigment-dispersing resin is preferred, and a form of resin particles containing a pigment (hereinafter also referred to as "pigment-containing resin particles") is more preferred. Here, the form of the pigment-containing resin particles is not particularly limited, as long as the particles are formed by at least a pigment and a pigment-dispersing resin, and the particles are those in which the pigment-dispersing resin is adsorbed to the pigment in the aqueous ink. Examples of the form of the pigment-containing resin particles include a particle form in which the pigment is encapsulated in the pigment-dispersing resin, a particle form in which the pigment is uniformly dispersed in the pigment-dispersing resin, a particle form in which the pigment is exposed on the surface of the pigment-dispersing resin particles, etc., and mixtures thereof are also included.

[0013] (Pigment-dispersing resin) The pigment-dispersing resin may be either a water-soluble resin or a water-insoluble resin. Here, with respect to the "water-soluble" and "water-insoluble" of the resin, when a resin dried at 105°C for 2 hours until a constant weight is reached is dissolved in 100 g of water at 25°C until saturation is reached, if the dissolved amount exceeds 10 g, it is determined as "water-soluble", and if it is 10 g or less, it is determined as "water-insoluble". In addition, when the pigment dispersion resin has an anionic group as described below and the anionic group is neutralized with a neutralizing agent, it is determined by the dissolution amount measured under the condition that the neutralizing agent is present under the condition that the mass ratio of the pigment dispersion resin to the neutralizing agent is the same as that in the aqueous ink of the present invention. One kind of the pigment dispersion resin may be used alone, or two or more kinds may be used in combination.

[0014] Examples of the pigment dispersion resin include vinyl resins obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds), polyester resins, and polyurethane resins. As the pigment dispersion resin, those synthesized as appropriate may be used, or commercially available products may be used. Among these, from the viewpoints of improving tape peelability, storage stability of the ink, and tape peelability and abrasion resistance, vinyl resins are preferable.

[0015] From the viewpoint of improving the storage stability of the ink, the pigment dispersion resin preferably has a crosslinked structure. In this case, the pigment dispersion resin preferably has a structure containing a constituent component of a polymer having a linear two-dimensional structure which may have a branched chain and a constituent component derived from a crosslinking agent. Such a crosslinked structure is considered that a polymer having a linear two-dimensional structure which may have a branched chain has become a three-dimensional structure by a constituent component derived from a crosslinking agent. Examples of the polymer having a linear two-dimensional structure which may have a branched chain include vinyl resins obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds), polyester resins, and polyurethane resins, and the vinyl resins described below are preferable. The crosslinking agent is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule from the viewpoint of improving the storage stability of the ink, more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group with 3 to 8 carbon atoms, and still more preferably trimethylolpropane polyglycidyl ether, pentaerythritol polyglycidyl ether, neopentyl glycol diglycidyl ether, 1,6 - hexanediol diglycidyl ether, 1,4 - butanediol diglycidyl ether, 1,4 - cyclohexanedimethanol diglycidyl ether, and diethylene glycol diglycidyl ether, and still more preferably trimethylolpropane polyglycidyl ether. When the crosslinking agent is a polyfunctional epoxy compound, the epoxy equivalent of the crosslinking agent is preferably 90 or more, more preferably 100 or more, still more preferably 110 or more, and preferably 300 or less, more preferably 200 or less, still more preferably 150 or less.

[0016] As the vinyl resin as the pigment - dispersing resin, from the viewpoint of improving the dispersion stability of the pigment and thus improving the storage stability of the ink, it preferably contains a structural unit derived from an anionic - group - containing monomer. In the present specification, the "anionic group" refers to an anionic group or a group that can be ionized to become an anionic group. Examples of the anionic group include a carboxy group (-COOM), a sulfonic acid group (-SO 3 M), and a phosphoric acid group (-OPO 3 M 2 ). In the above chemical formula, M represents a hydrogen atom, an alkali metal, ammonium, or an organic ammonium. Examples of the vinyl resin include a homopolymer of an anionic - group - containing monomer, a copolymer of an anionic - group - containing monomer and a hydrophobic monomer, and a copolymer of an anionic - group - containing monomer, a hydrophobic monomer, and a nonionic monomer. Here, the "hydrophobicity" of the hydrophobic monomer means that when the monomer is dissolved until saturated in 100 g of ion - exchanged water at 25°C, the dissolved amount is less than 10 g. Nonionic monomers are monomers with high affinity for water and water-soluble organic solvents, such as monomers containing hydroxy groups or polyalkylene glycol chains. When the vinyl resin is a copolymer, it may be any of a random copolymer, a block copolymer, an alternating copolymer, and a graft copolymer.

[0017] Examples of the anionic group-containing monomers include carboxy group-containing monomers, sulfonic acid group-containing monomers, and phosphoric acid group-containing monomers. Among these, carboxy group-containing monomers are preferred, (meth)acrylic acid is more preferred, and methacrylic acid is even more preferred. Examples of the hydrophobic monomers include (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having 1 to 22 carbon atoms; aromatic group-containing monomers such as styrene-based monomers and aromatic group-containing (meth)acrylates; and styrene-based macromonomers. The molecular weight of the aromatic group-containing monomers, preferably styrene-based monomers, is preferably less than 500. The styrene-based macromonomer is a compound having a polymerizable functional group at one end and a number average molecular weight of 500 or more and 100,000 or less. Among these, the hydrophobic monomers are preferably styrene-based monomers, more preferably one or more selected from the group consisting of α-methylstyrene, 2-methylstyrene, vinyltoluene, and divinylbenzene, and even more preferably one or more selected from the group consisting of styrene and α-methylstyrene. Examples of the nonionic monomers include polyalkylene glycol mono(meth)acrylates such as polyethylene glycol mono(meth)acrylate; and alkoxypolyalkylene glycol mono(meth)acrylates such as methoxypolyethylene glycol mono(meth)acrylate and octoxypolyethylene glycol mono(meth)acrylate. Note that “(meth)acrylic acid” means at least one selected from the group consisting of acrylic acid and methacrylic acid. Also, “(meth)acrylate” means at least one selected from the group consisting of acrylate and methacrylate. Each monomer of the vinyl resin can be used alone or in combination of two or more thereof.

[0018] When the vinyl resin is a copolymer, from the viewpoints of improving the storage stability of the ink and the tape peeling resistance and rubbing resistance, the vinyl resin preferably contains a structural unit derived from one or more anionic group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid, a structural unit derived from one or more hydrophobic monomers selected from the group consisting of (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having 1 to 22 carbon atoms, aromatic group-containing monomers, and styrene-based macromers, and a structural unit derived from one or more nonionic monomers selected from the group consisting of polyalkylene glycol mono(meth)acrylates and alkoxypolyalkylene glycol mono(meth)acrylates. More preferably, it contains a structural unit derived from one or more anionic group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid, a structural unit derived from one or more hydrophobic monomers selected from the group consisting of aromatic group-containing monomers and styrene-based macromers, and a structural unit derived from one or more nonionic monomers selected from the group consisting of polyalkylene glycol mono(meth)acrylates and alkoxypolyalkylene glycol mono(meth)acrylates. Even more preferably, it contains a structural unit derived from one or more anionic group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid, a structural unit derived from one or more hydrophobic monomers selected from the group consisting of aromatic group-containing monomers and styrene-based macromers, and a structural unit derived from one or more nonionic monomers selected from the group consisting of alkoxypolyalkylene glycol mono(meth)acrylates.

[0019] When the vinyl resin is a copolymer of an anionic group-containing monomer and a hydrophobic monomer, or a copolymer of an anionic group-containing monomer, a hydrophobic monomer, and a nonionic monomer, the content of the structural unit derived from each monomer component in all the structural units of the vinyl resin is as follows. The content of the structural unit derived from the anionic group-containing monomer in all the structural units of the vinyl resin is preferably 5% by mass or more, more preferably 10% by mass or more, still more preferably 13% by mass or more, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability of the ink, and, from the same viewpoint as above, is preferably 40% by mass or less, more preferably 30% by mass or less, still more preferably 20% by mass or less. The content of the structural unit derived from the hydrophobic monomer in all the structural units of the vinyl resin is preferably 45% by mass or more, more preferably 50% by mass or more, still more preferably 55% by mass or more, from the viewpoint of improving the storage stability of the ink and improving the tape peeling resistance and rubbing resistance, and, from the same viewpoint as above, is preferably 75% by mass or less, more preferably 70% by mass or less, still more preferably 65% by mass or less. When the vinyl resin contains a structural unit derived from a nonionic monomer, the content of the structural unit derived from the nonionic monomer in all the structural units of the vinyl resin is preferably 10% by mass or more, more preferably 15% by mass or more, still more preferably 20% by mass or more, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability of the ink, and is preferably 40% by mass or less, more preferably 35% by mass or less, still more preferably 30% by mass or less. The vinyl resin can be obtained, for example, by addition polymerization of a raw material monomer containing an anionic group-containing monomer, a hydrophobic monomer, or a nonionic monomer by a known method.

[0020] From the viewpoint of improving the dispersion stability of the pigment and the storage stability of the ink, the acid value of the vinyl resin is preferably 40 mgKOH / g or more, more preferably 50 mgKOH / g or more, still more preferably 60 mgKOH / g or more. And from the viewpoint of improving the tape peeling resistance and rubbing resistance, it is preferably 800 mgKOH / g or less, more preferably 500 mgKOH / g or less, still more preferably 300 mgKOH / g or less, and even more preferably 200 mgKOH / g or less. The acid value of the vinyl resin can be determined by the method described in the examples, but can also be calculated from the mass ratio of the constituent monomers. Further, the acid value of the vinyl resin having a crosslinked structure can also be calculated by the following formula. Acid value of vinyl resin having a crosslinked structure (mgKOH / g) = [Acid value of vinyl resin before crosslinking (mgKOH / g) × [(100 - crosslinking rate (mol%)) / 100]] In this specification, the crosslinking rate (mol%) of the vinyl resin having a crosslinked structure is a value calculated from the acid value of the vinyl resin before crosslinking and the equivalent of the crosslinkable functional group of the crosslinking agent.

[0021] From the viewpoints of improving the dispersion stability of the pigment and the storage stability of the ink, and improving the tape peeling resistance and rubbing resistance, the weight average molecular weight of the vinyl resin is preferably 5,000 or more, more preferably 10,000 or more, still more preferably 30,000 or more. And from the same viewpoints as above, it is preferably 100,000 or less, more preferably 80,000 or less, still more preferably 60,000 or less. The weight average molecular weight of the vinyl resin can be measured by the method described in the examples.

[0022] Examples of commercially available vinyl resins include polyacrylic acid such as "Aron AC-10SL" (manufactured by Toagosei Co., Ltd.); styrene / acrylic resins such as "Joncryl 67", "Joncryl 611", "Joncryl 678", "Joncryl 680", "Joncryl 690", "Joncryl 819" (all manufactured by BASF Japan Ltd.).

[0023] When the form of the pigment in the aqueous ink of the present invention is in the form of pigment-containing resin particles, the pigment-containing resin particles are preferably prepared by dispersing a pigment, a pigment dispersion resin, a neutralizing agent, a surfactant, etc., if necessary, by a known method to obtain an aqueous dispersion (hereinafter also referred to as "pigment aqueous dispersion") and then blending it into the aqueous ink. As a method for producing the pigment aqueous dispersion, a method is preferably exemplified in which a pigment mixture liquid containing a pigment, a pigment dispersion resin, an organic solvent, and water is dispersed to obtain a dispersion-treated product, and then the organic solvent is removed from the obtained dispersion-treated product. Further, if necessary, a cross-linking agent may be added to the obtained pigment aqueous dispersion to cross-link the pigment dispersion resin. Specific examples of such a method include the methods described in paragraphs

[0022] to

[0026] of JP-A-2022-104084. When the pigment is blended into the aqueous ink as a pigment aqueous dispersion, the average particle diameter of the pigment-containing resin particles in the aqueous dispersion is preferably 30 nm or more, more preferably 50 nm or more, still more preferably 70 nm or more, from the viewpoints of improving the dispersion stability of the pigment and the storage stability of the ink, and improving the tape peeling resistance and rubbing resistance. And from the same viewpoints as above, it is preferably 600 nm or less, more preferably 550 nm or less, still more preferably 500 nm or less, and even more preferably 450 nm or less. The average particle diameter of the pigment-containing resin particles in the pigment aqueous dispersion can be measured by the method described in the examples.

[0024] <Fixing resin> The aqueous ink of the present invention contains, as a fixing resin, a structural unit derived from a cyclic ether alkyl acrylate (a-1) having a cyclic ether structure in the molecule (hereinafter also referred to as "cyclic ether alkyl acrylate (a-1)") and a structural unit derived from one or more carboxy group-containing monomers (a-2) selected from the group consisting of acrylic acid and methacrylic acid (hereinafter also referred to as "carboxy group-containing monomer (a-2)"). The fixing resin may be used alone or in combination of two or more.

[0025] Cyclic ether alkyl acrylate (a-1) preferably has a 3-membered ring, 4-membered ring, 5-membered ring or 6-membered ring cyclic ether structure in the molecule from the viewpoints of improving the storage stability of the ink and the tape peel resistance and rubbing resistance. More preferably, it is at least one selected from the group consisting of tetrahydrofurfuryl acrylate, (3-ethyl-3-oxetanyl)methyl acrylate, (2-methyl-2-ethyl-1,3-dioxolan-4-yl)methyl acrylate, and cyclic trimethylolpropane formal acrylate. Even more preferably, it is at least one selected from the group consisting of tetrahydrofurfuryl acrylate, (3-ethyl-3-oxetanyl)methyl acrylate, and cyclic trimethylolpropane formal acrylate. Even more preferably, it is tetrahydrofurfuryl acrylate. From the viewpoint of improving the tape peel resistance, acrylic acid is preferred as the carboxy group-containing monomer (a-2).

[0026] The vinyl resin used as the fixing resin preferably contains a structural unit derived from a hydrophobic monomer (a-3) in addition to the structural units derived from the cyclic ether alkyl acrylate (a-1) and the carboxy group-containing monomer (a-2) from the viewpoint of improving the tape peel resistance. Here, the "hydrophobicity" of the hydrophobic monomer means that when the monomer is dissolved until saturated in 100 g of ion-exchanged water at 25°C, the dissolved amount is less than 10 g as described above.

[0027] Examples of the hydrophobic monomer (a-3) include (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having 1 to 22 carbon atoms, aromatic group-containing monomers such as styrene-based monomers and aromatic group-containing (meth)acrylates, and styrene-based macromonomers, in the same manner as the hydrophobic monomers exemplified in the above-described pigment dispersion resin. The molecular weight of the aromatic group-containing monomer, preferably the styrene-based monomer, is preferably less than 500. The styrene-based macromonomer is a compound having a number average molecular weight of 500 or more and 100,000 or less and having a polymerizable functional group at one end. Among these, the hydrophobic monomer (a-3) is preferably at least one selected from the group consisting of (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having 1 to 22 carbon atoms and aromatic group-containing monomers, and more preferably at least one selected from the group consisting of (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having 1 to 22 carbon atoms and aromatic group-containing (meth)acrylates.

[0028] As the hydrophobic monomer (a-3), from the viewpoint of improving tape peel resistance, those having a glass transition temperature (Tg) of 10 °C or lower when made into a homopolymer are preferred. As such a hydrophobic monomer (a-3), from the viewpoint of improving tape peel resistance, an alkyl acrylate having an alkyl group preferably having 1 to 10 carbon atoms, more preferably 2 to 8 carbon atoms, is exemplified. Specific examples thereof preferably include one or more selected from the group consisting of methyl acrylate (Tg: 8 °C), ethyl acrylate (Tg: -20 °C), propyl acrylate (Tg: 3 °C), isopropyl acrylate (Tg: -3 °C), butyl acrylate (Tg: -55 °C), isobutyl acrylate (Tg: -33 °C), isopentyl acrylate (Tg: -45 °C), hexyl acrylate (Tg: -57 °C), octyl acrylate (Tg: -65 °C), 2-ethylhexyl acrylate (Tg: -70 °C), and benzyl acrylate (Tg: 6 °C). Among these, the hydrophobic monomer (a-3) is more preferably one or more selected from the group consisting of methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, and benzyl acrylate from the viewpoint of improving tape peel resistance, still more preferably one or more selected from the group consisting of butyl acrylate, isobutyl acrylate, and benzyl acrylate, and still more preferably one or more selected from the group consisting of isobutyl acrylate and benzyl acrylate. Note that the numerical values in the parentheses above indicate the glass transition temperature (Tg) when each monomer is made into a homopolymer. As the glass transition temperature (Tg) of the homopolymer of each monomer, for example, the values described in Polymer Handbook Third Edition (Wiley-Interscience 1989) can be used.

[0029] The vinyl resin can contain structural units derived from other monomers other than the monomers (a-1) to (a-3) within a range that does not inhibit the effects of the present invention. Examples of other monomers include the nonionic monomers exemplified in the above pigment dispersion resin.

[0030] From the viewpoint of improving tape peeling resistance and abrasion resistance, the content of the structural unit derived from the cyclic ether alkyl acrylate (a-1) in all the structural units of the vinyl resin is preferably 25% by mass or more, more preferably 30% by mass or more, still more preferably 40% by mass or more, even more preferably 50% by mass or more, and even more preferably 55% by mass or more. From the viewpoint of improving the storage stability of the ink, it is preferably 96% by mass or less, more preferably 95% by mass or less, still more preferably 90% by mass or less, and even more preferably 85% by mass or less. From the viewpoint of improving the storage stability of the ink, the content of the structural unit derived from the carboxy group-containing monomer (a-2) in all the structural units of the vinyl resin is preferably 2% by mass or more, more preferably 4% by mass or more, and still more preferably 8% by mass or more. From the viewpoint of improving tape peeling resistance and abrasion resistance, it is preferably 25% by mass or less, more preferably 20% by mass or less, and still more preferably 17% by mass or less. From the viewpoint of improving tape peeling resistance, the content of the structural unit derived from the hydrophobic monomer (a-3) in all the structural units of the vinyl resin is preferably 3% by mass or more, more preferably 5% by mass or more, and still more preferably 7% by mass or more. From the same viewpoint as above, it is preferably 70% by mass or less, more preferably 65% by mass or less, and still more preferably 60% by mass or less. The vinyl resin can be obtained, for example, by addition polymerization of raw material monomers including a cyclic ether alkyl acrylate (a-1), a carboxy group-containing monomer (a-2), a hydrophobic monomer (a-3), or a nonionic monomer as required, by a known method.

[0031] From the perspective of improving the storage stability of the ink, the acid value of the vinyl resin is preferably 15 mgKOH / g or more, more preferably 30 mgKOH / g or more, still more preferably 50 mgKOH / g or more, and even more preferably 70 mgKOH / g or more. From the perspective of improving tape peel resistance and abrasion resistance, it is preferably 200 mgKOH / g or less, more preferably 160 mgKOH / g or less, still more preferably 140 mgKOH / g or less, and even more preferably 130 mgKOH / g or less. The acid value of the vinyl resin can be determined by the method described in the examples, but can also be calculated from the mass ratio of the constituent monomers.

[0032] From the perspective of improving tape peel resistance, the glass transition temperature Tg of the vinyl resin is preferably 45°C or less, more preferably 40°C or less, still more preferably 30°C or less, even more preferably 20°C or less, even more preferably 10°C or less, and even more preferably 5°C or less. From the perspective of improving the storage stability of the ink, it is preferably -30°C or more, more preferably -20°C or more, and still more preferably -15°C or more. The glass transition temperature of the vinyl resin is calculated by the method described in the examples.

[0033] From the perspective of improving tape peel resistance, the weight average molecular weight of the vinyl resin is preferably 50,000 or more, more preferably 100,000 or more, still more preferably 150,000 or more. From the same perspective as above, from the perspective of improving the storage stability of the ink, it is preferably 1,000,000 or less, more preferably 900,000 or less, and still more preferably 600,000 or less. The weight average molecular weight of the vinyl resin can be measured by the method described in the examples.

[0034] At least a part of the carboxy groups of the vinyl resin is preferably neutralized with a neutralizing agent from the viewpoint of improving the storage stability of the ink. The neutralizing agent is preferably at least one selected from the group consisting of alkali metal hydroxides, organic amines, and ammonia, more preferably at least one selected from the group consisting of alkali metal hydroxides and organic amines, and still more preferably an alkali metal hydroxide. Examples of the alkali metal hydroxide include sodium hydroxide and potassium hydroxide. Examples of the organic amine include alkanolamines, alkylamines, aminoalkanediols, alkoxyamines, and heterocyclic amines. Among these, at least one selected from the group consisting of alkanolamines and alkylamines is preferable, and alkanolamines are more preferable. Preferable examples of the alkanolamine include alkanolamines having 2 to 8 carbon atoms miscible with water, such as monoethanolamine, monoisopropanolamine, monoisobutanolamine, N-methylethanolamine, N,N-dimethylethanolamine, and N-methyldiethanolamine. From the viewpoint of improving the storage stability of the ink, the degree of neutralization of the carboxy groups of the vinyl resin is preferably 30 mol% or more, more preferably 40 mol% or more, still more preferably 50 mol% or more, and from the same viewpoint as above, preferably 100 mol% or less, more preferably 90 mol% or less, still more preferably 80 mol% or less, and even more preferably 70 mol% or less. Here, the degree of neutralization can be determined by the following formula as the equivalent amount of the neutralizing agent used with respect to the carboxy groups of the vinyl resin. When the equivalent amount of the neutralizing agent used is 100 mol% or less, it is synonymous with the degree of neutralization. When the equivalent amount of the neutralizing agent used exceeds 100 mol%, it means that the neutralizing agent is in excess with respect to the carboxy groups of the vinyl resin, and the degree of neutralization of the vinyl resin at this time is regarded as 100 mol%. Equivalent amount of neutralizing agent (mol%) = [(mass of neutralizing agent added (g) / equivalent of neutralizing agent) / 〔(acid value of vinyl resin (mgKOH / g) × mass of vinyl resin (g)) / (56.1×1000)〕]×100

[0035] The fixing resin is preferably used as resin particles not containing a pigment, and from the viewpoint of improving the productivity of the aqueous ink, it is preferably formulated in the aqueous ink as an aqueous dispersion of polymer particles not containing a pigment. The fixing resin may be a synthetic one as appropriate or a commercially available product. The aqueous dispersion of polymer particles not containing a pigment can be produced by, for example, obtaining the vinyl resin before neutralization of the fixing resin constituting the polymer particles not containing a pigment by a known polymerization method, and then stirring a mixture containing the vinyl resin, water, and, if necessary, a neutralizing agent while heating, and then cooling the mixture. When the fixing resin is formulated as an aqueous dispersion of resin particles not containing a pigment, the average particle diameter of the resin particles not containing a pigment in the aqueous dispersion is preferably 10 nm or more, more preferably 30 nm or more, still more preferably 50 nm or more, and even more preferably 70 nm or more, from the viewpoint of the storage stability of the aqueous ink, and is preferably 300 nm or less, more preferably 200 nm or less, still more preferably 150 nm or less, and even more preferably 130 nm or less. The average particle diameter of the resin particles not containing a pigment in the aqueous dispersion can be measured by the same method as the method for measuring the average particle diameter of the pigment-containing resin particles in the pigment aqueous dispersion described in the examples.

[0036] <Water-soluble organic solvent> The aqueous ink of the present invention contains a water-soluble organic solvent from the viewpoints of improving the storage stability of the ink and the tape peeling resistance and rubbing resistance. In the present invention, the "water-soluble organic solvent" refers to an organic solvent whose dissolution amount is 5 mL or more when the organic solvent is dissolved in 100 mL of water at 25°C, as described above. The boiling point of the water-soluble organic solvent is preferably 90°C or higher, more preferably 130°C or higher, still more preferably 150°C or higher, and is preferably 260°C or lower, more preferably 250°C or lower, still more preferably 240°C or lower. When two or more water-soluble organic solvents are used, the boiling point value is the weighted average value weighted by the content (mass%) of each water-soluble organic solvent.

[0037] (Alkylene glycol alkyl ether) From the viewpoint of improving the storage stability of the ink, as well as the tape peeling resistance and rubbing resistance, the water-soluble organic solvent preferably contains an alkylene glycol alkyl ether. From the viewpoint of improving the storage stability of the aqueous ink, as well as the tape peeling resistance and rubbing resistance, the number of carbon atoms of the alkyl group in the alkylene glycol alkyl ether is preferably 1 or more, more preferably 2 or more, still more preferably 3 or more, and from the same viewpoint as above, preferably 6 or less, more preferably 4 or less. The alkyl group may be linear or branched. Here, the number of carbon atoms of the above alkyl group is the number of carbon atoms of one alkyl group. When the alkylene glycol alkyl ether has a plurality of alkyl groups, it is preferable that the number of carbon atoms of each alkyl group is within the above range. As the alkylene oxide group of the alkylene glycol alkyl ether, one or more selected from the group consisting of an ethylene oxide group and a propylene oxide group are preferably exemplified. From the viewpoint of improving the storage stability of the aqueous ink, as well as the tape peeling resistance and rubbing resistance, the alkylene glycol alkyl ether is preferably one or more selected from the group consisting of alkylene glycol monoalkyl ethers and alkylene glycol dialkyl ethers.

[0038] Examples of the alkylene glycol monoalkyl ether include ethylene glycol monoalkyl ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, and ethylene glycol monoisobutyl ether; diethylene glycol monoalkyl ethers such as diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, and diethylene glycol monoisobutyl ether; propylene glycol monoalkyl ethers such as propylene glycol monomethyl ether, propylene glycol monopropyl ether, and propylene glycol monobutyl ether; dipropylene glycol monoalkyl ethers such as dipropylene glycol monomethyl ether and dipropylene glycol monopropyl ether; and tripropylene glycol monoalkyl ethers such as tripropylene glycol monomethyl ether. Examples of the alkylene glycol dialkyl ether include diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, diethylene glycol diethyl ether, and triethylene glycol dimethyl ether. The alkylene glycol alkyl ether may be used alone or in combination of two or more.

[0039] Among these, alkylene glycol alkyl ethers are preferably alkylene glycol monoalkyl ethers from the viewpoints of improving the storage stability of the ink and the tape peeling resistance and rubbing resistance, more preferably one or more selected from the group consisting of diethylene glycol monoalkyl ethers, propylene glycol monoalkyl ethers, and dipropylene glycol monoalkyl ethers, still more preferably one or more selected from the group consisting of diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, and dipropylene glycol monopropyl ether, and even more preferably one or more selected from the group consisting of propylene glycol monopropyl ether, propylene glycol monobutyl ether, dipropylene glycol monopropyl ether, and diethylene glycol monoisobutyl ether.

[0040] (Other water-soluble organic solvents other than alkylene glycol alkyl ethers) The water-soluble organic solvent preferably further contains other water-soluble organic solvents other than alkylene glycol alkyl ethers. The other water-soluble organic solvents can be appropriately selected according to the purpose. Examples of the other water-soluble organic solvents include polyhydric alcohols, nitrogen-containing heterocyclic compounds, amides, amines, and sulfur-containing compounds. Among these, the water-soluble organic solvent preferably further contains polyhydric alcohols from the viewpoints of improving the storage stability of the ink and the tape peeling resistance and rubbing resistance.

[0041] Examples of the polyhydric alcohol include alkane diols such as ethylene glycol, propylene glycol, 1,2-pentanediol, 1,2-hexanediol, and 1,2-octanediol; polyalkylene glycols such as diethylene glycol and triethylene glycol; glycerin; and trimethylolpropane. Among these, the polyhydric alcohol is preferably at least one selected from the group consisting of propylene glycol, 1,2-hexanediol, triethylene glycol, and glycerin, and more preferably propylene glycol.

[0042] The aqueous ink of the present invention may further contain various additives such as a surfactant, a humectant, a wetting agent, a wetting and penetrating agent, a viscosity modifier, an antifoaming agent, a preservative, a fungicide, and a rust preventive, if necessary. Examples of the surfactant include nonionic surfactants, anionic surfactants, amphoteric surfactants, and the like. Among these, nonionic surfactants are preferred. Examples of the nonionic surfactant include polyoxyalkylene alkyl ether type surfactants, acetylene glycol-based surfactants, polyhydric alcohol type surfactants, fatty acid alkanolamides, silicone-based surfactants, and fluorine-based surfactants. Among these, the nonionic surfactant is preferably at least one selected from the group consisting of acetylene glycol-based surfactants and silicone-based surfactants. Examples of the acetylene glycol-based surfactant preferably include acetylene glycol having 8 to 22 carbon atoms and an ethylene oxide adduct of the acetylene glycol, and more preferably 2,4,7,9-tetramethyl-5-decyne-4,7-diol or an ethylene oxide adduct thereof. Examples of the silicone-based surfactant include polyether-modified silicone, amino-modified silicone, carboxy-modified silicone, fatty acid-modified silicone, alcohol-modified silicone, aliphatic alcohol-modified silicone, epoxy-modified silicone, fluorine-modified silicone, and alkyl-modified silicone. Examples of commercially available nonionic surfactants include the KF series from Shin-Etsu Chemical Co., Ltd., the BYK series from BYK-Chemie GmbH, the Surfynol series from Nisshin Chemical Industry Co., Ltd. and Air Products & Chemicals, Inc., and the acetylenol series from Kawaken Fine Chemicals Co., Ltd.

[0043] The aqueous ink of the present invention is preferably obtained by mixing a water dispersion of pigment-containing resin particles, a water dispersion of a fixing resin, a water-soluble organic solvent, and, if necessary, various additives such as water and a surfactant. There is no particular limitation on the mixing method of each component.

[0044] (Composition and Physical Properties of Aqueous Ink for Inkjet Printing) From the viewpoint of printing density, the content of the pigment in the aqueous ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, still more preferably 3% by mass or more, and preferably 10% by mass or less, more preferably 8% by mass or less, still more preferably 6% by mass or less. The mass ratio of the pigment content to the total content of the pigment and the pigment dispersant in the aqueous ink of the present invention [pigment / (pigment + pigment dispersant)] is preferably 0.4 or more, more preferably 0.5 or more, still more preferably 0.6 or more from the viewpoint of improving the storage stability of the ink, and preferably 0.9 or less, more preferably 0.8 or less, still more preferably 0.7 or less from the same viewpoint as above. When the pigment dispersant is a crosslinked product of a pigment-dispersing resin with a crosslinking agent, the content of the pigment dispersant in the ink of the present invention is the total content of the pigment-dispersing resin before crosslinking and the crosslinking agent. From the viewpoint of improving tape peel resistance and scratch resistance, the content of the fixing resin in the aqueous ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, still more preferably 4% by mass or more, and from the viewpoint of improving the storage stability of the ink, preferably 12% by mass or less, more preferably 8% by mass or less, still more preferably 6% by mass or less. When the form of the pigment in the aqueous ink of the present invention is a form in which the pigment is dispersed with a pigment dispersion resin, the mass ratio of the content of the fixing resin to the total content of the pigment dispersion resin and the fixing resin in the aqueous ink of the present invention [fixing resin / (pigment dispersion resin + fixing resin)] is preferably 0.50 or more, more preferably 0.60 or more, still more preferably 0.70 or more from the viewpoint of improving tape peel resistance and rubbing resistance, and preferably 0.90 or less, more preferably 0.80 or less from the viewpoint of improving the storage stability of the ink.

[0045] The content of the water-soluble organic solvent in the aqueous ink of the present invention is preferably 15% by mass or more, more preferably 20% by mass or more, still more preferably 23% by mass or more from the viewpoints of improving the storage stability of the ink and tape peel resistance and rubbing resistance, and preferably 43% by mass or less, more preferably 40% by mass or less, still more preferably 37% by mass or less from the same viewpoints as above. The content of the alkylene glycol alkyl ether in the aqueous ink of the present invention is preferably 0.5% by mass or more, more preferably 1% by mass or more, still more preferably 3% by mass or more from the viewpoint of improving tape peel resistance and rubbing resistance, and preferably 30% by mass or less, more preferably 20% by mass or less, still more preferably 17% by mass or less from the same viewpoints as above. In the present invention, the content of the alkylene glycol alkyl ether in the water-soluble organic solvent is preferably 5% by mass or more, more preferably 10% by mass or more, still more preferably 15% by mass or more from the viewpoint of improving tape peel resistance and rubbing resistance, and preferably 30% by mass or less, more preferably 25% by mass or less, still more preferably 20% by mass or less from the same viewpoints as above.

[0046] The content of water in the aqueous ink of the present invention is preferably 40% by mass or more, more preferably 50% by mass or more, still more preferably 60% by mass or more from the viewpoint of reducing environmental load, and preferably 80% by mass or less, more preferably 75% by mass or less, still more preferably 70% by mass or less.

[0047] The viscosity of the aqueous ink of the present invention at 32 °C is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, still more preferably 4 mPa·s or more, and is preferably 12 mPa·s or less, more preferably 9 mPa·s or less, still more preferably 7 mPa·s or less. The viscosity of the aqueous ink can be measured using an E-type viscometer. The pH of the aqueous ink of the present invention at 25 °C is preferably 7.0 or more, more preferably 7.2 or more, still more preferably 7.5 or more. Also, from the viewpoints of member resistance and skin irritation, the pH is preferably 11 or less, more preferably 10 or less, still more preferably 9.5 or less. The pH of the aqueous ink can be measured by a conventional method.

[0048] [Inkjet printing method] The inkjet printing method of the present invention is a method of printing on a low-absorbency printing substrate using the aforementioned aqueous ink. In the inkjet printing method of the present invention, from the viewpoint of ejection property, the piezo type is preferable as the method of ejecting the aqueous ink. Examples of the low-absorbency printing substrate used in the inkjet printing method of the present invention include low-absorbency coated paper and resin films. Examples of the coated paper include general-purpose glossy paper and multicolor foam gloss paper. Examples of the resin film include films using synthetic resins. Examples of such synthetic resins include polyolefin resins such as polyvinyl chloride resin, polyethylene resin, and polypropylene resin; polyester resins such as polyethylene terephthalate resin. The resin film may be a biaxially stretched film, a uniaxially stretched film, or an unstretched film. Also, from the viewpoint of improving tape peel resistance and abrasion resistance, films using polyolefin resins or polyester resins are preferably corona discharge treated. Among these, from the viewpoint of improving tape peel resistance and abrasion resistance, a synthetic resin printing substrate is preferable as the low liquid-absorbing printing substrate, and a low liquid-absorbing printing substrate using one or more synthetic resins selected from the group consisting of polyvinyl chloride resin, polypropylene resin, and polyethylene terephthalate resin is more preferable. One or more selected from the group consisting of a printing substrate using polyvinyl chloride resin, a printing substrate corona discharge-treated using polypropylene resin, and a printing substrate corona discharge-treated using polyethylene terephthalate resin is even more preferable, and a printing substrate using polyvinyl chloride resin is even more preferable. When polyvinyl chloride resin is used as the low liquid-absorbing printing substrate, it is considered that the highly polar cyclic ether structure contained in the vinyl resin used as the fixing resin and the polyvinyl chloride resin polarized by chlorine atoms exhibit the interaction of dipole-dipole interaction. Further, in the case of a printing substrate corona discharge-treated using polypropylene resin or polyethylene terephthalate resin as the low liquid-absorbing printing substrate, since hydroxy groups and carboxy groups are present on the surface of the printing substrate by the corona discharge treatment, the highly polar cyclic ether structure contained in the vinyl resin used as the fixing resin and these functional groups exhibit the interaction of hydrogen bonding, and it is presumed that the abrasion resistance can be further improved.

Examples

[0049] In the following production examples, examples, and comparative examples, "parts" and "%" are "parts by mass" and "mass%" unless otherwise specified. The measurement methods or calculation methods for each physical property, etc. are as follows.

[0050] (1) Measurement of weight average molecular weight of vinyl resin It was determined by gel permeation chromatography. The measurement sample was prepared by mixing 0.1 g of the polymer with 10 mL of the following eluent in a glass vial, stirring at 25 °C for 10 hours with a magnetic stirrer, and filtering with a syringe filter ("DISMIC-13HP" manufactured by Advantec Co., Ltd., pore size: 0.2 μm, material: PTFE). The measurement conditions are shown below. GPC device: "HLC-8320GPC" manufactured by Tosoh Corporation Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", "TSKgel guardcolumn Super AW-H" manufactured by Tosoh Corporation Eluent: A solution prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively Flow rate: 0.5 mL / min Standard substances: Monodisperse polystyrene kits with known molecular weights, "PStQuick B (F-550, F-80, F-10, F-1, A-1000)", "PStQuick C (F-288, F-40, F-4, A-5000, A-500)" manufactured by Tosoh Corporation

[0051] (2) Measurement of acid value of vinyl resin The resin was dissolved in a titration solvent (toluene:acetone = 2:1 (volume ratio)) of toluene and acetone in an automatic potentiometric titrator (manufactured by Kyoto Electronics Industry Co., Ltd., motor-driven burette, model number: APB-610), and titrated with a 0.1 N potassium hydroxide / ethanol solution by potentiometric titration method. The inflection point on the titration curve was taken as the end point. The acid value (mgKOH / g) was calculated from the titration volume up to the end point of the potassium hydroxide solution

[0052] (3) Calculation of glass transition temperature (Tg) of vinyl resin The glass transition temperature of the vinyl resin can be calculated from the mass ratio of each monomer constituting the vinyl resin and the glass transition temperature of the homopolymer when each monomer is a homopolymer according to the following Fox equation 1 / Tg=(W 1 / Tg 1 )+(W 2 / Tg 2 )+···+(W m / Tg m ) W 1 +W 2 +···W m =1 In the above Fox equation, Tg is the glass transition temperature of the vinyl resin, and Tg1 、Tg 2 、···、Tg m is the glass transition temperature of the homopolymer formed from each monomer. The unit of temperature is K. Also, W 1 、W 2 、···、W m represents the mass ratio of each monomer in the vinyl resin. As the glass transition temperature of the homopolymer of each monomer in the Fox equation, for example, the values described in Polymer Handbook Third Edition (Wiley - Interscience 1989) can be used.

[0053] (4) Measurement of the solid content concentration of the vinyl resin solution Using an infrared moisture meter ("FD - 230" manufactured by Kett Science Laboratory Co., Ltd.), after drying 1.0 g of the measurement sample at a drying temperature of 150 °C and a measurement mode of 96 (monitoring time 2.5 minutes / variation width 0.05%), the moisture (%) of the measurement sample was measured, and the solid content concentration (%) was calculated by the following formula. Solid content concentration (%) = 100 - moisture (%) of the measurement sample

[0054] (5) Measurement of the solid content concentration of the pigment aqueous dispersion Weighed 10.0 g of sodium sulfate that had been made constant in a desiccator into a 30 mL ointment container, added about 1.0 g of the sample and mixed it, then weighed it accurately, held it at 105 °C for 2 hours to remove the volatile components, left it in the desiccator for an additional 15 minutes, and measured the mass. Taking the mass of the sample after removing the volatile components as the solid content, dividing it by the mass of the added sample to obtain the solid content concentration (%).

[0055] (6) Measurement of the average particle diameter of the pigment - containing resin particles in the pigment aqueous dispersion, the average particle diameter of the resin particles not containing the pigment in the aqueous dispersion, and the average particle diameter of the particles contained in the aqueous ink Cumulant analysis was performed using a laser particle analysis system (ELS-8000 manufactured by Otsuka Electronics Co., Ltd.), and the obtained cumulant average particle size was measured as the average particle size of pigment-containing resin particles in the pigment dispersion or the average particle size of particles contained in the aqueous ink. As the measurement sample, a dispersion diluted with water was used so that the concentration of the particles to be measured was 5×10 -3 % (in terms of solid content concentration). The measurement conditions were a temperature of 25°C, an angle of 90° between the incident light and the detector, and an integration count of 100 times. The refractive index of water (1.333) was input as the refractive index of the dispersion medium.

[0056] (7) Measurement of the viscosity of the aqueous ink Using an E-type viscometer (TV-25 manufactured by Toki Sangyo Co., Ltd., standard cone rotor 1°34’×R24, rotation speed 50 rpm), the viscosity of the aqueous ink at 32°C was measured.

[0057] (8) Measurement of the pH of the aqueous ink Using a desktop pH meter (F-71 manufactured by Horiba, Ltd.) equipped with a pH electrode (6337-10D manufactured by Horiba, Ltd.), the pH of the aqueous ink at 25°C was measured.

[0058] Production Example 1-1 (Production of an aqueous dispersion of vinyl resin A1) (1) Production of vinyl resin A1 As an initial charge into a reaction vessel equipped with a stirrer, a reflux condenser, and a dropping tank, 2.7 parts of tetrahydrofurfuryl acrylate (hereinafter referred to as "THFA"), 0.5 part of acrylic acid, 1.4 parts of isobutyl acrylate, 3.4 parts of methyl ethyl ketone (hereinafter referred to as "MEK"), and 0.4 part of water were charged, and the temperature of the reaction vessel was maintained at 77°C and stirred for 10 minutes. Next, a mixture of 24.6 parts of THFA, 4.2 parts of acrylic acid, 12.2 parts of isobutyl acrylate, 44.4 parts of MEK, 4.9 parts of water, and 1.4 parts of 4,4’-azobis(4-cyanovaleric acid) as a polymerization initiator was continuously added to the reaction vessel over 5 hours. After the addition was completed, the polymerization reaction was continued for 1 hour, and the polymerization reaction was terminated by cooling to room temperature, obtaining a MEK solution (solid content concentration 45%) of vinyl resin A1. The physical properties of vinyl resin A1 are shown in Table 1. (2) Production of aqueous dispersion of vinyl resin A1 To 18.8 parts of the MEK solution of vinyl resin A1 obtained above, 2.4 parts of MEK was added to dilute it so that the solid content concentration became 40%. Next, 2.0 parts of a 5N aqueous sodium hydroxide solution was added so that the neutralization degree with respect to the carboxy group of vinyl resin A1 became 60 mol%, and it was stirred at 25°C. Then, 73.8 parts of water was added over 1 hour. After the addition was completed, MEK was distilled off with an evaporator, obtaining an aqueous dispersion (solid content concentration 25%, average particle diameter of resin particles not containing pigment 92.5 nm) in which vinyl resin A1 was dispersed as resin particles not containing pigment.

[0059] Production Examples 1-2 to 1-22 and Comparative Production Example 1-1 (Production of aqueous dispersions of vinyl resins A2 to A22 and AC1) In Production Example 1-1, except that the raw material monomer composition or neutralizing agent constituting the vinyl resin was changed to the conditions shown in Table 1, and the amount of the neutralizing agent was changed as necessary so that the neutralization degree of the vinyl resin became 60 mol%, in the same manner as in Production Example 1-1, aqueous dispersions in which each of the vinyl resins shown in Table 1 was dispersed as resin particles not containing pigment were obtained. In addition, each notation other than the components used in Production Example 1-1 in Table 1 is as follows. OXE-10: 3-ethyl-3-oxetanyl acrylate MEDOL-10: (2-methyl-2-ethyl-1,3-dioxolan-4-yl)methyl acrylate CTFA: Cyclic trimethylolpropane formal acrylate THFMA: Tetrahydrofurfuryl methacrylate NH 3 : Ammonia DMAE: 2-(Dimethylamino)ethanol

[0060]

Table 1

[0061] Production Example 2-1 (Production of Cyan Pigment Aqueous Dispersion 1) (1) Synthesis of Vinyl Resin 16 parts of methacrylic acid, 44 parts of styrene, 30 parts (15 parts of active ingredient) of styrene macromonomer (“AS-6S” manufactured by Toagosei Co., Ltd., number average molecular weight: 6,000, solid content 50%), 25 parts of methoxypolyethylene glycol methacrylate (“Blemmer PME-200” manufactured by NOF Corporation) were mixed to prepare 115 parts of a monomer mixture solution. 18 parts of MEK, 0.03 part of 2-mercaptoethanol as a chain transfer agent, and 10% (11.5 parts) of the monomer mixture solution were placed in a reaction vessel and mixed, and nitrogen gas substitution was performed. On the other hand, the remaining 90% (103.5 parts) of the monomer mixture solution, 0.27 part of the chain transfer agent, 42 parts of MEK, and 3 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (“V-65” manufactured by Fujifilm Wako Pure Chemical Corporation) as a polymerization initiator were mixed, and the resulting mixture solution was placed in a dropping funnel. While stirring the mixed solution in the reaction vessel under a nitrogen atmosphere, the temperature was raised to 75°C, and the mixed solution in the dropping funnel was dropped over 3 hours. Then, after 2 hours at 75°C, a solution prepared by dissolving 3 parts of the polymerization initiator in 5 parts of MEK was added, and further aged at 75°C for 2 hours and at 80°C for 2 hours. Further, 50 parts of MEK were added to obtain a solution of a vinyl resin (acid value: 104 mgKOH / g, weight average molecular weight: 50,000) (solid content concentration: 45%).

[0062] (2) Production of Cyan Pigment Aqueous Dispersion 1 95.2 parts of the vinyl resin solution obtained in the above (1) and 53.7 parts of MEK were mixed as a pigment dispersant (pigment-dispersing resin), and 13.7 parts of 5N aqueous sodium hydroxide solution, 0.5 part of 25% aqueous ammonia, and 341.8 parts of ion-exchanged water were added thereto as neutralizing agents. Further, 100 parts of a cyan pigment (manufactured by DIC Corporation, "FASTOGEN BLUE CA5380 15:3", C.I. Pigment Blue 15:3) was added to obtain a pigment mixture (neutralization degree of vinyl resin: 72 mol%). The obtained pigment mixture was mixed for 1 hour at 7,000 rpm and 20 °C using a dispersing blade, and then further subjected to a dispersion treatment of 15 passes at a pressure of 180 MPa using a high-pressure homogenizer (manufactured by Microfluidics, "Microfluidizer", model: M-140K). MEK was removed from the obtained pigment dispersion at 60 °C under reduced pressure, and further a part of the water was removed, followed by centrifugation. The liquid layer portion was recovered and filtered through a membrane filter (manufactured by Sartorius, "Mini Sartorius Syringe Filter", pore size: 5 μm, material: cellulose acetate) to remove coarse particles, thereby obtaining a pigment aqueous dispersion (solid content concentration: 22%). To 100 parts of the obtained pigment aqueous dispersion, 0.45 part of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Corporation, "Denacol EX-321L", epoxy equivalent: 130) (corresponding to a crosslinking ratio of 30 mol%) and 15.2 parts of ion-exchanged water were added, and heat treatment was performed at 70 °C for 3 hours while stirring (solid content concentration: 22%). After cooling to room temperature, the liquid layer portion was recovered and filtered through a membrane filter (manufactured by Sartorius, "Mini Sartorius Syringe Filter", pore size: 5 μm, material: cellulose acetate) to remove coarse particles, thereby obtaining cyan pigment aqueous dispersion 1 (solid content concentration: 22%, pigment content: 15.2%, content of crosslinked pigment-dispersing resin: 6.8%, acid value of crosslinked pigment-dispersing resin: 73 mgKOH / g, average particle diameter of cyan pigment-containing resin particles: 105.2 nm).

[0063] Production Example 2-2 (Production of Black Pigment Aqueous Dispersion 2) In Production Example 2-1, except that the cyan pigment was changed to a black pigment (Cabot's "MONARCH 717", carbon black), in the same manner as in Production Example 2-1, a black pigment aqueous dispersion 2 (solid content concentration: 22%, pigment content: 15.2%, content of crosslinked pigment dispersion resin: 6.8%, acid value of crosslinked pigment dispersion resin: 73 mgKOH / g, average particle diameter of black pigment-containing resin particles: 101.0 nm) was obtained.

[0064] Example 1 (Preparation of Aqueous Ink I-1) To obtain the ink composition shown in Table 2 (total 100 parts), 6 parts of cyan pigment aqueous dispersion 1 (solid content concentration: 22%, pigment content: 15.2%, content of crosslinked pigment dispersion resin: 6.8%) as a solid content, 5 parts of an aqueous dispersion of vinyl resin A1 obtained in Production Example 1 (solid content concentration: 25%) as a fixing resin, 22 parts of propylene glycol, 5 parts of propylene glycol monopropyl ether, 1 part of an acetylene glycol-based surfactant ("Surfinol 104-PG50" manufactured by Nissin Chemical Industry Co., Ltd. (propylene glycol solution of 2,4,7,9-tetramethyl-5-decyne-4,7-diol, active ingredient 50%)) (hereinafter referred to as "Surfinol 104-PG50") in its original form, and 61 parts of ion-exchanged water were added and stirred, and then filtered through a membrane filter ("Mini Sartorius Syringe Filter" manufactured by Sartorius, pore size: 5 μm, material: cellulose acetate) to obtain aqueous ink I-1 (viscosity: 4.2 mPa·s, pH: 8.3). (Inkjet Printing) In an environment at a temperature of 32°C, an inkjet printing evaluation apparatus (manufactured by Trytec Co., Ltd.) equipped with a printing head ("KJ4B-HD06MHG-STDV" manufactured by Kyocera Corporation, piezo type) was filled with aqueous ink I-1. The printing head voltage was set to 26 V, the driving frequency was set to 30 kHz, the ejected droplet volume was set to 7 pL, the printing head temperature was set to 32°C, and the printing head resolution was set to 600 dpi. A solid image with a Duty of 100% was formed on a polyvinyl chloride film ("Scotch Graphic Film IJ1220" manufactured by 3M Japan Limited) (hereinafter also referred to as "PVC"), which was a low-absorbency printing substrate heated to 50°C, to obtain a printed matter.

[0065] Examples 2 to 25 and Comparative Example 1 In Example 1, except that the ink composition shown in Table 2 was changed, aqueous inks I-2 to I-25 and I-C1 were obtained in the same manner as in Example 1. Subsequently, in Example 1, except that each aqueous ink shown in Table 2 was used instead of aqueous ink I-1, a solid image with a Duty of 100% was formed on a polyvinyl chloride film "Scotch Graphic Film IJ1220" of a low-absorbency printing substrate by an inkjet recording method in the same manner, and a printed matter was obtained.

[0066] Example 26 In Example 1, except that the low-absorbency printing substrate was changed to a polyethylene terephthalate film (manufactured by Futamura Chemical Co., Ltd. "FE2001#25") having one-sided corona treatment (hereinafter also referred to as "PET"), a solid image with a Duty of 100% was formed on the corona-treated surface of the printing substrate in the same manner, and a printed matter was obtained.

[0067] Example 27 In Example 1, except that the low-absorbency printing substrate was changed to a biaxially stretched polypropylene film (manufactured by Futamura Chemical Co., Ltd. "FOR-AQ#25") having one-sided corona treatment (hereinafter also referred to as "OPP"), a solid image with a Duty of 100% was formed on the corona-treated surface of the printing substrate in the same manner, and a printed matter was obtained.

[0068] [Evaluation] Each aqueous ink and each printed matter obtained in the examples and comparative examples were evaluated by the following (1) to (3). The results are shown in Table 2.

[0069] (1) Evaluation of storage stability of aqueous ink The aqueous ink was stored in a sealed container at a constant temperature of 60°C, taken out after 28 days, and the average particle size after storage was measured. The average particle size change rate after storage at 60°C for 28 days was calculated by the following formula (rounding off the decimal part), and the storage stability was evaluated according to the following evaluation criteria. Average particle size change rate (%) = [(Average particle size after storage / Average particle size before storage) - 1] × 100 If the absolute value of the average particle size change rate is less than 15%, there is no practical problem. (Evaluation criteria) A: The absolute value of the average particle size change rate is less than 5%. B: The absolute value of the average particle size change rate is 5% or more and less than 10%. C: The absolute value of the average particle size change rate is 10% or more and less than 15%. D: The absolute value of the average particle size change rate is 15% or more and less than 20%. E: The absolute value of the average particle size change rate is 20% or more or measurement is impossible due to aggregation.

[0070] (2) Evaluation of tape peel resistance A tape (Nitto Denko Corporation's "Cellotape (registered trademark)", 18 mm width, model number: CT-18S) was attached to the solid image portion of the obtained printed matter, and the edge of the tape was quickly peeled off at a 90° angle. For the solid image portion after peeling, binarization processing was performed using the printing density value that is half the printing density value of the solid image portion before tape peeling as the threshold value, and the peeled area and non-peeled area were calculated by image analysis. The area ratio of the non-peeled area was calculated as the coating film residual rate (%) after the tape peel test. The larger the value of the coating film residual rate (%), the better the tape peel resistance. If it is 70% or more, there is no practical problem. (Evaluation criteria) A: The coating film residual rate after the tape peel test is 90% or more. B: The coating film residual rate after the tape peel test is 80% or more and less than 90%. C: The coating film residual rate after the tape peel test is 70% or more and less than 80%. D: The coating film residual rate after the tape peel test is 60% or more and less than 70%. E: The coating film residual rate after the tape peel test is less than 60%.

[0071] (3) Evaluation of abrasion resistance The solid image portion of the obtained printed matter was rubbed against waterproof abrasive paper (Nippon Kenji Co., Ltd.'s "WTCC-S", particle size: 320) at 200 g / cm 2A load was applied and the sample was rubbed back and forth 10 times. The remaining film area of the solid image part after rubbing was calculated by image analysis. For the solid image part after rubbing, a binarization process was performed using, as a threshold value, a printing density value that is half the printing density value of the solid image part before the rubbing test, to calculate the remaining film area and the non-remaining area by image analysis. The area ratio of the remaining film area was calculated as the film remaining rate (%) after the rubbing test. The higher the value of the film remaining rate (%), the better the rubbing resistance. If it is 70% or more, there is no practical problem. (Evaluation Criteria) A: The film remaining rate after the rubbing test is 90% or more. B: The film remaining rate after the rubbing test is 80% or more and less than 90%. C: The film remaining rate after the rubbing test is 70% or more and less than 80%. D: The film remaining rate after the rubbing test is 60% or more and less than 70%. E: The film remaining rate after the rubbing test is less than 60%.

[0072] [Table 2]

[0073] From Table 2, it can be seen that the aqueous ink of the examples of the present invention is superior in storage stability and in the tape peeling resistance and rubbing resistance of the obtained printed matter, compared to the aqueous ink of the comparative examples. In Comparative Example 1, since it contains a structural unit derived from cyclic ether alkyl methacrylate (THFMA) having a cyclic ether structure in the molecule instead of cyclic ether alkyl acrylate (THFA) having a cyclic ether structure in the molecule as the fixing resin, it is inferior in storage stability. Also, since the glass transition temperature of the fixing resin is high and the internal stress relaxation during tape peeling is not sufficient, it can be seen that the tape peeling resistance is inferior. [Industrial Applicability]

[0074] According to the present invention, there can be provided an inkjet printing aqueous ink and an inkjet printing method that are excellent in storage stability and, when used for printing on a low liquid-absorbing printing substrate such as a synthetic resin, are excellent in the tape peeling resistance and rubbing resistance of the obtained printed matter.

Claims

1. A water-based ink for inkjet printing on low-liquid-absorbent printing substrates, The water-based ink contains a pigment, a pigment dispersant, a fixing resin, a water-soluble organic solvent, and water. The fixing resin is a vinyl resin comprising a structural unit derived from a cyclic ether alkyl acrylate (a-1) having a cyclic ether structure in its molecule, and a structural unit derived from one or more carboxyl group-containing monomers (a-2) selected from the group consisting of acrylic acid and methacrylic acid, in an inkjet water-based ink.

2. The aqueous inkjet ink according to claim 1, wherein the cyclic ether alkyl acrylate (a-1) is one or more selected from the group consisting of tetrahydrofurfuryl acrylate, (3-ethyl-3-oxetanyl)methyl acrylate, (2-methyl-2-ethyl-1,3-dioxolan-4-yl)methyl acrylate, and cyclic trimethylolpropaneformal acrylate.

3. The aqueous inkjet ink according to claim 1, wherein the content of the constituent units derived from the cyclic ether alkyl acrylate (a-1) in all constituent units of the vinyl resin is 25% by mass or more and 96% by mass or less.

4. The aqueous inkjet ink according to claim 1, wherein the acid value of the vinyl resin is 15 mg KOH / g or more and 200 mg KOH / g or less.

5. The water-based ink for inkjet printing according to claim 1, wherein the glass transition temperature Tg of the vinyl resin is -30°C or higher and 45°C or lower.

6. The aqueous inkjet ink according to claim 1, wherein at least a portion of the carboxyl groups of the vinyl resin is neutralized with one or more selected from the group consisting of alkali metal hydroxides and organic amines.

7. The water-based inkjet ink according to claim 1, wherein the form of the pigment is a form in which the pigment is dispersed in a pigment dispersion resin as the pigment dispersant.

8. The water-based inkjet ink according to claim 7, wherein the mass ratio of the content of the fixing resin to the total content of the pigment dispersion resin and the fixing resin [fixing resin / (pigment dispersion resin + fixing resin)] is 0.50 or more and 0.90 or less.

9. The aqueous ink for inkjet printing according to claim 1, wherein the water-soluble organic solvent comprises an alkylene glycol alkyl ether.

10. The water-based inkjet ink according to claim 1, wherein the low liquid-absorbing printing substrate is a synthetic resin printing substrate used in printing.

11. The water-based inkjet ink according to claim 10, wherein the synthetic resin is one or more selected from the group consisting of polyvinyl chloride resin, polypropylene resin, and polyethylene terephthalate resin.

12. An inkjet printing method comprising printing on a low liquid-absorbent printing substrate using an aqueous ink for inkjet printing described in any one of claims 1 to 11.