Water-based ink for inkjet printing

Abstract

To provide a water-based ink for inkjet printing having excellent intermittent discharge 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 vinyl-based resin particles containing a pigment, a water-soluble organic solvent and water, a vinyl-based resin A constituting the vinyl-based resin particles containing a pigment contains 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 the content of a structural unit derived from the cyclic ether alkyl acrylate (a-1) in the total structural unit of the vinyl-based resin A is 30 mass% or more and 90 mass% or less; 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 ejecting 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. Particularly from the viewpoints of the weather resistance and water resistance of printed matter, inks using pigments as colorants have become the mainstream.

[0003] For example, Patent Document 1 discloses an environmentally friendly aqueous inkjet ink capable of recording an image in which pigments are stably and highly finely dispersed and which is excellent in durability, glossiness, color development property, and adhesion to various printing substrates. For the purpose of providing an aqueous pigment dispersion liquid, a structural unit (i) derived from at least one of (meth)acrylic acid and itaconic acid, and a structural unit (ii) derived from a (meth)acrylate derived from a biological material are included, the acid value is 30 to 250 mgKOH / g, the content of the structural unit (ii) is 50% by mass or more, the number average molecular weight is 1,000 to 30,000, the molecular weight distribution (weight average molecular weight / number average molecular weight) is 2.5 or less, and at least a part of the carboxy groups are neutralized with an alkali. An aqueous pigment dispersion liquid which is a polymer, an aqueous inkjet ink containing the aqueous pigment dispersion liquid, and the like are 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 for indoor and outdoor posting. On the other hand, as a specific problem of using synthetic resin as a printing substrate, improvement in the adhesion of the ink coating film to the substrate of 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 an advertising billboard or the like is posted, 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 adhesion of the ink coating film to the substrate of the printed material is required. In particular, when printing on synthetic resin using aqueous ink, it is necessary for the ink coating film formed by the aqueous ink to adhere sufficiently to the synthetic resin. In addition, different from the printing substrate of paper, synthetic resin has 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 in the ink. However, in the case of an ink containing such a hydrophobic organic solvent, the dispersion stability of the pigment decreases during ink concentration in the inkjet nozzle, and after a predetermined time has elapsed without discharging the ink from the inkjet nozzle, the dischargeability when discharging the ink again, so-called intermittent discharge stability, may decrease. Therefore, in order to improve the productivity of printed materials, improvement in intermittent discharge stability is required. However, when the aqueous inkjet ink of Patent Document 1 is used for inkjet printing on synthetic resin as a low liquid-absorbing printing substrate, it has been found that the tape peel resistance of the ink coating film of the obtained printed material and the intermittent discharge stability of the ink are insufficient. An object of the present invention is to provide an aqueous ink for inkjet printing and an inkjet printing method using the aqueous ink, which are excellent in intermittent discharge stability and, when used for printing on a low liquid-absorbing printing substrate, are excellent in the adhesion of the ink coating film of the obtained printed material (tape peel resistance and abrasion resistance). [Means for Solving the Problems]

[0006] The present inventors have found that an aqueous ink for inkjet printing containing vinyl resin particles containing a pigment, a water-soluble organic solvent, and water, wherein the vinyl resin constituting the vinyl resin particles containing the pigment has a cyclic ether structure in the molecule. A structural unit derived from a cyclic ether alkyl acrylate having the same and a structural unit derived from one or more carboxy group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid, and the content of the structural unit derived from the cyclic ether alkyl acrylate in all the structural units of the vinyl resin is within a predetermined range, whereby the above problems can be solved. That is, the present invention provides the following [1] and [2]. [1] An aqueous ink for inkjet printing on a low-absorbency printing substrate, the aqueous ink contains vinyl resin particles containing a pigment, a water-soluble organic solvent, and water, vinyl resin A constituting the vinyl resin particles containing the pigment contains 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 the content of the structural unit derived from the cyclic ether alkyl acrylate (a-1) in all the structural units of the vinyl resin A is 30% by mass or more and 90% by mass or less, an aqueous ink for inkjet printing. [2] An inkjet printing method of printing on a low-absorbency printing substrate using the aqueous ink according to [1]. [Effects of the Invention]

[0007] According to the present invention, there are provided an aqueous ink for inkjet printing having excellent intermittent ejection stability and excellent adhesion of an ink coating film of a printed matter to a substrate (tape peeling resistance and rubbing resistance) when used for printing on a low-absorbency printing substrate, and an inkjet printing method using the aqueous ink.

Mode for Carrying Out the Invention

[0008] [Aqueous Ink for Inkjet Printing] The aqueous ink for inkjet printing of the present invention (hereinafter, also simply referred to as "aqueous ink") is an aqueous ink for inkjet printing on a low liquid-absorbing printing substrate, and the aqueous ink contains vinyl resin particles containing a pigment (hereinafter, also referred to as "pigment-containing vinyl resin particles"), a water-soluble organic solvent, and water. The vinyl resin A constituting the vinyl resin particles containing the pigment has 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. The content of the structural unit derived from the cyclic ether alkyl acrylate (a-1) in all the structural units of the vinyl resin A is 30% by mass or more and 90% by mass or less. 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 this specification, the "low liquid-absorbing property" of the low liquid-absorbing printing substrate is a concept including low liquid-absorbing property and non-liquid-absorbing property. 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 or more and 10 g / m 2 or less. 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 absorber (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.

[0009] According to the present invention, it is excellent in intermittent discharge stability and, when used for printing on a low liquid-absorbing printing substrate, it is excellent in the substrate adhesion (tape peeling resistance and rubbing resistance) of the ink coating film of the obtained printed matter. Although the reason is not clear, it is considered as follows. The aqueous ink of the present invention is obtained by dispersing pigment-containing vinyl resin particles in an aqueous medium. The vinyl resin constituting the pigment-containing vinyl resin particles contains 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 the 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 substrate adhesion of the ink coating film is improved. Furthermore, depending on the type of synthetic resin constituting the printing substrate, dipole-dipole interaction and hydrogen bond interaction are expressed 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 expressed 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 substrate adhesion of the ink coating film with respect 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 the 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 contained in the vinyl resin can strongly express the interaction with the synthetic resin, and strong interaction can be efficiently expressed. Also, since it has an effect of relaxing the stress when the ink coating film peels off from the printing substrate, it is considered that the substrate adhesion of the ink coating film with respect to the synthetic resin used as the low liquid-absorbing printing substrate can be further improved. Furthermore, the vinyl resin, 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, in which the electrostatic repulsive force can be efficiently exhibited, further contains a structural unit derived from a cyclic ether alkyl acrylate. Different from the case of containing a structural unit derived from a cyclic ether alkyl methacrylate, it is unexpectedly a stable dispersion system, and it is considered to have an effect of improving the dispersion stability of the pigment even in an aqueous ink containing a water-soluble organic solvent. As a result, even when the ink is exposed to the air in the vicinity of the inkjet nozzle for a long time and the moisture in the ink volatilizes while a predetermined time elapses without discharging the ink from the inkjet nozzle, and the content ratio of the water-soluble organic solvent in the ink increases, aggregation of the pigment can be suppressed, and it is considered that the intermittent discharge property can be improved. Hereinafter, the intermittent discharge stability of the aqueous ink will be simply referred to as "intermittent discharge 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 will be simply referred to as "tape peel resistance" and "rubbing resistance", respectively.

[0010] <Vinyl resin particles containing a pigment> The vinyl resin particles containing a pigment (pigment-containing vinyl resin particles) according to the present invention are formed of at least a pigment and a vinyl resin A and are dispersed in an aqueous medium. Here, the form of the pigment-containing vinyl resin particles is not particularly limited, as long as the particles are formed of at least a pigment and a vinyl resin A, and the particles are those in which the vinyl resin A is adsorbed to the pigment in the aqueous ink. Examples of the form of the pigment-containing vinyl resin particles include a particle form in which the pigment is encapsulated in the vinyl resin A, a particle form in which the pigment is uniformly dispersed in the vinyl resin A, a particle form in which the pigment is exposed on the particle surface of the vinyl resin A, etc., and mixtures thereof are also included.

[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 inorganic pigments include, for example, metal oxides such as carbon black, titanium oxide, iron oxide, red iron oxide, chromium oxide, and nacreous pigments. Particularly in black ink, carbon black is preferred. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Specific examples of organic pigments 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 achromatic inks, achromatic pigments such as white, black, and gray can be used, and in chromatic inks, 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] (Vinyl resin A) The vinyl resin A (hereinafter also referred to as "vinyl resin A") constituting the pigment-containing vinyl resin particles according to the present invention has a function as a pigment dispersant that exhibits a pigment dispersion effect and a function as a fixing agent to a printing substrate. The vinyl resin may be either a water-soluble resin or a water-insoluble resin. Here, regarding the "water solubility" and "water insolubility" of the resin, when the resin dried at 105°C for 2 hours until a constant weight is dissolved in 100 g of water at 25°C until saturation is reached, if the dissolved amount exceeds 10 g, it is judged as "water-soluble", and if it is 10 g or less, it is judged as "water-insoluble". Further, when the carboxy group of the vinyl-based resin A is neutralized with a neutralizing agent as described later, it is judged based on the dissolved amount measured under the condition that the neutralizing agent is present under the condition that the mass ratio of the vinyl-based resin A to the neutralizing agent is the same as that in the aqueous ink of the present invention. The vinyl-based resin A may be used alone or in combination of two or more.

[0013] The vinyl-based resin A contains 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. The vinyl-based resin A may be any of a random copolymer, a block copolymer, an alternating copolymer, and a graft copolymer.

[0014] From the viewpoint of improving the intermittent ejection stability, tape peel resistance, and rubbing resistance, the 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, more preferably 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 trimethylolpropane formal acrylate, still more preferably one or more selected from the group consisting of tetrahydrofurfuryl acrylate, (3-ethyl-3-oxetanyl)methyl acrylate, and cyclic trimethylolpropane formal acrylate, and even more preferably tetrahydrofurfuryl acrylate. From the viewpoint of improving the intermittent ejection stability and tape peel resistance, acrylic acid is preferred as the carboxy group-containing monomer (a-2).

[0015] From the viewpoints of improving intermittent discharge stability, tape peeling resistance, and abrasion resistance, vinyl resin A preferably contains, in addition to the structural units derived from cyclic ether alkyl acrylate (a-1) and the structural units derived from carboxy group-containing monomer (a-2), further structural units derived from hydrophobic monomer (a-3). Here, the "hydrophobicity" of the hydrophobic monomer means that, as described above, 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.

[0016] 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. 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.

[0017] As the hydrophobic monomer (a-3), from the viewpoints of improving intermittent discharge stability, tape peeling resistance, and rubbing 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 viewpoints of improving intermittent discharge stability, tape peeling resistance, and rubbing resistance, alkyl acrylates having an alkyl group with 1 to 10 carbon atoms, more preferably 2 to 8 carbon atoms, are preferably 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, as the hydrophobic monomer (a-3), from the viewpoints of improving intermittent discharge stability, tape peeling resistance, and rubbing resistance, more preferably it is one or more selected from the group consisting of methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, and benzyl acrylate, still more preferably it is one or more selected from the group consisting of butyl acrylate, isobutyl acrylate, and benzyl acrylate, and even more preferably it is 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.

[0018] Vinyl resin A 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 nonionic monomers. Here, the nonionic monomer is a monomer having a high affinity for water or a water-soluble organic solvent, for example, a monomer containing a hydroxy group or a polyalkylene glycol chain. Examples of nonionic monomers include polyalkylene glycol mono(meth)acrylates such as polyethylene glycol mono(meth)acrylate; alkoxypolyalkylene glycol mono(meth)acrylates such as methoxypolyethylene glycol mono(meth)acrylate and octoxypolyethylene glycol mono(meth)acrylate.

[0019] From the viewpoint of improving tape peel resistance and scratch resistance, the content of the structural unit derived from cyclic ether alkyl acrylate (a-1) in all the structural units of vinyl resin A is 30% by mass or more, preferably 40% by mass or more, more preferably 50% by mass or more, still more preferably 55% by mass or more. From the viewpoint of improving intermittent discharge stability, it is 90% by mass or less, preferably 85% by mass or less, more preferably 75% by mass or less, still more preferably 70% by mass or less, and even more preferably 65% by mass or less. From the viewpoint of improving intermittent discharge stability, the content of the structural unit derived from the carboxy group-containing monomer (a-2) in all the structural units of vinyl resin A is preferably 5% by mass or more, more preferably 10% by mass or more, still more preferably 13% by mass or more, and even more preferably 18% by mass or more. From the viewpoint of improving tape peel resistance and scratch resistance, it is preferably 40% by mass or less, more preferably 35% by mass or less, and still more preferably 30% by mass or less. The content of the structural unit derived from the hydrophobic monomer (a-3) in all the structural units of the vinyl resin A is preferably 3% by mass or more, more preferably 5% by mass or more, still more preferably 10% by mass or more, from the viewpoints of improving intermittent discharge stability, tape peel resistance, and abrasion resistance, and, from the same viewpoints as above, preferably 50% by mass or less, more preferably 40% by mass or less, still more preferably 30% by mass or less, and even more preferably 20% by mass or less. The vinyl resin A 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 needed, by a known method.

[0020] The vinyl resin A may have a structure crosslinked with a crosslinking agent. In this case, the vinyl resin A 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 the crosslinking agent. Such a crosslinked structure is considered to be a three-dimensional structure formed by a polymer having a linear two-dimensional structure which may have a branched chain and a constituent component derived from the crosslinking agent. The crosslinking agent is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule, more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group with 3 to 8 carbon atoms, still more preferably one or more selected from the group consisting of 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 even more preferably trimethylolpropane polyglycidyl ether. When the crosslinking agent is a polyfunctional epoxy compound, the epoxy equivalent weight 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.

[0021] From the viewpoint of improving the intermittent discharge stability, the acid value of the vinyl resin A is preferably 60 mgKOH / g or more, more preferably 100 mgKOH / g or more, still more preferably 130 mgKOH / g or more, and from the viewpoints of improving the tape peel resistance and abrasion resistance, it is preferably 260 mgKOH / g or less, more preferably 240 mgKOH / g or less, still more preferably 220 mgKOH / g or less. The acid value of the vinyl resin A can be determined by the method described in the examples, but can also be calculated from the mass ratio of the constituent monomers. Further, when the vinyl resin A has a crosslinked structure, the acid value of the vinyl resin A can also be calculated by the following formula. Acid value of vinyl resin A having a crosslinked structure (mgKOH / g) = [Acid value of vinyl resin A before crosslinking (mgKOH / g) × [(100 - crosslinking rate (mol%)) / 100]] In this specification, the crosslinking rate (mol%) of the vinyl resin A having a crosslinked structure is a value calculated from the acid value of the vinyl resin A before crosslinking and the equivalent weight of the crosslinkable functional group of the crosslinking agent.

[0022] From the viewpoint of improving the tape peel resistance, the glass transition temperature Tg of the vinyl resin A is preferably 40°C or less, more preferably 30°C or less, still more preferably 20°C or less, even more preferably 15°C or less, and from the viewpoint of improving the intermittent discharge stability, it is preferably -20°C or more, more preferably -10°C or more, still more preferably -6°C or more, even more preferably -2°C or more. The glass transition temperature of the vinyl resin is calculated by the method described in the examples.

[0023] At least a part of the carboxy groups of the vinyl resin A is preferably neutralized with a neutralizing agent from the viewpoint of improving intermittent discharge stability. 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 alkanolamine, alkylamine, aminoalkanediol, alkoxyamine, and heterocyclic amine. Among these, at least one selected from the group consisting of alkanolamine and alkylamine is preferable, and alkanolamine is more preferable. As the alkanolamine, alkanolamines having 2 to 8 carbon atoms miscible with water such as monoethanolamine, monoisopropanolamine, monoisobutanolamine, N-methylethanolamine, N,N-dimethylethanolamine, and N-methyldiethanolamine are preferably mentioned. From the viewpoint of improving intermittent discharge stability, the degree of neutralization of the carboxy groups of the vinyl resin A is preferably 15 mol% or more, more preferably 20 mol% or more, still more preferably 25 mol% or more, and from the same viewpoint as above, preferably 60 mol% or less, more preferably 50 mol% or less, still more preferably 40 mol% or less. Here, the degree of neutralization can be determined by the following formula as the equivalent amount of the neutralizing agent used for the carboxy groups of the vinyl resin A. 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 A, 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 A (mgKOH / g) × mass of vinyl resin A (g)) / (56.1×1000))]]×100

[0024] The vinyl resin particles containing a pigment according to the present invention are preferably prepared by dispersing a pigment, a vinyl resin A, a neutralizing agent, a surfactant, etc., if necessary, by a known method to form an aqueous dispersion (hereinafter also referred to as "pigment aqueous dispersion") and then blending it into an aqueous ink. As a method for producing the pigment aqueous dispersion, a method is preferably employed in which a pigment mixture containing a pigment, a vinyl resin A, 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 crosslinking agent may be added to the obtained pigment aqueous dispersion to crosslink the pigment-dispersed resin. Specific examples of such a method include the methods described in paragraphs

[0022] to

[0026] of JP-A-2022-104084. The vinyl resin A may be previously blended as an aqueous dispersion into the pigment mixture. The average particle diameter of the vinyl resin particles containing a pigment in the pigment aqueous dispersion is preferably 50 nm or more, more preferably 70 nm or more, still more preferably 90 nm or more, from the viewpoints of improving the dispersion stability of the pigment and the intermittent ejection stability, and improving the tape peeling resistance and the rubbing resistance. From the same viewpoints as above, it is preferably 600 nm or less, more preferably 500 nm or less, still more preferably 300 nm or less, and even more preferably 200 nm or less. The average particle diameter of the vinyl resin particles containing a pigment in the pigment aqueous dispersion can be measured by the method described in the examples.

[0025] <Water-soluble organic solvent> The aqueous ink of the present invention contains a water-soluble organic solvent from the viewpoints of improving the intermittent ejection stability, the tape peeling resistance, and the rubbing resistance. In the present invention, the "water-soluble organic solvent" refers to an organic solvent in which, as described above, when 100 mL of water at 25°C is dissolved with the organic solvent, the dissolved amount thereof is 5 mL or more. 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 using two or more water-soluble organic solvents, the boiling point value shall be the weighted average value weighted by the content (% by mass) of each water-soluble organic solvent.

[0026] (Alkylene glycol alkyl ether) From the viewpoints of improving intermittent discharge stability, tape peel resistance, and rubbing resistance, the water-soluble organic solvent preferably contains an alkylene glycol alkyl ether. From the viewpoints of improving intermittent discharge stability, tape peel resistance, and rubbing resistance, the number of carbon atoms of the alkyl group of 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 viewpoints as above, it is 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 viewpoints of improving intermittent discharge stability, tape peel resistance, and rubbing resistance, the alkylene glycol alkyl ether is preferably one or more selected from the group consisting of alkylene glycol monoalkyl ether and alkylene glycol dialkyl ether.

[0027] Examples of alkylene glycol monoalkyl ethers 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 alkylene glycol dialkyl ethers 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.

[0028] Among these, alkylene glycol alkyl ethers are preferably alkylene glycol monoalkyl ethers from the viewpoints of improving intermittent discharge stability, tape peel resistance, and scratch resistance. More preferably, they are one or more selected from the group consisting of diethylene glycol monoalkyl ethers, propylene glycol monoalkyl ethers, and dipropylene glycol monoalkyl ethers. Even more preferably, they are 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. Even more preferably, they are 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.

[0029] (Water-soluble organic solvents other than alkylene glycol alkyl ethers) The water-soluble organic solvent preferably further contains a water-soluble organic solvent other than alkylene glycol alkyl ethers. The other water-soluble organic solvent can be appropriately selected according to the purpose. Examples of the other water-soluble organic solvent include polyhydric alcohols, nitrogen-containing heterocyclic compounds, amides, amines, and sulfur-containing compounds. Among these, the water-soluble organic solvent preferably further contains a polyhydric alcohol from the viewpoints of improving tape peel resistance, scratch resistance, and blistering resistance.

[0030] 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.

[0031] The aqueous ink of the present invention may further contain various additives such as a fixing resin, 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.

[0032] (Fixing resin) The aqueous ink of the present invention may contain, as a fixing resin, a vinyl resin B containing a structural unit derived from a cyclic ether alkyl acrylate (b-1) having a cyclic ether structure in the molecule (hereinafter also referred to as "cyclic ether alkyl acrylate (b-1)") and a structural unit derived from at least one carboxy group-containing monomer (b-2) selected from the group consisting of acrylic acid and methacrylic acid (hereinafter also referred to as "carboxy group-containing monomer (b-2)"). From the viewpoint of improving tape peelability resistance, the vinyl resin B preferably contains, in addition to the structural unit derived from the cyclic ether alkyl acrylate (b-1) and the structural unit derived from the carboxy group-containing monomer (b-2), a structural unit derived from a hydrophobic monomer (b-3). Here, the "hydrophobicity" of the hydrophobic monomer means that, as described above, 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. The cyclic ether alkyl acrylate (b-1), the carboxy group-containing monomer (b-2), and the hydrophobic monomer (b-3) are each preferably the same as those exemplified for the cyclic ether alkyl acrylate (a-1), the carboxy group-containing monomer (a-2), and the hydrophobic monomer (a-3) described above. In addition, the vinyl resin B can contain structural units derived from monomers other than the monomers (b-1) to (b-3) as long as the effects of the present invention are not inhibited. Examples of other monomers include the nonionic monomers exemplified for the vinyl resin A described above. The fixing resin may be used alone or in combination of two or more.

[0033] From the viewpoint of improving tape peel resistance and scratch resistance, the content of the structural unit derived from the cyclic ether alkyl acrylate (b-1) in all the structural units of the vinyl resin B 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 intermittent discharge stability, 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 intermittent discharge stability, the content of the structural unit derived from the carboxy group-containing monomer (b-2) in all the structural units of the vinyl resin B 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 peel resistance and scratch 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 peel resistance, the content of the structural unit derived from the hydrophobic monomer (b-3) in all the structural units of the vinyl resin B 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 B can be obtained, for example, by addition polymerization of raw material monomers including a cyclic ether alkyl acrylate (b-1), a carboxy group-containing monomer (b-2), a hydrophobic monomer (b-3), or a nonionic monomer as required, by a known method.

[0034] From the perspective of improving intermittent discharge stability, the acid value of vinyl resin B 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 vinyl resin B can be determined by the method described in the examples, but it can also be calculated from the mass ratio of the constituent monomers.

[0035] From the perspective of improving tape peel resistance, the glass transition temperature Tg of vinyl resin B is preferably 45°C or lower, more preferably 40°C or lower, still more preferably 35°C or lower, even more preferably 30°C or lower, and even more preferably 25°C or lower. From the perspective of improving intermittent discharge stability, it is preferably -30°C or higher, more preferably -20°C or higher, and still more preferably -15°C or higher. The glass transition temperature of vinyl resin B is calculated by the method described in the examples.

[0036] From the perspectives of improving intermittent discharge stability, tape peel resistance, and abrasion resistance, the weight average molecular weight of vinyl resin B is preferably 50,000 or more, more preferably 100,000 or more, and still more preferably 150,000 or more. From the same perspectives as above, 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.

[0037] At least a part of the carboxy groups of the vinyl resin B is preferably neutralized with a neutralizing agent from the viewpoint of improving intermittent discharge stability. 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 alkanolamine, alkylamine, aminoalkanediol, alkoxyamine, and heterocyclic amine. Among these, at least one selected from the group consisting of alkanolamine and alkylamine is preferable, and alkanolamine is 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 intermittent discharge stability, 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 for 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

[0038] 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 appropriately synthesized or a commercially available product may be used. 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. 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, even more preferably 70 nm or more, and preferably 300 nm or less, more preferably 200 nm or less, still more preferably 150 nm or less, even more preferably 130 nm or less, from the viewpoint of intermittent ejection stability. 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 vinyl resin particles in the pigment aqueous dispersion described in the examples.

[0039] (Surfactant) 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. As the acetylene glycol-based surfactant, acetylene glycol having 8 to 22 carbon atoms and an ethylene oxide adduct of the acetylene glycol are preferably used, and 2,4,7,9-tetramethyl-5-decyne-4,7-diol or an ethylene oxide adduct thereof is more preferably used. 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, alkyl-modified silicone, and the like. Examples of commercially available nonionic surfactants include the KF series of Shin-Etsu Chemical Co., Ltd., the BYK series of BYK-Chemie GmbH, the Surfynol series of Nisshin Chemical Industry Co., Ltd. and Air Products & Chemicals, Inc., and the Acetylenol series of Kawaken Fine Chemicals Co., Ltd.

[0040] The aqueous ink of the present invention is preferably obtained by mixing a water dispersion of pigment-containing vinyl resin particles, a water-soluble organic solvent, and, if necessary, water, a water dispersion of a fixing resin, and various additives such as a surfactant. There is no particular limitation on the mixing method of each component.

[0041] (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 3% by mass or more, still more preferably 5% by mass or more, and preferably 15% by mass or less, more preferably 10% by mass or less, still more preferably 7% by mass or less. The mass ratio of the pigment content to the total content of the pigment and vinyl resin A in the aqueous ink of the present invention [pigment / (pigment + vinyl resin A)] is preferably 0.3 or more, more preferably 0.4 or more, still more preferably 0.5 or more, from the viewpoint of improving intermittent discharge stability, and, from the same viewpoint as above, preferably 0.9 or less, more preferably 0.8 or less, still more preferably 0.7 or less, even more preferably 0.6 or less. When the vinyl resin A is crosslinked with a crosslinking agent, the content of the vinyl resin A in the ink of the present invention is the total content of the vinyl resin A before crosslinking and the crosslinking agent. When the aqueous ink of the present invention contains a fixing resin, the content of the fixing resin in the aqueous ink is preferably 0.5% by mass or more, more preferably 0.8% by mass or more, still more preferably 1% by mass or more, from the viewpoints of improving intermittent discharge stability, tape peel resistance, and rubbing resistance, and, from the same viewpoints as above, preferably 7% by mass or less, more preferably 5% by mass or less, still more preferably 3% by mass or less. When the aqueous ink of the present invention contains vinyl resin B as a fixing resin, the mass ratio of the content of the vinyl resin B to the total content of the vinyl resin A and vinyl resin B in the aqueous ink [vinyl resin B / (vinyl resin A + vinyl resin B)] is preferably 0.5 or more, more preferably 0.6 or more, still more preferably 0.7 or more, from the viewpoints of improving tape peel resistance and rubbing resistance, and, from the viewpoint of improving intermittent discharge stability, preferably 0.9 or less, more preferably 0.8 or less.

[0042] 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 intermittent discharge stability, tape peel resistance, and rubbing resistance, and, from the same viewpoints as above, preferably 43% by mass or less, more preferably 40% by mass or less, still more preferably 37% by mass or less. From the viewpoint of improving tape peelability and rubbing resistance, the content of alkylene glycol 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, and from the same viewpoint as above, it is preferably 30% by mass or less, more preferably 20% by mass or less, still more preferably 17% by mass or less. In the present invention, from the viewpoint of improving tape peelability and rubbing resistance, the content of 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, and from the same viewpoint as above, it is preferably 30% by mass or less, more preferably 25% by mass or less, still more preferably 20% by mass or less.

[0043] From the viewpoint of reducing environmental load, the water content 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, and is preferably 80% by mass or less, more preferably 75% by mass or less, still more preferably 70% by mass or less.

[0044] 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.

[0045] [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 performance, the piezo type is preferable as a method of ejecting aqueous ink. Examples of the low liquid-absorbing printing substrate used in the inkjet printing method of the present invention include low liquid-absorbing 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 made of 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. Moreover, a film made of a polyolefin resin or a polyester resin is preferably subjected to corona discharge treatment. Among these, from the viewpoint of improving tape peel resistance and abrasion resistance, a printing substrate made of synthetic resin 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, and one or more selected from the group consisting of a printing substrate using polyvinyl chloride resin, a printing substrate subjected to corona discharge treatment using polypropylene resin, and a printing substrate subjected to corona discharge treatment using polyethylene terephthalate resin is further 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 A and the polyvinyl chloride resin polarized by chlorine atoms exhibit dipole-dipole interaction. Further, in the case of a printing substrate subjected to corona discharge treatment using polypropylene resin or polyethylene terephthalate resin as the low liquid-absorbing printing substrate, since hydroxyl groups and carboxyl groups are present on the surface of the printing substrate by corona discharge treatment, it is presumed that the highly polar cyclic ether structure contained in the vinyl resin A and these functional groups exhibit hydrogen bond interaction, and the abrasion resistance can be further improved.

Example

[0046] 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.

[0047] (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 apparatus: "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 substance: 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

[0048] (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., electric burette, model number: APB-610), and titrated with a 0.1 N potassium hydroxide / ethanol solution by potentiometric titration. 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.

[0049] (3) Calculation of the glass transition temperature (Tg) of the vinyl resin The glass transition temperature of the vinyl resin can be calculated from the mass ratio of each monomer constituting the fixing 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, Tg 1 , Tg 2 , ···, Tg m are the glass transition temperatures of the homopolymers when each monomer is a homopolymer. The unit of temperature is K. Also, W 1 , W 2 , ···, W m represent the mass ratios of the respective monomers in the vinyl resin. As the glass transition temperatures of the homopolymers of the respective monomers in the Fox equation, for example, the values described in Polymer Handbook Third Edition (Wiley-Interscience 1989) can be used.

[0050] (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.), 1.0 g of the measurement sample was dried under the conditions of a drying temperature of 150 °C and a measurement mode of 96 (monitoring time 2.5 minutes / variation width 0.05%), and then 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

[0051] (5) Measurement of the solid content concentration of the pigment dispersion Weighed 10.0 g of sodium sulfate that had been equilibrated in a desiccator into a 30 mL ointment container, added approximately 1.0 g of the sample thereto, mixed them, weighed accurately, held at 105 °C for 2 hours to remove volatile components, left it in the desiccator for an additional 15 minutes, and measured the mass. The mass of the sample after removing volatile components was taken as the solid content, and divided by the mass of the added sample to obtain the solid content concentration (%).

[0052] (6) Measurement of the average particle diameter of pigment-containing vinyl resin particles in the pigment aqueous dispersion Cumulant analysis was performed using a laser particle analysis system (“ELS-8000” manufactured by Otsuka Electronics Co., Ltd.), and the obtained cumulant average particle diameter was measured as the average particle diameter of pigment-containing vinyl resin particles in the pigment aqueous dispersion. As the measurement sample, a dispersion diluted with water so that the concentration of the particles to be measured was 5×10 -3 % (in terms of solid content concentration) was used. The measurement conditions were a temperature of 25 °C, an angle of 90° between the incident light and the detector, and an integration number of 100 times, and the refractive index of water (1.333) was input as the refractive index of the dispersion medium.

[0053] (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.

[0054] (8) Measurement of the pH of the aqueous ink Using a desktop pH meter (“F-71” manufactured by Horiba, Ltd.) with a pH electrode (“6337-10D” manufactured by Horiba, Ltd.), the pH of the aqueous ink at 25 °C was measured.

[0055] Production Example 1 (Production of the aqueous dispersion D1 of pigment-containing vinyl resin particles) (1) Production of vinyl resin A1 As an initial charge into a reaction vessel equipped with a stirrer, a reflux condenser, and a dropping funnel, 2.7 parts of tetrahydrofurfuryl acrylate (hereinafter referred to as "THFA"), 1.2 parts of acrylic acid, 0.7 part of isobutyl acrylate, 6.1 parts of methyl ethyl ketone (hereinafter referred to as "MEK"), and 0.7 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 24.5 parts of THFA, 10.5 parts of acrylic acid, 5.8 parts of isobutyl acrylate, 62.2 parts of MEK, 6.9 parts of water, 0.7 part of 4,4’-azobis(4-cyanovaleric acid) as a polymerization initiator, and 0.7 part of 3-mercaptopropionic acid as a chain transfer agent 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 to obtain a MEK solution (solid content concentration 40%) of vinyl resin A1. The physical properties of vinyl resin A1 are shown in Table 1.

[0056] (2) Production of an aqueous dispersion of vinyl resin A1 To 19.3 parts of the MEK solution of vinyl resin A1 obtained above, 2.8 parts of MEK was added and diluted so that the solid content concentration became 35%. Next, 2.0 parts of a 5N aqueous sodium hydroxide solution was added so that the neutralization degree with respect to the carboxy groups of vinyl resin A1 became 30 mol%, and it was stirred at 25 °C. Then, 76.0 parts of water was added over 1 hour. After the addition was completed, MEK was distilled off with an evaporator to obtain an aqueous dispersion of vinyl resin A1 (solid content concentration 25%).

[0057] (3) Production of an aqueous dispersion D1 of pigment-containing vinyl resin particles To 76.9 parts of the aqueous dispersion of vinyl resin A1 (solid content concentration 25%) obtained above, 3.0 parts of MEK and 1.4 parts of ion-exchanged water were added, and further 18.8 parts of a cyan pigment ("FASTOGEN BLUE CA5380 15:3" manufactured by DIC Corporation, C.I. Pigment Blue 15:3) was added to obtain a pigment mixture (mass ratio [pigment / (pigment + vinyl resin A)] = 0.5). 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 10 passes at a pressure of 180 MPa using a high-pressure homogenizer (Microfluidics "Microfluidizer", model: M-140K). From the obtained pigment dispersion, MEK was removed at 60 °C under reduced pressure, and further a part of the water was removed and centrifuged. Then, the liquid layer portion was recovered and filtered through a membrane filter (Sartorius "Mini Sartorius Syringe Filter", pore size: 5 μm, material: cellulose acetate) to remove coarse particles, and an aqueous dispersion D1 (solid content concentration: 22%) of pigment-containing vinyl resin particles was obtained. The average particle diameter of the pigment-containing vinyl resin particles in the obtained aqueous dispersion D1 of pigment-containing vinyl resin particles is shown in Table 1.

[0058] Production Examples 2 to 17 and Comparative Production Example 1 (Production of Aqueous Dispersions D2 to D17 and DC1 of Pigment-Containing Vinyl Resin Particles) In Production Example 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 degree of neutralization of the vinyl resin was 30 mol%, in the same manner as in Production Example 1, an aqueous dispersion (solid content concentration: 22%) of each pigment-containing vinyl resin particle was obtained. In addition, each notation other than the components used in Production Example 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

[0059]

Table 1

[0060] Example 1 (Preparation of Aqueous Ink I-1) As shown in Table 2, the aqueous dispersion D1 of pigment-containing vinyl resin particles was added as a total content of 11 parts of pigment and vinyl resin A (pigment content: 5.5 parts, vinyl resin A content: 5.5 parts), 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-decyn-4,7-diol, active ingredient 50%), 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: 4.1 mPa·s, pH: 8.2). (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, piezo type, manufactured by Kyocera Corporation) was filled with aqueous ink I-1. The printing head voltage was set to 26V, the driving frequency was set to 30kHz, the discharge droplet volume was set to 7pL, 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.

[0061] Examples 2 to 20 and Comparative Example 1 In Example 1, except that the ink composition shown in Table 2 was changed, aqueous inks I-2 to I-20 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 the polyvinyl chloride film "Scotch Graphic Film IJ1220" of the low-absorbency printing substrate by an inkjet recording method in the same manner, and a printed matter was obtained.

[0062] Example 21 In Example 1, a solid image with a duty of 100% was formed in the same manner except that the low liquid-absorbing printing substrate was changed to a polyethylene terephthalate film (“FE2001#25” manufactured by Futamura Chemical Co., Ltd.) (hereinafter also referred to as “PET”), and a printed matter was obtained.

[0063] Example 22 In Example 1, a solid image with a duty of 100% was formed in the same manner except that the low liquid-absorbing printing substrate was changed to a biaxially stretched polypropylene film (“FOR-AQ#25” manufactured by Futamura Chemical Co., Ltd.) (hereinafter also referred to as “OPP”), and a printed matter was obtained.

[0064] [Evaluation] Each aqueous ink and each printed matter obtained in the examples and comparative examples were evaluated according to the following (1) to (3). The results are shown in Table 2.

[0065] (1) Evaluation of tape peelability A tape (“Cellotape (registered trademark)” manufactured by Nichiban Co., Ltd., 18 mm width, model number: CT-18S) was attached to the solid image portion of the obtained printed matter, and the end of the tape was quickly peeled off at a 90° angle. For the solid image portion after peeling, binarization processing was performed using a printing density value that is half the printing density value of the solid image portion before tape peeling as a 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%.

[0066] (2) Evaluation of Rub Resistance The solid image part of the obtained printed matter was rubbed 10 times back and forth with a water-resistant abrasive paper ("WTCC-S" manufactured by Nippon Kenji Co., Ltd., grit size: 320) under a load of 200 g / cm 2 . The remaining area of the coating film in the solid image part after rubbing was calculated by image analysis. For the solid image part after rubbing, using the printing density value half of that of the solid image part before the rubbing test as the threshold value, binary processing was performed to calculate the remaining area and non-remaining area of the coating film by image analysis, and the area ratio of the remaining area of the coating film was calculated as the coating film remaining rate (%) after the rubbing test. The larger the value of the coating film remaining rate (%), the better the rub resistance. If it is 70% or more, there is no practical problem. (Evaluation Criteria) A: The coating film remaining rate after the rubbing test is 90% or more. B: The coating film remaining rate after the rubbing test is 80% or more and less than 90%. C: The coating film remaining rate after the rubbing test is 70% or more and less than 80%. D: The coating film remaining rate after the rubbing test is 60% or more and less than 70%. E: The coating film remaining rate after the rubbing test is less than 60%.

[0067] (3) Evaluation of Intermittent Discharge Stability Using the above-mentioned printing evaluation apparatus, after printing 10 solid images of 20 mm × 20 mm at 100% duty of the ink, it was left without printing for 5 minutes. Then, from the state of the printed matter of the solid image obtained by printing 1 solid image of 20 mm × 20 mm, the discharge recovery rate (%) was calculated by the following formula as the ratio of the discharge area of the printed matter of the solid image immediately after leaving for 5 minutes (that is, the printed matter of the 10th solid image obtained by the command of printing 10 sheets) to the discharge area of the printed matter of the solid image immediately before leaving for 5 minutes (that is, the printed matter of the solid image obtained by the command of printing 1 sheet immediately after leaving), and the intermittent discharge property was evaluated. In this evaluation, the printing substrate was not heated. Discharge recovery rate (%) = [(Discharge area of the printed matter of the solid image immediately after leaving for 5 minutes) / (Discharge area of the printed matter of the solid image immediately before leaving for 5 minutes)] × 100 The larger the value of the discharge recovery rate (%), the better the intermittent discharge stability. If the discharge recovery rate is 85% or more, there is no practical problem. (Evaluation Criteria) A: The ejection recovery rate is 95% or more. B: The ejection recovery rate is 90% or more and less than 95%. C: The ejection recovery rate is 85% or more and less than 90%. D: The ejection recovery rate is 80% or more and less than 85%. E: The ejection recovery rate is less than 80%.

[0068]

Table 2

[0069] From Table 2, it can be seen that the aqueous ink of the examples of the present invention is superior in intermittent ejection 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 which constitutes the pigment-containing vinyl resin particles, it is inferior in intermittent ejection stability, and also, since the glass transition temperature of the vinyl 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

[0070] According to the present invention, it is possible to provide an aqueous ink for inkjet printing and an inkjet printing method which are excellent in intermittent ejection stability and in the tape peeling resistance, rubbing resistance of the obtained printed matter when used for printing on a low liquid-absorbing printing substrate such as a synthetic resin.

Claims

1. A water-based ink for inkjet printing on low-liquid-absorbent printing substrates, The water-based ink contains vinyl resin particles containing pigment, a water-soluble organic solvent, and water. The vinyl resin A constituting the vinyl resin particles containing the pigment comprises a constituent unit derived from a cyclic ether alkyl acrylate (a-1) having a cyclic ether structure in its molecule, and a constituent unit derived from one or more carboxyl group-containing monomers (a-2) selected from the group consisting of acrylic acid and methacrylic acid. A water-based ink for inkjet printing, wherein the content of constituent units derived from the cyclic ether alkyl acrylate (a-1) in all constituent units of the vinyl resin A is 30% by mass or more and 90% by mass or less.

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 water-based inkjet ink according to claim 1, wherein the glass transition temperature of the vinyl resin A is -20°C or higher and 40°C or lower.

4. The aqueous inkjet ink according to claim 1, wherein the acid value of the vinyl resin A is 60 mg KOH / g or more and 260 mg KOH / g or less.

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

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

7. 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.

8. The water-based inkjet printing ink according to claim 7, wherein the synthetic resin is one or more selected from the group consisting of polyvinyl chloride resin, polypropylene resin, and polyethylene terephthalate resin.

9. An inkjet printing method comprising printing on a low liquid absorption printing substrate using an aqueous ink for inkjet printing according to any one of claims 1 to 8.