Water-based ink for inkjet printing

Abstract

To provide an aqueous ink for inkjet recording having high substrate adhesion even in a non-liquid absorbing recording medium.SOLUTION: There is provided an aqueous ink for inkjet recording containing a pigment and a copolymer (P), wherein the copolymer (P) contains a constituent unit (A) derived from acrylonitrile and a constituent unit (S) derived from styrene and satisfies the following conditions 1 and 2. Condition 1: the copolymer (P) has a glass transition temperature of 50°C or more and 120°C or less. Condition 2: the content of the constituent unit (A) derived from acrylonitrile in the copolymer (P) is 8 mass% or more and 50 mass% or less in the whole constituent unit of the copolymer (P).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-based ink for ink-jet printing. [Background technology]

[0002] In the commercial printing field for product packaging, advertisements, etc., in addition to printing on conventional highly liquid-absorbent substrates such as plain paper and copy paper, there is a growing demand for printing on substrates made of low-liquid-absorbent coated paper such as offset coated paper and non-liquid-absorbent resin films. Furthermore, ink-jet recording methods using water-based pigment inks have been attracting attention from the viewpoints of reducing environmental impact, saving energy, and improving the water resistance and durability of printed matter. When an image is formed on a substrate with low or no liquid absorption, the liquid component is absorbed slowly or not at all, resulting in insufficient ink adhesion to the substrate (hereinafter also referred to as "substrate adhesion") and a problem of poor image quality. Therefore, various proposals have been made to improve the ink's substrate adhesion and image quality.

[0003] For example, Patent Document 1 discloses an emulsion for water-based inkjet inks that has a specific gel content and a core / shell structure, obtained by emulsion polymerization of an aliphatic conjugated diene monomer and a monomer having an ethylenically unsaturated bond other than the aliphatic conjugated diene monomer, as an emulsion that can provide water-based inkjet inks that have good print stability, print density, and excellent print quality such as abrasion resistance and gloss. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-45304 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the ink using the aqueous inkjet ink emulsion of Patent Document 1 does not have sufficient substrate adhesion when used on a non-liquid-absorbing recording medium. The present invention relates to a water-based ink for ink-jet printing that has high substrate adhesion even on non-liquid-absorbing recording media. [Means for solving the problem]

[0006] The present inventors have found that the above-mentioned problems can be solved by providing a water-based ink for inkjet printing containing a pigment and a copolymer (P) containing an acrylonitrile-derived structural unit (A) and a styrene-derived structural unit (S), wherein the glass transition temperature of the copolymer (P) and the content of the acrylonitrile-derived structural unit (A) in the copolymer (P) are within specific ranges.

[0007] That is, the present invention provides: A water-based ink for ink-jet printing containing a pigment and a copolymer (P), The present invention relates to a water-based ink for ink-jet printing, wherein the copolymer (P) contains a structural unit (A) derived from acrylonitrile and a structural unit (S) derived from styrene, and satisfies the following conditions 1 and 2: Condition 1: The glass transition temperature of the copolymer (P) is 50°C or higher and 120°C or lower. Condition 2: The content of the acrylonitrile-derived structural unit (A) in the copolymer (P) is 8% by mass or more and 50% by mass or less of all structural units in the copolymer (P). [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a water-based ink for ink-jet printing that has high substrate adhesion even when used on a non-liquid-absorbing recording medium. DETAILED DESCRIPTION OF THE INVENTION

[0009] According to the present invention, it is possible to provide a water-based ink for ink-jet printing that has high substrate adhesion even when used on a non-liquid-absorbing recording medium. The reason for this is not clear, but is thought to be as follows. The ink of the present invention contains, as a resin to be blended into the ink, a copolymer (P) containing acrylonitrile-derived structural units (A) at a specific mass ratio. This improves affinity with non-liquid-absorbing recording media due to the balance between the hydrophilicity of the nitrile groups derived from acrylonitrile and the hydrophobicity of the phenyl groups of the styrene-derived structural units (S) contained therein, as well as interactions due to the π electrons of the phenyl groups. Furthermore, by setting the glass transition temperature of the copolymer (P) to be 50°C or higher and 120°C or lower, it is possible to prevent the film formed by the copolymer (P) from softening at room temperature, and it is therefore believed that high substrate adhesion to the recording medium can be achieved.

[0010] [Water-based ink for inkjet printing] The water-based ink of the present invention contains at least a pigment, a copolymer (P) containing an acrylonitrile-derived structural unit (A) and a styrene-derived copolymer (S), and water.

[0011] [Pigments] The pigment used in the present invention is not particularly limited, and may be either an inorganic pigment or an organic pigment. Specific examples of inorganic pigments include carbon black, metal oxides such as titanium oxide, iron oxide, red iron oxide, and chromium oxide, and pearlescent pigments. Carbon black is particularly preferred for black inks. 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; and 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 threne pigments. The hue is not particularly limited, and achromatic pigments such as white, black, and gray, and chromatic pigments such as yellow, magenta, cyan, blue, red, orange, and green can all be used. The pigments can be used alone or in combination of two or more.

[0012] From the viewpoint of print density of the water-based ink, the content of the pigment in the water-based ink of the present invention is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 2% by mass or more, and is preferably 8% by mass or less, more preferably 6% by mass or less, even more preferably 5% by mass or less.

[0013] [Copolymer (P)] The copolymer (P) used in the present invention functions as a fixing resin (fixing aid) that improves the substrate adhesion and print density of the water-based ink. When the water-based ink contains the copolymer (P), the affinity between the copolymer (P) and non-absorbent recording media allows a smooth coating film to be obtained even on non-absorbent recording media. Furthermore, when the pigment is dispersed in a pigment dispersing polymer, the affinity between the copolymer (P) and the pigment dispersing polymer prevents local aggregation of pigment particles, improving substrate adhesion without reducing print density.

[0014] When the glass transition temperature of the copolymer (P) is 50°C or higher and 120°C or lower, the film formed by the copolymer (P) does not soften at room temperature and exhibits sufficient strength, thereby improving the adhesion of the water-based ink to the substrate. From the viewpoint of the adhesion of the water-based ink to the substrate, the glass transition temperature of the copolymer (P) is 50° C. or higher, preferably 85° C. or higher, more preferably 100° C. or higher, and 120° C. or lower, preferably 115° C. or lower, more preferably 110° C. or lower. The glass transition temperature of the copolymer (P) can be calculated from the glass transition temperatures of the homopolymers of all the monomers constituting the copolymer (P) using the following Fox's equation. 1 / Tg=(W1 / Tg1)+(W2 / Tg2)+···+(W m / Tg m ) W1+W2+W m =1 In the Fox formula, Tg is the glass transition temperature of the copolymer, and Tg1, Tg2, ..., Tg mis the glass transition temperature of the homopolymer of each polymer monomer. The unit of temperature is K. Also, W1, W2, ..., W m represents the mass ratio of each polymerized monomer to the entire copolymer. As the glass transition temperature of the homopolymer of each polymerizable monomer in the Fox equation, for example, the values described in Polymer Handbook Third Edition (Wiley-Interscience 1989) can be used.

[0015] The copolymer (P) contains structural units (A) derived from acrylonitrile and structural units (S) derived from styrene. From the viewpoint of substrate adhesion of the water-based ink, the content of the structural unit (A) relative to the mass of all structural units of the copolymer (P) is 8% by mass or more, preferably 9% by mass or more, more preferably 12% by mass or more, and 50% by mass or less, preferably 48% by mass or less, more preferably 40% by mass or less, even more preferably 38% by mass or less, and still more preferably 25% by mass or less.

[0016] Furthermore, from the viewpoint of further improving the adhesion of the water-based ink to the substrate, the mass ratio of the structural unit (A) to the total amount of the structural unit (A) and the structural unit (S) in the copolymer (P) [(A) / ((A)+(S))] is preferably 0.06 or more, more preferably 0.1 or more, even more preferably 0.15 or more, and is preferably 0.55 or less, more preferably 0.5 or less, even more preferably 0.4 or less, and most preferably 0.3 or less.

[0017] Increasing the weight-average molecular weight of copolymer (P) strengthens the bonding strength with the substrate surface, allowing the water-based ink to achieve even higher substrate adhesion. Therefore, the weight-average molecular weight of copolymer (P) is preferably 450,000 or more, more preferably 500,000 or more, and even more preferably 700,000 or more, and is preferably 3.8 million or less, more preferably 3 million or less, and even more preferably 2.5 million or less. The weight-average molecular weight of copolymer (P) can be measured by the method described in the Examples.

[0018] In addition to the structural units (A) and (S), the copolymer (P) may contain structural units derived from vinyl monomers. Examples of such vinyl monomers include acrylic acid, methacrylic acid, and alkyl esters thereof. Among these, from the viewpoint of the adhesion of the water-based ink to the substrate, one or more selected from acrylic acid and methacrylic acid are preferred. From the viewpoint of the adhesion of the water-based ink to the substrate, the content of the structural units derived from vinyl monomers other than the structural units (A) and (S) in the copolymer (P) is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less, relative to the mass of all structural units in the copolymer (P). Furthermore, when the structural units (A) and (S) in copolymer (P) derived from other vinyl monomers are one or more selected from acrylic acid and methacrylic acid, the total content of acrylic acid and methacrylic acid relative to the mass of all structural units in copolymer (P) is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less, from the viewpoint of substrate adhesion of the water-based ink. Furthermore, from the viewpoint of dispersion stability of copolymer (P), the total content of acrylic acid and methacrylic acid relative to the mass of all structural units in copolymer (P) is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more.

[0019] The copolymer (P) may have a constituent unit derived from an aliphatic conjugated diene as a constituent unit, but the content of the constituent unit derived from an aliphatic conjugated diene in all constituent units derived from all monomers is preferably 15 mass% or less, more preferably 10 mass% or less, even more preferably 6 mass% or less, and it is even more preferable that the copolymer (P) does not contain any constituent unit derived from an aliphatic conjugated diene.

[0020] From the viewpoint of improving the adhesion of the aqueous ink to a substrate, the content of copolymer (P) in the aqueous ink of the present invention is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably 4% by mass or more, and is preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 6% by mass or less.

[0021] The mass ratio of copolymer (P) to pigment (copolymer (P) / pigment) in the water-based ink of the present invention is preferably 0.5 or more, more preferably 0.8 or more, and even more preferably 1 or more, from the viewpoint of improving the adhesion of the water-based ink to the substrate, and is preferably 5 or less, more preferably 3 or less, and even more preferably 2 or less, from the viewpoint of print density.

[0022] (Method for producing copolymer (P)) The copolymer (P) can be prepared by copolymerizing a mixture of acrylonitrile, styrene and other monomers by known polymerization methods.

[0023] During the polymerization, a polymerization initiator or a polymerization chain transfer agent can be used. As the polymerization initiator, known radical polymerization initiators can be used, such as inorganic peroxides such as potassium persulfate, azo compounds such as 2,2'-azobisisobutyronitrile and 2,2'-azobis(2,4-dimethylvaleronitrile), and organic peroxides such as t-butyl peroxyoctoate and benzoyl peroxide. The amount of the radical polymerization initiator is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, and even more preferably 0.15 parts by mass or more, per 100 parts by mass of the monomer mixture, and is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 2 parts by mass or less. As the polymerization chain transfer agent, known chain transfer agents such as mercaptans such as octyl mercaptan and 2-mercaptoethanol, and thiuram disulfides can be used.

[0024] A surfactant can also be used during polymerization. Examples of the surfactant include nonionic surfactants, anionic surfactants, and cationic surfactants. From the viewpoint of improving the dispersion stability of the polymer particles, anionic surfactants are preferred. Examples of the anionic surfactant include fatty acid salts, alkylbenzenesulfonates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene aralkylaryl ether sulfates, and polyoxyethylene alkyl ether sulfates. Polyoxyethylene alkyl ether sulfates are more preferred.

[0025] Furthermore, copolymer (P) can have a core-shell structure by stepwise polymerizing a mixture of monomers with different formulations. However, from the viewpoint of improving the adhesion of the water-based ink to the substrate, it is preferable that copolymer (P) does not have a core-shell structure.

[0026] Preferred polymerization conditions vary depending on the types of polymerization initiator, monomer, and solvent used, but typically the polymerization temperature is preferably 40° C. or higher, more preferably 45° C. or higher, and preferably 90° C. or lower, more preferably 85° C. or lower. The polymerization time is preferably 1 hour or longer, more preferably 5 hours or longer, and preferably 20 hours or shorter, more preferably 10 hours or shorter. The polymerization atmosphere is preferably a nitrogen gas atmosphere or an inert gas atmosphere such as argon.

[0027] The copolymer (P) is preferably used as an aqueous dispersion of the copolymer (P) using water as the main dispersion medium. Furthermore, the obtained copolymer (P) can be purified by removing unreacted monomers and the like by reprecipitation, membrane separation, chromatography, extraction, or the like.

[0028] [Other ingredients] The water-based ink of the present invention may contain, in addition to the pigment and copolymer (P), a pigment dispersing polymer, a water-soluble organic solvent, a surfactant, and the like.

[0029] (Pigment dispersing polymer) From the viewpoint of pigment dispersion stability, the pigment dispersing polymer preferably has acid groups, and it is preferable that at least a portion of the acid groups be neutralized with a neutralizing agent, which is believed to increase the charge repulsion force exhibited after neutralization, suppress aggregation of pigment particles in the water-based ink, suppress thickening, and improve storage stability. Examples of the acid group include a group that exhibits acidity by dissociating and releasing a hydrogen ion, such as a carboxy group (-COOM), a sulfonic acid group (-SO3M), or a phosphate group (-OPO3M2), or a dissociated ionic form thereof. Of these, the carboxy group (-COOM) is preferred from the viewpoint of improving the storage stability and ejection stability of the water-based ink.

[0030] From the viewpoint of improving the jetting stability of the aqueous ink, the acid value of the pigment dispersion polymer is preferably 50 mgKOH / g or more, more preferably 70 mgKOH / g or more, even more preferably 90 mgKOH / g or more, and preferably 300 mgKOH / g or less, more preferably 270 mgKOH / g or less, even more preferably 250 mgKOH / g or less. When the acid value is within the above range, the amount of acid groups and their neutralized acid groups is sufficient, ensuring the dispersion stability of the pigment and, as a result, the jetting stability of the aqueous ink. This is also preferable in terms of the affinity between the pigment dispersion polymer and the aqueous medium. The acid value of the pigment dispersing polymer can be calculated from the mass ratio of the constituent monomers, or can be determined by dissolving or swelling the pigment dispersing polymer in an appropriate organic solvent for measuring acid values (e.g., methyl ethyl ketone) and titrating the resulting solution.

[0031] From the viewpoint of improving the dispersion stability of pigment molecules in ink, the pigment dispersion polymer more preferably contains a structural unit derived from (a-1) an ionic monomer (hereinafter also referred to as "component (a-1)") and a structural unit derived from (a-2) a hydrophobic monomer (hereinafter also referred to as "component (a-2)"). The pigment dispersion polymer may further contain a structural unit derived from (a-3) a hydrophobic monomer (hereinafter also referred to as "component (a-3)").

[0032] <(a-1) Ionic Monomer> (a-1) The ionic monomer includes anionic monomers and cationic monomers, with anionic monomers being preferred. Specific examples of the (a-1) ionic monomer include those described in paragraph

[0017] of JP 2018-83938 A. Among these, carboxylic acid monomers such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, and 2-methacryloyloxymethylsuccinic acid are preferred, and acrylic acid and methacrylic acid are more preferred.

[0033] <(a-2) Hydrophobic Monomer> (a-2) The term "hydrophobic" in the hydrophobic monomer means that when the monomer is dissolved in 100 g of ion-exchanged water at 25° C. until saturation, the amount of the dissolved monomer is less than 10 g. Specific examples of the (a-2) hydrophobic monomer include those described in paragraphs

[0020] to

[0022] of JP 2018-83938 A. Among these, alkyl(meth)acrylates having an alkyl group with 1 to 22 carbon atoms, particularly 6 to 18 carbon atoms, vinyl monomers having an aromatic group with 6 to 22 carbon atoms, and macromonomers having a polymerizable functional group at one end are preferred, with one or more selected from styrene, α-methylstyrene, and benzyl(meth)acrylate being more preferred.

[0034] Examples of the macromonomer having a polymerizable functional group at one end include compounds having a number average molecular weight of 500 or more and 100,000 or less, preferably 1,000 or more and 10,000 or less, and in which the polymerizable functional group is an acryloyloxy group or a methacryloyloxy group. The macromonomer is preferably an aromatic group-containing monomer-based macromonomer, and examples of the aromatic group-containing monomer constituting the macromonomer include the aromatic group-containing monomers described above. Specific examples of commercially available styrene-based macromonomers include AS-6(S), AN-6(S), and HS-6(S) manufactured by Toagosei Co., Ltd.

[0035] <(a-3) Nonionic Monomer> (a-3) The nonionic monomer is a monomer that has a high affinity with water or a water-soluble organic solvent, and is, for example, a monomer that contains a hydroxyl group or a polyalkylene glycol chain. Specific examples of the component (a-3) include those described in paragraph

[0018] of JP-A No. 2018-83938. Among these, methoxypolyethylene glycol (n=1 to 30) (meth)acrylate is preferred. The monomer components contained in each of the above components (a-1) to (a-3) can be used alone or in combination of two or more.

[0036] The content of the structural units derived from components (a-1) to (a-3) in the pigment dispersion polymer is as follows, from the viewpoint of improving the storage stability and ejection stability of the water-based ink. The content of component (a-1) in the pigment dispersion polymer is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 12% by mass or more, and preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less. The content of the component (a-2) in the pigment dispersion polymer is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and is preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less. When the component (a-3) is contained, its content in the pigment dispersion polymer is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less.

[0037] Furthermore, from the viewpoint of improving the storage stability and ejection stability of the water-based ink, the mass ratio of the component (a-1) to the component (a-2) [component (a-1) / component (a-2)] is preferably 0.1 or more, more preferably 0.15 or more, even more preferably 0.2 or more, and is preferably 3 or less, more preferably 2 or less, even more preferably 1 or less, even more preferably 0.5 or less. Furthermore, when the (a-3) component is contained, the mass ratio of the (a-1) component to the total of the (a-2) component and the (a-3) component [(a-1) component / [(a-2) component+(a-3) component]] is preferably 0.03 or more, more preferably 0.05 or more, even more preferably 0.1 or more, and is preferably 1 or less, more preferably 0.8 or less, even more preferably 0.6 or less, even more preferably 0.5 or less.

[0038] <Production of pigment dispersing polymers> The pigment dispersion polymer can be produced by copolymerizing a mixture of the above monomer components (a-1) to (a-3) by a known polymerization method. The polymerization method is preferably a solution polymerization method. Specifically, the polymer can be produced and purified using the polymerization initiator and polymerization chain transfer agent shown in the above "(Production method of copolymer (P))".

[0039] The number-average molecular weight of the pigment dispersing polymer is preferably 3,000 or more, more preferably 5,000 or more, even more preferably 10,000 or more, and even more preferably 30,000 or more, from the viewpoint of improving the storage stability and ejection stability of the water-based ink, and is preferably 100,000 or less, more preferably 70,000 or less, and even more preferably 60,000 or less, from the viewpoint of pigment dispersion stability. If the number-average molecular weight of the pigment dispersing polymer is within the above range, the adsorption force to the pigment is sufficient and dispersion stability can be achieved. The number-average molecular weight can be measured by the method described in the examples.

[0040] <Method for producing water-based pigment dispersion> The pigment can be dispersed in a water-based pigment dispersion by a known method using a pigment dispersing polymer. Examples of the dispersion method include the following methods. First, the pigment and the pigment-dispersing polymer are mixed in water. At this time, a neutralizing agent may be added to neutralize the pigment-dispersing polymer. There are no particular restrictions on the order in which the pigment and the pigment-dispersing polymer are mixed in water, but when adding the neutralizing agent, it is preferable to add the pigment-dispersing polymer to water, and then mix the neutralizing agent and the pigment in this order. The pigment and the pigment dispersing polymer can be mixed by a conventional method such as using an ultrasonic homogenizer.

[0041] An organic solvent may be used when performing the dispersion treatment. There are no particular restrictions on the organic solvent as long as it can be removed from the aqueous dispersion after the dispersion treatment. As the organic solvent, aliphatic alcohols, ketones, ethers, esters, etc. having 1 to 3 carbon atoms are preferred, and from the viewpoint of improving the wettability to the pigment, the solubility of the polymer dispersant, and the adsorption of the polymer dispersant to the pigment, ketones having 4 to 8 carbon atoms are more preferred, methyl ethyl ketone and methyl isobutyl ketone are even more preferred, and methyl ethyl ketone is even more preferred.

[0042] In the dispersion treatment, pigment particles can be atomized to a desired particle size by main dispersion using shear stress alone, but from the viewpoint of obtaining a uniform aqueous pigment dispersion, it is preferable to pre-disperse the pigment mixture and then further carry out main dispersion.

[0043] When pre-dispersing the pigment mixture, a commonly used mixing and stirring device such as an anchor blade or a disper blade can be used, but among these, a high-speed stirring and mixing device is preferred. The pre-dispersion temperature is preferably 0° C. or higher, and preferably 40° C. or lower, more preferably 30° C. or lower, and even more preferably 25° C. or lower. The pre-dispersion time is preferably 0.5 hours or higher, more preferably 0.8 hours or higher, and preferably 30 hours or lower, more preferably 10 hours or lower, and even more preferably 5 hours or lower.

[0044] Examples of means for applying shear stress for this dispersion include kneading machines such as roll mills and kneaders, high-pressure homogenizers such as Microfluidizer (manufactured by Microfluidics), and media-type dispersers such as paint shakers and bead mills. Commercially available media-type dispersers include Ultra Apex Mill (manufactured by Hiroshima Metal & Machinery Co., Ltd.) and Picomill (manufactured by Asada Iron Works Co., Ltd.). A combination of these devices can also be used. Of these, it is preferable to use a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment. When the main dispersion is carried out using a high-pressure homogenizer, the pigment particle size can be controlled to a desired size by controlling the processing pressure and the number of passes. From the viewpoints of productivity and economy, the treatment pressure is preferably 60 MPa or more, more preferably 100 MPa or more, even more preferably 130 MPa or more, and is preferably 250 MPa or less, more preferably 230 MPa or less. The number of passes is preferably 3 or more, more preferably 8 or more, and is preferably 20 or less, more preferably 16 or less.

[0045] In the present invention, the pigment is preferably dispersed by a pigment-dispersing polymer, and possible forms include a state in which the pigment-dispersing polymer is adsorbed to the pigment (i.e., in the form of pigment-adsorbed polymer particles), a state in which the pigment-dispersing polymer contains the pigment (i.e., in the form of particles in which the pigment is encapsulated, or in the form of pigment-containing polymer particles including particles in which the pigment is uniformly dispersed), etc. Among these, the form of pigment-containing polymer particles is more preferred from the viewpoint of more stable dispersion of the pigment. The pigment is preferably dispersed using a pigment-dispersing polymer having a crosslinked structure. In the present invention, the pigment-dispersing polymer having a crosslinked structure is a polymer containing a component derived from the pigment-dispersing polymer and a component derived from a crosslinking agent. Here, the crosslinking agent refers to a compound having two or more functional groups in the molecule that can react with the reactive sites of the pigment-dispersing polymer. In the present invention, in order to form a state in which the pigment is dispersed in a pigment-dispersing polymer having a crosslinked structure, the pigment-adsorbing polymer particles or pigment-containing polymer particles in the pigment dispersion obtained above may be mixed with a crosslinking agent to cause a reaction, i.e., a crosslinking treatment may be carried out. From the viewpoint of efficient reaction with the pigment-adsorbing polymer particles or pigment-containing polymer particles in a medium mainly composed of water, and from the viewpoints of the storage stability and ejection stability of the water-based ink, the water solubility (mass ratio) of the crosslinking agent is preferably 50% or less, more preferably 40% or less, and even more preferably 35% or less. Here, the water solubility (mass ratio) refers to the solubility (%) when 10 parts by mass of the crosslinking agent is dissolved in 90 parts by mass of water at room temperature (25°C).

[0046] The crosslinking treatment is carried out by heating a mixture of the pigment dispersion and the crosslinking agent. The heating temperature is preferably 50° C. or higher and 90° C. or lower. From the viewpoints of completion of the crosslinking reaction and economic efficiency, the heating time is preferably 0.5 hours or longer, more preferably 1 hour or longer, and even more preferably 1.5 hours or longer, and is preferably 12 hours or shorter, more preferably 8 hours or shorter, and even more preferably 6 hours or shorter. The progress of the crosslinking reaction can be confirmed by measuring the change in pH of the pigment dispersion during the crosslinking treatment, and the crosslinking reaction can be considered complete when no further change in pH occurs. It is desirable to continue the heat treatment until the crosslinking reaction is completely completed.

[0047] The crosslinking agent is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule, more preferably a compound having two or more glycidyl ether groups, and even more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 4 carbon atoms. The epoxy equivalent of the crosslinking agent is preferably 90 or more, more preferably 100 or more, even more preferably 110 or more, and preferably 300 or less, more preferably 200 or less, even more preferably 150 or less. The number of epoxy groups in the crosslinking agent is 2 or more per molecule from the viewpoint of efficiently reacting with the acid groups to enhance the storage stability of the aqueous pigment dispersion, and is preferably 6 or less per molecule, and from the viewpoint of market availability, is more preferably 4 or less, and even more preferably 3 or less.

[0048] Suitable examples of the crosslinking agent include one or more selected from polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether, with trimethylolpropane polyglycidyl ether being more preferred.

[0049] [Water-soluble organic solvent] The water-based ink of the present invention may contain a water-soluble organic solvent to improve leveling properties. The water-soluble organic solvent is not particularly limited, and examples thereof include glycols and glycol ethers having a boiling point of 100°C or higher and 300°C or lower to improve drying properties. The glycols and glycol ethers having a boiling point of 100°C or higher and 300°C or lower may be used alone or in combination of two or more.

[0050] Specific examples of glycols having a boiling point of 100°C or higher and 300°C or lower include those described in paragraph

[0054] of JP 2018-83938 A. Specific examples of glycol ethers having a boiling point of 100°C or higher and 300°C or lower include those described in paragraphs

[0051] and

[0052] of JP 2018-83938 A. Among these, ethylene glycol and propylene glycol are preferred, and propylene glycol is more preferred.

[0051] The total amount of water-soluble organic solvents in the ink is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, from the viewpoint of improving drying properties, and is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less, from the viewpoint of improving leveling properties.

[0052] [Surfactant] The water-based ink of the present invention may contain a surfactant from the viewpoint of improving leveling properties. As the surfactant, an acetylene glycol surfactant and a polyether-modified silicone surfactant are preferred. The acetylene glycol surfactant and the polyether-modified silicone surfactant may be used alone or in combination of two or more thereof.

[0053] Specific examples of acetylene glycol surfactants include those described in paragraphs

[0056] and

[0057] of JP 2018-83938 A. Specific examples of polyether-modified silicone surfactants include those described in paragraph

[0061] of JP 2018-83938 A. Among these, 2,4,7,9-tetramethyl-5-decyne-4,7-diol and polyether-modified silicone are preferred.

[0054] From the viewpoint of improving wettability to the printing substrate, the content of the surfactant in the ink is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, even more preferably 0.5% by mass or more, and is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less. From the viewpoint of improving wettability to the printing substrate, the content of the acetylene glycol surfactant in the ink is preferably 0.1% by mass or more, more preferably 0.4% by mass or more, and is preferably 3% by mass or less, more preferably 1% by mass or less. The mass ratio of the acetylene glycol-based surfactant to the sum of all surfactants in the ink [acetylene glycol-based surfactant / total surfactants] is preferably 0.1 or more, more preferably 0.7 or more, and is preferably 1 or less, more preferably 0.9 or less.

[0055] [Method for preparing water-based ink for ink-jet printing] The water-based ink for ink-jet recording of the present invention can be prepared by mixing the pigment or the water-based pigment dispersion and the copolymer (P) in water. When preparing the water-based ink, the above-mentioned water-soluble organic solvent and surfactant, as well as various additives commonly used in water-based inks, such as a humectant, wetting agent, penetrating agent, viscosity adjuster, pH adjuster, antifoaming agent, preservative, antifungal agent, and antirust agent, may be further added, if necessary. Furthermore, after mixing, filtration treatment using a filter or the like can be carried out.

[0056] [Non-liquid-absorbent recording medium] Non-liquid-absorbing recording media for printing with the water-based ink of the present invention include non-liquid-absorbing media such as processed paper, synthetic paper, art paper, low-liquid-absorbent coated paper, and resin film. Examples of low-liquid-absorbency coated paper include general-purpose glossy paper and multicolor foam glossy paper. Examples of resin films include resin films made of polyvinyl chloride, polymethyl methacrylate, polyethylene terephthalate, polybutylene terephthalate, polypropylene, polystyrene, polyamide, acrylonitrile-butadiene-styrene copolymer (ABS), acrylonitrile-styrene copolymer, polycarbonate (PC), a polymer alloy in which PC and ABS are mixed in a 1:1 ratio, triacetyl cellulose, etc. Among these, resin films made of polyester resins such as polyvinyl chloride and polyethylene terephthalate, polypropylene, etc. are preferred as non-liquid-absorbing recording media. [Example]

[0057] In the following Production Examples, Synthesis Examples, Examples and Comparative Examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. In the following examples, the various physical properties were measured by the following methods.

[0058] (1) Weight average molecular weight (Mw) of the polymer The weight average molecular weight (Mw) of the copolymer (P) and the pigment dispersing polymer was measured by the following gel permeation chromatography method. The measurements were performed using gel permeation chromatography (GPC) using a Tosoh GPC system (HLC-8320GPC) and Tosoh columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, and TSKgel guardcolumn Super AW-H) at a flow rate of 0.5 mL / min, with N,N-dimethylformamide containing 60 mmol / L phosphoric acid and 50 mmol / L lithium bromide as the eluent. The measurements were performed using a monodisperse polystyrene kit (Tosoh PStQuick B (F-550, F-80, F-10, F-1, and A-1000) and PStQuick C (F-288, F-40, F-4, A-5000, and A-500)) with known molecular weights as standards. The measurement sample was prepared by mixing 0.1 g of polymer with 10 mL of the eluent in a glass vial, stirring with a magnetic stirrer at 25°C for 10 hours, and filtering with a syringe filter (Advantec Co., Ltd., DISMIC-13HP, PTFE, 0.2 μm).

[0059] (2) Solid content concentration The solids concentrations of the aqueous dispersion of the copolymer (P), the pigment-dispersed polymer solution, and the aqueous dispersion of the pigment-containing polymer particles were measured by the following method. 10.0 g of sodium sulfate, maintained at a constant weight in a desiccator, was weighed into a 30 mL polypropylene container (inner diameter 40 mm, height 30 mm), and 1.0 g of the sample was added thereto and mixed to obtain a mixture. The mixture was then weighed, maintained at 105°C for 2 hours to remove volatiles, and then left in the desiccator for 15 minutes, after which the mass of the mixture after devolatilization was measured. The mass of the mixture after devolatilization minus the mass of sodium sulfate was determined as the solid content of the sample after devolatilization, and this was divided by the mass of the sample before devolatilization to obtain the solid content concentration (%).

[0060] (3) Average particle size of pigment-containing polymer particles The average particle size of the pigment-containing polymer particles was measured as a cumulant average particle size by the following method. The cumulant average particle size measured using a laser particle analysis system "ELS-8000" (cumulant analysis) manufactured by Otsuka Electronics Co., Ltd. was used as the average particle size of the pigment-containing polymer particles. The measurement conditions were a temperature of 25°C, an angle of 90° between the incident light and the detector, and 100 integration times. The refractive index of water (1.333) was input as the refractive index of the dispersion solvent. The measured concentration was 5 x 10 -3 It was done in %.

[0061] (4) Measurement of glass transition temperature Approximately 0.02 g to 0.03 g of particles of copolymer (P) were used as a measurement sample, which was heated to 200°C using a differential scanning calorimeter (manufactured by Perkin Elmer, trade name: Pyris 6 DSC) and then cooled from that temperature to -30°C at a rate of 10°C / min. The measurement sample was heated at a rate of 10°C / min, and the glass transition temperature (Tg) was determined as the temperature at the intersection of an extension of the baseline below the maximum endothermic peak temperature and a tangent line showing the maximum slope from the rising part of the peak to the peak apex. The measurement samples were prepared by freeze-drying aqueous dispersions P-1 to P-8 and P-11 to P-13 at -10°C for 9 hours using a freeze dryer (manufactured by Tokyo Rikakikai Co., Ltd., product name: FDU-2100).

[0062] Production Example 1 (Production of Water Dispersion P-1 of Copolymer (P)) In a reaction vessel equipped with a dropping funnel, the monomers shown in the "Initial Charge Monomer Composition" in Table 1, potassium persulfate as a polymerization initiator, a surfactant (polyoxyethylene alkyl ether sodium sulfate, manufactured by Kao Corporation, product name: Latemul E-118B), and ion-exchanged water were placed and mixed, and nitrogen gas replacement was performed to obtain an initial charge monomer solution. Also, the monomers shown in the "Monomer Composition to be Dropped" in Table 1, the polymerization initiator, the surfactant, and ion-exchanged water were placed and mixed to obtain a dropping monomer solution, and the dropping monomer solution was placed in the dropping funnel and nitrogen gas replacement was performed. Under a nitrogen atmosphere, the initial monomer solution in the reaction vessel was heated from room temperature to 80°C over 30 minutes while stirring. While maintaining the temperature at 80°C, the monomer from the dropping funnel was gradually added dropwise to the reaction vessel over 3 hours. After the addition, the mixture was stirred for 1 hour while maintaining the temperature inside the reaction vessel. The mixture was then filtered through a 200-mesh wire screen to obtain aqueous dispersion P-1 (solid content 41.6%) containing particles of copolymer (P), an acrylonitrile-styrene-containing polymer. The resulting copolymer (P) particles were confirmed to have no core-shell structure, as a single glass transition temperature was measured using a differential scanning calorimeter.

[0063] Production Examples 2 to 7, Comparative Production Examples 1 and 2 (Production of Water Dispersions P-2 to P-7, P-11 to P-12 of Copolymer (P)) In Production Example 1, the "initial charge monomer composition" and "dropped monomer composition" were changed as shown in Table 1, and the same procedure was followed as in Production Example 1 to obtain aqueous dispersions P-2 to P-7 and P-11 to P-12 containing particles of copolymer (P). The particles of copolymer (P) contained in the obtained aqueous dispersions P-2 to P-5, P-7, P-11, and P-12 were confirmed to have a single glass transition temperature measured by a differential scanning calorimeter, and therefore were confirmed to have no core-shell structure. On the other hand, the particles contained in aqueous dispersion P-6 were confirmed to have a core-shell structure, and therefore were confirmed to have multiple glass transition temperatures measured by a differential scanning calorimeter.

[0064] Production Example 8 (Production of Water Dispersion P-8 of Copolymer (P)) The monomers, polymerization initiator, surfactant, and ion-exchanged water shown in the "Initial Charge Monomer Composition" in Table 1 were mixed in an autoclave equipped with a stirrer and reacted for 6 hours at 50°C. The mixture was then filtered through a 200-mesh wire screen to obtain aqueous dispersion P-8 containing particles of copolymer (P), an acrylonitrile-styrene-butadiene-containing polymer. The obtained copolymer (P) particles did not have a core-shell structure. The content of butadiene-derived structural units in copolymer (P) was also determined. 1When confirmed by 1 H-NMR, the content of structural units derived from butadiene in all structural units of the copolymer (P) was 5 mass %. 1 The H-NMR measurement was carried out as follows. The aqueous dispersion P-8 was freeze-dried at -10°C for 9 hours using a freeze dryer (Tokyo Rikakikai Co., Ltd., product name: FDU-2100), and the residue was dissolved in deuterated chloroform to obtain a sample. The sample was analyzed using an NMR measurement device (Agilent, product name: MR400). 1 H-NMR was measured, and the content of the butadiene-derived structural units in all structural units of the copolymer (P) was determined from the ratio of the integrated value of the peak derived from butadiene to the integrated value of the peak derived from acrylonitrile.

[0065] Comparative Production Example 3 (Production of Water Dispersion P-13 of Copolymer (P)) In Production Example 8, an aqueous dispersion P-13 containing particles of copolymer (P) was obtained in the same manner as in Production Example 8, except that the "initial monomer composition" was changed as shown in Table 1. The obtained particles of copolymer (P) did not have a core-shell structure. In addition, as in Production Example 8, the butadiene content in copolymer (P) was 1 When confirmed by 1 H-NMR, the content of structural units derived from butadiene in all structural units of the copolymer (P) was 20 mass %.

[0066] [Table 1]

[0067] Preparation Example 1 (Preparation of polymer solution for pigment dispersion) 16 parts of methacrylic acid, 44 parts of styrene, 30 parts of styrene macromonomer (manufactured by Toagosei Co., Ltd., trade name: AS-6S, number average molecular weight 6,000, solids content 50%), and 25 parts of methoxypolyethylene glycol methacrylate (manufactured by NOF Corporation, trade name: Blemmer PME-200) were mixed to prepare 115 parts of a monomer mixture. In a reaction vessel, 18 parts of methyl ethyl ketone, 0.03 parts of 2-mercaptoethanol as a chain transfer agent, and 10% (11.5 parts) of the monomer mixture were placed and mixed, and the inside of the vessel was thoroughly purged with nitrogen gas. The remaining 90% (103.5 parts) of the monomer mixture, 0.27 parts of the chain transfer agent, 3 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (Fujifilm Wako Pure Chemical Industries, Ltd., product name: V-65) as a polymerization initiator, and 42 parts of methyl ethyl ketone were mixed and placed in a dropping funnel. The mixture in the reaction vessel was heated to 75°C under a nitrogen atmosphere while stirring, and the mixture in the dropping funnel was added dropwise over 3 hours. After 2 hours at 75°C after the completion of the dropping, a solution of 3 parts of the polymerization initiator in 5 parts of methyl ethyl ketone was added, and the mixture was further aged at 75°C for 2 hours and then at 80°C for 2 hours. An additional 50 parts of methyl ethyl ketone was added to obtain a solution of a pigment dispersion polymer (weight average molecular weight: 50,000). The solids concentration of the pigment dispersion polymer solution was 45% by mass.

[0068] Preparation Example 2 (Preparation of aqueous dispersion of pigment-containing polymer particles) 95.2 parts of the pigment dispersion polymer solution prepared above was mixed with 53.9 parts of methyl ethyl ketone, and 15.0 parts of 5N aqueous sodium hydroxide and 0.5 parts of 25% aqueous ammonia were added as neutralizers. 341.3 parts of ion-exchanged water and 100 parts of cyan pigment (CI Pigment Blue 15:3, manufactured by DIC Corporation, product name: TGR-SD) were then added to obtain a pigment mixture. The degree of neutralization was 78.8 mol%. The pigment mixture was mixed using a disperser blade at 7000 rpm and 20°C for 1 hour. The resulting dispersion was dispersed 15 times using a Microfluidizer (manufactured by Microfluidics, product name: High-Pressure Homogenizer M-140K) at a pressure of 180 MPa to obtain a dispersion of pigment-containing polymer particles. The resulting pigment-containing polymer particle dispersion was decompressed at 60°C to remove methyl ethyl ketone and a portion of the water, then centrifuged and the liquid layer was separated. The liquid layer was filtered through a filter (manufactured by Sartorius, trade name: Minisart Syringe Filter, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, yielding an aqueous dispersion of pigment-containing polymer particles. The solids concentration of this aqueous dispersion was 25% by mass (pigment 17.5% by mass, pigment dispersing polymer 7.5% by mass). To 100 parts of the resulting aqueous dispersion of pigment-containing polymer particles, 0.45 parts of a crosslinker (manufactured by Nagase ChemteX Corporation, trade name: Denacol EX321L) and 15.23 parts of ion-exchanged water were added, and the mixture was heated at 70°C for 3 hours with stirring. After cooling to room temperature, the liquid layer was filtered through a filter (manufactured by Sartorius, product name: Minisart Syringe Filter) to remove coarse particles, and ion-exchanged water was added to adjust the solids concentration to 22.0% by mass (pigment 15.1% by mass, pigment dispersing polymer 6.9% by mass (including 0.4% by mass of components derived from the crosslinking agent)), yielding an aqueous dispersion of pigment-containing polymer particles with a crosslinked structure. The cumulant average particle size of the pigment-containing polymer particles was 100 nm.

[0069] Example 1 (Preparation of Water-Based Ink 1) Aqueous ink 1 was prepared by mixing 12 parts (5 parts as solids) of aqueous dispersion P-1 obtained in Production Example 1, 22.7 parts (5 parts as solids) of the aqueous dispersion of pigment-containing polymer particles obtained in Preparation Example 2, 25 parts of propylene glycol as a water-soluble organic solvent, 0.5 parts of an acetylene glycol surfactant (manufactured by Nissin Chemical Industry Co., Ltd., product name: Surfynol 104PG-50), and 0.1 parts of a polyether-modified silicone surfactant (manufactured by Nissin Chemical Industry Co., Ltd., product name: Silface SAG005), and adding ion-exchanged water to bring the total to 100 parts.

[0070] Examples 2 to 8 and Comparative Examples 1 to 3 (Preparation of Water-Based Inks 2 to 8 and 11 to 13) Water-based inks 2 to 8 and 11 to 13 were obtained in the same manner as in Preparation Example 1, except that the water dispersion P-1 in Example 1 was replaced with water dispersions P-2 to P-8 and P-11 to P-13, respectively.

[0071] <Evaluation of adhesion to substrate> The water-based inks obtained in the examples and comparative examples were used to obtain prints by the following (1) inkjet printing method, and the substrate adhesion was evaluated by the following method (2).

[0072] (1) Inkjet printing method In an environment with a temperature of 32°C, an inkjet printing evaluation device (manufactured by Tritec Corporation) equipped with a print head (manufactured by Kyocera Corporation, product name: KJ4B-HD06MHG-STDV, piezo type) was filled with the water-based inks obtained in the examples and comparative examples. The settings were print head voltage: 26 V, drive frequency: 30 kHz, ejected droplet volume: 7 pL, print head temperature: 32°C, and print head resolution: 600 dpi. A solid image was printed at 100% duty on a resin film (manufactured by 3M Japan Limited, polyvinyl chloride, product name: Scotchcal Graphic Film IJ1220) heated to 60°C.

[0073] (2) Evaluation of substrate adhesion The resulting print was placed in an 80°C dryer and dried for 5 minutes. The print was then removed and left in a room at 25°C for 30 minutes. Tape (Nichiban Co., Ltd., Cellotape®, 18mm wide, CT-18S) was applied to the solid image area, and the edge of the tape was quickly peeled off at a 90° angle. The image area after peeling was binarized using a print density value that was half the print density of the solid image area before tape removal as a threshold value. The peeled and non-peeled areas were then calculated by image analysis, and the area percentage (%) of the non-peeled area was calculated. The higher the value, the better the adhesion to the substrate; a value of 80% or higher is practically acceptable. The results are shown in Table 2.

[0074] [Table 2]

[0075] The results of the examples and comparative example 3 show that when the glass transition temperature of the copolymer (P) is 50° C. or higher and 120° C. or lower, high substrate adhesion is achieved in non-liquid-absorbing recording media. Furthermore, from the results of the Examples and Comparative Examples 1 and 2, it can be seen that when the acrylonitrile content of the copolymer (P) is in the range of 8% by mass or more and 50% by mass or less, high substrate adhesion is achieved in non-liquid-absorbing recording media. The copolymer (P) particles contained in the water-based ink of Example 3 and the resin particles contained in the water-based ink of Example 6 were nearly identical in structure and weight average molecular weight, except that the resin particles contained in the water-based ink of Example 6 had a core-shell structure with a core portion that was not copolymer (P) and a shell portion that was copolymer (P), but there was a 10% difference in substrate adhesion. This shows that the water-based ink containing copolymer (P) particles without a core-shell structure has higher substrate adhesion on non-liquid-absorbing recording media than the water-based ink containing particles with a core-shell structure that includes copolymer (P).

Claims

1. A water-based ink for ink-jet printing containing a pigment and a copolymer (P), The weight average molecular weight of the copolymer (P) is 450,000 or more and 3,800,000 or less, The copolymer (P) contains a structural unit (A) derived from acrylonitrile and a structural unit (S) derived from styrene, and satisfies the following conditions 1 and 2. Condition 1: The glass transition temperature of the copolymer (P) is 50° C. or higher and 120° C. or lower Condition 2: The content of the acrylonitrile-derived structural unit (A) in the copolymer (P) is 8% by mass or more and 50% by mass or less of all structural units in the copolymer (P).

2. 2. The water-based ink for ink-jet printing according to claim 1, wherein the mass ratio of the acrylonitrile-derived structural unit (A) to the total of the acrylonitrile-derived structural unit (A) and the styrene-derived structural unit (S) in the copolymer (P), [(A) / ((A)+(S))], is 0.06 or more and 0.55 or less.

3. 3. The water-based ink for ink-jet printing according to claim 1, wherein the copolymer (P) does not have a core-shell structure.

4. 4. The water-based ink for ink-jet printing according to claim 1, wherein the content of the structural units derived from an aliphatic conjugated diene in the copolymer (P) is 15% by mass or less based on all structural units in the copolymer (P).

5. The water-based ink for ink-jet printing according to any one of claims 1 to 4, further comprising a water-soluble organic solvent, and one or more surfactants selected from the group consisting of polyether-modified silicone surfactants and acetylene glycol surfactants.

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