Cleaning fluid, ink set, and cleaning method

Abstract

The present invention provides a cleaning liquid for cleaning areas where the discharged aqueous inkjet ink containing a pigment and an alkali-soluble resin has adhered, in an inkjet printer. The cleaning liquid comprises a surfactant, a preservative, and water. The surfactant is a compound having a polyoxyalkylene skeleton and an HLB of 9 to 20, and the preservative is a compound having a hydantoin skeleton, with the proportion of the hydantoin skeleton compound in the cleaning liquid being 0.05 to 0.3% by mass. The cleaning liquid does not corrode a silicon nozzle plate and exhibits excellent storage stability, corrosion protection, and cleaning properties for aqueous inkjet inks.

Description

Technical Field

[0001] The present invention relates to a cleaning liquid, an ink set, and a cleaning method. Background Art

[0002] A cleaning liquid is known for cleaning the area to which the aqueous inkjet ink ejected from an inkjet printer containing a pigment and an alkali-soluble resin is attached (Patent Documents 1-6). Regarding such cleaning liquids, there are known cleaning liquids containing preservatives to prevent the growth of mold and the like (Patent Documents 1-2), or cleaning liquids containing deliquescent agents such as sorbitol or 1,3-bis(2-hydroxyethyl)-5,5-dimethyl-2,4-imidazolidinedione to prevent the drying of the cleaning liquid (Patent Documents 3-6). In addition, a nozzle plate is usually provided at the area from which the aqueous inkjet ink as described above is ejected, and the nozzle plate has a plurality of ejection ports for ejecting ink (Patent Document 7).

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: International Publication No. 2016 / 042883

[0006] Patent Document 2: International Publication No. 2018 / 003235

[0007] Patent Document 3: Japanese Patent Application Laid-Open No. 2018-87290

[0008] Patent Document 4: Japanese Patent Application Publication No. 2018-100341

[0009] Patent Document 5: Japanese Patent Application Publication No. 2019-38212

[0010] Patent Document 6: Japanese Patent Application Laid-Open No. 2020-82577

[0011] Patent Document 7: Japanese Patent Application Laid-Open No. 2011-111527 Summary of the Invention

[0012] [Problems to be Solved by the Invention]

[0013] The nozzle plates in inkjet printers, as described above, typically have metal oxides or nitrides on their surfaces to prevent ink erosion. However, when using nozzle plates with silicon surfaces, cleaning solutions containing high amounts of deliquescing agents such as 1,3-bis(2-hydroxyethyl)-5,5-dimethyl-2,4-imidazolidinedione present the following issues: the cleaning solution reduces the water repellency of the plate and reduces the storage stability of the cleaning solution.

[0014] The present invention has been made in view of the above-mentioned actual situation, and an object of the present invention is to provide a cleaning liquid that does not corrode a silicon nozzle plate and has excellent storage stability, corrosion resistance, and cleaning properties for aqueous inkjet ink.

[0015] [Methods for solving the problem]

[0016] That is, the present invention relates to a cleaning liquid for cleaning areas to which the ejected aqueous inkjet ink containing a pigment and an alkali-soluble resin is attached in an inkjet printer. The cleaning liquid contains a surfactant, a preservative, and water. The surfactant is a compound having a polyoxyalkylene skeleton and an HLB of 9 to 20. The preservative contains a compound having a hydantoin skeleton. The ratio of the compound having a hydantoin skeleton in the cleaning liquid is 0.05 to 0.3% by mass.

[0017] Furthermore, the present invention relates to an ink set comprising the cleaning liquid and an aqueous inkjet ink containing a pigment and an alkali-soluble resin.

[0018] The present invention also relates to a cleaning method for supplying the cleaning liquid to a portion to which the discharged aqueous inkjet ink has adhered in an inkjet printer that discharges aqueous inkjet ink containing a pigment and an alkali-soluble resin.

[0019] [Effects of the Invention]

[0020] The detailed mechanism of action of the cleaning solution of the present invention is still unclear, but is presumed as follows. However, the present invention is not limited to this mechanism of action.

[0021] The cleaning solution of the present invention comprises a surfactant, a preservative, and water. The surfactant is a compound having a polyoxyalkylene skeleton and an HLB of 9 to 20, and the preservative is a compound having a hydantoin skeleton. The ratio of the hydantoin skeleton compound in the cleaning solution is 0.05 to 0.3% by mass. The compound having a polyoxyalkylene skeleton and an HLB of 9 to 20 has excellent solubility in water, resulting in excellent storage stability of the cleaning solution of the present invention. Furthermore, the specific amount of the hydantoin skeleton compound, unlike conventional benzisothiazoline compounds used as preservatives, has low affinity for silicon-based materials. Therefore, the cleaning solution of the present invention does not corrode silicon nozzle plates, and exhibits excellent anti-corrosion effects and cleaning properties for aqueous inkjet inks. DETAILED DESCRIPTION

[0022] The cleaning liquid of the present invention is used for cleaning areas to which the discharged aqueous inkjet ink containing a pigment and an alkali-soluble resin is attached in an inkjet printer that ejects the aqueous inkjet ink. The cleaning liquid comprises a surfactant, a preservative, and water. The surfactant comprises a compound having a polyoxyalkylene skeleton and an HLB of 9 to 20, and the preservative comprises a compound having a hydantoin skeleton. The ratio of the compound having a hydantoin skeleton in the cleaning liquid is 0.05 to 0.3% by mass.

[0023] Surfactants

[0024] The surfactant is a compound having a polyoxyalkylene backbone and an HLB value of 9 to 20. The HLB value is an indicator of the hydrophilicity and lipophilicity of a surfactant, calculated using the Griffin method. A smaller HLB value indicates a higher lipophilicity, while a larger HLB value indicates a higher hydrophilicity. The surfactants may be used alone or in combination of two or more.

[0025] The compound having a polyoxyalkylene backbone and an HLB of 9 to 20 may be any known surfactant without particular limitation, as long as it has such a backbone and satisfies the aforementioned HLB values. Examples thereof include nonionic surfactants, cationic surfactants, anionic surfactants, and betaine surfactants. Specific examples of such surfactants include silicone surfactants, fluorochemical surfactants, and acetylene surfactants. Among these, compounds having a polyoxyethylene backbone are preferred, with nonionic surfactants, cationic surfactants, silicone surfactants, and acetylene surfactants being more preferred.

[0026] From the perspective of ensuring the cleaning properties for inks of various compositions, the ratio of the surfactant in the cleaning liquid is preferably 0.05 mass % or more, more preferably 0.1 mass % or more, and from the perspective of ensuring the temporal stability of the cleaning liquid, it is preferably 1 mass % or less, more preferably 0.8 mass % or less.

[0027] Preservatives

[0028] The preservative includes a compound having a hydantoin skeleton. The compound having a hydantoin skeleton can be used without particular limitation as long as it has antiseptic properties. From the perspective of dissolving in a cleaning solution, water-soluble compounds are preferred. Examples of the compound having a hydantoin skeleton include 1,3-bis(2-hydroxyethyl)-5,5-dimethyl-2,4-imidazolidinidinedione, 1,3-dibromo-5,5-dimethylhydantoin, 1,3-dichloro-5,5-dimethylhydantoin, and bromochloro-5,5-dimethylhydantoin. The preservative can be used alone or in combination of two or more. In addition, the term "water-soluble" refers to a compound that can dissolve 5 parts by mass or more in 100 parts by mass of water at 25°C.

[0029] The ratio of the compound having a hydantoin skeleton in the cleaning liquid is 0.05 to 0.3% by mass. From the perspective of ensuring good cleaning properties, the ratio of the compound having a hydantoin skeleton in the cleaning liquid is preferably 0.08% by mass or more, more preferably 0.1% by mass or more. From the perspective of ensuring the temporal stability of the cleaning liquid, the ratio is preferably 0.25% by mass or less, more preferably 0.2% by mass or less.

[0030] To enhance the antiseptic effect, the preservative may further include preservatives other than the hydantoin skeleton-containing compound. Examples of such preservatives include carbamates such as iodopropynyl butylcarbamate; isothiazolinones such as 1,2-benzisothiazolin-3-one; pyrithiones such as sodium pyrithione; and benzimidazoles such as thiabendazole.

[0031] From the viewpoint of ensuring good cleaning properties, the ratio of the water-soluble preservative in the cleaning liquid is preferably 0.001 mass % or more, more preferably 0.005 mass % or more, and from the viewpoint of ensuring the temporal stability of the cleaning liquid, it is preferably 0.1 mass % or less, more preferably 0.05 mass % or less.

[0032] From the viewpoint of ensuring good cleaning properties, the ratio of the preservative (the total of the compound having a hydantoin skeleton and preservatives other than the compound) in the cleaning liquid is preferably 0.05 mass % or more, more preferably 0.1 mass % or more, and from the viewpoint of ensuring the temporal stability of the cleaning liquid, it is preferably 0.3 mass % or less, more preferably 0.25 mass % or less, and even more preferably 0.2 mass % or less.

[0033] Water

[0034] Examples of the water include ion-exchanged water, pure water, distilled water, and industrial water. The water may be used alone or in combination of two or more.

[0035] From the viewpoint of making the cleaning liquid low in viscosity, the ratio of water in the cleaning liquid is preferably 60% by mass or more, more preferably 70% by mass or more, and from the viewpoint of appropriately containing cleaning components, etc., it is preferably 99.9% by mass or less, more preferably 99.5% by mass or less.

[0036] From the perspective of suppressing water evaporation from the cleaning solution, the cleaning solution may also contain a moisturizing agent. Examples of the moisturizing agent include one or more media selected from the group consisting of polyalkylene glycols, alkylene glycols, and glycerol. These moisturizing agents may be used alone or in combination of two or more.

[0037] Examples of the polyalkylene glycol include polyethylene glycol and polypropylene glycol. Examples of the alkylene glycol include ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, 1,2,6-hexanetriol, thiodiethanol, 1,3-butylene glycol, and 1,5-pentanediol. Preferred moisturizers include propylene glycol, diethylene glycol, and glycerin, as they exhibit excellent effects in suppressing water evaporation.

[0038] When the moisturizing agent is used, the ratio of the moisturizing agent in the cleaning solution is preferably 5% by mass or more, more preferably 10% by mass or more, from the viewpoint of ensuring moisturizing properties, and is preferably 40% by mass or less, more preferably 30% by mass or less, from the viewpoint of ensuring the temporal stability of the cleaning solution.

[0039] The cleaning solution may also contain a pH adjuster. Examples of the pH adjuster include: alcohol amines such as diethanolamine, triethanolamine, and 2-amino-2-ethyl-1,3-propanediol; alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide; ammonium hydroxides such as ammonium hydroxide and quaternary ammonium hydroxide; Hydroxides, alkali metal carbonates, etc.

[0040] From the perspective of preventing rust on printer parts, the pH value of the cleaning liquid is preferably 5 or higher, more preferably 6 or higher, and from the perspective of preventing corrosion of the attachment site of the printer, the pH value is preferably 12 or lower, more preferably 9 or lower.

[0041] In addition, the cleaning liquid may optionally contain an organic solvent, a defoaming agent, etc. Of these, organic solvents that may corrode components of the aqueous inkjet printer are preferably limited in amount to avoid corroding the components, and more preferably not to be added.

[0042] The preparation method of the cleaning liquid is not particularly limited. The cleaning liquid may be prepared as a single agent by mixing all the components, or as two agents by supplying the cleaning liquid to the areas where the aqueous inkjet ink has adhered as described below.

[0043] The viscosity of the cleaning liquid at 25° C. may be 5 cps or less, preferably 1 to 3 cps. The viscosity can be measured, for example, with an E-type viscometer (trade name “RE100L Viscometer”, manufactured by Toki Sangyo Co., Ltd.).

[0044] <Ink Set>

[0045] The ink set of the present invention comprises the cleaning liquid and an aqueous inkjet ink containing a pigment and an alkali-soluble resin.

[0046] Pigments

[0047] The pigment may be any organic pigment or inorganic pigment used in aqueous inkjet inks without particular limitation. Examples of the organic pigment include dye lake pigments, azo pigments, benzimidazolone pigments, phthalocyanine pigments, quinacridone pigments, anthraquinone pigments, dioxazine pigments, indigo pigments, thioindigo pigments, perylene pigments, pyrenone pigments, diketopyrrolopyrrole pigments, isoindolinone pigments, nitro pigments, nitroso pigments, flavonoid pigments, quinophthalone pigments, pyranthrone pigments, indanthrene pigments, and the like. In addition, examples of the inorganic pigment include carbon black, titanium oxide, zinc white, red iron, graphite, iron black, chromium oxide green, aluminum hydroxide, and the like. The pigment may also be a pigment surface-treated with a known surface treatment agent. The pigment may be used alone or in combination of two or more.

[0048] <Alkali-soluble resin>

[0049] The alkali-soluble resin is not particularly limited as long as it is an alkali-soluble resin used for pigment dispersion of conventional inks or coatings or can be used as a binder and can be dissolved in an aqueous medium in the presence of an alkaline compound. Preferably, it contains one or more resins with anionic groups such as carboxyl groups, sulfonic acid groups, and phosphonic acid groups (-P(=O)(OH2)).

[0050] The alkali-soluble resin preferably has a hydrophobic portion in the molecule mainly for improving adsorption to the pigment. Examples of the hydrophobic portion introduced into the molecule include hydrophobic groups such as long-chain alkyl groups, alicyclic groups, and aromatic cyclic hydrocarbon groups.

[0051] From the perspective of improving solubility in aqueous media, the acid value of the alkali-soluble resin is preferably 40 mgKOH / g or higher, more preferably 70 mgKOH / g or higher. Furthermore, from the perspective of improving the water resistance of printed materials, the acid value of the alkali-soluble resin is preferably 300 mgKOH / g or lower, more preferably 250 mgKOH / g or lower. The acid value is the theoretical acid value obtained by arithmetically calculating the number of milligrams of potassium hydroxide theoretically required to neutralize 1 gram of the alkali-soluble resin based on the composition of the monomers used to synthesize the alkali-soluble resin.

[0052] From the perspective of improving the blocking resistance of printed materials, the glass transition temperature of the alkali-soluble resin is preferably 0°C or higher, more preferably 10°C or higher. From the perspective of improving the folding resistance of printed materials, the glass transition temperature of the alkali-soluble resin is preferably 100°C or lower, more preferably 80°C or lower.

[0053] The glass transition temperature of the alkali-soluble resin is a theoretical glass transition temperature determined by the following Wood's equation when the alkali-soluble resin is an acrylic copolymer resin.

[0054] Wood's formula: 1 / Tg=W1 / Tg1+W2 / Tg2+W3 / Tg3+……+Wx / Tgx

[0055] [Wherein, Tg1 to Tgx represent the glass transition temperatures of the homopolymers of monomers 1, 2, 3, ..., x constituting the alkali-soluble resin, W1 to Wx represent the polymerization fractions of monomers 1, 2, 3, ..., x, and Tg represents the theoretical glass transition temperature. The glass transition temperature in Wood's formula is an absolute temperature.]

[0056] The glass transition temperature of the alkali-soluble resin is a theoretical glass transition temperature obtained by thermal analysis when the alkali-soluble resin is other than an acrylic copolymer resin. The thermal analysis method is in accordance with JIS K7121 (Determination of Transition Temperature of Plastics). For example, the glass transition temperature can be measured using a PerkinElmer Pyris 1 DSC at a heating rate of 20°C / min and a nitrogen flow rate of 20 ml / min.

[0057] From the perspective of improving water resistance of printed materials, the weight average molecular weight of the alkali-soluble resin is preferably 5,000 or more, more preferably 10,000 or more. From the perspective of improving solubility in aqueous media, the weight average molecular weight of the alkali-soluble resin is preferably 100,000 or less, more preferably 50,000 or less.

[0058] The weight-average molecular weight can be measured by gel permeation chromatography (GPC). For example, a Water 2690 (manufactured by Waters Corporation) can be used as the GPC apparatus, and a PLgel 5μ MIXED-D (manufactured by Polymer Laboratories) column can be used. Chromatography can be performed under the following conditions: tetrahydrofuran as the developing solvent, a column temperature of 25°C, a flow rate of 1 ml / min, an RI detector, a sample injection concentration of 10 mg / ml, and an injection volume of 100 μL. The weight-average molecular weight can be determined as polystyrene-equivalent weight-average molecular weight.

[0059] Examples of the alkali-soluble resin include acrylic acid copolymer resins, maleic acid copolymer resins, polyester resins obtained by polycondensation, and polyurethane resins. Materials for synthesizing such alkali-soluble resins are disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-94825. Acrylic acid copolymer resins, maleic acid copolymer resins, polyester resins, and polyurethane resins obtained using the materials described in this publication may be used. Furthermore, resins obtained using other materials may also be used. The alkali-soluble resins may be used alone or in combination of two or more.

[0060] As the acrylic copolymer resin, for example, one obtained by polymerizing a mixture of other monomers copolymerizable with an anionic group-containing monomer in a solvent in the presence of a common free radical generator (e.g., benzoyl peroxide, t-butyl perbenzoate, azobisisobutyronitrile, etc.) can be used.

[0061] Examples of the anionic group-containing monomer include monomers having at least one anionic group selected from the group consisting of a carboxyl group, a sulfonic acid group, and a phosphonic acid group. Among them, monomers having a carboxyl group are particularly preferred.

[0062] Examples of the monomer having a carboxyl group include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, 2-carboxyethyl (meth)acrylate, 2-carboxypropyl (meth)acrylate, maleic anhydride, fumaric anhydride, and maleic acid half ester. Examples of the monomer having a sulfonic acid group include sulfoethyl methacrylate. Examples of the monomer having a phosphonic acid group include phosphoethyl methacrylate.

[0063] As the other monomer copolymerizable with the anionic group-containing monomer, it is preferred that a hydrophobic group-containing monomer be included from the viewpoint of improving adsorption to the pigment.

[0064] Examples of the hydrophobic group-containing monomer include, for example, monomers having a long-chain alkyl group, such as (meth)acrylic acid, and free-radical polymerizable unsaturated carboxylic acids having 8 or more carbon atoms (e.g., 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, 2-hydroxystearyl (meth)acrylate, etc.), alkyl vinyl ethers having 8 or more carbon atoms (e.g., dodecyl vinyl ether, etc.), and vinyl esters of fatty acids having 8 or more carbon atoms (e.g., vinyl 2-ethylhexanoate, vinyl laurate, vinyl stearate, etc.). Examples of monomers having an alicyclic hydrocarbon group include cyclohexyl (meth)acrylate, and examples of monomers having an aromatic hydrocarbon group include benzyl (meth)acrylate, styrene, α-methylstyrene, vinyltoluene, and other styrene-based monomers. These hydrophobic group-containing monomers may be used alone or in combination of two or more.

[0065] As the other monomer copolymerizable with the anionic group-containing monomer, a hydrophilic group-containing monomer may be included from the viewpoint of suppressing aggregation of the alkali-soluble resin in an aqueous medium.

[0066] Examples of the hydrophilic group-containing monomer include, for example, monomers having an (poly)oxyalkylene chain, esters of one-terminal alkyl-terminated (poly)alkylene glycols such as methoxypolyethylene glycol, methoxypolyethylene glycol polypropylene glycol, ethoxypolyethylene glycol, ethoxypolyethylene glycol polypropylene glycol, propoxypolyethylene glycol, propoxypolyethylene glycol polypropylene glycol, and free radically polymerizable unsaturated carboxylic acids such as (meth)acrylic acid, or ethylene oxide adducts and / or propylene oxide adducts of free radically polymerizable unsaturated carboxylic acids such as (meth)acrylic acid; and examples of the basic group-containing monomer include vinyl pyrrolidones such as 1-vinyl-2-pyrrolidone and 1-vinyl-3-pyrrolidone, vinyl pyridines such as 2-vinylpyridine, 4-vinylpyridine, 5-methyl-2-vinylpyridine, and 5-ethyl-2-vinylpyridine, 1-vinylimidazole, and the like. Examples of the monomers having a hydroxyl group include vinyl imidazoles such as oxazole, 1-vinyl-2-methylimidazole, vinyl piperidines such as 3-vinylpiperidine and N-methyl-3-vinylpiperidine, nitrogen-containing derivatives of (meth)acrylic acid such as dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, tert-butylaminoethyl (meth)acrylate, (meth)acrylamide, N-hydroxymethyl (meth)acrylamide, N-butoxymethyl (meth)acrylamide, N-methoxy (meth)acrylamide, N-ethoxy (meth)acrylamide, N-dimethylacrylamide, and N-propylacrylamide; examples of monomers having a hydroxyl group include hydroxyalkyl (meth)acrylic acid esters such as hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate; and examples of monomers having an epoxy group include glycidyl (meth)acrylate. These hydrophilic group-containing monomers may be used alone or in combination of two or more.

[0067] Examples of other copolymerizable monomers other than the hydrophobic group-containing monomer and the hydrophilic group-containing monomer include alkyl (meth)acrylates having less than 8 carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, and hexyl (meth)acrylate. These other copolymerizable monomers other than the hydrophobic group-containing monomer and the hydrophilic group-containing monomer may be used alone or in combination of two or more.

[0068] Surfactants

[0069] The aqueous inkjet ink may contain a surfactant. These surfactants can be any known surfactant used in aqueous inkjet inks, such as nonionic surfactants, cationic surfactants, anionic surfactants, and betaine surfactants. Specific examples of these surfactants include silicone surfactants, fluorochemical surfactants, and acetylene surfactants. These surfactants may be used alone or in combination of two or more.

[0070] <Water-soluble solvent>

[0071] The aqueous inkjet ink may use a water-soluble solvent. The water-soluble solvent may be a known water-soluble solvent used in aqueous inkjet inks, such as lower alcohols such as ethanol and propanol; polyols such as glycerol; (poly)alkylene glycols such as (poly)ethylene glycol and (poly)propylene glycol, and their alkyl ethers. These water-soluble solvents may be used alone or in combination of two or more.

[0072] Hereinafter, the ratio of each component in the aqueous inkjet ink will be described.

[0073] The ratio of the pigment in the aqueous inkjet ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, from the perspective of increasing the printed image density. Furthermore, from the perspective of improving ejection stability, it is preferably 10% by mass or less, more preferably 8% by mass or less. If the pigment is a white pigment, the ratio of the white pigment in the aqueous inkjet ink of the present invention is preferably 4% by mass or more, more preferably 8% by mass or more, and preferably 30% by mass or less, more preferably 20% by mass or less.

[0074] From the perspective of improving the dispersibility of the pigment, the content of the alkali-soluble resin is preferably 5 parts by mass or more, more preferably 15 parts by mass or more, per 100 parts by mass of the pigment. From the perspective of reducing the viscosity of the aqueous inkjet composition, the content of the alkali-soluble resin is preferably 100 parts by mass or less, more preferably 80 parts by mass or less, and even more preferably 60 parts by mass or less, per 100 parts by mass of the pigment.

[0075] When the surfactant is used in the aqueous inkjet ink, from the viewpoint of improving dot expansion and full-page uniformity of printed matter, the ratio of the surfactant in the aqueous inkjet ink is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and from the viewpoint of improving storage stability, it is preferably 3% by mass or less, more preferably 2% by mass or less.

[0076] When the water-soluble solvent is used in the aqueous inkjet ink, from the viewpoint of improving ejection stability, the ratio of the water-soluble solvent in the aqueous inkjet ink is preferably 15% by mass or more, more preferably 20% by mass or more, and from the viewpoint of improving the drying properties of the coating film, preferably 60% by mass or less, more preferably 50% by mass or less.

[0077] From the viewpoint of improving the drying properties of the coating film, the ratio of water in the aqueous inkjet ink is preferably 40% by mass or more, more preferably 50% by mass or more, and from the viewpoint of improving ejection stability, it is preferably 70% by mass or less, more preferably 60% by mass or less.

[0078] <Basic compounds>

[0079] From the perspective of dissolving the alkali-soluble resin, the aqueous inkjet ink preferably contains an alkaline compound. Examples of such alkaline compounds include inorganic alkaline compounds such as sodium hydroxide and potassium hydroxide; and organic alkaline compounds such as ammonia, methylamine, ethylamine, monoethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, N,N-dibutylethanolamine, diethanolamine, N-methyldiethanolamine, triethanolamine, morpholine, N-methylmorpholine, and N-ethylmorpholine. These alkaline compounds may be used alone or in combination of two or more.

[0080] The ratio of the alkaline compound in the aqueous inkjet ink may be any amount sufficient to dissolve the alkali-soluble resin in the medium. However, from the perspective of improving the dispersion stability of the alkali-soluble resin, it is generally preferably 0.05% by mass or more, more preferably 0.1% by mass or more. Furthermore, from the perspective of improving the water resistance of printed materials, it is preferably 1% by mass or less, more preferably 0.5% by mass or less.

[0081] The aqueous inkjet ink may further contain known additives such as resins, resin emulsions, wax emulsions, pigment dispersants, mildew inhibitors, rust inhibitors, thickeners, antioxidants, ultraviolet absorbers, storage life enhancers, defoaming agents, and pH adjusters, depending on the purpose.

[0082] <Method for preparing aqueous inkjet ink>

[0083] There are no particular limitations on the method for preparing (manufacturing) the aqueous inkjet ink. The components may be added and mixed sequentially or simultaneously. For example, the following methods may be mentioned: (1) mixing an aqueous resin varnish obtained by dissolving an alkali-soluble resin in water in the presence of the alkaline compound, a pigment, and a pigment dispersant added as needed, and then preparing a pigment dispersion (ink base) using various dispersers, such as a ball mill, a grinder, a roller mill, a sand mill, a stirred mill, etc., and then adding the remaining materials to prepare an aqueous inkjet ink; or (2) after dispersing the pigment using the above method, obtaining a resin-coated pigment in which an alkali-soluble resin is precipitated on the surface of the pigment by an acid precipitation method or an ion exchange method described in Japanese Re-Publication Patent No. WO2005 / 116147, then neutralizing the obtained resin-coated pigment with an alkaline compound, redispersing it in water using various dispersers (such as a high-speed stirring device), and then adding the remaining materials to prepare an aqueous inkjet ink; etc.

[0084] The initial viscosity of the aqueous inkjet ink after production is in the range of 2.0 to 15.0 mPa·s, preferably 3.0 to 12.0 mPa·s at 25° C. The viscosity can be measured, for example, with an E-type viscometer (trade name "RE100L Viscometer", manufactured by Toki Sangyo Co., Ltd.).

[0085] Cleaning method

[0086] The cleaning method of the present invention comprises supplying a cleaning liquid to an area where the aqueous inkjet ink has deposited, in an inkjet printer that ejects an aqueous inkjet ink containing a pigment and an alkali-soluble resin. The cleaning liquid may be supplied to the area where the aqueous inkjet ink has deposited, either individually or in a premixed state.

[0087] In addition, the part where the aqueous inkjet ink is ejected in the inkjet printer is usually equipped with a nozzle plate having a plurality of ejection ports for ejecting ink, but the cleaning liquid of the present invention does not corrode the silicon nozzle plate and is therefore suitable for inkjet printers having such a silicon nozzle plate.

[0088] Example

[0089] Hereinafter, the present invention will be described using examples and the like, but the present invention is not limited to these examples.

[0090] <Production Example 1>

[0091] Production of Pigment Dispersion (Black Ink Base)

[0092] 20 parts by mass of an alkali-soluble resin (acrylic acid / n-butyl acrylate / benzyl methacrylate / styrene copolymer, weight-average molecular weight 30,000, acid value 185 mgKOH / g, glass transition temperature 40°C) was dissolved in a mixed solution of 2.5 parts by mass of potassium hydroxide and 77.5 parts by mass of water to obtain an aqueous resin varnish having a solid content of 20% by mass of the alkali-soluble resin. Subsequently, 64.3 parts by mass of water was added to 23.7 parts by mass of the aqueous resin varnish and mixed to prepare a pigment dispersion resin varnish. To this pigment dispersion resin varnish, 12 parts by mass of carbon black (trade name "Printex 90", manufactured by Degussa) was further added as a pigment. After stirring and mixing, the mixture was milled using a wet circulation mill to produce a black pigment dispersion (black ink base).

[0093] Preparation of aqueous inkjet ink

[0094] To 34 parts by mass of the black pigment dispersion were added 30 parts by mass of water, 35 parts by mass of propylene glycol, and 1 part by mass of an acetylene surfactant (trade name "Surfynol 465"), and the mixture was stirred and mixed to prepare an aqueous inkjet ink.

[0095] <Examples 1 to 13, Comparative Examples 1 to 11>

[0096] Preparation of cleaning solution

[0097] In each Example and Comparative Example, the raw materials were added to a beaker according to the compositions shown in Tables 1 and 2 and stirred using a stirring device to prepare cleaning solutions for each Example and Comparative Example. The cleaning solutions in the Examples had a pH of 6.5 to 8.5 and a viscosity of approximately 1.0 to 1.8 cps at 25°C.

[0098] The aqueous inkjet ink and cleaning liquid obtained above were used to perform evaluations according to the following method. The results are shown in Tables 1 and 2.

[0099] Storage stability

[0100] The cleaning solutions obtained above were placed in glass bottles and their viscosity (mPa·s) at 25°C was measured using a viscometer (RE100L, manufactured by Toki Sangyo Co., Ltd.). The bottles were then tightly stoppered and stored at 60°C for one month. The viscosity after storage (25°C) was measured using a viscometer. Storage stability was evaluated using the viscosity change rate (60°C, (viscosity after one month - viscosity before storage) / viscosity before storage).

[0101] [Evaluation Criteria]

[0102] ○: The viscosity change rate is less than 5%.

[0103] Δ: The viscosity change rate is 5% or more and less than 10%.

[0104] ×: The additive is separated and uneven.

[0105] <Anti-corrosion effect>

[0106] Bacterial evaluation: 0.6 g of bacterial culture solution cultured on ordinary agar medium was inoculated, mixed with 30 g of the washing solution obtained above in a petri dish, and cultured at 30°C for 7 days.

[0107] Evaluation of mold and yeast: 0.6 g of a mold and yeast suspension cultured on potato dextrose agar was inoculated, mixed with 30 g of the washing solution obtained above in a petri dish, and cultured at 28°C for 7 days.

[0108] Then, the residual state of microorganisms in each petri dish was evaluated.

[0109] [Evaluation Criteria]

[0110] ○: No microbial growth was observed in any of the culture dishes.

[0111] △: The number of colonies in any culture dish was less than 10.

[0112] ×: The number of colonies in at least one petri dish was 10 or more.

[0113] Cleaning performance for aqueous inkjet inks

[0114] An inkjet printer PX-105 (manufactured by Seiko Epson) was filled with the aqueous inkjet ink. After continuous full-page printing on 10 A4 sheets of paper to confirm ink filling, the printer was placed in a 50°C oven with the ink still in the recording head. After this period, a nozzle check print was performed, revealing multiple nozzles experiencing poor ink ejection (missing needles). The cleaning solution obtained above was then filled into the recording head in the initial filling mode to evaluate its cleaning properties for the aqueous inkjet ink.

[0115] [Evaluation Criteria]

[0116] ○: All nozzles were restored by the initial filling sequence alone, and ejection defects were improved.

[0117] △: All nozzles were restored after 1 to 3 cleaning operations, and the ejection failure was improved.

[0118] ×: Needle missing still occurred even after the cleaning operation was performed three or more times, and the ejection defect was not improved.

[0119] Corrosion resistance to silicon nozzle plates

[0120] A 2 cm×2 cm silicon nozzle plate was immersed in the cleaning solution obtained above and stored at 60° C. for 1 month. The liquid repellency of the nozzle plate after storage was evaluated by measuring the contact angle of water.

[0121] [Evaluation Criteria]

[0122] ○: The contact angle of water is 90° or more.

[0123] △: The contact angle of water is 80° or more and less than 90°.

[0124] ×: The water contact angle is less than 80°.

[0125] [Table 1]

[0126]

[0127] [Table 2]

[0128]

[0129] In Tables 1 and 2, Emulgen 1150S-60 represents polyoxyethylene alkyl ether (manufactured by Kao, HLB 19);

[0130] RHEODOL TW-L120 stands for polyoxyethylene sorbitan monolaurate (manufactured by Kao, HLB 17);

[0131] NYMEEN S-220 stands for polyoxyethylene stearamine (manufactured by NOF, HLB 15);

[0132] Surfynol 465 represents polyoxyethylene acetylene glycol ether (manufactured by EVONIK, HLB 13);

[0133] KF-355A indicates polyether-modified silicone (manufactured by Shin-Etsu Chemical Co., Ltd., HLB12);

[0134] DOWSI FZ-2104Fluid represents polyether-modified silicone (manufactured by DOW, HLB9);

[0135] Surfynol 440 represents polyoxyethylene acetylene glycol ether (manufactured by EVONIK, HLB8);

[0136] UNISAFE 10P-8 stands for polyoxyethylene polyoxypropylene cetyl ether (manufactured by NOF Corporation, HLB8);

[0137] KF-352A indicates polyether-modified silicone (manufactured by Shin-Etsu Chemical Co., Ltd., HLB7);

[0138] DMDMH stands for 1,3-bis(2-hydroxyethyl)-5,5-dimethyl-2,4-imidazolidinedione;

[0139] PROXEL GXL represents a composition containing 20% ​​by mass of 1,2-benzisothiazolin-3-one (manufactured by Lonna Corporation).

Claims

1. A cleaning liquid, characterized in that The invention is used for cleaning the part to which the aqueous inkjet ink containing pigment and alkali-soluble resin is attached in an inkjet printer. The cleaning solution comprises a surfactant, a preservative, and water. The surfactant is a compound having a polyoxyalkylene skeleton and an HLB of 9 to 20. The preservative comprises a compound having a hydantoin skeleton, The ratio of the compound having a hydantoin skeleton in the cleaning liquid is 0.05 to 0.3% by mass.

2. The cleaning solution according to claim 1, characterized in that The moisturizing agent comprises one or more moisturizing agents selected from the group consisting of polyalkylene glycol, alkylene glycol, and glycerin.

3. The cleaning solution according to claim 1 or 2, characterized in that Contains pH adjuster.

4. The cleaning solution according to claim 1 or 2, characterized in that The compound having a hydantoin skeleton is 1,3-bis(2-hydroxyethyl)-5,5-dimethyl-2,4-imidazolidinedione.

5. The cleaning solution according to claim 1 or 2, characterized in that The preservative further includes a preservative other than the compound having a hydantoin skeleton.

6. An ink set, characterized in that: The cleaning liquid according to any one of claims 1 to 5 and an aqueous inkjet ink containing a pigment and an alkali-soluble resin.

7. A cleaning method, characterized in that: In an inkjet printer that ejects an aqueous inkjet ink containing a pigment and an alkali-soluble resin, a liquid is supplied to a portion to which the ejected aqueous inkjet ink adheres. The cleaning solution according to any one of claims 1 to 5.

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