JP7528575B2 - Pretreatment liquid, inkjet recording liquid set containing said pretreatment liquid and image forming method - Google Patents

Description

The present invention relates to a pretreatment liquid, an inkjet recording liquid set containing the pretreatment liquid, and an image forming method.

The inkjet recording method allows for easy and inexpensive image creation, and is therefore used in a variety of printing fields, including photography, various types of printing, marking, and special printing such as color filters.

There are several types of inkjet inks used in inkjet recording methods, including aqueous inks made of water and organic solvents, non-aqueous inks that contain organic solvents but essentially no water, hot-melt inks that are printed by heating and melting ink that is solid at room temperature, and actinic radiation-curable inks that are cured by exposure to actinic radiation after printing. These inks are used according to the application.

In particular, water-based inks are generally odorless and highly safe, and are therefore being considered for use not only in home printers but also in industrial printers.

For example, Patent Document 1 describes a pretreatment liquid containing a first aggregating agent selected from orthophosphoric acid, phosphorous acid, hypophosphorous acid, and salts thereof, and a second aggregating agent selected from organic acids or salts thereof and polyvalent metal salts. According to Patent Document 1, by using the above-mentioned first and second aggregating agents in combination, it is possible to increase the coagulation properties of the ink and obtain high-quality images.

Patent Document 2 describes a primer ink for inkjet recording that contains a resin selected from a fluorine-acrylic composite resin, a silicone-acrylic composite resin, and a vinyl chloride-acrylic composite resin, water, and an organic solvent. According to Patent Document 2, the use of the primer ink can improve the adhesion of an image recorded with inkjet ink on a non-absorbent recording medium.

JP 2011-161643 A JP 2018-076544 A

The inventors have conducted research and found that when images were formed on low-absorbency recording media such as coated paper using the pretreatment liquid described in Patent Document 1, images with good fixation and sufficient image quality were obtained. On the other hand, images with the desired fixation and quality could not always be obtained on non-absorbency recording media such as film substrates.

In addition, when an image was formed on a non-absorbent recording medium using the inkjet recording primer ink described in Patent Document 2, the image did not have the desired adhesion and water resistance.

The present invention has been made in consideration of these points, and has as its first object to provide a pretreatment liquid that has good fixation to low-absorbency or non-absorbency recording media and can form images with excellent image quality and water resistance. The second object of the present invention is to provide an inkjet recording liquid set that includes the pretreatment liquid, and an image forming method that uses the pretreatment liquid.

The pretreatment liquid of the present invention is a pretreatment liquid that is applied to the surface of a recording medium before the inkjet ink is applied in an image forming method using an inkjet ink, and contains at least a water-insoluble resin and a pigment aggregating agent, and further contains a phosphate ester compound represented by general formula (1) or a salt thereof.

(In formula (1), R is a linear or branched alkyl group having 1 to 8 carbon atoms, and n is 1 or 2.)

The inkjet recording liquid set of the present invention is an inkjet recording liquid set that includes an inkjet ink and a pretreatment liquid that are applied to the surface of a recording medium, and the inkjet ink includes at least a pigment, a pigment dispersant, a water-soluble organic solvent, and water.

The image forming method of the present invention includes a step of applying the above-mentioned pretreatment liquid to the surface of a recording medium, and a step of applying an inkjet ink to the surface of the applied pretreatment liquid to form an image.

According to the present invention, it is possible to provide a pretreatment liquid that has good fixation to low-absorbency recording media or non-absorbency recording media, and is capable of forming images with excellent image quality and excellent water resistance. The present invention also provides an inkjet recording liquid set that includes the pretreatment liquid, and an image forming method that uses the pretreatment liquid.

FIG. 1 is a schematic diagram showing an exemplary configuration of an image forming apparatus according to an embodiment of the present invention.

As a result of intensive research into the above-mentioned problems, the present inventors have found that in an image forming method using an inkjet ink, by providing a pretreatment liquid applied to the surface of a recording medium before the inkjet ink is applied, which contains at least a water-insoluble resin and a coagulant, and further contains a phosphate ester compound represented by general formula (1) or a salt thereof, it is possible to form an image that has good fixability to low-absorbency or non-absorbency recording media, excellent image quality, and excellent water resistance.

Conventionally, when an aqueous ink is applied to the surface of a low-absorbency or non-absorbency recording medium with low polarity, it is difficult to achieve both the wettability and pinning properties of the ink, making it difficult to obtain an image with the desired image quality. In contrast, in the present invention, by using a pretreatment liquid containing a phosphoric acid ester compound represented by general formula (1) or a salt thereof, it is possible to obtain an image with good image quality on a low-absorbency or non-absorbency recording medium.

According to the new findings of the present inventors, the above phosphoric acid ester compound or its salt has a polar group, and when the compound is contained in a pretreatment liquid and applied to the surface of the recording medium, the polar group is oriented toward the surface to which the aqueous ink is applied (the surface opposite the recording medium). This is believed to allow the aqueous ink droplets to be appropriately wetted and spread out. In addition, the above phosphoric acid ester compound or its salt is unlikely to cause unexpected side reactions such as reacting with the pigment aggregating agent contained in the pretreatment liquid and precipitating, so the ink droplets react with the pigment aggregating agent in a state in which they are appropriately wetted and spread out on the surface of the pretreatment liquid and are pinned. This is believed to suppress color bleeding of the aqueous ink droplets, thereby allowing images with good image quality to be obtained.

Furthermore, the hydrophilic phosphate ester compound or its salt can diffuse into both the aqueous ink and the aqueous pretreatment liquid, promoting a crosslinking reaction between ionic sites (carboxy groups) derived from the pigment or pigment dispersant in the coating film (image) and ionic sites (hydrophilic groups) of the water-insoluble resin, thereby improving the water resistance of the coating film (image) that is formed.

The following describes an embodiment of the present invention, but the present invention is not limited to the following embodiment.

1. Pretreatment Liquid The pretreatment liquid according to one embodiment of the present invention contains at least a water-insoluble resin and a flocculant.

1-1. Water-insoluble resin The water-insoluble resin is a resin that is originally water-insoluble but has a form in which the resin disperses in an aqueous medium as microscopic particles, and is a water-insoluble resin that is forcibly emulsified and dispersed in water using an emulsifier or the like, or a water-insoluble resin that can be self-emulsified to form a stable aqueous dispersion by itself without using an emulsifier or dispersion stabilizer by introducing a hydrophilic functional group into the molecule. These resins are usually used in a state of being emulsified and dispersed in water or a water/alcohol mixed solvent. In the present invention, the "water-insoluble resin" refers to a resin that is insoluble in water in the weak acidic or weak basic range, and has a solubility of 0.5% or less in an aqueous solution of pH 4 to 10 (25°C).

The water-insoluble resin according to one embodiment of the present invention is preferably a resin having a composite structure with a core portion and a shell portion (hereinafter, simply referred to as a "composite resin"). In order to adhere an image to a low-absorption or non-absorption recording medium with low polarity, it is necessary to form a coating film with a relatively low polarity. For this purpose, it is preferable to include a resin with low polarity in the pretreatment liquid. However, although a resin with low polarity can impart good adhesion to the recording medium, the wetting and spreading of the droplets of the water-based ink is insufficient. In contrast, when a composite resin having a core-shell structure is used as the resin, the core portions and the shell portions are fused together during the formation of the coating film to form a sea-island structure. In particular, by using a hydrophobic resin for the core portion and a hydrophilic resin for the shell portion, good adhesion to the recording medium can be imparted and the droplets of the water-based ink can be appropriately wetted and spread.

In addition, the composite resin may be made of the same type of resin as the core resin and the shell resin, or may be made of different types of resin.

Examples of the above composite resins include composite resins in which the core portion is an acrylic resin segment and the shell portion is a urethane resin segment, and composite resins in which the core portion and the shell portion are acrylic resin segments.

(Acrylic resin segment)
The acrylic resin segment can be obtained by using a copolymer with an acrylic acid ester component, a methacrylic acid ester component, or a styrene component.

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

Examples of the styrene component include styrene, 4-methylstyrene, 4-hydroxystyrene, 4-acetoxystyrene, 4-acetylstyrene, and styrene sulfonic acid.

These ingredients may be used alone or in combination of two or more.

The number average molecular weight (Mn) of the acrylic resin segment is preferably 1000 to 50000, and more preferably 2000 to 20000. When the number average molecular weight (Mn) of the acrylic resin is 1000 to 50000, the solubility in hydrophilic organic solvents is good, and it is possible to promote the miniaturization of the particle size of the emulsion dispersion. The number average molecular weight (Mn) is a value measured by gel permeation chromatography (GPC), and can be determined, for example, from a calibration curve prepared with a polystyrene standard sample using Shimadzu Corporation's "RID-6A" (column: Tosoh Corporation's "TSK-GEL", solvent: tetrahydrofuran (THF), column temperature: 40°C).

The acrylic resin segment used as the core portion may be one produced by a known method, or a commercially available acrylic resin may be used.

Commercially available examples of the above acrylic resins include acrylic emulsions such as SE-841A (manufactured by Taisei Fine Chemical Co., Ltd.), Delpet 60N, 80N (manufactured by Asahi Kasei Corporation, "Delpet" is a registered trademark of the company), Dianale BR52, BR80, BR83, BR85, BR88 (manufactured by Mitsubishi Chemical Corporation, "Dianale" is a registered trademark of the company), KT75 (manufactured by Denka Co., Ltd.), and Vinyblan 2680, 2682, 2684, 2685 (manufactured by Nissin Chemical Industry Co., Ltd., "Vinyblan" is a registered trademark of the company).

(Urethane resin segment)
The urethane resin segment has a hydrophilic group. By introducing the hydrophilic group, the urethane resin can be imparted with a function of acting as an emulsifier for the acrylic resin segment, and a composite resin that is an emulsified dispersion of the acrylic resin segment can be obtained.

Examples of the hydrophilic group include a carboxy group (-COOH) and its salts, a sulfonic acid group (-SO ₃ H) and its salts, etc. The salts include alkali metal salts such as sodium salts and potassium salts, amine salts, etc. Of the hydrophilic groups, a carboxy group or its salts is preferred.

The urethane resin segment that can be used for the shell portion is preferably an aqueous dispersion of a self-emulsifying polyurethane having a water-soluble functional group in the molecule, or an aqueous dispersion of a forced-emulsifying polyurethane emulsified under strong mechanical shear force in combination with a surfactant. The polyurethane resin in the aqueous dispersion can be obtained by reacting a polyol with an organic polyisocyanate and a hydrophilic group-containing compound.

Examples of the above polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyolefin polyols, etc.

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

Examples of the above polyether polyols include polyethylene glycol, polypropylene glycol, polyethylene polytetramethylene glycol, polypropylene polytetramethylene glycol, polytetramethylene glycol, etc.

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

Examples of the organic polyisocyanates include aromatic isocyanates such as tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), polymeric MDI, xylylene diisocyanate (XDI), and tetramethylxylylene diisocyanate (TMXDI), aliphatic isocyanates such as hexamethylene diisocyanate (HMDI), and alicyclic isocyanates such as isophorone diisocyanate (IPDI) and 4,4'-dicyclohexylmethane diisocyanate (hydrogenated MDI, H12MDI). These may be used alone or in combination of two or more types.

Examples of the hydrophilic group-containing compound include carboxylic acid-containing compounds such as 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolbutyric acid, 2,2-dimethylolvaleric acid, and glycine, as well as their sodium, potassium, and amine salt derivatives, and sulfonic acid-containing compounds such as taurine (i.e., aminoethylsulfonic acid) and ethoxypolyethylene glycol sulfonic acid, as well as their sodium, potassium, and amine salt derivatives.

The urethane resin segment can be obtained by a known method. For example, the above-mentioned polyol, polyisocyanate, and hydrophilic group-containing compound are mixed and reacted at 30 to 130°C for 30 minutes to 50 hours to obtain a urethane prepolymer.

The urethane prepolymer is polymerized by extending the chain with a chain extender to become a polyurethane resin having hydrophilic groups. The chain extender is preferably water and/or an amine compound. By using water or an amine compound as the chain extender, it is possible to react with free isocyanate in a short time and efficiently extend the isocyanate-terminated prepolymer.

Examples of amine compounds as chain extenders include aliphatic polyamines such as ethylenediamine and triethylenediamine, aromatic polyamines such as metaxylenediamine and toluylenediamine, and polyhydrazino compounds such as hydrazine and adipic acid dihydrazide. The amine compounds may contain, in addition to the polyamines, monovalent amines such as dibutylamine or methyl ethyl ketoxime as a reaction terminator to the extent that the polymerization is not significantly inhibited.

In addition, in the synthesis of the urethane prepolymer, a solvent that is inactive with isocyanates and capable of dissolving the urethane prepolymer may be used. Examples of such solvents include dioxane, methyl ethyl ketone, dimethylformamide, tetrahydrofuran, N-methyl-2-pyrrolidone, toluene, propylene glycol monomethyl ether acetate, etc. These hydrophilic organic solvents used in the reaction stage are preferably finally removed.

In addition, in the synthesis of the urethane prepolymer, a catalyst such as an amine catalyst (e.g., triethylamine, N-ethylmorpholine, triethyldiamine, etc.), a tin-based catalyst (e.g., dibutyltin dilaurate, dioctyltin dilaurate, tin octoate, etc.), or a titanium-based catalyst (e.g., tetrabutyl titanate, etc.) may be added to promote the reaction.

The composite resin in which the core portion is an acrylic resin segment and the shell portion is a urethane resin segment may be prepared by a method in which the acrylic resin is emulsified in water using the hydrophilic urethane prepolymer, and an amine or an aqueous solution thereof is added as a chain extender to extend the chain of the urethane prepolymer (to increase the molecular weight), or by a method in which the hydrophilic urethane prepolymer is emulsified in water, and an amine or an aqueous solution thereof is added as a chain extender to extend the chain of the urethane prepolymer to prepare an aqueous dispersion of the urethane resin, and the acrylic resin is emulsified in the aqueous dispersion of the urethane resin.

The composite resin in which the core and shell portions are acrylic resins may also be prepared by a method of emulsifying an acrylic resin having hydrophobic groups with an aqueous dispersion of an acrylic resin having hydrophilic groups.

In addition, the above-mentioned composite resin may be a commercially available product. Examples of the above-mentioned commercially available products include WEM-202U, WEM-202C, WEM-3000, UW-319SX, and UW-600 (all manufactured by Taisei Fine Chemical Co., Ltd.).

The composite resin according to one embodiment of the present invention can form a sea-island structure by drying or heating the pretreatment liquid applied to the surface of the recording medium. Here, the "sea-island structure" refers to a structure in which discontinuous phases (island phases) that form an interface with the continuous phase (sea phase) are dispersed in the continuous phase (sea phase).

In the present invention, the composite resin in which the core portion is an acrylic resin segment and the shell portion is a urethane resin segment can form an island phase in which the urethane resin segment forms a sea phase and the acrylic resin segment is scattered in an island shape. Since the acrylic resin segment in the core portion is a hydrophobic resin, it has good adhesion to non-absorbent recording media such as polypropylene (PP) and polyethylene terephthalate (PET) having low polarity. In addition, since the urethane resin segment in the shell portion is a hydrophilic resin, the surface of the non-absorbent recording medium having low polarity becomes more hydrophilic, so that the droplets of the applied aqueous ink can be appropriately wetted and spread. This makes it possible to form an image with good image quality. In this way, by using a composite resin containing a hydrophilic resin and a hydrophobic resin, it becomes possible to provide an image that satisfies both good substrate adhesion and image quality for non-absorbent recording media.

In the present invention, even in a composite resin in which both the core and shell portions are acrylic resin segments, the above-mentioned sea-island structure is formed by using, for example, an acrylic resin segment having a hydrophobic group and an acrylic resin segment having a hydrophilic group, making it possible to provide an image that satisfies both good fixation properties and image quality for non-absorbent recording media.

The content of the composite resin is preferably 1.0% by mass or more and 30.0% by mass or less, and more preferably 5.0% by mass or more and 20.0% by mass or less, based on the total mass of the pretreatment liquid. By making the content 1.0% by mass or more, it is possible to form an image that has good fixability to non-absorbent recording media and good image quality. In addition, by making the content 30.0% by mass or less, the storage stability of the pretreatment liquid is improved.

In addition, in the pretreatment liquid according to one embodiment of the present invention, in addition to the composite resins described above, acrylic resins, urethane resins, polyester resins, and olefin resins can be used as the water-insoluble resin.

The acrylic resin may be the acrylic resin segment described above.

(urethane resin)
The urethane resin may be an aqueous dispersion of a self-emulsifying polyurethane having a water-soluble functional group in the molecule, or an aqueous dispersion of a forced-emulsifying polyurethane emulsified under strong mechanical shearing force in combination with a surfactant. The polyurethane resin in the aqueous dispersion may be obtained by reacting a polyol with an organic polyisocyanate and a hydrophilic group-containing compound.

The molecular weight of the above urethane resin is preferably as large as possible by introducing a branched structure or an internal crosslinked structure, and is preferably 50,000 to 10,000,000. The molecular weight can be determined in the same manner as the number average molecular weight (Mn) described above.

The urethane resin may be a commercially available product. Examples of commercially available urethane resins include WBR-016U (manufactured by Taisei Fine Chemical Co., Ltd.), Superflex 620, Superflex 650, Superflex 500M, Superflex E-2000 (all manufactured by Daiichi Kogyo Seiyaku Co., Ltd., "Superflex" is a registered trademark of the company), Permarine UC-20 (manufactured by Sanyo Chemical Industries, Ltd., "Permarine" is a registered trademark of the company), and Parasurf UP-22 (manufactured by Ohara Palladium Chemical Co., Ltd.).

(Polyester resin)
The polyester resin can be obtained by a known method using a polyhydric alcohol component and a polycarboxylic acid component such as a polycarboxylic acid, a polycarboxylic anhydride, or a polycarboxylic ester.

The number average molecular weight (Mn) of the polyester resin is preferably 1,000 to 50,000, and more preferably 2,000 to 20,000. The number average molecular weight (Mn) can be determined in the same manner as the method for determining the number average molecular weight (Mn) described above.

As the polyester resin, commercially available products may be used. Examples of commercially available polyester resins include ELITEL KA-5034, ELITEL KA-5071S, ELITEL KA-1449, ELITEL KA-0134, ELITEL KA-3556, ELITEL KA-6137, ELITEL KZA-6034, ELITEL KT-8803, ELITEL KT-8701, ELITEL KT-9204, ELITEL KT-8904, ELITEL KT-0507, and ELITEL KT-9511. These may be used alone or in combination of two or more types. All of the commercially available products are manufactured by Unitika Ltd., and "ELITEL" is a registered trademark of the company.

(Olefin resin)
Examples of the olefin resin that can be used include polyethylene, polypropylene, ethylene-propylene copolymers, random copolymers or block copolymers (e.g., ethylene-propylene-butene copolymers) of ethylene and/or propylene with other comonomers (e.g., 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, and α-olefin comonomers having 2 to 6 carbon atoms). Also usable are copolymers of two or more of the above other comonomers, and mixtures of two or more of the above polymers.

The above olefin resins may be produced by known methods, and there are no particular limitations on the production method or degree of modification.

The olefin resin preferably has a weight average molecular weight (Mw) of 20,000 to 100,000. The weight average molecular weight (Mw) can be determined using the measuring device described above.

The olefin resin may be a commercially available product. Examples of commercially available olefin resins include Arrowbase SB-1200 (manufactured by Unitika Ltd., "Arrowbase" is a registered trademark of the company), Auroren 150A, Auroren AE-301 (manufactured by Nippon Paper Industries Co., Ltd., "Auroren" is a registered trademark of the company), Superclone E-415 (manufactured by Nippon Paper Industries Co., Ltd., "Superclone" is a registered trademark of the company), and Hardlen Na-1001 (manufactured by Toyobo Co., Ltd., "Hardlen" is a registered trademark of the company).

Among the water-insoluble resins other than the above composite resins, urethane resins are preferred from the viewpoint of improving image quality, and acrylic resins, polyester resins, and polyolefin resins are preferred from the viewpoint of adhesion to low-absorbency recording media or non-absorbency recording media.

The content of water-insoluble resins other than the above composite resins is preferably 1.0% by mass or more and 30.0% by mass or less, and more preferably 2.0% by mass or more and 20.0% by mass or less, based on the total mass of the pretreatment liquid.

1-2. Pigment flocculant Examples of pigment flocculants include polyvalent metal salts, organic acids, and cationic polymers. The pretreatment liquid preferably contains at least one of an organic acid and a polyvalent metal salt as the pigment flocculant.

(Polyvalent metal salt)
Examples of the polyvalent metal salt include water-soluble salts such as calcium salts, magnesium salts, aluminum salts, and zinc salts. The polyvalent metal salt can aggregate anionic components (such as anionic resin emulsions) in the water-based inkjet ink by salting out.

(Organic Acid)
Examples of organic acids include formic acid, acetic acid, propionic acid, isobutyric acid, oxalic acid, fumaric acid, glutaric acid, malic acid, citric acid, malonic acid, succinic acid, maleic acid, benzoic acid, 2-pyrrolidone-5-carboxylic acid, lactic acid, acrylic acid and its derivatives, methacrylic acid and its derivatives, and compounds having a carboxy group including acrylamide and its derivatives, sulfonic acid derivatives, and phosphoric acid and its derivatives. Organic acids can aggregate pigments contained in inkjet inks. Organic acids can also aggregate anionic components (such as anionic resin emulsions) in water-based inkjet inks by varying the pH.

(Cationic Polymer)
Examples of the cationic polymer include diallyldimethylammonium chloride polymer, polyallylamine polymer, polyvinylamine polymer, polyethyleneimine polymer, etc. The cationic polymer can aggregate anionic components (such as anionic resin emulsion) in the inkjet ink.

By including the pigment aggregating agent in the pretreatment liquid, the coloring material components in the inkjet ink applied to the surface of the pretreatment liquid can be aggregated, thereby suppressing bleeding of the formed image. In particular, since the polyvalent metal salt and organic acid have low molecular weights and are easily diffused in the aqueous inkjet ink, the coloring material components in the inkjet ink can be aggregated more quickly. Therefore, the inkjet ink of the present invention can improve the pinning property of the inkjet ink even when printing at high speed, so that images with good image quality can be obtained.

The content of the pigment aggregating agent is preferably 0.5% by mass or more and 5.0% by mass or less, and more preferably 1.0% by mass or more and 3.0% by mass or less, relative to the total mass of the pretreatment liquid. When the content of the pigment aggregating agent is 0.5% by mass or more, relative to the total mass of the pretreatment liquid, it is possible to promote pigment aggregation in the ink droplets on the pretreatment liquid, thereby improving image quality. When an organic acid is used as the pigment aggregating agent, the content is preferably an amount that allows the pH of the pretreatment liquid to be adjusted to less than the first dissociation constant of the organic acid (e.g., 3.5 or less).

The content of the pigment flocculant can be measured by a known method. For example, when the pigment flocculant is a polyvalent metal salt, it can be measured by ICP emission spectrometry, when the pigment flocculant is an organic acid, it can be measured by high performance liquid chromatography (HPLC), and when the pigment flocculant is a cationic polymer, it can be measured by gel permeation chromatography (GPC).

1-3. Phosphate Compound or Salt Thereof The pretreatment liquid according to one embodiment of the present invention contains a phosphate compound represented by general formula (1) or a salt thereof.

(In formula (1), R is a linear or branched alkyl group having 1 to 8 carbon atoms, and n is 1 or 2.)

Examples of the phosphate ester compound represented by the above general formula (1) or its salt include methyl acid phosphate, ethyl acid phosphate, n-propyl acid phosphate, n-butyl acid phosphate, 2-ethylhexyl acid phosphate, n-octyl acid phosphate, and salts thereof.

The above phosphate ester compounds or salts thereof may be synthesized using known techniques, or commercially available products may be used. Commercially available examples of the phosphate esters represented by the above general formula (1) and their salts include Phoskex A-1, A-2, A-3, A-4, A-8, Phospair-16, 37, and 41 (all manufactured by SC Organic Chemical Co., Ltd.).

Moreover, the above R is more preferably a linear or branched alkyl group having 1 to 4 carbon atoms, and even more preferably a linear or branched alkyl group having 1 to 3 carbon atoms. When R has 1 to 4 carbon atoms, the molecular weight of the phosphate ester compound or its salt is small, so that the phosphate ester compound can instantly diffuse into the ink when the ink is applied. This can further enhance the effect of improving the water resistance of the coating film by the phosphate ester compound acting as a crosslinking accelerator. Also, when R has 1 to 4 carbon atoms, the storage stability of the pretreatment liquid can be further improved.

Furthermore, the above phosphate ester compound or its salt is oriented with its polar group (hydroxy group) facing the surface of the coating film (the surface that comes into contact with the inkjet ink) during coating film formation, improving the wetting and spreading of the aqueous inkjet ink droplets applied to that surface. In particular, the above phosphate ester compound or its salt, which is a linear or branched alkyl group having 1 to 4 carbon atoms, has a low molecular weight and can instantly diffuse into the inkjet ink droplets when they land. This improves the wettability, making it possible to obtain images with good image quality.

The content of the phosphoric acid ester compound or its salt is preferably 0.1% by mass or more and 5.0% by mass or less, more preferably 0.5% by mass or more and 4.0% by mass or less, and even more preferably 1.0% by mass or more and 3.0% by mass or less, based on the total mass of the pretreatment liquid. When the content of the phosphoric acid ester compound or its salt is 0.1% by mass or more and 5.0% by mass or less, based on the total mass of the pretreatment liquid, the progression of unexpected side reactions such as precipitation due to reaction with water-insoluble resins or coagulants in the pretreatment liquid can be suppressed. This improves the storage stability of the pretreatment liquid.

The pretreatment liquid according to an embodiment of the present invention preferably contains a surfactant. Examples of the surfactant include anionic surfactants, cationic surfactants, nonionic surfactants, betaine surfactants, silicone-based surfactants, fluorine-based surfactants, and acetylene glycol-based surfactants.

The pretreatment liquid according to an embodiment of the present invention preferably contains a water-soluble organic solvent. The water-soluble organic solvent more preferably contains glycols and alkanediols.

(Glycols)
Examples of glycols include ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tripropylene glycol, tetraethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, neopentyl glycol, glycerin, trimethylolethane, trimethylolpropane, tributyrolpropane, pentaerythritol, and polyfunctional glycols including sorbitol.

(Alkanediols)
Examples of alkanediols include 1,2-butanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,5-pentanediol, 2-methyl-2,4-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,2-hexanediol, 1,8-octanediol, 1,2-octanediol, and 1,9-nonanediol. Examples of such hexanediol include 3-methyl-1,5-pentanediol, 2-methyl-1,3-propanediol, 2,2-dimethyl-1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, 2,2-dibutyl-1,3-propanediol, 2-methyl-2,4-pentanediol, 2-methyl-1,8-octanediol, 1,4-cyclohexanediol, and 1,4-cyclohexanedimethanol.

Also, the organic solvent may include ethanol, propanol, isopropanol, butanol, 2-butanol, tert-butanol, isobutanol, pentanol, isopentanol, hexanol, isohexanol, octanol, isooctanol, 2-ethylhexyl alcohol, nonyl alcohol, isononyl alcohol, tert-nonyl alcohol, decanol, dodecanol, dodecahexanol, dodecaoctanol, allyl alcohol, oleyl alcohol, 1-methoxy-1-propanol, 3-methoxy-1-butanol, cyclohexyl alcohol, benzyl alcohol, 3-phenylpropanol, diethylene glycol, triethylene glycol, tripropylene glycol, tetraethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, neopentyl glycol, glycerin, trimethylolethane, trimethylolpropane, tributyrolpropane, pentaerythritol, and polyfunctional glycols including sorbitol, such as methanol.

The water-soluble organic solvents may be used alone or in combination of two or more.

The boiling point of the water-soluble organic solvent is preferably 250°C or less, and more preferably 230°C or less. If the boiling point of the water-soluble organic solvent is 250°C or less, it dries quickly and is therefore suitable for high-speed printing.

The content of the water-soluble organic solvent is preferably 10.0% by mass or more and 40.0% by mass or less, and more preferably 10.0% by mass or more and 30.0% by mass or less, based on the total mass of the pretreatment liquid. By having the content of the water-soluble organic solvent be 10.0% by mass or more and 40.0% by mass or less, based on the total mass of the pretreatment liquid, the wettability of the pretreatment liquid to a low-absorbency recording medium or a non-absorbency recording medium can be improved.

1-6. Water and other components The pretreatment liquid according to one embodiment of the present invention contains water. The water is not particularly limited, and may be ion-exchanged water, distilled water, or pure water. In addition, the pretreatment liquid may further contain other ingredients such as a crosslinking agent, a fungicide, and a bactericide, as long as the effects of the present invention are not impaired.

2. Ink-jet ink The ink-jet ink according to one embodiment of the present invention preferably contains at least a crosslinking agent, a pigment, and water.

2-1. Pigment The pigment can be selected from, for example, a yellow pigment, a red or magenta pigment, a blue or cyan pigment, a green pigment, a black pigment, and a white pigment, depending on the color of the image to be formed.

Pigments that can be used in the present invention include known organic and inorganic pigments. Examples of organic and inorganic pigments include azo pigments such as azo lakes, insoluble azo pigments, condensed azo pigments, and chelate azo pigments; polycyclic pigments such as phthalocyanine pigments, perylene and perylene pigments, anthraquinone pigments, quinacridone pigments, dioxandine pigments, thioindigo pigments, isoindolinone pigments, and quinophthaloni pigments; dye lakes such as basic dye lakes and acid dye lakes; organic pigments such as nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments; and inorganic pigments such as carbon black.

Further examples of organic and inorganic pigments include the following pigments listed in the Color Index:

Examples of red or magenta pigments include Pigment Red 3, 5, 19, 22, 31, 38, 43, 48:1, 48:2, 48:3, 48:4, 48:5, 49:1, 53:1, 57:1, 57:2, 58:4, 63:1, 81, 81:1, 81:2, 81:3, 81:4, 88, 104, 108, 112, 122, 123, 144, 146, 149, 166, 168, 169, 170, 177, 178, 179, 184, 185, 208, 216, 226, 257, Pigment Violet 3, 19, 23, 29, 30, 37, 50, and 88, and Pigment Orange 13, 16, 20, and 36.

Examples of blue or cyan pigments include Pigment Blue 1, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 17-1, 22, 27, 28, 29, 36, and 60.

Examples of green pigments include Pigment Green 7, 26, 36, and 50.

Examples of yellow pigments include Pigment Yellow 1, 3, 12, 13, 14, 17, 34, 35, 37, 55, 74, 81, 83, 93, 94, 95, 97, 108, 109, 110, 137, 138, 139, 153, 154, 155, 157, 166, 167, 168, 180, 185, and 193.

Examples of black pigments include Pigment Black 7, 26, and 28.

Examples of white pigments include calcium carbonate, barium sulfate, titanium dioxide (rutile, anatase, brookite), zinc oxide, zinc sulfide, antimony oxide, zirconium oxide, white hollow resin particles, and polymer particles.

In addition, a pigment that can be self-dispersed in water (self-dispersing pigment) can be used as the pigment. A self-dispersing pigment is a pigment that has at least sulfonic acid, sulfonate, carboxylic acid, or carboxylate as a functional group on its surface, and is dispersible in water even in the absence of a dispersant.

Commercially available products can be used as the self-dispersing pigment. Commercially available examples of the self-dispersing pigment include CAB-O-JET 200, 300, and 400 (manufactured by Cabot Corporation; "CAB-O-JET" is a registered trademark of the company), BONJET BLACK CW-1, CW-1S, and CW-2 (manufactured by Orient Chemical Industry Co., Ltd.; "BONJET" is a registered trademark of the company), and the like.

The content of the above yellow pigment, red or magenta pigment, blue or cyan pigment, green pigment, and black pigment is preferably 0.1% by mass or more and 15.0% by mass or less, more preferably 0.1% by mass or more and less than 10.0% by mass, and even more preferably 0.1% by mass or more and 8.0% by mass or less, based on the total mass of the inkjet ink. When the content of the above pigments is 0.1% by mass or more, based on the total mass of the inkjet ink, the color development of the obtained image is sufficient. When the content of the pigments is 15.0% by mass or less, based on the total mass of the ink, the viscosity of the ink does not increase too much, and it can be stably ejected onto a recording medium.

The content of the white pigment is preferably 2.0% by mass or more and 30.0% by mass or less, more preferably 5.0% by mass or more and less than 20.0% by mass, and even more preferably 6.0% by mass or more and 15.0% by mass or less, relative to the total mass of the inkjet ink. By making the white pigment 5% by mass or more, a sufficiently white cured film (image) can be obtained. On the other hand, by making the white pigment 30% by mass or less, the inkjet ink can be stably ejected and an image with sufficient concealment can be formed.

The ink-jet ink according to an embodiment of the present invention preferably contains a pigment dispersant. By containing a pigment dispersant, adhesion to low-absorbent and non-absorbent recording media is improved.

Examples of pigment dispersants include polymeric dispersants.

Examples of polymeric dispersants include block copolymers and random copolymers consisting of two or more monomers selected from styrene, styrene derivatives, vinylnaphthalene derivatives, acrylic acid, acrylic acid derivatives, maleic acid, maleic acid derivatives, itaconic acid, itaconic acid derivatives, fumaric acid, and fumaric acid derivatives, as well as salts thereof, polyoxyalkylenes, and polyoxyalkylene alkyl ethers.

The pigment may be further made more dispersible by the addition of a dispersing aid, if necessary.

The pigment may also be dispersed using a copolymer resin.

Examples of copolymer resins include water-soluble resins such as styrene-acrylic acid-acrylic acid alkyl ester copolymer, styrene-acrylic acid copolymer, styrene-maleic acid copolymer, styrene-maleic acid-acrylic acid alkyl ester copolymer, styrene-methacrylic acid copolymer, styrene-methacrylic acid-acrylic acid alkyl ester copolymer, styrene-maleic acid half ester copolymer, vinylnaphthalene-acrylic acid copolymer, vinylnaphthalene-maleic acid copolymer, etc.

The content of the pigment dispersant is preferably 10.0% by mass or more and 200.0% by mass or less based on the total mass of the pigment. If the content of the dispersant is 10.0% by mass or more based on the total mass of the pigment, the dispersion stability of the pigment is increased, and if the content of the dispersant is 200.0% by mass or less based on the total mass of the pigment, the ejection of the ink from the inkjet head is more likely to be stable.

In addition, it is preferable to disperse the above-mentioned pigments using a dispersing machine together with the above-mentioned pigment dispersant and other additives required for the desired purpose.

Examples of dispersing machines include conventionally known ball mills, sand mills, line mills, high-pressure homogenizers, etc. Among the above dispersing machines, dispersion using a sand mill is preferred from the viewpoint of achieving a sharp particle size distribution of the inkjet ink produced by dispersion.

The beads used for dispersion using a sand mill are preferably made of zirconia or zircon with a bead diameter of 0.3 to 3 mm, in order to prevent contamination by bead fragments and ionic components.

The particle size of the dispersed pigment can be measured using a commercially available particle size measuring device that uses dynamic light scattering, electrophoresis, etc.

The ink-jet ink according to an embodiment of the present invention preferably contains an organic solvent. As the organic solvent, glycols and alkanediols that can be used in the pretreatment liquid described above can be used.

The inkjet ink according to an embodiment of the present invention may contain a surfactant. Examples of the surfactant include anionic surfactants other than phosphate ester compounds, cationic surfactants, nonionic surfactants, betaine surfactants, silicone-based surfactants, fluorine-based surfactants, and acetylene glycol-based surfactants.

The surfactant is preferably present in an amount of 0.001% by mass or more and 5.0% by mass or less, and more preferably 0.001% by mass or more and 1.0% by mass or less, relative to the total mass of the aqueous inkjet ink.

2-5. Other Components The inkjet ink may contain pH adjusters, oil droplet fine particles, UV absorbers, anti-fading agents, fluorescent brighteners, polysaccharides, viscosity adjusters, resistivity adjusters, film-forming agents, UV absorbers, antioxidants, antifungal agents, rust inhibitors, and the like. These components may be used alone or in combination of two or more. The water contained in the inkjet ink is not particularly limited, and may be ion-exchanged water, distilled water, or pure water.

In addition, the inkjet ink according to one embodiment of the present invention may contain, as an additive, a phosphate ester compound represented by general formula (1) or a salt thereof contained in the pretreatment liquid described above.

(In formula (1), R is a linear or branched alkyl group having 1 to 8 carbon atoms, and n is 1 or 2.)

The inkjet ink according to one embodiment of the present invention may also contain a crosslinking agent. The type of crosslinking agent is not particularly limited as long as it can be used in aqueous inkjet inks, but it is preferable that the crosslinking agent is selected from the group consisting of epoxy compounds, vinyl ether compounds, oxazoline compounds, and carbodiimide compounds. The crosslinking agent may be synthesized using known techniques, or a commercially available product may be used.

3. Recording medium In one embodiment of the present invention, the type of recording medium that can be used is not limited, and can be an absorbent recording medium, a low absorbent recording medium, or a non-absorbent recording medium. In the present invention, a low absorbent recording medium or a non-absorbent recording medium is preferable.

Examples of the above absorbent recording media include plain paper ranging from thin to thick paper, medium-quality paper, fine paper, and recycled paper.

In the present invention, the low-absorbency recording medium or non-absorbency recording medium is defined based on the measurement results of the wettability of the recording medium surface with water shown below.

That is, a 0.5 μL drop of water is dropped onto the recording surface of the recording medium, and the rate of decrease in the contact angle (comparison of the contact angle 0.5 milliseconds after impact with the contact angle 5 seconds after impact) is measured to characterize the absorption performance of the recording medium. More specifically, in terms of the nature of the recording medium, a non-absorbent recording medium is a recording medium having a contact angle decrease rate of less than 1.0%, and a low-absorbent recording medium is a recording medium having a contact angle decrease rate of 1.0% or more and less than 5.0%. Absorbency is defined as a recording medium having a contact angle decrease rate of 5.0% or more. The contact angle can be measured, for example, using a portable contact angle meter "PCA-1" (manufactured by Kyowa Interface Science Co., Ltd.).

Examples of the low absorbency recording medium include coated printing papers such as art paper, coated paper, and cast coated paper.

Another example of a non-absorbent recording medium is film.

The above-mentioned films include known plastic films. Examples of the above-mentioned plastic films include polyester (PET) films, polyethylene (PE) films, polypropylene (PP) films, nylon (NY) films, polystyrene (PS) films, ethylene-vinyl acetate copolymer (EVA) films, polyvinyl chloride (PVC) films, polyvinyl alcohol (PVA) films, polyacrylic acid (PAA) films, polycarbonate films, polyacrylonitrile films, and biodegradable films such as polylactic acid films.

To impart gas barrier properties, moisture resistance, and aroma retention, one or both sides of the film may be coated with polyvinylidene chloride or a metal oxide may be vapor-deposited. The film may also be anti-fogging. The film may also be corona discharged, ozone treated, etc.

The film may be an unstretched film or a stretched film, or it may be a multi-layered recording medium in which a layer such as a PVA coating is provided on the surface of an absorbent recording medium such as paper, making the area to be recorded non-absorbent.

4. Image Forming Method The image forming method according to one embodiment of the present invention is an image forming method using an inkjet recording liquid set including the inkjet ink and the pretreatment liquid described above. Specifically, the image forming method includes a step of applying the pretreatment liquid to a surface of a low-absorbency recording medium or a non-absorbency recording medium (hereinafter, also simply referred to as a "pretreatment liquid applying step"), and a step of applying an inkjet ink to the surface of the applied pretreatment liquid (hereinafter, also simply referred to as an "ink applying step").

4-1. Pretreatment Liquid Application Step The pretreatment liquid application step is a step of applying a pretreatment liquid to the surface of a low-absorbency recording medium or a non-absorbency recording medium.

The above pretreatment liquid is the pretreatment liquid described above.

The method of applying the pretreatment liquid to the low-absorbency recording medium or non-absorbency recording medium is not particularly limited, but examples include roller coating, curtain coating, spray coating, and inkjet methods. Among these, the roller coating method is preferred from the viewpoint of efficient application even when the viscosity is relatively high. In the roller coating method, a roller coater or the like can be connected to an inkjet device.

The thickness of the pretreatment liquid applied to the surface of the low-absorbency recording medium or non-absorbency recording medium is preferably 0.3 μm or more and 3.0 μm or less, and more preferably 0.5 μm or more and 2.0 μm or less. When the thickness of the pretreatment liquid is 0.3 μm or more, it is easy to increase the adhesion of the image and the lamination strength while suppressing ink bleeding. When the thickness of the pretreatment liquid is 3.0 μm or less, the deformation stress due to moisture and heat can be reduced, so that the adhesion of the image and the lamination strength are less likely to be impaired.

4-2. Ink Application Step The ink application step is a step of applying the inkjet ink of the inkjet recording liquid set by an inkjet method to the surface to which the pretreatment liquid has been applied.

The inkjet ink is the inkjet ink described above.

The pigment may be any known organic or inorganic pigment. It may also be a pigment that is self-dispersible in water (self-dispersing pigment).

The inkjet ink may also contain a phosphate ester compound represented by the above general formula (1) or a salt thereof.

The inkjet ink may also contain a crosslinking agent. There are no particular limitations on the crosslinking agent as long as it can be used in aqueous inkjet inks, but it is preferable that the crosslinking agent is selected from the group consisting of epoxy compounds, vinyl ether compounds, oxazoline compounds, and carbodiimide compounds.

The inkjet method is not particularly limited, and a printer equipped with an inkjet head loaded with ink can be used. Specifically, ink is ejected as droplets from the nozzles of the inkjet head based on a digital signal, and these droplets are caused to land on the surface of a pretreatment liquid applied to the surface of a low-absorbency recording medium or a non-absorbency recording medium, thereby performing printing.

The inkjet head may be either an on-demand type or a continuous type. Examples of on-demand type inkjet heads include electro-mechanical conversion types, including single cavity type, double cavity type, bender type, piston type, shear mode type, and shared wall type, as well as electro-thermal conversion types, including thermal inkjet type and bubble jet type ("Bubble Jet" is a registered trademark of Canon Inc.).

Among the above inkjet heads, it is preferable to use an inkjet head that uses a piezoelectric element as the electromechanical conversion element used in the electromechanical conversion method (also called a piezo-type inkjet head).

The inkjet head may be either a scanning type or a line type inkjet head, but a line type is preferable.

A line-type inkjet head is an inkjet head whose length is equal to or greater than the width of the printing range. A line-type inkjet head may be one that is equal to or greater than the width of the printing range, or multiple heads may be combined to form a head that is equal to or greater than the width of the printing range.

In addition, multiple heads can be arranged side by side with their nozzles in a staggered arrangement to increase the overall resolution of the heads.

The transport speed of the recording medium can be set, for example, between 1 and 120 m/min. The faster the transport speed, the faster the image formation speed. According to the present invention, even at a very high linear speed of 50 to 120 m/min, which is applicable to a single-pass inkjet image formation method, it is possible to obtain an image with high ink fixation and good image quality.

The image forming method according to one embodiment of the present invention may include a step of drying the pretreatment liquid before the ink application step.

It is preferable to dry the pretreatment liquid under conditions that remove the solvent components of the pretreatment liquid, such as water and water-soluble organic solvents, while preventing the resin particles contained in the pretreatment liquid from completely fusing. The drying temperature of the pretreatment liquid may be, for example, 50°C or higher and 100°C or lower. The drying time of the pretreatment liquid may be, for example, 3 seconds or higher and 30 seconds or lower.

The pretreatment liquid may be dried using a non-contact heating drying device such as a drying oven or a hot air blower, or a contact heating drying device such as a hot plate or a heated roller.

The drying temperature can be obtained by measuring one of the following throughout the entire drying period of the pretreatment liquid: (a) when a non-contact heating drying device such as a drying oven or hot air blower is used, the ambient temperature such as the temperature inside the oven or the hot air temperature; (b) when a contact heating drying device such as a hot plate or hot roller is used, the temperature of the contact heating part; or (c) the surface temperature of the surface to be dried. It is more preferable to measure the surface temperature of the surface to be dried (c).

The image forming method according to one embodiment of the present invention may also include a step of drying the ink applied to the surface of the pretreatment liquid after the ink application step.

The ink is dried to remove the main solvent components of the ink, such as water and organic solvents. The ink drying temperature can be, for example, 50°C or higher and 100°C or lower. The ink drying time can be, for example, 3 seconds or higher and 30 seconds or lower.

The ink can be dried in the same manner as the drying of the pretreatment liquid described above. The drying temperature of the ink can also be measured in the same manner as the drying temperature of the pretreatment liquid described above.

The image forming method according to one embodiment of the present invention can be performed on film stored in a roll.

The carboxylic acid and phosphate ester compounds remaining in the coating film (image) formed using the image forming method according to one embodiment of the present invention can be confirmed, for example, by X-ray photoelectron spectroscopy (XPS).

The image formed by the image forming method according to one embodiment of the present invention has good water resistance because the use of a pretreatment liquid containing a phosphate ester compound represented by general formula (1) or a salt thereof promotes a crosslinking reaction between ionic sites of pigment particles, pigment dispersants, etc. contained in the inkjet ink and ionic sites of a water-insoluble resin contained in the pretreatment liquid.

1, the image forming apparatus 100 according to the present invention has a head carriage 120 having an inkjet head that ejects droplets of the inkjet ink of the inkjet recording liquid set and causes the ink to land in an area on a recording medium 110, and a pretreatment liquid application unit 130 for applying a pretreatment liquid to the recording medium 110. The image forming apparatus 100 may further have a dryer 140 that dries the pretreatment liquid applied to the surface of the recording medium 110, and a dryer 150 that dries the inkjet ink applied to the surface of the pretreatment liquid.

In FIG. 1, the pretreatment liquid application unit 130 and the head carriage 120 are arranged in this order from the upstream side along the transport direction of the recording medium 110 (the direction of the arrow in the figure), but the arrangement order of these is not limited to this order and can be set arbitrarily.

However, it is preferable to arrange the pretreatment liquid application unit 130 and the head carriage 120 in this order.

The head carriage 120 is equipped with, for example, four inkjet heads 121, and each inkjet head 121 has nozzles 122 from which yellow, magenta, cyan and black ink is ejected to land droplets of inkjet ink on the desired areas of the recording medium 110.

The pretreatment liquid application unit 130 may be configured to apply the pretreatment liquid to an area larger than the area on the recording medium 110 where the droplets of the inkjet ink land. For example, the pretreatment liquid application unit 130 may include a dispenser 132 that supplies the pretreatment liquid to the application roller 131, and the application roller 131 that applies the supplied pretreatment liquid in the form of a film.

The configuration of the pretreatment liquid application unit 130 is not limited to this, and may be configured to eject pretreatment liquid from the inkjet head 121 and land it on the recording medium 110.

The dryers 140 and 150 can be known dryers such as a hot air dryer that blows hot air and an irradiator that irradiates infrared rays or ionizing radiation. The dryer 140 is provided downstream of the pretreatment liquid application unit 130 and upstream of the head carriage 120, and dries the pretreatment liquid before the ejection of droplets of the inkjet ink. The dryer 150 is provided downstream of the head carriage 120, and dries the inkjet ink after the ejection of droplets of the inkjet ink.

The present invention will be explained in more detail using the following tests, but the present invention is not limited to the following tests.

1. Preparation of Inkjet Ink Ink was prepared as follows.

1-1. Preparation of Pigment Dispersion A mixture of 18.0 parts by mass of pigment (Pigment Blue 15:3), 31.5 parts by mass of pigment dispersant (acrylic dispersant having a carboxy group neutralized with sodium hydroxide ("Joncryl 819" manufactured by BASF Corporation, acid value 75 mg KOH/g, solid content 20 mass%), 20.0 parts by mass of ethylene glycol, and 30.5 parts by mass of ion-exchanged water was premixed, and then dispersed for 6 hours using a sand grinder filled with 0.5 mm zirconia beads at a volume ratio of 50%, to obtain a pigment dispersion with a pigment solid content of 18.0%.

1-2. Preparation of Inkjet Ink 29.8 parts by mass of the above pigment dispersion was stirred while adding the following additives in order to prepare an ink composition, which was then filtered through a 0.8 μm filter to obtain an inkjet ink. There was no substantial change in the composition before and after filtration.
Pigment dispersion 29.8 parts by weight Ethylene glycol 15.0 parts by weight Propylene glycol 10.0 parts by weight Silicone surfactant (KF-351A, Shin-Etsu Chemical Co., Ltd.) 0.2 parts by weight Ion-exchanged water 45.0 parts by weight

2. Preparation of Pretreatment Solution Next, a pretreatment solution was prepared as follows.

2-1. Preparation of pretreatment liquid 1 The components shown below were added in order with stirring, and then filtered through a 5.0 μm filter to obtain pretreatment liquid 1. There was no substantial change in composition before and after filtration.

Water-insoluble resin (A-1) 15.0 parts by mass Propylene glycol (B-1) 10.0 parts by mass 1,2-hexanediol (B-2) 10.0 parts by mass Pigment flocculant (C-1) 2.0 parts by mass Additive (D-1) 0.05 parts by mass Silicone surfactant (E) 1.0 part by mass Ion-exchanged water 62.0 parts by mass

2-2. Preparation of Pretreatment Solutions 2 to 18 Tables 1 and 2 show the contents (units: parts by mass) of each component in pretreatment solutions 1 and 2 to 18.

The abbreviations shown in Tables 1 and 2 are as follows:

(Water-insoluble resin)
A-1: Acrylic resin (SE-841A, manufactured by Taisei Fine Chemical Co., Ltd.)
A-2: Polyester resin (Elite KT-0507, manufactured by Unitika Ltd.)
A-3: Urethane resin (WBR-016U, manufactured by Taisei Fine Chemical Co., Ltd.)
A-4: Polyolefin resin (Arrowbase CB-1010, manufactured by Unitika Ltd., "Arrowbase" is a registered trademark of the company)
A-5: Composite resin (acrylic urethane emulsion, WEM-202C, manufactured by Taisei Fine Chemical Co., Ltd.)
A-6: Composite resin (self-crosslinking acrylic emulsion, UW-319SX, manufactured by Taisei Fine Chemical Co., Ltd.)

(Water-soluble organic solvent)
B-1: Propylene glycol B-2: 1,2-hexanediol

(Pigment flocculant)
C-1: Cationic polymer (PAS-H-1L, manufactured by Nittobo Medical Co., Ltd.)
C-2: Organic acid (glutaric acid)
C-3: Polyvalent metal salt (calcium chloride)

(Additive)
D-1: Phosphate compound 1 (Phoslex A-8, both R's have 8 carbon atoms)
D-2: Phosphate compound 2 (Phospair A-16, oleylamine salt of phosphate, both R's have 8 carbon atoms)
D-3: Phosphate compound 3 (Phospair A-37, coconut amine salt of phosphate, both R's have 8 carbon atoms)
D-4: Phosphate compound 4 (Phospair-41, beef tallow amine salt of phosphoric acid ester, both R's have 8 carbon atoms)
D-5: Phosphate compound 5 (Phoslex A-1, both R's have 1 carbon atom)
D-6: Phosphate compound 6 (Phoslex A-2, both R's have 2 carbon atoms)
D-7: Phosphate compound 7 (Phoslex A-3, both R's have 3 carbon atoms)
D-8: Phosphate compound 8 (Phoslex A-4, both R's have 4 carbon atoms)
D-9: Phosphate compound 9 (Phoslex A-12, both R's have 12 carbon atoms)
D-10: Phosphate ester type anionic surfactant (polyoxypropylene alkyl ether phosphate salt), Newcol 1000-FCP, manufactured by Nippon Nyukazai Co., Ltd., "Newcol" is a registered trademark of the company)
D-11: Sulfate ester compound (Alscope NS-230, manufactured by Toho Chemical Industry Co., Ltd., "Alscope" is a registered trademark of the company)

(Surfactant)
E: Silicone surfactant (BYK-333, manufactured by BYK-Chemie)

All of the above phosphate ester compounds D-1 to D-9 are manufactured by SC Organic Chemical Co., Ltd.

2-3. Formation of Images for Evaluation Images 1 to 18 for evaluation were formed in the following manner.

(Method of applying pretreatment liquid)
Each of pretreatment solutions 1 to 18 was applied onto a recording medium (OPP film (product name: FOS, manufactured by Futamura Corporation)) using a bar coater #10, and then dried at 60° C. for 5 minutes to produce a recording medium having a coating film formed by drying the pretreatment solution to a thickness of 3.2 μm.

(Image forming method)
The prepared inkjet ink was applied to two independently driven heads of an inkjet recording device (manufactured by Konica Minolta, Inc., 360 dpi, ejection amount 14 pL) having an inkjet recording head equipped with a piezoelectric inkjet nozzle, and the nozzles were arranged alternately to prepare a 720 dpi x 720 dpi head module, which was then installed on a stage conveyor so that the nozzle rows were perpendicular to the conveying direction. The inkjet ink was filled in the inkjet of the head module, and the inkjet recording device was configured so that a solid image could be recorded by a single pass method on a resin layer formed on the surface of a substrate conveyed by a stage conveyor. Using the head, droplets of the inkjet ink were ejected so that a 720 dpi x 720 dpi solid image with an ink deposition amount of 11.2 cc/ ^m2 was formed.

3. Evaluation 3-1. Image quality evaluation (evaluation method)
The images produced by the above method were visually evaluated. In the following evaluations, images with a rating of △ or higher were judged to be suitable for practical use.

(Evaluation criteria)
◎: A good image in which the ink wettability is very good, the image is uniform without uneven density, and no ink loss is observed. ○: An image in which the ink wettability is good, there are areas of differing density, but no ink loss is observed, and it is an acceptable image for practical use. △: An image in which the ink wettability is slightly insufficient, there are areas where the ink has lost its ink, and there are slight whiteouts. ×: An image in which the ink wettability is insufficient, there are many areas in the image where the ink has lost its ink, and whiteouts are noticeable.

3-2. Evaluation of storage stability of pretreatment liquid (evaluation method)
The average particle size of the resin particles in pretreatment solutions 1 to 18 after storage for one week at 60°C was measured using a particle size measuring device: Zetasizer 1000HS manufactured by Marballoon Co., Ltd., and the particle size increase rate was calculated using the following formula and evaluated according to the following criteria. The closer the value is to 100%, the better the storage stability is.

(Evaluation criteria)
The evaluation was performed according to the following criteria: In the following evaluation, a rating of △ or higher was judged to be preferable for practical use.
◎: The increase rate of the average particle size is less than 130%. ○: The increase rate of the average particle size is 130% or more and less than 140%. △: The increase rate of the average particle size is 140% or more and less than 150%. ×: The increase rate of the average particle size is 150% or more.

The particle size increase rate was calculated by the following formula: The average particle size after storage is the particle size after storage at 60° C. for one week.
Particle size increase rate (%)=(average particle size after storage/average particle size before storage)×100

3-3. Evaluation of adhesion to substrate (evaluation method)
The solid image formed by the above recording method was cut with a cutter into a 5×5 grid at 1 mm intervals, and a tape peeling test was carried out using the cross-cut method.

(Evaluation criteria)
The evaluation was performed according to the following criteria: In the following evaluation, a rating of △ or higher was judged to be preferable for practical use.
◎: No peeling due to tape ○: Peeling of 1 to 3 squares of the grid pattern was observed, but this was at a practically acceptable level △: Peeling of 3 to 6 squares of the grid pattern was observed, but this was at a good level ×: Peeling of 6 or more squares of the grid pattern was observed, and this was at an unacceptable level

3-4. Water resistance evaluation (evaluation method)
Images 1 to 18 formed using the above inkjet inks and pretreatment liquids 1 to 18 were stored at 40° C. for 3 days, and then cut into strips of 10 cm×1 cm so that the solid portion was the cut edge to obtain test pieces. The test pieces were treated with hot water for 30 minutes, and the condition of the test pieces after treatment was visually confirmed.

(Evaluation criteria)
The evaluation was performed according to the following criteria: In the following evaluation, a rating of △ or higher was judged to be preferable for practical use.
◎: No peeling at all on the test piece. ○: Part of the test piece has peeled off, but not significantly. △: Major peeling has occurred on the test piece. ×: The entire image has peeled off from the test piece film.

The results of the above evaluations are shown in Tables 3 and 4.

As shown in Tables 3 and 4, it was found that an image with good image quality can be obtained by including a phosphate ester compound or its salt in the pretreatment liquid. This is because the phosphate ester compound or its salt has a polar group, and when the phosphate ester compound or its salt is included in the pretreatment liquid and applied to the surface of the recording medium, the polar group can be oriented with respect to the surface to which the aqueous ink is applied. This is thought to be because the aqueous ink droplets are appropriately wetted and spread out. In addition, the phosphate ester compound or its salt is unlikely to cause unexpected side reactions such as reacting with the pigment aggregating agent contained in the pretreatment liquid and sedimenting, so the ink droplets react with the pigment aggregating agent in a state where they are appropriately wetted and spread out on the surface of the pretreatment liquid and are pinned. This is thought to be because color bleeding of the aqueous ink droplets is suppressed.

It was also found that the pretreatment liquid containing the above phosphoric acid ester compound or its salt has excellent storage stability. This is thought to be because it is less likely to react not only with the pigment flocculant in the pretreatment liquid, but also with the water-insoluble resin contained in the pretreatment liquid.

It was also found that the storage stability of the pretreatment liquid was improved by adjusting the content of the phosphate ester compound or its salt represented by general formula (1) to 0.1% by mass or more and 5.0% by mass or less. This is thought to be because adjusting the content of the phosphate ester compound or its salt to 0.1% by mass or more and 5.0% by mass or less can suppress the progression of unexpected side reactions, such as precipitation due to reaction with the water-insoluble resin or flocculant in the pretreatment liquid.

It was also found that by including a composite resin as the water-insoluble resin, it is possible to obtain images with good image quality that have high adhesion to both low-absorbency and non-absorbency recording media. This is thought to be because by including a hydrophobic resin as the core portion and a hydrophilic resin as the shell portion, the surface of the non-absorbency recording medium with low polarity becomes more hydrophilic, allowing the applied droplets of water-based ink to wet and spread appropriately, and the hydrophobic resin can also adhere to the non-absorbency recording medium with low polarity.

It was also found that a pretreatment liquid containing a phosphate ester compound or its salt improves the water resistance of the coating film (image) that is formed. This is thought to be because the hydrophilic phosphate ester compound or its salt is compatible with the aqueous ink and the aqueous pretreatment liquid, and can promote the crosslinking reaction between the ionic site (carboxy group) derived from the pigment or pigment dispersant in the coating film (image) and the ionic site (hydrophilic group) of the water-insoluble resin.

The inkjet recording liquid set of the present invention is expected to broaden the range of applications for inkjet printing and contribute to the advancement and dissemination of technology in this field.

Reference Signs List 100 Image forming apparatus 110 Recording medium 120 Head carriage 121 Inkjet head 122 Nozzle 130 Pretreatment liquid application unit 131 Application roller 132 Dispenser 140, 150 Dryer

Claims (7)

In an image forming method using an inkjet ink, a pretreatment liquid is applied to a surface of a recording medium before the inkjet ink is applied, the pretreatment liquid including at least a water-insoluble resin, a pigment aggregating agent , a water-soluble organic solvent, and water,
The pretreatment liquid further contains a phosphoric acid ester compound represented by general formula (1) or a salt thereof.
Pretreatment solution.

(In formula (1), R is a linear or branched alkyl group having 1 to 8 carbon atoms, and n is 1 or 2.) The pretreatment liquid according to claim 1, wherein the content of the phosphate ester compound or its salt is 0.1% by mass or more and 5.0% by mass or less with respect to the total mass of the pretreatment liquid. The pretreatment liquid according to claim 1 or 2, which contains at least one of an organic acid and a polyvalent metal salt as the pigment flocculant. The pretreatment solution according to any one of claims 1 to 3, wherein R is a linear or branched alkyl group having 1 to 4 carbon atoms. The pretreatment liquid according to any one of claims 1 to 4, wherein the water-insoluble resin has a composite structure having a core portion and a shell portion. An inkjet recording liquid set comprising an inkjet ink to be applied to a surface of a recording medium and the pretreatment liquid according to any one of claims 1 to 5 ,
The inkjet ink contains at least a pigment, a pigment dispersant, a water-soluble organic solvent, and water.
Inkjet recording liquid set. 6. An image forming method comprising: a step of applying the pretreatment liquid according to claim 1 to a surface of a recording medium; and a step of applying an inkjet ink to the surface of the applied pretreatment liquid to form an image.