Inkjet recording method

Abstract

To provide an inkjet recording method which can suppress color irregularities even when printing on a low liquid-absorbing recording medium such as a resin film, and exhibits superior continuous ejection performance by suppressing ejection failure even during continuous printing.SOLUTION: An inkjet recording method for recording on a low liquid-absorbing recording medium using an inkjet recording apparatus equipped with an inkjet head that ejects an aqueous ink onto the low liquid-absorbing recording medium, wherein the low liquid-absorbing recording medium is a resin film, the aqueous ink contains a colorant (A), a water-soluble organic solvent (B), and water, the inkjet recording apparatus includes at least four or more kinds of the aqueous ink including black ink, cyan ink, magenta ink, and yellow ink, and the aqueous inks are ejected in the order of cyan ink, magenta ink, black ink, and yellow ink.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an inkjet recording method.

Background Art

[0002] The inkjet recording method is a method of directly ejecting ink droplets from very fine nozzles onto a recording medium and attaching them to obtain characters and images. This method has many advantages such as being easy to achieve full color, being inexpensive, being able to use plain paper as a recording medium, and being non-contact with the object to be printed, so it has become extremely popular. On the other hand, in addition to printing on conventional highly absorbent recording media called plain paper and copy paper, there has been a demand for printing on recording media for commercial printing using low-absorbent coated paper such as offset coated paper, or films of non-absorbent resins such as polyvinyl chloride resin, polypropylene resin, and polyester resin. When printing is performed by an inkjet recording method on these low-absorbent recording media, since the absorption of the liquid component is slow or not absorbed, it takes a long time to dry, and color unevenness is likely to occur.

[0003] Patent Document 1 discloses an inkjet recording method for obtaining a good image without unevenness or color mixing without thermally deforming a low-absorbent recording medium regardless of the presence or absence of an aggregating action. An inkjet recording apparatus having an inkjet head for ejecting an aqueous ink onto a low-absorbent recording medium is used to record on the recording medium. The aqueous ink contains a colorant (A), an organic solvent (C), and water. The inkjet recording apparatus includes three or more aqueous inks having different static surface tensions. The recording medium is heated to 30 to 75°C, and the aqueous ink having a high static surface tension is ejected onto the recording medium in order.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the inkjet recording method of Patent Document 1, it is disclosed that black ink, cyan ink, magenta ink, and yellow ink are printed in this order, similar to the order of conventional general inks. When printing according to the printing order of the inks, when the recording medium is heated to 30 to 75°C, the vicinity of the nozzles of the inkjet head is exposed to high temperatures, making it particularly easy for ejection failures of black ink to occur. The reason is considered as follows. The inkjet head located behind in the printing order (that is, the inkjet heads for cyan ink, magenta ink, and yellow ink in the above) contains water vapor due to the drying of the previously printed ink compared to the periphery of the black inkjet head. Therefore, the vicinity of the nozzles becomes a high-humidity atmosphere, and ejection failures are less likely to occur. On the other hand, black ink has high cohesiveness compared to other color inks, and in the above, since it is at the head of the printing order, the vicinity of the nozzles becomes a low-humidity atmosphere, and ejection failures may occur. Therefore, in order to suppress ejection failures, a printing order with low-cohesiveness yellow ink at the head was considered, but in this color order, there was a problem that color unevenness was likely to occur during solid printing when there was uneven heating on the recording medium. An object of the present invention is to provide an inkjet recording method that can suppress color unevenness even when printing on a low-absorbency recording medium such as a resin film, and that has excellent continuous ejection properties capable of suppressing ejection failures even when performing continuous printing.

Means for Solving the Problems

[0006] The present inventors have found that by using an inkjet recording apparatus equipped with an inkjet head that ejects aqueous ink onto a low-absorbency recording medium and ejecting the aqueous ink in a specific order, the above problems can be solved. The present invention relates to the following [1]. 〔1〕An inkjet recording method for recording on a low liquid-absorbing recording medium using an inkjet recording apparatus including an inkjet head that discharges an aqueous ink, wherein the low liquid-absorbing recording medium is a resin film, the aqueous ink contains a colorant (A), a water-soluble organic solvent (B), and water, the inkjet recording apparatus includes at least four or more kinds of the aqueous inks including black ink, cyan ink, magenta ink, and yellow ink, and the aqueous inks are discharged in the order of cyan ink, magenta ink, black ink, and yellow ink.

Advantages of the Invention

[0007] According to the present invention, it is possible to provide an inkjet recording method that can suppress color unevenness even when printing is performed on a low liquid-absorbing recording medium such as a resin film, and that is excellent in continuous dischargeability capable of suppressing discharge failures even when continuous printing is performed.

Embodiments for Carrying Out the Invention

[0008] [Inkjet Recording Method] The inkjet recording method of the present invention is an inkjet recording method for recording on a low liquid-absorbing recording medium using an inkjet recording apparatus including an inkjet head that discharges an aqueous ink, wherein the low liquid-absorbing recording medium is a resin film, the aqueous ink contains a colorant (A), a water-soluble organic solvent (B), and water, the inkjet recording apparatus includes at least four or more kinds of the aqueous inks including black ink, cyan ink, magenta ink, and yellow ink, and the aqueous inks are discharged in the order of cyan ink, magenta ink, black ink, and yellow ink.

[0009] In this specification, "recording" is a concept including printing and typing for recording characters and images, and "recorded matter" is a concept including printed matter and typed matter on which characters and images are recorded. In addition, "low liquid absorbency" is a concept that includes low liquid absorbency and non-liquid absorbency of water and / or ink, and the low liquid absorbency can be evaluated by the water absorbency of pure water. More specifically, the water absorption amount of the recording medium at a contact time of 100 msec between the recording medium and pure water is 0 g / m 2 or more and 10 g / m 2 or less, preferably 0 g / m 2 or more and 6 g / m 2 or less. The water absorption amount is measured by the method described in the examples. Furthermore, "aqueous system" means that water occupies the largest proportion in the medium contained in the ink, and the medium may be only water, or may include a mixed solvent of water and one or more organic solvents.

[0010] According to the inkjet recording method of the present invention, even when printing is performed on a low liquid absorbency recording medium such as a resin film, color unevenness can be suppressed, and an effect of excellent continuous discharge property is achieved. The reason is not clear, but it is considered as follows. Generally, when printing, in order to improve the drying property of the ink, the low liquid absorbency recording medium is heated. At this time, if there is uneven heating of the low liquid absorbency recording medium, especially when solid printing is performed, due to the difference in temperature on the low liquid absorbency recording medium, the physical properties of the solid printing surface differ depending on the printing location. Therefore, when printing the next ink on top of the previously printed ink, due to the difference in the physical properties of the solid printing of the previously printed ink, the wet spreading property of the next printed ink changes, and as a result, color unevenness occurs. In the conventional inkjet recording method, from the viewpoint of obtaining a good recording, black ink, cyan ink, magenta ink, and yellow ink are printed in this order. However, especially yellow ink is likely to recognize color unevenness for the above reasons, and in the conventional printing order, color unevenness may become prominent. On the other hand, it is considered that the inkjet recording method of the present invention can reduce color unevenness by moving the printing order of yellow ink, which is particularly likely to recognize color unevenness, to the rear. In addition, since black ink has a higher cohesiveness than inks of other colors, ink ejection failures are likely to occur during continuous printing. Here, focusing on the humidity around the inkjet head, the humidity around the rear inkjet head tends to be higher than that around the front inkjet head because moisture due to the drying of the previously printed ink is included. Therefore, in the inkjet recording method of the present invention, by arranging the printing order of black ink to the rear, the humidity around the inkjet head that ejects black ink can be made high, so it is considered that aggregation of black ink can be prevented and continuous ejection performance can be improved. Based on the above, as a result of examining a printing order that can further reduce color unevenness and has excellent continuous ejection performance, by ejecting the aqueous ink in the order of cyan ink, magenta ink, black ink, and yellow ink for printing, the printing method of the present invention is considered to have the effect of suppressing color unevenness and having excellent continuous ejection performance.

[0011] From the viewpoint of productivity, the inkjet recording method of the present invention is preferably a method of recording in a one-pass system. Further, by recording in a one-pass system, the printing conditions can be made constant, so that color unevenness can be further suppressed.

[0012] <Low-absorbency recording medium> The inkjet recording method of the present invention is used to record on a low-absorbency recording medium using an inkjet recording apparatus including an inkjet head that ejects aqueous ink onto the recording medium. The recording medium used in the present invention is a low-absorbency resin film. The form of the resin film may be a single-sheet form or a roll form. Examples of the resin film include transparent synthetic resin films, such as polyester films, vinyl chloride films, polypropylene films, polyethylene films, nylon films, etc. These films may be biaxially stretched films, uniaxially stretched films, or non-stretched films. Among these, polyester films and stretched polypropylene films are more preferred, and polyester films such as corona discharge-treated polyethylene terephthalate (PET) films and stretched polypropylene films such as corona discharge-treated biaxially stretched polypropylene (OPP) films are more preferred. There is no particular limitation on the thickness of the resin film. A thin film with a thickness of less than 1 to 20 μm may be used, but from the viewpoints of suppressing appearance defects of the resin film and availability, it is preferably 20 μm or more, more preferably 30 μm or more, still more preferably 35 μm or more, and preferably 100 μm or less, more preferably 80 μm or less, still more preferably 75 μm or less. Examples of commercially available products of transparent synthetic resin films include Lumirror T60 (manufactured by Toray Industries, Inc., PET), Taiko FE2001 (manufactured by Futamura Chemical Co., Ltd., corona discharge-treated PET), Taiko FOR-AQ (manufactured by Futamura Chemical Co., Ltd., corona discharge-treated OPP), PVC80B P (manufactured by Lintec Corporation, vinyl chloride), Kinas KEE70CA (manufactured by Lintec Corporation, polyethylene), Yupo SG90 PAT1 (manufactured by Lintec Corporation, PP), Bonyl RX (manufactured by Kojin Film & Chemicals Co., Ltd., nylon), etc.

[0013] <Inkjet recording apparatus> The inkjet recording apparatus used in the present invention includes at least four or more kinds of the aqueous inks including cyan ink, magenta ink, yellow ink, and black ink. That the inkjet recording apparatus includes a plurality of aqueous inks means that it has ejection ports of an inkjet head that ejects each of the plurality of aqueous inks. For example, when the inkjet recording apparatus includes four kinds of aqueous inks, it has ejection ports of an inkjet head that ejects each of the four kinds of aqueous inks. In this case, the number of inkjet heads may be 1 or 2. The inkjet recording apparatus includes four or more kinds of aqueous inks, and preferably includes eight or less kinds of aqueous inks, more preferably seven or less kinds of aqueous inks, and still more preferably six or less kinds of aqueous inks. In particular, from the viewpoint of further improving the continuous ejection property and further suppressing color unevenness, the inkjet recording apparatus preferably includes four kinds of aqueous inks. In the inkjet recording method of the present invention, examples of combinations of a plurality of aqueous inks include combinations of four kinds of black ink, cyan ink, magenta ink, and yellow ink; combinations of five kinds of black ink, cyan ink, magenta ink, yellow ink, and white ink; combinations of seven kinds of black ink, cyan ink, magenta ink, yellow ink, red ink, green ink, and blue ink; combinations of eight kinds of black ink, cyan ink, magenta ink, yellow ink, red ink, green ink, blue ink, and white ink, and the like.

[0014] The inkjet recording apparatus used in the present invention preferably includes an inkjet head that ejects an aqueous ink onto a recording medium that moves in the feeding direction in order to heat a low-absorbency recording medium, and an under-heater that heats the recording medium from the surface on the back side of the resin film facing the head. That is, in the inkjet recording method of the present invention, it is preferable to heat the recording medium with an inkjet head that ejects an aqueous ink onto a low-absorbency recording medium and an under-heater provided on the surface on the back side of the recording medium facing the head.

[0015] (Inkjet head) Examples of the inkjet head include a serial head method and a line head method, and the line head method is preferred. The line head method is a method in which the head is fixed, the recording medium is moved in the conveyance direction, and ink droplets are ejected from the nozzle openings of the head in conjunction with this movement and adhered to the recording medium, and an image or the like can be recorded by a one-pass method.

[0016] The ink droplet ejection method is preferably the piezo method. In the piezo method, a large number of nozzles communicate with pressure chambers respectively, and the wall surface of the pressure chamber is vibrated by a piezo element to eject ink droplets from the nozzles. Note that the thermal method can also be adopted. From the viewpoint of the efficiency of high-speed printing and the like, the applied voltage of the recording head is preferably 5 V or more, more preferably 10 V or more, still more preferably 15 V or more, and preferably 40 V or less, more preferably 35 V or less, still more preferably 30 V or less. From the viewpoint of the efficiency of high-speed printing and the like, the driving frequency is preferably 2 kHz or more, more preferably 5 kHz or more, still more preferably 8 kHz or more, and preferably 80 kHz or less, more preferably 70 kHz or less, still more preferably 60 kHz or less.

[0017] (Recording conditions) From the viewpoint of further suppressing color unevenness, the ejected ink droplet amount per drop is preferably 0.5 pL or more, more preferably 1.0 pL or more, still more preferably 1.5 pL or more, even more preferably 1.8 pL or more, and preferably 20 pL or less, more preferably 15 pL or less, still more preferably 13 pL or less.

[0018] The recording head resolution is preferably 400 dpi (dots per inch) or more, more preferably 500 dpi or more, still more preferably 550 dpi or more.

[0019] During recording, the temperature inside the head, preferably inside the line head, is preferably 20°C or higher, more preferably 25°C or higher, still more preferably 30°C or higher, and preferably 45°C or lower, more preferably 40°C or lower, still more preferably 38°C or lower, from the viewpoint of reducing the viscosity of the ink and improving continuous ejection performance.

[0020] The surface temperature of the recording medium facing the area where the recording head, preferably the line head, ejects the ink is preferably 25°C or higher, more preferably 30°C or higher, still more preferably 35°C or higher, from the viewpoint of further suppressing color unevenness, and preferably 70°C or lower, more preferably 60°C or lower, still more preferably 50°C or lower, from the viewpoint of suppressing thermal deformation of the recording medium. The surface temperature of the recording medium can be adjusted by adjusting the temperature of the under-heater.

[0021] The recording speed is preferably 5 m / min or more and 100 m / min or less in terms of the conveyance speed conversion of the recording medium (conversion of the conveyance speed with respect to the direction in which the recording medium moves during recording), from the viewpoints of productivity and operability. Also, the lower limit value of the recording speed is more preferably 10 m / min or more, still more preferably 15 m / min or more, and even more preferably 20 m / min or more in terms of the conveyance speed conversion of the recording medium, from the viewpoint of productivity.

[0022] The adhesion amount of the aqueous ink on the recording medium is preferably 0.1 g / m or more as a solid content, 2 and preferably 25 g / m or less, more preferably 20 g / m or less, 2 from the viewpoints of improving the image quality of the recorded matter and the recording speed. 2

[0023] (Under-heater) The underheater provided in the inkjet recording apparatus is disposed on the surface opposite to the surface of the recording medium, facing the recording head for aqueous ink, and heats the recording medium. The underheater can be, for example, a heater having a stainless steel or ceramic plate of a hot water type or a thermoelectric type. When there are a plurality of recording heads that eject a plurality of inks, the underheater is preferably disposed at a position facing each recording head. The distance between the underheater and the recording medium is preferably 0.05 mm or more, more preferably 0.10 mm or more, still more preferably 0.20 mm or more, and preferably 5.0 mm or less, more preferably 4.0 mm or less, still more preferably 3.0 mm or less. In the inkjet recording method of the present invention, in order to record an image by ejecting one kind of ink and to make it difficult for color unevenness of each ink to occur even when the next ink is subsequently ejected, fixing and curing means for applying thermal energy such as a heater can be further provided.

[0024] <aqueous ink> The aqueous ink used in the present invention contains a colorant (A), a water-soluble organic solvent (B), and water.

[0025] (colorant (A)) As the colorant (A), either a pigment or a dye can be used, but a pigment is preferable from the viewpoint of imparting weather resistance and water resistance to the recorded matter. The pigment may be either an inorganic pigment or an organic pigment. Further, if necessary, a extender pigment can be used in combination with them. Examples of the inorganic pigment include carbon black, metal oxides, metal sulfides, metal chlorides, and the like. In black ink, carbon black is preferable. Examples of carbon black include furnace black, thermal lamp black, acetylene black, channel black, and the like. In addition, in white ink, metal oxides such as titanium oxide, zinc oxide, silica, alumina, magnesium oxide, and the like can be mentioned, and titanium oxide is preferable. In the present invention, from the viewpoint of using black ink in aqueous ink, carbon black is preferable as the pigment of the black ink.

[0026] Examples of the organic pigment include azo pigment, diazo pigment, phthalocyanine pigment, quinacridone pigment, isoindolinone pigment, dioxazine pigment, perylene pigment, perinone pigment, thioindigo pigment, anthraquinone pigment, quinophthalone pigment and the like. In the present invention, from the viewpoint of using cyan ink, magenta ink and yellow ink in aqueous ink, it is preferable to use an organic pigment having a chromatic color exhibiting at least the hue of cyan, magenta or yellow. Specific examples of the organic pigment having a chromatic color exhibiting the hue of cyan are preferably products of each product number of C.I. Pigment Blue, and more preferably C.I. Pigment Blue 15:3 (P.B.15:3). Specific examples of the organic pigment having a chromatic color exhibiting the hue of magenta are preferably C.I. Pigment Red, and more preferably C.I. Pigment Red 150 (P.R.150). Specific examples of the organic pigment having a chromatic color exhibiting the hue of yellow are preferably C.I. Pigment Yellow, and more preferably C.I. Pigment Yellow 74 (P.Y.74). In addition, in the present invention, when further using an aqueous ink other than black ink, cyan ink, magenta ink and yellow ink, an organic pigment having a chromatic color exhibiting another hue may be further used. The organic pigment having a chromatic color exhibiting another hue is not particularly limited, and any chromatic pigments such as blue, red, orange, green and the like can be used. Specific examples of the preferable organic pigment having a chromatic color exhibiting another hue include products of one or more selected from C.I. Pigment Yellow, C.I. Pigment Red, C.I. Pigment Orange, C.I. Pigment Violet, C.I. Pigment Blue, and C.I. Pigment Green. The above pigments and dyes can be used alone or in combination of two or more.

[0027] The colorant (A) is preferably a pigment as described above. As the form of the pigment, there are (i) the form of a self-dispersing pigment (a pigment that can maintain a dispersed state without a dispersant), (ii) the form of pigment particles obtained by dispersing a pigment with a surfactant, and (iii) the form of polymer particles containing a pigment. From the viewpoint of further improving the continuous discharge property and further suppressing color unevenness, the form of the pigment is preferably the form of polymer particles containing a pigment. That is, when the colorant (A) is a pigment, the colorant (A) is preferably polymer particles containing a pigment, and more preferably water-insoluble polymer particles containing a pigment in which the pigment is contained in the water-insoluble polymer described below. In the present specification, the form of polymer particles containing a pigment means particles in which the polymer contains the pigment, particles in which a part of the pigment is exposed on the surface of particles composed of the polymer and the pigment, particles in which the polymer is adsorbed on a part of the pigment, and mixtures thereof.

[0028] 〔Self-dispersing pigment〕 A self-dispersing pigment means a pigment that can be dispersed in an aqueous medium without using a surfactant or a resin by directly bonding one or more hydrophilic functional groups (anionic hydrophilic groups such as carboxy groups or cationic hydrophilic groups such as quaternary ammonium groups) to the surface of the pigment via another atomic group such as an alkanediyl group having 1 to 12 carbon atoms. Commercially available products of self-dispersing pigments include the CAB-O-JET series manufactured by Cabot Japan Co., Ltd. The self-dispersing pigment is preferably used as a pigment aqueous dispersion dispersed in water.

[0029] 〔Water-insoluble polymer particles containing a pigment〕 The water-insoluble polymer particles containing a pigment (hereinafter also referred to as "pigment-containing polymer particles") include the above-described pigment and a water-insoluble polymer described below. Further, in the present invention, from the viewpoint of improving the dispersion stability of the colorant (A) and further improving the continuous discharge property, it is preferable that the water-insoluble polymer has a crosslinked structure.

[0030] In this specification, the "water-insoluble" property of a water-insoluble polymer means that when a polymer dried to a constant weight is dissolved in 100 g of water at 25°C, the dissolved amount is 10 g or less, preferably 5 g or less, more preferably 1 g or less. When the water-insoluble polymer is an anionic polymer, it is the dissolved amount when the anionic groups of the polymer are 100% neutralized with NaOH. In addition, the form of existence of the water-insoluble polymer in the aqueous ink includes a particulate form in which the polymer encapsulates (capsules) the pigment, a particulate form in which the pigment is uniformly dispersed in the polymer, a particulate form in which the pigment is exposed from the particle surface of the polymer, a form in which the polymer is adsorbed on the pigment, and a form in which the polymer is not adsorbed on the pigment, etc., and a form in which these are mixed is also included. From the viewpoint of the dispersion stability of the pigment, in the present invention, a form of pigment-containing polymer particles is preferable, and a pigment-encapsulated form in which the water-insoluble polymer contains the pigment is more preferable.

[0031] ≪Water-insoluble polymer≫ As the water-insoluble polymer, from the viewpoint of improving the dispersion stability of the colorant (A) and further improving the continuous discharge property, a vinyl-based polymer obtained by addition polymerization of vinyl monomers is preferable. As the vinyl polymer, a vinyl polymer obtained by copolymerizing a monomer mixture containing (a-1) an ionic monomer and (a-2) a hydrophobic monomer is preferred, a vinyl polymer obtained by copolymerizing a monomer mixture containing (a-1) an ionic monomer, (a-2) a hydrophobic monomer, and (a-3) a macromonomer is more preferred, and a vinyl polymer obtained by copolymerizing a monomer mixture containing (a-1) an ionic monomer, (a-2) a hydrophobic monomer, (a-3) a macromonomer, and (a-4) a nonionic monomer is even more preferred. That is, it is preferable that the vinyl polymer has a structural unit derived from (a-1) an ionic monomer and a structural unit derived from (a-2) a hydrophobic monomer, it is more preferable that the vinyl polymer has a structural unit derived from (a-1) an ionic monomer, a structural unit derived from (a-2) a hydrophobic monomer, and a structural unit derived from (a-3) a macromonomer, and it is even more preferable that the vinyl polymer has a structural unit derived from (a-1) an ionic monomer, a structural unit derived from (a-2) a hydrophobic monomer, a structural unit derived from (a-3) a macromonomer, and a structural unit derived from (a-4) a nonionic monomer.

[0032] · (a-1) Ionic monomer Examples of the (a-1) ionic monomer (hereinafter also referred to as the “(a-1) component”) include anionic monomers and cationic monomers, and anionic monomers are preferred. Examples of the anionic monomer include carboxylic acid monomers, sulfonic acid monomers, phosphoric acid monomers, and the like. Examples of the carboxylic acid monomer include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, 2-methacryloyloxymethyl succinic acid, and the like. Among the above, from the viewpoint of further improving the dispersion stability of the pigment-containing polymer particles in the aqueous ink, carboxylic acid monomers are preferred, one or more selected from acrylic acid and methacrylic acid are more preferred, and methacrylic acid is even more preferred.

[0033] · (a-2) Hydrophobic monomer (a-2) Hydrophobic monomers (hereinafter also referred to as "(a-2) component") include alkyl (meth)acrylate esters, aromatic group-containing monomers, etc. As the alkyl (meth)acrylate ester, those having an alkyl group with 1 to 22 carbon atoms are preferred. For example, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, amyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, (iso)stearyl (meth)acrylate, etc. can be mentioned. As the aromatic group-containing monomer, a vinyl monomer having an aromatic group with 6 to 22 carbon atoms, which may have a substituent containing a hetero atom, is preferred, and a styrene-based monomer and an aromatic group-containing (meth)acrylate ester are more preferred. As the styrene-based monomer, one or more selected from styrene, 2-methylstyrene, and divinylbenzene are preferred, and styrene is more preferred. As the aromatic group-containing (meth)acrylate ester, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, etc. are preferred, and benzyl (meth)acrylate is more preferred.

[0034] ·(a-3) Macromonomer (a-3) Macromonomer (hereinafter also referred to as "(a-3) component") is a compound having a polymerizable functional group at one end and a number average molecular weight of 500 or more and 100,000 or less. As the polymerizable functional group present at one end, an acryloyloxy group or a methacryloyloxy group is preferred. The number average molecular weight of the (a-3) macromonomer is preferably 1,000 or more and 10,000 or less. The number average molecular weight is measured by gel permeation chromatography using polystyrene as a standard substance. (a-3) As the macromonomer, from the viewpoint of improving the dispersion stability of the pigment-containing polymer particles in the aqueous ink, an aromatic group-containing monomer-based macromonomer and a silicone-based macromonomer are preferable, and an aromatic group-containing monomer-based macromonomer is more preferable. Examples of the aromatic group-containing monomer constituting the aromatic group-containing monomer-based macromonomer include the aromatic group-containing monomers described in the above (a-2) hydrophobic monomer, and styrene and benzyl (meth)acrylate are preferable, and styrene is more preferable. Specific examples of the styrene-based macromonomer include AS-6, AS-6S, AN-6, AN-6S, HS-6, HS-6S, etc. of Toagosei Co., Ltd.

[0035] ·(a-4) Nonionic monomer (a-4) The nonionic monomer (hereinafter also referred to as the "(a-4) component") is preferably used as a monomer component from the viewpoint of improving the dispersion stability of the pigment-containing polymer particles in the aqueous ink. Examples of the (a-4) nonionic monomer include polyalkylene glycol (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, polypropylene glycol (n = 2 to 30, n represents the average number of added moles of oxyalkylene groups. The same applies hereinafter) (meth)acrylate, polyethylene glycol (n = 2 to 30) (meth)acrylate, alkoxypolyalkylene glycol (meth)acrylates such as methoxypolyethylene glycol (n = 1 to 30) (meth)acrylate, and phenoxy (ethylene glycol·propylene glycol copolymer) (n = 1 to 30, ethylene glycol in which: n = 1 to 29) (meth)acrylate, etc. Among these, alkoxypolyalkylene glycol (meth)acrylate is preferable, methoxypolyethylene glycol (n = 1 to 30) (meth)acrylate is more preferable, and methoxypolyethylene glycol (n = 1 to 30) methacrylate is even more preferable. Specific examples of commercially available (a-4) nonionic monomers include NK Ester M-20G, 40G, 90G, 230G, etc. from Shin-Nakamura Chemical Co., Ltd., Brenmer PE-90, 200, 350, etc., PME-100, 200, 400, etc., PP-500, 800, 1000, etc., AP-150, 400, 550, etc., 50PEP-300, 50POEP-800B, 43PAPE-600B, etc. from NOF Corporation. The above components (a-1) to (a-4) can be used alone or in admixture of two or more thereof.

[0036] The content of the structural units derived from the components (a-1) to (a-4) in the water-insoluble polymer is as follows from the viewpoint of the dispersion stability of the pigment-containing polymer particles. The content of the component (a-1) is preferably 3% by mass or more, more preferably 5% by mass or more, still more preferably 7% by mass or more, and is preferably 40% by mass or less, more preferably 30% by mass or less, still more preferably 20% by mass or less. The content of the component (a-2) is preferably 25% by mass or more, more preferably 30% by mass or more, still more preferably 35% by mass or more, and is preferably 60% by mass or less, more preferably 55% by mass or less, still more preferably 50% by mass or less. The content of the component (a-3) is 0% by mass or more, preferably 5% by mass or more, more preferably 8% by mass or more, still more preferably 10% by mass or more, and is preferably 30% by mass or less, more preferably 25% by mass or less, still more preferably 20% by mass or less. The content of the component (a-4) is 0% by mass or more, preferably 5% by mass or more, more preferably 10% by mass or more, still more preferably 15% by mass or more, and is preferably 50% by mass or less, more preferably 45% by mass or less, still more preferably 40% by mass or less.

[0037] ·Crosslinking agent When the water-insoluble polymer has a crosslinked structure, as the crosslinking agent to be used, when it contains an anionic monomer as the (a-1) component (that is, when the water-insoluble polymer has a structural unit derived from an anionic monomer), a compound having a functional group that reacts with the anionic group of the structural unit derived from the anionic monomer is preferable, a compound having two or more of the functional groups is more preferable, and a compound having two or more and six or less of the functional groups is still more preferable. As specific examples of the above crosslinking agent, one or more selected from a compound having two or more epoxy groups in the molecule, a compound having two or more oxazoline groups in the molecule, and a compound having two or more isocyanate groups in the molecule are preferable, and a compound having two or more epoxy groups in the molecule is preferable.

[0038] Specific examples of the compound having two or more epoxy groups in the molecule include one or more selected from polyglycidyl ethers such as cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, and hydrogenated bisphenol A type diglycidyl ether. Among these, one or more selected from cyclohexanedimethanol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether are preferable, and trimethylolpropane polyglycidyl ether is more preferable.

[0039] ≪Production of water-insoluble polymer≫ The water-insoluble polymer can be obtained by copolymerization using a known polymerization method, for example, solution polymerization. From the viewpoint of improving the productivity of the aqueous dispersion of the pigment-containing polymer particles described later, the organic solvent contained therein is used as the organic solvent to be used in Step I described later without removing the solvent used in the polymerization reaction, and thus it is preferably used as it is as a water-insoluble polymer solution.

[0040] From the viewpoint of obtaining a good image without unevenness or color mixing, the weight average molecular weight of the water-insoluble polymer is preferably 5,000 or more, more preferably 10,000 or more, still more preferably 20,000 or more, and preferably 500,000 or less, more preferably 400,000 or less, still more preferably 300,000 or less, and even more preferably 200,000 or less.

[0041] ≪Production of Particles of Water-Insoluble Polymer Containing Pigment (Pigment-Containing Polymer Particles)≫ The pigment-containing polymer particles can be efficiently produced as an aqueous dispersion by a method having the following Step I, Step II, and, if necessary, Step III. Step I: A step of subjecting a mixture containing a water-insoluble polymer, an organic solvent, a pigment, and water (hereinafter also referred to as "pigment mixture") to a dispersion treatment to obtain a dispersion of pigment-containing polymer particles Step II: A step of removing the organic solvent from the dispersion obtained in Step I to obtain an aqueous dispersion of pigment-containing polymer particles (hereinafter also referred to as "pigment aqueous dispersion") Step III: A step of mixing the aqueous dispersion obtained in Step II with a crosslinking agent and subjecting it to a crosslinking treatment to obtain an aqueous dispersion

[0042] · Step I In Step I, first, a method of dissolving a water-insoluble polymer in an organic solvent, and then adding and mixing a pigment, water, and, if necessary, a neutralizing agent, a surfactant, etc. to the obtained organic solvent solution to obtain an oil-in-water type dispersion is preferred. There is no limitation on the organic solvent for dissolving the water-insoluble polymer, but when the water-insoluble polymer is synthesized by solution polymerization, the solvent used in the polymerization may be used as it is. When the water-insoluble polymer is an anionic polymer, the anionic groups in the water-insoluble polymer may be neutralized using a neutralizing agent. Examples of the neutralizing agent include hydroxides of alkali metals, ammonia, organic amines, and the like. There is no particular limitation on the dispersion treatment method in Step I, but preferably, after the pigment mixture is preliminarily dispersed, further shear stress is applied to perform the main dispersion, and it is preferable to control the average particle size of the pigment particles to a desired particle size. Examples of the preliminary dispersion device include commonly used mixing and stirring devices such as anchor blades and disperser blades, but a high-speed stirring and mixing device is preferred. In addition, examples of the main dispersion device include kneaders such as roll mills and kneaders, high-pressure homogenizers such as Microfluidizer (manufactured by Microfluidic), media-type dispersers such as paint shakers and bead mills. From the viewpoint of reducing the particle size of the pigment, a high-pressure homogenizer is preferred. When performing the main dispersion using a high-pressure homogenizer, the pigment can be controlled to have a desired particle size by controlling the treatment pressure and the number of passes.

[0043] · Step II In Step II, an organic solvent can be removed from the dispersion obtained in Step I by a known method to obtain a pigment aqueous dispersion. It is preferable that the organic solvent in the obtained pigment aqueous dispersion is substantially removed, but it may remain slightly as long as the object of the present invention is not impaired. The obtained pigment aqueous dispersion is one in which solid water-insoluble polymer particles containing a pigment are dispersed in a medium having water as the main medium. Here, the form of the pigment-containing polymer particles is not particularly limited, but as described above, the pigment inclusion state containing the pigment is more preferable.

[0044] · Step III Step III is an optional step, but when the water-insoluble polymer has a crosslinked structure, it is preferable to perform Step III from the viewpoint of improving the dispersion stability of the colorant (A) and further improving the continuous discharge property. The crosslinking agent used in Step III is as described above.

[0045] The solid content concentration of the obtained pigment aqueous dispersion is preferably 10% by mass or more, more preferably 15% by mass or more, and preferably 35% by mass or less, more preferably 30% by mass or less, from the viewpoint of improving the dispersion stability of the pigment aqueous dispersion. The solid content concentration of the pigment aqueous dispersion is measured by the method described in the examples. The average particle diameter of the pigment-containing polymer particles in the pigment aqueous dispersion is preferably 40 nm or more, more preferably 60 nm or more, still more preferably 80 nm or more, even more preferably 85 nm or more, and preferably 180 nm or less, more preferably 160 nm or less, still more preferably 150 nm or less, from the viewpoint of reducing coarse particles and improving continuous dischargeability. The average particle diameter of the pigment-containing polymer particles is measured by the method described in the examples.

[0046] <<Fixing Aid Polymer>> The aqueous ink preferably contains a fixing aid polymer from the viewpoints of further improving the fixing property of the aqueous ink and further suppressing color unevenness. The fixing aid polymer is preferably used as polymer particles that do not contain a pigment.

[0047] Examples of the fixing aid polymer include condensation resins such as polyurethane and polyester, vinyl polymers such as acrylic resin, styrene resin, styrene-acrylic resin, butadiene resin, styrene-butadiene resin, vinyl chloride resin, vinyl acetate resin, and acrylic silicone resin. Among these, acrylic resin is preferable from the viewpoint of accelerating the drying property on the recording medium and improving the fixing property of the image. Also, the fixing aid polymer is preferably used as a dispersion liquid containing polymer particles from the viewpoint of improving the productivity of the aqueous ink. The fixing aid polymer may be produced by, for example, an emulsion polymerization method or the like, or a commercially available product may be used.

[0048] Examples of commercially available fixing aid polymers include Neocryl A1127 (manufactured by DSM NeoResins, an anionic self-crosslinking aqueous acrylic resin); acrylic resins such as Joncryl 390 (manufactured by BASF Japan Ltd.); urethane resins such as WBR-2018 and WBR-2000U (manufactured by Dainippon Fine Chemical Co., Ltd.); styrene-butadiene resins such as SR-100 and SR-102 (manufactured by Nippon A&R Co., Ltd.); styrene-acrylic resins such as Joncryl 7100, Joncryl 7600, Joncryl 537J, Joncryl 538J, Joncryl 780, and Joncryl PDX-7164 (manufactured by BASF Japan Ltd.); and vinyl chloride resins such as Vinibran 700 and Vinibran 701 (manufactured by Nisshin Chemical Industry Co., Ltd.). Examples of the form of the fixing aid polymer include particles dispersed in water.

[0049] From the viewpoint of fixing performance, the weight average molecular weight of the fixing aid polymer is preferably 10,000 or more, more preferably 20,000 or more, still more preferably 30,000 or more, and preferably 2.5 million or less, more preferably 1 million or less. Also, from the viewpoint of improving the dispersion stability of the colorant (A) and further improving the continuous dischargeability, the average particle diameter of the fixing aid polymer particles in the dispersion containing the fixing aid polymer particles or in the ink is preferably 10 nm or more, more preferably 30 nm or more, still more preferably 50 nm or more, and preferably 300 nm or less, more preferably 200 nm or less, still more preferably 150 nm or less, and even more preferably 130 nm or less.

[0050] (Water-soluble organic solvent (B)) From the viewpoint of further suppressing color unevenness, the water-soluble organic solvent (B) preferably has a boiling point of 90°C or higher and lower than 250°C. The boiling point of the water-soluble organic solvent (B) is preferably 130°C or higher, more preferably 140°C or higher, still more preferably 150°C or higher, and preferably 245°C or lower, preferably 240°C or lower, preferably 235°C or lower. Examples of such water-soluble organic solvents (B) preferably include glycol ethers (b-1) and polyhydric alcohols (b-2). That is, the water-soluble organic solvent (B) preferably contains glycol ethers (b-1) and polyhydric alcohols (b-2).

[0051] 〔Glycol ether (b-1)〕 Specific examples of the glycol ether (b-1) (hereinafter also referred to as the “(b-1) component”) include alkylene glycol monoalkyl ethers, alkylene glycol dialkyl ethers, etc. From the viewpoint of further improving the continuous discharge property and further suppressing color unevenness, alkylene glycol monoalkyl ethers are preferred. The number of carbon atoms of the alkyl group of the alkylene glycol monoalkyl ether is preferably 1 or more, more preferably 2 or more, still more preferably 3 or more, and preferably 6 or less, more preferably 4 or less. The alkyl group of the alkylene glycol monoalkyl ether may be linear or branched. Specific examples of the alkylene glycol monoalkyl ether include ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monoisobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, dipropylene glycol monobutyl ether, dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether, and the like. Among these, one or more selected from ethylene glycol monoisopropyl ether (boiling point 144 °C), ethylene glycol monopropyl ether (boiling point 151 °C), diethylene glycol monomethyl ether (boiling point 194 °C), diethylene glycol monoisopropyl ether (boiling point 207 °C), diethylene glycol monoisobutyl ether (boiling point 230 °C), and diethylene glycol monobutyl ether (boiling point 230 °C) are preferred, one or more selected from ethylene glycol monoisopropyl ether, diethylene glycol monoisopropyl ether, and diethylene glycol monoisobutyl ether are more preferred, and diethylene glycol monoisobutyl ether is even more preferred.

[0052] [Polyhydric alcohol (b-2)] Examples of the polyhydric alcohol (b-2) (hereinafter also referred to as the "(b-2) component") include 1,2-alkanediols such as ethylene glycol, propylene glycol, 1,2-butanediol, 1,2-pentanediol, and 1,2-hexanediol, diethylene glycol, polyethylene glycol, dipropylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 3-methyl-1,3-butanediol, 1,5-pentanediol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol, 1,2,4-butanetriol, 1,2,3-butanetriol, and petriol. Among these, from the viewpoint of improving the dispersion stability of the colorant (A) and further improving the continuous discharge property, one or more selected from alkanediols having 2 to 6 carbon atoms and polypropylene glycol having a molecular weight of 500 to 1000 are preferred, one or more selected from propylene glycol (boiling point 188 °C), diethylene glycol (boiling point 245 °C), 1,2-hexanediol (boiling point 223 °C), and polypropylene glycol having a molecular weight of 500 to 1000 are more preferred, one or more selected from propylene glycol, diethylene glycol, and polypropylene glycol having a molecular weight of 500 to 1000 are even more preferred, and propylene glycol is even more preferably.

[0053] [Other organic solvents] In the present invention, as other organic solvents other than the above-mentioned water-soluble organic solvent (B), other alcohols usually blended in aqueous inks, alkyl ethers of the alcohols, glycol ethers, nitrogen-containing heterocyclic compounds such as NMP, amides, amines, sulfur-containing compounds, etc. can be contained. For example, 1,6-hexanediol (boiling point 250 ° C), triethylene glycol (boiling point 285 ° C), tripropylene glycol (boiling point 273 ° C), polypropylene glycol (boiling point 250 ° C or higher), glycerin (boiling point 290 ° C), etc. can be used in combination with a solvent having a boiling point of less than 250 ° C.

[0054] (Surfactant (C)) The aqueous ink used in the present invention preferably contains a surfactant (C) from the viewpoint of further suppressing color unevenness. Examples of the surfactant (C) include nonionic surfactants, anionic surfactants, and amphoteric surfactants, and nonionic surfactants are preferred. Examples of the nonionic surfactant include acetylene glycol-based surfactants, polyoxyalkylene alkyl ether-based surfactants, polyhydric alcohol-based surfactants, fatty acid alkanolamides, silicone-based surfactants, and fluorine-based surfactants. Among these, as the surfactant (C), from the viewpoint of further suppressing color unevenness, it preferably contains one or more selected from a silicone-based surfactant (c-1) and an acetylene glycol-based surfactant (c-2), and more preferably contains both the silicone-based surfactant (c-1) and the acetylene glycol-based surfactant (c-2).

[0055] [Silicone-based surfactant (c-1)] The silicone-based surfactant (c-1) (hereinafter also referred to as the “(c-1) component”) can be appropriately selected according to the purpose, but from the viewpoint of suppressing the increase in ink viscosity, improving the continuous discharge property, and further suppressing color unevenness, a polyether-modified silicone-based surfactant is preferred. Since the polyether-modified silicone-based surfactant can suppress the increase in ink viscosity and the color mixing of inks, it is considered that color unevenness can be further suppressed. As the polyether group of the polyether-modified silicone-based surfactant, for example, a polyethyleneoxy group, a polypropyleneoxy group, or a polyalkyleneoxy group in which an ethyleneoxy group (EO) and a propyleneoxy group (trimethyleneoxy group or propane-1,2-diyl oxy group; PO) are added in a block or random manner is suitable, and a compound in which a polyether group is grafted to the silicone main chain or a compound in which polyether groups are block-bonded to both ends of the silicone main chain can be used. Specific examples of the polyether-modified silicone-based surfactant include PEG-3 dimethicone, PEG-9 dimethicone, PEG-9 methyl ether dimethicone, PEG-10 dimethicone, PEG-11 methyl ether dimethicone, PEG / PPG-20 / 22 butyl ether dimethicone, PEG-32 methyl ether dimethicone, PEG-9 polydimethylsiloxyethyl dimethicone, lauryl PEG-9 polydimethylsiloxyethyl dimethicone, and the like. Commercially available products of the polyether-modified silicone-based surfactant include the KF series manufactured by Shin-Etsu Chemical Co., Ltd., Silface SAG005 manufactured by Nisshin Chemical Industry Co., Ltd., BYK-348 manufactured by BYK-Chemie Japan Co., Ltd., and the like.

[0056] 〔Acetylene glycol-based surfactant (c-2)〕 Examples of the acetylene glycol-based surfactant (c-2) (hereinafter also referred to as the “(c-2) component”) include acetylene glycol having 8 to 22 carbon atoms and ethylene adducts of the acetylene glycol. Examples of the acetylene glycol surfactant (C-2) include 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol, 3,6-dimethyl-4-octyne-3,6-diol, 3,5-dimethyl-1-hexyne-3-ol, 2,4-dimethyl-5-hexyne-3-ol, and ethylene oxide adducts thereof. 2,4,7,9-Tetramethyl-5-decyne-4,7-diol is more preferred. Examples of commercially available products of the acetylene glycol surfactant (C-2) include Surfynol 104PG50 (propylene glycol 50% by mass solution of 2,4,7,9-tetramethyl-5-decyne-4,7-diol), Surfynol 440 (ethylene oxide 3.5 mol adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol), Surfynol 465 (ethylene oxide 10 mol adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol), Surfynol DF110D (dipropylene glycol 32% by mass solution of 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol), etc. manufactured by Nissin Chemical Industry Co., Ltd. The surfactant (C) can be used alone or in combination of two or more. Further, within a range not inhibiting the effects of the present invention, it may contain surfactants other than the above.

[0057] In the aqueous ink used in the present invention, in addition to the above components, various additives such as commonly used humectants, wetting agents, penetrants, defoamers, preservatives, fungicides, and rust preventives can be added.

[0058] <Content of each component of the aqueous ink> The aqueous ink used in the present invention can be obtained by appropriately mixing and stirring the above components. The content and physical properties of each component of the obtained aqueous ink are as follows.

[0059] (Content of the pigment (A)) The content of the pigment (A) in the aqueous ink is preferably 2.0% by mass or more, more preferably 4.0% by mass or more, and still more preferably 6.0% by mass or more from the viewpoint of improving the recording density of the aqueous ink. Further, from the viewpoints of reducing the ink viscosity during solvent volatilization, improving the continuous ejection property, and further suppressing color unevenness, it is preferably 30.0% by mass or less, more preferably 20% by mass or less, still more preferably 15% by mass or less, and even more preferably 10.0% by mass or less.

[0060] (Content of water-insoluble polymer) When the colorant (A) is water-insoluble polymer particles containing a pigment, the content of the water-insoluble polymer in the aqueous ink is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and still more preferably 3.0% by mass or more from the viewpoints of improving the continuous ejection property and further suppressing color unevenness, and is preferably 10.0% by mass or less, more preferably 8.0% by mass or less, and still more preferably 6.0% by mass or less.

[0061] (Content of fixing aid polymer) The content of the fixing aid polymer in the aqueous ink is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and still more preferably 3.0% by mass or more from the viewpoint of the fixing property of the aqueous ink, and is preferably 10.0% by mass or less, more preferably 8.0% by mass or less, and still more preferably 6.0% by mass or less.

[0062] (Total content of polymers in the aqueous ink) The total content of the water-insoluble polymer and the fixing aid polymer in the aqueous ink is preferably 2.0% by mass or more, more preferably 4.0% by mass or more, and still more preferably 6.0% by mass or more from the viewpoints of the fixing property of the aqueous ink, improving the continuous ejection property, and further suppressing color unevenness, and is preferably 20.0% by mass or less, more preferably 16.0% by mass or less, and still more preferably 12.0% by mass or less.

[0063] (Content of water-soluble organic solvent (B)) From the perspective of improving the continuous ejection property of the ink, the content of the water-soluble organic solvent (B) in the aqueous ink is preferably 12.0% by mass or more, more preferably 15.0% by mass or more, still more preferably 18.0% by mass or more, and is preferably 50.0% by mass or less, more preferably 45.0% by mass or less, still more preferably 40.0% by mass or less. From the perspective of improving the continuous ejection property, the content of the glycol ether (b-1) in the aqueous ink is preferably 0.5% by mass or more, more preferably 0.7% by mass or more, still more preferably 0.9% by mass or more, and is preferably 15.0% by mass or less, more preferably 12.0% by mass or less, still more preferably 8.0% by mass or less. From the perspective of improving the continuous ejection property, the content of the polyhydric alcohol (b-2) in the aqueous ink is preferably 10.0% by mass or more, more preferably 12.0% by mass or more, still more preferably 15.0% by mass or more, and is preferably 45.0% by mass or less, more preferably 40.0% by mass or less, still more preferably 35.0% by mass or less. From the perspective of imparting appropriate drying property and further suppressing color unevenness in high-speed printing, the content of the high-boiling organic solvent having a boiling point of 250 °C or higher in the aqueous ink used in the present invention is preferably 5% by mass or less, more preferably 4% by mass or less, still more preferably 3% by mass or less.

[0064] (Mass ratio of glycol ether (b-1) to polyhydric alcohol (b-2)) The mass ratio [(b-1) / (b-2)] of the glycol ether (b-1) to the polyhydric alcohol (b-2) is preferably 0.01 or more, more preferably 0.05 or more, still more preferably 0.10 or more from the perspective of further suppressing color unevenness. It is preferably 1.00 or less, more preferably 0.50 or less, still more preferably 0.30 or less.

[0065] (Content of surfactant (C)) The content of surfactant (C) in the aqueous ink is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, still more preferably 0.8% by mass or more, and even more preferably 1.0% by mass or more, from the viewpoints of suppressing an increase in ink viscosity, improving the continuous discharge property of the ink, and further suppressing color unevenness. And it is preferably 5.0% by mass or less, more preferably 4.0% by mass or less, still more preferably 3.0% by mass or less, and even more preferably 2.0% by mass or less. The content of silicone surfactant (c-1) in the aqueous ink is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, still more preferably 0.3% by mass or more, and preferably 1.1% by mass or less, more preferably 0.9% by mass or less, still more preferably 0.7% by mass or less, from the same viewpoints as above. The content of acetylene glycol surfactant (c-2) in the aqueous ink is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, still more preferably 0.8% by mass or more, and preferably 4.0% by mass or less, more preferably 2.0% by mass or less, still more preferably 1.5% by mass or less, from the same viewpoints as above.

[0066] (Water content) The water content in the aqueous ink is preferably 20% by mass or more, more preferably 30% by mass or more, still more preferably 40% by mass or more, and preferably 85% by mass or less, more preferably 80% by mass or less, still more preferably 75% by mass or less, from the viewpoints of suppressing an increase in ink viscosity, improving the continuous discharge property of the ink, and further suppressing color unevenness.

[0067] <Physical properties of aqueous ink> The viscosity of the aqueous ink at 32 °C is preferably 2.0 mPa·s or more, more preferably 3.0 mPa·s or more, still more preferably 4.0 mPa·s or more, and preferably 12 mPa·s or less, more preferably 9.0 mPa·s or less, still more preferably 7.0 mPa·s or less, from the viewpoint of improving the continuous discharge property of the aqueous ink. The pH of the aqueous ink is preferably 6.5 or higher, more preferably 7.0 or higher, still more preferably 7.5 or higher, from the viewpoints of storage stability and the like and further suppressing color unevenness, and is preferably 11.0 or lower, more preferably 10.0 or lower, still more preferably 9.0 or lower, from the viewpoints of member resistance and skin irritation. The viscosity and pH of the aqueous ink are measured by the methods described in the examples.

[0068] The average particle diameter of the particles contained in the aqueous ink is preferably 40 nm or more, more preferably 60 nm or more, still more preferably 80 nm or more, even more preferably 85 nm or more, from the viewpoint of improving the continuous ejection property of the aqueous ink, and is preferably 220 nm or less, preferably 180 nm or less, still more preferably 160 nm or less, even more preferably 150 nm or less. The average particle diameter and the static surface tension are measured by the methods described in the examples.

Examples

[0069] In the following production examples, examples and comparative examples, "parts" and "%" are "parts by mass" and "mass%" unless otherwise specified. The measurement methods for each physical property are as follows.

[0070] (1) Measurement of the weight average molecular weight of the water-insoluble polymer Using a solution prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively, as an eluent, gel permeation chromatography [GPC apparatus (HLC-8120GPC) manufactured by Tosoh Corporation, column (TSK-GEL, α-M × 2) manufactured by Tosoh Corporation, flow rate: 1 mL / min], using monodisperse polystyrene with a known molecular weight in advance as a standard substance for measurement.

[0071] (2) Measurement of the solid content concentration of the pigment dispersion Weighed 10.0 g of sodium sulfate that had been constant-weighted in a desiccator into a 30-ml polypropylene container (φ = 40 mm, height = 30 mm), added approximately 1.0 g of the sample thereto, mixed them, weighed accurately, maintained at 105 °C for 2 hours to remove volatile components, left it in the desiccator for 15 minutes, and measured the mass. The mass of the sample after removing volatile components was taken as the solid content, and divided by the mass of the added sample to obtain the solid content concentration.

[0072] (3) Measurement of Pigment-Containing Polymer Particles and Average Particle Diameter of Polymer Particles Cumulant analysis was performed and measured using a laser particle analysis system “ELS-8000” (manufactured by Otsuka Electronics Co., Ltd.). The measurement conditions were a temperature of 25 °C, an angle of 90° between the incident light and the detector, and an integration number of 100 times. The refractive index of water (1.333) was input as the refractive index of the dispersion solvent. The measurement concentration was 5×10 -3 Mass% (in terms of solid content concentration).

[0073] (4) Measurement of Viscosity of Pigment Aqueous Dispersion and Aqueous Ink Using an E-type viscometer “TV-25” (manufactured by Toki Sangyo Co., Ltd., using a standard cone rotor of 1°34’×R24, rotation speed of 50 rpm), the viscosity was measured at 32 °C.

[0074] (5) Measurement of pH of Pigment Aqueous Dispersion and Aqueous Ink Using a desktop pH meter “F-71” (manufactured by Horiba, Ltd.) and a pH electrode “6337-10D” (manufactured by Horiba, Ltd.), the pH of the aqueous ink at 25 °C was measured.

[0075] (6) Water Absorption Amount of Recording Medium at Contact Time of 100 msec between Recording Medium and Pure Water Using an automatic scanning liquid absorbance meter (manufactured by Kumagai Riki Kogyo Co., Ltd., KM500win), the transfer amount at a contact time of 100 ms of pure water was measured under the conditions of 23 °C and a relative humidity of 50%, and taken as the water absorption amount at 100 msec. The measurement conditions are shown below. “Spiral Method” Contact Time : 0.010~1.0 (sec) Pitch (mm): 7 Length Per Sampling (degree): 86.29 Start Radius (mm): 20 End Radius (mm): 60 Min Contact Time (ms): 10 Max Contact Time (ms): 1000 Sampling Pattern (1 - 50): 50 Number of Sampling Points (> 0): 19 「Square Head」 Slit Span (mm) : 1 Slit Width (mm) : 5

[0076] Production Example 1 (Synthesis of Water - Insoluble Polymer) 16 parts of methacrylic acid (manufactured by Fujifilm Wako Pure Chemical Corporation), 44 parts of styrene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 30 parts of styrene macromonomer "AS - 6S" (manufactured by Toagosei Co., Ltd., number - average molecular weight 6,000, solid content 50%), and 25 parts of methoxypolyethylene glycol methacrylate "Blemmer PME - 200" (manufactured by NOF Corporation) were mixed to prepare 115 parts of a monomer mixture (100 parts as the total content of monomers). 18 parts of methyl ethyl ketone, 0.03 part of 2 - mercaptoethanol as a chain transfer agent, and 10% (11.5 parts) of the above monomer mixture were placed in a reaction vessel and mixed, and sufficient nitrogen gas substitution was carried out. On the other hand, 90% (103.5 parts) of the remaining monomer mixture, 0.27 part of the chain transfer agent, 42 parts of methyl ethyl ketone, and 3 parts of the polymerization initiator 2,2'-azobis(2,4-dimethylvaleronitrile) "V-65" (manufactured by Fujifilm Wako Pure Chemical Corporation) were mixed, and the resulting mixture was placed in a dropping funnel. While stirring the mixed solution in the reaction vessel under a nitrogen atmosphere, the temperature was raised to 75°C, and the mixed solution in the dropping funnel was added dropwise over 3 hours. After 2 hours had elapsed at 75°C from the end of the dropwise addition, a solution prepared by dissolving 3 parts of the polymerization initiator in 5 parts of methyl ethyl ketone was added, and the mixture was further aged at 75°C for 2 hours and at 80°C for 2 hours. Then, 50 parts of methyl ethyl ketone was added to obtain a solution of a water-insoluble polymer (weight average molecular weight: 50,000). The solid content concentration of the water-insoluble polymer solution was 45% by mass.

[0077] Production Example 2-1 (Production of an aqueous dispersion of black pigment-containing polymer particles) 95.2 parts of the water-insoluble polymer solution obtained in Production Example 1 was dissolved in 53.9 parts of methyl ethyl ketone, and 15.0 parts of a 5N aqueous sodium hydroxide solution and 0.5 part of 25% aqueous ammonia as neutralizing agents, and 341.3 parts of deionized water were added thereto. Next, 100 parts of C.I. Pigment Black 7 (P.B.7, manufactured by Cabot Corporation, carbon black pigment) as a black pigment was added to obtain a pigment mixture. The degree of neutralization was 78.8 mol%. The pigment mixture was mixed for 1 hour at 7000 rpm and 20°C using a dispersing blade, and then further subjected to a dispersion treatment of 15 passes at a pressure of 180 MPa using a microfluidizer "High Pressure Homogenizer M-140K" (manufactured by Microfluidics). After the dispersion treatment, methyl ethyl ketone and a part of the water were removed at 60°C under reduced pressure. Then, centrifugation was performed, and the liquid layer portion was filtered through a filter "Mini Sartorius Syringe Filter" (manufactured by Sartorius, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, obtaining a dispersion-treated product (solid content concentration 25% by mass). To 100 parts of the obtained dispersion-treated product (solid content concentration: 25% by mass), 0.45 part of Denacol EX321L (manufactured by Nagase ChemteX Corporation) and 15.0 parts of deionized water were added, and heat treatment was performed at 70 °C for 3 hours while stirring. After cooling to room temperature, the liquid layer portion was filtered through a filter "Mini Sartorius Syringe Filter" (manufactured by Sartorius, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, and an aqueous dispersion of polymer particles containing a black pigment (solid content concentration: 22.0% by mass) was obtained. The average particle diameter of the polymer particles containing a black pigment was 100 nm. The physical properties of the aqueous dispersion of polymer particles containing a black pigment are shown in Table 1.

[0078] Production Examples 2-2 to 2-4 (Production of Aqueous Dispersions of Polymer Particles Containing Cyan, Magenta, and Yellow Pigments) In Production Example 2-1, except that the black pigment was changed to a cyan pigment (manufactured by DIC Corporation, P.B. 15:3), a magenta pigment (manufactured by Fuji Pigment Co., Ltd., P.R. 122), or a yellow pigment (manufactured by Dainichi Seika Kogyo Co., Ltd., P.Y. 74), and the composition of the aqueous dispersion of pigment-containing polymer particles was changed to the composition shown in Table 1, an aqueous dispersion of polymer particles containing cyan, magenta, and yellow pigments (solid content concentration: 22% by mass) was obtained in the same manner as in Production Example 2-1. The physical properties of the aqueous dispersion of each pigment-containing polymer particle are shown in Table 1.

[0079]

Table 1

[0080] Production Example 3 (Production of Fixing Aid Polymer Emulsion) In a 1000 mL separable flask, 145 parts of methyl methacrylate (manufactured by FUJIFILM Wako Pure Chemical Corporation), 50 parts of 2-ethylhexyl acrylate (manufactured by FUJIFILM Wako Pure Chemical Corporation), 5 parts of methacrylic acid (manufactured by FUJIFILM Wako Pure Chemical Corporation), 18.5 parts of Latemul E118B (manufactured by Kao Corporation, emulsifier, active ingredient 26%), 96 parts of deionized water, and potassium persulfate (manufactured by FUJIFILM Wako Pure Chemical Corporation) were charged, and stirring was performed with a stirring blade (300 rpm) to obtain a monomer emulsion. 4.6 parts of Latemul E118B, 186 parts of deionized water, and 0.08 part of potassium persulfate were placed in a reaction vessel, and sufficient nitrogen gas substitution was performed. While stirring with a stirring blade (200 rpm) under a nitrogen atmosphere, the temperature was raised to 80 °C, and the monomer emulsion was charged into a dropping funnel. This monomer emulsion was added dropwise over 3 hours and reacted. Deionized water was added to this reaction solution to obtain a fixing aid polymer emulsion with a solid content of 41.6% by weight. The average particle size of this polymer emulsion was 100 nm.

[0081] Production Example 4-1 (Production of Black Ink) 46.3 parts by mass of an aqueous dispersion of black pigment-containing polymer particles (solid content: 22.0% by mass) obtained in Production Example 2-1 (including 7.0 parts by mass of pigment and 3.2 parts by mass of water-insoluble polymer), 12.0 parts by mass of the fixing aid polymer emulsion (solid content: 41.6% by weight) obtained in Production Example 3 (including 5.0 parts by mass of the fixing aid polymer), 4.0 parts by mass of diethylene glycol monoisobutyl ether (boiling point: 230 °C) as the (b-1) component, 23.0 parts by mass of propylene glycol (boiling point: 188 °C) as the (b-2) component, 0.5 part by mass of a silicone-based surfactant (manufactured by Shin-Etsu Chemical Co., Ltd., Silface SAG005), and 1.2 parts by mass of an acetylene glycol-based surfactant (manufactured by Nisshin Chemical Industry Co., Ltd., Surfynol 104PG50) were added. Further, deionized water was added so that the total amount became 100 parts by mass and mixed. The obtained mixture was filtered through a filter "Mini Sartorius Syringe Filter" (manufactured by Sartorius, pore size: 5.0 μm, material: cellulose acetate) to obtain a black ink (aqueous ink). The physical properties of the black ink are shown in Table 2.

[0082] Production Examples 4-2 to 4-4 (Production of Cyan, Magenta, and Yellow Inks) In Production Example 4-1, except that the aqueous dispersion of black pigment-containing polymer particles was changed to the aqueous dispersions of cyan, magenta, and yellow pigment-containing polymer particles obtained in Production Examples 2-2 to 2-4, and the composition of the aqueous ink was changed to the composition shown in Table 2, cyan, magenta, and yellow aqueous inks were obtained in the same manner as in Production Example 4-1. Also, the four-color inks obtained in Production Examples 4-1 to 4-4 were designated as Ink Set 1. The physical properties of each aqueous ink are shown in Table 2.

[0083] Production Examples 4-5 to 4-8, 4-9 to 4-12, 4-13 to 4-16, and 4-17 to 4-20 In Production Examples 4-1 to 4-4, except that the composition of the aqueous ink was changed to the composition shown in Table 2, in the same manner as in Production Examples 4-1 to 4-4, the four-color inks obtained in Production Examples 4-5 to 4-8 were designated as Ink Set 2, the inks obtained in Production Examples 4-9 to 4-12 were designated as Ink Set 3, the inks obtained in Production Examples 4-13 to 4-16 were designated as Ink Set 4, and the inks obtained in Production Examples 4-17 to 4-20 were designated as Ink Set 5. The physical properties of each aqueous ink are shown in Table 2.

[0084]

Table 2

[0085] Example 1 On corona-treated PET (manufactured by Futamura Chemical Co., Ltd., Taiko polyester film FE2001, water absorption of the recording medium at a contact time of 100 msec between the recording medium and pure water: 0 g / m 2 ), using Ink Set 1 obtained in Production Examples 4-1 to 4-4, with the printing order being cyan ink, magenta ink, black ink, and yellow ink in that order, an image was formed by the following inkjet recording method. (Inkjet recording method) In an environment with a temperature of 25 ± 1°C and a relative humidity of 30 ± 5%, an aqueous ink was filled into a one-pass printing evaluation apparatus (manufactured by Trytec Co., Ltd.) having a roll-to-roll conveyance equipped with an inkjet head (manufactured by Kyocera Corporation, "KJ4B-HD06MHG-STDV", piezo type). At this time, in Example 1, the aqueous ink was filled in the order of cyan ink, magenta ink, black ink, and yellow ink from the inkjet head on the upstream side in the feeding direction of the recording medium so that the printing order was in this order. The interval between each inkjet head filled with each aqueous ink was set to 55 cm. A head voltage of 26 V, a frequency of 10 kHz, an appropriate discharge volume of 3 pL, a head temperature of 32°C, a resolution of 600 dpi, a pre-discharge flushing count of 200 shots, and a negative pressure of -4.0 kPa were set, and the recording medium was fixed to the printing evaluation apparatus in a direction such that the longitudinal direction and the conveyance direction of the recording medium were the same. The printing evaluation apparatus includes an inkjet head that discharges an aqueous ink onto the recording medium and an under-heater that heats the recording medium from the back side of the surface of the recording medium facing the head. The distance between the under-heater and the recording medium was set to 0.25 mm, and the distance between the inkjet head and the recording medium was set to 1.0 mm. The surface temperature of the under-heater was set to 40°C, and the temperature of the recording medium was set to 40°C. A print command was transferred to the printing evaluation apparatus, and inks were discharged in the order of cyan ink, magenta ink, black ink, and yellow ink to print a solid brown image. Then, it was dried with a hot air dryer at 60°C for 5 minutes to obtain a printed matter. Note that the printing duty of each color of the solid brown image was set to a total of 260% with black at 30%, cyan at 30%, magenta at 100%, and yellow at 100%.

[0086] (Evaluation of color unevenness) The solid image of the printed matter obtained by the above inkjet recording method was visually confirmed, and the color unevenness was evaluated according to the following evaluation criteria. The evaluation results are shown in Table 3. In the following evaluation criteria, if the evaluation is 3 to 5, there is no problem in practical use. 〔Evaluation criteria for color unevenness〕 5: The solid image is uniform. 4: There is shading unevenness in less than 10% of the area of the solid image. 3: There is shading unevenness in 10% or more and less than 20% of the area of the solid image. 2: There is shading unevenness in 20% or more and less than 50% of the area of the solid image. 1: There is obvious shading unevenness across the entire area of the solid image.

[0087] (Evaluation of continuous ejection property) In the above inkjet recording method, each ink set was filled into an inkjet head (number of nozzles per color: 2,656, total number of nozzles for all colors: 10,624) so as to have a predetermined printing order, and the continuous ejection property was evaluated. After continuously ejecting ink for 80 minutes at an appropriate ejection liquid volume of 3 pL, nozzle check patterns for each color were printed respectively. The printed nozzle check patterns for each color were visually observed, and the number of nozzles of the inkjet head where landing deviation occurred was counted. The continuous ejection property was evaluated according to the following evaluation criteria based on the total number of nozzles for all colors where landing deviation occurred. The evaluation results are shown in Table 3. 〔Evaluation criteria for continuous ejection property〕 5: No landing deviation is observed. 4: The number of nozzles where landing deviation occurs is less than 5 in the total for all colors. 3: The number of nozzles where landing deviation occurs is 5 or more and less than 20 in the total for all colors. 2: The number of nozzles where landing deviation occurs is 20 or more and less than 200 in the total for all colors. 1: The number of nozzles where landing deviation occurs is 200 or more in the total for all colors.

[0088] Examples 2 to 5 In Example 1, evaluation was carried out in the same manner as in Example 1 except that the ink set was changed to the ink set shown in Table 3.

[0089] Example 6 In Example 1, evaluation was carried out in the same manner as in Example 1 except that the temperature of the under-heater was changed to 30°C.

[0090] Example 7 In Example 1, evaluation was performed in the same manner as in Example 1, except that the temperature of the underheater was changed to 45°C.

[0091] Comparative Example 1 In Example 1, evaluation was performed in the same manner as in Example 1, except that the printing order of the aqueous inks was changed to the order of black ink, cyan ink, magenta ink, and yellow ink.

[0092] Comparative Example 2 In Example 1, evaluation was performed in the same manner as in Example 1, except that the printing order of the aqueous inks was changed to the order of yellow ink, magenta ink, cyan ink, and black ink.

[0093]

Table 3

[0094] From Table 3, it can be seen that the inkjet printing methods of Examples 1 to 7 are excellent in continuous discharge properties, capable of suppressing color unevenness even when printing on a low liquid-absorbing recording medium such as a resin film, and also capable of suppressing discharge defects even when performing continuous printing, as compared with Comparative Examples 1 and 2.

Claims

1. An inkjet recording method for recording on a low liquid-absorbing recording medium using an inkjet recording apparatus including an inkjet head that discharges an aqueous ink onto the low liquid-absorbing recording medium, wherein the low liquid-absorbing recording medium is a resin film, the aqueous ink contains a colorant (A), a water-soluble organic solvent (B), and water, the inkjet recording apparatus includes at least four or more types of the aqueous ink including black ink, cyan ink, magenta ink, and yellow ink, and discharges the aqueous ink in the order of cyan ink, magenta ink, black ink, and yellow ink.

2. The inkjet recording method according to claim 1, wherein the colorant (A) is water-insoluble polymer particles containing a pigment.

3. The inkjet recording method according to claim 1 or 2, wherein the boiling point of the water-soluble organic solvent (B) is 90°C or higher and less than 250°C.

4. The inkjet recording method according to claim 1 or 2, wherein the water-soluble organic solvent (B) contains a glycol ether (b-1) and a polyhydric alcohol (b-2).

5. The inkjet recording method according to claim 4, wherein the mass ratio [(b-1) / (b-2)] of the glycol ether (b-1) to the polyhydric alcohol (b-2) is 0.01 or more and 1.0 or less.

6. The inkjet recording method according to claim 1 or 2, further including an under heater provided on a surface opposite to the surface of the recording medium facing the inkjet head to heat the recording medium.

7. The inkjet recording method according to claim 1 or 2, wherein the recording speed is 5 m / min or more and 75 m / min or less in terms of the conveyance speed of the low liquid-absorbing recording medium.

8. The inkjet recording method according to claim 1 or 2, wherein recording is performed in a one-pass method.

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