Patent Publication Number: US-2012033010-A1

Title: Aqueous inkjet ink

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from U.S. Provisional Application No. 61/370,620 filed on Aug. 4, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an aqueous inkjet ink, a method for inkjet printing, and an inkjet printing apparatus. 
     BACKGROUND 
     In an aqueous inkjet ink, a color material such as a dye or a pigment is dispersed in an aqueous medium. 
     Even when an aqueous medium having high volatility such as water is used, an aqueous inkjet ink should have a property suitable for ejection from an inkjet head. Further, the aqueous inkjet ink for use in printing on a paper medium is required to suppress the deformation of a paper medium such as curling or cockling as much as possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The single FIGURE shows an example of an inkjet printing apparatus to which an embodiment is applied. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an aqueous inkjet ink includes a color material in an amount of from 2 to 20% by mass of the total amount of the ink, water in an amount of from 41 to 70% by mass of the total amount of the ink, and a water-soluble organic solvent in an amount of from 20 to 53% by mass of the total amount of the ink. The water-soluble organic solvent contains a compound represented by the following general formula 1, and the compound represented by the following general formula 1 accounts for 20 to 50% by mass of the total amount of the aqueous inkjet ink. When the amount of the compound represented by the following general formula 1 is equal to the amount of the water-soluble organic solvent, the total amount of the water-soluble organic solvent is the compound represented by the following general formula 1. The sum of the amounts of the color material, the water, and the water-soluble organic solvent is 100% by mass or less of the aqueous inkjet ink. 
     
       
         
         
             
             
         
       
     
     In the general formula 1, a, b, and c may be the same or different, each representing an integer of 1 or more, and satisfy the inequation: 3≦a+b+c≦15. 
     Hereinafter, embodiments will be specifically described. 
     In an inkjet printing apparatus shown in the FIGURE, paper cassettes  100  and  101  contain paper P of different sizes, respectively. A paper feed roller  102  or  103  takes out the paper P in response to the selected paper size from the paper cassette  100  or  101  and conveys the paper P to conveying roller pairs  104  and  105  and a resist roller pair  106 . 
     A tension is given to a conveying belt  107  by a driving roller  108  and two driven rollers  109 . Through-holes are provided at predetermined intervals in the conveying belt  107 , and for the purpose of adsorbing the paper P onto the conveying belt  107 , a negative pressure chamber  111  connected to a fan  110  is installed in the inside of the conveying belt  107 . Conveying roller pairs  112 ,  113 , and  114  are installed downstream in the paper conveying direction of the conveying belt  107 . A conveying unit is constituted by the conveying belt  107 , the driving roller  108 , the driven rollers  109 , and the conveying roller pairs  112 ,  113 , and  114 . 
     Four rows of inkjet heads which eject inks onto paper according to image data are disposed above the conveying belt  107 . An inkjet head  115 C which ejects a cyan (C) ink, an inkjet head  115 M which ejects a magenta (M) ink, an inkjet head  115 Y which ejects a yellow (Y) ink, and an inkjet head  115 Bk which ejects a black (Bk) ink are arranged in this order from the upstream. Further, these inkjet heads  115 C,  115 M,  115 Y, and  115 Bk are provided with a cyan (C) ink cartridge  116 C, a magenta (M) ink cartridge  116 M, a yellow (Y) ink cartridge  116 Y, and a black (Bk) ink cartridge  116 Bk, respectively, each of which contains an ink of each color. These cartridges are connected to the inkjet heads via tubes  117 C,  117 M,  117 Y, and  117 Bk, respectively. 
     An image forming action of the inkjet printing apparatus having such a structure will be described below. 
     First, image processing for printing by an image processing unit (not shown) is initiated, and image data for printing are transferred to the respective inkjet heads  115 C,  115 M,  115 Y, and  115 Bk. Also, the paper P (paper medium P) of a selected paper size is taken out one by one from the paper cassette  100  or  101  by the paper feed roller  102  or  103  and conveyed to the conveying roller pairs  104  and  105  and the resist roller pair  106 . The resist roller pair  106  corrects a skew of the paper P and conveys the paper P to each of the inkjet heads  115 C,  115 M,  115 Y, and  115 Bk at a given timing. 
     The negative pressure chamber  111  draws air through the holes of the conveying belt  107 , and therefore, the paper P is conveyed in a state of being adsorbed onto the conveying belt  107  on a lower side of the inkjet heads  115 C,  115 M,  115 Y, and  115 Bk. In this manner, the respective inkjet heads  115 C,  115 M,  115 Y, and  115 Bk and the paper P can keep a fixed space from each other. The ink of each color is ejected from each of the inkjet heads  115 C,  115 M,  115 Y, and  115 Bk in synchronization with the timing for conveying the paper P from the resist roller pair  106 . Thus, a color image is formed at a desired position on the paper P. The paper P having an image formed thereon is discharged to a paper discharge tray  118  by the conveying roller pairs  112 ,  113 , and  114 . 
     Each ink cartridge stores an aqueous inkjet ink according to one embodiment. 
     The aqueous inkjet ink according to this embodiment contains water, a water-soluble organic solvent, and a color material. The amount of water is from 41 to 70% by mass of the total amount of the ink, and the amount of the water-soluble organic solvent is from 20 to 53% by mass of the total amount of the ink. Further, the water-soluble organic solvent contains a polyoxyethylene glyceryl ether represented by the following general formula 1, and the polyoxyethylene glyceryl ether accounts for 20 to 50% by mass of the total amount of the ink. 
     
       
         
         
             
             
         
       
     
     In the general formula 1, a, b, and c may be the same or different, each representing an integer of 1 or more, and satisfy the inequation: 3≦a+b+c≦15. 
     An ink medium in the aqueous inkjet ink according to this embodiment is constituted by water and a given polyoxyethylene glyceryl ether, both of which are blended in predetermined amounts, respectively. In the ink medium, a color material is dissolved or dispersed. As described above, the amount of water is from 41 to 70% by mass of the total amount of the ink, and the amount of the water-soluble organic solvent is from 20 to 53% by mass of the total amount of the ink. Further, the polyoxyethylene glyceryl ether accounts for 20 to 50% by mass of the total amount of the ink. Since such an ink medium is used, the aqueous inkjet ink according to this embodiment can form a high-quality image on a paper medium while suppressing the deformation of the paper medium as much as possible. 
     The aqueous inkjet ink according to this embodiment has an excellent ability to suppress the deformation of paper, and therefore, there is no fear that paper after printing cannot be conveyed. As a result, it became possible to use the aqueous inkjet ink in high-speed printing at a printing rate of 50 ppm (page per minute) or more. 
     The paper medium as used herein generally refers to a medium made of paper to be used for printing. The paper medium is broadly divided into coated paper coated with a material for increasing print properties such as art paper or coat paper and non-coated paper to be used for utilizing the properties of paper itself. The paper medium is applied to a variety of uses such as books, documents, newspapers, packages, printer sheets, etc. The paper medium also includes corrugated cardboard, containers made of paper, and thick paper such as cardboard. For example, so-called plain paper such as copy paper to be used in a copier or a printer for office or home use is a typical paper medium. 
     According to one embodiment, a color material is contained in an ink medium containing water and a water-soluble organic solvent containing a given polyoxyethylene glyceryl ether in predetermined amounts, respectively. 
     As the color material, either of a dye and a pigment may be used. As the dye, any of a variety of dyes to be used in an inkjet ink such as a direct dye, an acidic dye, a food dye, a reactive dye, or a disperse dye can be used. 
     Specific examples of the dye include the following dyes: C.I. Direct Yellow 8, C.I. Direct Yellow 11, C.I. Direct Yellow 24, C.I. Direct Yellow 26, C.I. Direct Yellow 27, C.I. Direct Yellow 28, C.I. Direct Yellow 33, C.I. Direct Yellow 44, C.I. Direct Yellow 50, C.I. Direct Orange 6, C.I. Direct Orange 8, C.I. Direct Orange 29, C.I. Direct Orange 102, C.I. Direct Red 1, C.I. Direct Red 2, C.I. Direct Red 4, C.I. Direct Red 13, C.I. Direct Red 17, C.I. Direct Red 20, C.I. Direct Red 33, C.I. Direct Red 37, C.I. Direct Red 44, C.I. Direct Red 46, C.I. Direct Red 62, C.I. Direct Red 75, C.I. Direct Blue 1, C.I. Direct Blue 2, C.I. Direct Blue 6, C.I. Direct Blue 15, C.I. Direct Blue 22, C.I. Direct Blue 25, C.I. Direct Blue 76, C.I. Direct Blue 77, C.I. Direct Blue 86, C.I. Direct Blue 108, and C.I. Direct Blue 120. 
     The dye is contained in the ink in an amount of from 2 to 20% by mass of the total amount of the ink. If the dye is contained within this range, a printed matter having a necessary image density can be formed without causing any problems with respect to the storage stability or ejection performance of the ink. 
     If a pigment is used as the color material, the water resistance and light resistance of the aqueous inkjet ink can be further increased. 
     The pigment is not particularly limited, and either of an inorganic pigment and an organic pigment may be used. Examples of the inorganic pigment include titanium oxide and iron oxide. Further, a carbon black produced by a known method such as a contact method, a furnace method, or a thermal method can be used. 
     As the organic pigment, for example, an azo pigment (such as an azo lake pigment, an insoluble azo pigment, a condensed azo pigment, or a chelate azo pigment), a polycyclic pigment (such as a phthalocyanine pigment, a perylene pigment, a perinone pigment, an anthraquinone pigment, a quinacridone pigment, a dioxazine pigment, a thioindigo pigment, an isoindolinone pigment, or a quinophthalone pigment), a dye chelate (such as a basic dye type chelate, or an acid dye type chelate), a nitro pigment, a nitroso pigment, aniline black, or the like can be used. 
     Specific examples of the carbon black which is used in a black ink include No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, and No. 2200B (all of which are manufactured by Mitsubishi Chemical Corporation), Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, and Raven 700 (all of which are manufactured by Columbian Chemicals Company), Regal 400R, Regal 330R, Regal 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, and Monarch 1400 (all of which are manufactured by Cabot Corporation), and Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW18, Color Black FW200, Color Black S150, Color Black S160, Color Black S170, Printex 35, Printex U, Printex V, Printex 140U, Special Black 6, Special Black 5, Special Black 4A, and Special Black 4 (all of which are manufactured by Degussa AG). 
     Specific examples of the pigment which is used in a yellow ink include C.I. Pigment Yellow 1, C.I. Pigment Yellow 2, C.I. Pigment Yellow 3, C.I. Pigment Yellow 12, C.I. Pigment Yellow 13, C.I. Pigment Yellow 14C, C.I. Pigment Yellow 16, C.I. Pigment Yellow 17, C.I. Pigment Yellow 73, C.I. Pigment Yellow 74, C.I. Pigment Yellow 75, C.I. Pigment Yellow 83, C.I. Pigment Yellow 93, C.I. Pigment Yellow 95, C.I. Pigment Yellow 97, C.I. Pigment Yellow 98, C.I. Pigment Yellow 109, C.I. Pigment Yellow 110, C.I. Pigment Yellow 114, C.I. Pigment Yellow 128, C.I. Pigment Yellow 129, C.I. Pigment Yellow 138, C.I. Pigment Yellow 150, C.I. Pigment Yellow 151, C.I. Pigment Yellow 154, C.I. Pigment Yellow 155, C.I. Pigment Yellow 180, and C.I. Pigment Yellow 185. 
     Specific examples of the pigment which is used in a magenta ink include C.I. Pigment Red 5, C.I. Pigment Red 7, C.I. Pigment Red 12, C.I. Pigment Red 48 (Ca), C.I. Pigment Red 48 (Mn), C.I. Pigment Red 57 (Ca), C.I. Pigment Red 57:1, C.I. Pigment Red 112, C.I. Pigment Red 122, C.I. Pigment Red 123, C.I. Pigment Red 168, C.I. Pigment Red 184, C.I. Pigment Red 202, and C.I. Pigment Violet 19. 
     Specific examples of the pigment which is used in a cyan ink include C.I. Pigment Blue 1, C.I. Pigment Blue 2, C.I. Pigment Blue 3, C.I. Pigment Blue 15:3, C.I. Pigment Blue 15:4, C.I. Pigment Blue 15:34, C.I. Pigment Blue 16, C.I. Pigment Blue 22, C.I. Pigment Blue 60, C.I. Vat Blue 4, and C.I. Vat Blue 60. 
     Since the pigment is used in an inkjet ink, it is preferred that the average particle diameter of the pigment is within a range of from about 10 to 300 nm. It is more preferred that the average particle diameter of the pigment is within a range of from about 10 to 200 nm. 
     The average particle diameter of the pigment can be determined using a particle size distribution analyzer by means of a dynamic light scattering method. Examples of the particle size distribution analyzer include HPPS (Malvern Instruments Ltd.). 
     The pigment can be used in a state of a pigment dispersion. The pigment dispersion can be prepared by, for example, dispersing the pigment in water or an alcohol with a dispersant. Examples of the dispersant include a surfactant, a water-soluble resin, and a water-insoluble resin. Alternatively, a self-dispersible pigment may be used. The self-dispersible pigment is a pigment which can be dispersed in water or the like without using a dispersant, and to which at least one functional group selected from a carbonyl group, a carboxyl group, a hydroxyl group, and a sulfone group or a salt thereof is bound through a surface treatment. Examples of the surface treatment include a vacuum plasma treatment, a diazo coupling treatment, and an oxidation treatment. The self-dispersible pigment is obtained by grafting a functional group or a molecule containing a functional group on the surface of a pigment through such a surface treatment. 
     The content of the pigment in the ink is within a range of from 2 to 20% by mass of the total amount of the ink. If the content of the pigment is within this range, a printed matter having a necessary image density can be formed without causing any problems with respect to the storage stability or ejection performance of the ink. An ink containing the pigment in an amount of from 3 to 10% by mass of the total amount of the ink has a viscosity suitable for ejection from an inkjet head over a wide temperature range of from about 20 to 50° C. Also, the quality of an image formed using the ink is favorable. 
     The pigment dispersion is mixed with the ink medium composed of water and a water-soluble organic solvent containing a given polyoxyethylene glyceryl ether, whereby the aqueous inkjet ink according to this embodiment is prepared. 
     As described above, according to one embodiment, the amount of water is from 41 to 70% by mass of the total amount of the ink. If the amount of water in the ink is too small, a viscosity within a range suitable as an inkjet ink cannot be ensured, and therefore, the ejection performance from the inkjet head is lowered. Moreover, the dispersion stability of the pigment is lowered, and therefore, the clogging of the inkjet head is liable to occur. On the other hand, if the amount of water in the ink is too large, it is not easy to suppress the deformation of paper. The amount of water is preferably from 45 to 65% by mass of the total amount of the ink. 
     In the aqueous inkjet ink according to one embodiment, from 20 to 53% by mass of the total amount of the ink is the water-soluble organic solvent. If the amount of the water-soluble organic solvent in the ink is too small, it is not easy to suppress the deformation of paper. On the other hand, if the amount of the water-soluble organic solvent in the ink is too large, a viscosity within a range suitable as an inkjet ink cannot be ensured, and therefore, the ejection performance from the inkjet head is lowered. Moreover, the dispersion stability of the pigment is lowered, and therefore, the clogging of the inkjet head is liable to occur. The amount of the water-soluble organic solvent is more preferably from 30 to 50% by mass of the total amount of the ink. The sum of the amounts of the color material, water, and the water-soluble organic solvent is 100% by mass or less of the aqueous inkjet ink. 
     Incidentally, if the amount of the water-soluble organic solvent does not exceed the amount of water, the storage stability of the aqueous inkjet ink can be maintained at a high level. 
     In the water-soluble organic solvent, a polyoxyethylene glyceryl ether represented by the following general formula 1 is contained. The amount of the polyoxyethylene glyceryl ether is from 20 to 50% by mass of the total amount of the ink. 
     
       
         
         
             
             
         
       
     
     In the general formula 1, a, b, and c may be the same or different, each representing an integer of 1 or more, and satisfy the inequation: 3≦a+b+c≦15. 
     In order to suppress the deformation of paper, the value of (a+b+c) in the general formula 1 is set to 3 or more. If the value of (a+b+c) is too large, a viscosity within a range suitable as an inkjet ink cannot be ensured. Moreover, there is a tendency that the dispersion stability of the pigment is decreased, and therefore, the upper limit of the value of (a+b+c) is set to 15. 
     Examples of the polyoxyethylene glyceryl ether represented by the general formula 1 include Uniox G-450 and Uniox G-750 (both of which are manufactured by NOF Corporation). The value of (a+b+c) is said to be about 8 in the case of Uniox G-450, and about 15 in the case of Uniox G-750. 
     If the inkjet ink includes an ink medium containing a polyoxyethylene glyceryl ether as described above, the ejection performance from the inkjet head is improved, and also a suppressive effect on the deformation of paper is increased. The present inventors considered the suppression of deformation of paper as follows. When paper comes into contact with the aqueous ink, the paper is deformed generally due to a mechanism as described below. First, because of water in the aqueous ink, hydrogen atoms forming hydrogen bonds in cellulose fiber which is a main component of paper are dissociated due to swelling and the hydrogen bonds are cleaved. When water evaporates and the paper dries, the hydrogen atoms in the cellulose fiber are reassociated and form hydrogen bonds again. This reassociation occurs in different states or at different positions than before the hydrogen bonds are cleaved, and as a result, the paper is deformed and curling or the like is caused. 
     The above-described polyoxyethylene glyceryl ether contains an ethylene oxide group. This ethylene oxide group can reassociate hydrogen atoms and again to form hydrogen bonds which were cleaved by the penetration of water molecules. In this manner, the deformation of paper is suppressed. 
     By using the ink medium including the water-soluble organic solvent containing such a polyoxyethylene glyceryl ether and water in combination, the excellent properties thereof is maximally exhibited, and the performance of suppressing the deformation of paper is markedly improved. 
     Incidentally, the amount of the polyoxyethylene glyceryl ether is from 20 to 50% by mass of the total amount of the ink. If the amount of the polyoxyethylene glyceryl ether is too small, the effect thereof cannot be obtained. On the other hand, if the amount of the polyoxyethylene glyceryl ether is too large, the nozzle is clogged with the ink and ejection becomes not easy. The amount of the polyoxyethylene glyceryl ether is preferably from 20 to 40% by mass of the total amount of the ink. 
     In the water-soluble organic solvent, a compound as shown below may be further contained. Examples of the compound include a polyhydric alcohol having 2 to 10 carbon atoms, a lower alkyl ether of a polyhydric alcohol wherein the lower alkyl group has 1 to 6 carbon atoms and the polyhydric alcohol has 2 to 6 carbon atoms, a heterocyclic compound, and a sulfur-containing compound. 
     Examples of the polyhydric alcohols having 2 to 10 carbon atoms include glycerin, polyethylene glycol, polypropylene glycol, ethylene glycol, propylene glycol, triethylene glycol, 1,2-hexanediol, and trimethylolpropane. 
     Examples of the lower alkyl ether of a polyhydric alcohol wherein the lower alkyl group has 1 to 6 carbon atoms and the polyhydric alcohol has 2 to 6 carbon atoms include an ethylene glycol monoalkyl ether, a diethylene glycol monoalkyl ether, a triethylene glycol monobutyl ether, and a triethylene glycol monoalkyl ether. 
     Examples of the heterocyclic compound include 2-pyrrolidone, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, and N-ethylmorpholine. Examples of the sulfur-containing compound include sulfolane, dimethylsulfoxide, and 3-sulfolene. 
     Further, a compound selected from diacetone alcohol, diethanolamine, and triethanolamine may be contained in the water-soluble organic solvent. 
     Among these compounds, glycerin, polyethylene glycol, polypropylene glycol, ethylene glycol, propylene glycol, 2-pyrrolidone, triethylene glycol monobutyl ether, and 1,2-hexanediol are preferred. These compounds also function as a moisture retaining agent. 
     When the aqueous inkjet ink according to this embodiment is obtained, for example, the water-soluble organic solvent containing water and a polyoxyethylene glyceryl ether and a pigment dispersion are mixed. To the ink according to this embodiment, an additive as described below can be added as needed. 
     Since the ink according to this embodiment is for use in inkjet printing, it is necessary that the ink have a viscosity suitable for ejection from a nozzle of a head in an inkjet printer. Specifically, the viscosity of the ink at 25° C. is preferably from 5 to 30 mPa·s, more preferably from 3 to 15 mPa·s. 
     In order to adjust the ejection performance or the like of the aqueous inkjet ink, a surfactant may be additionally added thereto within a range that does not deteriorate the effects. 
     As the surfactant, either of an anionic surfactant and a nonionic surfactant may be used. Examples of the anionic surfactant include an alkylbenzene sulfonate salt, a sulfosuccinate ester salt, and a phosphate ester salt of a higher alcohol. Specific examples thereof include sodium dodecylbenzene sulfonate and sodium dioctyl sulfosuccinate. 
     Examples of the nonionic surfactant include a fatty acid salt, an ester salt of a higher alcohol, an ethylene oxide adduct of a higher alcohol, an ethylene oxide adduct of an alkylphenol, an ethylene oxide adduct of a polyhydric alcohol fatty acid ester, and acetylene glycol and an ethylene oxide adduct thereof. Specific examples thereof include polyoxyethylene nonylphenyl ether and polyoxyethylene lauryl ether, and also include Surfynol 104, Surfynol 440, Surfynol 465, and Surfynol TG (all of which are manufactured by Air Products and Chemicals, Inc.), and Orphin E1010 (manufactured by Nissin Chemical Industry Co., Ltd.). 
     If the surfactant is contained in an amount of from about 0.01 to 10% by mass of the total amount of the ink, the effect thereof can be exhibited without causing any problems. The amount of the surfactant is more preferably within a range of from 0.1 to 5% by mass of the total amount of the ink. 
     Among the above-described surfactants, for example, a nonionic surfactant or the like can be used as a dispersant for preparing a pigment dispersion. 
     In the aqueous inkjet ink according to this embodiment, an antiseptic or antifungal agent can be blended as needed. Examples of the antiseptic or antifungal agent include sodium pyridinethion-1-oxide, ethyl p-hydroxybenzoate, 1,2-benzisothiazolin-3-one and a salt thereof, sodium dehydroacetate, 2-phenoxyethanol, and sodium benzoate. 
     Among these, 1,2-benzisothiazolin-3-one and a salt thereof are preferred, and the compound is commercially available under the name of Proxel from Avecia Biotechnology Inc. 
     If the antiseptic or antifungal agent is contained in an amount of from about 0.001 to 3% by mass of the total amount of the ink, the effect thereof can be exhibited without causing any problems. The amount of the antiseptic or antifungal agent is more preferably from 0.01 to 1% by mass of the total amount of the ink. 
     The aqueous inkjet ink according to this embodiment preferably further contains an antioxidant. The present inventors considered the effect of the antioxidant as follows. The aqueous inkjet ink is sometimes exposed to a high temperature of 50° C. or higher when being transported. In that case, an ethylene oxide group of a polyoxyethylene glyceryl ether represented by the general formula 1 is cleaved and an acid is generated. Owing to the antioxidant, the generation of an acid is suppressed, and a variation in the properties of the ink such as pH due to a thermal history is suppressed. As a result, the storage stability of the ink is increased. 
     The above-described cleavage of an ethylene oxide group is sometimes caused when the ink is exposed to the atmosphere. When printing is performed on paper, the aqueous inkjet ink according to this embodiment is exposed to the atmosphere, the cleavage of an ethylene oxide group proceeds over time. An acid generated by the cleavage of an ethylene oxide group forms a new hydrogen bond in cellulose fiber, which results in an increase in the deformation of paper. Owing to the antioxidant, the generation of an acid is suppressed, and therefore, the formation of a new hydrogen bond is reduced. In this manner, even after some time has passed since printing is performed, the deformation of paper such as curling can be suppressed. 
     Examples of the antioxidant include a phenol-based antioxidant (including a hindered phenol-based antioxidant), an amine-based antioxidant, a sulfur-based antioxidant, and a phosphorus-based antioxidant. 
     As the phenol-based antioxidant, from the viewpoint of the weather resistance and the stability of the aqueous dispersion, 2,6-di-tert-butyl-4-methylphenol, 2,2′-methylenebis(4-ethyl-6-tert-butylphenol), tetrakis-[methylene-3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate]methane, 2,6-di-tert-butyl-p-cresol (BHT), and the like are preferred. 
     Examples of the amine-based antioxidant include phenyl-β-naphthylamine, α-naphthylamine, N,N′-di-sec-butyl-p-phenylenediamine, phenothiazine, and N,N′-diphenyl-p-phenylenediamine. 
     Examples of the sulfur-based antioxidant include dilauryl 3,3′-thiodipropionate, distearyl thiodipropionate, laurylstearyl thiodipropionate, and dimyristyl 3,3′-thiodipropionate. 
     Examples of the phosphorus-based antioxidant include triphenyl phosphite, octadecyl phosphite, and trinonyl phenyl phosphate. 
     Examples of other antioxidants include ascorbic acid or an alkali metal salt thereof, a sterically hindered phenol compound such as dibutylhydroxytoluene and butylhydroxyanisole, isopropyl citrate, dl-α-tocopherol, nordihydroguaiaretic acid, and propyl gallate. 
     Among the above-described antioxidants, a phenol-based antioxidant and ascorbic acid or a salt thereof are preferred, and 2,6-di-tert-butyl-p-cresol (BHT) and ascorbic acid are particularly preferred. 
     If the antioxidant is contained in an amount of from about 0.01 to 1% by mass of the total amount of the ink, the effect thereof can be obtained without causing any problems. The amount of the antioxidant is more preferably from 0.05 to 0.5% by mass of the total amount of the ink. 
     To the aqueous inkjet ink according to this embodiment, an ultraviolet absorber such as a benzophenone-based, benzotriazole-based, salicylate-based, cyanoacrylate-based, or nickel complex salt-based ultraviolet absorber; a light stabilizer such as a hindered amine-based light stabilizer; an antiozonant such as a quinoline-based or phenylenediamine-based antiozonant; or the like can be further added. 
     The above-described additives can be used alone or in combination of two or more types thereof. By adding such an additive, the storage stability of the ink or the quality of an obtained printed image can be further increased. 
     Hereinafter, specific examples of the aqueous inkjet ink will be shown. 
     Water, a water-soluble organic solvent, a surfactant, and a color material were mixed with one another, whereby an ink sample was obtained. In the following Table 1, the contents of water and the water-soluble organic solvent in each ink sample expressed in % by mass are summarized. The remainder of each ink sample is composed of the surfactant and a pigment as the color material. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Water-soluble organic solvent 
                   
               
            
           
           
               
               
               
               
               
            
               
                 No. 
                 Water 
                 AO1 
                 AO2 
                 AO3 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 56 
                 36 
                 0 
                 0 
               
               
                 2 
                 46 
                 36 
                 0 
                 10 
               
               
                 3 
                 46 
                 46 
                 0 
                 0 
               
               
                 4 
                 56 
                 0 
                 36 
                 0 
               
               
                 5 
                 46 
                 0 
                 36 
                 10 
               
               
                 6 
                 46 
                 0 
                 46 
                 0 
               
               
                 7 
                 56 
                 9 
                 0 
                 27 
               
               
                 8 
                 41 
                 36 
                 0 
                 15 
               
               
                 9 
                 56 
                 0 
                 5 
                 31 
               
               
                 10 
                 41 
                 0 
                 36 
                 15 
               
               
                 11 
                 75 
                 0 
                 9 
                 8 
               
               
                   
               
            
           
         
       
     
     As the surfactant, Surfynol 465 (manufactured by Nissin Chemical Industry Co., Ltd.) was used, and the amount thereof was set to 1% by mass of the total amount of the ink sample. 
     The color material was added in a state of a self-dispersible pigment dispersion. The used pigment dispersion was a carbon black dispersion liquid CAB-JET-300 (manufactured by Cabot Corporation), in which a carbon black is dispersed in water. The average particle diameter of the pigment is about 120 nm. The amount of water contained in the pigment dispersion is included in the amount of water in the above Table 1. 
     In the preparation of each ink sample, first, the respective components were mixed according to the formulation shown in the table, and the resulting mixture was stirred for 1 hour using a stirrer. 
     Thereafter, the mixture was filtered through a 1-μm membrane filter, whereby the sample was obtained. 
     Incidentally, the water-soluble organic solvents represent the following compounds, respectively. 
     AO1: Uniox G-450 (manufactured by NOF Corporation) 
     AO2: Uniox G-750 (manufactured by NOF Corporation) 
     AO3: glycerin 
     Both Uniox G-450 and Uniox G-750 are compounds represented by the following general formula 1. 
     
       
         
         
             
             
         
       
     
     Uniox G-450 has a value of (a+b+c) in the above general formula 1 of about 8, and Uniox G-750 has a value of (a+b+c) in the above general formula 1 of about 15. 
     The viscosity of each of the obtained ink samples was measured. The viscosity was measured using a cone-plate type viscometer, VISCOMETER TV-22 (manufactured by Toki Sangyo Co., Ltd.) with a cone rotor (0.8°×R24) under the condition of 25° C. and 20 rpm. 
     The viscosities of the respective ink samples are summarized in the following Table 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Viscosity 
               
               
                   
                 No. 
                 (mPa · s) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 1 
                 10.0 
               
               
                   
                 2 
                 14.5 
               
               
                   
                 3 
                 27.5 
               
               
                   
                 4 
                 9.0 
               
               
                   
                 5 
                 13.0 
               
               
                   
                 6 
                 25.5 
               
               
                   
                 7 
                 6.5 
               
               
                   
                 8 
                 15.0 
               
               
                   
                 9 
                 5.5 
               
               
                   
                 10 
                 13.5 
               
               
                   
                 11 
                 2.8 
               
               
                   
                   
               
            
           
         
       
     
     For each of the obtained ink samples, the performance of suppressing curling of paper was examined. The evaluation method was as follows. 
     Solid printing was performed on plain paper at 100 duty using an inkjet printing apparatus provided with a piezoelectric head CF1 manufactured by Toshiba Tec Corporation, whereby a print sample was obtained. As the plain paper, Toshiba Copy Paper was used. The print sample immediately after printing was placed on a flat desk, and after 1 minute, the height of each of the four corners of the print sample lifted from the surface of the desk was measured. The curing suppressing performance immediately after printing was evaluated based on the following evaluation criteria. 
     A: less than 10 mm 
     B: 10 mm or more but less than 20 mm 
     C: 20 mm or more 
     If the height of curl is less than 20 mm, there is no problem from a practical viewpoint, and therefore, the curling suppressing performance immediately after printing can be evaluated to be favorable. 
     Each print sample evaluated for the curling suppressing performance immediately after printing was stored in a thermostat bath at 25° C. and 55 RH. After 3 days, the curling suppressing performance of the print sample was examined in the same manner as above. Also, the curling suppressing performance after 3 days was evaluated based on the same evaluation criteria as above. 
     Further, the presence or absence of deformation of paper immediately after printing was evaluated based on the paper conveying performance in an actual apparatus. By using an inkjet printing apparatus, a solid image was formed on the same plain paper as described above. The inkjet printing apparatus used here was provided with a piezoelectric head CF1 manufactured by Toshiba Tec Corporation, and the conveying speed corresponded to about 100 ppm. In order to form one pixel, 3 droplets of 4 pl of the ink were continuously ejected from one nozzle and allowed to land on the same position. The solid image was formed at 600 dpi (dots per inch) on the entire surface of one side of the paper. 
     The paper conveying performance was evaluated based on the following criteria. The case where the paper can be conveyed is acceptable. 
     A: A paper jam is not caused and the paper can be conveyed, and the height of curl is less than 10 mm. 
     B: A paper jam is not caused and the paper can be conveyed, and the height of curl is less than 20 mm. 
     C: The paper cannot be conveyed due to a paper jam. 
     Further, the paper conveying performance was evaluated also when both-side printing was performed. In the both-side printing, the paper on which one-side printing was performed under the above-mentioned condition was automatically fed again and printing was also performed on the other side of the paper under the same condition. The paper conveying performance was evaluated based on the following criteria. 
     A: A paper jam is not caused and the paper can be conveyed. 
     B: The paper is not neatly discharged, but can be conveyed. 
     C: The paper cannot be conveyed due to a paper jam. 
     The evaluation results of the paper conveying performance are summarized in the following Table 3 along with the results of the curling suppressing performance. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Curling suppressing 
                   
                 Paper conveying 
                   
               
               
                   
                 performance 
                   
                 performance 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Immediately 
                 After 3 days 
                 One-side 
                 Both-side 
               
               
                 No. 
                 after printing 
                 from printing 
                 printing 
                 printing 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 A 
                 A 
                 A 
                 A 
               
               
                 2 
                 B 
                 B 
                 B 
                 B 
               
               
                 3 
                 A 
                 A 
                 A 
                 A 
               
               
                 4 
                 A 
                 B 
                 A 
                 B 
               
               
                 5 
                 B 
                 B 
                 B 
                 B 
               
               
                 6 
                 A 
                 B 
                 A 
                 B 
               
               
                 7 
                 B 
                 C 
                 B 
                 C 
               
               
                 8 
                 A 
                 B 
                 A 
                 B 
               
               
                 9 
                 C 
                 C 
                 C 
                 C 
               
               
                 10 
                 A 
                 B 
                 A 
                 B 
               
               
                 11 
                 C 
                 C 
                 C 
                 C 
               
               
                   
               
            
           
         
       
     
     As shown in the above Table 3, the ink samples No. 1 to No. 6, No. 8, and No. 10 exhibit favorable curling suppressing performance, and also exhibit acceptable paper conveying performance. All of these ink samples contain water in an amount ranging from 41 to 70% by mass of the total amount of the ink, and also contain a water-soluble organic solvent in an amount ranging from 20 to 53% by mass of the total amount of the ink. Further, the water-soluble organic solvent contains a specific polyoxyethylene glyceryl ether, which accounts for 10% by mass or more but less than 50% by mass of the total amount of the ink. 
     In the case of the ink samples No. 1 to No. 6, the increasing ratio of the viscosity was less than 10% even after each ink sample was stored in a thermostat bath at 65° C. for 1 week. This means that these ink samples have excellent storage stability. As shown in the above Table 1, it was confirmed that when water is contained in an amount equal to or more than that of the water-soluble organic solvent, the storage stability of the ink sample is also increased. 
     The ink sample No. 7 exhibits poor curling suppressing performance after 3 days from printing and also exhibits poor paper conveying performance in both-side printing. As shown in the above Table 1, in the ink sample No. 7, the amount of Uniox G-450 as the polyoxyethylene glyceryl ether is 9% by mass of the total amount of the ink. Therefore, it is presumed that the cause of the poor performance is because the amount of the polyoxyethylene glyceryl ether is small. 
     In the case of the ink sample No. 9, all of the properties were evaluated as “C”. It is shown in the above Table 1 that in this ink sample No. 9, the amount of Uniox G-750 as the polyoxyethylene glyceryl ether is 5% by mass. Since the amount of the polyoxyethylene glyceryl ether is smaller than in the case of the ink sample No. 7, the desired properties cannot be obtained. 
     In the case of the ink sample No. 11, all of the properties were evaluated as “C”. In this ink sample No. 11, the amount of water is 75% by mass of the total amount of ink, which is large, and therefore, the deformation of paper cannot be suppressed. 
     In order to obtain an aqueous inkjet ink with which an image can be formed while suppressing the deformation of paper, the ink should contain water in an amount of from 41 to 70% by mass of the total amount of the ink and a water-soluble organic solvent in an amount of from 20 to 53% by mass of the total amount of the ink. Further, it is necessary that a given polyoxyethylene glyceryl ether account for 20 to 50% by mass of the total amount of the ink. Since the ink samples No. 1 to No. 6, No. 8, and No. 10 satisfied all the requirements, the desired properties were obtained. 
     The aqueous inkjet ink according to this embodiment can form an image on a paper medium without causing the deformation of the paper medium such as curling. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.