Ink jet ink composition

Abstract

To provide an ink jet ink composition and the like capable of achieving compatibility among good color development, good abrasion resistance of a recorded matter, and good clogging recoverability.SOLUTION: An ink jet ink composition comprises a pigment, a lactam having a 6- to 8-membered lactam ring, a solvent component, and a water-soluble urethane resin dissolved in the solvent component, wherein the pigment contains a self-dispersing pigment having a carboxyl group introduced therein, the content of the water-soluble urethane resin is 0.3 mass% or more relative to the total amount of the ink jet ink composition, the solvent component contains water, and the ink jet ink composition is an aqueous ink.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to ink-jet ink compositions. [Background technology]

[0002] Inkjet recording methods are capable of recording high-resolution images using relatively simple equipment and have been rapidly developing in various fields. Among these, various studies have been conducted to improve performance during high-speed printing. For example, Patent Document 1 discloses an inkjet recording device including a line head in which nozzles for ejecting ink are formed in a direction intersecting the transport direction of a recording medium so as to cover the intersecting direction of the printing area of ​​the recording medium. The inkjet recording device further includes a drying unit for drying the recording medium, a transport unit for transporting the recording medium, and a control unit, wherein the ink contains colloidal silica, and the control unit controls the drying unit or the transport unit so that drying of the ink begins within 0.4 seconds after the ink is applied to the recording medium. Printing using the inkjet recording device described in Patent Document 1 allows for accurate stacking of recording media, even during high-speed printing. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2020-006556 Summary of the Invention [Problem to be solved by the invention]

[0004] The ink is still insufficient in terms of excellent color development, abrasion resistance, and clogging recovery. [Means for solving the problem]

[0005] The present invention provides an inkjet ink composition comprising a pigment, a lactam having a 6- to 8-membered lactam ring, a solvent component, and a water-soluble urethane resin dissolved in the solvent component, wherein the pigment comprises a self-dispersing pigment having an introduced carboxyl group, the content of the water-soluble urethane resin is 0.3 mass% or more relative to the total amount of the inkjet ink composition, and the solvent component contains water, making the inkjet ink a water-based ink. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 2 is a schematic diagram of a recording apparatus used in the present embodiment. [Figure 2] FIG. 2 is a schematic diagram of a recording apparatus used in the present embodiment. [Figure 3] 1 is a table showing the results of examples. [Figure 4] 1 is a table showing the results of examples. DETAILED DESCRIPTION OF THE INVENTION

[0007] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail with reference to the drawings as necessary, but the present invention is not limited to this, and various modifications are possible without departing from the spirit of the present invention. In the drawings, the same elements are given the same reference numerals, and redundant explanations will be omitted. Furthermore, positional relationships such as up, down, left, and right will be based on the positional relationships shown in the drawings unless otherwise specified. Furthermore, the dimensional ratios of the drawings are not limited to those shown in the drawings.

[0008] 1. Inkjet ink composition The inkjet ink composition of this embodiment contains a pigment, a lactam having a 6- to 8-membered lactam ring, a solvent component, and a water-soluble urethane resin dissolved in the solvent component, wherein the pigment contains a self-dispersing pigment having an introduced carboxyl group, the content of the water-soluble urethane resin is 0.3 mass% or more relative to the total amount of the inkjet ink composition, and the solvent component contains water, making the inkjet ink a water-based ink.

[0009] When recording on an absorbent recording medium such as plain paper using an aqueous ink to obtain a recorded product, it is preferable for the ink to have high color development so that even thin lines of text are easily legible. In this regard, self-dispersing pigments generally tend to have excellent color development because the hydrophilic functional groups introduced into their surface react with calcium salts contained in the absorbent recording medium, such as plain paper, causing the pigment to remain on the surface of the recording medium and making it difficult for it to penetrate. On the other hand, self-dispersing pigments that tend to remain on the surface of the recording medium have poor abrasion resistance and may peel off when rubbed, for example, when marking with a line marker. In addition to this line marker resistance, when the recorded area is rubbed, the ink may transfer to components such as printer rollers or to other recording media, causing contamination. This type of contamination is particularly noticeable in inkjet printers that perform high-speed printing, such as line-type business inkjet printers.

[0010] In this regard, the inkjet ink composition of this embodiment contains a specific amount of a water-soluble urethane resin dissolved in a solvent component, along with a self-dispersing pigment into which a carboxyl group has been introduced, which has excellent color development and abrasion resistance. As a result, after the inkjet ink composition is ejected onto a recording medium, the water-soluble urethane resin forms a film on the surface, making the inkjet ink composition less likely to peel off from the recording medium and further improving the abrasion resistance (line marker resistance) of the recorded material. Furthermore, because this film is likely to form before the ink composition has completely dried, ink transfer tends to be suppressed immediately after printing.

[0011] Furthermore, water-soluble urethane resins are less likely to produce foreign matter due to precipitation or the like at the gas-liquid interface between the inkjet ink composition and the atmosphere. On the other hand, if the inkjet ink composition is left for a long period of time and the solvent component dries, foreign matter is more likely to be produced at the gas-liquid interface, and the inkjet ink composition is more likely to partially solidify. Therefore, it has been found that when an inkjet ink composition that has been left for a long period of time is ejected from an inkjet head, clogging of the inkjet head occurs, and the clogging is difficult to resolve even by head cleaning, i.e., the clogging recovery ability is deteriorated. It is believed that the poor redispersibility of the solidified material formed by mixing the pigment and the water-soluble urethane resin leads to the deterioration of clogging recovery ability.

[0012] Therefore, the inkjet ink composition of this embodiment further contains a lactam having a 6- to 8-membered lactam ring. This improves the moisture retention of the inkjet ink composition, making it less likely to dry. As a result, even if the inkjet ink composition is left standing for a long period of time, the solvent does not dry easily, preventing the pigment and the water-soluble urethane resin from mixing and forming a solid, and providing excellent clogging recovery. Furthermore, because the ink composition can be prevented from drying in the nozzles of the inkjet head, deflection of the ink composition and ejection failures can be prevented, and ejection stability tends to be excellent.

[0013] Therefore, it is believed that the inkjet ink composition of this embodiment can achieve good color development, good abrasion resistance of the recorded material, and good clogging recovery properties all at the same time.

[0014] Each component of the inkjet ink composition of this embodiment will be described in detail below. Note that, hereinafter, the inkjet ink composition of this embodiment will also be simply referred to as the "ink composition."

[0015] 1.1.Colorants The ink composition of this embodiment contains a pigment as a coloring material. The content of the pigment relative to the total amount of the ink composition is not particularly limited, but is preferably, for example, 0.5 to 15.0 mass %. The coloring material may be used alone or in combination of two or more types.

[0016] 1.1.1.Pigments The pigment of this embodiment includes a self-dispersion pigment having a carboxyl group introduced therein. This tends to improve the color development of the ink composition. Furthermore, self-dispersion pigments tend to have high color development, and recording using an ink composition containing a self-dispersion pigment having a carboxyl group introduced therein tends to result in better abrasion resistance of the recorded material compared to ink compositions containing other self-dispersion pigments. Furthermore, the carboxyl groups of the self-dispersion pigment interact with components contained in recording media such as plain paper, making it easier for the pigment to remain on the surface of the recording media, thereby improving the color development.

[0017] A self-dispersing pigment is a pigment that can be dispersed in a solvent such as water without the use of a dispersant such as a resin, and is, for example, a pigment in which a hydrophilic group such as a carboxyl group is bonded to the pigment directly or via another atomic group, and the pigment is dispersed by the hydrophilic group. Examples of hydrophilic groups other than the carboxyl group include, but are not limited to, oxygen-containing hydrophilic groups such as a ketone group, a hydroxyl group, an ester group, a lactone group, and an alkylene oxide group; sulfur-containing hydrophilic groups such as a sulfonic acid group and a sulfinic acid group; phosphorus-containing hydrophilic groups such as a phosphate group and a phosphonic acid group; and amino groups. Among these, oxygen-containing hydrophilic groups are preferred.

[0018] In this embodiment, the term "hydrophilic group" includes not only the hydrophilic group but also the ion or salt of the hydrophilic group. For example, the term "carboxyl group" includes not only the carboxyl group but also an ion in which a hydrogen atom is removed from the carboxyl group, and a salt in which another atom such as sodium is bonded in place of the hydrogen atom of the carboxyl group. In other words, the carboxyl group may be an ion or a salt.

[0019] The method for introducing carboxyl groups into a pigment is not particularly limited. For example, carbon atoms on the pigment particle surface may be oxidized to form carboxyl groups by physical treatment with vacuum plasma or chemical treatment using an oxidizing agent such as sodium hypochlorite or ozone. Alternatively, carboxyl groups may be introduced onto the pigment particle surface by treating the pigment particle surface with a carboxyl group-containing compound, such as a carboxylic acid or its salt, such as 4-amino-1,2-benzenedicarboxylic acid. Here, "treating" includes chemically bonding the carboxyl group-containing compound to the pigment particle surface, or chemically bonding a carboxyl group-containing compound derived from the carboxyl group-containing compound to the pigment particle surface. It also includes introducing carboxyl groups onto the pigment surface by generating them through oxidation treatment.

[0020] As the self-dispersing pigment into which a carboxyl group has been introduced, a self-dispersing pigment into which a carboxyl group has been introduced by oxidizing carbon atoms on the surface of the pigment particle is preferred from the viewpoint of improving the color development of the recording medium.

[0021] The ink composition of this embodiment may contain a self-dispersing pigment other than a self-dispersing pigment having a carboxyl group introduced therein. For example, a self-dispersing pigment having a phosphonic acid group introduced therein is preferred from the viewpoint of further improving color development. Furthermore, a self-dispersing pigment having a sulfonic acid group introduced therein is preferred from the viewpoint of further improving the abrasion resistance of recorded materials. On the other hand, a self-dispersing pigment having a carboxyl group introduced therein is excellent in both color development and abrasion resistance of recorded materials.

[0022] Furthermore, the ink composition of this embodiment may contain a resin-dispersed pigment in addition to the self-dispersed pigment. The resin-dispersed pigment is a pigment that can be dispersed in a solvent such as water by a resin, for example, a pigment whose surface is coated with a resin. The resin in the resin-dispersed pigment may be a dispersant, as described below. The use of the resin-dispersed pigment can improve the abrasion resistance of the recorded material. On the other hand, the self-dispersed pigment tends to have higher color development than the resin-dispersed pigment.

[0023] As the pigment, either inorganic pigments or organic pigments can be used. The pigments may be used alone or in combination of two or more. The pigments referred to here are pigments that are the base pigments for self-dispersing pigments. Self-dispersing pigments can be obtained by subjecting the pigments referred to here to surface treatments or the like.

[0024] Examples of inorganic pigments that can be used include carbon blacks (CI (Colour Index Generic Name) Pigment Black 7) such as furnace black, lamp black, acetylene black, and channel black, iron oxide, and titanium oxide. Black pigments such as carbon black are particularly useful and preferable because they can be used to produce black ink.

[0025] Examples of organic pigments include azo pigments such as insoluble azo pigments, condensed azo pigments, azo lakes, and chelate azo pigments; polycyclic pigments such as phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxane pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments; dye chelates (for example, basic dye chelates, acid dye chelates, etc.); dye lakes (basic dye lakes, acid dye lakes), nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments.

[0026] The volume average particle diameter (D50) of the pigment is preferably 80 to 200 nm, and more preferably 90 to 150 nm. The volume average particle diameter can be measured as the D50 value using a particle size distribution analyzer that uses laser diffraction scattering as its measurement principle. Examples of particle size distribution analyzers include a particle size distribution analyzer that uses dynamic light scattering as its measurement principle (e.g., "Microtrac UPA" manufactured by Nikkiso Co., Ltd.).

[0027] The content of the self-dispersing pigment having a carboxyl group introduced therein is preferably 0.5% by mass or more, or preferably 15% by mass or less, relative to the total amount of the ink composition. It is further preferably 1.0 to 12.0% by mass, more preferably 3.0 to 10.0% by mass, even more preferably 4.0 to 8.0% by mass, and particularly preferably 5.0 to 7.0% by mass. The content may be preferably 3 to 12% by mass, relative to the total amount of the ink composition. When the content of the self-dispersing pigment having a carboxyl group introduced therein is within the above range, color development and abrasion resistance tend to be further improved.

[0028] The content of the pigment relative to the total amount of the ink composition is not particularly limited, but is, for example, 3.0 to 12.0% by mass.

[0029] The content of the self-dispersing pigment having a carboxyl group introduced therein relative to the total amount of pigment is not particularly limited, but is, for example, 80 to 100 mass %, 90 to 100 mass %, or 95 to 100 mass %.

[0030] Dispersants When the ink composition of this embodiment contains a resin-dispersed pigment, it may contain a dispersant to disperse the pigment. The dispersant is present near the surface of the pigment particles, surrounding the surface. For example, the dispersant is adsorbed or adhered to the pigment. In this respect, the dispersant differs from the water-soluble resin and resin particles described below. Note that in this embodiment, the dispersant for dispersing the pigment is not included in the resin described below. Dispersants that function by adhering to the pigment are less likely to contribute to film formation, unlike the resins described below. The dispersants may be used alone or in combination of two or more.

[0031] The dispersant is not particularly limited, but examples thereof include dispersants commonly used in preparing pigment dispersions, such as polymer dispersants, and specific examples thereof include those containing one or more of polyoxyalkylene polyalkylene polyamines, vinyl polymers and copolymers, acrylic polymers and copolymers, polyesters, polyamides, polyimides, polyurethanes, amino polymers, silicon-containing polymers, sulfur-containing polymers, fluorine-containing polymers, and epoxy resins as the main component.

[0032] Commercially available polymer dispersants include the Ajisper series manufactured by Ajinomoto Fine-Techno Co., Ltd., the Solsperse series (Solsperse 36000, etc.) available from Avecia and Noveon, the Disperbic series manufactured by BYK Additives & Instruments, and the Disparlon series manufactured by Kusumoto Chemicals Co., Ltd.

[0033] The content of the dispersant relative to the total amount of the ink composition is not particularly limited, but is, for example, 0.1 to 5.0% by mass.

[0034] 1.2.Resin The ink composition of this embodiment contains a water-soluble urethane resin in an amount of 0.3% by mass or more relative to the total amount of the ink composition. This tends to improve the abrasion resistance of recorded matter obtained using the ink composition of this embodiment and inhibit ink transfer. Furthermore, the water-soluble resin is less likely to produce foreign matter due to precipitation or the like at the gas-liquid interface between the inkjet ink composition and the atmosphere, and tends to further improve ejection stability. The water-soluble urethane resin includes one that is dissolved in a solvent component, which will be described later. The water-soluble urethane resin does not need to be completely dissolved in the solvent, but it is preferable that 90% by mass or more of the resin is dissolved in the solvent component. It is even more preferable that the resin is completely dissolved in the solvent component.

[0035] In addition to the water-soluble urethane resin, the resin contained in the ink composition of this embodiment may be a water-soluble resin other than the water-soluble urethane resin, resin particles, etc. The resin may be used alone or in combination of two or more types.

[0036] The resin content is preferably 0.3 to 3.0 mass%, 0.4 to 2.5 mass%, 0.4 to 2.0 mass%, 0.4 to 1.5 mass%, or 0.4 to 1.0 mass% relative to the total amount of the ink composition. When the resin content is within the above range, the abrasion resistance of the recorded matter tends to be improved.

[0037] 1.2.1.Water-soluble resin 1.2.1.1.Water-soluble urethane resin The water-soluble urethane resin refers to a water-soluble urethane resin having a polar group in its molecular structure. The polar group may be in the form of a salt. The polar group is preferably an acid group. Examples of the acid group include a carboxyl group, a sulfonic acid group, and a phosphorus-containing group such as a phosphate group. The water-soluble urethane resin may be used alone or in combination of two or more types.

[0038] Water-soluble urethane resins have N-H bonds in their structure, which cause hydrogen bonds to form in the water-soluble urethane resin film, making the film stronger and resulting in a tendency for the recorded material to have excellent abrasion resistance. Furthermore, it is believed that the ink easily forms a film on the ink surface even before the ink adhered to the recording medium has sufficiently dried, resulting in excellent abrasion resistance and ink smear suppression. On the other hand, when the ink containing the self-dispersing pigment with a carboxyl group and the water-soluble urethane resin has dried, clogging recovery becomes an issue. It is believed that as the ink has dried, a strong solidified substance forms between the pigment and the water-soluble urethane resin.

[0039] In this embodiment, the term "water-soluble resin" refers to a resin that is water-soluble, meaning that it dissolves in water. The "water-soluble resin" can be confirmed by, for example, mixing 1% by mass of the resin into water at room temperature (25°C), stirring, and then checking whether the entire liquid does not appear cloudy or whether any undissolved residue is visible in the liquid. Furthermore, it is preferable that the resin be one that can exist in water in a state without having a particle size when measured by dynamic light scattering.

[0040] The water-soluble urethane resin has repeating units derived from polyisocyanate and polyol, and among them, those having repeating units derived from polyol having an acid group are preferred, and resins having repeating units derived from polyisocyanate, polyol having no acid group, and polyol having an acid group are preferred. The water-soluble urethane resin may further have repeating units derived from polyamine.

[0041] The polyisocyanate refers to a compound having two or more isocyanate groups in its molecular structure, and is not particularly limited, but examples thereof include aliphatic polyisocyanates and aromatic polyisocyanates.

[0042] The aliphatic polyisocyanate is not particularly limited, but examples thereof include polyisocyanates having a chain structure such as tetramethylene diisocyanate, dodecamethylene diisocyanate, hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, 2-methylpentane-1,5-diisocyanate, and 3-methylpentane-1,5-diisocyanate; and polyisocyanates having a cyclic structure such as isophorone diisocyanate, hydrogenated xylylene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 1,4-cyclohexane diisocyanate, methylcyclohexylene diisocyanate, and 1,3-bis(isocyanatomethyl)cyclohexane.

[0043] The aromatic polyisocyanate is not particularly limited, but examples thereof include tolylene diisocyanate, 2,2'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-dibenzyl diisocyanate, 1,5-naphthylene diisocyanate, xylylene diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, and α,α,α',α'-tetramethylxylylene diisocyanate.

[0044] A polyol is a compound having two or more hydroxy groups in its molecular structure. The polyol of the present embodiment is not particularly limited, but examples thereof include polyols having no acid group and polyols having an acid group.

[0045] The polyol having no acid group is not particularly limited, but examples thereof include polyether polyol, polyester polyol, and polycarbonate polyol.

[0046] The polyether polyol is not particularly limited, but examples thereof include addition polymers of alkylene oxides and polyols, and glycols.

[0047] The alkylene oxide is not particularly limited, and examples thereof include ethylene oxide, propylene oxide, butylene oxide, α-olefin oxide, etc. Examples of polyols to be addition polymerized with the alkylene oxide include diols such as 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 4,4-dihydroxyphenylpropane, 4,4-dihydroxyphenylmethane, hydrogenated bisphenol A, dimethylolurea and derivatives thereof, etc.; triols such as glycerin, trimethylolpropane, 1,2,5-hexanetriol, 1,2,6-hexanetriol, pentaerythritol, trimethylolmelamine and derivatives thereof, and polyoxypropylenetriol, etc.

[0048] Examples of glycols include (poly)alkylene glycols such as tetramethylene glycol, hexamethylene glycol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, and (poly)tetramethylene glycol; ethylene glycol-propylene glycol copolymers; and the like.

[0049] The polyester polyol is not particularly limited, but examples thereof include acid esters. The acid components constituting the acid esters include, but are not particularly limited to, aromatic dicarboxylic acids such as phthalic acid, naphthalenedicarboxylic acid, biphenyldicarboxylic acid, and tetrahydrophthalic acid; alicyclic dicarboxylic acids such as hydrogenated products of these aromatic dicarboxylic acids; and aliphatic dicarboxylic acids such as malonic acid, succinic acid, tartaric acid, oxalic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, alkylsuccinic acid, linoleic acid, maleic acid, fumaric acid, mesaconic acid, citraconic acid, and itaconic acid. Anhydrides, salts, and derivatives (alkyl esters, acid halides) of these compounds can also be used as the acid component. The components that form esters with the acid component include, but are not particularly limited to, polyols such as diols and triols; glycols such as (poly)alkylene glycols; and the like. Examples of polyols and glycols include those exemplified as components constituting the polyether polyols described above.

[0050] The polycarbonate polyol is not particularly limited, and may be, for example, a polycarbonate polyol produced by a known method, specifically, an alkanediol-based polycarbonate diol such as polyhexamethylene carbonate diol, etc. Further, a polycarbonate diol obtained by reacting a carbonate component such as alkylene carbonate, diaryl carbonate, or dialkyl carbonate, or phosgene with an aliphatic diol component, etc. may be used.

[0051] The polyol having an acid group is not particularly limited, but examples thereof include polyols having an acid group such as a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, a phosphonic acid group, etc. Among these, one or more phosphorus-containing groups such as a carboxyl group, a sulfonic acid group, and a phosphoric acid group are preferred, and a carboxyl group is more preferred.

[0052] The polyol having a carboxylic acid group is not particularly limited, but examples thereof include dimethylol acetic acid, dimethylol propionic acid, dimethylol butanoic acid, and dimethylol butyric acid.

[0053] The acid group of the polyol having an acid group may be in a salt state. The cation that forms such a salt is not particularly limited, but examples thereof include alkali metal ions and cations of organic amines. The alkali metal ions are not particularly limited, but examples thereof include lithium, sodium, and potassium. The organic amine cations are not particularly limited, but examples thereof include ammonium ions and dimethylamine.

[0054] The polyamine is not particularly limited, but examples thereof include monoamines having multiple hydroxy groups, such as dimethylolethylamine, diethanolmethylamine, dipropanolethylamine, and dibutanolmethylamine; bifunctional polyamines, such as ethylenediamine, propylenediamine, hexylenediamine, isophoronediamine, xylylenediamine, diphenylmethanediamine, hydrogenated diphenylmethanediamine, and hydrazine; and trifunctional or higher functional polyamines, such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, polyamidepolyamine, and polyethylenepolyimine.

[0055] The acid value of the water-soluble urethane resin is preferably 40 to 80 mgKOH / g, 45 to 80 mgKOH / g, or 50 to 80 mgKOH / g. When the acid value of the urethane resin is within the above range, the resin tends to have better abrasion resistance, ejection stability, and clogging recovery. The acid value of the water-soluble urethane resin is not particularly limited, but can be adjusted, for example, by the amount of polyol containing an acid group used. The acid value can be measured by potentiometric titration. For example, the method described in the Examples below can be used.

[0056] The weight-average molecular weight of the water-soluble urethane resin is preferably 5,000 to 150,000, 7,500 to 100,000, 10,000 to 50,000, 12,500 to 30,000, or 15,000 to 25,000. Having a weight-average molecular weight within the above range tends to further improve ejection stability. The weight-average molecular weight of the water-soluble urethane resin is not particularly limited, but can be adjusted, for example, by the reaction temperature and reaction time of the polyisocyanate and polyol. The weight-average molecular weight can be measured by GPC. For example, the method described in the Examples below can be used.

[0057] The number average molecular weight of the water-soluble urethane resin is preferably 2000 to 7000, more preferably 3500 to 5000. Having a number average molecular weight within the above range tends to further improve ejection stability. The number average molecular weight of the water-soluble urethane resin is not particularly limited, but can be adjusted, for example, by the reaction temperature and reaction time of the polyisocyanate and polyol. The number average molecular weight can be measured using the method described in the Examples below.

[0058] The content of the water-soluble urethane resin relative to the total amount of the ink composition is 0.3% by mass or more, preferably 0.3 to 3.0% by mass, more preferably 0.4 to 2.5% by mass, even more preferably 0.4 to 2.0% by mass, still more preferably 0.4 to 1.5% by mass, even more preferably 0.4 to 1.0% by mass, and particularly preferably 0.4 to 0.8% by mass, or 0.5 to 0.7% by mass. By ensuring that the resin content is within the above range, clogging recovery, ejection stability, abrasion resistance of the recorded material, and the like tend to be further improved.

[0059] 1.2.1.2. Other water-soluble resins The ink composition of this embodiment may or may not contain a water-soluble resin other than the water-soluble urethane resin. Examples of the water-soluble resin include, but are not limited to, a water-soluble acrylic resin, a water-soluble polyester resin, and a water-soluble amino resin. The water-soluble resin may be used alone or in combination.

[0060] The content of the other resins relative to the total amount of the ink composition is not particularly limited, but is, for example, 2.0% by mass or less, preferably 1.0% by mass or less, and more preferably 0.5% by mass or less.

[0061] 1.2.2.Resin particles The ink composition of this embodiment may or may not contain resin particles. The resin particles are not water-soluble resins, but resins dispersed in the solvent component of the ink composition. Examples of such resin particles include resin emulsions.

[0062] The resin particles are not particularly limited, but examples thereof include resin particles made of urethane-based resins, acrylic-based resins, fluorene-based resins, polyolefin-based resins, rosin-modified resins, terpene-based resins, polyester-based resins, polyamide-based resins, epoxy-based resins, vinyl chloride-based resins, or ethylene vinyl acetate-based resins. From the viewpoints of intermittency and clogging recovery, it is preferable to use these resin particles in the form of an emulsion. The resin particles may be used alone or in combination of two or more types.

[0063] The urethane resin is a general term for resins having a urethane bond, and is not particularly limited, but examples thereof include polyether-type urethane resins having an ether bond in the main chain, polyester-type urethane resins having an ester bond in the main chain, and polycarbonate-type urethane resins having a carbonate bond in the main chain. The urethane resin may be a preparation prepared by a known method, or a commercially available product.

[0064] Acrylic resin is a general term for polymers obtained by polymerizing at least an acrylic monomer such as (meth)acrylic acid or a (meth)acrylic acid ester as one component. The acrylic resin is not particularly limited, but examples include those obtained by polymerizing a (meth)acrylic monomer such as (meth)acrylic acid or a (meth)acrylic acid ester, and those obtained by copolymerizing a (meth)acrylic monomer with another monomer, such as a styrene-acrylic resin. The acrylic resin may be a prepared product prepared by a known method, or a commercially available product may be used.

[0065] The content of the resin particles relative to the total amount of the ink composition is not particularly limited, but is, for example, preferably 1.0% by mass or less, more preferably 0.5% by mass or less, even more preferably 0.1% by mass or less, particularly preferably 0.05% by mass or less, and may be 0% by mass (resin particles not included). When the content of the resin particles is within the above range or less, clogging recovery properties and ejection stability are more excellent, which is preferable.

[0066] 1.3. Lactams The ink composition of this embodiment contains lactams having a 6- to 8-membered lactam ring.

[0067] Lactams refer to compounds having a structure in which a ring is formed by dehydration condensation between a carboxy group and an amino group in a molecule. A lactam ring refers to a ring formed by dehydration condensation between a carboxy group and an amino group in a molecule. A 6- to 8-membered lactam ring refers to a lactam ring having 6 to 8 atoms, including atoms other than carbon atoms such as nitrogen atoms.

[0068] Furthermore, when the ink composition of this embodiment contains an acetylene glycol surfactant (described below) in addition to a lactam having a 6- to 8-membered lactam ring, the lactam makes the ink composition less likely to dry and maintains a moist state, preventing the acetylene glycol surfactant from phase separating and generating foreign matter, which tends to result in excellent ejection stability of the ink composition.

[0069] Lactams having a 6- to 8-membered lactam ring are not particularly limited, but examples thereof include 2-piperidone, which may or may not have a substituent, ε-caprolactam, which may or may not have a substituent, and ω-heptalactam, which may or may not have a substituent. Substituents include alkyl groups, alkynyl groups, alkenyl groups, halogen atoms, imino groups, amino groups, thiol groups, hydroxy groups, acyl groups, nitrile groups, formyl groups, amide groups, acryl halide groups (—CHCH—C(═O)—X; X is a halogen atom), ester groups, carboxy groups, alkoxy groups, thioalkoxy groups, nitro groups, and nitroso groups. Lactams having a 6- to 8-membered lactam ring may be used alone or in combination of two or more.

[0070] The content of lactams having a 6- to 8-membered lactam ring is preferably 0.1% by mass or more, or preferably 6.0% by mass or less, relative to the total amount of the ink composition. Furthermore, it is preferably 0.5 to 5.0 mass%, more preferably 1.0 to 4.0 mass%, even more preferably 1.1 to 3.5 mass%, and particularly preferably 1.2 to 3.0 mass%. When the content of lactams having a 6- to 8-membered lactam ring is within the above range, ejection stability, clogging recovery, etc. tend to be more excellent. In particular, when the content of lactams having a 6- to 8-membered lactam ring is within the above range, it is possible to prevent the discharge stability and clogging recovery ability from being reduced due to the excessive content of lactams having a 6- to 8-membered lactam ring.

[0071] 1.4.Surfactants The ink composition of this embodiment may contain a surfactant. The surfactant is not particularly limited, but examples thereof include acetylene glycol surfactants, silicone surfactants, and fluorine surfactants. The surfactants may be used alone or in combination of two or more.

[0072] The content of the surfactant is preferably 0.1 to 3.0% by mass, and more preferably 0.2 to 2.0% by mass, relative to the total amount of the ink composition. When the content of the surfactant is within the above range, the ejection stability of the ink composition tends to be further improved.

[0073] 1.4.1. Acetylene glycol surfactants The ink composition of this embodiment preferably contains an acetylene glycol-based surfactant. The acetylene glycol-based surfactant tends to reduce the surface tension of the ink composition and improve the ejection stability of the ink composition. On the other hand, the acetylene glycol-based surfactant has a relatively low solubility in water, and as the ink composition dries, the acetylene glycol-based surfactant tends to undergo phase separation and produce foreign matter. Examples of acetylene glycol surfactants include those with an HLB value of 6 or less and those with an HLB value of more than 6. Here, the HLB value is a value that represents the balance between the hydrophobicity and hydrophilicity of a surfactant, with a smaller HLB value representing a more hydrophobic surfactant and a larger HLB value representing a more hydrophilic surfactant. In the present invention, the HLB value is calculated by the Griffin method.

[0074] The content of the acetylene glycol surfactant relative to the total amount of the ink composition is preferably 0.1 to 2.5 mass%, 0.2 to 2.0 mass%, 0.3 to 1.8 mass%, 0.4 to 1.6 mass%, or 0.5 to 1.4 mass%, more preferably 0.6 to 1.0 mass%, and more preferably 0.7 to 0.8 mass%. When the content of the acetylene glycol surfactant is within the above range, the ejection stability of the ink composition tends to be further improved.

[0075] 1.4.1.1. Acetylene glycol surfactants with an HLB value of 6 or less The ink composition of this embodiment preferably contains an acetylene glycol surfactant with an HLB value of 6 or less. This tends to particularly improve the ejection stability of the ink composition. This is thought to be because an acetylene glycol surfactant with an HLB value of 6 or less reduces the surface tension of the inkjet ink composition, making the ink droplets smaller during ejection and reducing the risk of ink deflection. Furthermore, the ink composition tends to have improved permeability into the recording medium. Higher permeability is preferable because it reduces the likelihood of the recording medium to which the ink composition has adhered contaminating other recording media or components within the recording device when it comes into contact with them.

[0076] Acetylene glycol surfactants tend to improve the penetration of the ink composition into the recording medium, and this tendency is particularly pronounced for acetylene glycol surfactants with an HLB value of 6 or less. If the ink composition penetrates too far into the recording medium, the color development of the recorded material tends to decrease. On the other hand, the self-dispersing pigment containing a carboxyl group contained in the ink composition of this embodiment has high color development and prevents the ink composition from penetrating too far into the recording medium. Therefore, even when the ink composition of this embodiment contains an acetylene glycol surfactant, it is possible to achieve both good penetration of the ink composition into the recording medium and good color development of the recorded material.

[0077] Acetylene glycol surfactants have low solubility in water, and acetylene glycol surfactants with an HLB value of 6 or less have particularly low solubility in water. Therefore, as the ink composition dries, the acetylene glycol surfactant is prone to phase separation and the formation of foreign matter, which tends to deteriorate the ejection stability and clogging recovery of the ink composition. In this regard, the ink composition of this embodiment contains lactams having a 6- to 8-membered lactam ring, which makes the ink composition less likely to dry and allows it to maintain a moist state. Therefore, even when the ink composition of this embodiment contains an acetylene glycol surfactant, the ink composition does not dry easily, and deterioration in the ejection stability and clogging recovery of the ink composition due to the acetylene glycol surfactant can be suppressed.

[0078] From this viewpoint, the ratio (mass ratio) of the lactam content to the acetylene glycol surfactant content is preferably 0.5 to 8.0, 1.0 to 7.0, 1.5 to 6.0, or 2.0 to 5.0, and more preferably 2.5 to 4.0. When the ratio of the lactam content to the acetylene glycol surfactant content is within the above range, the ejection stability and clogging recovery properties tend to be excellent.

[0079] The ratio (mass ratio) of the lactam content to the acetylene glycol surfactant content having an HLB value of 6 or less is preferably 1.0 to 25.0, 2.0 to 20.0, 2.5 to 17.5, or 5.0 to 15.0. When the ratio of the lactam content to the acetylene glycol surfactant content having an HLB value of 6 or less is within the above range, ejection stability and clogging recovery tend to be excellent.

[0080] The ink composition of this embodiment also contains a water-soluble urethane resin. Because the water-soluble urethane resin has high compatibility with the acetylene glycol-based surfactant, the inclusion of the water-soluble urethane resin can prevent the acetylene glycol-based surfactant from phase separating and generating foreign matter, even when the ink composition is drying. As a result, even when the ink composition of this embodiment contains an acetylene glycol-based surfactant, it is possible to prevent the ink composition from experiencing deterioration in ejection stability and clogging recovery due to the acetylene glycol-based surfactant.

[0081] From this viewpoint, the ratio (mass ratio) of the content of the water-soluble urethane resin to the content of the acetylene glycol surfactant is preferably 0.1 to 5.0, 0.1 to 4.0, 0.2 to 3.0, or 0.3 to 2.0. When the ratio of the content of the water-soluble urethane resin to the content of the acetylene glycol surfactant is within the above range, ejection stability and clogging recovery tend to be excellent.

[0082] The ratio (mass ratio) of the content of the water-soluble urethane resin to the content of the acetylene glycol surfactant having an HLB value of 6 or less is preferably 0.5 to 10.0, 1.0 to 7.5, or 1.5 to 5.0. When the ratio of the content of the water-soluble urethane resin to the content of the acetylene glycol surfactant having an HLB value of 6 or less is within the above range, ejection stability and clogging recovery tend to be excellent.

[0083] The HLB value of the acetylene glycol surfactant is preferably 6 or less, 5 or less, or 4 or less. The lower limit of the HLB value is preferably 0 or more, 1 or more, or 2 or more. When the HLB value is within the above range, phase separation is less likely to occur, and ejection stability tends to be further improved. The upper and lower limits can be arbitrarily combined to form a suitable numerical range. For example, the suitable range of the HLB value of the acetylene glycol surfactant may be 0 to 6, 1 to 6, 2 to 6, 0 to 5, 1 to 5, 2 to 5, 0 to 4, 1 to 4, or 2 to 4.

[0084] The acetylene glycol surfactant having an HLB value of 6 or less is not particularly limited, and examples thereof include acetylene glycol as represented by the following formula (1) and alkylene oxide adducts of acetylene glycol as represented by the following formula (2). Use of such acetylene glycol surfactants tends to further improve ejection stability and penetration. Acetylene glycol surfactants having an HLB value of 6 or less may be used alone or in combination of two or more.

[0085] [ka]

[0086] R 1 ~R 4 are each independently an alkyl group having 1 to 4 carbon atoms which may have a substituent.

[0087] [ka]

[0088] R 1 ~R 4 each independently represents an alkyl group having 1 to 4 carbon atoms which may have a substituent, and m and n each independently represent 0 or an integer of 1 or more, and m+n=1 to 30 is satisfied.

[0089] m is preferably 1 to 15, 1 to 10, or 1 to 5. n is preferably 1 to 15, 1 to 10, or 1 to 5.

[0090] R 1 ~R 4 is not particularly limited, and examples thereof include methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, and tert-butyl. Examples of the substituent include an alkyl group, an alkynyl group, an alkenyl group, a halogen atom, an imino group, an amino group, a thiol group, a hydroxy group, an acyl group, a nitrile group, a formyl group, an amide group, an acryl halide group (-CH2CH2-C(=O)-X; X is a halogen atom), an ester group, a carboxy group, an alkoxy group, a thioalkoxy group, a nitro group, and a nitroso group.

[0091] Specific structures of acetylene glycol surfactants having an HLB value of 6 or less include compounds represented by formula (1) and compounds represented by formula (2), and in the compound represented by formula (2), n and m are preferably within the above ranges or less, 15 or less, 9 or less, and 8 or less, respectively. Although not particularly limited, examples include 2,4,7,9-tetramethyl-5-decyne-4,7-diol or its alkylene oxide adduct, in which the number of moles added (n, m) of the adduct are within the above ranges or less, 15 or less, 9 or less, and 8 or less, respectively.

[0092] Examples of product names of acetylene glycol surfactants having an HLB value of 6 or less include Olfine D-10PG (manufactured by Air Products Co., Ltd.), Surfynol 420, and Surfynol 104PG50 (manufactured by Nissin Chemical Industry Co., Ltd.).

[0093] The content of the acetylene glycol surfactant having an HLB value of 6 or less is preferably 0.1 to 1.0 mass%, 0.1 to 0.8 mass%, 0.1 to 0.6 mass%, 0.1 to 0.4 mass%, or 0.1 to 0.3 mass% relative to the total amount of the ink composition. When the content of the acetylene glycol surfactant having an HLB value of 6 or less is within the above range, ejection stability tends to be further improved.

[0094] The content of the acetylene glycol surfactant having an HLB value of 6 or less is preferably 10 to 60 mass%, 15 to 50 mass%, 20 to 40 mass%, or 20 to 35 mass% relative to the total amount of surfactants. When the content of the acetylene glycol surfactant having an HLB value of 6 or less is within the above range, ejection stability tends to be further improved.

[0095] The content of the acetylene glycol surfactant having an HLB value of 6 or less is preferably 10 to 60 mass%, 15 to 50 mass%, 20 to 40 mass%, or 20 to 35 mass% relative to the total amount of the acetylene glycol surfactant. When the content of the acetylene glycol surfactant having an HLB value of 6 or less is within the above range, ejection stability tends to be further improved.

[0096] 1.4.1.2. Acetylene glycol surfactants with an HLB value greater than 6 The ink composition of this embodiment may contain an acetylene glycol surfactant having an HLB value of greater than 6. Among acetylene glycol surfactants, those with an HLB value of more than 6 tend to improve ink penetration less than those with an HLB value of 6 or less, but on the other hand, they tend to cause less phase separation in the ink composition. Therefore, by including an acetylene glycol-based surfactant with an HLB value of greater than 6 together with an acetylene glycol-based surfactant with an HLB value of 6 or less, the penetration of the ink composition into the recording medium is further improved, making it less likely that unpenetrated ink composition will remain. This prevents the unpenetrated ink composition from contaminating the transport path, causing the dirt on the transport path to be transferred to another recording medium, or reducing transportability. Furthermore, by using an acetylene glycol-based surfactant with an HLB value of greater than 6 in combination with an acetylene glycol-based surfactant with an HLB value of 6 or less, ejection stability tends to be improved. Furthermore, the presence of an acetylene glycol-based surfactant with a high HLB value is preferable because it improves the compatibility of the acetylene glycol-based surfactant with a low HLB value with water.

[0097] The HLB value of the acetylene glycol surfactant is preferably greater than 6, 7 or greater, or 8 or greater. The upper limit of the HLB value is preferably 14 or less, 13 or less, or 12 or less. Having the HLB value within the above range tends to make phase separation less likely to occur and improve ejection stability. The upper and lower limits can be arbitrarily combined to form a suitable numerical range. For example, the preferred range of the HLB value of the acetylene glycol surfactant may be greater than 6 and 14 or less, greater than 6 and 13 or less, greater than 6 and 12, 7 to 14, 7 to 13, 7 to 12, 8 to 14, 8 to 13, or 8 to 12.

[0098] The specific structure of the acetylene glycol surfactant having an HLB value of greater than 6 is not particularly limited, but examples include 5,8-dimethyl-6-dodecyne-5,8-diol or its alkylene oxide adduct, 4,7-dimethyl-5-decyne-4,7-diol or its alkylene oxide adduct, and alkylene oxide adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol, in which any one or each of the mole numbers of addition of the adducts is 9 or more, 10 or more, or 16 or more. Furthermore, in the compound represented by formula (2) above, any one or each of the mole numbers of addition of the adducts (n, m) is 9 or more, 10 or more, or 16 or more. Acetylene glycol surfactants having an HLB value of greater than 6 may be used alone, or two or more may be used in combination.

[0099] An example of a product name of an acetylene glycol surfactant having an HLB value of more than 6 is Olfine E1010 (manufactured by Nissin Chemical Industry Co., Ltd.).

[0100] The content of the acetylene glycol surfactant having an HLB value of greater than 6 is preferably 0.1 to 1.5 mass%, 0.2 to 1.0 mass%, or 0.2 to 0.8 mass%, relative to the total amount of the ink composition. When the content of the acetylene glycol surfactant having an HLB value of greater than 6 is within the above range, ejection stability tends to be further improved.

[0101] The content of the acetylene glycol surfactant having an HLB value of greater than 6 is preferably 40 to 85 mass%, 45 to 80 mass%, or 50 to 80 mass%, relative to the total amount of surfactants. When the content of the acetylene glycol surfactant having an HLB value of greater than 6 is within the above range, ejection stability tends to be further improved.

[0102] The content of the acetylene glycol surfactant having an HLB value of greater than 6 is preferably 40 to 85 mass%, 45 to 80 mass%, or 50 to 80 mass%, relative to the total amount of the acetylene glycol surfactant. When the content of the acetylene glycol surfactant having an HLB value of greater than 6 is within the above range, ejection stability tends to be further improved.

[0103] 1.4.2. Silicone surfactants The ink composition of this embodiment may or may not contain a silicone surfactant. Silicone surfactants tend to foam easily and adversely affect ejection stability. Furthermore, silicone surfactants are not highly compatible with water-soluble urethane resins, and the water-soluble urethane resins do not inhibit phase separation of the silicone surfactant. Therefore, it is preferable that the ink composition does not contain a silicone surfactant. Examples of silicone surfactants include polysiloxane compounds and polyether-modified organosiloxanes. Commercially available silicone surfactants include, but are not limited to, BYK-306, BYK-307, BYK-333, BYK-341, BYK-345, BYK-346, BYK-347, BYK-348, BYK-349, and BYK-UV3500 (all trade names, manufactured by BYK Japan K.K.), KF-351A, KF-352A, KF-353, KF-354L, KF-355A, KF-615A, KF-945, KF-640, KF-642, KF-643, KF-6020, X-22-4515, KF-6011, KF-6012, KF-6015, and KF-6017 (all trade names, manufactured by Shin-Etsu Chemical Co., Ltd.).

[0104] The content of the silicone surfactant relative to the total amount of the ink composition is not particularly limited, but is, for example, 2.0% by mass or less, or even 0.1 to 2.0% by mass.

[0105] 1.4.3.Fluorosurfactants The ink composition of this embodiment may or may not contain a fluorine-based surfactant. Examples of fluorine-based surfactants include, but are not limited to, perfluoroalkyl sulfonates, perfluoroalkyl carboxylates, perfluoroalkyl phosphates, perfluoroalkyl ethylene oxide adducts, perfluoroalkyl betaines, and perfluoroalkyl amine oxide compounds. Examples of commercially available fluorine-based surfactants include, but are not limited to, S-144 and S-145 (manufactured by Asahi Glass Co., Ltd.); FC-170C, FC-430, and Fluorad-FC4430 (manufactured by Sumitomo 3M Limited); FSO, FSO-100, FSN, FSN-100, and FS-300 (manufactured by DuPont); and FT-250 and 251 (manufactured by Neos Corporation).

[0106] The content of the fluorine-based surfactant relative to the total amount of the ink composition is not particularly limited, but is, for example, 2.0% by mass or less, or even 0.1 to 2.0% by mass.

[0107] 1.5. Solvent Components The solvent component contains at least water and may further contain an organic solvent.

[0108] 1.5.1. Organic Solvents The ink composition of this embodiment may contain an organic solvent. The organic solvent is not particularly limited, but examples thereof include monohydric alcohols, polyols, and glycol ethers. The organic solvent may be used alone or in combination of two or more. A water-soluble organic solvent is preferred.

[0109] The monohydric alcohols are not particularly limited, but examples thereof include methanol, ethanol, 1-propanol, isopropanol, 1-butanol, 2-butanol, isobutanol, and 2-methyl-2-propanol.

[0110] Examples of polyols include alkanediols having 5 or more carbon atoms, such as 1,2-hexanediol. These are preferred because they can improve the penetration and ejection stability of the ink. They also increase the water solubility of acetylene glycol surfactants, particularly those with an HLB value of 6 or less, and have the effect of suppressing phase separation. Therefore, from the perspective of improving ejection stability, it is preferred that the ink composition contains an alkanediol having 5 or more carbon atoms. As the alkanediol having 5 or more carbon atoms, an alkanediol having 5 to 10 carbon atoms is more preferred. Other examples of polyols include alkanediols having four or less carbon atoms, those having three or more hydroxyl groups in the molecule, and those having an ether group in the molecular skeleton (intermolecular condensation products of alkanediols). Polyols are preferred because they improve the moisture retention of the ink and are excellent in ejection stability and clogging recovery properties.

[0111] The polyols are not particularly limited, but examples thereof include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, 1,3-propanediol, 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and glycerin.

[0112] Glycol ethers are compounds in which one or two of the two hydroxyl groups in an alkanediol are etherified, resulting in one or zero hydroxyl groups in the molecule. The etherification is preferably alkyl etherification. Examples include glycol monoethers and glycol diethers, with glycol monoethers being preferred. The glycol ethers are not particularly limited, but examples thereof include triethylene glycol monomethyl ether, triethylene glycol dimethyl ether, triethylene glycol monobutyl ether, diethylene glycol monobutyl ether, dipropylene glycol monopropyl ether, tetraethylene glycol monobutyl ether, tetraethylene glycol monooctyl ether, triethylene glycol monohexyl ether, and nonylethylene glycol monohexyl ether.

[0113] Among the polyols, polyols having a normal boiling point of 280° C. or higher are not particularly limited, but examples thereof include triethylene glycol, tetraethylene glycol, glycerin, etc. When the ink composition contains a polyol having a normal boiling point of 280° C. or higher, the ink composition tends to be less prone to drying and to have better clogging recovery properties.

[0114] Furthermore, the polyols having a normal boiling point of less than 280°C are not particularly limited, but examples thereof include ethylene glycol, diethylene glycol, pentaethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, 1,3-propanediol, 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol.

[0115] The content of the organic solvent is preferably 1% by mass or more, or 40% by mass or less, based on the total amount of the ink composition. It is more preferably 5.0 to 35.0% by mass, 10.0 to 30.0% by mass, or 15.0 to 25.0% by mass. When the content of the organic solvent is within the above range, clogging recovery is improved, and ejection stability tends to be further improved. The content of polyols may also be preferably within the above range.

[0116] The content of polyols having a normal boiling point of 280°C or higher is preferably 5 to 25% by mass, and more preferably 10 to 20% by mass, relative to the total amount of the ink composition. When the content of polyols having a normal boiling point of 280°C or higher is within the above range, clogging recovery properties tend to be further improved.

[0117] The content of polyols having a normal boiling point of 280°C or higher is preferably 50 to 85% by mass, and more preferably 60 to 80% by mass, based on the total amount of organic solvents. When the content of polyols having a normal boiling point of 280°C or higher is within the above range, clogging recovery properties tend to be further improved.

[0118] 1.5.2.Water The ink composition of this embodiment contains water as a solvent component. The water is not particularly limited, but examples thereof include ion-exchanged water, ultrafiltered water, reverse osmosis water, and distilled water.

[0119] The ink of this embodiment is a water-based ink, and a water-based ink is an ink composition in which the solvent component contained in the ink is at least primarily water.

[0120] The water content relative to the total amount of the ink composition is not particularly limited, but is, for example, 30.0% by mass or more, preferably 30.0 to 99.0% by mass, 40.0 to 95.0% by mass, 50.0 to 95.0% by mass, 52.5 to 90% by mass, or 55.0 to 80% by mass.

[0121] 1.6. Betaine The ink composition of this embodiment preferably contains betaine. Betaine refers to a compound that has a positive charge and a negative charge at non-adjacent positions within the same molecule, with no dissociable hydrogen bonded to the positively charged atom, forming an intramolecular salt and resulting in an uncharged molecule as a whole. The betaine of this embodiment is preferably one in which the positively charged moiety is a quaternary ammonium cation.

[0122] The ink composition containing betaine can prevent the ink composition from drying out in the nozzles of the inkjet head. This can prevent the ink composition from deflecting or failing to eject, and the ejection stability tends to be excellent. In addition, the ink composition is less likely to dry out, and can be kept in a moist state. As a result, the ink composition tends to have excellent clogging recovery properties. In addition, the ink composition containing betaine tends to suppress curling of the resulting recorded material.

[0123] The betaine is not particularly limited, but examples thereof include trimethylglycine, γ-butyrobetaine, homarine, trigonelline, carnitine, homoserine betaine, valine betaine, lysine betaine, ornithine betaine, alanine betaine, stachydrine, and glutamic acid betaine. Among these, it is preferable to contain one or more of trimethylglycine, γ-butyrobetaine, and carnitine, and it is more preferable to contain trimethylglycine. This tends to further improve the clogging recovery ability. Note that one type of betaine may be used alone, or two or more types may be used in combination.

[0124] The betaine content is preferably 1.0 to 10.0 mass%, 1.5 to 9.0 mass%, 2.0 to 8.0 mass%, or 2.5 to 7.5 mass% relative to the total amount of the ink composition. A betaine content of 1.0 mass% or more tends to result in excellent ejection stability, while a betaine content of 10.0 mass% or less can suppress phase separation of the acetylene glycol surfactant in the ink composition, tending to result in excellent clogging recovery.

[0125] 1.7.Other Ingredients In addition to the components described above, the ink composition of this embodiment may contain other known components that can be used in conventional ink compositions. Examples of such other components include, but are not limited to, solubilizers, viscosity adjusters, pH adjusters such as triethanolamine, antioxidants, preservatives, corrosion inhibitors, chelating agents for capturing specific metal ions that affect dispersion, and other additives, as well as organic solvents other than those described above. The other components may be used alone or in combination.

[0126] 2. Method for producing inkjet ink composition The ink composition of this embodiment is not particularly limited, and may be, for example, a mixture of the above components. Alternatively, a colorant dispersion may be prepared by dispersing a colorant and a dispersant in a solvent, and the resulting colorant dispersion may be mixed with the other components. The solvent for dispersing the colorant and dispersant is not limited to water.

[0127] 3. Recording Media The recording medium used for recording with the ink composition of this embodiment is not particularly limited, but examples thereof include absorbent recording media, low absorbent recording media, and non-absorbent recording media. The ink composition of this embodiment is preferably used for recording on absorbent recording media.

[0128] Absorbent recording media are not particularly limited, but examples include plain paper such as electrophotographic paper with high ink permeability, inkjet paper (paper specifically for inkjet printers with an ink absorbing layer made of silica particles or alumina particles, or an ink absorbing layer made of a hydrophilic polymer such as polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP)), and fabric. The absorbent recording medium is preferably plain paper or inkjet paper. It is preferable that the portion of the recording medium that absorbs the ink composition even without water contains a calcium salt. In this case, the color development of the ink of this embodiment is more excellent, which is preferable. For example, the entire recording medium or a layer that absorbs the ink composition. Examples of calcium salts include water-soluble calcium salts and poorly water-soluble calcium salts. Water-soluble calcium salts are preferred in terms of further enhancing the color development of the ink, but poorly water-soluble calcium salts are also acceptable. Examples of poorly water-soluble calcium salts include calcium carbonate. Examples of water-soluble calcium salts include calcium chloride.

[0129] The low-absorbency recording medium is not particularly limited, but examples thereof include art paper, coated paper, cast paper, and the like, which are used in general offset printing and have relatively low ink permeability.

[0130] Non-absorbent recording media are not particularly limited, but examples include films and plates of plastics such as polyvinyl chloride, polyethylene, polypropylene, polyethylene terephthalate (PET), polycarbonate, polystyrene, polyurethane, etc.; plates of metals such as iron, silver, copper, aluminum, etc.; metal plates and plastic films manufactured by vapor deposition of these various metals, and alloy plates such as stainless steel and brass; and recording media in which a plastic film such as polyvinyl chloride, polyethylene, polypropylene, polyethylene terephthalate (PET), polycarbonate, polystyrene, polyurethane, etc. is adhered (coated) to a paper substrate.

[0131] "Low-absorbency recording medium" or "non-absorbency recording medium" is defined as a medium that absorbs water in an amount of 10 mL / m2 or less within 30 msec from the start of contact in the Bristow method. 2 "Absorbent recording media" refers to recording media with a water absorption capacity of 10 mL / m 2 This refers to a recording medium that exceeds the specified limit. The Bristow method is the most widely used method for measuring liquid absorption in a short period of time, and is also adopted by the Japan Pulp and Paper Technical Association (JAPAN TAPPI). Details of the test method are described in Standard No. 51 "Paper and Paperboard - Liquid Absorbency Test Method - Bristow Method" of the "JAPAN TAPPI Paper and Pulp Test Method 2000 Edition."

[0132] 4. Recording device The recording apparatus of this embodiment is an inkjet recording apparatus used in printing using the ink composition of this embodiment. As an example of an inkjet recording apparatus, FIG. 1 shows a perspective view of a serial printer. As shown in FIG. 1, the serial printer 20 includes a conveying unit 220 and a recording unit 230. The conveying unit 220 conveys the recording medium F fed to the serial printer to the recording unit 230 and ejects the recording medium after recording outside the serial printer. Specifically, the conveying unit 220 has feed rollers and conveys the fed recording medium F in the sub-scanning direction T2.

[0133] The recording unit 230 also includes an inkjet head 231 that ejects ink, etc. onto the recording medium F sent from the conveying unit 220, a carriage 234 that carries these, and a carriage movement mechanism 235 that moves the carriage 234 in the main scanning directions S1 and S2 of the recording medium F.

[0134] In the case of a serial printer, an inkjet head 231 having a length smaller than the width of the recording medium is provided, and the head moves to perform recording in multiple passes. In a serial printer, the head 231 is mounted on a carriage 234 that moves in a predetermined direction, and the head moves in conjunction with the movement of the carriage, thereby ejecting the ink composition onto the recording medium F. In this way, recording is performed in two or more passes. A pass is also called a main scan. A sub-scan is performed to transport the recording medium between passes. In other words, main scans and sub-scans are performed alternately.

[0135] Although not shown, the serial printer may be provided with a drying mechanism. By providing a drying mechanism, the solvent and the like can be quickly evaporated and dispersed from the ink composition applied to the recording medium, thereby enabling the rapid formation of a recorded image and the like. The drying mechanism is not particularly limited as long as it has a configuration that promotes the evaporation and dispersion of the solvent and the like contained in the ink composition. Examples of the drying mechanism include a heating mechanism that applies heat to the recording medium, a blowing mechanism that blows air onto the ink composition, and a mechanism that combines these. Examples of the drying mechanism include a forced air heater, a radiant heater, an electric conduction heater, a high-frequency dryer, and a microwave dryer.

[0136] The inkjet device of this embodiment is not limited to the serial printer, but may be a line printer, which uses a line head, an inkjet head whose length is equal to or greater than the recording width of the recording medium, to perform recording on the recording medium in a single scan.

[0137] As an example of an inkjet recording apparatus, a line printer is shown in FIG. In the XYZ coordinate system shown in FIG. 2, the X direction indicates the length direction of the recording medium, the Y direction indicates the width direction of the recording medium on the transport path within the recording apparatus, and the Z direction indicates the height direction of the apparatus.

[0138] As an example, the recording device 10 is a line-type inkjet printer capable of high-speed, high-density printing. The recording device 10 includes a feed unit 12 that stores recording media P such as paper, a conveyance unit 14, a belt conveyance unit 16, a recording unit 8, an Fd (face-down) discharge unit 20 as an "discharge unit," an Fd (face-down) placement unit 22 as a "placement unit," a reversing path unit 24 as a "reversing conveyance mechanism," an Fu (face-up) discharge unit 26, and an Fu (face-up) placement unit 28.

[0139] The feeding unit 12 is disposed at the bottom of the recording device 10. The feeding unit 12 includes a feeding tray 30 that stores recording media P, and a feeding roller 32 that sends the recording media P stored in the feeding tray 30 to the transport path 11.

[0140] The recording medium P stored in the feed tray 30 is fed by a feed roller 32 along the conveying path 11 to the conveying unit 14. The conveying unit 14 includes a conveying drive roller 34 and a conveying driven roller 36. The conveying drive roller 34 is driven to rotate by a drive source (not shown). In the conveying unit 14, the recording medium P is nipped between the conveying drive roller 34 and the conveying driven roller 36 and conveyed to the belt conveying unit 16 located downstream of the conveying path 11.

[0141] The belt conveying section 16 includes a first roller 38 located upstream on the conveying path 11, a second roller 40 located downstream, an endless belt 42 rotatably attached to the first roller 38 and the second roller 40, and a support 44 that supports the upper section 42a of the endless belt 42 between the first roller 38 and the second roller 40.

[0142] The endless belt 42 is driven by the first roller 38 or the second roller 40, which is driven by a drive source (not shown), so as to move from the +X direction to the −X direction in the upper section 42a. Therefore, the recording medium P conveyed from the conveying unit 14 is further conveyed downstream of the conveying path 11 in the belt conveying unit 16.

[0143] The recording unit 8 includes a line-type inkjet head 48 and a head holder 46 that holds the inkjet head 48. The inkjet head 48 is disposed so as to face the upper section 42a of the endless belt 42 supported by a support body 44. Nozzles (not shown) are arranged below the inkjet head 48 in the Y direction. The inkjet head 48 ejects ink toward the recording medium P to perform recording when the recording medium P is transported in the upper section 42a of the endless belt 42. The recording medium P is transported downstream of the transport path 11 by the belt transport unit 16 while recording is being performed.

[0144] A first branching section 50 is provided downstream of the conveying path 11 of the belt conveying unit 16. The first branching section 50 is configured to be switchable between the conveying path 11 that conveys the recording medium P to the Fd discharge unit 20 or the Fu discharge unit 26 and a reversing path 52 of the reversing path unit 24 that reverses the recorded side of the recording medium P and conveys the recording medium P again to the recording unit 8. The recording medium P that is switched to the reversing path 52 by the first branching section 50 and conveyed has its recorded side reversed during the conveying process on the reversing path 52, and is conveyed again to the recording unit 8 so that the side opposite to the initially recorded side faces the inkjet head 48.

[0145] A second branch section 54 is further provided downstream of the first branch section 50 along the conveying path 11. The second branch section 54 is configured to be able to switch the conveying direction of the recording medium P so that the recording medium P is conveyed toward the Fd discharge section 20 or the recording medium P is conveyed toward the Fu discharge section 26.

[0146] The recording medium P transported from the second branching section 54 toward the Fd discharge section 20 is discharged from the Fd discharge section 20 and placed on the Fd placement section 22. At this time, the recording medium P is placed so that the recorded surface faces the Fd placement section 22. Also, the recording medium P transported from the second branching section 54 toward the Fu discharge section 26 is discharged from the Fu discharge section 26 and placed on the Fu placement section 28. At this time, the recording medium P is placed so that the recorded surface faces away from the Fu placement section 28.

[0147] A line-type inkjet head (line head) is a head used in a recording device in which the nozzle area formed in a direction intersecting the transport direction of the recording medium M is arranged so as to be able to cover the entire recording area in the intersecting direction of the recording medium M, and in which an image is formed by fixing one of the head or the recording medium M and moving the other. The nozzle area in the intersecting direction of the line head does not have to be able to cover the entire intersecting direction of all recording media M that the recording device supports.

[0148] In a line printer, the head is fixed and does not move, and printing is performed in one pass (single pass), so line printers have an advantage over serial printers in that they have a faster printing speed.

[0149] 5. Inkjet recording method The inkjet recording method of this embodiment includes an ink deposition step of ejecting the ink composition of this embodiment from an inkjet head and depositing it on a recording medium. If necessary, the method may also include other steps, such as a transport step of transporting the recording medium.

[0150] 5.1.Ink deposition process In the ink deposition step, the ink composition of this embodiment is ejected from an inkjet head and deposited on a recording medium. More specifically, a pressure generating means provided in the inkjet head is driven to eject the ink composition filled in the pressure generating chamber of the inkjet head from the nozzle.

[0151] The inkjet head used in the ink deposition step may be a line head that performs recording by a line method or a serial head that performs recording by a serial method. From the viewpoint of increasing the recording speed, it is preferable that the inkjet head is a line head.

[0152] In the line method using a line head, for example, an inkjet head having a width equal to or greater than the recording width of the recording medium is fixed to the recording device. The recording medium is then moved in the sub-scanning direction (the direction in which the recording medium is transported), and ink droplets are ejected from the nozzles of the inkjet head in conjunction with this movement to record an image on the recording medium.

[0153] In the serial method using a serial head, for example, the inkjet head is mounted on a carriage that can move in the width direction of the recording medium. The carriage is then moved in the main scanning direction (the width direction of the recording medium), and ink droplets are ejected from the nozzles of the inkjet head in conjunction with this movement to record an image on the recording medium.

[0154] 5.2.Transportation process The inkjet recording method using the ink composition of this embodiment may include a transport step. In the transport step, the recording medium is transported in a predetermined direction within the recording device. More specifically, the recording medium is transported from a paper feed section to a paper discharge section of the recording device using a transport roller or a transport belt provided within the recording device. During this transport process, the ink composition ejected from the inkjet head adheres to the recording medium, forming a recorded product. The ink adhesion step and the transport step may be performed simultaneously or alternately. When double-sided printing is performed, a step of inverting the recording medium while conveying it is performed between recording on one side and recording on the other side.

[0155] The recording speed for double-sided printing is preferably 10 sheets / minute or more, more preferably 15 sheets / minute or more, and even more preferably 20 sheets / minute or more. There is no upper limit to the recording speed, but it is preferably 50 sheets / minute or less, and more preferably 40 sheets / minute or less. In double-sided printing, there are two pages per sheet. Therefore, the number of pages is twice the number of sheets per minute. The recording speed can also be expressed in pages per minute. When the speed is in the above range or higher, the time from when the ink is applied until the recording medium is transported by the transport roller is short. Furthermore, when recording media recorded at such a speed are discharged and stacked in the disposal section of the recording device, there are more opportunities for the surfaces of the recording media to come into contact with each other before the ink composition penetrates or dries, making the effects of the present invention even more effective. Recording may be performed by single-sided printing. In single-sided printing, one sheet is one page, and the recording speed is expressed in pages per minute. In the case of single-sided printing, the above-mentioned recording speed range is also preferable. In the case of single-sided printing, the backside of one stacked recorded material may come into contact with the recorded surface of another recorded material, causing ink transfer stains. However, this embodiment is preferable because it can reduce ink transfer stains. [Example]

[0156] The present invention will be described in more detail below using examples and comparative examples. The present invention is not limited to the following examples. Unless otherwise specified, the experiments in the examples and comparative examples were carried out at room temperature (25°C) and 1 atmosphere.

[0157] 1. Preparation of Inkjet Ink Composition The inkjet ink compositions used in each example were obtained by placing each component in a stainless steel mixing tank so as to obtain the composition shown in the tables of Figures 3 and 4, mixing and stirring at room temperature, and removing impurities and foreign matter by filtration, etc., as necessary. The numerical values ​​for each component shown in each example in the figures represent mass % unless otherwise specified. Furthermore, the mass % of the colorant, water-soluble resin, and resin particles represents the solids concentration. Furthermore, each component in the tables represents the following. A pigment dispersion was prepared as follows, and this was used to prepare the ink.

[0158] Surfynol 104PG50 (manufactured by Air Products) Olfine E1010 (manufactured by Nissin Chemical Industry Co., Ltd.) BYK348 (manufactured by BYK Japan Co., Ltd.) Urethane resin particles (Superflex 420 (product name), manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)

[0159] <Carbon black pigment 1> Carbon black pigment 1 in the figure was prepared by the following method. 500 g of carbon black bulk powder prepared by the furnace method was added to 3750 g of ion-exchanged water and heated to 50°C while stirring with a dissolver. Subsequently, while the mixture was ground using a sand mill with 0.8 mm diameter zirconia beads, 5300 g of an aqueous solution of sodium hypochlorite (available chlorine concentration = 12%) was added dropwise to the mixture at 50 to 60°C over 3.5 hours. Grinding was continued for 30 minutes using the sand mill to obtain a reaction solution. This reaction solution was filtered through a 400-mesh wire screen to separate the zirconia beads, unreacted carbon black, and the reaction solution. A 5% aqueous solution of potassium hydroxide was added to the separated reaction solution to adjust the pH to 7.5. The solution was desalted and purified using an ultrafiltration membrane until the liquid conductivity reached 1.5 mS / cm. Further desalting and purification were performed using an electrodialysis device until the liquid conductivity reached 1.0 mS / cm. The resulting solution was concentrated until the carbon black concentration reached 17% by mass. This concentrated solution was centrifuged to remove coarse particles and then filtered through a 0.6 μm filter. Ion-exchanged water was added to the resulting filtrate, and the carbon black was diluted to a concentration of 15 mass % and dispersed to obtain Carbon Black Pigment 1. Carbon Black Pigment 1 is a self-dispersing pigment in which carbon atoms on the pigment particle surface are oxidized to form carboxyl groups.

[0160] <Carbon black pigment 2> Carbon black pigment 2 in the figure was prepared by the following method. A solution prepared by dissolving 5.0 g of concentrated hydrochloric acid in 5.5 g of water was cooled to 5°C, and 1.5 g of 4-amino-1,2-benzenedicarboxylic acid (treatment agent) was added. The container containing this solution was placed in an ice bath and cooled to below 10°C. A solution prepared by dissolving 1.8 g of sodium nitrite in 9 g of 5°C water was then added. After stirring for 15 minutes, 6.0 g of carbon black pigment was added under stirring and stirred for another 15 minutes to obtain a slurry. The resulting slurry was filtered through filter paper (trade name "Standard Filter Paper No. 2" manufactured by Advantec), and the resulting particles were thoroughly washed with water. The washed particles were dried in an oven at 110°C to obtain a self-dispersible pigment. Water was added to the resulting self-dispersible pigment to obtain a pigment content of 10.0% by mass, to prepare a dispersion. The sodium ions in the dispersion were then exchanged for potassium ions using an ion exchange method, yielding carbon black pigment 2, in which -CH-(COOK) groups were bonded to the pigment particle surface. Carbon black pigment 2 was in a state of being dispersed in water. Carbon black pigment 2 is a self-dispersing pigment in which a compound having a carboxyl group is introduced onto the surface of the pigment particle.

[0161] <Carbon black pigment 3> Carbon black pigment 3 in the figure was prepared by the following method. First, [2-(4-aminophenyl)-1-hydroxyethane-1,1-diyl]bisphosphonic acid, sodium salt, was prepared using the following procedure. A 500 mL three-neck flask was equipped with a condenser with a gas outlet at the top of the condenser, a thermometer, a dry nitrogen inlet, and a 100 mL pressure-equalizing addition funnel. To the flask, 32 g of phosphorous acid (380 mmol) and 160 mL of methanesulfonic acid (solvent) were first added. To this stirred mixture, 57.4 g of aminophenylacetic acid (380 mmol) was added in portions. The stirred mixture was heated to 65°C for 1-2 hours to completely dissolve the solids. The entire system was flushed with dry nitrogen, and the temperature was reduced to 40°C after all of the solids had dissolved. To the heated solution, 70 mL of PCl3 (800 mmol) was slowly added via the addition funnel. HCl gas evolved from the reaction, and this gas flowed through a gas outlet into a drying tube and then through a funnel into the concentrated NaOH solution in the beaker. After the addition was complete, the reaction mixture was stirred and heated at 40°C for 2 hours. After this time, the temperature was raised to 65-70°C, and the mixture was stirred overnight. The resulting clear, brown solution was cooled to room temperature and quenched by addition to 600 g of an ice / water mixture. This aqueous mixture was placed in a 1 L beaker and heated to 90-95°C for 4 hours (the top of the beaker can be covered with a glass plate). The mixture was then cooled to room temperature, and the pH of the mixture was adjusted to 4-5 with 50% NaOH solution (the NaOH solution was added slowly as the temperature rose as a result of quenching). The mixture was cooled to 5° C. in an ice bath for 2 hours, and then the resulting solid was collected by suction filtration, washed with 1 L of cold deionized water, and dried overnight at 60° C. to give a white or off-white solid product (yield: 48 g, 39%). [2-(4-aminophenyl)-1-hydroxyethane-1,1-diyl]bisphosphonic acid-sodium salt was thus obtained. Next, 20 g of carbon black pigment, 20 mmol of the compound obtained above, and 20 mmol of nitric acid were added to 200 ml of deionized water and stirred at 6,000 rpm for 30 minutes. Next, 20 mmol of sodium nitrite was slowly added to the mixture. Stirring was continued for 1 hour as described above. The pH was adjusted to 10 with NaOH. After 30 minutes, the resulting modified pigment was filtered with deionized water to obtain a pigment dispersion containing carbon black pigment 3. The pigment solids content of this pigment dispersion was adjusted to 12% by mass. Carbon black pigment 3 is a self-dispersing pigment with phosphonic acid groups introduced.

[0162] <Carbon black pigment 4> Cab-o-Jet 200 (manufactured by Cabot Corporation) was used as the carbon black pigment 4 in the figure. Carbon black pigment 4 is a self-dispersing pigment into which a sulfonic acid group has been introduced.

[0163] <Carbon black pigment 5> 3.0 g of a styrene-acrylic acid polymer dispersant (Joncryl 682, manufactured by BASF Japan Ltd.) and 1.8 g of triethanolamine were dissolved in 80.1 g of ion-exchanged water, and 15 g of carbon black and 0.1 g of an antifoaming agent (Surfynol DF110D, manufactured by Nissin Scientific Industry Co., Ltd.) were added. The mixture was dispersed using zirconia beads in a paint shaker, and a dispersion liquid containing dispersed carbon black pigment 5 was obtained.

[0164] Carbon black pigments 1 to 4 are self-dispersed pigments, while carbon black pigment 5 is a resin-dispersed pigment. All of the carbon black pigments had a volume average particle size (D50) of 110 to 130 nm.

[0165] <Urethane resin 1> Urethane resin 1 was prepared by the following method. First, a four-neck flask equipped with a stirrer, thermometer, nitrogen gas inlet tube, and reflux condenser was prepared. 41.7 parts by mass of isophorone diisocyanate, 40.1 parts by mass of polypropylene glycol (number average molecular weight 2000), 13.2 parts by mass of dimethylolpropionic acid, and 200.0 parts by mass of methyl ethyl ketone were placed in this four-neck flask and reacted at 80°C for 6 hours under a nitrogen gas atmosphere (primary reaction). Next, 0.6 parts by mass of ethylenediamine, 2.0 parts by mass of methanol, 2.4 parts by mass of dimethylolpropionic acid, and 100.0 parts by mass of methyl ethyl ketone were added. The residual ratio of isocyanate groups was confirmed by FT-IR, and the reaction was continued at 80°C until the desired residual ratio was reached (secondary reaction), yielding a reaction solution. The resulting reaction solution was cooled to 40°C, and then ion-exchanged water was added. A potassium hydroxide aqueous solution was then added while stirring at high speed with a homomixer. Methyl ethyl ketone was distilled off from the resulting liquid by heating under reduced pressure, and a liquid containing urethane resin 1 was obtained.

[0166] For the obtained urethane resin 1, hydrochloric acid was added to a liquid containing the urethane resin 1 to precipitate the urethane resin, which was then vacuum-dried overnight at 40°C. The resin was then dissolved in tetrahydrofuran to prepare a sample, and the acid value of urethane resin 1 was measured by potentiometric titration using a potassium hydroxide-methanol titrant, which was found to be 65 mgKOH / g. Furthermore, the weight-average molecular weight of the obtained urethane resin 1, measured by gel permeation chromatography (GPC), was found to be approximately 21,000 in terms of polystyrene.

[0167] <Urethane resin 2> Urethane resin 2 was prepared in the same manner as urethane resin 1, except that the amount of polypropylene glycol added was reduced and the amount of dimethylolpropionic acid added in the primary and secondary reactions was increased in the preparation of urethane resin 1. Furthermore, when the acid value and weight average molecular weight were measured using the same methods as for urethane resin 1, the acid value of urethane resin 2 was 75 mgKOH / g and the weight average molecular weight was approximately 21,000.

[0168] <Acrylic resin> The acrylic resin was prepared by the following method. A four-neck flask equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube was prepared. 200.0 parts by mass of ethylene glycol monobutyl ether was placed in this four-neck flask, and the mixture was stirred under a nitrogen gas atmosphere and heated to 130°C. 62.0 parts by mass of styrene monomer, 22.0 parts by mass of butyl acrylate, 16.0 parts by mass of acrylic acid, and 4.0 parts by mass of a polymerization initiator (t-butyl peroxide) were added dropwise over 3 hours. After aging for 2 hours, the ethylene glycol monobutyl ether was distilled off under reduced pressure to obtain the acrylic resin.

[0169] 2. Evaluation Method In the following evaluations, the recording device used was a PX-S270T (product name, manufactured by Seiko Epson Corporation), an inkjet printer with a double-sided printing mechanism, which was modified to become a line inkjet printer equipped with a line head, as shown in Figure 2.

[0170] 2.1.Color development The recording medium is "Xerox P paper" (also written as XP paper, manufactured by Fuji Xerox Co., Ltd., basis weight: 64 g / m 2 ) and "Copyplus paper" (also written as CO paper. Manufactured by Hammermill, basis weight: 75 g / m 2 The above recording device was filled with the ink of each example, and the amount of the ink composition of each example deposited was 6 mg / inch. 2 A solid pattern was recorded using the ink. The OD value (optical density) was then measured using an i1Pro2 (manufactured by X-Rite) and evaluated according to the following criteria. Note that CO paper contains a higher calcium salt content than XP paper, and tends to have stronger color development. [Evaluation criteria] A: The OD value of all recording media is 1.3 or more. B: The OD value of one recording medium is 1.3 or more, and the OD value of the other recording medium is less than 1.3. C: The OD value of all recording media is less than 1.3. D: The OD value of both recording media is less than 1.3, and at least one recording medium has an OD value of 1.1 or less.

[0171] 2.2. Abrasion resistance (line marker resistance) Using the same recording medium as in the color development test, a character string pattern of 55 pt size was recorded on the recording medium using the ink composition of each example. Immediately after recording, the recording medium was fixed on a horizontally placed flat surface, and one hour after recording, the character string was rubbed with a line marker "OPTEX CARE" (manufactured by Zebra Corporation), and then visually evaluated according to the following criteria. Note that if the evaluations differed between the two recording media, the lower evaluation was adopted. For example, if the ink composition of the example or comparative example was evaluated as equivalent to a B on Xerox P paper and a C on Copyplus paper, the ink composition of that example or comparative example was evaluated as a C. [Evaluation criteria] A: The color of the letters does not bleed even after rubbing twice. B: The color of the letters does not bleed when rubbed once, but the color of the letters bleeds when rubbed twice. C: The color of the letters bleeds after rubbing once.

[0172] Furthermore, using the ink composition of each example, 50 sheets were continuously printed on both sides using the above recording apparatus and recording medium, and the resulting printed sheets were sequentially discharged onto a paper discharge tray starting from the printed sheets at a printing speed of 15 sheets / min. The end faces of the stack of recorded materials piled up on the discharge tray in the recording medium conveyance direction were visually inspected for ink transfer stains caused by the rollers during conveyance. As a result, when an ink composition rated C according to the above evaluation criteria was used, stains on the end faces were noticeable, whereas when an ink composition rated B or A was used, stains on the end faces were not noticeable or not noticeable at all. It was found that ink that is less likely to cause transfer smearing is also less likely to peel off due to rubbing. Furthermore, when double-sided printing was performed in the same manner at a recording speed of 20 sheets per minute, there was a tendency for ink transfer smearing to increase.

[0173] 2.3.Discharge stability After confirming that the nozzle discharge state of the inkjet head of the above recording device was normal, the inkjet head was filled with the ink composition of each example, and a test pattern was printed on a recording medium. The inkjet head was then allowed to idle for 5 minutes while filled with the ink composition of each example, and the test pattern was then printed again to count the number of nozzles that exhibited ink landing position deviation. A landing position deviation of 50% or more relative to the distance between adjacent nozzles was defined as occurrence of landing position deviation, and evaluation was performed according to the following criteria. [Evaluation criteria] A: There are no nozzles where the landing position has shifted. B: The number of nozzles in which the landing position shift occurred is 1% or less of the total number of nozzles. C: The number of nozzles in which misalignment of the landing position occurred is more than 1% and 3% or less of the total number of nozzles. D: The number of nozzles in which misalignment of the ink droplet occurred exceeds 3% of the total number of nozzles.

[0174] 2.4.Clogging recovery After confirming that there were no non-ejecting nozzles in the inkjet head of the recording device, the head was left uncapped at 40°C for one day. Then, using one of the line heads (600 nozzles), 0.5 cc of ink was sucked from the nozzles, and the nozzle surface was wiped with a rubber wiper (cleaning). The number of cleanings performed until no non-ejecting nozzles were found was counted, and the results were evaluated according to the following criteria. [Evaluation criteria] A: Nozzles that do not eject ink can be eliminated by cleaning three times or less. B: Nozzles that do not eject ink are eliminated after 4 or 5 cleanings. C: No non-ejecting nozzles are eliminated after six cleanings. D: No nozzles that do not eject ink remain even after six cleanings.

[0175] From the above evaluation results, the inks of the present embodiment, which contain a self-dispersing pigment having a carboxyl group introduced therein, a lactam having a 6- to 8-membered lactam ring, a solvent component containing water, and a water-soluble urethane resin dissolved in the solvent component in an amount of 0.3 mass% or more relative to the total amount of the ink composition, all had excellent color development, abrasion resistance, and clogging recovery properties, and also had good ejection stability. In contrast, inks other than those of this embodiment were inferior in color development, abrasion resistance, or clogging recovery. [Explanation of symbols]

[0176] 1...line printer, 2...inkjet head, 3...IR heater, 4...platen heater, 5...heating heater, 6...cooling fan, 7...preheater, 8...blower fan, M...recording medium, SS...sub-scanning direction, 20...serial printer, 220...conveyor unit, 230...recording unit, 231...inkjet head, 234...carriage, 235...carriage movement mechanism, F...recording medium, S1, S2...main scanning direction, T2...sub-scanning direction

Claims

1. The ink contains a pigment, a lactam having a 6- to 8-membered lactam ring, a solvent component, and a water-soluble urethane resin dissolved in the solvent component, The pigment includes a self-dispersing pigment having a carboxyl group introduced therein, the content of the water-soluble urethane resin is 0.3% by mass or more relative to the total amount of the inkjet ink composition; the solvent component comprises water, The ink-jet ink composition is a water-based ink.

2. the content of the water-soluble urethane resin is 0.3 to 3.0% by mass relative to the total amount of the inkjet ink composition; The ink-jet ink composition of claim 1 .

3. the content of the lactams is 0.5 to 5.0% by mass relative to the total amount of the inkjet ink composition; The ink-jet ink composition of claim 1 .

4. Contains acetylene glycol surfactants, The ink-jet ink composition of claim 1 .

5. The HLB value of the acetylene glycol surfactant is 6 or less. The ink-jet ink composition of claim 4.

6. the content of the acetylene glycol surfactant is 0.2 to 2.0% by mass relative to the total amount of the inkjet ink composition; The ink-jet ink composition of claim 4.

7. the content of the self-dispersion pigment having a carboxyl group introduced therein is 3 to 12% by mass relative to the total amount of the inkjet ink composition; The ink-jet ink composition of claim 1 .

8. The acid value of the water-soluble urethane resin is 40 to 80 mgKOH / g. The ink-jet ink composition of claim 1 .

9. Contains betaine, The ink-jet ink composition of claim 1 .

10. Used for recording on absorbent recording media, The ink-jet ink composition of claim 1 .

11. an ink deposition step of ejecting the inkjet ink composition according to any one of claims 1 to 10 from an inkjet head and depositing it on a recording medium; Inkjet recording method.

12. the inkjet head is a line head; The inkjet recording method according to claim 11.

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