Ink, ink cartridge, and image-recording method

1. Field of the Invention

The present invention relates to an ink, an ink cartridge containing the ink, and an image-recording method in which the ink is used.

2. Description of the Related Art

In known image-recording methods, inks that contain a pigment dispersed with a polymer dispersant (hereinafter referred to as a “polymer-dispersed pigment”) as a coloring material are used to improve the scratch resistance of images. Although inks that contain a polymer-dispersed pigment can produce images having relatively high scratch resistance, the images have low optical densities. It is proposed in Japanese Patent Laid-Open No. 2003-226827 that the optical density of images can be improved by decreasing the water content of an ink containing a polymer-dispersed pigment to promote water evaporation and thereby accelerating aggregation of the pigment on a recording sheet. Japanese Patent Laid-Open No. 2003-226827 describes inks that contain a polymer-dispersed pigment and various organic solvents, such as a wetting agent and a penetrant.

An ink according to one aspect of the present invention contains a pigment dispersed with a polymer dispersant, polymer particles, a surfactant, a water-soluble organic solvent, and water, wherein the amount of anionic functional group on the surface of the polymer particles is 0.2 mmol/g or less, the surfactant is represented by the following general formula (1) and contains a fluorinated surfactant having a hydrophile-lipophile balance (HLB) of 11 or less as determined by a Griffin method,

R 1 (CR 2 R 3 ) n CH 2 CH 2 (OCH 2 CH 2 ) m OH  (1)

wherein R 1 denotes a fluorine atom or a hydrogen atom, R 2 and R 3 independently denote a fluorine atom or a hydrogen atom, and at least one of R 2 and R 3 denotes a fluorine atom, n is 1 or more and 30 or less, and m is 1 or more and 60 or less, and

the water-soluble organic solvent contains at least one water-soluble organic solvent selected from the following group A:

glycerin, ethylene glycol, diethylene glycol, poly(ethylene glycol) having a weight-average molecular weight of 10,000 or less, 1,3-propanediol, 1,4-butanediol, and diglycerol.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

FIG. 1 is a schematic view of an image-recording apparatus for use in an image-recording method according to an embodiment of the present invention.

FIG. 2 is a graph of an absorption curve from which the absorption coefficient Ka of a recording medium is determined.

The present inventors found that although inks described in Japanese Patent Laid-Open No. 2003-226827 produced images having somewhat improved optical densities, these optical densities are still below the desired level. The present inventors also found that a large amount of organic solvents, such as a wetting agent and a penetrant, introduced to decrease the water content of inks reduces the optical density and scratch resistance of images.

The present invention provides an ink that contains a polymer-dispersed pigment and produces images having high optical densities and scratch resistance. The present invention also provides an ink cartridge containing an ink according to an embodiment of the present invention and an image-recording method in which an ink according to an embodiment of the present invention is used.

The present invention will be further described with the embodiments.

As a result of extensive studies on a method for achieving high optical density and scratch resistance of images using an ink that contains a polymer-dispersed pigment, the present inventors arrived at one embodiment of the present invention, that is, a method of using polymer particles having a particular amount of anionic functional group on the surface thereof in combination with a particular surfactant and a particular water-soluble organic solvent. The following is a possible mechanism by which the advantages of the present invention can be achieved by such a method.

As a result of extensive studies, the present inventors found that use of a particular fluorinated surfactant in an ink can improve the optical density and scratch resistance of images as compared with use of another surfactant. This is probably because the particular fluorinated surfactant decreases the contact angle of an ink on a recording medium as compared with another surfactant. Use of the particular fluorinated surfactant decreases the contact angle of an ink on a recording medium and spreads the ink on the surface of the recording medium. This allows a polymer-dispersed pigment to be more easily remained in the area around the surface of the recording medium and improves the optical density and scratch resistance of images. The studies of the present inventors show that not all the fluorinated surfactants have this effect, and fluorinated surfactants represented by the following general formula (1) and having a hydrophile-lipophile balance (HLB) of 11 or less as determined by a Griffin method are particularly effective.

R 1 (CR 2 R 3 ) n CH 2 CH 2 (OCH 2 CH 2 ) m OH  (1)

In the general formula (1), R 1 denotes a fluorine atom or a hydrogen atom, R 2 and R 3 independently denote a fluorine atom or a hydrogen atom, and at least one of R 2 and R 3 denotes a fluorine atom, n is 1 or more and 30 or less, and m is 1 or more and 60 or less.

However, the use of such a particular fluorinated surfactant cannot sufficiently improve the optical density and scratch resistance of images on some recording media. For example, use of a recording medium having high ink absorbency, such as plain paper, sometimes results in an insufficient amount of polymer-dispersed pigment remained in the area around the surface of the recording medium and low optical densities and scratch resistance of images.

The present inventors further studied the type of water-soluble organic solvent. As a result, the present inventors found that use of a particular water-soluble organic solvent in combination with the particular fluorinated surfactant in an ink allows a polymer-dispersed pigment to be remained in the area around the surface of a recording medium even having high ink absorbency, such as plain paper. The particular water-soluble organic solvent is at least one water-soluble organic solvent selected from glycerin, ethylene glycol, diethylene glycol, poly(ethylene glycol) having a weight-average molecular weight of 10,000 or less, 1,3-propanediol, 1,4-butanediol, and diglycerol. The present inventors found from various experiments that use of such a water-soluble organic solvent and such a fluorinated surfactant in combination can improve the optical density and scratch resistance of images. Although there is no clear reason for improved optical density and scratch resistance of images due to the use of such a particular fluorinated surfactant and such a particular water-soluble organic solvent in combination, the present inventors think that the improved optical density and scratch resistance of images results from two or more hydroxyl groups in the molecular structure of the particular water-soluble organic solvent, hydroxyl groups bound to both terminal carbon atoms of the molecular structure, and high symmetry of the molecular structure.

As a result of further studies, the present inventors found that when an ink further contains polymer particles having 0.2 mmol/g or less of an anionic functional group on the surface thereof, the ink has an enhanced effect of improving the optical density and scratch resistance of images. Polymer particles having a smaller amount of anionic functional group on the surface thereof can more easily aggregate on a recording medium, and the polymer particles, together with a polymer-dispersed pigment, are more easily remained in the area around the surface of a recording medium and further increase the optical density of images. The polymer particles that are remained in the area around the surface of a recording medium and protect the polymer-dispersed pigment can improve scratch resistance.

As described above, when an ink contains a polymer-dispersed pigment, polymer particles having a particular amount of anionic functional group on the surface thereof, the particular fluorinated surfactant, and the particular water-soluble organic solvent, the polymer-dispersed pigment and the polymer particles can be remained in the area around the surface of any recording medium. Thus, due to the synergistic effects of these components, the advantages of the present invention, that is, high optical density and scratch resistance of images can be achieved.

[Ink]

An ink according to an embodiment of the present invention contains a polymer-dispersed pigment, polymer particles having 0.2 mmol/g or less of an anionic functional group on the surface thereof, a fluorinated surfactant represented by the general formula (1) and having HLB of 11 or less (hereinafter also referred to simply as a “surfactant represented by the general formula (1)”), a water-soluble organic solvent, and water. The components for use in an ink according to an embodiment of the present invention will be described below.

<Polymer-Dispersed Pigment>

An ink according to an embodiment of the present invention contains a pigment dispersed with a polymer dispersant (polymer-dispersed pigment).

Pigment

Examples of pigments for use in an ink according to an embodiment of the present invention include, but are not limited to, inorganic pigments, such as carbon black, organic pigments, and known pigments for use in ink jet inks. The pigment content (% by mass) of an ink according to an embodiment of the present invention is preferably 0.1% by mass or more and 15.0% by mass or less, more preferably 1.0% by mass or more and 8.0% by mass or less. A pigment content of less than 1.0% by mass may result in insufficient optical densities of images. A pigment content of more than 8.0% by mass may result in poor ink jet characteristics, such as low sticking resistance.

Method for Identifying Pigment Dispersed with Polymer Dispersant

A pigment dispersed with a polymer dispersant can be identified by the following method. An ink concentrated or diluted to a solid content of approximately 10% by mass is centrifuged at 12,000 rpm for 1 hour. After a water-soluble organic solvent and a polymer that does not contribute to dispersion of the pigment are transferred to a liquid phase, precipitated components including the pigment are collected. If the precipitated components including the pigment include a polymer, the pigment can be considered to be a polymer-dispersed pigment. A polymer contained as a main component in the precipitated components including the pigment contributes to dispersion of the pigment (a polymer dispersant), and a polymer contained in the liquid phase does not contribute to dispersion of the pigment.

Polymer Dispersant

A polymer dispersant for use in an ink according to an embodiment of the present invention may be any known polymer dispersant for use in ink jet inks. The polymer dispersant may be water-soluble. Thus, a pigment dispersed with the polymer dispersant may be a pigment dispersed with a water-soluble polymer. The sentence “a polymer is water-soluble”, as used herein, means that the polymer has no particle size when the polymer is neutralized with an amount of alkali equivalent to the acid value of the polymer. Thus, the water-soluble polymer is different from polymer particles (having a particle size) described below. Examples of monomers for use in the preparation of such a polymer dispersant include, but are not limited to, the following monomers. The polymer dispersant may be synthesized from at least two of these monomers. At least one of the monomers may be a hydrophilic monomer. Examples of the monomers include, but are not limited to, styrene, vinylnaphthalene, aliphatic alcohol esters of α,β-ethylenically unsaturated carboxylic acids, acrylic acid, methacrylic acid, maleic acid, itaconic acid, fumaric acid, vinyl acetate, vinylpyrrolidone, acrylamide, and derivatives thereof. The hydrophilic monomer may be acrylic acid or methacrylic acid. In particular, a polymer dispersant for use in an ink according to an embodiment of the present invention may be a copolymer that includes a unit derived from acrylic acid and a unit derived from (meth)acrylic acid. The polymer dispersant may be a block copolymer, a random copolymer, a graft copolymer, or a salt thereof. The polymer dispersant may also be a natural polymer, such as rosin, shellac, or starch.

The polymer dispersant preferably has a polystyrene-equivalent weight-average molecular weight of 1,000 or more and 30,000 or less, more preferably 3,000 or more and 15,000 or less, as determined by gel permeation chromatography (GPC). The polymer dispersant preferably has an acid value of 50 mgKOH/g or more and 350 mgKOH/g or less, more preferably 80 mgKOH/g or more and 250 mgKOH/g or less. The polymer dispersant having an acid value in this range can improve pigment dispersion stability and ink ejection stability. The acid value of polymers can be measured by potentiometric titration. The polymer dispersant content (% by mass) of an ink according to an embodiment of the present invention is preferably 0.1% by mass or more and 5.0% by mass or less, more preferably 0.5% by mass or more and 3.0% by mass or less. The ratio of the pigment content (% by mass) to the polymer content (% by mass) of an ink according to an embodiment of the present invention is preferably 0.3 or more and 5.0 or less, more preferably 0.5 or more and 2.0 or less. In embodiments of the present invention, the component contents used for the calculation of the mass ratio are based on the mass of the ink.

The ratio of the pigment content to the polymer dispersant content of an ink according to an embodiment of the present invention is more than 3, preferably 3.3 or more, more preferably 4 or more and 10 or less, on a mass basis.

Content

The polymer-dispersed pigment content (the pigment and polymer dispersant content) of an ink according to an embodiment of the present invention is preferably 0.1% by mass or more and 5.0% by mass or less, more preferably 1.0% by mass or more and 4.0% by mass or less. A polymer-dispersed pigment content of less than 1.0% by mass may result in an insufficient effect of improving the optical density of images. A polymer-dispersed pigment content of more than 4.0% by mass may result in insufficient sticking resistance.

The total of the polymer-dispersed pigment content (the pigment and polymer dispersant content) and the polymer particle content of an ink according to an embodiment of the present invention is preferably 12% by mass or less, more preferably 10% by mass or less. A total content of more than 12% by mass may result in insufficient ink ejection stability.

The ratio of the polymer particle content to the polymer dispersant content of an ink according to an embodiment of the present invention is preferably 0.2 or more and 14.0 or less, more preferably 0.4 or more and 8.0 or less, on a mass basis. When this mass ratio is less than 0.2, this may result in an insufficient effect of improving the optical density of images. When the mass ratio is more than 14.0, this may result in insufficient ink ejection stability.

The ratio of the pigment content to the polymer dispersant and polymer particle content of an ink according to an embodiment of the present invention is preferably 0.30 or more and 4.50 or less, more preferably 0.60 or more and 4.00 or less, on a mass basis. When this mass ratio is less than 0.30, this may result in an insufficient effect of improving the optical density of images. When the mass ratio is more than 4.50, this may result in an insufficient effect of improving the scratch resistance of images.

The ratio of the polymer-dispersed pigment content (the pigment and polymer dispersant content) to the polymer particle content of an ink according to an embodiment of the present invention is preferably 0.6 or more and 14.0 or less, more preferably 1.0 or more and 10.0 or less, still more preferably 1.0 or more and 2.0 or less, on a mass basis. When this mass ratio is less than 0.6, this may result in a low polymer-dispersed pigment content and an insufficient effect of improving the optical density of images. When the mass ratio is more than 14.0, this may result in a low polymer particle content and an insufficient effect of improving the scratch resistance of images.

<Polymer Particles>

An ink according to an embodiment of the present invention contains polymer particles having 0.2 mmol/g or less of an anionic functional group on the surface thereof. The amount of anionic functional group on the surface of the polymer particles is preferably 0.15 mmol/g or less, more preferably 0.10 mmol/g or less. The amount of anionic functional group on the surface of the polymer particles is preferably more than 0 mmol/g, more preferably 0.03 mmol/g or more.

Examples of the anionic functional group include, but are not limited to, —COOM, —SO 3 M, —PO 3 HM, and —PO 3 M 2 , wherein “M” denotes a hydrogen atom, an alkali metal, ammonium, or an organic ammonium.

The term “polymer particles”, as used herein, refers to a polymer dispersed in a solvent in a state in which the polymer has a particle size. In accordance with an embodiment of the present invention, the polymer particles preferably has a 50% cumulative volume-average particle size (D 50 ) of 1 nm or more and 100 nm or less, more preferably 5 nm or more and 50 nm or less. D 50 of polymer particles may be measured in a polymer particle dispersion diluted 50-fold (on a volume basis) with pure water using UPA-EX150 (manufactured by Nikkiso Co., Ltd.) under the measurement conditions of SetZero: 30 s, the number of measurements: 3, measurement time: 180 seconds, and refractive index: 1.5.

The polymer particles may be known polymer particles and may be at least one selected from polyurethane polymer particles and acrylic polymer particles. The polyurethane polymer particles and acrylic polymer particles will be described below.

<1> Polyurethane Polymer Particles

Physical Properties of Polyurethane Polymer Particles

Polyurethane polymer particles for use in an ink according to an embodiment of the present invention preferably have a polystyrene-equivalent weight-average molecular weight (Mw) of more than 5,000 and 150,000 or less, more preferably 8,000 or more and 100,000 or less, as determined by gel permeation chromatography (GPC). When the polystyrene-equivalent weight-average molecular weight (Mw) is 5,000 or less, the polyurethane polymer particles may have low strength and an insufficient effect of improving the scratch resistance of images. When the polystyrene-equivalent weight-average molecular weight (Mw) is more than 150,000, the ink may have insufficient storage stability and ejection stability. The weight-average molecular weight of the polymer particles may be determined with an apparatus Alliance GPC 2695 (manufactured by Waters), four columns of Shodex KF-806M (manufactured by Showa Denko K.K.) in series, and a refractive index (RI) detector and may be calculated using polystyrene standard samples PS-1 and PS-2 (manufactured by Polymer Laboratories).

Polyurethane polymer particles for use in an ink according to an embodiment of the present invention preferably have an acid value of 100 mgKOH/g or less, more preferably 5 mgKOH/g or more and 30 mgKOH/g or less. The acid value of the polyurethane polymer particles can be measured by titrimetry. For example, the acid value may be measured by potentiometric titration of polymer particles dissolved in THF using an automatic potentiometric titrator AT510 (manufactured by Kyoto Electronics Manufacturing Co., Ltd.) and a potassium hydroxide ethanol titrant.

Polyurethane polymer particles for use in an ink according to an embodiment of the present invention preferably have a glass transition temperature (Tg) of −80° C. or more, more preferably −50° C. or more. The glass transition temperature (Tg) is preferably 120° C. or less, more preferably 100° C. or less.

Polyurethane Polymer Particle Content

The polyurethane polymer particle content of an ink according to an embodiment of the present invention is preferably 0.1% by mass or more and 10.0% by mass or less. A polyurethane polymer particle content of less than 0.1% by mass may result in an insufficient effect of improving the scratch resistance of images. A polyurethane polymer particle content of more than 10.0% by mass may result in insufficient ink ejection stability.

The ratio of the polyurethane polymer particle content to the fluorinated surfactant content of an ink according to an embodiment of the present invention is preferably 0.5 or more and 4.0 or less, more preferably 1.0 or more and 3.0 or less, on a mass basis. When this mass ratio is less than 0.5, this may result in a low polyurethane polymer particle content and an insufficient effect of improving the scratch resistance of images. When the mass ratio is more than 4.0, this may result in a low fluorinated surfactant content, an insufficient amount of polymer-dispersed pigment remained in the area around the surface of a recording medium, and an insufficient effect of improving the optical density and scratch resistance of images.

Method for Producing Polyurethane Polymer Particles

Polyurethane polymer particles for use in an ink according to an embodiment of the present invention may be produced by any known method, for example, by the following method. A polyol having no acid group is well stirred and dissolved in an organic solvent, such as methyl ethyl ketone, and is allowed to react with a polyisocyanate and a diol having an acid group, thereby producing an urethane prepolymer solution. The urethane prepolymer solution is then neutralized. After ion-exchanged water is added to the urethane prepolymer solution, the urethane prepolymer solution is agitated at a high speed and is emulsified in a homo mixer. After emulsification, a chain extension agent is added to the emulsion, and the urethane prepolymer is subjected to a chain extension reaction.

The materials of polyurethane polymer particles for use in an ink according to an embodiment of the present invention will be described below.

(1) Polyisocyanate

Polyurethane polymer particles for use in an ink according to an embodiment of the present invention can include a polyisocyanate-derived unit. The term “polyisocyanate”, as used herein, refers to a compound having two or more isocyanate groups. Examples of polyisocyanates for use in the present invention include, but are not limited to, aliphatic polyisocyanates, alicyclic polyisocyanates, aromatic polyisocyanates, and araliphatic polyisocyanates. The percentage of a polyisocyanate-derived unit in the polyurethane polymer particles can be 10.0% by mass or more and 80.0% by mass or less.

Examples of the aliphatic polyisocyanates include, but are not limited to, 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. Examples of the alicyclic polyisocyanates include, but are not limited to, isophorone diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane 4,4′-diisocyanate, 1,4-cyclohexane diisocyanate, methylcyclohexylene diisocyanate, and 1,3-bis(isocyanatomethyl)cyclohexane. Examples of the aromatic polyisocyanates include, but are not limited to, 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, and 1,4-phenylene diisocyanate. Examples of the araliphatic polyisocyanates include, but are not limited to, dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, and α,α,α,α-tetramethylxylylene diisocyanate. These polyisocyanates may be used alone or in combination. In accordance with an embodiment of the present invention, among these polyisocyanates, at least one selected from isophorone diisocyanate, hexamethylene diisocyanate, and dicyclohexylmethane 4,4′-diisocyanate may be used.

(2) Polyol Having No Acid Group

Polyurethane polymer particles for use in an ink according to an embodiment of the present invention can include a unit derived from a polyol having no acid group. The percentage of a unit derived from a polyol having no acid group in the polyurethane polymer particles is preferably 0.1% by mass or more and 80.0% by mass or less.

Examples of the polyol having no acid group include, but are not limited to, polyester polyols, polyether polyols, and polycarbonate diols. The polyol having no acid group preferably has 13 or more and 250 or less carbon atoms. The polyol having no acid group preferably has a polystyrene-equivalent number-average molecular weight of 600 or more and 4,000 or less as determined by GPC.

Examples of the polyester polyols include, but are not limited to, esters of acid components and polyalkylene glycols, dihydric alcohols, or tri- or higher-valent polyhydric alcohols. Examples of the acid components of the polyester polyols include, but are not limited to, aromatic dicarboxylic acids, alicyclic dicarboxylic acids, and aliphatic dicarboxylic acids. Examples of the aromatic dicarboxylic acids include, but are not limited to, isophthalic acid, terephthalic acid, orthophthalic acid, 1,4-naphthalenedicarboxylic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, biphenyldicarboxylic acid, and tetrahydrophthalic acid. Examples of the alicyclic dicarboxylic acids include, but are not limited to, hydrogenated compounds of the aromatic dicarboxylic acids. Examples of the aliphatic dicarboxylic acids include, but are not limited to, malonic acid, succinic acid, tartaric acid, oxalic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, alkylsuccinic acid, linolenic acid, maleic acid, fumaric acid, mesaconic acid, citraconic acid, and itaconic acid. Reactive derivatives of these acid components, such as acid anhydrides, alkyl esters, and acid halides, can also be used as acid components of the polyester polyols. The acid components of the polyester polyols may be used alone or in combination. Examples of the polyalkylene glycols include, but are not limited to, poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol), and ethylene glycol-propylene glycol copolymers. Examples of the dihydric alcohols include, but are not limited to, hexamethylene glycol, tetramethylene glycol, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, 4,4′-dihydroxyphenylpropane, and 4,4′-dihydroxyphenylmethane. Examples of the tri- or higher-valent polyhydric alcohols include, but are not limited to, glycerin, trimethylolpropane, 1,2,5-hexanetriol, 1,2,6-hexanetriol, and pentaerythritol. These polyester polyols may be used alone or in combination.

Examples of the polyether polyols include, but are not limited to, polyalkylene glycols and addition polymers of alkylene oxides and dihydric alcohols or tri- or higher-valent polyhydric alcohols. Examples of the polyalkylene glycols include, but are not limited to, poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol), and ethylene glycol-propylene glycol copolymers. Examples of the dihydric alcohols include, but are not limited to, hexamethylene glycol, tetramethylene glycol, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, 4,4′-dihydroxyphenylpropane, and 4,4′-dihydroxyphenylmethane. Examples of the tri- or higher-valent polyhydric alcohols include, but are not limited to, glycerin, trimethylolpropane, 1,2,5-hexanetriol, 1,2,6-hexanetriol, and pentaerythritol. Examples of the alkylene oxides include, but are not limited to, ethylene oxide, propylene oxide, butylene oxide, and α-olefin oxides. These polyether polyols may be used alone or in combination.

The polycarbonate diols may be produced by conventional methods. Examples of the polycarbonate diols include, but are not limited to, polycarbonate diols produced by reactions between a carbonate component, such as an alkylene carbonate, diary carbonate, or dialkyl carbonate, or phosgene and an aliphatic diol component. These polycarbonate diols may be used alone or in combination.

Among the polyols having no acid group, polyether polyols may be used. Polyether polyols impart moderate flexibility to a polymer film and tend to improve the scratch resistance of images. Furthermore, polyether polyols have relatively high hydrophilicity and therefore improve ink ejection stability. Among polyether polyols, poly(propylene glycol) may be used.

(3) Diol Having Acid Group

Polyurethane polymer particles for use in an ink according to an embodiment of the present invention can include a unit derived from a diol having an acid group. The term “a diol having an acid group”, as used herein, refers to a diol having an acid group, such as a carboxy group, a sulfonic acid group, or a phosphate group. The diol having an acid group may be in the form of a salt with an alkali metal, such as Li, Na, or K, ammonia, or an organic amine, such as dimethylamine. The diol having an acid group can be dimethylolpropionic acid or dimethylolbutanoic acid. These diols having an acid group may be used alone or in combination. The percentage of a unit derived from a diol having an acid group in the polyurethane polymer particles is preferably 5.0% by mass or more and 40.0% by mass or less.

(4) Chain Extension Agent

In accordance with an embodiment of the present invention, a chain extension agent may be used to produce polyurethane polymer particles. A chain extension agent is a compound that can react with a residual isocyanate group in a polyisocyanate unit of a urethane prepolymer. The residual isocyanate group is an isocyanate group that did not form a urethane bond. Examples of the chain extension agent include, but are not limited to, polyvalent amine compounds, such as trimethylolmelamine and derivatives thereof, dimethylolurea and derivatives thereof, dimethylolethylamine, diethanolmethylamine, dipropanolethylamine, dibutanolmethylamine, ethylenediamine, propylenediamine, diethylenetriamine, hexylenediamine, triethylenetetramine, tetraethylenepentamine, isophoronediamine, xylylenediamine, diphenylmethanediamine, hydrogenated diphenylmethanediamine, and hydrazine, polyamide polyamine, and polyethylene polyimine. Examples of the chain extension agent also include, but are not limited to, ethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, poly(ethylene glycol), 3-methyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, hydrogenated bisphenol A, glycerin, trimethylolpropane, and pentaerythritol. These chain extension agents may be used alone or in combination.

<2> Acrylic Polymer Particles

Physical Properties of Acrylic Polymer Particles

Acrylic polymer particles for use in an ink according to an embodiment of the present invention preferably have a polystyrene-equivalent weight-average molecular weight (Mw) of more than 100,000 and 3,000,000 or less, more preferably 300,000 or more and 1,000,000 or less, as determined by gel permeation chromatography (GPC). When the polystyrene-equivalent weight-average molecular weight (Mw) is 100,000 or less, the acrylic polymer particles may have low strength and an insufficient effect of improving the scratch resistance of images. When the polystyrene-equivalent weight-average molecular weight (Mw) is more than 150,000, the ink may have insufficient storage stability and ejection stability. The weight-average molecular weight of the polymer particles may be determined with an apparatus Alliance GPC 2695 (manufactured by Waters), four columns of Shodex KF-806M (manufactured by Showa Denko K.K.) in series, and a refractive index (RI) detector and may be calculated using polystyrene standard samples PS-1 and PS-2 (manufactured by Polymer Laboratories).

Acrylic polymer particles for use in an ink according to an embodiment of the present invention preferably have an acid value of 150 mgKOH/g or less, more preferably 25 mgKOH/g or more and 140 mgKOH/g or less. The acid value of the acrylic polymer particles can be measured by titrimetry. For example, the acid value may be measured by potentiometric titration of polymer particles dissolved in THF using an automatic potentiometric titrator AT510 (manufactured by Kyoto Electronics Manufacturing Co., Ltd.) and a potassium hydroxide ethanol titrant.

Acrylic polymer particles for use in an ink according to an embodiment of the present invention preferably have a glass transition temperature (Tg) of −20° C. or more, more preferably −10° C. or more, still more preferably 25° C. or more. The glass transition temperature (Tg) is preferably 120° C. or less, more preferably 100° C. or less.

Acrylic Polymer Particle Content

The acrylic polymer particle content of an ink according to an embodiment of the present invention is preferably 0.1% by mass or more and 10.0% by mass or less. An acrylic polymer particle content of less than 0.1% by mass may result in an insufficient effect of improving the scratch resistance of images. An acrylic polymer particle content of more than 10.0% by mass may result in insufficient ink ejection stability.

The ratio of the acrylic polymer particle content to the fluorinated surfactant content of an ink according to an embodiment of the present invention is preferably 0.5 or more and 4.0 or less, more preferably 1.0 or more and 3.0 or less, on a mass basis. When this mass ratio is less than 0.5, this may result in a low acrylic polymer particle content and an insufficient effect of improving the scratch resistance of images. When the mass ratio is more than 4.0, this may result in a low fluorinated surfactant content, an insufficient amount of polymer-dispersed pigment remained in the area around the surface of a recording medium, and an insufficient effect of improving the optical density and scratch resistance of images.

Monomer for Acrylic Polymer Particles

Examples of a monomer for acrylic polymer particles for use in an ink according to an embodiment of the present invention include, but are not limited to, alkyl(meth)acrylates, such as methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, butyl(meth)acrylate, hexyl(meth)acrylate, octyl(meth)acrylate, lauryl(meth)acrylate, and stearyl(meth)acrylate; and (meth)acrylic acids, such as acrylic acid and methacrylic acid. The acrylic polymer particles may be formed of a homopolymer of such a monomer or a copolymer of such a monomer and another monomer. Examples of the other monomer include, but are not limited to, vinyl esters, olefins, styrenes, crotonic acids, itaconic acids, maleic acids, fumaric acids, acrylamides, ally compounds, vinyl ethers, vinyl ketones, glycidyl esters, and unsaturated nitriles. When the acrylic polymer particles are formed of a copolymer, a unit derived from an alkyl(meth)acrylate or (meth)acrylic acid preferably constitutes 50% by mol or more of the copolymer.

<3> Method for Measuring Amount of Anionic Functional Group on Surface of Polymer Particles

The amount of anionic functional group on the surface of polymer particles can be measured by colloid titration of an aqueous dispersion of the polymer particles. In the exemplary embodiments described below, the amount of anionic functional group on the surface of polymer particles in a polymer particle dispersion was measured by colloid titration utilizing potential difference using an automatic potentiometric titrator (AT-510; manufactured by Kyoto Electronics Manufacturing Co., Ltd.) equipped with a stream potential titration unit (PCD-500). Methyl glycol chitosan was used as a titration reagent.

In order to measure the amount of anionic functional group on the surface of polymer particles in an ink, first, the polymer particles must be separated from a pigment. An ink is centrifuged at 23° C. and at 440,000 G for 2 hours. A supernatant containing polymer particles is collected. The amount of anionic functional group on the surface of the polymer particles can be measured by the method described above.

<Surfactant>

An ink according to an embodiment of the present invention contains a surfactant represented by the following general formula (1) and having HLB of 11 or less.

R 1 (CR 2 R 3 ) n CH 2 CH 2 (OCH 2 CH 2 ) m OH  (1)

In general formula (1), R 1 denotes a fluorine atom or a hydrogen atom, R 2 and R 3 independently denote a fluorine atom or a hydrogen atom, and at least one of R 2 and R 3 denotes a fluorine atom, n is 1 or more and 30 or less, and m is 1 or more and 60 or less.

As described above, the fluorinated surfactant has HLB of 11 or less, as determined by a Griffin method. The fluorinated surfactant preferably has HLB of 6 or more and 11 or less. HLB determined by a Griffin method is defined by “20× the sum total of formula weights of hydrophilic moieties/molecular weight”. In the general formula (1), “CH 2 CH 2 (OCH 2 CH 2 ) m OH” is a “hydrophilic moiety”.

Examples of the fluorinated surfactant represented by the general formula (1) and having HLB of 11 or less include, but are not limited to, FS-3100, FS-30, FSO, and FSN-100 (manufactured by Du Pont), Megaface F-444 (manufactured by DIC Corporation), and DSN403N (manufactured by Daikin Industries, Ltd.).

The amount of surfactant represented by the general formula (1) is preferably 0.1% by mass or more and 5.0% by mass or less of the ink.

An ink according to an embodiment of the present invention may further contain a surfactant that is different from the surfactant represented by the general formula (1). For example, the other surfactant may be a nonionic surfactant, such as acetylene glycol or an ethylene oxide adduct of acetylene glycol. In this case, the amount of surfactant other than the surfactant represented by the general formula (1) is preferably 0.1% by mass or less of the ink.

<Water and Water-Soluble Organic Solvent>

An ink according to an embodiment of the present invention contains water and a water-soluble organic solvent. Water can be deionized water (ion-exchanged water). The water content of the ink is preferably 50% by mass or more and 90% by mass or less.

The term “water-soluble organic solvent”, as used herein, refers to an organic solvent having water solubility of 500 g/1 or more at 20° C. The water-soluble organic solvent may be any known water-soluble organic solvent for use in inks. Examples of such a water-soluble organic solvent include, but are not limited to, alcohols, glycols, alkylene glycols, poly(ethylene glycol), nitrogen-containing compounds, and sulfur-containing compounds. These water-soluble organic solvents may be used alone or in combination. The water-soluble organic solvent content of an ink according to an embodiment of the present invention is preferably 50% by mass or less, more preferably 5% by mass or more and 45% by mass or less.

An ink according to an embodiment of the present invention contains at least one water-soluble organic solvent selected from the group A consisting of glycerin, ethylene glycol, diethylene glycol, poly(ethylene glycol) having a weight-average molecular weight of 10,000 or less, 1,3-propanediol, 1,4-butanediol, and diglycerol. In accordance with an embodiment of the present invention, in particular, the total amount of water-soluble organic solvent(s) selected from the group A can be greater than the total amount of water-soluble organic solvent(s) not belonging to the group A. The ratio of the total amount of water-soluble organic solvent(s) selected from the group A to the total amount of water-soluble organic solvent(s) not belonging to the group A in the ink is preferably more than 4, more preferably 4.1 or more, still more preferably 10 or more, on a mass basis.

In accordance with an embodiment of the present invention, the total amount of water-soluble organic solvent(s) selected from the group A is preferably 50% by mass or less, more preferably 5% by mass or more and 45% by mass or less, still more preferably 10% by mass or more and 40% by mass or less, of the ink.

<Additive Agents>

An ink according to an embodiment of the present invention may also contain an additive agent, such as a surfactant other than the surfactants described above, a pH adjuster, an anticorrosive, a preservative, a fungicide, an antioxidant, a reducing inhibitor, an evaporation promoter, and/or a chelator.

In particular, an ink according to an embodiment of the present invention can contain at least one additive agent selected from the following group B (hereinafter also referred to as a “group B additive agent”):

tetritol, pentitol, hexitol, heptitol, octitol, and poly(ethylene glycol) having a weight-average molecular weight of more than 10,000 and 100,000 or less.

The group B additive agent in an ink can improve ink ejection stability while high optical density and scratch resistance are maintained.

The total amount of tetritol, pentitol, hexitol, heptitol, and octitol of the group B is preferably 3% by mass or more and 20% by mass or less, more preferably 5% by mass or more and 10% by mass or less, of the ink. The amount of poly(ethylene glycol) having a weight-average molecular weight of more than 10,000 and 100,000 or less of the group B is preferably 0.0001% by mass or more and 10% by mass or less, more preferably 0.001% by mass or more and 5% by mass or less, of the ink. An ink according to an embodiment of the present invention preferably has a viscosity of 2 cP or more and 10 cP or less.

[Ink Cartridge]

An ink cartridge according to an embodiment of the present invention includes an ink storage portion for storing an ink according to an embodiment of the present invention. The ink storage portion may include an ink containing chamber for storing liquid ink and a chamber for housing a negative-pressure-generating member. The negative-pressure-generating member can store ink by the action of a negative pressure. Alternatively, an ink cartridge according to an embodiment of the present invention may include no ink containing chamber for storing liquid ink and include an ink storage portion that includes a negative-pressure-generating member for storing the whole accommodating amount of ink. Alternatively, an ink cartridge according to an embodiment of the present invention may include an ink storage portion and a recording head.

[Image-Recording Method]

An image-recording method according to an embodiment of the present invention includes an ink-applying process for applying an ink according to an embodiment of the present invention to a recording medium. An image-recording method according to an embodiment of the present invention may further include a conveyance process for conveying a recording medium and a heating process for heating the recording medium to which an ink has been applied.

FIG. 1 is a schematic view of an image-recording apparatus for use in an image-recording method according to an embodiment of the present invention. In the image recording apparatus illustrated in FIG. 1 , recording is performed on a roll of recording medium, and the recording medium is rolled up after recording. The image recording apparatus includes a recording medium supply unit 1 configured to hold a roll of recording medium and supply the recording medium, an ink-applying unit 2 configured to apply inks to the recording medium, a heating unit 3 configured to heat the recording medium, and a recording medium collecting unit 4 configured to roll the recording medium on which an image is recorded. The recording medium is subjected to the treatments in the units as described above while being conveyed using a conveying unit including a pair of rolls and a belt along a recording medium transport path represented by a solid line in the figure. The recording medium rolled up with the recording medium collecting unit 4 may be supplied to another apparatus to cut the recording medium into a desired size or bind the recording media.

In accordance with an embodiment of the present invention, in a conveyance process for conveying a recording medium, the recording medium is preferably conveyed at a speed of 50 m/min or more, more preferably 100 m/min or more.

In accordance with an embodiment of the present invention, a recording medium is conveyed under tension. In other words, an image-recording apparatus can include a tension-applying unit configured to produce tension. More specifically, a conveying mechanism between the recording medium supply unit 1 and the recording medium collecting unit 4 in FIG. 1 may include a tension-applying unit configured to apply tension to a recording medium and/or a tension control unit configured to control the tension of the recording medium. Tension applied to a recording medium can prevent fibers of the recording medium from being swollen with water contained in ink. When fibers of a recording medium are swollen, ink permeates the recording medium at a higher speed due to increased gaps between the swollen fibers of the recording medium, and the ink deeply permeates the recording medium. This may result in insufficient optical densities of images. As described above, tension applied to a recording medium can prevent fibers of the recording medium from being swollen with water contained in ink and thereby suppress the decrease in optical density of images resulting from the high permeation speed.

The tension applied to a recording medium is preferably 20 N/m or more. The swelling of fibers of a recording medium with water contained in ink can be more efficiently suppressed at a tension of 20 N/m or more. The tension applied to a recording medium is preferably 30 N/m or more, more preferably 40 N/m or more and 100 N/m or less.

An ink-applying process and a heating process will be described below.

(1) Ink-Applying Process

In accordance with an embodiment of the present invention, an ink is applied to a recording medium in an ink-applying process. An ink can be applied to a recording medium using an ink jet system. In other words, an image-recording method according to an embodiment of the present invention can be an ink jet recording method. The ink jet system may be a thermal ink jet system or a piezoelectric ink jet system. In the thermal ink jet system, an ink is ejected through an ejection port of a recording head by the action of thermal energy. In the piezoelectric ink jet system, an ink is ejected through an ejection port of a recording head using a piezoelectric element.

The recording head may be of a serial type or a full-line type. The recording head of the serial type is scanned in a direction across the recording medium conveyance direction. In the recording head of the full-line type, a plurality of nozzles are arranged to cover the expected maximum width of recording media. Ink jet recording heads of the full-line type allow images to be recorded at higher speeds. Ink jet recording heads of the full-line type can have nozzles arranged in a direction perpendicular to the recording medium conveyance direction. Ink jet recording heads of the full-line type for each color can be arranged in parallel along the conveyance direction.

(2) Heating Process

In accordance with an embodiment of the present invention, in a heating process, a recording medium to which the ink has been applied can be heated to a surface temperature of 70° C. or more. The phrase “the surface temperature of a recording medium to which an ink has been applied”, as used herein, refers to the surface temperature of a recording medium at a position that the recording medium reaches 0.5 seconds after an ink has been applied to the recording medium. More specifically, the surface temperature of an ink recording area X of a recording medium is measured at a position “V×0.5/60 (m)” separated in the recording medium conveyance direction from a position at which an ink is applied to the ink recording area X (directly under a recording head in the case of an ink jet recording head of the full-line type), wherein V (m/min) denotes the transfer speed of the recording medium. In the exemplary embodiments of the present invention, the surface temperature of a recording medium was measured with a noncontact infrared thermometer digital radiation temperature sensor FT-H20 (manufactured by Keyence Corp.) at a position generally vertically separated by 10 cm from a surface of the recording medium.

In accordance with an embodiment of the present invention, the surface temperature of a recording medium to which an ink has been applied is preferably 80° C. or more. In order to prevent the thermal deformation of a recording medium, the surface temperature of the recording medium is preferably 140° C. or less. A recording medium may be heated with a heater from the front side (the side to which an ink is applied) and/or the back side of the recording medium.

In accordance with an embodiment of the present invention, heating in the heating process may be continuously performed before, during, and after the application of an ink. In accordance with an embodiment of the present invention, before an ink is applied to a recording medium, the recording medium is not heated, or even when the recording medium is heated, the surface temperature of the recording medium is preferably less than 70° C., more preferably 60° C. or less, still more preferably 40° C. or less.

In the heating process, a recording medium may be heated while the recording medium is pressed, for example, with a pressure roller. Pressing a recording medium can improve the fixability of images. A recording medium may be pressed during part of the heating process rather than throughout the heating process. A recording medium may be pressed stepwise. The heating process may be followed by a pressing process.

<Recording Medium>

In an image-recording method according to an embodiment of the present invention, a recording medium to which an ink is applied may be any generally used recording medium. Examples of such a recording medium include, but are not limited to, permeable recording media, such as plain paper and glossy paper; less permeable recording media, such as print sheets; and non-permeable recording media, such as glass, plastics, and films. In particular, recording media for use in the present invention can be highly permeable recording media having a water absorption coefficient Ka of 0.3 mL/m 2 ·ms 1/2 or more.

In accordance with an embodiment of the present invention, the absorption coefficient Ka of a recording medium is calculated by using a Bristow method described in a JAPAN TAPPI paper pulp test method No. 51, “Kami oyobi itagami no ekitai kyushusei shiken hobo (liquid absorption test method for paper and paper board)”. The Bristow method is described in many commercially available books and is not described in detail herein. The Bristow method is defined by the wetting time Tw, the absorption coefficient Ka (mL/m 2 ·ms 1/2 ), and the roughness index Vr (mL/m 2 ). FIG. 2 illustrates an absorption curve. The absorption curve illustrated in FIG. 2 is based on a permeability model, in which a liquid on a recording medium begins to permeate the recording medium after a wetting time Tw. The slope of a straight line after the wetting time Tw represents the absorption coefficient Ka. The absorption coefficient Ka corresponds to the permeation speed of the liquid in the recording medium. As illustrated in FIG. 2 , the wetting time Tw is the time to reach an intersection point AB between an approximate straight line A, which is drawn using the least squares method and from which the absorption coefficient Ka is calculated, and a straight line B of V=Vr wherein V denotes the amount of transferred liquid and Vr denotes the roughness index. In accordance with an embodiment of the present invention, the liquid permeating the recording medium is water having a temperature of 25° C. Thus, the absorption coefficient Ka is a water absorption coefficient at 25° C.

A recording medium for use in an ink jet recording method according to an embodiment of the present invention may be a recording medium having a desired size or a rolled recording medium, which is cut into a desired size after the image formation. As described above, it is easy to apply tension to a rolled recording medium.

The present invention will be further described with the following exemplary embodiments and comparative examples. However, the present invention should not be limited to these exemplary embodiments. Unless otherwise specified, “part” in the exemplary embodiments is on a mass basis.

[Preparation of Pigment Dispersion]

<Preparation of Pigment Dispersion A>

A 500-mL recovery flask equipped with a mechanical agitator was placed in a chamber of an ultrasonic wave generator and was charged with 1 g of a polymer dispersant styrene-acrylic acid random copolymer (acid value: 80 mgKOH/g) and 120 mL of a solvent tetrahydrofuran. Ultrasonic waves were applied to the polymer dispersant while stirring. 5 g of carbon black FW18PS (manufactured by Cabot Corporation) and 120 mL of tetrahydrofuran in another container were mixed in a planetary mixer (manufactured by Kurabo Industries Ltd.) until the pigment surface became wet with the solvent. The wet carbon black was then mixed well with the polymer dispersant in the 500-mL recovery flask. Aqueous potassium hydroxide was then added dropwise such that the neutralization ratio of the polymer dispersant was 100%, thereby causing phase inversion. The mixture was then premixed for 60 minutes and was dispersed with Nanomizer NM2-L200AR (manufactured by Yoshida Kikai Co., Ltd.) for 2 hours. Tetrahydrofuran was distilled off from the dispersion liquid using a rotatory evaporator. After concentration adjustment, a pigment dispersion liquid A was obtained. The pigment content was 6.0% by mass, and the pigment content/polymer dispersant content ratio was 5.0.

<Preparation of Pigment Dispersions B to D>

A pigment dispersion B (pigment content/polymer dispersant content ratio: 3.3), a pigment dispersion C (pigment content/polymer dispersant content ratio: 2.5), and a pigment dispersion D (pigment content/polymer dispersant content ratio: 2.0) were prepared in the same manner as in the preparation of the pigment dispersion A except that the amount of polymer dispersant was 1.5, 2.0, and 2.5 g, respectively.

[Preparation of Ink]

Inks were prepared by mixing and dispersing the raw materials of each composition (% by mass) listed in the following tables 2 to 8 and passing the mixture through a glass filter AP20 (manufactured by MILLIPORE). In the tables, surfactants represented by the general formula (1) are referred to as “General formula (1)”, and surfactants not represented by the general formula (1) are referred to as “Other than general formula (1)”. In the tables, water-soluble organic solvents of the group A are referred to as “Group A”, and water-soluble organic solvents not belonging to the group A are referred to as “Other than group A”. Additive agents of the group B are referred to as “Group B”, and additive agents not belonging to the group B are referred to as “Other than group B”.

Group A:

Glycerin, ethylene glycol, diethylene glycol, poly(ethylene glycol) having a weight-average molecular weight of 10,000 or less, 1,3-propanediol, 1,4-butanediol, and diglycerol.

Group B:

Tetritol, pentitol, hexitol, heptitol, octitol, and poly(ethylene glycol) having a weight-average molecular weight of more than 10,000 and 100,000 or less.

The following are the names and physical properties of the polymer particles and the abbreviations of the surfactants in the tables.

<Polymer Particles>

<Surfactant>

(1) Fluorinated Surfactants Represented by General Formula (1)

(1-1) Fluorinated Surfactants Represented by the General Formula (1) and Having HLB of 11 or Less

F-444: Megaface F-444 (manufactured by DIC Corporation) [HLB: 8.5]

FSO: Zonyl FSO (manufactured by Du Pont) [HLB: 9.5]

FS-3100: Capstone FS-3100 (manufactured by Du Pont) [HLB: 9.8]

DSN403N: Unidyne DSN-403N (manufactured by Daikin Industries, Ltd.) [HLB: 10.0]

FS-30: Capstone FS-30 (manufactured by Du Pont) [HLB: 11.0]

(1-2) Fluorinated Surfactants Represented by the General Formula (1) and Having HLB of More than 11

S-242: Surflon S-242 (manufactured by AGC Seimi Chemical Co., Ltd.) [HLB: 12.0]

S-243: Surflon S-243 (manufactured by AGC Seimi Chemical Co., Ltd.) [HLB: 15.0]

(2) Surfactants Other than General Formula (1)

(2-1) Fluorinated Surfactants Other than the General Formula (1)

Ftergent 250: Ftergent 250 (manufactured by NEOS Co. Ltd.) [HLB: 10.4]

(2-2) Surfactants Other than Fluorinated Surfactants

AE100: Acetylene glycol surfactant Acetylenol E100 (manufactured by Kawaken Fine Chemicals Co., Ltd.) [HLB: 16.3]

[Evaluation of Images]

<Preparation of Image Samples>

A 3 cm×3 cm solid image (recording duty: 100%) was recorded on a recording medium OK Prince High Quality (basis weight: 64 g/m 2 ) (manufactured by Oji Paper Co., Ltd.) using an ink jet recording apparatus equipped with a piezoelectric ink jet head KJ4 (manufactured by Kyocera Corporation, nozzle density: 600 dpi) illustrated in FIG. 1 under the conditions listed in Table 6 (the type of ink used, the surface temperature of the recording medium to which an ink is applied, and tension applied to the recording medium with a tension-applying unit) (image sample 1). Three lines each having a width of 3 mm and one line having a width of 17 mm (recording duty for each line: 100%) were recorded in parallel on a recording medium DL9084 (basis weight: 91 g/m 2 ) (manufactured by Mitsubishi Paper Mills, Ltd.) (image sample 2). The recording conditions included a temperature of 25° C., a relative humidity of 55%, an ink ejection frequency of 39 kHz, a recording medium transfer speed of 100 m/s, and an ink ejection volume of approximately 13 pl per dot during recording. In the tables, “−” in the column of “Tension applied to recording medium” means that no particular tension other than tension resulting from conveyance was applied to the recording medium with the tension-applying unit. A recording duty of 100% for the ink jet recording apparatus refers to the conditions where one dot of ink (13 ng) was applied to a unit area of 1/600 inches× 1/600 inches at a resolution of 600 dpi×600 dpi. The surface temperature of the recording medium to which an ink was applied was measured with a noncontact infrared thermometer digital radiation temperature sensor FT-H20 (manufactured by Keyence Corporation) at a position generally vertically separated by 10 cm from a surface of the recording medium.

Of the evaluation criteria for the following evaluation items, AA to B represent acceptable levels, and C represents an unacceptable level.

<Optical Density of Images>

The optical density of the image sample 1 was measured with a reflection densitometer RD19I (manufactured by GretagMacbeth). The optical density of images was rated according to the following criteria. Tables 9 to 12 show the results.

AA: The optical density was 1.5 or more.

A: The optical density was 1.4 or more and less than 1.5.

B: The optical density was 1.3 or more and less than 1.4.

C: The optical density was less than 1.3.

<Scratch Resistance of Images>

Within 3 minutes of the image sample 2 being recorded, a sheet of OK Top Coat+ (basis weight: 105 g/m 2 ) (manufactured by Oji Paper Co., Ltd.) was placed on the image sample 2, and a 500-g weight was placed on the sheet. The contact area between the 500-g weight and the sheet was 12.6 cm 2 . A scratch resistance test was performed by moving the OK Top Coat+ sheet once at a speed of 10 cm/s relative to the image sample 2. The weight crossed the four lines recorded on the image sample 2 at right angles. Ink that adhered to the 12.6-cm 2 area of the OK Top Coat+ on which the weight was placed was read with a scanner (a multifunction device iR3245F manufactured by CANON KABUSHIKI KAISHA, 600 dpi, gray scale, photograph mode). The percentage of area having a brightness of less than 128 out of 256 tones (the percentage of an ink adhesion area) was determined. The scratch resistance of images was rated according to the following criteria. Tables 9 to 12 show the results.

AA: The percentage of the ink adhesion area was 1% or less.

A: The percentage of the ink adhesion area was more than 1% and 3% or less.

B: The percentage of the ink adhesion area was more than 3% and 5% or less.

C: The percentage of the ink adhesion area was more than 5%.

[Evaluation of Ejection Stability]

A yellow ink tank of an ink jet printer PX-205 (manufactured by Seiko Epson Corporation) was charged with an ink. An A4-size yellow image ({R, G, B}={255, 255, 0} on a RGB 256 gray scale) of standard print quality was continuously printed on 10 sheets to prepare image samples. The image samples were visually inspected and were rated according to the following criteria for ejection stability. Tables 9 to 12 show the results.

A: Streaks and unevenness of color were not observed on the 10 image samples.

B: Streaks and unevenness of color were observed on some of the image samples.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2014-088604 filed Apr. 22, 2014, which is hereby incorporated by reference herein in its entirety.