JP2022018661A - Ink for inkjet - Google Patents

Abstract

To provide ink for inkjet which is excellent in ejection stability and enables formation of an image excellent in image density and scratch resistance.SOLUTION: Ink for inkjet contains carbon black, an aqueous medium, and a pigment dispersion resin. A content ratio of the carbon black is 2.5 mass% or more and 5.5 mass% or less. An oil absorption of the carbon black is 40 mL/100 g or more and 70 mL/100 g or less. A number average molecular weight of the pigment dispersion resin is 25,000 or more and 55,000 or less. The pigment dispersion resin includes a first pigment dispersion resin existing in a state of being deposited to the carbon black, and a second pigment dispersion resin existing in a state of being dispersed in the aqueous medium. A content of the second pigment dispersion resin to 100 pts.mass of the pigment dispersion resin is 25 pts.mass or more and 55 pts.mass or less.SELECTED DRAWING: None

Description

The present invention relates to ink jet inks.

In recent years, inkjet recording devices have been increasingly used for mass printing in the business field as the image quality and recording speed have improved. Along with this, the inkjet ink used in the inkjet recording apparatus is required to be able to form an image having excellent ejection stability and excellent image density and scratch resistance.

On the other hand, when an image is formed on non-smooth recording paper using an inkjet ink containing an aqueous medium, the image density of the formed image tends to decrease. Specifically, the inkjet ink containing an aqueous medium has a long time required from landing on the recording medium to drying, and has a good affinity with cellulose. Therefore, the inkjet ink containing an aqueous medium easily penetrates into a recording paper (particularly, a non-smooth recording paper having no coating layer), and the pigment permeates the recording paper after landing on the recording paper to obtain an image density. Decreases.

Further, in recent years, with the progress of high-speed printing of inkjet recording devices, a penetrant may be added to the inkjet ink in order to increase the drying speed of the inkjet ink after landing on the recording medium. By adding a penetrant to the inkjet ink, the speed at which the aqueous medium penetrates the recording medium can be increased. As a result, the ink for inkjet is dried faster after landing on the recording medium. However, since the penetrant increases the speed at which the pigment permeates the recording medium, the above-mentioned decrease in image density becomes remarkable.

It has been proposed to use a self-dispersing pigment as a method for improving the image density of an image formed in an inkjet ink (for example, Patent Document 1). Inkjet inks containing self-dispersing pigments are characterized by being excellent in image density of formed images. On the other hand, the inkjet ink containing the self-dispersing pigment tends to have low scratch resistance of the formed image. Therefore, it has been proposed to add a binder resin to an inkjet ink containing a self-dispersing pigment (for example, Patent Document 2).

JP-A-2006-239915 Japanese Unexamined Patent Publication No. 2013-237823

However, when the binder resin is added to the inkjet ink, the image density of the formed image tends to decrease. As described above, the inkjet inks described in Patent Documents 1 and 2 do not satisfy all of the ejection stability, the image density of the formed image, and the scratch resistance.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an inkjet ink capable of forming an image having excellent ejection stability and excellent image density and scratch resistance.

The inkjet ink according to the present invention contains carbon black, an aqueous medium, and a pigment-dispersed resin. The content ratio of the carbon black is 2.5% by mass or more and 5.5% by mass or less. The oil absorption of the carbon black is 40 mL / 100 g or more and 70 mL / 100 g or less. The number average molecular weight of the pigment-dispersed resin is 25,000 or more and 55,000 or less. The pigment dispersion resin includes a first pigment dispersion resin existing in a state of being adhered to the carbon black and a second pigment dispersion resin existing in a state of being dispersed in the aqueous medium. The content of the second pigment-dispersed resin with respect to 100 parts by mass of the pigment-dispersed resin is 25 parts by mass or more and 55 parts by mass or less.

The inkjet ink according to the present invention can form an image having excellent ejection stability and excellent image density and scratch resistance.

Hereinafter, embodiments of the present invention will be described. In addition, in this specification, acrylic and methacrylic may be generically referred to as "(meth) acrylic". The oil absorption of carbon black is measured according to the method described in ASTM D2414 "Standard Test Method for Carbon Black Oil Absorption (OAN)". Each component described in the present specification may be used alone or in combination of two or more.

<Inkjet ink>
Hereinafter, the ink jet ink according to the embodiment of the present invention (hereinafter, may be referred to as ink) will be described. The ink of the present invention contains carbon black, an aqueous medium, and a pigment-dispersed resin. The content ratio of carbon black is 2.5% by mass or more and 5.5% by mass or less. The oil absorption of carbon black is 40 mL / 100 g or more and 70 mL / 100 g or less. The number average molecular weight of the pigment-dispersed resin is 25,000 or more and 55,000 or less. The pigment dispersion resin includes a first pigment dispersion resin existing in a state of being adhered to carbon black and a second pigment dispersion resin existing in a state of being dispersed in an aqueous medium (hereinafter, may be referred to as a free resin). .. The content of the free resin with respect to 100 parts by mass of the pigment-dispersed resin is 25 parts by mass or more and 55 parts by mass or less.

The ink of the present invention is suitable as, for example, an ink used in an inkjet recording apparatus provided with a line head.

By providing the above-mentioned configuration, the ink of the present invention can form an image having excellent ejection stability and excellent image density and scratch resistance. The reason is presumed to be as follows. The ink of the present invention contains carbon black (low structure carbon black) having a relatively low oil absorption as a pigment, and also contains a pigment dispersion resin having a relatively large molecular weight. Since low structure carbon black has a relatively small particle size, it can cover the surface of the recording medium without gaps. Further, since the pigment dispersion resin having a relatively large molecular weight does not easily penetrate into the recording medium, it is possible to effectively suppress the penetration of low structure carbon black into the recording medium. The ink of the present invention forms an image having excellent image density by using low structure carbon black in combination with a pigment dispersion resin having a relatively large molecular weight, even though the content ratio of low structure carbon black is relatively small. can. Further, the ink of the present invention has a low viscosity because the content ratio of low structure carbon black is relatively small. Therefore, the ink of the present invention is excellent in ejection stability. Further, a part of the pigment dispersion resin contained in the ink of the present invention exists as a free resin. The free resin stays on the surface of the recording medium in the image formed by the ink of the present invention and functions as a binder for protecting the pigment. As a result, the ink of the present invention can form an image having excellent scratch resistance.

[Carbon black]
In the ink of the present invention, carbon black functions as a black pigment. The oil absorption of carbon black is 40 mL / 100 g or more and 70 mL / 100 g or less, preferably 60 mL / 100 g or more and 70 mL / 100 g or less. By setting the oil absorption amount of carbon black to 40 mL / 100 g or more, the production cost of carbon black can be reduced. By setting the oil absorption amount of carbon black to 70 mL / 100 g or less, the image density of the formed image can be improved.

When the ink of the present invention contains two or more types of carbon black, the oil absorption amount of the above-mentioned mixture of two or more types of carbon black is defined as the oil absorption amount of the carbon black contained in the ink of the present invention.

The content ratio of carbon black in the ink of the present invention is 2.5% by mass or more and 5.5% by mass or less, preferably 3.5% by mass or more and 4.5% by mass or less. By setting the content ratio of carbon black to 2.5% by mass or more, the image density of the formed image can be improved. By setting the content ratio of carbon black to 5.5% by mass or less, the ejection stability of the ink of the present invention can be improved.

The ink of the present invention preferably contains only carbon black as a pigment. However, the ink of the present invention may further contain a pigment other than carbon black in addition to carbon black.

[Aqueous medium]
The aqueous medium is a medium containing water. The aqueous medium may function as a solvent or a dispersion medium. Examples of the aqueous medium include an aqueous medium containing only water and an aqueous medium containing water and a water-soluble organic solvent. As the aqueous medium, an aqueous medium containing water and a water-soluble organic solvent is preferable.

The water content in the ink of the present invention is preferably 40.0% by mass or more and 85.0% by mass or less, and more preferably 50.0% by mass or more and 70.0% by mass or less. By setting the water content to 40.0% by mass or more and 85.0% by mass or less, the ejection stability of the ink of the present invention can be improved.

Examples of the water-soluble organic solvent contained in the aqueous medium include glycol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, thiodiglycols, glycerin and dimethyl sulfoxide.

Examples of the glycol compound include ethylene glycol, 1,3-propanediol, propylene glycol, 1,5-pentanediol, 1,2-octanediol, 1,8-octanediol, and 3-methyl-1,5-pentane. Examples thereof include diol, diethylene glycol, triethylene glycol and tetraethylene glycol.

Examples of the glycol ether compound include diethylene glycol diethyl ether, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, triethylene glycol monomethyl ether, and triethylene glycol monoethyl. Examples include ether, triethylene glycol monobutyl ether and propylene glycol monomethyl ether.

Examples of the lactam compound include 2-pyrrolidone and N-methyl-2-pyrrolidone.

Examples of the nitrogen-containing compound include 1,3-dimethylimidazolidinone, formamide and dimethylformamide.

Examples of the acetate compound include diethylene glycol monoethyl ether acetate.

As the water-soluble organic solvent contained in the aqueous medium, 2-pyrrolidone, glycerin, 3-methyl-1,5-pentanediol, triethylene glycol monobutyl ether or 1,2-octanediol are preferable.

The content of the water-soluble organic solvent in the ink of the present invention is preferably 10.0% by mass or more and 55.0% by mass or less, and more preferably 25.0% by mass or more and 40.0% by mass or less. By setting the content ratio of the water-soluble organic solvent to 10.0% by mass or more and 55.0% by mass or less, the ejection stability of the ink of the present invention can be further improved.

[Pigment dispersion resin]
The pigment dispersion resin includes a first pigment dispersion resin existing in a state of being adhered to carbon black and a free resin existing in a state of being dispersed in an aqueous medium. The first pigment dispersion resin functions as a dispersant for dispersing carbon black in an aqueous medium. The free resin improves the image density of the image formed by the ink of the present invention.

The number average molecular weight of the pigment-dispersed resin is 25,000 or more and 55,000 or less, preferably 35,000 or more and 45,000 or less. By setting the number average molecular weight of the pigment dispersion resin to 25,000 or more, the image density of the formed image can be improved. By setting the number average molecular weight of the pigment dispersion resin to 55,000 or less, it is possible to suppress the viscosity of the ink of the present invention from becoming excessively high and improve the ejection stability of the ink of the present invention.

The content of the free resin with respect to 100 parts by mass of the pigment-dispersed resin is 25 parts by mass or more and 55 parts by mass or less, preferably 35 parts by mass or more and 45 parts by mass or less. By setting the content of the free resin to 25 parts by mass or more, the scratch resistance of the formed image can be improved. By setting the content of the free resin to 55 parts by mass or less, the ejection stability of the ink of the present invention can be improved.

The mass of the free resin can be measured by the method described in Examples or a method similar thereto. The total of the content of the first pigment-dispersed resin and the content of the free resin is usually equal to the content of the pigment-dispersed resin.

Examples of the pigment dispersion resin include (meth) acrylic resin, styrene- (meth) acrylic resin, polyvinyl resin, polyester resin, amino resin, epoxy resin, urethane resin, polyether resin, polyamide resin, phenol resin, and silicone resin. Examples thereof include a fluororesin, a styrene-maleic acid copolymer, a styrene-maleic acid half ester copolymer, a vinylnaphthalene-acrylic acid copolymer, and a vinylnaphthalene-maleic acid copolymer.

The (meth) acrylic resin is a polymer of (meth) acrylic acid or (meth) acrylic acid alkyl ester.

Examples of the (meth) acrylic acid alkyl ester include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, and n-hexyl (meth) acrylate. , N-octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate and 2-hydroxyethyl (meth) acrylate.

As the pigment dispersion resin, a (meth) acrylic resin is preferable, a copolymer of (meth) acrylic acid and a (meth) acrylic acid alkyl ester is more preferable, and n-butyl methacrylate, butyl acrylate and 2-methacrylic acid are more preferable. A copolymer of hydroxyethyl and acrylic acid (hereinafter, may be referred to as a copolymer (A)) is more preferable.

The content ratio of n-butyl methacrylate in all the monomers used as the raw material of the copolymer (A) is, for example, 60% by mass or more and 80% by mass or less. The content of butyl acrylate in all the monomers used as the raw material of the copolymer (A) is, for example, 2% by mass or more and 8% by mass or less. The content ratio of 2-hydroxyethyl methacrylate in all the monomers used as the raw material of the copolymer (A) is, for example, 2% by mass or more and 8% by mass or less. The content ratio of acrylic acid in all the monomers used as the raw material of the copolymer (A) is, for example, 10% by mass or more and 30% by mass or less.

The content of the pigment-dispersed resin with respect to 100 parts by mass of carbon black is preferably 35.0 parts by mass or more and 65.0 parts by mass or less, and more preferably 45.0 parts by mass or more and 55.0 parts by mass or less. By increasing the content of the pigment-dispersed resin described above, the content of the pigment-dispersed resin (free resin) that does not adhere to carbon black is increased. By setting the content of the pigment-dispersed resin to be 35.0 parts by mass or more and 65.0 parts by mass or less, the content of the free resin can be easily adjusted within the above range.

The content ratio of the pigment-dispersed resin in the ink of the present invention is preferably 0.5% by mass or more and 4.0% by mass or less, and more preferably 1.3% by mass or more and 2.8% by mass or less. By setting the content ratio of the pigment dispersion resin to 0.5% by mass or more, the scratch resistance of the formed image can be further improved. By setting the content ratio of the pigment dispersion resin to 4.0% by mass or less, the ejection stability of the ink of the present invention can be further improved.

[Surfactant]
The ink of the present invention preferably further contains a surfactant. The surfactant improves the compatibility and dispersion stability of each component contained in the ink of the present invention. In addition, the surfactant improves the permeability (wetting property) of the ink of the present invention to the recording medium. As the surfactant, a nonionic surfactant is preferable.

Examples of the nonionic surfactant include polyoxyethylene dodecyl ether, polyoxyethylene hexadecyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan monooleate ether, monodecanoyl sucrose, and polyethylene oxide addition of acetylene glycol. Things can be mentioned. As the nonionic surfactant, an ethylene oxide adduct of acetylene glycol is preferable.

When the ink of the present invention contains a surfactant, the content ratio of the surfactant in the ink of the present invention is preferably 0.01% by mass or more and 2.0% by mass or less, and 0.1% by mass or more and 0. More preferably, it is 5% by mass or less.

[Additive]
The ink of the present invention may further contain known additives (eg, dissolution stabilizers, anti-drying agents, antioxidants, viscosity regulators, pH regulators, and fungicides), if necessary. ..

[Ink manufacturing method]
The ink of the present invention can be produced, for example, by mixing an aqueous medium, a pigment dispersion liquid containing carbon black and a pigment dispersion resin, and a component (for example, a surfactant) added as needed. The pigment dispersion liquid can be prepared by adding carbon black to an aqueous solution containing a pigment dispersion resin and then performing a dispersion treatment. Examples of the apparatus used for the dispersion treatment include a bead mill. In the production of the ink of the present invention, foreign matter and coarse particles may be removed by a filter (for example, a filter having a pore size of 5 μm or less) after the dispersion treatment.

Hereinafter, examples of the present invention will be described. However, the present invention is not limited to the following examples.

[Synthesis of Pigment Dispersion Resin (R-3)]
A flask equipped with a dropping funnel, a nitrogen introduction tube, a reflux condenser, a thermometer and a stirrer was used as a reaction vessel. After adding 50 g of methyl ethyl ketone to the reaction vessel, the temperature of the content was heated to 75 ° C. while nitrogen bubbling the content (methyl ethyl ketone). Separately, a monomer mixture containing the monomers shown in Table 1 below, 50 g of methyl ethyl ketone, and 500 mg of a polymerization initiator (AIBN) was prepared. Using a dropping funnel, the monomer solution was added dropwise to the reaction vessel over 3 hours. After the dropping, the contents of the reaction solution were heated under reflux for 6 hours. Then, after allowing the contents of the reaction vessel to cool, the same amount of methyl ethyl ketone as the methyl ethyl ketone volatilized by the above-mentioned heat-reflux treatment was added to the reaction vessel. As a result, a resin solution (solid content concentration: 50% by mass) containing the pigment dispersion resin (R-3) was obtained.

[Synthesis of Pigment Dispersion Resins (R-1) to (R-2) and (R-4) to (R-5)]
Pigment-dispersed resins (R-1) to (R-2) and (R-1) and (R-2) by the same method as the synthesis of the pigment-dispersed resin (R-3) except that the time of the heat-refluxing treatment was changed as shown in Table 2 below. R-4) to (R-5) were synthesized.

[Preparation of pigment dispersion resin (R-6)]
A pigment-dispersed resin containing a urethane resin (“S46000” manufactured by Lubrizol) was used as the pigment-dispersed resin (R-6).

[Measurement of number average molecular weight]
The number average molecular weights of the pigment dispersion resins (R-1) to (R-6) were measured using gel filtration chromatography (“HLC-8220GPC” manufactured by Tosoh Corporation) under the following conditions. The measurement results are shown in Table 2 below.

(GPC condition)
Column: TSKgel GMHXL (manufactured by Tosoh Corporation)
Number of columns: 2 (series connection)
Column temperature: 40 ° C
Carrier solvent: THF (tetrahydrofuran)
Carrier flow rate: 1 mL / min Detector: RI (refractive index) detector Calibration curve: Calibration curve prepared by measuring using standard polystyrene

[Preparation for carbon black]
The following carbon blacks (C-1) to (C-4) were prepared. The oil absorption of each carbon black is shown in Table 3 below.
Carbon black (C-1): "MONARCH (registered trademark) 800" manufactured by Cabot Corporation
Carbon Black (C-2): "Carbon Black # 950" manufactured by Mitsubishi Chemical Corporation
Carbon Black (C-3): "Carbon Black # 980" manufactured by Mitsubishi Chemical Corporation
Carbon Black (C-4): "MCF88" manufactured by Mitsubishi Chemical Corporation

[Preparation of Pigment Dispersion Liquids (D-1) to (D-13)]
The composition of any of the pigment dispersion resins (R-1) to (R-6), any of carbon black (C-1) to (C-4), and ion-exchanged water are shown in Tables 4 and 5 below. It was mixed so as to be. The obtained mixed solution was premixed by stirring at 5000 rpm for 1 hour using a high-speed rotating disper (TK Robomix + TK homodisper 2.5 type, manufactured by Primix Corporation). Then, the dispersion treatment was carried out for 2 hours using a bead mill (“SC mill” manufactured by Nippon Coke Industries Co., Ltd.). In the dispersion treatment, zirconia beads (diameter 0.2 mm) were used as the medium. Further, in the dispersion treatment, the filling rate of the media in the vessel of the bead mill was set to 80% by volume. As a result, pigment dispersion liquids (D-1) to (D-13) were obtained.

(Measurement of free rate)
For each pigment dispersion liquid, the content of the free resin with respect to 100 parts by mass of the pigment dispersion resin was measured. The measurement sample (any of the pigment dispersions (D-1) to (D-13)) was centrifuged at 140000 rpm for 3 hours with a small ultracentrifuge (“CS150-FNX” manufactured by Hitachi Koki Holdings Co., Ltd.). After the centrifugation, the supernatant of the measurement sample was collected, and the collected supernatant was dried. The mass of the obtained dried product was measured. The mass of the measured dry matter was taken as the mass of the free resin contained in the measurement sample. Based on the mass of the free resin, the content of the free resin with respect to 100 parts by mass of the pigment-dispersed resin was calculated. The calculation results are shown in Tables 4 and 5 below.

In Tables 4 and 5 below, "pigment-dispersed resin [parts by mass]" indicates the content of the pigment-dispersed resin with respect to 100 parts by mass of carbon black. "Free resin [parts by mass]" indicates the content of the free resin with respect to 100 parts by mass of the pigment-dispersed resin.

[Preparation of ink (I-1)]
2. While stirring the contents of a beaker equipped with a stirrer (“Three One Motor BL-600” manufactured by Shinto Kagaku Co., Ltd.) at a rotation speed of 400 rpm, add 3.0 parts by mass of 2-pyrrolidone and glycerin to the above-mentioned beaker. 0 parts by mass, 15.0 parts by mass of 3-methyl-1,5-pentanediol, 8.0 parts by mass of triethylene glycol monobutyl ether, 0.8 parts by mass of 1,2-octanediol, and nonionic surface activity. Agent (“Surfinol (registered trademark) 440” manufactured by Nisshin Chemical Industry Co., Ltd.) 0.2 parts by mass and 26.7 parts by mass of pigment dispersion (D-3) (Carbon Black (C-1) 4.0) (Containing by mass and 2.0 parts by mass of the pigment dispersion resin (R-2)) and 40.3 parts by mass of ion-exchanged water were added in this order. In the obtained mixed solution, the mixed solution was filtered using a filter having a pore size of 5 μm in order to remove foreign matter and coarse particles. As a result, ink (I-1) was obtained.

[Preparation of inks (I-2) to (I-17)]
The inks (I-2) to (I-17) were prepared by the same method as the preparation of the ink (I-1) except that the following points were changed. In the preparation of the inks (I-2) to (I-17), the type and addition of the pigment dispersion liquid so that the compositions of carbon black, the pigment dispersion resin and the ion-exchanged water are as shown in Tables 6 and 7 below. The amount and the amount of ion-exchanged water added were changed. In Tables 6 and 7 below, the content ratio of the ion-exchanged water indicates the total content ratio of the ion-exchanged water contained in the pigment dispersion liquid and the ion-exchanged water added at the time of preparing the ink.

The inks (I-1) to (I-7) and (I-15) to (I-16) were used as the inks of Examples 1 to 9, respectively. The inks (I-8) to (I-14) and (I-17) were used as the inks of Comparative Examples 1 to 8, respectively.

<Evaluation>
Inks (I-1) to (I-7) and (I-15) to (I-16) of Examples 1 to 9 and inks (I-8) to Comparative Examples 1 to 8 by the following methods. Regarding (I-14) and (I-17), the image density of the formed image, the ejection stability (specifically, the ink adhesion to the nozzle and the landing accuracy), and the scratch resistance of the formed image. Was evaluated. The evaluation results are shown in Table 8 below.

In the evaluation, an inkjet recording device equipped with a line head (a testing machine manufactured by Kyocera Document Solutions Co., Ltd.) was used as an evaluation machine. The ink tank of the recording head of the evaluation machine was filled with an evaluation target (any of inks (I-1) to (I-17)).

[Image density]
In the evaluation of image density, plain paper (“C ² ” manufactured by Fuji Xerox Co., Ltd., A4 size PPC paper) was used as a recording medium. The image density was evaluated in a normal temperature and humidity environment (temperature 25 ° C., humidity 50% RH environment). A solid image of 150 mm × 200 mm was formed on five recording media using an evaluation machine. In the formation of the solid image, the ink ejection amount was set to 11 pL per pixel. Next, five recording media were stored all day and night in a normal temperature and humidity environment. Five recording media after storage were used as evaluation targets for image density. Next, using a reflection densitometer (“RD-19” manufactured by X-Rite), the image densities of the solid images formed on the five evaluation targets were measured. In the measurement of the image density, the image density was measured at 10 points randomly selected from the solid image for each evaluation target. The average value of the image densities measured at a total of 50 points of the five evaluation targets was taken as the evaluation value (OD) of the image densities. The image density was determined according to the following criteria.

(Evaluation criteria for image density)
A (good): OD is 1.15 or more B (bad): OD is less than 1.15

[Ink adhesion]
In the evaluation of ink adhesion, superfine paper (matte coated paper for inkjet, A4 size) was used as a recording medium. The evaluation of ink adhesion was performed in a normal temperature and humidity environment (temperature 25 ° C., humidity 50% RH environment).

Solid images were continuously printed on 5000 recording media using an evaluation machine. In continuous printing, the paper transport speed was set to 350 mm / sec. After continuous printing, a cleaning operation was performed on the line head of the evaluator. Specifically, a small amount of ink was ejected from the line head of the evaluator (purge operation). Next, the ink ejection surface was wiped off with a wiper (wipe operation). As a result, the ink adhering to the ink ejection surface of the line head was removed. Next, the ink ejection surface of the line head was observed with a microscope to confirm the presence or absence of residual ink. Ink adhesion was determined according to the following criteria.

(Evaluation criteria for ink adhesion)
A (especially good): No ink adheres to the ink ejection surface B (good): Slight ink adheres to the ink ejection surface C (defective): Ink clearly adheres to the ink ejection surface Is

[Impact accuracy]
In the evaluation of landing accuracy, "Super Fine Paper (matte coated paper for inkjet)" manufactured by Seiko Epson Corporation was used as the recording medium. Further, in the evaluation of the impact accuracy, an evaluation machine after the above-mentioned evaluation of ink adhesion (that is, an evaluation machine after continuous printing on 5000 sheets of recording media and a cleaning operation) was used. The landing accuracy was evaluated in a normal temperature and humidity environment (temperature 25 ° C., humidity 50% RH environment).

By ejecting ink from the line head of the evaluator, a striped image formed by a plurality of fine lines was formed on the recording medium. In the formation of the striped image, the line width of the thin lines was set to 1 pixel, and the spacing between adjacent thin lines (line-to-line pitch) was set to 3 pixels. The striped image formed on the recording medium was read with a microscope. Specifically, the distance A between the specific thin line a and the thin line b located 16 pixels away from the thin line a was measured at 204 points. It should be noted that there were three other thin lines between the thin line a and the thin line b. The variation (3σ) of the measured interval A was calculated using image processing software (manufactured by Kyocera Document Solutions Co., Ltd.). The calculated variation of the interval A (3σ) was used as the evaluation value of the landing accuracy. The landing accuracy was determined according to the following criteria.

(Evaluation criteria for landing accuracy)
A (good): 3σ is less than 15 B (bad): 3σ is 15 or more

The ink ejection stability is good when the ink adhesion is A (especially good) or B (good) and the landing accuracy is A (good), and the ink adhesion is C (bad). Or, when the landing accuracy is B (defective), it is determined to be defective.

[Scratch resistance]
In the evaluation of scratch resistance, plain paper (“C2” manufactured by Fuji Xerox Co., Ltd., A4 size PPC paper) was used as the recording medium. A 4 cm × 5 cm solid image was formed on one recording medium using an evaluation machine. In the formation of the solid image, the ink ejection amount was set to 11 pL per pixel. Next, a blank sheet (unprinted plain paper described above) was placed on the surface (the surface on the solid image side) of the recording medium on which the solid image was formed. Next, while applying a load of 1 kg to the blank paper using a weight, one side of the blank paper was rubbed 5 times on the solid image. Then, using a reflection densitometer (“RD-19” manufactured by X-Rite), the image density of the non-printing portion (the portion where the solid image was not formed) of the recording medium on which the solid image was formed was measured. Among the image densities measured in the non-printed area, the highest image densit (OD _MAX ) was used as the evaluation value of scratch resistance. The scratch resistance was determined according to the following criteria.

(Evaluation criteria for scratch resistance)
A (good): OD _MAX is 0.02 or more B (bad): OD _MAX is less than 0.02

As shown in Tables 6 to 8, the inks (I-1) to (I-7) and (I-15) to (I-16) of Examples 1 to 9 are composed of carbon black, an aqueous medium, and an aqueous medium. It contained a pigment-dispersed resin. The content ratio of carbon black was 2.5% by mass or more and 5.5% by mass or less. The oil absorption of carbon black was 40 mL / 100 g or more and 70 mL / 100 g or less. The number average molecular weight of the pigment-dispersed resin was 25,000 or more and 55,000 or less. The pigment-dispersed resin contained a first pigment-dispersed resin existing in a state of being adhered to carbon black and a free resin existing in a state of being dispersed in an aqueous medium. The content of the free resin with respect to 100 parts by mass of the pigment-dispersed resin was 25 parts by mass or more and 55 parts by mass or less. The inks (I-1) to (I-7) and (I-15) to (I-16) of Examples 1 to 9 form an image having excellent ejection stability and excellent image density and scratch resistance. did it.

On the other hand, since the inks (I-8) to (I-14) and (I-17) of Comparative Examples 1 to 8 did not have the above-mentioned configuration, the ejection stability was poor or the image was formed. The image density and scratch resistance were poor.

Specifically, the ink (I-8) of Comparative Example 1 had an oil absorption of carbon black of more than 70 mL / 100 g. In the ink (I-8), it is determined that the image density of the formed image is lowered because the carbon black is not the low structure carbon black. In addition, the ink (I-8) had poor ejection stability.

The ink of Comparative Example 2 (I-9) and the ink of Comparative Example 8 (I-17) had a number average molecular weight of the pigment-dispersed resin of less than 25,000. In the ink (I-9) and the ink (I-17), it is determined that the image density of the formed image is lowered because the pigment dispersion resin easily penetrates into the recording medium.

The ink (I-10) of Comparative Example 3 had a number average molecular weight of the pigment-dispersed resin of more than 55,000. It is determined that the ink (I-10) has an excessively large number average molecular weight of the pigment-dispersed resin, so that the viscosity is increased and the ejection stability is lowered.

The ink (I-11) of Comparative Example 4 had a carbon black content of less than 2.5% by mass. It is determined that the ink (I-11) lacks carbon black, so that the image density of the formed image is reduced.

The ink (I-12) of Comparative Example 5 had a carbon black content of more than 5.5% by mass. It is judged that the viscosity of the ink (I-12) increased due to the excess of carbon black, and the ejection stability decreased.

The ink (I-13) of Comparative Example 6 had a free resin content of 25 parts by mass or less with respect to 100 parts by mass of the pigment-dispersed resin. It is determined that the ink (I-13) lacks the free resin, so that the scratch resistance of the formed image is lowered.

The ink (I-14) of Comparative Example 7 had a free resin content of more than 55 parts by mass with respect to 100 parts by mass of the pigment-dispersed resin. It is judged that the ejection stability of the ink (I-14) was lowered because the free resin was excessive.

The ink of the present invention can be used to form an image on a recording medium.

Claims (3)

Contains carbon black, an aqueous medium, and a pigment-dispersed resin,
The content ratio of the carbon black is 2.5% by mass or more and 5.5% by mass or less.
The oil absorption of the carbon black is 40 mL / 100 g or more and 70 mL / 100 g or less.
The number average molecular weight of the pigment-dispersed resin is 25,000 or more and 55,000 or less.
The pigment dispersion resin includes a first pigment dispersion resin existing in a state of being adhered to the carbon black and a second pigment dispersion resin existing in a state of being dispersed in the aqueous medium.
An inkjet ink in which the content of the second pigment-dispersed resin is 25 parts by mass or more and 55 parts by mass or less with respect to 100 parts by mass of the pigment-dispersed resin. The inkjet ink according to claim 1, wherein the content of the pigment-dispersed resin with respect to 100 parts by mass of the carbon black is 35.0 parts by mass or more and 65.0 parts by mass or less. The inkjet ink according to claim 1 or 2, wherein the pigment dispersion resin contains a (meth) acrylic resin.