JP2002241656A - Ink and ink set - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ink capable of providing a good character quality of a black character, high image density and excellent light resistance, capable of enabling a reciprocating printing process to be carried out and a gray image to be formed when forming a color image, and capable of providing an image having no density unevenness of the black ink between reciprocal passages by using the black ink and an ink set in which the ionicity is different from each other. SOLUTION: This ink containing at least (a) a cationic pigment dispersion, (b) a water-soluble organic solvent, (c) water and (d) a salt of a weak acid and a weak base, having >=2 quotient of (a molar conductivity at 0.01 mol/1) divided by (a molar conductivity at 0.1 mol/l) is characterized in that the ink has >=40 mv ζ-potential and >=40 dyne/cm surface tension.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an ink, an ink set, an ink cartridge, a recording unit, and an ink jet recording apparatus.

[0002]

2. Description of the Related Art Conventionally, as a black ink for writing instruments (fountain pens, felt-tip pens, water-based ballpoint pens, etc.) and a black ink for ink-jet, it is a black colorant having high density of printed matter and excellent in fastness and the like. Inks using carbon black have been proposed.

Particularly, in recent years, inks have been developed so that good recording can be performed on plain paper such as copy paper, report paper, notebooks, stationery, bond paper, and continuous slip paper generally used in offices in recent years. Detailed research and development have been carried out from various aspects such as the composition and physical properties of. For example, JP-A-61-283875 and JP-A-64-607
No. 4 discloses an aqueous facial ink containing carbon black and a dispersant.

Conventionally, when a color image is printed using black ink together with ink of another color (for example, magenta, cyan, yellow, etc.), a black image portion, a color image portion and a black image portion are formed on a recording medium. In some cases, a phenomenon (hereinafter, referred to as "bleeding") in which the color blurs or the ink is mixed unevenly and the image quality deteriorates may be recognized. With respect to such bleeding, there is a means for improving the permeability of the ink into the recording medium by adding a so-called surfactant.
-65269. Japanese Patent Application Laid-Open No. Sho 55-66976 proposes a means using a volatile solvent as the main solvent for the ink. However, for these prior art inks, image density reduction,
In some cases, the image quality and the ejection stability were reduced.

[0005] In particular, when a black ink is formed to have a higher color density as a character than other colors and to form a sharp edge without an unnatural line thickening, an image having a more preferable appearance is obtained. However, when full-color printing is performed with such an ink, the above-described bleeding between colors becomes a problem. As a solution to such a problem, it is known to prevent intercolor bleeding by using an ink set having different ionicity between the black ink and the color ink.

[0006]

As a means for preventing the bleeding, the present inventor studied an ink set having different ionicity between the black ink and the color ink. Discovered that there are still problems with the following phenomena: For example, as shown in FIG. 1, when an attempt is made to form a gray image using such an ink set, when the gray level is high (bright), the gray level is formed using yellow, magenta, and cyan process blacks ( Region 1), then
As the brightness becomes lower (darker) gray, black ink is added to process black (area 2) and only the last black ink is applied (area 3) (FIG. 1).
In the graph, the abscissa indicates the brightness of the image, the ordinate indicates the ejection amount, the plot of Δ indicates the ejection amount of process black, and the plot of ○ indicates the ejection amount of black ink. By using this method, an image without graininess can be formed even in a region having a high brightness.

In order to improve the printing speed, there is a reciprocating printing process in which printing is performed both when the carriage moves forward and backward. When the reciprocating printing process, the formation of the gray image, and the ink set of the black ink and the color ink having different ionic properties are to be used together, the order of the black ink and the color ink ejection in the area 2 in FIG. The inventor of the present invention has found a problem that an image density difference (hereinafter, referred to as “black reciprocal density unevenness”) occurs due to the reverse rotation of the image.

The problem to be solved by the present invention is to improve the character quality of black characters, the image density, the light resistance, and the reciprocal printing process when forming a color image. And the use of an ink set having different ionicity between the black ink and the color ink to provide an image having no reciprocal density unevenness of the black ink in the area 2 in FIG. Another object is to provide an image without bleeding between black ink and color ink and between color ink and color ink.

[0009]

In order to solve the above-mentioned problems, an ink according to a first embodiment of the present invention comprises at least (a) a cationic pigment dispersion, (b) a water-soluble organic solvent, , (C) water and (d) a weak acid and a weak base having a molar conductivity at 0.01 mol / l and a molar conductivity at 0.1 mol / l of 2 or more. Salt, the ζ potential is 40 mv or more, and the surface tension is 40 dyne /
cm or more.

According to the first embodiment, by selecting a pigment as a coloring material, it is possible to obtain a high image density with excellent light fastness and a surface tension of 40 mN / m.
(Dyne / cm) or more can improve the character quality. Further, by adding a salt, the occurrence of the black reciprocating density unevenness can be prevented.

In a second embodiment, the ink contains the black ink, at least a water-soluble anionic dye, a water-soluble organic solvent, and water, and has a surface tension of 32 mN / m (dyne / dyne). cm) or less.

According to the second embodiment, the surface tension of the color ink is set to 32 mN / m (dyne / cm).
By suppressing the bleeding below, bleeding between the color ink and the color ink can be prevented, and bleeding between the black ink and the color ink can also be prevented by making the ionicity of the black ink and the color ink different. it can.

Although it is not clear why the above-mentioned reciprocation unevenness can be prevented, the following is predicted. First, in the area 2 of FIG. 1, when the color ink is printed first and then the black ink is printed, the black ink is also given permeability because the surface tension of the color ink is low, and the image density is reduced. As it goes down. Conversely, when the black ink is printed first and then the color ink is printed, the ionicity of the black ink and the color ink are different, so a reaction between the inks occurs here and the color material is solid-liquid on the paper. It is separated and hardly affected by the permeability in the color ink. Therefore, black reciprocating density unevenness occurs.

Therefore, when a salt is added to the black ink, solid-liquid separation after adhering to the paper surface of the black ink itself,
It is thought to occur quickly. It is considered that, because the solid-liquid separation speed of the black ink is high, even if the color ink is printed first, the color ink is hardly affected by the permeability, and a decrease in image density can be prevented. However, here, the salt to be added sometimes acts in a direction to reduce the stability of the ink. For example, a salt that is a strong electrolyte attracts the hydrated molecules of the cationic group of the pigment dispersion, and thus the dispersion stability of the pigment may decrease.

Therefore, (molar conductivity at 0.01 mol / l) ÷ (molar conductivity at 0.1 mol / l) is 2
By selecting a salt of a weak acid and a weak base as described above,
The hydration molecules of the surface functional groups of the dispersed pigment particles are not reduced. Regarding the concentration of the salt added, in the case of a strong electrolyte, if the concentration is slightly increased, the compression of the electric double layer of the dispersed pigment particles is promoted, and as a result, the electric potential is reduced, and the pigment particles are reduced. Adversely affect the dispersion stability of the polymer. (Mole conductivity at 0.01 mol / l) ÷
In the case of a salt of a weak acid and a weak base having a (molar conductivity at 0.1 mol / l) of 2 or more, the concentration dependence is not remarkable and the ζ potential hardly decreases. That is, the present inventor has discovered that the solid-liquid separation speed of the black ink itself after adhering to the paper surface can be increased without impairing the dispersion stability of the pigment particles. In the present invention, the conductivity is a value measured at 25 ° C. using DS-12 (manufactured by Horiba, Ltd.).

[0016]

Next, the present invention will be described in more detail with reference to preferred embodiments. As the pigment dispersion used in the present invention, a self-dispersion type pigment in which at least one kind of cationic group is bonded directly or via another atomic group to the surface of the pigment, or a pigment using a cationic dispersant , Dispersed in water, or a mixture thereof is applicable. The amount of the pigment in the ink is in the range of 0.1 to 15% by weight, preferably 1 to 10% by weight, based on the total amount of the ink.

Next, the self-dispersion type carbon black preferably used in the present invention will be described in detail. As the self-dispersible carbon black, those having cationic properties are preferably used. Examples of the cationically charged carbon black include those in which at least one selected from the following quaternary ammonium groups is bonded to the surface of the carbon black.

[0018] In the above formula, R represents an alkyl group having 1 to 12 carbon atoms, a substituted or unsubstituted phenyl group, or a substituted or terminally substituted naphthyl group. Incidentally, the above cationic groups include, for example, NO ₃ ⁻ and CH ₃ COO ⁻ as counter ions.

As a method for producing the above-mentioned cationically charged self-dispersible carbon black, for example, a method of bonding an N-ethylpyridyl group having the following structure will be described. A method of treating carbon black with 3-amino-N-ethylpyridinium bromide. Since carbon black charged cationically by the introduction of a cationic group on the surface of carbon black has excellent water dispersibility due to repulsion of ions, even when it is contained in an aqueous ink, a dispersant or the like is used. Maintains a stable dispersion state without the addition of.

Incidentally, various cationic groups as described above may be directly bonded to the surface of carbon black. Alternatively, another atomic group may be interposed between the carbon black surface and the cationic group, and the cationic group may be indirectly bonded to the carbon black surface. Here, specific examples of the other atomic groups include, for example, those having 1 to 12 carbon atoms.
A substituted or unsubstituted phenylene group and a substituted or unsubstituted naphthylene group. Here, examples of the substituent of the phenylene group and the naphthylene group include a linear or branched alkyl group having 1 to 6 carbon atoms.

Incidentally, in the present invention, two or more of the above-mentioned self-dispersible carbon blacks may be used as a color material of an ink appropriately selected. Further, the amount of the self-dispersible carbon black added to the ink is preferably in the range of 0.1 to 15% by weight, particularly 1 to 10% by weight based on the total weight of the ink. Within this range, the self-dispersion type carbon black can maintain a sufficiently dispersed state in the ink.

In the present invention, the pigment dispersion may be one in which a pigment is dispersed in water using a cationic dispersant. Examples of the pigment include pigments of various hues in addition to carbon black.

The black pigment which can be used in the present invention is, for example, carbon black produced by a furnace method or a channel method, having a primary particle diameter of 15 to
40mμm, specific surface area by BET method is 50 ~ 300m
² / g, DBP oil absorption 40-150ml / 100g,
Those having characteristics such as a volatile content of 0.5 to 10% by weight and a pH value of 2 to 9 are preferably used. As a commercially available carbon black having such characteristics, for example, 2300, no. 900, MCF88, N
o. 33, no. 40, no. 45, no. 52, MA
7, MA8), # 2200B (above, manufactured by Mitsubishi Kasei), R
aven1255 (all made in Colombia), Regal
400R, Regal330R, Regal660R,
MogulL (above, made by Cabot), Color B
rack FW1, ColorBlack FW18,
Color Black S170, Color BL
ack S150, Printex 35, Print
ex U (all manufactured by Degussa) and the like, and any of them can be preferably used. In addition to the above, pigments newly produced for the present invention can of course also be used.

The yellow pigment which can be used in the present invention includes C.I. I. Pigment Yellow 1,
C. I. Pigment Yellow 2, C.I. I.
Pigment Yellow 3, C.I. I. Pigm
ent Yellow 16, C.I. I. Pigment
Yellow 83 and the like, but are not limited thereto.

The magenta pigment which can be used in the present invention includes C.I. I. Pigment Red 5, C.I.
I. Pigment Red 7, C.I. I. Pigme
ntRed 12, C.I. I. Pigment Red
48 (Ca), C.I. I. Pigment Red 48
(Mn), C.I. I. Pigment Red 57 (C
a), C.I. I. Pigment Red 112, C.I.
I. Pigment Red 122 and the like,
It is not limited to these.

The cyan pigment which can be used in the present invention includes C.I. I. Pigment Blue 1, C.I.
I. Pigment Blue 2, C.I. I. Pigm
ent Blue 3, C.I. I. Pigment Bl
ue 22, C.I. I. batBlue 4, C.I. I. v
at Blue 6 and the like, but are not limited thereto.

The pigment dispersant has cationic properties. For example, a dispersant having a cationic hydrophilic group can be suitably used. As the cationic dispersant that can be used in the present invention, it is preferable to use a copolymer of a cationic monomer and a hydrophobic monomer. Examples of the cationic monomer include the following monomers and their quaternized compounds. N, N-dimethylaminoethyl methacrylate, N, N-dimethylaminoethyl acrylate, N, N-dimethylacrylamide, N, N-dimethylmethacrylamide, N, N-dimethylaminopropylacrylamide and N, N-dimethylaminopropyl methacryl Amides.

The above compound can be quaternized by a conventional method using methyl chloride, dimethyl sulfate, benzyl chloride, epichlorohydrin and the like. Next, examples of the hydrophobic monomer include styrenes such as styrene, α-methylstyrene, and vinyl toluene, acrylonitrile, and acrylic esters of (meth) acrylic acid. As described above, when copolymerizing a cationic monomer and a hydrophobic monomer, the cationic monomer in the copolymer,
The weight ratio with the hydrophobic monomer is from 10:90 to 60:40.
It is preferable to set it in the range.

A method for dispersing a pigment in water using the above dispersant, for example, as a method for producing a black ink, includes preparing a pigment in a liquid mixture comprising a water-soluble resin and an aqueous medium containing at least water. After adding and stirring,
The pigment is sufficiently dispersed by using a dispersing means described below, and if necessary, centrifugation is performed to obtain a desired pigment dispersion. Next, appropriately selected additive components as described above are added to the pigment dispersion and stirred to obtain a black ink. If necessary to dissolve the resin, it is necessary to add an acid as described above.

Further, in the above-mentioned production method, it is effective to carry out premixing for 30 minutes or more before stirring and dispersing the aqueous medium containing the pigment. Such a premixing operation is preferable because the wettability of the pigment surface can be improved and the adsorption of the dispersant on the pigment surface can be promoted.

In the present invention, the dispersing machine used for the dispersion treatment of the pigment may be any commonly used dispersing machine, such as a ball mill, a roll mill and a sand mill. Among them, a high-speed sand mill is particularly preferably used. Examples of such a material include a super mill, a sand grinder, a bead mill, an agitator mill, a Glen mill, a Dyno mill, a pearl mill, and a Kobol mill (all trade names).

The salt of a weak acid and a weak base used in the present invention can be used in the ink (molar conductivity at 0.01 mol / l) ÷
(Molar conductivity at 0.1 mol / l) is added to the ink at a ratio of 2 or more. Specifically, a salt of a weak acid and a weak base is contained in the ink at 0.05 to 5.0% by weight.
Is preferable. When the content is within this range, the above-mentioned condition of the molar conductivity is satisfied, and there is no serious problem such as long-term storage stability of the ink and nozzle clogging.

More specific examples of the weak acid include:
Aliphatic carboxylic acids such as formic acid, acetic acid, propionic acid, etc., glyconic acid, glyceric acid, gluconic acid, etc., oxy acids, aromatic carboxylic acids such as benzoic acid or derivatives thereof, and phosphoric acid etc. or their derivatives The derivative is fisted. More specific examples of the weak base include ammonia, primary amine, secondary amine, and tertiary amine.

As the liquid medium of the ink used in the present invention,
Water or an aqueous medium in which a water-soluble organic solvent as described below is mixed with water is mentioned. As a water-soluble organic solvent suitably used, for example, alkyl alcohols having 1 to 4 carbon atoms (for example, methyl alcohol, Ethyl alcohol, n-
Propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, etc.), ketones or keto alcohols (eg, amides such as dimethylformamide, dimethylacetamide, acetone, diacetone alcohol, etc.), ethers (Eg, tetrahydrofuran, dioxane, etc.), polyalkylene glycols (eg, polyethylene glycol, polypropylene glycol, etc.), and alkylene glycols in which the alkylene group contains 2 to 6 carbon atoms (eg, ethylene glycol, propylene glycol, butylene) Glycol, triethylene glycol, 1,2,6-hexanetriol, thiodiglycol, hexylene glycol, diethylene glycol, etc.), and alkyls such as polyhydric alcohols. Ethers (eg, ethylene glycol methyl ether, ethylene glycol ethyl ether, triethylene monomethyl ether, triethylene glycol monoethyl ether, etc.),
Furthermore, N-methyl-2-pyrrolidone, 2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone and the like can be mentioned. The total amount of these water-soluble organic solvents in the ink is 2 to 6 with respect to the total amount of the ink.
0% by weight, a more preferred range is 5 to 25% by weight.

A particularly preferred water-soluble organic solvent for constituting the ink of the present invention is glycerin, and its addition amount is preferably 2 to 30% by weight, more preferably 5 to 15% by weight in the ink. It is suitable. A more preferred water-soluble organic solvent is a mixed solvent containing glycerin and a polyhydric alcohol (for example, diethylene glycol and ethylene glycol). The mixing ratio of glycerin and other polyhydric alcohol is glycerin: other polyhydric alcohol. 1 with alcohol
It is preferable to be in the range of 0: 5 to 10:50 (weight ratio). Other polyhydric alcohols used with glycerin include, for example, diethylene glycol, ethylene glycol, polyethylene glycol and propylene glycol. These glycerin or a mixture of glycerin and a polyhydric alcohol can be used by further mixing with another water-soluble organic solvent.

The pigment ink as described above has a 分散 potential of 40 m in consideration of the long-term dispersion stability of the pigment.
It is preferably at least v. The ζ potential of the ink is
For example, it can be measured by using an electrophoretic light scattering method, and specifically, it can be measured using ELS-8000 (standard cell) manufactured by Otsuka Electronics Co., Ltd.

The pigment ink as described above has a surface tension of 40 mN / m (dyne / cm) at 25 ° C.
In the case described above, a printed matter with sharp edges can be formed on plain paper, which is preferable. The surface tension of the ink can be measured using, for example, CBVP-A3 manufactured by Kyowa Interface Chemical Co., Ltd.

As described above, at least (a) a cationic pigment dispersion, (b) a water-soluble organic solvent, (c) water, and (d) (molar conductivity at 0.01 mol / l) Rate) ÷
(Molar conductivity at 0.1 mol / l) containing a salt of a weak acid and a weak base of 2 or more, a ζ potential of 40 mv or more, and a surface tension of 40 mN / m (dyne / cm).
By using the above-mentioned ink, by selecting a pigment as a coloring material, it is possible to obtain a high image density with excellent light fastness and a surface tension of 40 mN / m (dy).
ne / cm) or more can improve character quality. In addition, the addition of a salt can prevent the occurrence of the black reciprocating density unevenness.
Further, the ink of the present invention is suitably used as various recording inks, but can be particularly preferably used for ink jet. Further, an ink cartridge having an ink containing section containing the ink, an ink containing section containing the ink, and a recording unit having a recording head for ejecting the ink by an inkjet method,
In addition, the ink jet recording apparatus is suitably used in an ink jet recording apparatus including an ink container for storing the ink and a recording head for discharging the ink by an ink jet method.

Further, the present invention provides a black ink among the pigment inks, at least a water-soluble anionic dye,
It contains a water-soluble organic solvent and water, and has a surface tension of 32.
an ink set comprising mN / m (dyne / cm) or less color ink.

The water-soluble anionic dye used in the color ink used in the present invention includes a color index (COLOUR).
INDEX) is not particularly limited as long as it is a water-soluble dye, a direct dye, a reactive dye or the like. In addition, the dye is not particularly limited even if it is not recorded in the color index. Among the water-soluble dyes mentioned here, the solubility pH
Dependencies are naturally included. These dyes are
It is preferably used in the ink of the present invention in an amount of 1 to 10% by weight, more preferably 1 to 5% by weight.

As the water-soluble dye of the color ink used in the present invention, a dye having at least one --COOM group (M represents an alkali metal or ammonium) in a molecule can be suitably used. Specifically, for example, dyes represented by the following general formulas (1) to (4) can be effectively used in the present invention.

[0042]

[0043]

[0044]

[0045] The amount of the dye is not particularly limited, but is preferably 0.1 to 15% by weight, more preferably 0.1 to 10% by weight, based on the total weight of the ink. The liquid medium of the color ink is the same as the liquid medium of the pigment ink.

In the ink of the present invention, in addition to the above-mentioned materials, a surfactant, a preservative, an antioxidant, and other various auxiliary materials for controlling physical properties can be added. As surfactants, nonionic surfactants, amphoteric surfactants,
Cationic surfactants and the like can be used. The purpose of adding these surfactants is various, such as increasing the permeability of the ink, the wettability with the ejection device member, the flow characteristics, and the dispersion stability. The ink of the present invention may contain various additives such as a pH adjuster for imparting functionality and a preservative in addition to the surfactant.

The components for adjusting the electrical conductivity of the color ink using an anionic dye in the present invention include alkali metals, primary to tertiary amines, quaternary ammonium hydroxides, and inorganic acids / organic acid salts. Is mentioned.

The surface tension of the above color inks is, for example, 32 dyne / cm or less in order to prevent the color inks from mixing with each other even when the color inks are applied adjacently on a recording medium. Preferably, there is. The surface tension of the color ink is the same as the measurement of the surface tension of the pigment ink.

As described above, the bleeding between the color inks can be prevented by using the ink set including the black ink and the color ink, and the ionicity of the black ink and the color ink can be prevented. , The bleeding between the black ink and the color ink can be prevented. Further, the ink set of the present invention can be suitably used for ink jet.

[0050]

Next, the present invention will be described more specifically with reference to examples and comparative examples. (Example 1) Create ~ area ~ Black pigment dispersion 1 with 230m ² / g, DBP oil absorption of 70 ml /
100 g of carbon black 10 g and 3-amino-N-
After thoroughly mixing 3.06 g of ethylpyridinium bromide in 72 g of water, 1.62 g of nitric acid was added dropwise to the mixture.
Stirred to ° C. After a few more minutes, add 1.07 to 5 g of water.
g of sodium nitrite was added, and the mixture was further stirred for 1 hour. The obtained slurry is filtered through filter paper (trade name: Toyo Filter Paper No. 2; manufactured by Advantis Co., Ltd.), the pigment particles are sufficiently washed with water, dried in an oven at 110 ° C., and water is added to the pigment to obtain a pigment. A 10% by weight aqueous pigment solution was prepared. A group represented by the following chemical formula was introduced into the surface of carbon black by the above method.

Thereafter, a strongly basic ion exchange resin (product name: Diaion SA-10A: manufactured by Nippon Rensui Co., Ltd.)
100 g of the pigment aqueous solution was added to 10 g, and the mixture was stirred for 2 hours to ion-exchange the pyridinium group to an OH type. Thereafter, the ion exchange resin and the aqueous pigment solution were separated with a mesh filter having a pore diameter of 100 μm, and the aqueous pigment solution was adjusted to pH 5 with acetic acid.

Preparation of Black Ink 1 Black pigment dispersion 1 50 parts by weight Glycerin 10 parts by weight Trimethylolpropane 7 parts by weight Ammonium acetate 1 part by weight Acetyleneol EH 0.1 part by weight Ion-exchanged water remainder The materials described above were stirred and mixed, and filtered through a membrane filter having a pore size of 2 μm to obtain a cationic dispersion ink 1. When the ζ potential of this ink was measured, it was 45 mv.
Met.

Example 2 Preparation of Black Pigment Dispersion 2 A surface area of 230 m ² / g and a DBP oil absorption of 70 ml / g
100 g of carbon black 10 g and 3-amino-N-
After 2.06 g of ethylpyridinium bromide was thoroughly mixed with 72 g of water, 1.62 g of nitric acid was added dropwise thereto, and
Stirred to ° C. After a few more minutes, add 1.07 to 5 g of water.
g of sodium nitrite was added, and the mixture was further stirred for 1 hour. The obtained slurry is filtered through filter paper (trade name: Toyo Filter Paper No. 2; manufactured by Advantis Co., Ltd.), the pigment particles are sufficiently washed with water, dried in an oven at 110 ° C., and water is added to the pigment to obtain a pigment. A 10% by weight aqueous pigment solution was prepared. A group represented by the following chemical formula was introduced into the surface of carbon black by the above method.

Thereafter, a strongly basic ion exchange resin (product name: Diaion SA-10A; manufactured by Nippon Rensui Co., Ltd.)
100 g of the pigment aqueous solution was added to 10 g, and the mixture was stirred for 2 hours to ion-exchange the pyridinium group to an OH type. Thereafter, the ion exchange resin and the aqueous pigment solution were separated with a mesh filter having a pore diameter of 100 μm, and the aqueous pigment solution was adjusted to pH 5 with acetic acid.

Preparation of Black Ink 2 Black pigment dispersion 2 50 parts by weight Glycerin 10 parts by weight Trimethylolpropane 7 parts by weight Ammonium benzoate 1 part by weight Acetylenol EH 0.1 part by weight Ion exchange water Remainder The materials shown above were stirred and mixed, and filtered through a membrane filter having a pore size of 2 μm to obtain a cationic dispersion ink 2. The 尚 potential of this ink was 49 mv when measured.

Comparative Example 1 Preparation of Black Pigment Dispersion 3 A surface area of 230 m ² / g and a DBP oil absorption of 70 ml / g
100 g of carbon black 10 g and 3-amino-N-
After thoroughly mixing 3.06 g of ethylpyridinium bromide in 72 g of water, 1.62 g of nitric acid was added dropwise to the mixture.
Stirred to ° C. After a few more minutes, add 1.07 to 5 g of water.
g of sodium nitrite was added, and the mixture was further stirred for 1 hour. The obtained slurry is filtered through filter paper (trade name: Toyo Filter Paper No. 2; manufactured by Advantis Co., Ltd.), the pigment particles are sufficiently washed with water, dried in an oven at 110 ° C., and water is added to the pigment to obtain a pigment. A 10% by weight aqueous pigment solution was prepared. A group represented by the following chemical formula was introduced into the surface of carbon black by the above method.

Thereafter, a strongly basic ion exchange resin (product name: Diaion SA-10A: manufactured by Nippon Rensui Co., Ltd.)
100 g of the pigment aqueous solution was added to 10 g, and the mixture was stirred for 2 hours to ion-exchange the pyridinium group to an OH type. Then, with a mesh filter with a pore diameter of 100 microns,
The ion exchange resin and the aqueous pigment solution were separated, and the aqueous pigment solution was adjusted to pH 5 with acetic acid.

Preparation of Black Ink 3 50 parts by weight of black pigment dispersion 3 10 parts by weight of glycerin 7 parts by weight of trimethylolpropane 0.1 part by weight of acetylenol EH Remaining ion-exchanged water Stir the above materials The resulting mixture was mixed and filtered through a membrane filter having a pore size of 2 μm to obtain a cationic dispersion ink 3. The ζ potential of this ink was measured and found to be 50 mv.

(Comparative Example 2)-Preparation of Black Pigment Dispersion 4-The surface area was 230 m ² / g and the DBP oil absorption was 70 ml / g.
100 g of carbon black 10 g and 3-amino-N-
After 2.06 g of ethylpyridinium bromide was thoroughly mixed with 72 g of water, 1.62 g of nitric acid was added dropwise thereto, and
Stirred to ° C. After a few more minutes, add 1.07 to 5 g of water.
g of sodium nitrite was added, and the mixture was further stirred for 1 hour. The obtained slurry is filtered through filter paper (trade name: Toyo Filter Paper No. 2; manufactured by Advantis Co., Ltd.), the pigment particles are sufficiently washed with water, dried in an oven at 110 ° C., and water is added to the pigment to obtain a pigment. A 10% by weight aqueous pigment solution was prepared. A group represented by the following chemical formula was introduced into the surface of carbon black by the above method.

Thereafter, a strongly basic ion exchange resin (product name: Diaion SA-10A; manufactured by Nippon Rensui Co., Ltd.)
100 g of the pigment aqueous solution was added to 10 g, and the mixture was stirred for 2 hours to ion-exchange the pyridinium group to an OH type. Thereafter, the ion exchange resin and the aqueous pigment solution were separated with a mesh filter having a pore diameter of 100 μm, and the aqueous pigment solution was adjusted to pH 5 with acetic acid.

Preparation of Black Ink 4-50 parts by weight of black pigment dispersion 4-10 parts by weight of glycerin-7 parts by weight of trimethylolpropane-1 part by weight of sodium chloride-0.1 part by weight of acetylenol EH-The remaining part of ion-exchanged water The above-mentioned materials were stirred and mixed, and filtered through a membrane filter having a pore diameter of 2 μm to obtain a cationic dispersion ink 4. The ζ potential of this ink was 35 mv when measured.

(Preparation of Color Ink) ・ C. I. 3 parts by weight of Direct Yellow 86-10 parts by weight of glycerin-10 parts by weight of urea-1 part by weight of lauryl sulfate-The rest of ion-exchanged water After sufficiently stirring the above components, the mixture was filtered through a membrane filter having a pore diameter of 0.2 μm to give color ink A. I got

C. I. Acid Red 289 3 parts by weight-Glycerin 10 parts by weight-Urea 10 parts by weight-Lauryl sulfuric acid 1 part by weight-Ion-exchanged water balance I got

C. I. Direct Blue 199 3 parts by weight-Glycerin 10 parts by weight-Urea 10 parts by weight-Lauryl sulfuric acid 1 part by weight-Ion-exchanged water balance I got

The following evaluations were performed for the above Examples and Comparative Examples. Table 1 shows the evaluation results. (Black Reciprocating Density Unevenness) An ink jet printer (BJC-600; manufactured by Canon) was filled with the black inks of the above Examples and Comparative Examples and the above color inks.
Next, an image in which colors Y, M, and C were printed in the same area with 10% duty and black with 60% duty in the same area was printed back and forth, and evaluated based on the following criteria. ：: Difference in image density between back and forth is less than 0.05. X: The image density difference between reciprocation is 0.05 or more.

(Storage Stability) Each of the above inks was stored in a Teflon (registered trademark) jar at 60 ° C. for one month, and evaluated according to the following criteria. ：: There is no significant difference in physical property values compared to the initials. Δ: The average particle diameter is 1.2 times or more as compared with the initial. X: The average particle diameter is twice or more as compared with the initial.

(Bleeding) An ink jet printer (BJC-600; manufactured by Canon Inc.) was filled with the black inks of the above Examples and Comparative Examples and the above color inks. The printed image is a 10 cm square, divided by 5 × 5 squares (size of one square: 2 cm × 2 cm), and solid printing is performed alternately with black ink and each color ink. The boundary of the part could be evaluated. Bleeding at the boundary between the black ink and each color ink was evaluated according to the following criteria. ：: In all colors, the boundary between the two colors is clear, and no bleeding or color mixing is observed at the boundary. Δ: Some of the colors have a sharp boundary between the two colors, and some are unrecognizable. X: In all colors, the boundary between two colors cannot be recognized.

[0068]

[0069]

According to the present invention, by selecting a pigment as a coloring material, it is possible to obtain an excellent light fastness and a high image density. Further, by setting the surface tension to 40 dyne / cm or more, a character can be obtained. The present invention provides an inkjet ink that can improve the quality and can prevent the occurrence of the black reciprocating density unevenness by adding a salt. Further, according to the present invention, by suppressing the surface tension of the color ink to 32 dyne / cm or less, it is possible to prevent bleeding between the color inks, and if the ionicity of the black ink and the color ink is different. By doing so, an ink jet ink set that can prevent bleeding between black ink and color ink is provided.

[Brief description of the drawings]

FIG. 1 is a view for explaining a method of forming a gray image using an ink set.

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Claims (10)

[Claims] At least (a) a cationic pigment dispersion, (b) a water-soluble organic solvent, (c) water, and (d)
An ink containing a salt of a weak acid and a weak base, wherein (molar conductivity at 0.01 mol / l) ÷ (molar conductivity at 0.1 mol / l) is 2 or more, The ink has a ζ potential of 40 mv or more and a surface tension of 40 dyne / cm.
An ink characterized by the above. 2. The ink according to claim 1, wherein the salt of the weak acid and the weak base is a salt composed of an organic acid or an inorganic acid and ammonia or a derivative thereof. 3. The ink according to claim 1, wherein the salt of the weak acid and the weak base accounts for 0.05 to 5.0% by weight of the ink. 4. The ink according to claim 1, wherein the ink is a black ink. 5. The ink according to claim 1, wherein the ink is for inkjet. 6. An ink set comprising the black ink according to claim 4 and a color ink, wherein the color ink comprises at least a water-soluble anionic dye, a water-soluble organic solvent, and water. Includes and has a surface tension of 32m
N / m (dyne / cm) or less. 7. The ink set according to claim 6, wherein the ink set is for inkjet. 8. An ink cartridge comprising an ink container containing the ink according to claim 1. Description: 9. A recording unit, comprising: an ink container that stores the ink according to claim 5; and a recording head that ejects the ink by an inkjet method. 10. An ink jet recording apparatus, comprising: an ink container for storing the ink according to claim 5; and a recording head for discharging the ink by an ink jet method.