JP2000230134A - Water-based pigment ink - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a water-based pigment ink having a high paper fixing density and improved in blackness by using a carbon black excellent in dispersion performances in water as a black pigment. SOLUTION: This water-based pigment ink is obtained by dispersing a carbon black having >=50 m2/g nitrogen adsorption specific surface area (N2SA), >=60 ml/100 g DBP oil absorption volume, >=110 tinting strength (Tint) and >=3 μequivalents/m2 sum total of carboxyl groups and hydroxyl groups and further characteristics of (1) 1.1-2.1 ratio (Dupa50%/Dst) of the average particle diameter Dupa50% (nm) of an agglomerate to the Stokes mode diameter (Dst) (nm) of an aggregate, (2) 120-180 (nm) value of the average particle diameter Dupa50% (nm) of the agglomerate and (3) <=350 (nm) value of the maximum particle diameter Dupa99% (nm) of the agglomerate in an aqueous medium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aqueous pigment ink using an easily water-dispersible carbon black having excellent dispersibility in water as a black pigment.

[0002]

2. Description of the Related Art Since carbon black is hydrophobic and has low wettability with water, it is extremely difficult to stably disperse it in water at a high concentration. This is because there are very few functional groups having high affinity for water molecules existing on the surface of the carbon black, for example, hydrophilic hydrogen-containing functional groups such as carboxyl groups and hydroxyl groups. Therefore, when used as a black pigment in an aqueous pigment ink in which carbon black is dispersed in water, it is necessary to improve the water dispersibility by modifying the surface properties of the carbon black.

[0003] Aqueous pigment inks have attracted attention as writing instruments, especially as recording liquids for ink jet printers in recent years. For example, JP-A-3-97770 discloses an aqueous medium, polyoxyethylene styryl phenyl ether and primary particles. An ink jet recording liquid characterized by containing carbon black having a diameter of 20 to 40 nm, a DBP oil absorption of 40 to 120 ml / 100 g, and a pH of 7.0 or more has been proposed. This is to improve the storage stability of the recording liquid by using polyoxyethylene styryl phenyl ether as a dispersing agent. A commercial product is used as the target carbon black, and there is no intention to modify the carbon black. It has not been.

It has long been known to improve the dispersibility of carbon black in water by oxidizing carbon black to form a hydrophilic functional group on the surface. Japanese Patent No. 18186 discloses a method for producing carbon oxide, which comprises oxidizing carbon black with an aqueous solution of hypohalite, and then washing the organic black with an organic solvent when separating and collecting the carbon black from the reaction system. And JP-A-57-1
Japanese Patent No. 59856 discloses a method for producing a water-dispersible modified carbon black, which comprises subjecting carbon black to low-temperature oxidation plasma treatment.

Further, JP-A-8-3498 discloses an aqueous pigment ink containing water and carbon black, wherein the carbon black has a surface active hydrogen content of 1.5 mmol / g or more, and In a method for producing an aqueous pigment ink containing water and carbon black, (a) obtaining an acidic carbon black, and (b) further oxidizing the acidic carbon black with hypohalite in water. And a process for finely dispersing carbon black having an oil absorption of 100 ml / 100 g or less in an aqueous medium; and JP-A-8-319444. Oxidizing the carbon black with a salt.

In the above-mentioned JP-A-8-3498 and JP-A-8-319444, the water dispersibility is improved by oxidizing carbon black and adding a large amount of active hydrogen which is a hydrophilic functional group to the surface. Thus, an aqueous pigment ink having excellent long-term dispersion stability can be obtained. However, in order for carbon black to disperse in water and maintain a stable dispersion state, the amount of hydrophilic functional groups present at the contact interface between the surface of the carbon black particles and the water molecules functions greatly, and merely the carbon black unit weight It is difficult to accurately judge whether the dispersibility is good or not simply by regulating the amount of the functional group present per hit.

Therefore, the present inventors focused on the amount of hydrophilic hydrogen-containing functional groups present per unit surface area of carbon black as a new index for accurately judging the quality of dispersion performance, and proceeded with research. A water-dispersible carbon black having a total amount of carboxyl groups and hydroxyl groups of 3 μeq / m ² or more per unit surface area among the hydrogen-containing functional groups to be produced, and a method for producing the same have been developed and proposed (Japanese Patent Application No. Hei 9-1997).
No. 332405).

[0008]

However, forming only a hydrophilic functional group on the surface of the carbon black increases the dispersibility of the carbon black in water and limits the long-term dispersion stability. There is. Therefore,
As a result of further research, it was found that the dispersing performance such as easy dispersibility and dispersion stability in water was closely related to the aggregation form of carbon black particles, and the nitrogen adsorption specific surface area (N ₂ SA)
There 80 m ² / g or more, a carbon black DBP oil absorption was oxidized following carbon black 70 ml / 100 g, the Stokes mode diameter Dst of the aggregate (nm) and the agglomerate mean particle size D _UPA of _(nm) A water-dispersible carbon black (Japanese Patent Application No. 9-332404) having a characteristic of a ratio D _UPA / Dst of 1.5 to 2.0 was proposed.

Further, the present inventors have determined that the nitrogen adsorption specific surface area (N ₂ S
A) is 50 m ² / g or more, a carbon black having DBP oil absorption of less 80 ml / 100 g, (1) the Stokes mode diameter Dst (nm aggregate of carbon black) and an average particle diameter Dupa50% of the agglomerate (nm (Dupa 50% / Dst) is 1.1 to 1.5, and (2) the average particle size Dupa of agglomerate
The value of 50% (nm) is 40 to 140 (nm), the value of (3) the maximum particle size Dupa99% (nm) of agglomerate is 250 (nm) or less,
And the total amount of carboxyl groups and hydroxyl groups among the hydrogen-containing functional groups present on the surface is 3 μeq per unit surface area
A water-dispersible carbon black characterized by having a characteristic of at least / m ² was developed (Japanese Patent Application No. 10-78481).

However, it has been found that when these water-dispersible carbon blacks are used as black pigments for aqueous pigment inks, there are some difficulties in terms of ink performance. That is, there is a problem that the blackness is insufficient due to a decrease in the paper fixing density.

The inventors of the present invention have conducted intensive studies on the development of a water-dispersible carbon black which is more suitable as a black pigment for aqueous pigment inks, and as a result, the present invention has been achieved. When printing on plain paper, specialty paper, OHP sheet, art paper, etc., an aqueous pigment ink that can provide excellent paper fixing density, print quality, ejection stability, light resistance, storage stability, etc. in a well-balanced manner. To provide.

[0012]

The aqueous pigment ink according to the present invention for achieving the above object has a nitrogen adsorption specific surface area.
(N ₂ SA) is 50 m ² / g or more, DBP oil absorption is 60 ml / 100 g or more, tinting strength (Tint) is 110 or more, and the sum of carboxyl groups and hydroxyl groups present on the surface is 3 μeq / m ² or more. The composition is characterized in that carbon black having the following characteristics is dispersed in an aqueous medium. (1) The ratio of the average particle diameter Dupa50% (nm) of the agglomerate to the Stokes mode diameter Dst (nm) of the aggregate (Dupa50% /
Dst) is 1.1 to 2.1 (2) The value of the average particle size Dupa 50% (nm) of the agglomerate is 12
0 to 180 (nm) (3) The maximum particle size of agglomerate Dupa99% (nm) is 35
0 (nm) or less However, 50% of Dupa is 50% in the cumulative frequency distribution curve of the agglomerate particle size created from the degree of frequency modulation of scattered light by irradiating a laser beam to an aqueous dispersion of carbon black.
Cumulative frequency value, Dupa99% is 99% in the same distribution curve
Indicates the value of the cumulative frequency. Dst represents the Stokes equivalent diameter of the maximum frequency in the aggregate Stokes equivalent diameter distribution by the centrifugal sedimentation method.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The carbon black targeted in the present invention has a nitrogen adsorption specific surface area (N ₂ SA) of at least 50 m ² / g, a DBP oil absorption of at least 60 ml / 100 g, and a tinting strength (Tint).
Is used. Carbon black having characteristics of 110 or more is used. When N ₂ SA is small, the particle size is large, so that when the ink is used, the filterability and ejection stability are reduced. On the other hand, when N ₂ SA is large, the particle size is small, so that when printing, the paper fixing density is reduced. As a result, the print density is impaired. Preferably, those having an N ₂ SA of 80 to 120 m ² / g are used.

When the DBP oil absorption is small, the paper fixing density decreases. Conversely, when the DBP oil absorption increases, the fluidity and dispersion stability of the dispersion liquid decrease, and the filterability deteriorates. Therefore, the DBP oil absorption is set to 60 ml / 100 g or more, but is preferably set in the range of 60 to 120 ml / 100 g. The tinting strength (Tint) is a characteristic related to the particle diameter (specific surface area) and structure (DBP) of carbon black, and furthermore, the aggregate diameter. Tint is 110 to prevent a decrease in filterability of the aqueous pigment ink. This is set as above.

In order to improve the dispersibility in water, it is advantageous to have a functional group having a high affinity for water molecules on the surface of the carbon black particles. In the present invention, a carboxyl group (—COOH) and a hydroxyl group (—O—O) containing active hydrogen which greatly function in dispersibility in water are contained.
H) the total amount present on the surface of the carbon black particles is 3
By setting the value to μeq / m ² or more, the water dispersibility is improved. When this value is less than 3 μeq / m ² , it is difficult to improve the dispersion performance because the amount of the hydrophilic functional group is small. This surface functional group can be formed by oxidizing carbon black, and a commonly used oxidizing agent such as oxygen, ozone or halogen acid, persulfuric acid, and salts thereof is used for the oxidizing treatment.

The values measured for these functional groups by the following method are used. Carboxyl group content: 2 to 5 g of carbon black was added to 50 ml of 0.976N sodium hydrogen carbonate and shaken for 6 hours. Then, the carbon black was filtered off from the reaction solution, and a 0.05N aqueous hydrochloric acid solution was added to the filtrate. pH 7.0
The neutralization titration test is performed with a 0.05N aqueous solution of sodium hydroxide until the carboxyl group is measured. This measured value is the nitrogen adsorption specific surface area of carbon black (N ₂ SA; m ² / g)
Is defined as the amount of carboxyl groups formed per unit surface area of carbon black (μeq / m ² ).

Hydroxyl content: 2,2'-Diphenyl-
1-picrylhydrazyl (DPPH) is dissolved in carbon tetrachloride to prepare a solution having a concentration of 5 × 10 ⁻⁴ mol / l, and 0.1 to 0.6 g of carbon black is added to the solution, and the solution is kept at a constant temperature of 60 ° C. Stir in the bath for 6 hours. Thereafter, carbon black is separated from the reaction solution by filtration, and the filtrate is measured for hydroxyl groups by an ultraviolet absorption spectrophotometer. The value measured in this way was divided by the nitrogen adsorption specific surface area of carbon black (N ₂ SA; m ² / g),
The amount of hydroxyl groups formed per unit surface area of carbon black (μeq / m ² ).

The nitrogen adsorption specific surface area (N ₂ SA), DBP
In addition to the prerequisite property requirements such as oil absorption, tinting strength (Tint) and functional group content, the water-based pigment ink of the present invention is required to disperse carbon black having the following properties in an aqueous medium. Become. (1) The ratio of the average particle diameter Dupa50% (nm) of the agglomerate to the Stokes mode diameter Dst (nm) of the aggregate (Dupa50% /
Dst) is 1.1 to 2.1 (2) The value of the average particle size Dupa 50% (nm) of the agglomerate is 12
0 to 180 (nm) (3) The maximum particle size of agglomerate Dupa99% (nm) is 35
0 (nm) or less

When carbon black is dispersed in water, the carbon black particles are easily dispersed in water, and in order to maintain the dispersed state stably, the carbon black particles are dispersed in water in a finer aggregated form and reagglomerated. To form a large aggregated form. In other words, the aggregate form of carbon black is such that several to several tens of basic fine particles of carbon black are fused and bonded in an irregular and complex chain form to form an aggregate. Are formed of aggregates (agglomerates) that are entangled with each other or adhere and re-aggregate.

Therefore, the aggregated form of carbon black dispersed in water is such that the aggregates, which are the smallest aggregate units of carbon black, are dispersed in water, and the aggregates in which the aggregates are reaggregated are less agglomerated. The state is effective for improving the dispersion performance, and the dispersion state free of agglomerates is ideal. However, aggregates exhibiting a complicated three-dimensional morphology tend to be entangled and re-agglomerated, and it is almost impossible to obtain a dispersed state free of agglomerates.

However, when carbon black is dispersed as a black pigment in an aqueous medium and used as an aqueous pigment ink, it is not sufficient to have only excellent dispersion performance. For example, when the dispersing performance is high, it is advantageous in terms of storage stability, viscosity, filterability and the like of the ink, but there is a problem that the blackness is lowered because the paper fixing density is low when printing.

In the present invention, the aggregation form of the carbon black particles is specified to stably disperse the carbon black in water when using an aqueous pigment ink, and to achieve both the paper fixing density and the filterability. That is,
To regulate the value of the ratio Dupa50% / Dst between the average particle diameter Dupa50% (nm) of the agglomerate and the Stokes mode diameter Dst (nm) of the aggregate in the range of 1.1 to 2.1, it is necessary to set 1.1. If it is lower than the value, the paper fixing density is remarkably reduced when printing is performed.
If the ratio exceeds, the filterability is significantly reduced. Further, the value of the average particle diameter Dupa50% (nm)
The reason for specifying the range of 0 to 180 nm is that if it is less than 120 nm, the paper fixing density decreases while passing through the gap between paper fibers, while
This is because if it exceeds 0 nm, the filterability decreases. Further, the value of the maximum particle size Dupa99% (nm) of agglomerate is set to 350 nm or less, because if it exceeds 350 nm, the ejection stability and the filterability will be significantly reduced.

As the characteristics Dst and Dupa, values obtained by the following measuring methods are used. (1) Stokes mode diameter Dst (nm) of aggregate: JI
SK6221 (1982) 5 A dispersion having a carbon black concentration of 100 mg / l is prepared by mixing a dried carbon black sample with a 20% by volume aqueous ethanol solution containing a small amount of a surfactant based on "How to prepare a dried sample". This is sufficiently dispersed with an ultrasonic wave to obtain a sample. A disk centrifuge (manufactured by Joyes Lobel, UK) was set to a rotation speed of 6000 rpm, and a spin solution (2% by weight glycerin aqueous solution,
After adding 15 ml (5 ° C.), 1 ml of a buffer solution (20% by volume aqueous ethanol solution, 25 ° C.) is injected. Then
After adding 0.5 ml of the carbon black dispersion liquid at a temperature of 25 ° C. with a syringe, centrifugal sedimentation was started, and at the same time, the recorder was operated, and the distribution curve shown in FIG. 1 (horizontal axis: carbon black dispersion liquid was added with a syringe). , The vertical axis: the absorbance at a specific point changed with the centrifugal sedimentation of carbon black). Each time T is read from this distribution curve, and is substituted into the following equation (Equation 1) to calculate the Stokes equivalent diameter corresponding to each time.

[0024]

(Equation 1)

In Equation 1, η is the viscosity of the spin liquid (0.935
cp), N is the disk rotation speed (6000 rpm), r ₁ is the radius of the carbon black dispersion injection point (4.56 cm), r ₂ is the radius to the absorbance measurement point (4.82 cm), ρ _CB is the density of carbon black (g / cm ³ ), ρ ₁ is the density of the spin liquid (1.00178
g / cm ³ ).

The Stokes equivalent diameter of the maximum frequency in the Stokes equivalent diameter and absorbance distribution curve (FIG. 2) obtained in this manner is calculated as the Stokes mode diameter Dst of the aggregate.
(nm).

(2) Average particle size of agglomerate Dupa 50%,
Maximum particle size Dupa99% (nm): Carbon black is dispersed in water to prepare a dispersion of 0.1 to 0.5 g / l, and a heterodyne laser Doppler type particle size distribution analyzer (Microtrac, UPAmode19340) is used. The dispersion is irradiated with laser light to measure the particle size of the agglomerate in the dispersion from the degree of frequency modulation of the scattered light. The carbon black in the dispersion liquid undergoes Brownian motion, and the frequency of scattered light is modulated by the size of the dispersed carbon black aggregate due to the Doppler effect. Therefore, the intensity of the Brownian motion differs depending on the size of the aggregate,
The size of the aggregate in a state of being dispersed in water, that is, the particle size of agglomerate can be measured. A cumulative frequency distribution curve is created from the agglomerate particle size measured in this way, and the value of 50% cumulative frequency is defined as the average particle size Dupa of agglomerate 50% (nm), and the value of 99% cumulative frequency is defined as the maximum agglomerated particle size Dupa99. % (nm).

The aqueous pigment ink of the present invention can be obtained by dispersing carbon black having these characteristics in a desired concentration in an aqueous medium such as water. In addition,
The dispersion concentration of the carbon black is set in consideration of the dispersion performance and the ink performance, but is desirably set to 60% by weight or less in order to achieve dispersion stability.

Hereinafter, examples of the present invention will be specifically described in comparison with comparative examples.

Examples 1-3, Comparative Examples 1-3 Nitrogen adsorption specific surface area (N ₂ SA), DBP oil absorption, coloring power (Tint)
Of carbon black having different concentrations were added to 3000 ml of an aqueous solution of ammonium persulfate having different concentrations.
Stir and mix at 300 ° C and 300 rpm for different reaction times.
It was oxidized. Next, the carbon black separated by filtration is dispersed in pure water to form an aqueous sodium hydroxide solution.
(Concentration: 0.5N), purified by an ultrafiltration membrane to separate the remaining salts, then filtered to separate carbon black, washed with water and dried to obtain carbon black with different functional groups. .

For each carbon black thus obtained, nitrogen adsorption specific surface area (N ₂ SA), DBP oil absorption, coloring power (Tint), carboxyl group (C group) and hydroxyl group (H group), aggregate Stokes mode diameter Dst (n
m), the average particle size Dupa of agglomerate 50% (nm) and the maximum particle size Dupa 99% (nm) were measured. Table 1 shows the obtained results.

[0032]

[Table 1]

Next, these carbon blacks were dispersed in pure water at a concentration of 20% by weight to prepare an aqueous pigment ink, and the dispersing performance and the ink performance were evaluated by the following methods. Table 2 shows the obtained results.

Heating stability: The sample was packed in a closed container, and the viscosity change was measured in a 70 ° C. incubator for 1 to 4 weeks to compare the dispersion stability at the time of heating. The viscosity was measured using a rotary viscometer [Yamaichi Denki Co., Ltd., VM-100A-L
].

Particle size measurement: The particle size of the sample and the sample subjected to the heating stability test were measured using a heterodyne laser Doppler system particle size distribution analyzer (Microtrac, UPA model 9340). In this measurement device, when laser light is applied to particles that are performing Brownian motion in a suspension, the frequency of scattered light is modulated by the Doppler effect. The intensity of Brownian motion, that is, the particle diameter is measured from the degree of frequency modulation.

Print density: The carbon black dispersion density of the aqueous pigment ink was diluted to 3% by weight.
Using ROX 4024 paper, printing was performed with a # 3 bar coder, and the optical density was measured using a Macbeth densitometer (RD-927, manufactured by Colmorgen).

Filterability: 200 g of the aqueous pigment ink is 90
φ filter paper (NO.2) and membrane pore size 0.8 μm, 0.65 μm, 0.45
A filtration test was performed using a μm filter under a reduced pressure of 20 Torr, and the passing amount was measured.

[0038]

[Table 2]

The characteristics required of the aqueous pigment ink are storage stability, filterability, paper print density (blackness), nitrogen adsorption specific surface area (N ₂ SA) of carbon black, DBP oil absorption,
There is a property that depends on coloring power (Tint). Further, as shown in the examples, the agglomerate form of carbon black in a state where dispersibility in water was imparted by oxidation treatment was Dup
It can be seen that good properties are shown when a50% is 120 to 180 nm and Dupa99% is 350 nm. Therefore, the nitrogen adsorption specific surface area (N ₂ SA), DBP oil absorption,
Carbon black exhibiting tinting properties (Tint) and the above-mentioned agglomerate form satisfies all the properties required for aqueous pigments.

However, the carbon black in the agglomerate form does not always satisfy both the filterability and the blackness, but only one of the properties. In Comparative Example 1, the DBP oil absorption was out of the range, the average agglomerate particle size was 70.2 nm, and the particles were dispersed in water in a very fine state. For this reason, although the filterability is good, the fineness of the particle size reduces the blackness by passing through the fibers of the paper. Comparative Example 2 satisfies the nitrogen adsorption specific surface area (N ₂ SA), DBP oil absorption, and coloring power (Tint), but has a broad particle size distribution and a maximum agglomerate particle size of 350 nm or more. This maximum particle size affects the filterability. 2 Filter paper passes through, but with a 0.8 μm membrane filter, 8
The result was that only 0% passed. Comparative Example 3 has coloring power
(Tint) is small and out of range. When dispersed as agglomerates in water, the maximum particle size increases. Comparative Example 2
As in the case of No. 5, the maximum particle size hindered the filtration. 2 It also became difficult to pass through the filter paper.

As described above, the nitrogen adsorption specific surface area (N ₂ SA), DB
If the conditions of oil absorption and coloring power (Tint) are out of the range and the particle state of carbon black dispersed in water is out of the specified range, all or some of the required properties such as filterability and blackness It turns out that it decreases.

[0042]

As described above, according to the water-based pigment ink of the present invention, the water-dispersible carbon black which is excellent in dispersibility in water by specifying the properties of carbon black, particularly the agglomerated form of particles, can be used in an aqueous medium. Because it is dispersed inside,
It is possible to maintain a stable dispersion state and increase the paper fixing density, that is, it is possible to improve the blackness, and it is an excellent paper fixing when printing on plain paper, special paper, OHP sheet, art paper, etc. An aqueous pigment ink having a good balance of density, print quality, ejection stability, light resistance, storage stability, and the like can be provided.

[Brief description of the drawings]

FIG. 1 is a distribution curve showing changes in absorbance due to centrifugal sedimentation of carbon black and elapsed time from the addition of a carbon black dispersion at the time of measuring Dst.

FIG. 2 is a distribution curve showing the relationship between Stokes equivalent diameter and absorbance obtained at the time of measuring Dst.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4J037 AA02 DD05 DD07 DD17 DD30 EE02 EE19 EE28 EE43 FF05 FF15 4J039 BA04 BE01 CA06 EA19 EA35 EA41 EA42 EA44 FA02 GA24

Claims (2)

[Claims] 1. A nitrogen adsorption specific surface area (N ₂ SA) of at least 50 m ² / g, a DBP oil absorption of at least 60 ml / 100 g, a coloring power (Tint) of at least 110, and a sum of carboxyl groups and hydroxyl groups present on the surface. Is 3 μeq / m ² or more, and carbon black having the following properties is dispersed in an aqueous medium. (1) The ratio of the average particle diameter Dupa50% (nm) of the agglomerate to the Stokes mode diameter Dst (nm) of the aggregate (Dupa50% /
Dst) is 1.1 to 2.1 (2) The value of the average particle size Dupa 50% (nm) of the agglomerate is 12
0 to 180 (nm) (3) The maximum particle size of agglomerate Dupa99% (nm) is 35
0 (nm) or less However, 50% of Dupa is 50% in the cumulative frequency distribution curve of the agglomerate particle size created from the degree of frequency modulation of scattered light by irradiating a laser beam to an aqueous dispersion of carbon black.
Cumulative frequency value, Dupa99% is 99% in the same distribution curve
Indicates the value of the cumulative frequency. Dst represents the Stokes equivalent diameter of the maximum frequency in the aggregate Stokes equivalent diameter distribution by the centrifugal sedimentation method. _2. The value of the nitrogen adsorption specific surface area (N ₂ SA) is 80 to 1
20 (m ² / g), the value of DBP oil absorption is 60 to 120 (ml / 100
The easily water-dispersible carbon black according to claim 1, which is g).