JPH11148026A - Easily water-dispersible carbon black - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a carbon black which easily disperses into water. SOLUTION: This carbon black is obtained by the oxidation of carbon black having a nitrogen-adsorbing specific area (N2 SA) of 80 m<2> /g or larger and a DBP oil-absorbing value of 70 ml/100 g or smaller. The resulting carbon black has a DUPA/Dst ratio of 1.5-2.0, wherein DUPA and Dst are an average particle size (nm) of the agglomerate and a Stokes mode diameter (nm) of the aggregate, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to carbon black having excellent dispersibility in water.

[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 the number of functional groups having high affinity for water molecules existing on the surface of the carbon black, for example, hydrophilic functional groups such as a carboxyl group and a hydroxyl group is extremely small. Therefore, when it is 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 of carbon black.

[0003] Aqueous pigment inks have attracted attention as writing materials, especially as recording liquids for ink jet printers in recent years. For example, JP-A-3-97770 discloses an aqueous medium, polyoxyethylenestyrylphenyl 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 intended to improve the storage stability of the recording liquid by using polyoxyethylene styryl phenyl ether as a dispersant. A commercially available carbon black is used as the target carbon black, and there is no intention to modify 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 hydrophilic functional groups on the surface. For example, Japanese Patent Application Laid-Open No. 48-18186 discloses a method of oxidizing carbon black with an aqueous solution of hypohalite, and then washing with an organic solvent to separate and collect the oxidized carbon black from the reaction system. A method for producing carbon black is disclosed in
JP-A-59856 discloses a method for producing a water-dispersible modified carbon black, which comprises subjecting carbon black to low-temperature oxidative 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 method for producing an aqueous pigment ink. JP-A-8-319444 further includes a step of finely dispersing carbon black having an oil absorption of 100 ml / 100 g or less in an aqueous medium; and a step of oxidizing the carbon black with a hypohalite. A method for producing an aqueous pigment ink is disclosed.

In the above-mentioned JP-A-8-3498 and JP-A-8-319444, carbon black is oxidized to contain a large amount of active hydrogen, which is a hydrophilic functional group, on the surface, so that water dispersibility is improved. An object of the present invention is to obtain an aqueous pigment ink which is excellent and has excellent long-term dispersion stability.

[0007]

However, the formation of only a hydrophilic functional group on the surface of carbon black enhances the dispersibility of carbon black in water, and the dispersion stability of carbon black over a long period of time or the particle size of the carbon black dispersion. There is a limit in maintaining the uniformity of the surface. Therefore, the present inventors have conducted detailed research on the dispersion state of carbon black in water, and as a result, the dispersibility of carbon black in water, such as easy dispersibility and dispersion stability, is closely related to the aggregation form of carbon black particles. I found a relationship.

[0008] That is, 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 aggregates (aggregates). Are entangled with each other or adhered and aggregated (agglomerates). Therefore, in order to improve the dispersion performance of carbon black, it is effective to reduce the ratio of aggregate and agglomerate in water, that is, the ratio of agglomerate, due to a change in the aggregation form that occurs during the dispersion process.

The present invention has been completed on the basis of the above findings, and one of the objects of the present invention is to make it possible to easily microdisperse in water and to stably maintain a dispersed state for a long period of time. An object of the present invention is to provide an easily water-dispersible carbon black having excellent dispersibility. Further, the other purpose is to disperse this carbon black in water at a high concentration in a micro concentration, for example, as a recording liquid used in an ink jet printer, etc., which has excellent ejection stability, paper fixing density, print quality, light resistance,
An object of the present invention is to provide an aqueous pigment ink having good storage stability and the like.

[0010]

The water-dispersible carbon black of the present invention for achieving the above object has a nitrogen adsorption specific surface area (N ₂ SA) of 80 m ² / g or more and a DBP oil absorption of 7 or more.
A carbon black obtained by oxidizing carbon black of 0 ml / 100 g or less, wherein the ratio D _UPA / Dst of the Stokes mode diameter Dst (nm) of the aggregate to the average particle diameter D _UPA (nm) of the agglomerate is 1. It is characterized by having a characteristic of 5 to 2.0.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the characteristics of carbon black that the nitrogen adsorption specific surface area (N ₂ SA) is limited to 80 m ² / g or more and the DBP oil absorption is limited to 70 ml / 100 g or less are as follows.
This is a prerequisite for providing excellent dispersion performance. That is, when carbon black is dispersed in water, if its specific surface area is small, the contact interface with water molecules is also small, so that the dispersibility is reduced. Also, if the DBP oil absorption is large, that is, if the structure level is high, it is dispersed in water. The aggregate unit of the carbon black in the degraded state becomes large, and it tends to settle during long-term dispersion. Therefore, it is necessary that the carbon black is dispersed in water in smaller aggregation units, and when the dispersion aggregation units are large, the filterability is reduced when dispersed in water. Therefore, it becomes impossible to maintain the ejection stability at a high level. That is, when the nitrogen adsorption specific surface area (N ₂ SA) deviating from the prerequisite characteristics is less than 80 m ² / g and the DBP oil absorption exceeds 70 ml / 100 g, it becomes difficult for carbon black to be micro-dispersed in water. The nitrogen adsorption specific surface area (N ₂ SA) and DBP oil absorption are values measured by the following methods. Nitrogen adsorption specific surface area (N ₂ SA); ASTM D3037-88 Method B DBP oil absorption; JIS K6221-82 A method

The water-dispersible carbon black of the present invention comprises:
The value of the ratio D _UPA / Dst between the Stokes mode diameter Dst (nm) of the aggregate and the average particle diameter D _UPA (nm) of the agglomerate is 1 for carbon black having the above characteristics.
By using carbon black having a selectivity of 5 to 2.0, the amount of the hydrogen-containing surface functional group is imparted by oxidation treatment, and the carbon black easily disperses in water.

When carbon black is dispersed in water, the carbon black particles must be dispersed in water in a finer agglomerated form in order to enhance the dispersing performance such as easy dispersibility in water and dispersion stability. is necessary. In the aggregate form of carbon black, several to several tens of basic particles of carbon black are fused and bonded to form aggregates, which are the minimum aggregate units, and these aggregates are entangled or adhered to each other. Consists of aggregated agglomerates. Therefore, it is ideal that the aggregate is dispersed in water in the state of the aggregate that is the minimum aggregation unit of carbon black, and the aggregate is not dispersed in the agglomerate.

According to the present invention, the value of the ratio D _UPA / Dst between the Stokes mode diameter Dst (nm) of the carbon black aggregate and the average particle diameter D _UPA (nm) of the agglomerate is 1.5 to 1.5.
2.0. The value of D _UPA / Dst is an index indicating the form of aggregation of carbon black dispersed in water, and the larger the value of D _UPA / Dst, the more the form of aggregation of carbon black dispersed is the ratio of agglomerate. The smaller the value, the lower the ratio of agglomerates and the higher the ratio of aggregates. And D
_The closer the value of _UPA / Dst is to 1, the greater the degree of microdispersion in the aggregate state, and under the ideal dispersion state where all the agglomerates are loosened, _DUPA is D
D _UPA / Dst = 1 in accordance with st. In the present invention, the dispersion performance is improved by setting the value of D _UPA / Dst in a suitable range, and more preferably, 1.5 to 1.
Set to the range of 8.

As the above characteristics Dst and D _UPA , values obtained by the following measuring methods are used. (1) Stokes mode diameter Dst (nm) of aggregate: JI
SK6221 (1982) 5 A dispersion of carbon black concentration 100 mg / l was 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 at a temperature of 25 ° C. with a syringe, centrifugal sedimentation was started, and at the same time, the recorder was operated to distribute the distribution curve shown in FIG. , 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 substituted into the following equation (Equation 1) to calculate the Stokes equivalent diameter corresponding to each time.

[0016]

(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 the 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 D of agglomerate_UPA(n
m): 0.1 to 0.5 g /
l) and prepare a heterodyne laser Doppler
Particle size distribution analyzer (Microtrac, UPAmode1)
9340) by irradiating the dispersion with laser light,
The degree of frequency modulation of carbon black in the dispersion
Average particle size D_UPA(nm). Carbon in dispersion
Black has a Brownian motion, due to the Doppler effect.
Depending on the size of the carbon black aggregates dispersed
The frequency of the scattered light is modulated. Therefore, large aggregates
Because the intensity of the Brownian motion differs depending on the size, water
Size of aggregates in the state of being dispersed inside,
Agglomerate average particle size D _UPAMeasuring (nm)
Can be. The principle of the measurement is described in JP-A-3-170844.
This is described in detail in the official gazette.

In the present invention, the water-dispersible carbon black is obtained by oxidizing the carbon black having the above-mentioned properties. The oxidation treatment method is not particularly limited, but wet oxidation is preferred. Oxidizing agents include oxygen atoms such as oxygen, ozone, halogen acids, halogen acids, hypohalous acids, halogen acids such as hypohalous acids, salts of halogen acids, and persulfates such as peroxo-2-sulfates. Is used, but when carboxyl groups or hydroxyl groups formed on the carbon black surface by wet oxidation treatment are modified into salts, the hydrophilicity with water is further improved and the dispersibility is improved. Hypohalites, persulfates and the like. As the salt, an ammonium salt or an alkali metal salt such as sodium, potassium or lithium is preferable.

It is more preferable that the carbon black is previously subjected to an oxygen oxidation treatment or an ozone oxidation treatment and then subjected to a wet oxidation treatment, since wet oxidation is promoted and addition of a hydrophilic functional group is promoted. After neutralization, the carbon black is separated by filtration, and the separated carbon black is removed by electrodialysis, ultrafiltration, etc. to remove salts and concentrated by removing the oxidizing agent, or by washing with water. After that, the water-dispersible carbon black of the present invention can be obtained by a method such as drying.

Hydrogen-containing surface functional groups such as carboxyl groups and hydroxyl groups are formed on the surface of the oxidized carbon black, and the affinity for water molecules is improved.
In the present invention, the amount of the hydrogen-containing surface functional group is desirably formed on the carbon black surface at 3 μeq / m ² or more. When the amount of these functional groups is less than 3 μeq / m ^{2, the} amount of the formed hydrophilic carboxyl groups and hydroxyl groups is small, so that the dispersibility in water is reduced. In the process of dispersing carbon black in water, the amount of hydrophilic functional groups present at the contact interface between carbon black and water molecules plays an important role. Cannot be evaluated accurately. Therefore, the amount of the hydrogen-containing surface functional group existing per unit surface area of carbon black is used as an index.

The values of these hydrogen-containing surface functional groups measured by the following method are used. Carboxyl group content: After adding 2 to 5 g of carbon black to 50 ml of 976N sodium hydrogencarbonate and shaking for 6 hours, the carbon black was separated by filtration from the reaction solution, and a 0.05N aqueous hydrochloric acid solution was added to the filtrate.
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 ).

[0025]

EXAMPLES Examples of the present invention will be specifically described below in comparison with comparative examples.

Examples 1 to 4 and Comparative Examples 1 to 4 Using carbon black samples having different nitrogen adsorption specific surface area (N ₂ SA) and DBP oil absorption, each carbon black sample 1
50 g is put in an ozone treatment container, and ozone is
It was circulated at a flow rate of hr and treated for 1 hour. Each of the ozone-treated carbon blacks was mixed with sodium hypochlorite having an effective chlorine concentration of 6% or 3,000 ml of a 1.5 N aqueous solution of ammonium peroxodisulfate, and oxidized while changing the reaction temperature and reaction time while stirring. Next, those oxidized with sodium hypochlorite are diluted with dilute hydrochloric acid, and those oxidized with ammonium peroxodisulfate are neutralized with an aqueous solution of caustic soda, and the remaining salts are removed by electrodialysis. The carbon black dispersion concentration is 20% by weight. The water was evaporated to obtain a water-dispersible carbon black.

With respect to the obtained water-dispersible carbon black, the Stokes mode diameter Dst (nm) of the aggregate and the average particle diameter D _UPA (nm) of the agglomerate were measured, and the ratio D _UPA / Dst was calculated. Further, the carboxyl group and the hydroxyl group were measured, and the sum of the carboxyl group amount and the hydroxyl group amount was defined as the hydrogen-containing surface functional group amount. In the measurement of the Stokes mode diameter Dst of the aggregate, the measurement was performed by dispersing a dried sample in a measurement liquid. Table 1 shows the results thus obtained.

[0028]

[Table 1]

Next, in order to evaluate the dispersing performance of these oxidized carbon blacks in water, a dispersion prepared by adjusting the carbon black dispersion concentration to 20% by weight was tested by the following method. Dispersion performance was evaluated. The results are shown in Table 2.

(A) Measurement of Viscosity: The sample was placed in a closed container, and the viscosity was measured one week after the temperature was maintained at 70 ° C., and the viscosity stability upon heating was compared. The viscosity was measured with a rotational vibration type viscometer (VM-100A-L, manufactured by Yamaichi Electric Co., Ltd.). After freezing at −20 ° C. for 24 hours, the cycle of thawing at room temperature and measuring the viscosity was repeated, and the stability during freezing and thawing was compared based on the change in viscosity. The viscosity was measured with a rotational vibration type viscometer (VM-100A-L, manufactured by Yamaichi Electric Co., Ltd.).

[0031] (b) measurement of the average particle diameter D _UPA of the agglomerate; by measuring the 70 ° C. temperature to the holding to one week of the agglomerate of the dispersion water after the average particle diameter D _UPA (nm), the change Compared.

(C) Filterability: Dispersed water was filtered with 90φ filter paper (NO.
2) and a filtration test was carried out under a reduced pressure of 20 Torr using filters of 3 μm, 0.8 μm, and 0.45 μm in pore size, and the amount of filter paper passed was compared.

(D) Print density: The dispersion water was diluted to a carbon black concentration of 3% by weight, printed on a XEROX 1321 paper using a # 3 bar coder, and a Macbeth densitometer (RD-produced by Colmorgen) was used.
918) was used to measure the optical density.

[0034]

[Table 2]

From the results of Tables 1 and 2, the nitrogen adsorption specific surface area (N ₂ SA) is 80 m ² / g or more, and the DBP oil absorption is 70 ml / 100 g.
The following carbon blacks are oxidized carbon blacks, and the Stokes mode diameter Dst (n
m) and the average particle size D _UPA (nm) of agglomerates D _UPA
It is recognized that the carbon blacks of Examples 1 to 4 in which the value of / Dst is 1.5 to 2.0 have excellent dispersibility in water and filterability. That is, the dispersion water obtained by dispersing the carbon blacks of Examples 1 to 4 in water was heated to 70 ° C., and the viscosity after one week passed hardly changed. The change was slight, indicating that the dispersion stability was excellent. This is because the microdispersed aggregates and agglomerates are reassembled in water and the formation of larger aggregates is suppressed, and thus the filterability of the dispersed water is high.

On the other hand, in the dispersion water in which the carbon blacks of Comparative Examples 1 to 4, which do not satisfy the requirements of the present invention, are dispersed in water, heating and freezing and thawing at 70 ° C. are repeated.
The viscosity is constant or reduced and stable, but D _UPA / D
Since the st is large, the aggregate of the carbon black itself is large, and the filterability is poor because the agglomerates are re-aggregated.

[0037]

As described above, according to the present invention, an easily water-dispersible carbon black having excellent dispersibility, which can be easily dispersed in water and can stably maintain a dispersed state for a long period of time, can be obtained. Provided. An aqueous pigment obtained by dispersing the water-dispersible carbon black in water has excellent ejection stability as a recording liquid used in an ink jet printer, for example, and has a high paper fixing density, print quality, light resistance, and storage stability. Etc. are good. Therefore, it is extremely useful as a water-based pigment ink used in a wide range of application fields such as recording liquids for ink jet printers and various writing instruments.

[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 measurement of Dst.

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

Claims (1)

[Claims] 1. A carbon black obtained by oxidizing carbon black having a nitrogen adsorption specific surface area (N ₂ SA) of 80 m ² / g or more and a DBP oil absorption of 70 ml / 100 g or less, and a Stokes mode diameter Dst ( nm) and the average particle size D _UPA (nm) of the agglomerate D _UPA / Dst is 1.5 to
An easily water-dispersible carbon black having a property of 2.0.