JP2001164147A - Carbon black pigment for water base ink - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon black pigment which excels in water dispersibility and is suited in use for a water base ink. SOLUTION: The carbon black pigment for a water base ink is obtained by subjecting carbon black having a nitrogen adsorption specific area (N2SA) of >=120 m2/g, a DBP absorption of <=80 cm3/100 g, and a tint strength (Tint) of >=120 to oxidation treatment to modify it in such a manner that the amount of hydrogen-containing functional groups in the surfaces of carbon black particles is >=3 μmol/m2, and has a ratio of the average particle diameter (Dupa 50%) (nm) of carbon black agglomerate to Stokes, diameter (Dst) (nm) of the carbon black aggregate of 0.8-1.1, an average particle diameter (Dupa 50%) (nm) of the agglomerate of 20-60 (nm), a maximum particle diameter (Duoa 99%) (nm) of the agglomerate of <=160 (nm) and, at the same time, a precipitation residual ratio of <=10 wt.% as the agglomeration properties of particles.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an excellent dispersion performance,
The present invention relates to a carbon black pigment for aqueous inks that can be easily and stably dispersed in water and can be suitably used as a pigment for black aqueous inks.

[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.

It has been known for a long time to improve the dispersibility of carbon black in water by oxidizing carbon black to form hydrophilic functional groups on the surface. Japanese Patent Application Laid-Open No. H05-159856 discloses a method of oxidizing carbon black with an aqueous solution of hypohalite, and Japanese Patent Application Laid-Open No. 57-159856 discloses a method of oxidizing carbon black with low-temperature oxygen plasma. However, the oxidation treatment using low-temperature plasma has a difficulty in promoting a uniform reaction to a large amount of carbon black.

Also, a method for producing a water-based ink for improving dispersibility in water by using a coupling agent or a surfactant to carbon black which has been subjected to light oxidation treatment (Japanese Patent Application Laid-Open Nos. 4-189877 and 4-189877). -359072, 5-43759 and 5-125306) are also known. However, it is difficult to maintain the dispersion performance for a long period of time due to deterioration due to oxidation or decomposition of the surfactant or the like due to temperature change and temporal change.

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. A process for finely dispersing carbon black having an oil absorption of 100 ml / 100 g or less in an aqueous medium is disclosed in 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. An object of the present invention is to obtain an aqueous pigment ink which is excellent and has excellent long-term dispersion stability. 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 plays a significant role, and simply carbon black is used. It is difficult to accurately judge the quality of the dispersibility based on the amount of the functional group per unit weight.

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 modified by oxidation treatment. Carbon black, wherein the total amount of carboxyl groups and hydroxyl groups among the hydrogen-containing functional groups present on the surface is 3 μm per unit surface area.
A water-dispersible carbon black characterized by a eq / m ² or more (JP-A-11-148027) was proposed.

Further, there is a limit in increasing the dispersibility of carbon black in water only by forming a hydrophilic functional group on the surface of carbon black, and there is a limit in maintaining the dispersion stability over a long period of time. As a result, we found that dispersion performance such as easy dispersibility in water and dispersion stability was closely related to the aggregation form of carbon black particles, and the nitrogen adsorption specific surface area (N ₂ SA) was 80 m ² / g. As described above, carbon black obtained by oxidizing carbon black having a DBP oil absorption of 70 ml / 100 g or less, wherein the Stokes mode diameter Dst (nm) of the aggregate and the average particle diameter Dupa (nm) of the agglomerate
A water-dispersible carbon black (Japanese Patent Application Laid-Open No. H11-148026) has been proposed which has a characteristic that the value of Dupa / Dst is 1.5 to 2.0.

[0009]

However, when carbon black is used as a pigment such as an ink for an ink jet or a water-based ink, for example, the ejection stability from a nozzle of an ink jet recording apparatus is excellent. It is important that the dispersion state is stable and the storage stability is excellent even when the aqueous dispersion of carbon black is stored at a high temperature for a long period of time.

The performance of dispersing carbon black in water, particularly the dispersion stability, is affected by the size of the carbon black particles, that is, the form of aggregation of the carbon black particles, and sedimentation of the carbon black starts due to the size of the aggregates and reaggregation. . Therefore, the present inventors further studied the aggregation form of carbon black particles and the sedimentation of carbon black in the aqueous dispersion in order to improve the water dispersion performance of carbon black, and identified the aggregation form of carbon black particles. By using the amount of sedimentation caused by forcible sedimentation by applying an external force to the carbon black in the aqueous dispersion as an evaluation criterion, by regulating the evaluation criterion value, the dispersibility in water is excellent, and a stable dispersion state is achieved. It was confirmed that it could be maintained for a long time.

The present invention has been completed on the basis of the above findings, and an object of the present invention is to maintain a stable dispersion state even when an aqueous dispersion of carbon black is left at a high temperature for a long time. For example, when used as a recording liquid for an ink jet printer, it is excellent in paper fixing density, printing quality, light resistance, etc., and also has a high ejection stability from a nozzle of an ink jet recording apparatus, and has excellent dispersing performance. It is to provide a carbon black pigment for ink.

[0012]

The carbon black pigment for aqueous ink of the present invention for achieving the above object has a nitrogen adsorption specific surface area (N ₂ SA) of at least 120 m ² / g and a DBP absorption of 80 cm ^3. / 100 g or less, carbon black having a specific coloring power (Tint) of 120 or more is oxidized to modify the amount of hydrogen-containing functional groups on the surface of the carbon black particles to 3 μmol / m ² or more,
The value of the ratio Dupa50% / Dst between the Stokes mode diameter Dst (nm) of the aggregate of carbon black and the average particle diameter Dupa50% (nm) of agglomerate is 0.8 to 1.1, and the average particle diameter Dupa50% of the agglomerate ( (nm) is 20 to 60 (nm), the maximum particle size of agglomerate Dupa99% (nm) is 160 (nm) or less, and the precipitation residue ratio is 10 wt% or less. The feature of. However, Dst is the Stokes equivalent diameter of the maximum frequency in the aggregate Stokes equivalent diameter distribution by the centrifugal sedimentation method, and Dupa50% is the agglomerate particle diameter created from the degree of frequency modulation of scattered light by irradiating an aqueous dispersion of carbon black with laser light. The value of 50% cumulative frequency in the cumulative frequency distribution curve and the value of Dupa99% indicate the value of 99% cumulative frequency in the distribution curve. The percentage of sediment residue was determined by measuring the amount (M1) of the sediment residue after centrifuging the aqueous dispersion of carbon black (concentration: 20 wt%) at a gravitational acceleration of 9.8 × 20,000 m / s ² for 30 minutes. Weight ratio to the weight (M0) of carbon black in the aqueous dispersion before treatment
(M1 / M0).

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION As a characteristic of carbon black targeted in the present invention, a nitrogen adsorption specific surface area (N ₂ SA) is 1
20 m ² / g or more, a DBP absorption 80 cm ^3/100 g or less, to regulate the ratio coloring force (Tint) to 120 or more is to be the prerequisite for imparting excellent dispersing performance. That is, when carbon black is dispersed in water, if the specific surface area is small, the area of the interface in contact with water molecules is also small, so that the dispersion performance is reduced and the nitrogen adsorption specific surface area (N ₂ SA) is 1%.
When it is less than 20 m ² / g, when the aqueous ink is used, the precipitation residue ratio becomes large, and the filterability and the ejection stability decrease.

If the DBP absorption is large, that is, if the structure level is high, the aggregation units of the carbon black particles in the state of being dispersed in water also become large, and the particles are likely to settle during the long-term dispersion. Residue rate increases and filterability and discharge stability decrease. Therefore, DBP absorption amount is set below 80 cm ^3/100 g. Further, if the specific coloring power (Tint) is small, the particle size distribution of the carbon black particles becomes broad, and similarly, the filterability and the ejection stability are reduced.

The carbon black having these characteristics is modified by an oxidation treatment so that the amount of the hydrogen-containing functional group on the surface of the carbon black particles becomes 3 μmol / m ² or more. Known oxidation means is applied for the oxidation treatment, for example, dry oxidation treatment with oxygen or ozone, or hypochlorite, chlorite, chlorate, persulfate, perborate, percarbonate, etc. It can be performed by an appropriate method such as wet oxidation treatment in which carbon black is put into an aqueous solution and stirred and mixed. The oxidation treatment is performed to form a hydrogen-containing functional group such as a carboxyl group or a hydroxyl group having a high affinity for water molecules on the surface of the carbon black particles, and the amount of these functional groups is 3 μmol / m ² or more. It is oxidatively modified so that When the value is less than 3 μmol / m ² , it is difficult to improve the dispersion performance because the amount of the hydrophilic functional group is small.

The carbon black pigment for an aqueous ink of the present invention is a carbon black having the above-mentioned properties,
The agglomerated form of the particles has the following particle aggregating properties. Ratio Dupa50% / Dst of Stokes mode diameter Dst (nm) of aggregate and average particle diameter Dupa50% (nm) of agglomerate
Is 0.8 to 1.1. The value of the average particle size Dupa50% (nm) of agglomerate is 20 ~
60 (nm). The maximum particle size of agglomerate Dupa99% (nm) is 160
(nm) or less.

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

Therefore, the aggregate 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 re-aggregated are less agglomerated. The state is effective in improving the dispersion performance.

However, when the aggregation form of the carbon black particles is reduced, it is advantageous for improving the dispersing performance. However, when the aqueous ink is used, the carbon black passes through the gap between the paper fibers, and the paper fixing density decreases. Becomes On the other hand, when the aggregation form becomes large, the dispersibility deteriorates, and the ejection stability and the filterability of the ink decrease.

That is, the value of Dupa50% / Dst is set to 0.8 to
The reason for setting the range to 1.1 is that if this value exceeds 1.1, the ratio of agglomerates in which the aggregates reaggregate becomes high, and it becomes difficult to maintain a stable dispersion state. However, if this value is less than 0.8, the blackness is greatly reduced.

The reason why the value of the average particle size Dupa 50% (nm) of agglomerate is set in the range of 20 to 60 nm is that if it is less than 20 nm, the degree of blackness is greatly reduced. This is because the filterability is significantly reduced. The reason why the value of the maximum particle size Dupa 99% (nm) of agglomerate is set to 160 nm or less is that if it exceeds 160 nm, the rate of precipitation residue and the filterability decrease.

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 dry carbon black sample was mixed with a 20% by volume aqueous ethanol solution containing a small amount of a surfactant to obtain a carbon black concentration of 0.1 kg / m ³ (100 mg / l). ) To make a dispersion,
This is sufficiently dispersed with an ultrasonic wave to obtain a sample. One disk centrifuge (Joyes Lobel, UK)
After setting the rotation speed to 00 s ⁻¹ and adding 15 cm ³ of a spin solution (2% by weight glycerin aqueous solution, 25 ° C.), 1 cm ³ of a buffer solution (20% by volume ethanol aqueous solution, 25 ° C.) is injected. Then, after adding 0.5 cm ³ of the carbon black dispersion at a temperature of 25 ° C. with a syringe, centrifugal sedimentation was started,
At the same time, the recorder was operated to obtain the distribution curve (horizontal axis:
The time elapsed since the carbon black dispersion was added with a syringe, the vertical axis: the absorbance at a specific point changed with the centrifugal sedimentation of the carbon black) is created. 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.

[0023]

(Equation 1)

In Equation 1, η is the viscosity of the spin liquid (0.935
mPa · S), N is the disk rotation speed (100s ^-1 ), 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 solution (1.0
0178 g / cm ³ ).

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

(2) Agglomerate average particle size Dupa 50% (n
m), maximum particle diameter Dupa99% (nm): carbon black dispersed in water to prepare a dispersion of 0.1~0.5kg / m ^3, laser Doppler system particle size distribution measuring apparatus (manufactured by Microtrac Inc. The dispersion is irradiated with laser light using UPA mode 1 9340), and the particle size of agglomerate in the dispersion is measured 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,
Since the intensity of the Brownian motion varies 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).

Further, the carbon black pigment of the present invention is required to have a precipitation residue rate of 10% by weight or less. The sedimentation residue ratio indicates the situation in which when an external force is applied to an aqueous dispersion of carbon black, the carbon black is settled by the external force and precipitates. It is defined as a parameter indicating the dispersion stability of the pigment.

That is, the ratio of the sediment residue is the amount of the sediment residue (M1) after centrifuging the aqueous dispersion of carbon black (concentration: 20 wt%) at a gravitational acceleration of 9.8 × 20,000 m / s ² for 30 minutes. And a value measured as a weight ratio (M1 / M0) to the weight (M0) of the carbon black in the aqueous dispersion before the centrifugation treatment.

If the precipitation residue ratio exceeds 10% by weight, the ejection stability is reduced and the ink head is clogged.

An aqueous ink is prepared by dispersing a carbon black pigment having these characteristics in a desired concentration in an aqueous medium. For example, a carbon black pigment is mixed and dispersed in water at a set concentration, and p
Adjust H to 6-11. Next, the remaining salts are separated and purified by electrodialysis or a separation membrane (reverse osmosis membrane, ultrafiltration membrane, loose RO, etc.). The residual salt concentration in the carbon black dispersion is preferably separated and refined so that the conductivity is less than 0.5 S / m when the carbon black content is 20 wt%. Further, in order to achieve dispersion stability as an aqueous ink, it is desirable to set the concentration of the carbon black pigment to 60% by weight or less.

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

Examples 1-3 and Comparative Examples 1-2 Oxidation treatment was carried out using carbon black having the characteristics shown in Table 1. In the oxidation treatment, 150 g of carbon black was added to 3000 cm ³ of a 1.5 N aqueous sodium persulfate solution, the reaction temperature was 60 ° C., the reaction time was 10 hours, and the stirring speed was 5 s.
^{Performed under} the condition of ^-1 . Next, the filtered carbon black is dispersed in pure water, neutralized with an aqueous sodium hydroxide solution, purified by an ultrafiltration membrane, and the remaining salts are separated.The carbon black is separated, washed with water and dried. Thus, a sample of a carbon black pigment was prepared. For these samples, carboxyl groups and hydroxyl groups were measured as hydrogen-containing functional groups on the surface of the carbon black particles, and the particle aggregation properties were measured. The results are shown in Table 1.

[0033]

[Table 1] Note: The amount of hydrogen-containing functional groups is the sum of carboxyl groups and hydroxyl groups.

Next, these carbon black samples were dispersed as pigments in water, and the dispersion was centrifuged at 9.8 × 14500 m / s ² and the water flow rate was 3.33 cm ³ / cm by a continuous centrifuge.
s (200 ml / min). The carbon black dispersion after the centrifugation treatment is subjected to an ultrafiltration membrane [Asahi Kasei Corporation
, AHP-1010, molecular weight cut-off 50000], the remaining excess salt was separated and purified, and then concentrated to adjust the carbon black dispersion concentration to 20% by weight. These dispersions were tested by the following method to evaluate their dispersion performance. Table 2 shows the obtained results.

(A) Measurement of viscosity: The sample was placed in a closed container, and the viscosity was measured after 1, 2, 3 and 4 weeks while maintaining the temperature at 70 ° C., and the dispersion stability upon heating was measured. Compared.
The viscosity was measured using a rotary viscometer [Yamaichi Electric Co., Ltd., VM
-100A-L].

(B) Average particle size of agglomerate Dupa 50% (n
m), measurement of the maximum particle size Dupa 99% (nm); the agglomerate particle size in the dispersion liquid after 1, 2, 3 and 4 weeks was maintained at a temperature of 70 ° C. and the cumulative frequency of the agglomerate particle size Create a distribution curve and calculate the average particle size Dup of agglomerate
a50% (nm) and the maximum particle size Dupa99% (nm) were measured and the changes were compared.

(C) Filterability: The dispersion was treated with 90φ filter paper (NO.
2) and membrane pore size 3μm, 0.8μm, 0.65μm, 0.45μm
A filtration test was carried out under a reduced pressure of 2666 Pa (20 Torr) using the above filter, and the passing amounts were compared.

(D) Print density: The dispersion was diluted to a carbon black concentration of 3% by weight, and XEROX 4024 paper was used as copy paper, and printing was performed with a # 6 bar coater, and a Macbeth densitometer (RD manufactured by Kolmorgen Co., Ltd.) was used. -927) was used to measure the optical density.

(E) Precipitation residue rate; dispersion was 9.8 × 200
The amount of sediment residue (M1) after centrifugation for 30 minutes at a gravitational acceleration of 00 m / s ² and the weight of carbon black in the aqueous dispersion before centrifugation (M0) were measured. Weight ratio
(M1 / M0) was determined.

[0040]

[Table 2]

From the results shown in Tables 1 and 2, the dispersions of the Examples in which the carbon black pigment satisfying the characteristic requirements regulated by the present invention was dispersed in water had a low precipitation residue rate, storage stability, filterability, It can be seen that the paper fixing density is excellent. On the other hand, it is recognized that the dispersions of the comparative examples in which any of the carbon black characteristics are deviated are inferior in filterability, paper fixing density and the like.

[0042]

As described above, according to the carbon black pigment of the present invention, it has excellent water dispersibility, and the aqueous ink obtained by dispersing this carbon black pigment in water has a storage stability and a paper fixing density. , Filterability and the like are provided in a well-balanced manner. Therefore, the carbon black pigment of the present invention is extremely useful as a carbon black pigment for aqueous inks used in a wide range of application fields such as aqueous inks 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.

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Claims (1)

[Claims] 1. A nitrogen adsorption specific surface area (N ₂ SA) 120 m ² / g or more, DBP absorption amount of 80 cm ^3/100 g or less and a specific tinting strength (Tint)
Oxidizes carbon black having a particle size of 120 or more to reduce the amount of hydrogen-containing functional groups on the surface of the carbon black particles to 3 μmol / m ^2.
The ratio Dupa50% / Dst between the Stokes mode diameter Dst (nm) of the carbon black aggregate and the average particle diameter Dupa50% (nm) of the agglomerate is 0.8 to 1.
1. The value of the average particle size Dupa50% (nm) of agglomerate is 20
Carbon black for water-based ink, characterized in that it has a particle aggregating property of not more than 60 (nm), a maximum agglomerate particle diameter Dupa of 99% (nm) of not more than 160 (nm) and a precipitation residue ratio of not more than 10 wt%. Pigment. However, Dst is the Stokes equivalent diameter of the maximum frequency in the aggregate Stokes equivalent diameter distribution by the centrifugal sedimentation method, and Dupa50% is the agglomerate particle diameter created from the degree of frequency modulation of scattered light by irradiating an aqueous dispersion of carbon black with laser light. The value of 50% cumulative frequency in the cumulative frequency distribution curve and the value of Dupa99% indicate the value of 99% cumulative frequency in the distribution curve. The precipitation residue ratio was 9.8 × 20 using an aqueous dispersion of carbon black (concentration: 20 wt%).
Weight ratio (M1 /) of the amount of the precipitate residue (M1) after centrifugation at gravitational acceleration of 000 m / s ² for 30 minutes and the weight (M0) of the carbon black in the aqueous dispersion before centrifugation. M0).