JP5126757B2 - Water-based ink - Google Patents

Description

The present invention relates to a water-based ink containing a carbon black pigment suitable for various uses including those for inkjet printers.

  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 with water molecules present on the surface of carbon black. Accordingly, when used as an aqueous black ink in which carbon black is dispersed in water as a black pigment, it is necessary to improve the water dispersibility by modifying the surface properties of the carbon black.

  It has long been known to improve the dispersion performance of carbon black in water by oxidizing the carbon black to form hydrophilic functional groups on the surface. A method of oxidizing with an aqueous solution of halite is disclosed, and Patent Document 2 discloses a method of oxidizing carbon black with low-temperature oxygen plasma.

  Also known is a method for producing a water-based ink (for example, Patent Document 3) for improving dispersibility in water by using a coupling agent or a surfactant, etc., on a lightly oxidized carbon black. It is difficult to stably maintain the dispersion performance for a long period of time due to the deterioration due to oxidation and decomposition of the surfactant and the like due to change and change over time.

  Carbon black excellent in water dispersibility is useful as a pigment for water-based black ink, and has recently attracted attention as a recording liquid for ink jet printers including writing instruments. For example, Patent Document 4 contains an aqueous medium, polyoxyethylene styryl phenyl ether, and carbon black having a primary particle size of 20 to 40 nm, a DBP oil absorption of 40 to 120 ml / 100 g, and a pH of 7.0 or more. A characteristic inkjet recording liquid has been proposed. This is intended to improve the storage stability of the recording liquid by using polyoxyethylene styryl phenyl ether as a dispersant.

  Patent Document 5 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 water and carbon black. In the method for producing an aqueous pigment ink containing, an aqueous pigment ink comprising: (a) a step of obtaining acidic carbon black; and (b) a step of further oxidizing the acidic carbon black with hypohalite in water. Patent Document 6 discloses an aqueous pigment comprising 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 using hypohalite. An ink manufacturing method is disclosed.

The present applicant has paid attention to the fact that the water dispersibility of carbon black is based on the fact that the amount of hydrophilic functional groups present at the contact interface between the carbon black particle surface and water molecules performs a large function, and the quality of the water dispersibility is accurately determined. As a new indicator for determining the amount of hydrophilic hydrogen-containing functional groups present per unit surface area of carbon black, that is, carbon black modified by oxidation treatment, among the hydrogen-containing functional groups present on the surface, carboxyl An easily water dispersible carbon black (Patent Document 7) was developed, characterized in that the total amount of groups and hydroxyl groups is 3 μeq / m ² or more per unit surface area.

Further, the present applicant has found that dispersibility such as easy dispersibility in water and dispersion stability is closely related to the aggregate form of the carbon black particles, and the nitrogen adsorption specific surface area (N ₂ SA) is 80 m ^2. / G to carbon black having a DBP oil absorption of 70 ml / 100 g or less, and a ratio of the Stokes mode diameter Dst (nm) of the aggregate to the average particle diameter Dupa (nm) of the agglomerate Dupa / An easily water dispersible carbon black (Patent Document 8) having a characteristic of a Dst value of 1.5 to 2.0 was proposed.

  Further, since the area of the pores on the surface of the carbon black particles does not significantly affect the dispersibility in water, the applicant of the present application focuses on the CTAB specific surface area in which the external specific surface area excluding the pore area is measured. Aqueous carbon black dispersion for carbon black which satisfies the relationship of CTAB specific surface area and DBP absorption amount CTAB × DBP <185... 1), 5 <DBP / CTAB <12. A body (Patent Document 9) was developed and proposed.

JP-A-48-018186 JP-A-57-159856 Japanese Patent Laid-Open No. 04-189877 Japanese Patent Laid-Open No. 03-097770 Japanese Patent Laid-Open No. 08-003498 JP 08-319444 A Japanese Patent Laid-Open No. 11-148027 JP-A-11-148026 JP 2003-183541 A

  In the subsequent research, the carbon black in which N-vinyl-2-pyrrolidone is cationically polymerized and polyvinylpyrrolidone is graft-bonded to a carboxyl group generated on the surface of the carbon black by liquid phase oxidation of carbon black, It has been found that the water dispersibility is further improved and the pigment has good adhesion to the printing medium.

That is, the present invention has been developed based on this knowledge, the purpose of which is excellent in water dispersion performance and high adhesion to print media , suitable for various applications including inkjet printers , An object of the present invention is to provide a water-based ink containing a carbon black pigment having improved storage stability and printing density.

The water-based ink of the present invention for achieving the above object is based on N-vinyl-2-pyrrolidone (hereinafter referred to as NVP) based on the carboxyl group on the surface of carbon black particles produced by liquid phase oxidation of carbon black. There polyvinylpyrrolidone (hereinafter referred to as PVP) by cationic polymerization Ri is Na are grafted, said homopolymer N- vinyl-2-pyrrolidone together bound comprises carbon black pigment formed by separation and purification This is a structural feature.

Aqueous-in key of the present invention, a carbon black as a base point a carboxyl group of the carbon black particle surface which is generated by liquid phase oxidation, which contains carbon black pigment PVP is grafted by NVP to cationic polymerization The carbon black pigment has excellent water dispersion performance and improves adhesion to the printing medium. Accordingly, water- based inks such as water dispersions in which the carbon black pigment is dispersed in water are remarkably excellent in storage stability and printing density, and are extremely useful for various applications including those for inkjet printers.

  In the oxidation treatment of carbon black, liquid phase oxidation by a wet method is applied in order to uniformly oxidize carbon black, and persulfate, perborate, percarbonate, etc. are preferably used as the oxidizing agent. Examples of the salt include ammonium salts and alkali metal salts.

  Liquid phase oxidation is performed by putting carbon black in an aqueous oxidizing agent solution and heating and stirring, and a functional group such as —COOH group or —OH group is formed on the surface of carbon black by oxidation treatment. The degree of the oxidation treatment is controlled by setting the concentration of the oxidizer aqueous solution, the amount of carbon black added, the treatment temperature, the stirring speed, the treatment time, and the like under appropriate conditions.

  After the oxidation reaction, the carbon black slurry is desalted and removed by using an ultrafiltration membrane, an RO membrane, an electrodialysis membrane, or the like. For example, when the carbon black solid content concentration in the carbon black slurry is 3 wt%, the desalting is preferably performed until the electric conductivity becomes 200 μS / cm or less.

  The desalted carbon black slurry is dried by removing moisture by a dryer, and the dried carbon black is pulverized by a pulverizer such as a jet mill, a cutter mixer, or a ball mill. The pulverization is preferably performed so that, for example, the amount passing through a sieve having an opening of 300 μm is 90 wt% or more.

By adding the pulverized carbon black to the NVP solution, heating and stirring, the carboxyl group on the carbon black surface and NVP are cationically polymerized according to the reaction formula shown in the following formula (1), and PVP is graft-bonded. . In this case, the mixing amount ratio of NVP and carbon black (CB) is such that the weight (g) of NVP is CB weight (g) × CB DB absorption (cm 3/100 g) × 0.01 × ³ times or more. Is set. This is because when the weight ratio of NVP is less than 3 times, stirring becomes difficult and it is difficult to react uniformly.

The reaction is preferably performed by heating to 30 to 100 ° C. at a stirring speed of 200 rpm or more. When the heating temperature is less than 30 ° C., the reaction between NVP and the carboxyl group on the carbon black surface does not proceed sufficiently, and the reaction is not achieved. Conversely, if it exceeds 100 ° C. and the reaction is activated monomers to each other than the reaction with the carboxyl group increases the production of homopolymer, the water dispersibility is Ru impaired.

In order to neutralize unreacted carboxyl groups during this reaction, alkali metal hydroxide (sodium hydroxide, potassium hydroxide) or ammonia, organic amines (for example, tetramethylammonium hydroxide, tetraethylammonium hydroxide), etc. Add to neutralize.
For example, when sodium hydroxide is used, an unreacted carboxyl group is converted to a sodium salt as shown in the following formula (2). After neutralization, heat treatment (95 to 100 ° C.) is performed to stabilize the alkali.

  The carbon black dispersion neutralized with NVP with cationic polymerization and alkali is separated and purified by reverse osmosis membranes, ultrafiltration membranes, loose RO and other separation membranes. To do. In this case, it is desirable to separate and purify the residual salt concentration of the carbon black dispersion solution until the concentration of carbon black is 20 wt% and is less than 5 mS / cm. Next, the carbon black dispersion slurry is dried by evaporating and removing moisture with a dryer, and the resulting carbon black is pulverized to produce the carbon black pigment for aqueous ink of the present invention.

In this way, NVP is cationically polymerized on all or a part of the carboxyl groups based on the carboxyl groups formed on the carbon black surface by liquid phase oxidation, and the carboxyl groups that have not been grafted are neutralized with an alkali. aqueous ink of the present invention, including mosquitoes over carbon black pigment PVP is grafted to the carbon black surface by the water dispersion performance is remarkably improved.

That is, the carbon black pigment dispersion obtained by dispersing in water, the dispersion performance is high dispersion stability is improved, also, hardly occurs even reaggregation of the carbon black in the dispersion. Therefore, water dispersibility, storage stability, filterability, ejection stability, viscosity, etc. are remarkably improved as a water-based ink, and the polymer bonded to the carbon black surface serves as an anchor for printing media such as paper fiber structures. Therefore, the print density can be improved and improved.

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

Example As a carbon black, Tokai Carbon Co., Ltd. Seast 9H (trade name) was used. 150 g of the carbon black was added to 3000 ml of a 2.0 N sodium persulfate aqueous solution and stirred at a temperature of 60 ° C. and a stirring speed of 300 rpm. While processing for 10 hours, liquid phase oxidation was performed. After the oxidation treatment, the carbon black dispersion is separated and purified by using an ultrafiltration membrane (AHP-1010 manufactured by Asahi Kasei Co., Ltd., molecular weight cut off 50000), and the carbon black concentration is concentrated to 10 wt%. Processed. The electric conductivity of the dispersion after concentration was 0.6 mS / cm. Thereafter, water was removed by evaporation of the dispersion using a dryer to obtain dried carbon black, which was then pulverized by a cutter mixer to produce a carbon black powder that passed through a sieve having an opening of 300 μm. The carboxyl group of the carbon black thus liquid-phase oxidized was 5.2 μmol / m ² .

The carboxyl group was measured by the following method.
About 2-5 g of carbon black was put in 50 ml of 0.976N sodium hydrogen carbonate aqueous solution, shaken for about 6 hours, carbon black was separated by filtration, and the titration test of the filtrate was performed to determine the carboxyl formed on the carbon black surface. The basis weight was measured. The carboxyl group amount was divided by the nitrogen adsorption specific surface area of the carbon black sample to determine the carboxyl group amount (μmol / m ² ) per unit surface area.

100 g of the oxidized carbon black powder was added to 300 g of NVP manufactured by Nippon Shokubai Co., Ltd., and reacted at a temperature of 40 ° C. and a stirring speed of 200 rpm for 5 hours. After the reaction, the pH of the reaction solution was measured and found to be 3.2. Therefore, a 0.5 mol / dm ³ sodium hydroxide aqueous solution was added to neutralize and heated to 95 ° C. After that, in order to remove the homopolymer and excess alkali ions generated during the grafting process, it was purified and concentrated with an ultrafiltration membrane (Daisen Membrane Systems Co., Ltd. FUS1582, fractional molecular weight 150,000) to obtain a carbon black dispersion Was adjusted to 20 wt%.

Comparative Example A carbon black dispersion obtained by liquid-phase oxidation of carbon black was filtered by the same method as in Examples, and the filtered carbon black was neutralized with a sodium hydroxide solution. The neutralized oxidized carbon black dispersion is separated and purified by an ultrafiltration membrane (AHP-1010 manufactured by Asahi Kasei Co., Ltd., molecular weight cut off 50000), and concentrated to obtain a carbon black dispersion concentration of 20 wt. % Adjusted.

  With respect to these carbon black aqueous dispersions, water dispersion performance, ink performance, and the like were measured by the following methods, and the obtained results are shown in Table 1.

(1) Heating stability;
The sample was put in a sealed container and kept at a temperature of 70 ° C. in an incubator, and the viscosity was measured every week. The viscosity was measured with a rotational vibration viscometer (manufactured by Yamaichi Electronics Co., Ltd., VM-100A-L).

(2) Particle size of particle aggregates;
For each sample subjected to the heating stability test, the dispersion was irradiated with laser light using a heterodyne laser Doppler particle size distribution analyzer (UPAmodel 9340 manufactured by Microtrack), and the dispersion was determined from the degree of frequency modulation of the scattered light. The particle diameter of the particle aggregate (agglomerate) inside is measured. The cumulative frequency distribution curve was created from the measured particle size, the 50% cumulative frequency value being the average particle size Dupa 50% (nm), and the 99% cumulative frequency value being the maximum particle size Dupa 99% (nm).

(3) pH;
About each sample which performed the heating stability test, it measured by JISZ8802.

(4) electric conductivity;
About each sample which performed the heating stability test, it measured by JISK0131.

(5) surface tension;
Each sample subjected to the heating stability test was measured according to ASTM D1590.

(6) Filterability;
A sample 200 g was added to 90φ No. A filtration test was conducted under reduced pressure of 2666.4 Pa using 2 filter papers and membrane filters having membrane pore diameters of 3 μm, 0.8 μm, 0.65 μm, and 0.45 μm, and the passing amount was measured.

(7) Print density;
The sample was diluted to a carbon black concentration of 4 wt%, printed on XEROX 4024 paper with a # 6 bar coater, and the optical density was measured using a Macbeth densitometer (RD-927 manufactured by Colmogen).

(8) Adhesion test;
An aqueous dispersion with a carbon black concentration diluted to 4 wt% is printed on photographic-quality paper using a # 6 bar coater. After printing, it is dried at room temperature for 24 hours, and then gold cloth is used with a Gakushin type abrasion resistance tester. Using # 3, pressurization was performed at a weight of 250 g. After the treatment, the printed surface was measured for optical density with a Macbeth densitometer. The closer this value is to the print density, the better the adhesion.

In Examples and Comparative Examples, the storage stability, filterability, surface tension, blackness, etc. are almost the same.
However, the grafting had an effect, and the difference between the printing density (OD value) and the adhesion test (OD value) was slight in the examples, but the OD value was remarkably lowered in the comparative example without grafting. .

According to the present invention, high adhesion to the print medium which has excellent water dispersion properties, very useful in various applications including ink jet printers, storage stability, an improved water-based ink print density Can be provided.

Claims (1)

Carbon black as a base point a carboxyl group of the carbon black particle surface which is generated by liquid phase oxidation, Ri Na N- vinyl-2-pyrrolidone is polyvinyl pyrrolidone grafted by cationic polymerization, wherein the N- vinyl-2- A water-based ink comprising a carbon black pigment obtained by separating and purifying a homopolymer in which pyrrolidones are bonded to each other .