JP2009108215A - Pigment dispersion, method for producing the same and application of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pigment dispersion that can exhibit excellent storage stability, jetting stability and color developing properties of images and to provide a method for producing a pigment dispersion that can produce the pigment dispersion in a simple way without need of a process requiring a large amount of requisite energy such as dispersing by a mill. <P>SOLUTION: The pigment dispersion is produced by dispersing pigment particles in an aqueous medium wherein the pigment particles have a resin being present in at least a part of the surface thereof with an average particle size of not more than 80 nm and a maximum particle size of not more than 220 nm. The method for producing the pigment dispersion comprises spraying into (A) a supercritical fluid prepared by dissolving a pigment therein (B) a supercritical fluid prepared by dissolving a resin in the presence of an entrainer therein to precipitate a pigment particle having a resin present in at least a part of the surface thereof and then while rapidly expanding a supercritical fluid including the precipitated pigment particle, spraying the same into an aqueous medium. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a pigment dispersion useful for an aqueous ink used in ink jet recording, a method for producing the same, and an aqueous ink using the same.

  In general, dyes and pigments are known as pigments used in inks, but when inks are used as inkjet inks, there is a tendency that the fastness (image storage stability) of the output image is insufficient. Therefore, pigment ink is widely used.

  The pigment ink is usually obtained by dispersing a water-insoluble organic pigment in an aqueous medium. At that time, in order to secure performances such as storage stability, ejection stability, and image coloring property of the ink, it is important to make the dispersed pigment particles fine and make a stable dispersion. Therefore, as a method for obtaining such a pigment dispersion, an organic pigment having a particle size distribution in which 90% by weight of the pigment particles are less than 100 nm by high-speed mill dispersion using a dispersion medium having a particle size of 100 μm or less. There are proposed a method for obtaining a particle dispersion (Patent Document 1) and a method for obtaining fine pigment particles by depositing organic pigment particles in a supercritical fluid in the production of organic pigment ink (Patent Document 2). Yes.

Japanese Patent Laid-Open No. 9-176543 JP 2003-286427 A

  However, according to the method of Patent Document 1, although a dispersion of fine pigment particles can be obtained, a great amount of energy is required for dispersion, and a complicated process of separating the dispersion and beads is also required. In addition, since the organic pigment particle dispersion obtained by the method of Patent Document 1 coexists with a dispersant, it can be expected to have some effect of preventing reagglomeration of the finely divided pigment particles. Depending on the concentration of the dispersant and the like, it may be insufficient, and storage stability, ejection stability, and image colorability sufficient to be satisfactory when used as an inkjet ink may not be obtained. On the other hand, with the method of Patent Document 2, fine pigment particles can be obtained without requiring a great amount of energy as in mill dispersion, but by merely depositing pigment particles in water using a supercritical fluid, Even if pigment particles once precipitate as fine particles, they tend to agglomerate again in water, so when used in ink-jet inks, storage stability, ejection stability, and image quality that can be satisfactorily satisfied At present, no color developability is obtained.

  Accordingly, an object of the present invention is to provide a pigment dispersion that can exhibit excellent storage stability, ejection stability, and image colorability, and the pigment dispersion can be easily used without requiring a significant process such as mill dispersion. And a water-based ink using the pigment dispersion.

  The present inventors have intensively studied to solve the above problems. As a result, when the supercritical fluid (B) in which the resin is dissolved in the presence of the entrainer is sprayed in the supercritical fluid (A) in which the pigment is dissolved, the resin is in a state in which the pigment is taken in by the poor solvent action. Precipitation and formation of particles, and these particles are pigment particles in which a resin is present on at least a part of the surface, are less likely to cause re-aggregation after being once refined, and have storage stability and It has been found that a dispersion having excellent ink ejection stability when used as a water-based ink can be formed. Furthermore, when the supercritical fluid containing the pigment particles is sprayed in an aqueous medium while rapidly expanding, a strong shearing force is applied to the pigment particles, and the particles are further refined. The average particle size and the maximum particle size of the pigment particles are It has been found that a dispersion of pigment particles having such a particle size can provide good ink ejection stability and image colorability even when used as a water-based ink. The present invention has been completed based on these findings.

That is, the present invention has the following configuration.
(1) A dispersion in which pigment particles are dispersed in an aqueous medium, wherein the pigment particles are particles in which a resin is present on at least a part of the surface, and the average particle diameter is 80 nm or less, the maximum A pigment dispersion characterized by having a particle size of 220 nm or less.
(2) By spraying the supercritical fluid (B) in which the resin is dissolved in the presence of the entrainer into the supercritical fluid (A) in which the pigment is dissolved, the resin is present on at least a part of the surface. A method for producing a pigment dispersion, comprising: depositing pigment particles, and then spraying the supercritical fluid containing the pigment particles into an aqueous medium while rapidly expanding the pigment particles.
(3) The method for producing a pigment dispersion according to (2), wherein the substance forming the supercritical fluid is carbon dioxide.
(4) A water-based ink comprising the pigment dispersion according to (1) as an essential component.
(5) The water-based ink according to (4), which is used for inkjet recording.

According to the present invention, a pigment dispersion capable of exhibiting excellent storage stability, ejection stability, and image colorability and a water-based ink using the pigment dispersion do not require a process requiring a large amount of energy such as mill dispersion. It can be obtained by a simple method.
Specifically, since the pigment particles contained in the pigment dispersion of the present invention are pigment particles in which a resin is present on at least a part of the surface, re-aggregation after being once miniaturized is difficult to occur, and storage stability In addition, the ink ejection stability when used as a water-based ink is excellent. In addition, since the pigment particles have a fine particle diameter, the pigment dispersion of the present invention exhibits good ink ejection stability and image color development when used as a water-based ink. Furthermore, since the pigment particles have a narrow particle size distribution and uniformity, they are excellent in image color development and ink ejection stability when used as water-based ink.
In addition, since the water-based ink of the present invention exhibits excellent image coloring properties, a sufficient image density can be obtained even if the ink discharge amount is reduced. If the ink discharge amount can be reduced while securing the image density, it is possible to obtain an effect that the ink can be prevented from passing through the back side of the paper.
Furthermore, according to the method of producing the pigment dispersion of the present invention, according to the method of obtaining the pigment dispersion by dissolving the pigment particles once in the supercritical fluid and then precipitating the pigment dispersion, There is also an effect that it is possible to easily prevent the problem of foreign matter contamination that has been frequently generated in the necessary kneading step and dispersion (mill dispersion) step. In particular, when carbon dioxide is used as a supercritical fluid, the temperature and pressure during production can be set very low, so that it is highly environmentally and storable and requires no harmful organic solvents or wastewater treatment. There is also an advantage that the environmental load is small.

The pigment dispersion of the present invention is a dispersion obtained by dispersing pigment particles in an aqueous medium, and is characterized by the pigment particles.
That is, the pigment particles according to the present invention are particles in which a resin is present on at least a part of the surface. As a result, the pigment particles according to the present invention are unlikely to cause reaggregation in an aqueous medium, and as a result, the pigment dispersion of the present invention and the water-based ink using the pigment dispersion have good storage stability, Exhibits excellent ink ejection stability. Whether or not the resin is present on the surface of the pigment particles can be confirmed by, for example, electron microscope observation or elemental analysis, as will be described later in Examples.

  The pigment particles according to the present invention have an average particle size of 80 nm or less and a maximum particle size of 220 nm or less. Preferably, the average particle size is 70 nm or less and the maximum particle size is 180 nm or less. As described above, since the pigment particles according to the present invention are fine, the pigment dispersion according to the present invention and the water-based ink using the pigment dispersion exhibit good ink ejection stability and excellent image color development. In addition, the average particle diameter in this invention means a volume average particle diameter.

The pigment particles preferably have a narrow and uniform particle size distribution. Specifically, the particle size distribution index (PdI) is 0.2 or less, preferably 0.18 or less. In this case, when used as a water-based ink, it is possible to obtain further excellent image colorability and ink ejection stability.
In the present invention, the average particle size, the maximum particle size, and the particle size distribution index (PdI) of the pigment particles in the pigment dispersion can be measured, for example, by the method shown in the examples described later.

  The pigment in the present invention is not particularly limited as long as it is an organic pigment. For example, quinacridone pigments, quinacridone quinone pigments, dioxazine pigments, phthalocyanine pigments, anthrapyrimidine pigments, anthanthrone pigments, indanthrone pigments Pigment, flavanthrone pigment, perylene pigment, diketopyrrolopyrrole pigment, perinone pigment, quinophthalone pigment, anthraquinone pigment, thioindigo pigment, benzimidazolone pigment, isoindolinone pigment, azomethine pigment or Examples thereof include azo pigments.

  Although it does not specifically limit as resin in this invention, For example, a styrene acrylic resin, a fluorine resin, a polyester resin etc. are mentioned. Among these, styrene-acrylic resins and fluorine resins are particularly preferable.

  The content of the resin in the pigment particles is not particularly limited, but is preferably 15 to 30% by weight. If the amount of the resin is less than the above range, the reaggregation suppressing effect may be insufficient. On the other hand, if the amount is more than the above range, the resins may be easily aggregated.

  The aqueous medium in the present invention is preferably water, but is not limited thereto, and may be, for example, a mixed solvent of water and a water-soluble solvent such as alcohol.

  The content ratio of the pigment particles in the pigment dispersion of the present invention is not particularly limited. For example, when used in water-based ink, it is preferably 5 to 15% by weight.

The pigment dispersion of the present invention may contain other components other than the pigment, the resin, and the aqueous medium as necessary, as long as the effects of the present invention are not impaired. Examples of other components include substances and additives that form an entrainer described later and a supercritical fluid. These other components may be dissolved in the aqueous medium, dispersed and contained in the aqueous medium, or contained in the pigment particles together with the resin.
The pigment dispersion of the present invention as described above can be easily obtained by the method for producing the pigment dispersion of the present invention described later.

  The method for producing a pigment dispersion of the present invention comprises spraying a supercritical fluid (B) in which a resin is dissolved in the presence of an entrainer into a supercritical fluid (A) in which a pigment is dissolved, so It consists of a step of precipitating pigment particles partially containing a resin, and then a step of spraying the supercritical fluid containing the pigment particles into an aqueous medium while rapidly expanding.

Hereinafter, an embodiment of a production apparatus used in the production method of the present invention will be described with reference to FIG. 1 and a method for producing a pigment dispersion using the production apparatus will be described in detail.
In FIG. 1, a first agitation tank 1 and a second agitation tank 2 are equipped with stirrers 3 and 4 and pressure / temperature control devices 5 and 6, respectively, and form a supercritical fluid via high-pressure pumps 7 and 8. It is connected to a cylinder 9 containing a substance (supercritical fluid forming substance), and is brought into a supercritical state in each of the stirring tanks 1 and 2 by the pressure / temperature control devices 5 and 6 and the high-pressure pumps 7 and 8. A supercritical fluid is prepared. The first stirring tank 1 and the second stirring tank 2 communicate with each other through a valve 10, and the first stirring tank 1 has a water tank 12 containing an aqueous medium (for example, water) through a valve 11. Communicating with More specifically, the communication port of the water tank 12 communicating with the first stirring tank 1 via the valve 11 is provided in a portion filled with the aqueous medium, and the supercritical fluid in the first stirring tank 1 is in the aqueous medium. To be introduced. For the first stirring tank 1 and the second stirring tank 2, a pressure vessel is usually used.

  First, the supercritical fluid (A) in which the pigment is dissolved is prepared in the first stirring tank 1, and the supercritical fluid (B) in which the resin is dissolved in the presence of the entrainer is prepared in the second stirring tank 2, respectively. . Specifically, the pigment is put into the first stirring tank 1 and the resin and the entrainer are put into the second stirring tank 2 in advance, respectively, and then the valve 13 is opened to supply the supercritical fluid forming substance from the cylinder 9. Introduced into high-pressure pumps 7 and 8. The supercritical fluid forming substance is pressurized by the high-pressure pumps 7 and 8 and then supplied to the stirring tanks 1 and 2 via the opened valves 14 and 15, and the pressure / temperature control devices 5 and 6 , The temperature is controlled to become a supercritical state, that is, a supercritical fluid. Then, the pigment or resin and the entrainer are dissolved in each supercritical fluid by stirring with the stirrers 3 and 4 for a predetermined time to prepare the supercritical fluid (A) and the supercritical fluid (B).

  In the present invention, the supercritical fluid is a fluid in a so-called supercritical state that exceeds the critical temperature and critical pressure, and is slightly below the critical temperature and critical pressure, but the state change of the phase transition is extremely short. Therefore, it includes a fluid in a subcritical state that can be handled in substantially the same manner as the supercritical state. Examples of supercritical fluid forming substances that are well known include water, carbon dioxide, ethane, ethylene, propane, propylene, butane, hexane, methanol, ethanol, benzene, toluene, ammonia, pyridine, cyclohexane, and the like. It is done. The supercritical fluid forming substance (supercritical fluid) may be appropriately selected depending on the kind of the pigment or resin to be used, and two or more kinds may be used in combination as necessary. Moreover, the supercritical fluid forming substance may be different between the supercritical fluid (A) and the supercritical fluid (B), or may be the same.

  The supercritical fluid forming substance (that is, the supercritical fluid) used in the production method of the present invention is preferably carbon dioxide, among the above-mentioned ones. This is because carbon dioxide has a critical temperature of 31 ° C. and a critical pressure of 7.4 MPa, and can be handled under mild conditions of relatively low temperature and low pressure. Further, when carbon dioxide is used in the production method of the present invention, it can be easily recovered and recycled after finally obtaining a dispersion, so that it is possible to realize production that is environmentally friendly and has low energy costs. There is also.

  In the present invention, the entrainer is a dissolution accelerator that promotes the dissolution of the resin. Specifically, the entrainer may be appropriately selected according to the type of the resin. For example, ethanol, nitrogen, methane, ethane , Water and the like. Of these, ethanol is particularly preferable.

  The control pressure (set pressure) and the control temperature (set temperature) in the pressure / temperature control devices 5 and 6 may be appropriately set according to the supercritical fluid forming material to be used. For example, when carbon dioxide is used. Is preferably 10 to 20 MPa and a temperature of 32 to 50 ° C. for the temperature and pressure in the first stirring tank 1 (pressure / temperature control device 5). For the temperature control device 6), a pressure of 15-20 MPa and a temperature of 32-55 ° C. are preferred. Further, the stirring by the stirrers 3 and 4 may be performed to such an extent that the previously charged contents can be sufficiently dissolved. Usually, the stirring may be performed at a stirring speed of 150 to 300 rpm for 30 to 60 minutes.

  Next, the valve 10 is opened, and the supercritical fluid (B) is sprayed onto the supercritical fluid (A). Here, in order to perform spraying vigorously, the pressure in the first stirring tank 1 is preferably set lower than the pressure in the second stirring tank 2. In this way, when the supercritical fluid (B) in which the resin is dissolved in the presence of the entrainer is sprayed in the supercritical fluid (A) in which the pigment is dissolved, the concentration of the entrainer is drastically decreased and the solvent becomes poor. The resin is precipitated by the action, but at this time, the resin is precipitated in a state where the pigment is taken in, so that pigment particles in which the resin is present on at least a part of the surface are precipitated.

  Next, the valve 11 is opened, and the supercritical fluid in the first stirring tank 1 containing the precipitated pigment particles is sprayed while rapidly expanding into the aqueous medium in the water tank 12. Here, in order to spray while rapidly expanding, the pressure in the first stirring tank 1 may be set higher than the pressure in the water tank 12, and the water tank 12 is preferably open to atmospheric pressure. In this way, when the supercritical fluid is sprayed while rapidly expanding, a strong shearing force is imparted to the pigment particles in the supercritical fluid. Fine spherical fine particles having the aforementioned average particle size and maximum particle size are dispersed in the aqueous medium.

  In the method for producing a pigment dispersion of the present invention, for example, among additives described below, a charge control agent, wax, or a substance that becomes a precipitation nucleus at the time of pigment particle precipitation is added to a supercritical fluid (A ) And supercritical fluid (B).

  The water-based ink of the present invention contains the above-described pigment dispersion of the present invention as an essential component. The water-based ink of the present invention is obtained by obtaining the pigment dispersion by the above-described production method of the present invention, and then distilling off the substance used as the supercritical fluid, and if necessary, various conventionally known various inks that are applied to the normal ink. It is obtained by adding an additive.

  Examples of additives include charge control agents, waxes, dispersants (surfactants, etc.), drying inhibitors, penetration enhancers, ultraviolet absorbers, viscosity modifiers, surface tension modifiers, antifungal agents, and rust inhibitors. , PH adjusters, antifoaming agents and the like. These additives may be only one kind or two or more kinds. The amount used may be appropriately set according to the performance and the like within a range not impairing the effects of the present invention.

The water-based ink of the present invention can be applied to various recording methods, but is particularly preferably used for inkjet recording.
In the preferred water-based ink of the present invention, when an image is output on plain paper using the ink jet recording method, the relationship between the discharge amount L (pl) and the print density I satisfies the following formula.
ln (L) -1.3 ≦ I ≦ ln (L) −0.93
It can be said that the water-based ink satisfying the above formula has very good dispersibility of the pigment, good image coloring, and high image density with respect to the ink discharge amount.

EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in detail, this invention is not limited to a following example.
The physical properties of the pigment dispersion and the ink performance were evaluated by the following methods.

  <Volume Average Particle Diameter, Particle Size Distribution Index (PdI), and Maximum Particle Diameter of Pigment Dispersion in Pigment Dispersion> Measurement was performed using a dynamic light scattering particle size distribution analyzer (manufactured by HORIBA). If the particle size distribution index (PdI) was 0.2 or less, it was judged that the distribution was narrow.

  <Image Density> Using a reflection densitometer (manufactured by Gretag Macbeth), the image density when the ink discharge amount was 14 pl was measured.

Example 1
A pigment dispersion was produced using the production apparatus shown in FIG. Specifically, the first stirring tank 1 and the second stirring tank 2 of the manufacturing apparatus include stirrers 3 and 4 having U-shaped blades, pressure / temperature control devices 5 and 6, and a heater (not shown). A 700 mL pressure vessel built in each was used.
The first stirring tank 1 was charged with 10 g of magenta CI Pigment Red 238 pigment, and the second stirring tank 2 was charged with 100 g of styrene-acrylic resin (Mw: 4500, softening point: 140 ° C.) and 50 mL of ethanol as an entrainer. Next, the valve 13 is opened and the carbon dioxide contained in the cylinder 9 is introduced into the high-pressure pumps 7 and 8 and sufficiently pressurized. Then, the pressure / temperature control devices 5 and 6 are opened via the opened valves 14 and 15. The first stirring tank 1 was supplied at a temperature of 40 ° C. and a pressure of 13 MPa, and the second stirring tank 2 was supplied at a temperature of 40 ° C. and a pressure of 15 MPa. Then, both tanks 1 and 2 were stirred at a stirring speed of 1000 rpm for 0.5 minutes to dissolve the respective contents. Thereafter, the valve 10 is opened, and the contents of the second stirring tank 2 are sprayed into the first stirring tank 1 (at this time, the pressure in the first stirring tank 1 is 13 MPa, the pressure in the second stirring tank 2 is 15 MPa), pigment particles including a styrene-acrylic resin were precipitated. Next, the valve 11 is opened, and the contents of the first stirring tank 1 (supercritical fluid containing the precipitated pigment particles) are sprayed into 1000 mL of water accommodated in the water tank 12 while rapidly expanding, whereby the pigment dispersion is obtained. Obtained.

The presence of a resin (styrene-acrylic resin) on the surface of the pigment particles in the obtained pigment dispersion means that the particles are taken out from the pigment dispersion, dried and then subjected to electron microscope observation and elemental analysis. Confirmed by that.
Table 1 shows the physical properties of the obtained pigment dispersion and the results of performance evaluation when the pigment dispersion is used as an inkjet ink.

(Example 2)
Using the same production apparatus as in Example 1, 10 g of yellow CI pigment yellow 180 pigment was added to the first stirring tank 1, 100 g of fluororesin (softening point: 120 ° C.) and 70 mL of ethanol as an entrainer were added to the second stirring tank 2. In the same manner as in Example 1, except that carbon dioxide was supplied so that the first stirring tank 1 had a temperature of 40 ° C. and a pressure of 18 MPa, and the second stirring tank 2 had a temperature of 40 ° C. and a pressure of 20 MPa. A dispersion was obtained.

The presence of resin (fluorine resin) on the surface of the pigment particles in the obtained pigment dispersion was confirmed in the same manner as in Example 1.
Table 1 shows the physical properties of the obtained pigment dispersion and the results of performance evaluation when the pigment dispersion is used as an inkjet ink.

(Comparative Example 1)
A pigment dispersion was obtained by the melt-kneading pulverization method using the same material as in Example 1. That is, 10 g of magenta CI Pigment Red 238 pigment and 100 g of styrene-acrylic resin (Mw: 4500, softening point: 140 ° C.) were put into a Henschel mixer and mixed for 10 minutes. The resultant mixture was melt kneaded and dispersed at 170 ° C. using “MOS140-800” manufactured by Mine Co., Ltd., and then the obtained red melt kneaded material was further pulverized with a jet mill to obtain pigment particles. The pigment particles were put into 100 mL of water and dispersed with a ball mill to obtain a pigment dispersion.
Table 1 shows the physical properties of the obtained pigment dispersion and the results of performance evaluation when the pigment dispersion is used as an inkjet ink.

(Comparative Example 2)
Using the same material as in Example 2, a pigment dispersion was obtained by a melt-kneading pulverization method. That is, 10 g of yellow CI pigment yellow 180 pigment and 100 g of a fluororesin (softening point: 120 ° C.) were put into a Henschel mixer and mixed for 10 minutes, and the resulting mixture was mixed with Niedix (Mitsui Mine Co., Ltd. “ MOS 140-800 ") was melt-kneaded and dispersed at 140 [deg.] C., and the obtained yellow melt-kneaded product was further pulverized with a jet mill to obtain pigment particles. The pigment particles were put into 1000 mL of water and dispersed with a ball mill to obtain a pigment dispersion.
Table 1 shows the physical properties of the obtained pigment dispersion and the results of performance evaluation when the pigment dispersion is used as an inkjet ink.

It is a schematic diagram for demonstrating one Embodiment of the manufacturing apparatus used for the manufacturing method of the pigment dispersion of this invention.

Explanation of symbols

1: first stirring tank 2: second stirring tank 3, 4: stirrer 5, 6: pressure / temperature control device 7, 8: high pressure pump 9: cylinders 10, 11, 13-15: valve 12: water tank

Claims (5)

  A dispersion in which pigment particles are dispersed in an aqueous medium, wherein the pigment particles are particles in which a resin is present on at least a part of the surface, the average particle diameter is 80 nm or less, and the maximum particle diameter is A pigment dispersion characterized by being 220 nm or less.   Pigment particles in which the resin exists in at least a part of the surface by spraying the supercritical fluid (B) in which the resin is dissolved in the presence of the entrainer into the supercritical fluid (A) in which the pigment is dissolved And then spraying the supercritical fluid containing the pigment particles into an aqueous medium while rapidly expanding the pigment dispersion. The method for producing a pigment dispersion according to claim 2, wherein the substance forming the supercritical fluid is carbon dioxide.   An aqueous ink comprising the pigment dispersion according to claim 1 as an essential component.   The water-based ink according to claim 4, which is used for ink jet recording.

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