Pigments generally exhibit vivid color tone and high coloring power, and they are widely used in many fields. Examples of use applications in which pigments are used include paints, printing inks, electrophotographic toners, ink-jet inks, and color filters. Pigments are thus important compounds indispensable in everyday life at the present time. General properties, classification by use, and other aspects of pigments are described in, for example, “Dispersion Stabilization of Pigment and Surface Treatment Technique/Evaluation”, Technical Information Institute Co., Ltd., 2001, pp. 123-224. In particular, examples of pigments that require high performance, and that are of particular importance in practical use, are ink-jet ink pigments, and color filter pigments.
As the coloring material for ink-jet ink, dyes have been used, but they have points to be improved as to water resistance and light resistance. To improve them, pigments have come to be used. Images obtained from pigment inks have remarkable advantages of superior light resistance and water resistance compared with images obtained from dye-based inks. However, pigments are not easily formed uniformly and pulverized into fine-particles of a nanometer size (i.e., monodispersion), which can permeate pores in the surface of paper, and hence pigments in the image are poor in close bindness to the paper. Consequently, improvement thereof has been demanded.
An exemplary important application of a pigment includes a color filter used for a CCD sensor. In particular, recently, there is a demand for a reduction in thickness of the color filter along with an increase in the number of pixels of a digital camera. An organic pigment is used in the color filter, and a thickness of the color filter substantially depends upon a diameter of a particle of the organic pigment. Therefore, even in this field, there is a demand for producing fine particles that are stable in monodispersion at a nanometer size level under control.
A method of producing organic pigment fine particles is roughly classified into a brake-down method of producing fine particles from a bulk substance by crushing or the like, and a build-up method of producing fine particles by particle growth in a vapor phase or a liquid phase (see “Experimental Chemical Lecture, 4th Edition” edited by the Chemical Society of Japan (Maruzen Co., Ltd.), vol. 12, pp. 411-488, etc.). In general, the breakdown method (crushing method) is widely used. However, according to this method, it is difficult to obtain an organic substance with a nanometer size level, the productivity is remarkably low, and substances applicable in this method are limited.
As a procedure for compensating for the above disadvantage, encapsulating pigment fine particles with resin has been proposed. (Technical Information Association “Preparation and Dispersion/Aggregation Control of nano-fine particles and evaluation thereof” 2003, Ch. 1, Sec. 4, (various synthesis methods of polymer-nano-fine particles and problems); Kengo YASUI, Kozue SUNOUCHI (Dainippon Ink and Chemicals, Inc.) “Increase in Resolution with Aqueous Pigment Ink” DIC TECH REV, 2004, No. 10, pp. 11-18; Hiroshi HARADA, Sadahiro INOUE (Dainippon Ink and Chemicals, Inc.) “Microcapsulated Pigment Jet Ink” DIC Tech Rev, 2003, No. 9, pp. 1-7; Masao TANAKA (Dainippon Ink and Chemicals, Inc.) “Micro/Nano Capsul Technique, Microcapsulation of Pigment, and Application to Ink Jet” Industrial Material, 2004, Vol. 52, No. 6, pp. 42-45, etc.). According to this method, an attempt is made so as to enhance water resistance and light resistance, and to increase a gloss regarding printing. However, the recent demand for the increase in resolution has not been satisfied sufficiently. This is because those methods depend upon the breakdown method, and fine particles cannot be regulated to a desired minute size. Further, this method requires to fine particles by grinding as much as possible, and to adsorb resins to the particles completely, which takes a great amount of time and energy in terms of the productivity.
On the other hand, it is conducted that a pigment is dispersed with a dispersant. In this case, merely by allowing the dispersant to adsorb to the surface of the pigment, long-term stability, water resistance, and viscosity stability are insufficient. Further, when such the pigment is used as ink-jet ink, due to the strong shear force at a time of the discharge from a nozzle, the dispersant desorbs from the pigment, deteriorates, and further clogs the nozzle, etc.
As to the above-mentioned subjects, a number of methods have been studied. Examples of the methods include chemical reforming of the surface of a pigment by oxidation or the like, and a chemical bond formation or the like of a pigment surface and a dissoluble group by grafting or the like (see JP-A-H 10-316909, “JP-A” means unexamined published Japanese patent application.). However, according to any of those methods, it is difficult to effect a reaction completely. As a result, quality of an obtained dispersion liquid can not be improved. The increase in a content of a dispersant can also be considered so as to ensure the stability. However, there arise the decrease in printing performance and the excessive increase in viscosity.
Further, a method of adding a polymerizable surfactant to a pigment obtained by a breakdown method, followed by emulsion polymerization (see JP-A-2005-97517). However, in the emulsion polymerization disclosed herein, in order to allow a monomer completely to adsorb to the surfaces of pigment fine particles, a great amount of monomer is required. This method also depends upon the breakdown method, so that the size of a pigment is large, and it is necessary to make the pigment uniform in a finer size range. Further, according to this method, the energy consumption amount is large, since a crushing step is required, and it is necessary to switch the step facility so as to effect a polymerization reaction, resulting in low productivity.
Contrary to this, recently, a method of producing organic fine particles with a nanometer size by a build-up method has been studied. For example, there is a production method by a micro-chemical process (see European Patent Publication No. 1516896 A1). However, the recent high demand with respect to ink-jet ink etc. is not satisfied, and there is still a demand for further improvement and development.