Metal complex pigments are excellent in heat resistance and weather resistance, hence they have attained increased importance in recent years. Conventionally, metal phthalocyanine compounds, which are typical metal complex pigments, are very useful as pigments in the field of coloring material industries, and many investigations have been conducted on such compounds for a long time. Phthalocyanine pigments can exhibit vivid color tone and high coloring (tinctorial) power, and they are widely used as cyan colorants in many fields. Examples of use applications in which the pigments are used include paints, printing inks, electrophotographic toners, ink-jet inks, and color filters. The pigments are important compounds indispensable in everyday life at the present time. Practically particularly important applications of phthalocyanine pigments as color materials (colorants), which need to have high performance in particular, include pigments for inkjet inks or color filters.
As the coloring material for ink-jet ink, dyes have been used, but they have drawbacks as to water resistance and light resistance. To overcome the drawbacks, pigments have come to be used. As cyan pigments, use may be mainly made of copper phthalocyanine pigments. Images obtained from pigment inks have remarkable advantages of superior light resistance and water resistance compared with images obtained from dye-based inks. However, the former images have the problems that the pigment is not easily formed uniformly or pulverized into fine-particles of a nanometer size, which can permeate pores in the surface of paper, and then the pigment in the image is poor in contact or adherence property to the paper.
With an increase in the number of pixels in digital cameras, there is a need for a color filter used in a CCD sensor to be made thinner. Some color filters use organic pigments (for example, metal complex pigments, such as copper phthalocyanine compounds as cyan pigments and nickel azo compounds as yellow pigments are used). Since the thickness of the filter depends largely on the particle diameter of the organic pigment, there has been a need to produce stable fine-particles of a nanometer-size level.
In some fields other than the field of coloring material industries, phthalocyanine compounds (fine particles) are used in such fields in which semiconductivity or photoconductivity of the compounds are utilized. For example, investigations have been conducted on electrophotographic photoconductors or laser printer photoconductors, based on the photoconductivity of metal-free phthalocyanines, or a variety of metal phthalocyanines, such as copper phthalocyanine, vanadyl oxyphthalocyanine, aluminum chlorophthalocyanine, zinc phthalocyanine, hydroxygallium phthalocyanine, and titanyl phthalocyanine.
Some types of metal phthalocyanines have redox power, and thus attention has focused on their application to catalysts. Since phthalocyanine compounds have multiple functions as mentioned above, not only metal-free or copper phthalocyanines but also various types of metal phthalocyanines, especially fine particles thereof, are increasing in importance (see “Pigment Dispersion and Stabilization and Surface Treatment Techniques and Evaluation,” 2001, pp. 123-224, published by Technical Information Institute Co., Ltd., Japan; and Masato Tanaka and Shouji Korna, “Phthalocyanines: Their Basic Physical Properties and Application to Functional Materials,” 1991, pp. 55-124, published by Bun-Shin, Japan).
In general, the methods to produce pigment fine particles are roughly classified into the breakdown method, in which fine particles are produced from a bulk material by pulverization or the like, and the build-up method, in which fine particles are produced by particle-growth from a gas phase or liquid phase (see “Experimental Chemical Lecture, 4th Edition,” edited by the Chemical Society of Japan (Maruzen Co., Ltd.), vol. 12, pp. 411-488, 1993). The pulverizing method, which has been widely used hitherto, is a fine-particle-producing method having high practicability, but it has various problems, such as that its productivity is very low in producing pigment particles of nanometer size, and that the materials to which the method can be applied are limited. In recent years, investigations have been made to produce pigment fine-particles of nanometer size by the build-up method.
As one of the build-up method, a method called a reprecipitation method is proposed (see JP-A-6-79168 (“JP-A” means unexamined published Japanese patent application)). The reprecipitation method produces fine particles of an organic material by bringing a solution of an organic material dissolved in a good solvent, into contact with its poor solvent, to precipitate fine particles of the organic material. This method is effective as an efficient method of producing particles of nanometer size. However, it is difficult to find good solvents for pigments that are basically sparingly soluble in solvents, and it is therefore difficult to synthesize desired pigment fine particles at a high concentration. A method has been recently studied in which an amide-type solvent is used as a good solvent, to obtain pigment fine particles (see JP-A-2004-91560). However, the concentration of the fine particles of a metal phthalocyanine, which is a metal complex pigment obtained by the method, is considerably low, and therefore there is need for development of a new method.
As another method, a method, in which fine particles are produced using a micro-jet reactor in an acid-paste method that has been used for refining a metal phthalocyanine, is known (see JP-A-2002-155221). The acid-paste method comprises the steps of: dissolving a crude reaction product in a strong acid (generally concentrated sulfuric acid), with the benefit of high solubility of copper phthalocyanine or the like in a strong acid; and pouring the resulting solution into ice water, to precipitate particles. However, in this method, a highly oxidizing acid is used, and therefore, new decomposable impurities, which causes a degradation of the performance of the product for use in electronic materials, catalysts, or the like, become mixed in the solution, although their amount is very small (see P. A. Barrett, D. A. Frye, R. P. Linstead, “J. Chem. Soc.,” 1938, 1157). This problem cannot be solved by using a micro-jet reactor, and therefore, further improvement in the method is desired.
As to a method of producing a metal complex, for example, a metal phthalocyanine, there is an indirect synthetic method using an alkali metal phthalocyanine as a synthetic precursor. This method comprises the steps of: first, synthesizing an alkali metal phthalocyanine that is highly pure and relatively easy to dissolve in an organic solvent; dissolving or dispersing it in an organic solvent; and allowing it to react with salts of transition metals, such as copper, dissolved or dispersed in an organic solvent, to precipitate a transition metal phthalocyanine. This method use either (i) dilithium phthalocyanine or (ii) dipotassium phthalocyanine. These methods are further described in below.
(i) Metal-free phthalocyanines are also hardly soluble compounds in organic solvents, although they have slightly better solubility in an organic solvent than a phthalocyanine of a transition metal, such as copper. When alkali metal phthalocyanines are brought into contact with a solvent having acidity (for example, water or an alcohol), they are converted into hardly soluble metal-free phthalocyanines, and precipitated resultantly. Among the alkali metal phthalocyanines, however, dilithium phthalocyanine is relatively stable and soluble in absolute ethanol. Based on such properties, dilithium phthalocyanine can be used in the synthesis of transition metal phthalocyanines, through reaction with transition metal salts in absolute ethanol (see P. A. Barrett, D. A. Frye, R. P. Linstead, “J. Chem. Soc.,” 1938, 1157).
Actually, however, it is too much to say that dilithium phthalocyanine is easily soluble. Specifically, because the reaction does not proceed in a uniform solution; rather, in actually, it converts dilithium phthalocyanine dispersed in a liquid into metal phthalocyanine, and considerable time is required to complete the reaction. Therefore, the reaction with alcohol (solvent) is suppressed in an environment where the reaction with the transition metal ion is fast. If the reaction time becomes longer because of scaling up or the like, however, metal-free phthalocyanine by-products can be obtained in some cases.
(ii) In alcohols, dipotassium phthalocyanine is rapidly converted into metal-free phthalocyanine. In alcohols, therefore, it cannot be reacted with a transition metal salt. A method in which dipotassium phthalocyanine is allowed to react with a transition metal salt in a hydroxyl-free organic solvent is proposed (see JP-A-61-190562). A method of purifying metal-free phthalocyanine, which comprises the step of: heating dipotassium phthalocyanine together with an ether-series solvent, such as a crown ether or diglyme, dimethyl sulfoxide, and dimethylformamide, so as to form a soluble complex has been studied (U.S. Pat. No. 4,197,242). By combining the aforementioned methods, and based on the method using diglyme, a method of synthesizing a metal phthalocyanine, which comprises the steps of: preparing a solution of a dipotassium phthalocyanine bis(methoxyethyl)ether complex, and allowing it to react with transition metal salts has been thought. However, in this method, dipotassium phthalocyanine is uniformly dissolved, but the transition metal salts are dispersed in diglyme. Eventually, this method is a reaction using a dispersed material, and, in this point, this method is the same as the method described in (i).
As mentioned above, the above indirect synthetic method, in which the alkali metal phthalocyanine is reacted with the transition metal salt, is a reaction in which one of these compounds is in a nonuniformly dispersed state. It is therefore difficult to use this method for synthesis serving to attain particle size control at the same time. There has been no example of studies that disclose the production of fine particles to control particle sizes by using this synthesis method.