Methods for forming or displaying images by the subtractive color process using printing inks, coating materials, toner, ink-jet inks, or the like are generally performed by the combination of three primary colors yellow (Y), magenta (M), and cyan (C). Each pigment for colorants of such three primary colors is required to have the following characteristics:    1) high color rendering performance indicated by hue, chroma, lightness, and the like;    2) excellent manufacturability; and    3) image quality and durability on printing paper.
Firstly, as for the color rendering performance, when images represented on a color display are printed on a color printer, the images tend to be less sharp because the color reproduction range of the printer ink (YMC color space) is narrower than the color reproduction range on the color display (RGB color space). One of the causes thereof is the low lightness or chroma of pigments, and attempts to improve the lightness or chroma of pigments have been widely made.
Secondly, as for the manufacturability, pigments need to satisfy various physicochemical characteristics for preparing inks or toner. For example, it is necessary that a pigment dispersion should have suitable viscosity. Furthermore, in the case of the preparation of toner by an emulsion polymerization method as disclosed in Patent Document 1, appropriate surface characteristics are required such that monomers can be adsorbed on pigment surfaces.
Thirdly, as for the quality on printing paper, light resistance, strong affinity toward resin binders for obtaining physical strength (e.g., frictional strength), and the like are also required in addition to color imaging performance.
To respond to these needs, improvement in pigments for each hue has heretofore been made. For example, as for yellow pigments, many azo pigments have been developed because there is a wide option for preparing them and, accordingly, the control of their color rendering properties such as hue is relatively easy, and further manufacturability, etc. is also good. Among others, various azo pigments carrying a plurality of azo groups in their molecules have been developed, since such azo pigments, in addition to a strong color force, correspondingly have a high molecular weight, resulting in enhanced aggregation properties, and further are also preferable for light resistance, etc. For such pigments, on the ground that the absorption spectrum width can be narrowed and the sharpness can be easily improved, many pigments having a molecular structure where the intramolecular azo groups are present at symmetric positions have been developed. For example, Patent Document 2 describes C.I. Pigment Yellow 155.
The molecular structure as seen in C.I. Pigment Yellow 155 is preferred for conferring high color rendering properties, but is not always preferable in light of manufacturability or image intensity. For example, when the azo pigment C.I. Pigment Yellow 155 having two azo groups at symmetric positions as mentioned above is dispersed in a solvent, its viscosity becomes very high even in the absence of a binder. Therefore, uniform stirring is difficult when attempting emulsion polymerization by adding monomers to an aqueous dispersion of the pigment. As a result, it is difficult to obtain toner having uniform performance.
Since the viscosity is high even in resin-free systems as described above, the viscosity is further increased when a binder resin is added for the purpose of preparing, for example, ink jet inks, resists for color filters, or the like. Such a highly viscous pigment dispersion complicates appropriate dispersion (adjustment of pigment particle size and its distribution) and requires a large dispersion apparatus, dispersion procedures for a long time, etc., putting a heavy burden onto manufacture of products using the pigment. As further problems, too high a viscosity of pigment dispersions may cause difficult ink discharge at a ink jet head; or in the case of poor dispersion, coarse pigment particles may remain, which cause poor image quality or clogging of ink jet nozzles.
Against this backdrop, various approaches such as the change of the type of solvents or the lowering of pigment concentration have been carried out in order to reduce the viscosity of pigment dispersions. However, none of them are fully satisfactory under the present circumstances.
As mentioned above, regarding C.I. Pigment Yellow 155 excellent in color rendering properties, there is a strong need to reduce the viscosity of a dispersion thereof. If C.I. Pigment Yellow 155 that brings about a low viscosity of its dispersion can be prepared, this pigment can replace the conventional pigments for electrophotographic toner or ink-jet inks and enable more colorful color images to be obtained at low cost. Furthermore, if C inks among the C, M, Y, and K inks in the conventional process inks can be replaced with ones having high chroma, many applications such as colorants for high-chroma second colors, for example, green color using such inks, are expected.
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