Recently, electrophotographic image formation using an electrostatic image developing toner (hereinafter also denoted simply as toner) has been applicable to full-color prints as well as monochromatic prints as typified in conventional documentation. As such a full-color image forming apparatus can make printed sheets by the number as required on demand without printing plates, which are required in conventional printing, it has been employed mainly in a short-run printing field in which a small number of prints are often ordered (see for example, Japanese Patent O.P.I. Publication No. 2005-157314).
When making a full-color print used for catalogues or printed advertisements by using a toner, the toner is required to provide color reproduction so as to be faithful to an original image. In full-color image formation, yellow, magenta and cyan toner images are superimposed to reproduce a targeted color image and a color toner as a base is required to have superior color reproducibility in obtaining faithful color reproduction. Particularly in the catalogues or printed advertisements including a portrait image, a high chroma toner, which is capable of reproducing a color tone such as flesh tone faithfully, is required.
Particularly in recent years, opportunities have been increased which outputs a graphic image formed on a display employing a computer. The color gamut of an image formed according to a conventional color printing method or a conventional color electro-photographic method is far narrower than that of an image formed on a display, and therefore, it is difficult to output an image on the display on a paper to reproduce the color tone of the image as it is. Particularly, reproduction of a so-called secondary color, which is derived from superposition of the color toners, is difficult. In view of the above, study to increase the color gamut of the toner has been made in order to output on a paper an image with color gamut close to that of an image on a display.
Accordingly, study on various colorants has been made in order to achieve increased color gamut or enhanced color reproducibility of color toners. A colorant for a magenta color toner has been studied. As one of typical examples for a magenta toner, there is quinacridone pigment. A toner using quinacridone pigment is generally used, since it exhibits superior light resistance and has an appropriate color tone as a magenta color. However, the toner using quinacridone pigment, when used in combination with other color toners, is likely to produce color contamination, and is difficult to obtain a satisfactory color tone when a high chroma image on a display or computer graphics are output in which high requirements for color tone are made.
Attempts have been made in which quinacridone pigment is not used alone, but used in combination with a dye, whereby chroma is improved (see Japanese Patent O.P.I. Publication No. 2007-286148). Further, attempts have been made in which quinacridone pigment is used in combination with other pigments such as naphthol pigment (see Japanese Patent O.P.I. Publication No. 2006-267741), anthraquinone pigment (see Japanese Patent O.P.I. Publication No. 2006-154363) or a mixture of anthraquinone pigment and azo pigment (see Japanese Patent O.P.I. Publication No. 2007-140076).
However, these techniques are difficult to obtain high light resistance which quinacridone pigment inherently has or to stably maintain color tone for a long term. The toner employing quinacridone pigment as a colorant has problems in that it is difficult to reproduce the color tone of an image on the display as it is, and to obtain an image with high chroma and high light resistance maintained for a long time.