Conventionally, in color copiers and color printers utilizing the electrophotographic method, a toner has been generally employed wherein a colorant is dispersed into resin particles.
Another toner is also employed wherein the colorant is adhered on the surfaces of resin particles. For example, Japanese Patent Publication Open to Public Inspection Nos. 63-23166, 63-2075 and 4-243267 propose methods wherein the colorant is mechanically or chemically adhered on. However, according to such methods, the surface of the toner is only colored and it is difficult to obtain the sufficient coloring effect. Furthermore, problems have been caused such that the desorption, etc. is caused on the surface and electrification properties vary, and in the above-mentioned system in which a heat roller is employed, the roller surface is stained.
Accordingly, the toner has been widely employed wherein the colorant is dispersed into the interior of a resin particle. As properties required for color tone of such the toner, not only color reproduction and image transparency for an overhead projector (hereinafter referred to as OHP) but also light fastness is enumerated in order to maintain consistently these properties.
The above-mentioned OHP image transmission rate means the OHP image transmission rate, and a degree of the variation in hue between the color of light transmitted through the OHP image and the color of light obtained by the reflection of the transmitted light on paper. When a toner comprising a pigment as the colorant is employed, good light fastness is obtained. However, on account of insolubility of the pigment, a dispersed particle having a diameter of tens of nm to hundreds of nm is formed and problems such as the decrease in the transparency and the hue variation in the color of transmitted light are caused. When the toner is employed which comprises a disazo pigment such as C.I. PIGMENT YELLOW 12, 13, 14, 16, 17, etc. described, for example, in Japanese Patent Publication Open to Public Inspection Nos. 2-210363, 62-157051 and 62-255956, and C.I. PIGMENT YELLOW 185 described in Japanese Patent Publication Open to Public Inspection No. 6-118715, the pigment is insoluble and tends to coagulate to form dispersed particle having a diameter of tens of nm to hundreds of nm through the secondary particle and further tertiary particle. As a result, problems such as the decrease in saturation and transparency of the OHP image are caused.
As the countermeasures against those, the pigment is previously treated by a flushing method, a master batch method, etc. and the resulting treated pigment has been employed. When employing the countermeasure, the increase in cost is not avoided because of the increase in the number of the manufacturing process. On the other hand, when the toner comprising a dye as the colorant is employed, the transparency of the OHP image is excellent because the dye is soluble and is fully dispersed. However, there is provided a problem such that the light fastness is inferior to that of the pigment. There have been known dyes such as C.I. SOLVENT YELLOW 162 described in Japanese Patent Publication Open to Public Inspection No. 3-276161, C.I. DIRECT YELLOW 160 described in Japanese Patent Publication Open to Public Inspection No. 2-207274, C.I. SOLVENT YELLOW 112 described in Japanese Patent Publication Open to Public Inspection No. 2-207273, etc. These dyes enable to obtain the OHP image having high transparency and no hue variation. However, as compared to the pigment, the light fastness is inferior and the consistent properties cannot be obtained for a long period of time.