A cadmium pigment has been the only known orange colored pigment. The cadmium pigment is the pigment of a solid solution consisting of cadmium sulfide (CdS) as main component, and appropriate proportions of zinc sulfide (ZnS) cadmium selenide and mercury sulfide (HgS). The cadmium pigment has been widely used, since it has been considered as a coloring agent of high clearness for which there is virtually no substitute (Tsunashima, et al.: Latest Applied Pigment Technology, peLge 24, C.M.C. Co., Ltd.). The use of the cadimium pigment has, however, been discontinued almost completely, since it began to be avoided when the cadmium pollution in the environment became a problem.
On the other hand there have been sold, or proposed pigments which comprise a flaky substrate coated with iron oxide, or a metal oxide containing iron oxide, as described below, but no pearl pigment having an orange color has been developed as yet.
The inventor of this invention previously disclosed a transparent color pigment which comprises mica particles coated thereon with iron oxide and/or a hydrate thereof (see Japanese Patent Publication No. Hei 1-60511). This pigment is, however, an orange pigment having a high degree of transparency and a low hiding power, since its observation by a scanning electron microscope reveals that iron oxide is composed of needle crystals having a diameter of as large as 0.1 to 0.2 micron, and causing, therefore, large scattering of reflected light.
The pearl pigments which comprise mica, or like particles coated with iron oxide are made and sold for practical use by Merck as a series of products under the tradename "IRIODIN 500". They are, however, not pearl pigments having an orange color of high chroma. In connection with two series of pigments comprising mica as a flaky substrate with two coated system of titanium oxide and ferric oxide, respectively, as a coating metal oxide, Thurn-Muller, et al. report the values "a" (indicating red on the +side and green on the--side), and "b" (yellow on the +side and blue on the--side) of the Hunter color tone produced by each such pigment, and by varying with the amount of the coating (or its optical thickness) (Kontakte, No. 2, pages 35-43, 1992). It is understood that the most desirable orange color having the highest chroma is obtained when the maximum values of "a" and "b" coincide with each other. According to this literature, the pigments which comprise mica coated with titanium oxide show only interference colors, but as is obvious from the variations of the values "a" and "b", there is no coincidence between the maximum values of "a" and "b" with respect to any interference color, and there does not exist any range in which an interference orange color of high chroma is produced. Studies have also been made of the pigments which comprise mica coated with ferric oxide, and include the measuring result of variations of the each "a" and "b" value of combined interference color with absorption characteristic of ferric oxide (complementary colors to absorption colors). But the colors vary from bronze to copper, and from copper to sienna, and they fail to show any coincidence between the maximum values of "a" and "b", as the pigments in the system of containing titanium oxide.
There has also been proposed a red pigment having an orange to bluish red color which comprises sheet-like iron oxide particles, or sheet-like particles coated with iron oxide and an aluminum compound layer, or an aluminum compound layer containing a composite of iron oxide and alumina, being coated with such a layer having an appropriate optical thickness (see Japanese Patent Laid open No. Hei 6-100794). This pigment has reddish color generated by double layer structure such as combination of reflection (complementary) by the absorption of the iron oxide coated on the sheet-like particles, with interference color due to a second coating structure controlling the thickness of the outer aluminum oxide layer. The combined colors of interference and reflection are claimed to give a reddish color of high chroma having a by far sharper tone than that of the color (of reflection) obtained by a pigment containing only iron oxide. The pigment is also claimed to be able to develop a color of still improved chroma if the outer aluminum oxide layer is replaced by a composite oxide layer of iron and aluminum oxides.
These pigments are, however, produced by a known and commonly used process, i.e. neutralization decomposition, the urea process (uniform precipitation reaction), or thermal hydrolysis, employing iron and aluminum salts as materials for the coating metal compounds, and no pearl pigment having an orange color can be obtained.
Thus, only the cadmium pigment is known as an orange pigment, and there is a demand for the development of a safe and high-chroma orange colored pigment instead of it.