The invention relates to iron oxide pigments based on silica, titanium dioxide and/or zirconium oxide particles coated with iron oxide.
Pigments of this type which are based on monodisperse, spherical titanium dioxide particles are described in EP 0,192,485, corresponding to U.S. Pat. No. 4,675,251; these spherical titanium dioxide substrates preferably have a diameter in the submicron range of between 0.2 and 0.8 .mu.m and the iron oxide layer in particular has a thickness of between 10 and 300 .ANG., which corresponds to a weight proportion of not more than about 60% by weight, relative to the weight of the titanium dioxide substrate. If the iron oxide coating is present as an .alpha.- or .gamma.-Fe.sub.2 O.sub.3 modification, orange-red to red-brown pigments are obtained; if an Fe.sub.2 O.sub.3 -coated pigment is subjected at elevated temperatures of, for example, more than 400.degree. C. to a reducing gas atmosphere, a grey-black to black pigment which has magnetic properties and a magnetite coating is obtained. The pigments described in EP 0,192,485 have a high covering power and a high color strength.
However, the disadvantage of Fe.sub.2 O.sub.3 -coated pigments is that they have a very narrow color spectrum and are virtually limited to orange-red-red-brown hues; black pigments can only be obtained by conversion of the Fe.sub.2 O.sub.3 coating into a magnetite coating, which gives the pigments, at the same time, ferromagnetic properties, which is, however, undesirable in many applications.
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