Tin-containing lustrous pigments

DETAILED DESCRIPTION The subject of the invention is a lustrous pigment of mica flakes coated with a metal oxide layer consisting of a very uniform layer of tin dioxide, which can contain aluminum oxide.
Each of the oxides can be in hydrated form.
The subject of the invention is also a process for the production of these pigments by precipitating metal oxide hydrates from salt solutions on to the mica flakes and subsequently washing the products obtained, drying and possibly calcining wherein a solution of Sn(II) salt, preferably SnCl.
sub.
2, is used to produce the metal oxide hydrate and the precipitation is done in the presence of an oxidizing agent, preferably also in the presence of a water-soluble aluminum salt.
Surprisingly, contrary to statements in the literature, coating of mica flakes to form lustrous pigments with good gloss, is unsuccessful with Sn(IV) solutions but works using Sn(II) solutions in the presence of an oxidizing agent.
It therefore seems critical to the process of the invention to carry out the oxidation slowly and during the precipitation.
Under these conditions, surprisingly smooth tin dioxide hydrate layers are obtained on the mica, in uniformity not achieved hitherto.
Thus, pigments described in the literature as produced using SnO.
sub.
2 have been unimportant because no useable lustrous pigments were formed by the known processes.
It has also been found that addition of a small amount of aluminum salts, from which Al.
sub.
2 O.
sub.
3 is later formed, is advantageous for the formation of uniform layers.
Although the effect of aluminum ions and/or of aluminum oxide or of its hydrates is not completely understood, three phenomena seem to be especially important.
Co-precipitation of aluminum hydroxide hydrates prevents the formation of cracks which otherwise occur upon calcination of tin dioxide hydrate layers of high water content.
Addition of aluminum salts also appears to promote formation of smooth tin dioxide layers and quantitative deposition of the precipitate on the mica flakes