진주광택안료 제조에 있어서 TiO₂ 상전환에 관한 연구

Abstract

Recently, the pearlescent pigments, which create lustrous, iridescent and angle-dependent optical effects, are used extensively in a variety of industrial products, such as optical filters, cosmetics, plastics, printed products and paints. There are several pearlescent pigments of commercial importance. Among them, TiO₂/mica is the most widely used pigment in various applications. The TiO₂/mica pigment is typically produced by the deposition of TiO₂ layers on the mica in aqueous suspension followed by a calcinations process. The calcination of TiO₂/mica results in crystalline TiO₂. The phase transformation of anatase to rutile (A-R) and their proportion in the TiO₂ thin layer has a notable influence on the pearlescent appearance of TiO₂/mica pigment. Rutile has a higher refractive index than anatase and, therefore, yields a stronger pearlescent effect than anatase. Furthermore, the muscovite mica, which is used as a substrate for nearly all commercial products of TiO₂/mica pearlescent pigment, is known to have template effect, namely, muscovite mica is anatase directing. In addition, rutile is known to have a lower photocatalytic activity than anatase, suggesting that rutile transformation is more desirable to prevent the oxidation of surrounding binder in outdoor weathering. For above reasons, rutile modification is more desirable than anatase. It is well known that, by using an intermediate thin layer of tin oxide between mica and TiO₂, TiO₂ can be formed as rutile after calcinations. However, the use of tin oxide to make the rutile TiO₂/mica pearlescent pigment is limited in some cases. The pearlescent pigment containing tin oxide cannot be used to color polymer compositions which can be used in contact with food, and, in some countries, the presence of tin oxide is not permitted in cosmetic products. Recently, it was claimed that, without the use of any rutile directing additive or coating, the rutile formation can be achieved easily by the control of pH during the precipitation of hydrous titanium oxide. However the effect of pH control and the mechanism of A-R transformation in this process are not clear. The aim of this work is to clarify the effect of pH on the precipitation by hydrolysis of TiOCl2 over mica and to understand the mechanism of subsequent A-R transformation during calcination.