SURFACE CHARACTERIZATION OF SOME TIO2-BASED PIGMENTS .3. COATING OF THE PIGMENTS

Titanium dioxide (TiO2) pigments, subjected to different coating processes, have been characterized by microelectrophoresis (MEP), high-resolution transmission electron microscopy (HRTEM) and Fourier-transform infrared spectroscopy (FTIR). MEP and HRTEM measurements are in excellent agreement with the FTIR data, to which they add useful complementary information. Among the FTIR measurements, the adsorption of pyridine yields vital information on the composition and thermal stability of the surface layer. The coating overlayer is complete in all cases examined, and none of the ions introduced in the TiO2 phase (e.g. the photostabilizing Zn2+) is present in the surface layer after the coating process. Pre-coating with TiO2 from titanyl sulfate produces a porous overlayer of scale-like anatase crystallites. The vacuum thermal stability of the TiO2 overlayer is quite low. Coating with alumina yields a thick, homogeneous, and amorphous overlayer, whose chemical nature depends on the coating procedure. When derived from Na aluminate, it is an amorphous Al hydrate (similar to boehmite), but contains some Al3+ surface ions in a quasitetrahedral co-ordination. However, when derived from aluminium sulfate, it is amorphous aluminium basic sulfate, ionic and highly hydrated. The alumina overlayers of both types are quite unstable towards vacuum thermal treatments. Coating with silica yields an amorphous overlayer of variable thickness of regular hydrated silica, with no mixed TiO2-SiO2 phases. The SiO2 layer is chemically stable towards vacuum thermal treatments at high temperatures, whereas the hydration degree and the texture (surface area and porosity) are irreversibly modified.