SURFACE STUDY OF DWA-WETTED TIO2 PIGMENTS

We present a study on the dispersion of coated TiO2 pigments in acrylic-isocyanate automotive paints, based both on the surface characterization of the pigment particles and on studies of the films prepared from the paint. We investigated how the Al2O3-SiO2 coating on TiO2 affects the adsorption of an organic dispersing and wetting agent (DWA) and what the effect of this surfactant is on the dispersibility of the pigment. The study also shows the value of surface methods in the characterization of powder surfaces. Five Al2O3- and/or SiO2-coated TiO2 pigment samples were used as substrates. These were treated with varying amounts of a commercial organic DWA. The substrate surfaces were characterized with XPS and zeta potential measurements prior to the DWA treatment. The adsorption of the surfactant was analysed with carbon analysis, UV and IR spectroscopies, XPS techniques and with static SIMS. Results from both the XPS analyses and adsorption measurements showed that the substrate saturation point for DWA depended on the pigment surface, owing to variations in substrate surface acidity. The effect of the DWA treatment on pigment dispersion was studied in paints. The TiO2 pigments with and without the treatment were dispersed in acrylic-isocyanate automotive paints together with a DWA-wetted red pigment. The TiO2 dispersion from the paint films was studied with a mechanical rub-out test, x-ray microanalysis and XPS sputter profiling. The observed colour variations in rub-out tests were clearly connected with DWA treatment. X-ray microanalysis revealed differences in the TiO2 pigment dispersion stage that probably caused these variations.