Recycling of TiO2 Pigments from Waste Paint: Process Development, Surface Analysis, and Characterization

Abstract: Pigments are commonly used in paint, plastic and paper products and titanium dioxide (TiO2), the most important white pigment, accounts for approximately 70 % of the total volume of all pigments used today. Minerals containing TiO2 are relatively abundant in the earth’s crust. However, the production of TiO2 is energy intensive and carries a high environmental burden. Therefore, the paint industry is seeking replacements for the virgin TiO2 pigment used extensively in paint formulations today. The research work presented in this thesis was carried out to investigate the possibility to use secondary TiO2 pigments, recycled from waste paint, as an alternative to virgin pigments. Typically, commercial grade pigments are not pure TiO2. Rather, they are surface treated in order to make the pigments more compatible with the paint matrix and to facilitate optimum dispersion. Waste paint feedstock for a recycling process will therefore contain TiO2 pigments having different chemistries due to the variety of surface coatings. In this research three pigments, coated with different combinations of aluminium, silicon, and zirconium oxides were investigated. The TiO2 was recovered from paint in a three-step recycling process. First, the paint was pyrolysed at 500 °C in a nitrogen atmosphere to remove the volatile organic fraction. Next, the inorganic pyrolysed residues were oxidized in air at 500 °C to remove any residual organics and black carbon. After pyrolysis and oxidation the inorganic fraction was found to be contaminated with ionic salt residues from the decomposition of paint components during the recycling process. Therefore, the final step in the recycling process was to wash the residues in the presence of a mixture of ion exchangers yielding a pure secondary TiO2 product. In order to clarify the extent to which the pigment was affected by the recycling process, the characteristics of the recycled pigments were studied using techniques such as powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and measurement of the dynamic electrophoretic mobility. Of the three pigments studied, it was shown that a pigment coated with a combination of silicon and aluminium oxides was more prone to degradation in the recycling process compared to pigments coated with a combination of aluminium and zirconium oxides or only aluminium oxide. In addition, recycled pigment was used as a replacement for virgin pigment in a paint formulation. Results showed that the paint made from recycled TiO2 had a minor decrease in opacity, and that the effect on whiteness was insignificant when compared to a paint containing virgin pigment. However, surface defects due to poorly dispersed pigments decreased the gloss of the paint film. Even though the performance of the recycled pigment was lower than that of the virgin