Comparative study of the dispersion of three oxide pigments with sodium polymethacrylate dispersants in alkaline medium

The adsorption of sodium polymethacrylate (NaPMA) dispersants onto three oxide powders (hematite, goethite and rutile) in alkaline medium has been investigated together with the resulting stabilization of the suspensions. Under basic conditions of waterborne coatings formulation, hematite and rutile are negatively charged whereas goethite is almost neutral. Adsorption isotherms, electrophoretic mobility, particle size, IR spectroscopy and rheological measurements were used to investigate the polymer-pigments interactions. It was found that adsorption of the anionic NaPMA polymer on the three pigments was still possible even at pH above the isoelectric point (IEP). Depending on the pH and the type of oxide, electrostatic interactions, chemical complexation at the surface or/and hydrogen bonding were suggested to be the predominant contributions to the adsorption process. The adsorption at high pH was enough for preventing particles aggregation and ensuring an efficient stabilization of the suspensions. The viscosity measurements performed at pH close to the IEP's also suggested that besides the electrostatic stabilization, the NaPMA polymers provided a steric stabilization. The steric effect was larger with the polymers of higher molar mass. The comparison of the three oxide showed that the affinity for the surface site came mainly from chemical interaction with acidic surface sites in the order rutile > goethite > hematite. The strong adsorption on rutile came from the aluminum treatment of its surface.