Surface chemistry and rheological behaviour of titania pigment suspensions

The influence of pH and surface chemical state on rheological behaviour was investigated for variations of silica- and alumina-coated, aluminium-doped titania pigment suspensions. The variation in rheological properties corre lates with the change in the pigment surface properties, determined from el ectrophoresis measurements, atomic concentrations, chemical states and modi fied Auger parameters derived from X-ray photoelectron spectroscopy (XPS). Pigment suspensions exhibited a maximum yield stress and viscosity at or ne ar the isoelectric point (iep). At a pH where the magnitude of the zeta pot ential of the titania pigment is high, a low-viscosity, dispersed suspensio n was obtained. The pH of the maximum yield value of the pigment suspension increases with increasing aluminium hydroxyl group density and decreases w ith increasing silicon hydroxyl group density. Low-viscosity pigment disper sions were obtained with increasing aluminium surface concentration and fur ther reduced with an increase in the silicon surface concentration. Pigment particle attractions are chiefly dictated by van der Waals forces and hete roaggregation. The pigment aggregate strength therefore depends upon the Ha maker constant of the heterogeneous pigment based on the proportion of the respective surface groups. (C) 1999 Elsevier Science B.V. All rights reserv ed.