THE RELATIONSHIP BETWEEN PARTICLE AGGREGATION AND RHEOLOGY IN MIXED SILICA-TITANIA SUSPENSIONS

The rheological behaviour of concentrated suspensions of coarse SiO2 p articles containing various amounts of fine TiO2 has been studied usin g a viscometer which prevents settling of the suspended material. At a volume fraction of 0.468 the mixed SiO2/TiO2 suspensions exhibit shea r thinning behaviour, with the yield stress and viscosity being strong ly dependent on pH and the TiO2 content of the suspension. Four differ ent TiO2 samples are investigated, each having different inorganic sur face coatings. The TiO2 particles adsorb on the surface of the SiO2 pa rticles and coat them, resulting in suspensions with isoelectric point s corresponding to those of suspensions containing TiO2 alone. The cha nges in yield stress as a function of pH are influenced by the TiO2 co ntent of the suspension, and when the silica:TiO2 volume ratio is grea ter than similar to 0.03, the yield stress vs pH curve resembles that of a pure TiO2 suspension. The maximum yield stress is observed at a p H corresponding to the isoelectric point of the TiO2 component, and at this pH, networks of TiO2 aggregates can form. The formation and brea kdown of these aggregates has been monitored in concentrated suspensio ns using focused beam reflectance measurement (FBRM). FBRM results cle arly demonstrate that increases in the yield stress of the suspensions are accompanied by an increase in aggregate size. At a given pH, chan ges in plastic viscosity with increasing TiO2 content in the suspensio ns appear to result from an increase in the maximum packing density du e to the corresponding change in the particle size distribution. (C) 1 998 Elsevier Science S.A. All rights reserved.