Flocculation-dispersion characteristics of alumina using a wide molecular weight range of polyacrylic acids

The effects of polymer molecular weight on the flocculation/dispersion behavior of a model colloidal system consisting of alumina and polyacrylic acid (PAA) were investigated. Low molecular weight polymers, traditionally used as dispersants were discovered to flocculate alumina at ultra low concentrations. For molecular weights ranging from 2000 to 50 000 g mol−1, distinct concentration ranges were found to exist in which the polymer behaved as a flocculant and above which it behaved as a dispersant. With increasing molecular weight, the number of polymer molecules required to achieve a benchmark flocculation (80% flocculation) decreased down to the molecular weight of 250 000 g mol−1 PAA. Above this size, there was no significant advantage of increasing the molecular weight. It was observed that regardless of the sign of the zeta potential, flocculation of alumina could be obtained with PAA suggesting that in addition to electrostatics, other forces such as bridging by hydrogen bonding are also responsible for the flocculation.