Shear-induced aggregation of carboxylated polymer latices

Orthokinetic flocculation of latex particles containing high levels of surface carboxylation was studied by shearing for short times in a rotational viscometer and measuring changes in particle size using photon correlation spectroscopy. Results can be interpreted in terms of a comparison between the DLVO repulsive force between a pair of particles and the hydrodynamic shear force opposing it. This enables a prediction of the critical shear rate, , required to initiate aggregation. Experimental and calculated values of showed good agreement.The stability of the carboxylated latices under shear was much reduced at pH values >7 when the surface groups are ionised. The increase in suspension viscosity with pH was shown to be critical in determining the onset of aggregation via hydrodynamic forces.