Polymer coatings with a destabilizing action on suspensions

Abstract

The rheological behaviour of concentrated suspensions of spherical glass particles can be influenced drastically by surface pretreatment of the particles. This treatment consisted of silanization with mono- or difunctional organo-silicon compounds. Results from ESCA, IR and SEM analysis strongly suggest that silanization with the reagent dimethyldichlorosilane (DMDCS) results, depending on the processing, in a more or less homogeneous layer of oligomer and/or polymer. The particles were suspended in an apolar (DOP) and in a polar liquid (glycerol/water). All suspensions exhibited perfect or nearly Newtonian behaviour except DMDCS-coated glass in glycerol/water. With the latter, under oscillatory shear conditions, behaviour of a continuous coagulation network structure was found: dynamic moduli are hardly dependent on frequency while network structure breakdown occurred already at the lowest accessible deformation (10 −3). Under steady shear rate conditions pseudoplastic behaviour was found, with relative differential viscosities at high shear rates (10 2 s −1) approaching the value of equivalent suspensions of uncoated spheres. Results are interpreted with the giant floc model, according to which macroscopic shear is understood as microscopic slip along shear planes between coagulated domains. The origin of coagulation in these systems is not in van der Waals forces because bond strengths as judged from rheology are too high. Polymer bridging between coating polymers is suggested as flocculation mechanism.