Thixotropy and rheopexy of aggregated dispersions with wetting polymer

Abstract

Different types of thixotropy of aggregated dispersions of magnetic metal, nonmagnetic iron oxide and carbon black particles in organic solvents with a polymer containing pendant wetting groups are studied. Thixotropic behavior depends on the type of aggregation which determines the type of structural recovery in and after shear. Metal particles form denser aggregates combined into flocs or networks recoverable in or after shear, which results in positive thixotropy with up-shear stress-vs-shear-rate curves running above down-shear curves. By contrast, iron oxide and carbon black particles form looser aggregates which re-flocculate slower in shear and do not re-flocculate at all at rest after shear, which results in rheopexy with recovery strongly accelerated by shear and negatively thixotropic loops. Viscoelastic moduli of the latter systems are determined by their shear history, e.g., G′ decreases with the preceding shear rate and does not recover. The effect of the wetting polymer on time-dependant phenomena is twofold: It suppresses thixotropy by consolidating aggregates, but slows down recovery, so that rheopexy is maximized at certain polymer-to-pigment ratios. Furthermore, two types of rheopexy are discriminated. Recovery rate of carbon black dispersions is controlled by the current shear rate only, resulting in thixotropic loops strongly dependent on sweep time. On the other hand, recovery rate of iron oxide dispersions has a long memory of shear history, which leads to thixotropic loops almost insensitive to sweep time. The difference is attributed to slow shear-induced dilation/shrinkage of aggregates built from elongated iron oxide particles, as opposed to stable aggregates of carbon black.