Formulation effects on magnetic mix microstructure based on rheological, magnetic susceptability, and particle size measurements

Steady and oscillatory shear rheological, magnetic susceptibility, and particle size measurements are used to characterize the microstructure of metal particles dispersions in organic solvents with respect to changes induced by milled-in polymer only and polymer + dispersant . A hierarchical structure built of inseparable clusters, aggregates with their size fixed during mix preparation, and a volume-filling network of these aggregates is consistent with our observations. As polymer-to-particles ratio and/or number of active groups in the polymer increase, the aggregates decrease in size and the network becomes weaker and less brittle. For each polymer-to-particles ratio an optimum level of dispersant is established. Below the optimum dispersant level aggregates are too large, while above it aggregates appear to interact too strongly. A plateau at intermediate shear rates occurs unless an optimum dispersant level is maintained along with sufficient polymer. This plateau is attributed to wall-slip and may be used to discriminate dispersion quality.