PROBING MAGNETIC PAINTS THROUGH MAGNETORHEOLOGICAL AND SUSCEPTIBILITYMEASUREMENTS

Linear viscoelastic measurements with and without a superimposed magne tic field are used to characterize the structure of model magnetic pai nts comprised of metal particles, cyclohexanone and polyvinylchloride wetting resin. Simple network modeling is used to derive a formula for the field-induced increase of G' with the single parameter, critical field H-crit, which characterizes dispersion quality by how easily par ticles align under the magnetic field. Milling weakens the structure o f the paint, provided sufficient resin concentration is maintained, as revealed by a decrease of G' and H-crit with milling time. At low res in concentration G' increases with milling time indicating that millin g induces structural reorganization which, being unsupported by resin adsorption, results in forming a stronger network. Magnetorheological measurements are correlated with susceptibility measurements in a smal l alternating magnetic field which probes particle interactions withou t breaking the structure. As the resin concentration increases, rheolo gical spectra shift to shorter relaxation times due to structure trans formation from a network to small aggregates. Susceptibility spectra s hift to longer times due to weakening of particle interactions. (C) 19 98 The Society of Rheology.