Magnetic characterization of metal particle pigment dispersions

The authors address the continuing problem of dispersion characterization using a variety of fundamental magnetic measurement techniques. The response of wet metal particle lacquers when subjected to externally applied AC and DC magnetic fields is investigated as a function of dispersion milling time. Results indicate substantial physical rotation of the constituent single and aggregated particles present in the dispersion. The magnitude of the field required to cause physical rotation is found to be significantly lower than that observed for magnetic switching to take place. Examination of the B-H loops of the dispersion measured at 50 Hz AC as a function of milling time indicates a reduction in the measured coercivity and increase in squareness. Similarly, there is an increase in the dispersion DC susceptibility as a function milling time. All these results are consistent with a reduction in the overall aggregate size, and hence are believed to be a measure of the improvement in dispersion quality