Surface and colloid chemical studies of gamma iron oxides for magnetic memory media

The specific interaction of dispersants and of matrix polymers with the surface of gamma iron oxide particles determines the dispersibility of these powders in coating formulations for magnetic tapes and disks, and also determines the adhesion and other mechanical properties of the cast tapes. These specific interactions have been found to be Lewis acid—base interactions of the surface sites of the gamma iron oxides with the active sites of the polymers or dispersants. The FeOH surface sites of the gamma iron oxides can react with either acidic or basic molecules, and the energy of such bonds can be assessed and predicted by measurement of the molar heats of adsorption of test acids and bases with the FeOH sites. We have measured the molar heats of adsorption of test acids and bases with two different methods; primarily with a flow microcalorimeter having down-stream UV concentration-detection, but also by FTIR transmission measurements of the infrared spectral shifts of complexation of test acids or bases with the surface FeOH sites. Polymer adsorption onto gamma iron oxides from multicomponent systems (three solvents and two polymer species) was studied by another FTIR technique, photo-acoustic spectroscopy, and the results of such adsorption on the dispersibility and dispersion stability of the gamma iron oxides were assessed by photon correlation spectroscopy. Photon correlation spectra of solutions of the basic polymer polyvinylmethylether in basic organic solvents showed that some of the polymer was in the form of one micron aggregates which caused rapid flocculation of the oxide, but upon addition of the acidic epoxy resin the polymer was less aggregated and the oxide became well dispersed.