Sunil Priya Dasgupta

Degree and stability of magnetic dispersions: Sedimentation, rheological, and magnetic properties

Abstract There is a distinct correlation between the dispersion qualities of magnetic acicular particles as measured by slush grind rheology, sedimentation test, and coating magnetic properties. Sedimentation time measurement is a very sensitive tool to evaluate the degree of dispersion and dispersion stability. Sedimentation, rheology, and coating magnetic properties show that “steric stability” is essential to maintain the excellence of magnetic particle dispersion. Some surface active agents provide excellent pigment dispersion but poor stability with time, whereas other chemical compounds adsorbed on a particle surface can provide a very high degree of dispersion with excellent dispersion stability. Results of particle size evaluation from sedimentation measurements show five to six times improvement in the degree of dispersion due to suitable surface active agents.

Interaction characteristics of magnetic particles with surfactants, solvents, and binder resins

Abstract The Brabender plasticorder titration method provides an excellent and very reliable tool to evaluate and compare the surface interaction characteristics of magnetic particles. Thus, the method can be very usefully employed to evaluate efficiencies of surfactants, binder resins, and solvents in adsorbing on magnetic particles and in providing effective “disagglomeration” of magnetic particles during a dispersion process. The data obtained from solvent-surfactant profile plots are, therefore, meaningful to define “solvent and surfactant demands” of magnetic particles for optimum dispersion behavior. “Ball points” of several magnetic oxides have been determined by titrating them with solutions of different surfactants in organic solvents. The solvent and surfactant demands of these oxides determined from the data are dependent on surface characteristics of the particles, their bulk density, and surface area. Gafac RE610, an organic phosphate compound, is significantly more efficient in adsorbing on magnetic particles than Yelkin TS, a lecithin. Among several binder resins evaluated in this work, nitrocellulose has the most capability to adsorb on magnetic oxides. Toluene serves better as a solvent than either THF or cyclohexanone for adsorbing a surfactant on magnetic particles from a solution. The results demonstrate that important aspects of the fabrication of magnetic media are related to fundamental phenomena. More careful examination and better understanding of these phenomena may be essential to maintain as well as advance the current emphasis on magnetic media in high technology.

Dispersion characteristics of magnetic particles : surface charge, surface adsorption and acid-base interaction behavior

Abstract Electrophoretic mobility (EM) of magnetic oxide particles has been measured in aqueous as well as organic solvent media using a zeta meter and a mass transporter analyzer. EM has also been measured for the magnetic particles after treatment with surface active compounds and/or in the presence of a surfactant. The results show that the sign and magnitude of the EM values change depending on the type and polarity of the surfactants or surface-treating compounds. The sign reversal of the EM values when the medium is changed from water to organic solvent can be understood on the basis of acid-base interaction. The potential energy between particles calculated using the maximum available EM value in an organic solvent is far below the value required for dispersion stability. These results along with the calculation o magnetic potential energy show that the stability of these dispersions cannot be maintained by electrostatic potential alone. Treatment of these oxide particles with surface active compounds provide stable dispersions by a ‘stearic stabilization’ mechanism. Adsorption of surfactants such as organic phosphate compounds and binder resins such as nitrocellulose or polyurcthanes on magnetic oxide particles is due to acid-base interfacial interaction between oxide particles, organic macromolecules, and solvents. Adsorption of these macromolecules in multilayers seems to be a major cause for high dispersion viscosity and the poor magnetic properties of these dispersions on tapes. These results indicate flocculation of the particles, possibly by a bridging mechanism. Thus, surface treatment of oxide particles with proper compounds inhibits adsorption of the binder resins in multilayers, promotes better rheological properties of the formulated grinds, and improves magnetic performance of tapes coated with these grinds.