Phosphate esters as dispersants for the cathodic electrophoretic deposition of alumina suspensions

The stability of alumina ethanol suspensions prepared with phosphate esters (PEs) having three different alkoxy functional groups (ethyl, butyl, and butoxyethyl) and the corresponding effects on the electrophoretic deposition characteristics were investigated. The mechanism of particle charging was characterized by measuring the suspension pH, conductivity, zeta-potential, and deposition yield under various experimental conditions. The highest stability was observed for the suspension prepared with the butoxyethyl acid phosphate though the zeta-potentials of the three suspensions were very similar. This was attributed to the steric repulsion of the alkoxy functional group. The PEs made the alumina particles positively charged which allowed the cathodic deposition to occur. The zeta-potential, deposition yield and green density were functions of the PE addition amount. The highest green density was obtained at the highest zeta-potential, however, the deposition rate for this PE amount was the lowest. The over-addition of PE caused decreases in the zeta potential and green density, but an increase in the adhesion of the deposit to a metal substrate. Thick deposits with smooth surfaces were obtained from the over-added PE suspension.