Effect of Electrode Reactions during Aqueous Electrophoretic Deposition on Bulk Suspension Properties and Deposition Quality

Abstract

Non-aqueous solvents have been popularly used as suspending medium in Electrophoretic deposition (EPD) for several applications. The drawbacks of these solvents are that they cause environmental concerns and are expensive. Therefore, there is a great interest for the development aqueous EPD, which is low-cost and environmental friendly. There are several problems when using water as the suspending medium in EPD. First, there is gas evolution at the electrodes on application of the electric field. Hydrogen is generated at cathode and oxygen at anode. This results in incorporation of bubbles in deposits and the quality of deposit suffers. Several approaches, such as the use of pulse DC, asymmetric AC, and palladium electrode have been reported in the literature to overcome this problem.However, the biggest problem concerning aqueous EPD is reproducibility of deposition. As an example, in batch EPD of Al2O3 from aqueous suspension, we observed the best quality of deposition from the first deposit. Invariably, the amount and quality of deposit decreased progressively with increased number of deposition from the same suspension. For prolonged first deposition, the deterioration became severe and no deposition occurred from the second deposition onwards. This occurred even for cases when only a small fraction of the powder suspension is depleted in the first deposit. A closer look showed destabilized suspension and significant change in bulk pH of the suspension. Here, we measured the change in bulk pH as a function of time, and starting pH during water electrolysis as well as during aqueous EPD of Al2O3. The bulk pH of suspension increased with increasing time of deposition and with increasing applied voltages, resulting in progressive destabilization of suspension and decrease in deposit yield and quality. Use of suitable dosages of cationic dispersant polyethyleneimine (PEI) in the suspension decreased the extent of bulk pH variation during aqueous EPD.