Dispersion of alpha-alumina ultrafine powders using 2-phosphonobutane-1,2,4-tricarboxylic acid for the implementation of a DCC process

In this paper, we compare the effectiveness of 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) with that of 4,5-dihydroxy-1,3-benzenedisulfonic acid (Tiron) as electrostatic dispersants for concentrated alpha-alumina ultrafine powder suspensions. Three grades of alpha alumina powders with diameters in the range of 100–300 nm are tested. The comparison based upon direct measurements of electrokinetic properties, adsorption isotherms and rheology shows that both PBTCA and Tiron are good candidates for dispersion. In addition, the method of adsorption of PBTCA and Tiron onto the alumina surface is described. Like Tiron, PBTCA is adsorbed as a monolayer through an inner sphere complex. The key contribution of the phosphonate foot of the PBTCA molecule in the adsorption mechanism is demonstrated and the influence of alumina surface chemistry, especially hydroxylation, on adsorption is pointed out. Finally, it is shown that the concentrated suspensions could be used to implement a shaping process by direct coagulation casting (DCC).