The Role of Binder Adsorption for High Solid Loading Nano-Zirconia Extrusion Pastes
Burghardt, A; Satet, R; Eisele, U; Hoffmann, MJ
JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Band 95, Heft 6, S. 1901-1910, DOI: 10.1111/j.1551-2916.2012.05166.x
The viscosity behavior of extrusion pastes comprised of a PVB melt and varying amounts of nano-zirconia powder have been explored with special emphasis on the impact of a dispersant to reduce the viscosity. In contrast to suspensions with nanopowders, the maximum solid loading in the highly viscous extrusion pastes studied is not limited by attractive interparticle forces. However, at a solid loading >30 vol% the pastes had too high viscosity to be processed due to the high number of adsorbed polymer molecules. By adding a decoupling agent that reduced the work of adhesion of the polymer molecules on the zirconia particles, the viscosity of the pastes could be lowered significantly. The calculated work of adhesion is consistent with the viscosity behavior of the feedstock and is confirmed by wetting angles that have been experimentally determined. It is shown that the effect of the dispersant cannot be explained by electrostatic or steric stabilization of the particles in the melt. A unifying scheme is proposed where the limiting factors agglomeration and adsorption for a high solid loading combined with the necessary mode of function of the dispersant are illustrated as a function of the viscosity of the medium and the particle size. The viscosity-lowering decoupling effect of the dispersant was a prerequisite for reaching a solid loading of 50 vol% in the additional feedstock development. The feedstock with 50 vol% nano-zirconia could be extruded to a tape and sintered to a relative density of 97% at 1200 degrees C.