(596c) Coupled Effects of Additives and Solids Content On the Rheological Behavior of Ceramic Suspensions in Ceramic Foams Manufacture
Cellular ceramics (ceramic foams, honeycombs) are materials with specific properties such as: high superficial area, high permeability and low density (attributed to the cellular structure), low electrical conductivity, thermal and chemical stability (inherent properties associated to the ceramic materials), with many applications like catalytic supports, liquid metal filters, gas (particulate) filtration, and porous burners.
The ceramic foams are commonly produced by the replication method, it's consist in a impregnation a polymeric flexible template (i.e. polyurethane foam) with a ceramic suspension (slurry) containing the appropriate additives (dispersant, binder, defoamer, solvent), after that this template is submitted to a heat treatments to eliminate the additives, burn out organic compounds and sintering the ceramic structure. In this manufacture process reproducibility is a difficult task because the variability of the slurry characteristics that depend on the additive selection.
The slurry rheological behavior is a key factor in the ceramic foam manufacture, because it affects the ceramic coating on the polymer matrix during the impregnation. The aim of this work is study the combined effects of dispersant, binder, defoamer, solvent and solids content on slurries rheological behavior, to establish the influence in the impregnation step, and also to evaluate the effect on final physical properties of ceramic foams.
The ceramic foams are produced by impregnation of polyurethane foams of 10 pores per inch (FoamPartner GmbH, Switzerland) with different ceramic slurries. Coated polyurethane foams were compressed (50%), in order to avoid closed cells by removing the slurry excess, and dried for 24 h at room temperature. After that, the foams were submitted to thermal cycles in an electric furnace to 5 °C/min until 500 °C and 10 °C/min until 1600 °C with a soaking time of 2 h.
Slurries used in the impregnation step are manufactured using 3 different concentrations and type of binder, dispersant, and solvent (with and without defoamer), 3 different ammonia content to modify the suspension pH and two ceramic powders yttria stabilized zirconia or alumina. The mixtures were prepared in a planetary mill (Gabbrielli, Mill-2B, Italy) and homogenized with a high shear mixer (BYK, dispermat, Germany) with tooth profile impeller of 30 mm.
The results show that the critical factors in the impregnation step are the type and concentration of the binder (< 0,6 %wt), and the dispersant type (> 3.5 %wt), to promote polyurethane foam retention. Also, pseudoplastic and thixotropyc slurries behavior promote open cell structure and homogeneous coating in the polyurethane foam, and this is related to the ceramic foam strength.