BINDER DISTRIBUTION IN GREEN CERAMIC FOILS

Green ceramic foils with a thickness of 20 mu m were tape cast on a doctor-blade machine from an aqueous dispersion of barium titanate powder, containing poly vinyl alcohol, PVA as a binder and triethylene glycol, TEG as a plasticizer. After casting, the wet foils were dried in an oven. The specific aim of this study was to characterize the binder distribution in green foils and to find parameters that affect this distribution. Mercury Intrusion Porosimetry and Nitrogen Adsorption (B.E.T.) were used as techniques to analyse the binder distribution. Experiments were carried out with foils containing either a coarse (d(50) = 0.75 mu m) or a fine barium titanate powder (d(50) = 0.2 mu m). The moment in the drying process of foils, that the binder gets fixed in the ceramic structure determines the binder distribution in green foils. The earlier the PVA fixation process takes place, the more homogeneous is the binder distribution. The plasticizer triethylene glycol (TEG) promotes fixation of PVA by a gelation process. More TEG in the slip leads to a more homogeneous binder distribution in foils. In the case of a low TEG amount, small pores ave preferentially filled with binder over large ones and the lower side of foils is enriched with binder. Less hydrolysed PVA types are less susceptible to gelation with TEG, resulting in a more heterogeneous binder distribution.