Green bodies from modified Si3N4 powders Part II:: Hydrophobic green bodies

The quality of ceramic batches often depends on weather conditions, in particular humidity. This paper will report on the properties of hydrophobic batches prepared from modified silicon nitride powders. The water absorption of batches depends mainly on the polarity of the binder. Polar binders such as PEC absorb the highest amounts of water. However, the polarity of the powder surface is also of significant importance. Hence, for the preparation of real hydrophobic batches, both the powder and the binder must have low polarity. The compaction behaviour of such batches is more or less unaffected by moisture. The green densities of compacts depend almost only on the surface characteristics of the modified powders. The best results are obtained with powders modified by non-polar, long alkyl chains. These chains have to be immobilised via chemical bonds (covalent or ionic). Modification by adsorption, the typical route, is less effective. Three different powders with chemically bound long chains are described.