The effect of undispersed agglomerates on the relative viscosity of ceramic moulding suspensions

Abstract

In order to apply polymer processing methods [1-8] to the fabrication of technical ceramics it is necessary to disperse the ceramic powder in an organic vehicle, frequently a wax or polymer. It has been suggested that the shear stress imposed during mixing should exceed the agglomerate strength by a factor of 5-10 in order to destroy agglomerates and their relics [9]. The presence of undispersed agglomerates may be detrimental to the mechanical properties of the sintered product [10] but may also have a pronounced effect on suspension viscosity. This effect is considered to be so marked that viscosity has been used as a measure of dispersion [11, 12], sometimes without external corroboration. Undispersed agglomerates upset the distribution of occupied space and also change the effective particle size distribution. In the former case, agglomerates may not be filled with organic vehicle, for example because of the slow rate of pore filling, the back pressure of pore gas or powder surface treatments which offer unfavourable wetting characteristics. The binary ceramic vehicle system then becomes a ternary ceramic-vehicle-air system and the air behaves as added ceramic volume. The suspension viscosity is expected to rise compared with the fully dispersed suspension. The pore gas may be fully or partially replaced by vehicle through favourable pore filling thermodynamics and kinetics ably demonstrated by DeBruyne for a similar model system [13]. Under these circumstances the effective ceramic volume fraction is increased both by added air and by organic fluid abstracted from the volume fraction of vehicle. Again, the viscosity is expected to rise. The relative viscosity of a powder, ~/r, is very sensitive to ceramic volume fraction V, and one o f the best expressions for relating these quantitieS is [14]: