FLUIDITY AND DISPERSION OF ALUMINA SUSPENSION AT THE LIMIT OF THICKENING BY AMMONIUM POLYACRYLATES

The limit of thickening of an alumina suspension by ammonium polyacrylates (PAA) and its molecular weight dependence of the limit were determined from the lowering of the flow point to be a measure of simultaneous promotion of the thickening and the dispersion. PAA of a smaller molecular weight gave a lower flow point minimum and a thicker alumina suspension retaining fluidity up to 85 wt% alumina for PAA of molecular weight 2500. The suspension thickened to the limit has the smallest gap between the flow point and the wet point, supporting Daniel's statement on good dispersion. The average water layer thickness, calculated by dividing the amount of water of suspension at the limit of thickening by the particle numbers, indicated no linearity with the chain length of the PAA. The thickening for PAA with molecular weights smaller than 21 000 approximately 30 nm being accompanied by dilatant flow. The suspension at high solid loadings showed various extension features on the glass plate with changes in the PAA concentration around the flow point minimum. The flow behaviour of the alumina suspension around the limit of thickening was characterized by the Bingham model with two parameters of the yield stress, sigma-0, and the Bingham viscosity, eta. Increase in the fluidity on PAA addition was strongly attributed to a greater lowering of sigma-0 than of eta. A balanced ratio between the two parameters in the apparent viscosity under a suitable shear rate was suggested to be necessary for the flow of the castable thick suspension.