Influence of surfactant on rheological behaviors of injection–molded alumina suspensions

The effect of three different surfactants, namely, stearic acid (SA), oleic acid (OA) and 12-hydroxystearic acid (HSA), on the rheological properties of injection-molded alumina suspensions has been investigated over a temperature range of 120–150°C and a shear-rate range of 1000–15 000 s−1. The suspension mixtures exhibited a pseudoplastic flow behavior and a viscosity (η) order of ηSA<ηOA<ηHSA over the shear-rate range tested. The yield stress of the suspensions, on the other hand, followed a reverse order as that of the viscosity and the suspension flow was highly dependent on the working temperature employed. A particulate network within the mixtures in a form of powder agglomerates was suspected to play a major role in the flow behavior observed, over which, different levels of powder agglomeration were resulted from the varying surfactants used. A model experiment by repeatedly shearing the suspensions under a fixed stress level partially confirmed the hypothesis. The suspension viscosity was reduced in a power-law fashion as the number of shears was increased and eventually a minimum viscosity was reached. This suggests that the particle network restructured itself upon the shear was applied, leading to breakup of the network and improvements in the particle packing.