Rheology of injection-molded zirconia-wax mixtures

Abstract

Rheological behavior of highly-concentrated (with particle solids loading of 50–70 vol%) zirconia-wax mixtures was investigated. The particle surfaces were first modified by monolayer adsorption of stearic acid as a lubricant, followed by compounding with different fractions of low-molecular-weight wax. The mixtures roughly exhibit a pseudoplastic or Bingham behavior with a yield stress which increases exponentially with decreasing particle-particle separation. The maximum solid loading was determined experimentally to be 0.7, suggesting the particles arrange in an ordered dense packing configuration. Three models, namely, modified Elier's equation, Krieger-Dougherty equation, and Maron-Pierce equation were used to model the relative viscosity (η)-solid fraction (φ) relationship for the mixtures. The Krieger-Dougherty equation exhibits a good description on the mixture viscosity up to 65% solids concentration whilst the others are limited to 60% loading. Influence of temperature on mixture viscosity is critical particularly for higher solids concentration. For a 65% mixture, a pronounced particle-particle interaction dominates the mixture rheology at temperatures above approximately 75°C whilst below this temperature, the viscous flow of the organic vehicle turns to be important in dominating the mixture rheology. However, interparticle interaction becomes less significant when the solids concentration is below approximately 60%.