(450e) The Effect of Flow and Interaction Strength on the Microstructure and Properties of a Model Carbon Black Suspension

The microstructure of carbon black suspensions is highly relevant due to the widespread use of these materials in applications where flow is an important aspect of processing and end use. In these applications, key design parameters such as the viscosity and electrical conductivity of the suspension are ultimately determined by the state of the suspension microstructure, which is subject to change as a result of both shear and interaction strength. Additionally, while much research has been aimed at understanding the relationship between macroscopic properties of interest and the microstructure of carbon black suspensions, a direct measurement of the structure of these suspensions while under shear has been challenging. In this work, structure-property relationships for a commercially available conductive carbon black, Vulcan XC-72, suspended in propylene carbonate at a range of lithium salt concentrations are studied. The shear-induced microstructure of these suspensions is directly measured by performing Rheo-USANS (Ultra-Small Angle Neutron Scattering) experiments at both the NIST Center for Neutron Research and the Spallation Neutron Source at ORNL. In these experiments, it is found that a signature bifurcation in macroscopic properties exists that depends on the magnitude of the inverse Bingham number, Bi^-1, which relates the measured stress to the yield stress of the suspension. At high shear rates, where Bi^-1 > 1, the suspension is stable and exhibits a shear-thinning behavior that can be attributed to the erosion of open-structured carbon black agglomerates with increasing shear rate. The extent of this shear-induced breakdown of agglomerates is found to decrease with an increase in salt concentration. At low shear rates, where Bi^-1< 1, shear drives the densification of these agglomerates, which leads to an order of magnitude decrease in both the viscosity and electrical conductivity. Additionally, spatiotemporally resolved Rheo-SANS experiments performed at the NIST Center for Neutron Research show that this densification process leads to sedimentation of particles that can be observed as a long-lived decline in both the viscosity and conductivity. These experiments show that the shear-induced microstructure of carbon black suspensions gives rise to rich rheological and electrical behaviors that have implications for many applications of carbon black suspensions.

• Hipp, J.B., Richards, J.J., and Wagner, N.J., Rheol. 63(3), 423–436 (2019).