The Influence of Matrix Viscosity and Interfacial Properties on the Dispersion Kinetics of Carbon Black Agglomerates

The dispersion of carbon black agglomerates suspended in polydimethyl siloxane (PDMS) or polybutadiene (PBD) liquids has been studied. Agglomerates comprised of either a low-structure or a high-structure carbonblack were subjected to simple shear flow. Two characteristic length scales that affect the dispersion process are identified. One length scale (Lp) is a measure of the ease with which fluid can flow through the porous agglomerate structure. The second (δ) is a measure of the depth to which processing fluid has soaked into the agglomerate. Values of Lp were found to be independent of types or viscosity of the polymer but were sensitive to the structure and packing density within agglomerates. The parameter δ, which varies with time of immersion in the processing fluid, was found to be smaller at comparable immersion times for matrix fluids with high viscosity. Shearing experiments showed two kinetic regimes (fast and very slow dispersion). For each type of matrix liquid, there exists a critical packing density; agglomerates having a packing density lower than this critical value exhibit only fast erosion, but those with packing density higher than this critical value show dispersion kinetics that starts as fast but shifts to slow after a certain period of shearing. The critical packing density was higher in PBD than in PDMS for agglomerates of the low-structure carbon-black but was not significantly different for agglomerates of the high-structure carbon-black. The critical packing density was found to shift to higher values if higher viscosity PDMS was used. The dispersion kinetic regime for the different polymers liquids could be correlated with the value of δ/Lp. If δ/Lp remained less than a critical ratio, only fast erosion was observed. Measurement of fragment size distributions support the notion that fragments are produced primarily from the periphery of the agglomerate in the slow kinetic regime, but can be larger (up to the size of δ) for the fast kinetic regime.