THE INFLUENCE OF MATRIX VISCOSITY AND INTERFACIAL PROPERTIES ON THE DISPERSION KINETICS OF CARBON BLACK AGGLOMERATES H. Yamada, I. Manas-Zloczower* Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106 and D. L. Feke Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106 ABSTRACT 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 carbon- black 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. INTRODUCTION The quality of carbon black dispersion into rubber is one of the important factors affecting the physical properties of the resulting compounds. 1 3 The kinetics as well as the ultimate degree of dispersion attained are influenced by the nature of the elastomer, the morphology of carbon black, and the mixing conditions. 4 10 Many researchers 11 14 have studied and proposed models of agglomerate dispersion. However, not all factors governing the dispersion process are yet well understood. One of the little studied factors affecting dispersion mechanisms and kinetics is the agglomerate permeability (k), which depends on the overall packing density and the morphology of constituent aggregates. The permeability of the agglomerate determines the ease with which the processing fluid can flow through the agglomerate, and consequently affects the hydrodynamic stresses expe- rienced by the agglomerate. Additionally, the presence of matrix fluid can alter the cohesivity of the agglomerate and the infiltration process can induce some structural changes in the agglomer- ate. In our previous work, 15 the dispersion of carbon black agglomerates in polydimethyl siloxane (PDMS) liquids undergoing simple shear flow was investigated. Two characteristic length scales that affect the dispersion process have been identified, namely a length scale Lp(= √ k), which * Author to whom correspondence should be addressed. 1