Compound Processability and Molecular Weight Distribution of SBR

Nonuniformities in the mixing and extrusion behavior of nominally standard grades of emulsion- and solution-polymerized oil-extended SBR are associated with variations in molecular weight distribution but are not reflected by Mooney viscosity. The variations in the samples studied were representative of normal production material. The solution-polymerized polymers characteristically exhibit wider variations in compound die swell and have generally more rapid dispersion of carbon black than comparable emulsion-polymerized polymers. This is true, regardless of whether BIT or t′ point is considered as a measure of carbon black dispersibility. Generally, increasing polydispersity increases compound die swell and retards the rate of carbon-black dispersion. Utilization of unit work instead of mixing time as a measure for torque rheometer processability gives similar information on carbon-black dispersibility. However, unit work is preferred because of the promise it holds for scaleup and interlaboratory correlation. Delta Mooney values, and to a lesser extent peak Mooney torque, provide a useful basis for predicting the mixing and extrusion behavior of oil-extended SBR. The torque rheometer t′ point recently proposed as an index for carbonblack dispersibility is also useful for predicting compound die swell.