The rheological behavior of a very concentrated suspension (76.5 vol %), which serves as a widely used solid rocket fuel simulant, was characterized employing both torsional and capillary flows. No comprehensive studies of the rheology of concentrated suspensions have been carried out previously at such a high solids content. The suspension exhibited shear thinning over the apparent shear rate range of 3&3000 SC'. Significant slip at the wall was observed in both torsional and capillary flows with the slip velocity increasing from about 0.001 mm/s at a shear stress of 4 Pa to as high as 60 mm/s at 100 kPa. A flow visualization technique was applied for the first time to determine the wall slip velocities in torsional flow directly, to also provide the true deformation rate and feedback on yielding. The contribution of the slip of the suspension at the wall to the volumetric flow rate in capillary flow was found to increase with decreasing shear stress, giving rise to plug flow at sufficiently low shear stress values. The observed plug flow is related to the shear-thinning nature of the suspension and differs from the behavior of shear thickening suspensions, which may exhibit plug flow at high wall shear stress values, i.e., above a critical wall shear stress in capillary flow.

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Extrusion of filled polymer is commonly employed in diverse industries including compounding operations. The analysis of extrusion of filled polymers is complicated especially by the ubiquitous viscoplasticity and wall slip of the filled polymers. Furthermore, the role played by entrainment of air in the processor, the continuously evolving microstructure, and hence the rheological behavior of the filled polymer in the mixing volume of the extruder and the flow instabilities associated with the converging flows involving the filtration of the binder polymer present additional challenges to the analysis. Specialized techniques are also necessary to quantitatively describe the dispersive and the distributive degree of mixing of the compound. The principal tasks of this study included the collection of experimental data from twin‐screw extrusion using an instrumented and industrial‐scale co‐rotating extruder in conjunction with a well‐characterized filled polymer, which exhibits viscoplasticity and wall slip. The process allowed the adequate mixing of the ingredients and the removal of its air content. Next, the processing data were compared with the results of numerical simulation using the Finite Element Method. The predictions compared favorably with the experimental temperature and pressure distributions obtained under different sets of operating conditions. The distributive degree of mixing (spatial homogeneity) of the filled polymer upon exit from the die was also characterized employing a wide angle X‐ray diffraction technique in spite of the amorphous nature of both the filler and the binder polymer, i.e., hollow glass spheres and poly(dimethyl siloxane) polymer.