Unstable flows during compounding of highly filled suspensions

Extrusion processability and capillary flow of highly filled suspensions were studied. The experimental studies revealed flow instabilities, basically arising from the time dependent formation of mats, which induce filtering of the polymeric matrix out of the suspension. This demixing generally leads into oscillations in pressure as the mats developed are broken under progressively increasing stresses. Such flow instabilities are less likely to occur for suspensions incorporating either matrices with high viscosities or fillers with smaller particle sizes. Furthermore, operating conditions, which involve increasing the matrix viscosity (lower temperature), increasing the shear stress at the wall (high volumetric rates) and greater diameter dies, also can generate stable processing conditions for such suspensions. These experimental findings were further elucidated and can be explained in a coherent fashion by a proposed mechanism involving the slip and filtration rates in such highly filled suspensions.