Wall slip and compressibility like effects in capillary rheometer tests on complex polymer systems

Abstract

Using an appropriate set of capillary dies, the applicability of the Mooney wall slip method has been investigated with several filled compounds. Inconsistent results were obtained, for example, ‘negative’ slip velocity. With respect to data measured in practical capillary rheometry, a model was developed for treating (barrel) pressure data versus die length/diameter ratio at fixed applied apparent shear rate, i.e. the so called Bagley plots. A very simple equation was obtained

P_meas = 2P_ends - 1/β ln [exp (P_ends β) - 4σ_w0 β L/D ]

which yields the wall shear stress at zero pressure σ_w0, the ends pressure loss P _ends, and a factor β when fitted to experimental data by non-linear regression. Experimental results show that the factor β accounts for both wall slip and compressibility like effects. Negative β values indicate dominating wall slip effects, while positive values demonstrate compressibility like effects. Slip velocity is thus pressure dependent and consequently the combination of wall slip and a pressure dependent viscosity can mask the expected gap dependence in the analysis by Mooney.