The effect of processing additives on the wall slip of molten polyethylene

Abstract

Large amplitude oscillatory shear (LAOS) data, as a function of strain amplitude and frequency, are measured for seven molten plastics. A constitutive equation based on transient network theory, following the original formulation of Marrucci et al. (2 0 ), is able to adequately predict the nonlinear viscoelastic behavior of these polymer melts in LAOS. Evaluating this constitutive equation to predict the shear stresses arising in LAOS with wall slip, using a previously proposed slip boundary condition (18), shows that this slip boundary condition is insufficient to predict the observed unstable stresses that arise in LAOS with wall slip. The critical strain amplitude at which unstable stresses arise in LAOS has been shown to correspond with the critical shear rate for the onset of melt fracture in extrusion, for untreated molten polymers. This observation can be used to develop a procedure to evaluate the melt fracture "potential" of polymer melts. Polymer processing additives are shown to have two distinct types of effects on the wall slip of molten polymers in LAOS. Two additives are shown to decrease the critical strain amplitude for the onset of unstable stresses in LAOS while another additive increases the critical strain amplitude. This suggests that there could be two mechanisms by which processing additives suppress melt fracture in extrusion.