Measurements of slip velocity and frictional heating in the capillary extrusion of linear-low-density polyethylene with a fluoropolymer processing aid

The slip velocity and frictional or slip heating of linear-low density polyethylene with a fluoropolymer processing aid in capillary flow were measured by rheo-particle image velocimetry and thermal imaging. The pure polymer did not show slip before the stick-slip regime but exhibited strong slip when blended with the processing additive. However, for shear stresses beyond the stick-slip regime, the pure polymer and the blend exhibited the same flow behavior with slip. The slip velocity increased with the shear stress at two different rates before and after the stick-slip and the contribution of slip to the total flow rate exhibited a minimum. Significant rises in temperature were measured under slip and no slip conditions, being these much higher than the values predicted by the adiabatic flow assumption. Clear difference was made between viscous and frictional heating before the stick-slip regime, even though they could not be distinguished from one another at higher stresses. Overall, in the presence of slip, frictional and viscous heating act synergistically producing higher temperature rises in the melt. Finally, in contrast to predictions by numerical simulations of viscous heating, measured velocity profiles did not evidence the heating effects in the shear stress range analyzed in this work. POLYM. ENG. SCI., 56:837–845, 2016.