Stick and slip phenomena during extrusion of polyethylene melts as investigated by laser-Doppler velocimetry

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Abstract
The flow behavior of polyethylene melts in a slit die was investigated using laser-Doppler velocimetry. Two polyethylenes, a linear and a long-chain branched sample, were compared in order to get an insight into the influence of branching on stick and slip phenomena. For the long-chain branched polyethylene, velocity profiles were obtained which do not give any indication to wall slip in the range of shear rates applied. For the linear polyethylene, the velocity distributions in the three regions, commonly distinguished by different pressure-output relations in the literature, were measured. Pronounced wall slip velocities are detected at low apparent shear rates. The importance of this finding for the determination of viscosity functions is discussed. At higher output rates the well-known pressure oscillations are accompanied by velocity fluctuations of the same frequency but totally different shapes of amplitudes. Significant periodic features of the time dependence of the velocity are tried to be interpreted by a hypothesis of entanglement and disentanglement of the molecules close to the wall. Exceeding an upper critical output value, a flow region is reached which is distinguished by a constant pressure reading as a function of time again. Within this region a nearly ideal plug flow is observed, i.e., there is an indication for strong slip at the wall.