Measurement of Slip Velocity and True Flow Curve of Poly (vinyl chloride) Melts

Abstract

PVC melts have viscoelastic properties. Moreover PVC melts have additional flow characteristics such as slippage on the wall of solid surfaces. The velocities on the wall of most polymer melts are usually zero, but those of PVC melts are not zero but finite. In the present paper, the slip velocities were measured by a simple method using some grooved nozzles, which can stop the slippage on the wall over a certain shear rate range. The measured values of the slip velocities were found to be from 10 to 60% of the average velocities. It was found that grooved nozzles should be suitably designed for various compounds, flow conditions, geometries, etc. The true flow curves, defined as those in which the effect of the slippage is removed, had almost the same property as the flow curves of the other polymer melts without slippage. A simple model of two-layer flow was introduced in order to predict the slip velocities and deepen understanding of the slippage mechanism. A very thin layer of a low molecular weight liquid is assumed to ooze out on the wall from a bulk flow layer of the PVC compound. The slip velocities could be predicted approximately from the model. It may be concluded that the very thin layer really exists, since the slip velocities had almost the same values regardless of the differences in the surface metals of the nozzles.