Flow Visualization for Extensional Viscosity Assessment

Abstract

In many melt processing such as in blow molding, vacuum forming, and film extrusion, extensional or stretching flows play a crucial role and therefore, extensional rheology has a significant effect on the processability and also on the properties of final products. Extensional melt characterization is hence required in order to understand flow properties in various flow conditions. Despite a number of studies carried out in extensional melt characterization, it has not been adequately resolved how melts behave in extensional flows. The extensional viscosity assessments described in this work are based on flow data obtained from an actual extrusion line under real process conditions. The values of extensional viscosity in this work are obviously for nonsteady state flow conditions from a 180-degree entry angle slit die flow cell: the flow is subject to varying strain rate over the convergence path. The property determined from the assessment described here is therefore, referred to as an apparent extensional viscosity. Two methods, one based on a direct measurement of particle velocimetry on the center-line of a convergent flow and one based on a simple model velocity field inside the natural entry profile (both using stress birefringence for extensional stress quantification) are shown to provide routes to measuring the apparent (unsteady state) extensional viscosity of two polyolefins under practical process conditions, with good agreement between the two routes. Apparent extensional viscosity of melts has been measured at stretch rates in the range 0.1 to 80 s-1 in these studies.