Numerical simulation of extrusion of S-PVC formulations in a capillary rheometer

Abstract

The supermolecular structures present in S-PVC during processing affect the rheological properties of the melt. The objective of this work is to investigate the ability of viscoelastic models to describe measured material functions of unplasticized suspension polymerized PVC, as well as the possibility of reproducing, in numerical simulations, the seemingly contradictory elastic properties of the large entrance pressure drop and small extrudate swell observed in capillary extrusion of PVC. We report results for two formulations of unplasticized PVC compound with different morphologies and levels of gelatination. The shear viscosity and slip were measured in a capillary rheometer using the Mooney technique (also verified with non-isothermal numerical simulations), elongational viscosity was estimated by use of the Cogswell analysis, and the linear relaxation spectrum was determined from creep and oscillatory shear in a parallel disk rheometer. These data were used to determine the model parameters for the multi-mode Phan-Thien and Tanner model and the K-BKZ-Wagner model with the single exponential damping function. It is not obvious whether it is possible to fit all these data using the models, due to the samples' different morphology. Then we investigated the self-consistency of the models and data analysis in numerical simulations of the capillary extrusion. The simulated Bagley corrections were in agreement with experiments for both models – supporting the Cogswell analysis – but only the PTT model predicted the extrudate swell properly – supporting the merits of this model over the Wagner model. We conclude that it is possible to make a self-consistent description using the PTT model based on a spectrum of relaxation times, determined from dynamical measurements, and model parameters determined from steady-shear and elongational (Cogswell) viscosity.