Modeling of the rheological behavior of polyethylene/supercritical CO2 solutions

This paper deals with the modeling of the rheological behavior of polyethylene (PE)/supercritical CO2 solutions. Melt viscosity measurements were performed using a capillary tube die attached to a twin screw extruder, for different amounts of CO2 injected into the extruder barrel (from 0 to 4 wt%). The viscosity of the solutions was determined by measuring the pressure drop and the polymer flow rate through the die. A D-optimal experimental design was developed to determine the minimal number of experiments necessary with a minimization of the error on the model coefficients. The experimental results showed that the viscosity of PE/CO2 solutions depends on shear rate, temperature, pressure, and CO2 concentration. The pressure in the capillary was modeled by a second order polynomial. A theoretical model based on a power law was proposed to describe the pseudoplastic behavior of PE/CO2 solutions with different shear rates. Correction factors were included to take into account the effects of temperature, pressure, and CO2 concentration.