Numerical Analysis and Experimental Studies on the Role of Rheological Properties in Effecting Die Swell of Low Density Polyethylene, Polypropylene and Polystyrene: A Predictive Model Based on Newtonian Fluid

Abstract

Die swell measurements were performed on low density polyethylene, polypropylene, and polystyrene, in a Randcastle microtruder over a range of shear rate, length/diameter ratio (Li/Di) of die, and polymer melt temperature to accelerate efforts to develop reliable quantitative description of die swell phenomena observed in practical polymer processing operations. Die swell of these polymers increased with increasing shear rates. An increase in polymer melt temperature and the length/diameter ratio of the Microtruder die decreased the die swell of the polymers. A comparison of the experimental data with predictions from various existing models such as the ones reported by Bagley and Duffey, Mendelson et al., Tanner, White and Roman, Vinogradov and Malkin, Macosko and Kumar et al. revealed that existing models are not capable of accurately predicting the die swell of the materials. Therefore, a theoretical model based on strain energy density function, Gaussian network theory, and first normal stress difference with no adjustable parameters appropriate for determining die swell has been developed for Newtonian fluid. The experimental data for the polymers studied conformed excellently well with predictions from this proposed model. Thus for the first time a suitable quantitative model for exploring die swell phenomena in actual polymer processing equipment, such as an extruder, is established. Further, a special linear relationship between die swell and maximum recoverable deformation and a nonlinear relationship between die swell, storage and loss moduli have been established for these polymers. The first normal stress difference, calculated from the maximum recoverable deformation, has been found to vary strongly with shear stress and shear rate but independent of temperature for a specific polymer. Flow activation energies of the polymer melts decrease with increasing frequencies, indicating increased mobility of polymer chains at such frequencies.