SENSITIVITY ANALYSIS OF THE BAGLEY CORRECTION TO SHEAR AND EXTENSIONAL RHEOLOGY

The excess pressure losses due to end effects in the capillary flow of two linear low-density polyethylene resins (LLDPE) were studied. Thes e losses were first determined experimentally by using two methods: 1) by extrapolating experimental data of pressure drop versus length-to- radius ratios (L/R) to zero capillary length and 2) by means of using orifice dies (L/R congruent to 0). Both methods resulted in practicall y the same end corrections. Numerical simulation was also used to mode l this important aspect of experimental rheology. The constitutive equ ations used in the simulations are a multimode K-BKZ equation, a multi mode Phan-Thien/Tanner, and finally a purely viscous Carreau equation. It was found that the numerical predictions agreed qualitatively but underestimated the experimental data for the various geometries used t o determine the end effects. Furthermore, it is demonstrated that the entrance pressure loss is also insensitive to extensional rheology, wh ile it depends more strongly on the shear rheology. This finding raise s doubts as to the usefulness of end pressure (known also as Bagley co rrection) as a method of determining the extensional viscosity of poly mer melts at high rates.