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. These 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≅0). Both methods resulted in practically the same end corrections. Numerical simulation was also used to model this important aspect of experimental rheology. The constitutive equations used in the simulations are a multimode K-BKZ equation, a multimode 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 to determine the end effects. Furthermore, it is demonstrated that the entrance pressure loss is also insensitive to extensional rheology, while it depends more strongly on the shear rheology. This finding raises doubts as to the usefulness of end pressure (known also as Bagley correction) as a method of determining the extensional viscosity of polymer melts at high rates. Received: 18 December 1997 Accepted: 21 May 1998