Capillary extrusion flow of a fluoropolymer melt

Abstract

The capillary extrusion-forming flow of a fluoropolymer (FEP) melt was studied both experimentally and numerically. The excess pressure drop due to entry (related to the Bagley correction), the compressibility, and the effect of pressure and temperature on viscosity on the capillary data analysis have been examined. Using a series of capillary dies having different diameters, D, and length-to-diameter L/D ratios, a full rheological characterization has been carried out, and the experimental data have been fitted both with a viscous model (Cross) and a viscoelastic one (the Kaye—Bernstein, Kearsley, Zapas/Papanastasiou, Scriven, Macosko or K-BKZ/PSM model). For the viscous model, the viscosity is a function of both temperature and pressure. For the viscoelastic K-BKZ model, the time-temperature shifting concept has been used for the non-isothermal calculations, while the time-pressure shifting concept has been used to shift the relaxation moduli for the pressure-dependence effect. It was found that the viscous simulations gave good results in the range of apparent shear rates studied. The viscoelastic simulations gave slightly better results in reproducing the experimental data, especially for the entrance pressure losses for L/D = 0. It is concluded that pressure-dependence of the viscosity and viscoelastic effects are small to moderate in flow of the FEP melt, which is a linear polymer.