Abstract
Measurements of uniaxial, biaxial, and planar elongational viscosity and step stress relaxation of a low density polyethylene (LDPE) melt were carried out. The description of experimental uniaxial, biaxial, and planar elongational viscosities results by the Kaye-Bernstein-Kearsley-Zapas (K-BKZ) constitutive equation was examined. Three types of damping function forms, Wagner-Demarmels (WD) model, Papanastasiou-Scriven-Macosko (PSM) model, and Luo-Tanner (LT) model, have been proposed in the K-BKZ constitutive equation. These three damping function forms were examined to see which was able to describe the experimental shear, biaxial, and planar damping functions. The experimental shear and biaxial damping functions were described by the LT and PSM models better than the WD model, however, the experimental planar damping function was not described well by any of the three models. After estimating the best parameters in each model by fitting the three experimental damping functions, the uniaxial, biaxial, and planar elongational viscosity was calculated by the estimated parameters. The uniaxial elongational viscosity was predicted best by the LT model, then by the WD, and PSM models. For biaxial and planar elongational viscosity, none of the three models predicted well, thus the lack of suitability of the biaxial and planar elongational viscosity to the K-BKZ model was suggested.
