Abstract
Transient elongational viscosities of polyethylene melts from converging flow (entrance pressure losses) and from uniaxial stretching after extrusion (Rheotens test) are compared with steady‐state viscosities obtained by isothermal homogeneous drawing in different elongational rheometers. The laboratory tests allow predictions on differences of the drawability (minimum film thickness) in tubular film blowing. If the shear viscosities of two low‐density polyethylene (LDPE) melts are the same, the material exhibiting the lower level of elongational viscosity can be stretched to higher total strains before break. The shape of the die is shown to have a significant influence on the subsequent stretching behavior due to preorientations imposed during extrusion. Converging flow decreases drawability, whereas diverging dies yield an increase of the maximum pulling speed before break. The temperature dependence of the drawdown force in the Rheotens test can be predicted from the flow‐activation energy of the melt...
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