ESTIMATION
OF EXTENSIONAL PROPERTIES OF POLYETHYLENE MELTS FROM MELT STRENGTH MEASUREMENTS
Rahul
K. Gupta, Ranjit Prasad and Sati N. Bhattacharya*
Rheology and Materials Processing Centre, School of Civil &
Chemical Engineering, RMIT University, 124 La Trobe Street, Melbourne, Vic 3000
Australia
Extensional
viscosity plays an important role in many polymer processes such as film
blowing, fiber spinning, blow molding and sheet casting. Melt extension
measurements have found widespread use in determining the extensional flow
properties of different polymer melts as it is simple, reproducible and is
representative of many polymer processes. All of these test results have been
performed at a constant mass flow rate and can only represent a qualitative
comparison of melt extension because of the non isothermal stretching and
prehistory of the polymer melt in the die and extruder. This paper is aimed at
studying how uniaxial melt extension experiments for polyolefins can be used to
estimate the transient extensional viscosity. An analytical Wagner model in the
linear region of the drawing test was used to calculate the transient
extensional viscosity. An optimal extrusion and drawing parameters for a
reliable drawing test were obtained so that true and accurate melt strength of
a polymer could be obtained. Later, these results were compared to the
transient extensional viscosity obtained from a constant strain rate
Rheometrics Melt Extensional (RME) rheometer. The results agree within the
measurement accuracy with those measured from RME. Thus, extensional viscosity
calculated from uniaxial melt extension experiments can be used to estimate
transient extensional viscosity.
Keywords: LDPE, Wagner model, Transient extensional
viscosity, melt extension, RME