The melt extensibility of polypropylene

Abstract

AbstractA Göttfert Rheotens melt strength tester was used to measure the melt strength of several commercially available polypropylene (PP) samples. The melt strengths of the polymer melts investigated were relatively low, in the range 1–10 cN, and were found to be strongly dependent on the extrusion (extrusion flowrate and melt temperature) and drawing (draw distance and draw rate) parameters of the test. A two‐parameter exponential relationship was found between the melt strength of the polymers and the extrusion rate. The melt strength dependence on extrusion rate was defined by a constant parameter, B, which was in the range of 0.003–0.02 and strongly related to the polydispersity of the polymer. An equilibrium melt strength value was found as the extrusion rate tends to zero and was independent of draw distance. Arrhenius temperature dependent flow activation energies determined from the melt strength test were 32–42 kJ mol−1, agreeing with literature values cited for the narrower polydisperse polymers. The relatively low melt strength of conventional linear PPs was enhanced by melt blending with a commercial modifier, EP1, which was a blend of 50 wt% oligocyclopentadiene and 50 wt% PP. Above concentrations of 40 wt% EP1 the melt became phase separated and showed very different behaviour in melt strength. The transient initial stretching and draw ratio at the onset of instability for the Rheotens tensile flow curve were successfully correlated to the melt strength plateau.© 2001 Society of Chemical Industry