MELT STRENGTH BEHAVIOR OF POLYPROPYLENES

Melt strength data are presented for polypropylenes, differing widely in molecular weight and molecular weight distribution. Melt strength was assessed from uniaxial tensile experiments with a Rheotens apparatus. A new measuring procedure was applied by performing the stretching in front of the take-up wheels of the Rheotens instead of in between the wheels. It is shown that the melt strength of polypropylene is solely determined by its weight average molecular weight. At a given melt index level, narrow molecular weight distribution (controlled rheology) polypropylenes have a lower melt strength than the wider distribution reactor grades. At high temperatures, the melt strength follows an Arrhenius type temperature dependence, while at the lower test temperatures the melt strength of polypropylenes having a wider distribution is increased by flow-induced crystallisation.