Extensional rheology of polypropylene in relation to processing characteristics

Transient extensional rheological behaviour of several polypropylene (PP) homo- and co-polymer grades was studied at different temperatures. In addition, shear rheological characterisation was completed. The structure of PP grades was characterised by using gel-permeation chromatography (GPC), differential scanning calorimetry (DSC) and hot-stage optical microscopy (HS-OM). It was found that the extensional viscosity of the polymers followed the three-fold transient shear viscosity curves when the tests were conducted in fully molten state, as expected for linear polymer melts. However, the strain hardening behaviour was evident when PP was stretched in the semi-molten state, i. e. below its melting temperature (Tm). Similar to results of a recent study by Rauschenberger, three temperature zones of different extensional behaviour were found: linear viscoelastic behaviour and uniform drawing in the molten state (180°C), unstable drawing just above Tm (170°C) and stable drawing with strong strain hardening below Tm (165°C). The strain hardening behaviour was quantified and related to molecular structure and morphology of PP. It was found that none of the investigated parameters alone was sufficient to predict the extensional behaviour of PP. The definition of Extensional Viscosity Index (EVI) as the ratio of extensional viscosities at the Hencky strain of 3 and 1, respectively, was found to be practical for the evaluation of the polymers¿ processability for the important industrial processes, such as BOPP film manufacturing, where extensional deformation predominates.