Effect of processing history on the rheological properties, crystallization and foamability of branched polypropylene

The effects of processing history and post-processing annealing procedure on the rheological properties of long-chain branched polypropylene (LCB-PP) have been studied intensively. It is found that melt flow index, swelling ratio, storage modulus and complex viscosity at low frequency, as measures of melt viscoelasticity for LCB-PP, were greatly influenced even by the short-time processing in an internal mixer. Further, the relationship between the magnitude of shear modification and total shear strain was linear on a logarithmic scale. Moreover, the rheological properties of processed LCB-PP were recovered by vacuum-annealing to some extent. It also demonstrates that more intense processing history resulted in longer time required for the recovery of rheological properties. As far as the recovery process was concerned, high-pressure CO₂ saturation annealing was much more effective than vacuum-annealing and more sufficient recovery was brought by higher saturation pressure. This superiority of high-pressure CO₂ saturation annealing was attributed to the larger free volume for PP chain movement, which shorten the annealing time required for long branches to recover to their initial states. Besides, the peak temperature during the crystallization of LCB-PP was substantially improved after processing. Meanwhile, the foamability of LCB-PP was deteriorated after shearing process, which manifested as larger cell size and lower cell density.