Effects of degree of recycling on the rheology and processability of thermoplastic polymers

The effects of recycling on the processability of two injection molding grade thermoplastic polymers has been investigated. Recycling of a Polypropylene (PP) and a Polyamide (Nylon 6, PA6) has been simulated by repeated injection molding cycles, followed by blending of the processed materials with the virgin polymer. The flow properties of both the recycled materials and the blends have been evaluated by different rheological techniques, namely, capillary viscosity and MFI measurements, linear viscoelastic tests and solution viscosity number determination. The experimental results clearly indicate a qualitatively different behaviour of the two polymers upon recycling. PP shows a reduction of both viscosity and viscoelastic properties upon increasing the number of processing cycles, thus confirming the thermo-mechanical degradation of this polymer. PA6, conversely, shows an increase of both the melt and the number viscosity as the number of injection molding passages is increased. The results on PA6 suggest also that the thermally-induced re-gradation is accompanied by an increase of the degree of branching, that may be responsible for some peculiar features of the Virgin/Recycled blend rheology.