Effect of the additive level and of the processing temperature on the re-building of post-consumer pipes from polyethylene blends

One of the main shortcomings of the mechanical recycling is that the properties of the secondary materials are generally sensibly inferior with respect to the virgin ones. This feature implies, for sure, that they cannot be used for the same technological application but also other uses of the secondary material are impossible. The aim of this work is to study the re-building of recycled post-consumer pipes, made of a blend of different polyethylenes, by re-processing in the presence of a nitroxyl compound as radical generator. The processing temperature and the additive concentration have been varied in order to study the effect of these parameters on the final properties. The mechanical, rheological, solubility and thermal results indicate that low levels of additive and lower processing temperatures mostly cause long chain branching of the polyethylenes. On the contrary, at higher concentrations and temperatures cross-linked structures are formed. Dramatic variations of the melt viscosity are observed at low shear rates for the added materials, but in the window of the shear rates commonly used for processing, the viscosities are similar to the one of the as received material. Low processing temperatures and additive levels allow enhancing the ultimate properties, which approach the ones of the virgin materials. The elastic modulus is in any case lower than the one observed in the non-added material due to change in the molecular structure and in the crystallinity. The impact strength strongly increases in the presence of the additive. The presence of crosslinks that deviate the cracks and a local plastic deformation at the interface between the crosslinks and the matrix can be invoked to explain the results.