Crystallization behaviour of recycled polyolefins blends

A novel tailor-made thermal fractionation protocol, based on the Successive Self-nucleation and Annealing (SSA) method, was developed to investigate the complex chemical composition of PE/PP blends derived from recycling. The temperature regions where co-crystallization among the blend components do not occur were assessed, enabling the development of the quantitative method. Furthermore, a set-up for achieving Continuous Cooling Curve diagrams was designed, and allowed to study the crystallization kinetics at processing-relevant cooling conditions of the phases in the blends. An “inversion point” in the crystallization order of the two polymers arises from the difference in crystallization rates between PP and PE with increasing cooling rate. Mutual nucleating effects, found at the interface between the phases, correlate with the inversion point. Moreover, the order of crystallization of the two polymers at low cooling rates, i.e., before the inversion point, can be tuned by employing neat or nucleated PP. This demonstrates the importance of knowing and controlling the type of components in recycled blends. Finally, the nature of such nucleating effects was revealed by a novel approach for studying surface-induced crystallization in the blends. The method consists of detecting variations in the crystallization kinetics of the dispersed phase (PE) with changing the crystalline state of the matrix (PP) through self-nucleation. The enhancement of crystallization kinetics of PE that was achieved when increasing the lamellar thickness of PP, together with the very low value found for the interfacial free energy difference, are evidence that such nucleating effects occur through epitaxial growth.