Effect of Mechanical Recycling on Structural Modification and Mechanical Behavior of PET

Abstract

Climate change and the engendered environmental stress are putting pressure on the search for greener solutions. One striking example of environmental stress is plastic pollution (Geyer et al., Sci Adv. 2017, 3, e1700782). Besides reducing plastic consumption, recycling is today one of the applicable approaches (Damayanti and Wu, Polymers 2021, 13, 1475). In this context, poly(ethylene terephthalate) (PET) is in the spotlight. More than having unique properties, PET occupies the podium of recycled polymers worldwide today (Damayanti and Wu, Polymers 2021, 13, 1475; Welle, Resour. Conserv. Recycl. 2011, 55, 865–875). However, despite efforts in PET recycling and the extensive number of published papers on the subject, many challenges continue to restrain its development. One of the reasons for this is a lack of understanding of the structural modifications caused by processing. Moreover, these changes impact both the processing step and the final product properties. This study focused on the effect of the mechanical recycling process on the mechanical response of PET. By successive cycles of extrusion and injection molding, recycled PET (rPET) could be produced and characterized. The widely described chain scission was confirmed by size exclusion chromatography. The consequent loss of viscosity was observed during the processing and by rheological measurements, rendering the process more and more challenging. A modification of the crystallization kinetics was also brought to light by different techniques, such as differential scanning calorimetry and polarized light optical microscopy. Despite the different effects on the matrix, the mechanical characterization showed a stable response. Tensile properties showed a highly ductile material with more than 400% elongation for both virgin and rPET. The notch sensitivity of this material was also observed in Izod impact tests, remaining relatively low but stable. A better understanding of the relationship between structural modifications and final properties may enable fine-tuning of impact modifier structures to enhance rPET toughness and broaden its applications.