Effect of the Heat Treatment on the Mechanical, Thermomechanical and Morphological Properties of Recycled Polyethylene Terephthalate

The effects of thermally induced residual stresses on the mechanical, thermomechanical, and morphological properties of recycled polyethylene terephthalate (rPET) were systematically investigated. Specimens were subjected to free quenching at controlled bath temperatures (0, 10, 20, and 30°C) and compared against an annealed reference. Quenching at 20°C was identified as the optimal condition, maximizing Izod impact strength (~8.7 kJ/m2) and elongation at break (~5.1%) while minimizing elastic modulus (~0.95 GPa) and density. Differential scanning calorimetry (DSC) confirmed a moderate degree of crystallinity (χc = 33.6%) for this condition, reflecting an optimal balance between crystalline reinforcement and amorphous ductility. Dynamic mechanical analysis (DMA) corroborated these findings through an enhanced loss modulus peak and a characteristic residual stress profile featuring surface compressive stresses that impede crack propagation. These results establish quantitative microstructure–property relationships across multiple characterization scales and provide practical guidelines for optimizing the post-processing cooling stage in rPET injection moulding. These enhancements are attributed to the increased free volume retained upon rapid cooling, which promotes segmental chain mobility and augments the energy absorption capacity of the material under mechanical solicitation. The present findings establish quantitative microstructure–property relationships across multiple characterization scales, elucidating the mechanical behavior arising from the interplay between the crystalline and amorphous phases inherent to semi-crystalline of rPET.