Mechanical recycling of polyethylene terephthalate (PET) is a key strategy for circular packaging applications; however, repeated thermo-mechanical processing leads to progressive polymer degradation. In this study, the effect of controlled repeated extrusion on the degradation behavior of bottle-grade PET was systematically investigated under laboratory conditions. Mechanical recycling was simulated using a co-rotating twin-screw extruder, where PET was subjected to up to four consecutive processing cycles corresponding to a cumulative residence time of 8 min. Progressive processing resulted in chain scission, reflected by a decrease in intrinsic viscosity from approximately 0.80 to 0.65 dL·g⁻¹ and a corresponding reduction in molecular weight. Melt flow rate increased accordingly, indicating a gradual loss of melt strength. Differential scanning calorimetry revealed that the glass transition and melting temperatures remained nearly unchanged, while the degree of crystallinity increased from approximately 23.0% to 29.5%, accompanied by changes in crystallization behavior. These structural changes led to reduced ductility, with elongation at break decreasing from about 84% to 60%. Optical analysis showed systematic material darkening, and a strong linear correlation between lightness (L*) and intrinsic viscosity was observed. By isolating intrinsic thermo-mechanical degradation effects under controlled processing conditions, this study enables a clearer definition of realistic reuse limits for mechanically recycled bottle-grade PET. The results indicate that bottle-grade PET retains properties compatible with demanding applications only after a limited number of thermo-mechanical processing cycles, whereas further processing restricts its usability to less demanding applications such as fibers, films, and non-food packaging.

Residual polymeric contaminants from labels, closures, and adhesives can affect the quality and reprocessability of mechanically recycled polyethylene terephthalate (PET). Direct comparison of individual contaminants, however, is often complicated by differences in PET feedstock, contaminant concentration, and processing conditions among published studies. In this work, eight packaging-relevant contaminants—polypropylene (PP), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polystyrene (PS), glycol-modified PET (PETG), poly(lactic acid) (PLA), poly(vinyl chloride) (PVC), and a commercial hot-melt labeling adhesive (HMA)—were compared at a fixed model concentration of 3 wt.% in bottle-grade PET under identical thermomechanical processing conditions. Contaminant selection was supported by ATR-FTIR screening of commercial bottle labels. Melt flow rate, MFR-derived intrinsic viscosity, differential scanning calorimetry, tensile properties, and CIE L*a*b* color characteristics were evaluated over successive processing stages. PET/PP, PET/LDPE, PET/HDPE, PET/PS, and PET/PETG remained processable through three processing stages, whereas PET/PLA and PET/HMA could only be processed through the second stage, and PET/PVC lost stable strand-forming ability after initial compounding. Tg and Tm remained comparatively stable, while crystallization temperature and PET crystallinity showed contaminant- and processing-dependent changes. Elongation at break was generally more sensitive to contamination than tensile strength, and PVC produced the most pronounced discoloration. Industrial PET fractions collected at successive separation and washing/decontamination stages were evaluated as a complementary case study. Progressive purification was associated with improved MFR-derived IV, while TGA showed only minor differences among the fractions. The results demonstrate that contaminant identity strongly affects PET property evolution and reprocessability during repeated mechanical recycling.