Effects of Packaging-Relevant Contaminants on the Property Evolution and Reprocessability of PET During Repeated Mechanical Recycling

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.