Towards Sustainable Food Packaging: Mechanical Recycling Effects on Thermochromic Polymers Performance

Integrating thermochromic pigments (TPs) into food packaging offers significant benefits for monitoring temperature variations, improving food safety, and reducing waste. However, the recyclability of such materials remains underexplored, particularly regarding the retention of their optical and mechanical properties after repeated recycling. Addressing this gap, this research aims to evaluate how mechanical recycling affects key properties of polypropylene (PP) blends containing varying TP concentrations. Three formulations, PP100/TP0 (0% TP), PP98/TP2 (2% TP), and PP92/TP8 (8% TP), were subjected to five recycling cycles, with changes in thermal stability, color transition behavior, mechanical integrity, and surface morphology analyzed. The results indicate that PP100/TP0 maintained its mechanical integrity with minimal degradation (6% absolute crystallinity loss; color difference ΔE*_ab = 1.45) across recycling cycles. However, blends containing TPs exhibited progressive deterioration. P98/TP2 displayed moderate reductions in mechanical strength (-10.8%) and thermochromic efficiency (color change ΔE*_ab = 6.52), while PP92/TP8 showed significant degradation, including increased activation temperatures (+3.8 °C) and color vibrancy loss (42.9% loss in saturation). These effects were attributed to polymer breakdown, pigment aggregation, and altered crystallinity. Despite the limitations of recyclability, this study provides critical insights into the feasibility of TPs in sustainable, intelligent food packaging. Further research is required to enhance TP stability during reprocessing, ensuring long-term functionality in circular packaging systems.