Long-term thermal aging of recycled polypropylene: Evaluation of different alditols, thiosynergists, and acid scavengers as novel stabilizer combinations

The long-term thermal stability of recycled polypropylene (rPP) is a critical factor for its use in durable applications and is strongly affected by oxidative degradation and stabilizer depletion during the previous life cycle. Novel stabilizer systems for rPP based on bio-derived components such as alditols or thiosynergists and inorganic acid scavengers were systematically investigated and compared to conventional and commercial recyclate reference formulations. rPP from automotive battery cases was compounded with different additive combinations and subjected to accelerated oven aging at 150 °C and 135 °C. The aging behavior was evaluated using mechanical testing, melt volume rate measurements, determination of the carbonyl index, and molecular weight characterization through gel permeation chromatography. The results demonstrate that the choice of alditol, thiosynergist, and acid scavenger significantly influences the resistance to thermo-oxidative degradation. Several bio-based formulations show delayed embrittlement, reduced oxidation, and improved retention of molecular weight compared to standard phosphite-containing systems. The study highlights synergistic effects between stabilizer components, provides insight into structure-performance relationships relevant for the development of alternative stabilization strategies for rPP, and gives considerations of possible mechanisms of action for alditols. • Bio-based stabilizer systems improve long-term thermal stability of rPP. • Synergistic effects enable sustainable stabilization strategies for rPP. • Structure-performance relationship is identified for alditols in combination with thiosynergists.