Impact of Seasonal Composition on the Long-Term Performance of Post-Consumer Recycled Polypropylene

Driven by global sustainability goals, the use of recycled polymers is increasing, but their inconsistent quality poses a challenge for reliable use in technical applications. In this study, ten batches of an industrially used post-consumer polypropylene (PP) recyclate were examined over 15 months, all ordered with identical specifications for melt flow rate (MFR = 7–13 g/10 min) and density (0.97 ± 0.03 g/cm3). Despite fulfilling these specifications, considerable differences were found in rheological, thermal and mechanical properties. Filler content varied between 2% and 24%, strongly affecting mechanical performance. While Young's modulus and yield stress varied slightly, elongation at break differed greatly (10%–100%). Particularly large differences were found in the crack resistance. At the same stress level, the number of cycles until failure ranged between 3000 and 6.52 million cycles. These deviations are attributed to variations in molecular structure, filler content and polymeric and non-polymeric impurities. The findings highlight that, despite fulfilling the target specifications for MFR, considerable compositional differences can significantly affect the material performance. Standard parameters are therefore insufficient to assess the suitability of recycled PP for technical applications. Comprehensive characterization, narrower specification limits and improved sorting are essential for the reliable and sustainable use of recyclates in long-term applications.