Identifying the roots of property variability in post-consumer recycled polypropylene feedstocks

• Feedstock heterogeneity identified in 11 post-consumer recycled PP flake samples. • MFR and ethylene content alone are insufficient to fully assess rPP variability. • Rheological Interfacial relaxation time (λ max ) serves as heterogeneity descriptor. • λ max acts as a comparative quantifier of interfacial tension (α) in comparable rPPs. • MFR, Et%, and λ max together explain rPPs mechanical property variability. The properties of post-consumer recycled polypropylene (PCR-PP) vary widely depending on feedstock composition, making reliable replacement of virgin polypropylene (vPP) challenging. While advanced washing and sorting technologies can remove many contaminants, fundamental inconsistencies persist from heterogeneous proportions of polypropylene grades with distinct chemical compositions and chain architectures, trace impurities, and their interactions with PP matrix. Conventional quality indicators like melt flow rate (MFR) and ethylene content (Et%) fail to capture the full sources of variability, particularly the interactions between ethylene phases and the PP matrix, which strongly influence mechanical performance but cannot be easily measured. To address this, PCR-PP feedstocks in flake stage were selected and sampled based on the theory of sampling and representative sampling principles to reflect real feedstock diversity at laboratory scale. Eleven industrial PCR-PP feedstocks, collected from different suppliers and production periods, were analyzed to capture realistic variability across supply streams and establish a feedstock comparison framework. Comprehensive compositional, rheological, structural, and morphological analyses revealed that interfacial behavior between PP and ethylene phases is a deterministic factor governing mechanical performance variation. A novel comparative framework using viscoelastic interfacial relaxation time ( λ max ) was introduced, providing an indirect yet sensitive method to quantify these hidden inconsistencies. Incorporating this metric alongside conventional MFR and Et% fully explains the observed mechanical differences, offering new insights into the fundamental origins of PCR-PP variability and establishing a practical methodology for more reliable feedstocks evaluation. These findings support improved recycling strategies and facilitate broader industrial application of recycled polypropylene.