Rheological and thermal degradation behavior of polypropylene/low-density polyethylene blends under successive multiple processing cycles

This study explores how polypropylene (PP), low-density polyethylene (LDPE), and their blends degrade under repeated extrusion recycling cycles. To assess molecular, thermal, viscoelastic, and morphological changes, methods such as melt flow index (MFI), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), oxidation induction time (OIT), dynamic rheological measurements, and scanning electron microscopy (SEM) were used. Results showed that PP underwent progressive chain scission, resulting in a 44% rise in MFI and a significant decrease in molecular weight. In contrast, LDPE experienced a 35% reduction in MFI due to branching and partial crosslinking. The PP90/PE10 blend exhibited the highest degradation, marked by rapid MFI increase, lower OIT, and viscosity loss—effects linked to partial miscibility and interfacial stress that sped up oxidation and chain scission in PP. SEM images supported this mechanism by revealing many finely dispersed LDPE-rich areas and extensive interfacial regions in the 90/10 blend. Meanwhile, the 80/20 blend showed better stability, thanks to larger LDPE domains that distributed stress more evenly, which SEM imaging also confirmed. The crossover frequency (ωc) shifted to higher values for PP and the blends with more cycles, indicating faster relaxation and lower molecular weight. Conversely, LDPE shifted toward lower frequencies, consistent with crosslinking-driven elasticity. Overall, the study highlights the different degradation pathways of PP and LDPE and underscores the importance of blend composition and interfacial structure in influencing the recyclability and rheological behavior of PP/LDPE systems.