Rheological insights into the degradation behavior of PP/HDPE blends

The (re-)processing and degradation behavior of polypropylene (PP) blends with high-density polyethylene (HDPE) is a crucial topic in mechanical recycling due to the prevalence of PP/HDPE mixtures. In particular, small amounts of cross-contamination (below 10.0 wt.%) between these polyolefins are quite common. While much empirical data is available, systematic studies on such blends are scarce and rarely account for the polymers' chain structure. This study employs time-resolved rheology, atomic force microscopy (AFM), differential scanning calorimetry (DSC) and chemiluminescence techniques to study the degradation of different PP types containing 0.5 to 10.0 wt.% of HDPE. It is observed that the addition of small amounts of HDPE can result in a prominent change in the viscoelastic properties of PP during the thermo-oxidative degradation. The substantial evolution of elasticity in the blends is mainly related to the gelation of fine PE particles dispersed within the PP matrix. It is also shown that a smaller viscosity ratio between the blend components can lead to a more significant degradation through morphology alterations. Remarkably, time-resolved rheology studies revealed that even a small amount of HDPE (2.5 wt.%) can lead to a reduction in the thermo-oxidative stability of the PP/ HDPE blend through excessive branching/crosslinking of the HDPE phase. This effect was particularly more pronounced when the PP contained an ethylene comonomer in its backbone. The finer morphology of PE particles in PP copolymer can exacerbate the thermo-oxidative stability of the blend. These insights are essential in designing new life cycles for recycled PP cross-contaminated with HDPE, typically found in the light fraction of municipal solid plastic waste.