Additive-Assisted Upcycling of Recycled Polyolefins: Tailoring the Molecular Architecture and Processability of polyolefins

Mechanical recycling of polyolefins is often limited by the progressive deterioration of polymer microstructure during repeated melt processing, which leads to reduced molecular weight, loss of ductility, poorer flow behavior, and restricted end-use applications. In this context, the present work proposes a combined perspective on the upgrading of two major commodity polyolefins, high-density polyethylene (HDPE) and polypropylene (PP), through the use of commercially available repair additives able to direct degradation pathways toward more favorable molecular architectures. In both systems, the objective is to move beyond conventional downcycling and enable the production of recycled materials with improved and tunable processability and performance.

For HDPE, the study shows that thermo-mechanical degradation generates a heterogeneous microstructure arising from chain scission, branching, and crosslinking, with a dramatic reduction in elongation at break. By introducing Nexamite® R305 into degraded HDPE and subjecting the material to further reprocessing, it is possible to selectively promote long-chain branching. This modification significantly increases melt strength and improves processability under elongational flow, enabling the production of fibers with markedly enhanced elongation at break compared with untreated recycled HDPE.

For PP, multiple extrusion cycles cause a progressive decrease in zero-shear viscosity, indicating molecular weight loss and worsening flow characteristics. The introduction of Nexamite® R201, either in the early stages of recycling or after extensive degradation, mitigates this reduction and helps preserve or recover processability. Furthermore, under longer residence times, the additive promotes melt structuring phenomena, such as branching or crosslinking, allowing the achievement of recycled PP with adaptable rheological behavior.

Overall, the obtained results demonstrate that additive-assisted reactive reprocessing is an effective strategy to tailor the molecular architecture of recycled polyolefins, opening new opportunities for the upcycling of HDPE and PP into materials suitable for more demanding and higher-value applications.