Mechanical compatibilization and property improvements of immiscible HDPE/PP blends via differential speed rolling

Polyethylene (PE) and polypropylene (PP) comprise over 50% of global polymer production due to their low cost, processability, and tunable mechanical properties. However, their thermodynamic immiscibility and poor interfacial adhesion significantly limit the recyclability of mixed polyolefin waste. Conventional compatibilization strategies rely on chemical additives to overcome weak interfaces and subsequently improve the mechanical performance of PE/PP blends. In this work, we demonstrate a processing-driven strategy to leverage weak interfaces to mechanically compatibilize high density PE (HDPE)/PP (50:50) blends without chemical additives. Using differential speed rolling (DSR), a solid-state processing technique adapted from deformation processing of metallic alloys, we impart a shear gradient into a sheet of blended material. This shear gradient promotes domain coalescence, leading to hierarchical fiber formation, micro- and nanovoid formation, and controlled interfacial debonding. Consequently, blend toughness increases 20x (0.74 to 16.2 MJ/m 3 ) while density decreases by ∼2.4% (0.927 to 0.9 g/cm 3 ). In contrast, equal speed rolling (ESR) yields only modest mechanical improvements. The results suggest that controlled interfacial instability—rather than suppression of phase separation—can be leveraged to enhance the mechanical performance of immiscible blends. This work establishes DSR as a scalable, solvent-free route for valorizing immiscible polyolefin waste.