Effect of mechanical recycling on diffusion behavior and mechanical properties of polypropylene in fused filament fabrication

Abstract The influence of mechanical recycling on interlayer welding in fused filament fabrication (FFF) remains insufficiently quantified, particularly with respect to the competing effects of molecular degradation and diffusion-controlled weld formation. This study establishes a mechanistic diffusion-degradation framework that links recycling-induced material variation to interfacial bonding in polypropylene (PP). Two successive recycling cycles were performed under controlled conditions, and the resulting materials were processed into filaments and printed at nozzle temperatures of 200 °C (L-condition) and 240 °C (H-condition). Recycling reduced the melt viscosity by up to 78%, accompanied by decreases in density, specific enthalpy, and heat capacity, suggesting thermo-mechanical degradation and changes in molecular structure. These temperature-dependent physical properties were incorporated into a self-diffusion coefficient theory to calculate accumulated interfacial diffusion during deposition. After two cycles, interfacial diffusion increased by up to threefold at the low printing conditions, while increasing the nozzle temperature produced nearly an order-of-magnitude enhancement relative to the low-temperature condition. Enhanced diffusion reduced mechanical anisotropy by approximately 53% at the lower printing temperature; however, tensile strength decreased by about 11%, demonstrating that molecular degradation counteracts diffusion-driven strengthening of the weld. Despite progressive degradation, specimens printed at the high-temperature condition exhibited strength comparable to injection-molded counterparts from the same recycled feedstock. This quantitative integration of recycling-induced structural variation with diffusion-controlled weld formation provides insight into the diffusion-degradation interplay and offers a framework for optimizing recycled semicrystalline polymers to balance sustainability, anisotropy reduction, and structural performance in additive manufacturing.