Characterization and comparison of virgin and recycled polypropylene filament feedstocks for fused filament fabrication 3D printing

Polypropylene (PP) currently represents one of the most used commodity plastics for consumer goods, e.g., packaging and healthcare. Despite its use for long lasting applications, its properties are suitable for a wide range of single use disposable products, such as personal protective equipment. This fact leads to a significant amount of PP to be handled at their end of life, which can be reintroduced in new resource loops as a secondary raw material through alternative processes, e.g., additive manufactur ing. This work compares the thermal, rheological, and mechanical properties of virgin and recycled PP filament feedstocks for Fused Filament Fabrication (FFF) and their printability for potential applications. A commercial virgin PP filament and a 100% recycled one from the same manufacturer were selected for the tests. The filaments were 3D printed with a desktop size FFF 3D printer with a 0.4 mm diameter nozzle (Prusa i3 MK3S). The thermal and rheological characterization of the filaments before and after 3D printing was performed through Differential Scanning Calorimetry and flow stress ramp tests. The tensile properties were evaluated on two batches of 3D printed specimens following the ASTM D638-22 standard. Some 3D printed samples were fabricated to compare the printability of the two filaments. Despite the additional thermomechanical degradation in extruding the recycled filaments, the 3D printed samples reached good overall quality in terms of extrudate consistency and shape retention, achieving similar results with virgin and recycled PP. No detachment, delamination or warping occurred during or, after the fabrication of the samples. The tensile tests also showed comparable values between the two filaments, confirming the limited impact of thermomechanical degradation on the 3D printed parts. According to the tests, the recycled PP filament exhibits thermal, rheological, and mechanical properties comparable to the virgin feedstock. Its processability, dimensional stability, and achieved shape retention demonstrate the potential use of this recycled for 3D printing. These filaments can be used in different sectors, from furniture to assistive technolog y. Future work can consider products at their end of life as sources of recycled PP, e.g., disposable personal protective equipment, finding new ways to reintroduce them in new loops through 3D printing.