Sustainable additive manufacturing: Microstructural evolution and mechanical viability of recycled Flax/PP via fused granular fabrication

This study investigates the recyclability of flax fiber-reinforced polypropylene (Flax/PP) composites processed via fused granular fabrication (FGF), with a focus on microstructural evolution, thermal degradation, and mechanical performance under repeated recycling. Flax/PP composites were subjected to six consecutive recycling cycles (R0–R5). Mechanical testing, scanning electron microscopy (SEM), and infrared thermography were performed at each cycle to assess property degradation, while thermal analysis (DSC and TGA) was conducted at selected stages (R0, R2, and R5). Results show a progressive and irreversible decline in mechanical performance beyond two cycles: tensile strength dropped from ∼13 MPa (R0) to ∼7 MPa (R5), and elongation at break fell from ∼9 % to ∼3 %, accompanied by a shift from ductile to brittle failure. SEM observations revealed increasing fiber pull-out, interfacial debonding, and porosity growth from ∼12.5 % to ∼32.6 %, with maximum pore sizes exceeding 400 µm. DSC and TGA results confirmed thermal degradation of both flax fibers and the PP matrix, including decreased crystallinity and reduced thermal stability. By the sixth cycle, severe interlayer delamination and nozzle clogging rendered the material unprintable. These findings elucidate the degradation pathways of natural fiber composites under intensive melt processing and provide a foundation for improving recyclability. Future work will verify whether strategies such as virgin PP blending, optimized processing conditions, and the use of bio-based coupling agents can mitigate the observed thermal and interfacial deterioration.