Printability versus durability in 3D-printed biocomposites: Unraveling the distinct roles of flax and hemp fibers in PLA-based filaments

The expansion of Fused Deposition Modeling (FDM) towards low-volume industrial production and customized tooling requires bio-based materials that balance processability with long-term performance. This study addresses the critical trade-off between printability and environmental durability in polylactic acid (PLA) filaments reinforced with flax and hemp fibers (2.5–15 wt%). While both reinforcements improved the stiffness of the matrix, their distinct morphological and chemical characteristics led to divergent behaviors. Flax fibers, characterized by a smoother surface and lower lignin content, facilitated superior melt flow and interlayer adhesion, resulted in a tensile strength optimum at 10 wt%, followed by a slight decline likely due to fiber aggregation. A twofold increase in impact strength at 15 wt% was achieved, governed by energy dissipation during fiber pull-out and debonding mechanisms. Conversely, hemp fibers exhibited higher surface roughness and lignin content, which hindered interlayer fusion, evidenced by increased porosity in microstructural analysis, but provided exceptional resistance to degradation. Under accelerated weathering conditions, hemp-reinforced composites retained structural integrity significantly better than neat PLA and flax composites, attributed to the photoprotective and hydrophobic nature of lignin. These findings introduce a novel selection criterion for engineering biocomposites: flax for high-mechanical-performance components such as customized tooling or structural parts and hemp for applications requiring extended service life in harsh environments (e.g., outdoor fixtures and housing).