Boosting the reinforcing potential of flax co-products: Biomass fractionation and composite microstructure as keys

The growing demand for sustainable thermoplastic composites has increased interest in using agricultural co-products as reinforcements due to their low cost, wide availability, and low density. This requires an optimization of fractionation processes, i.e. milling and sieving procedures, to develop mechanically efficient composite microstructures. This study investigates the influence of flax shives (a co-product of the flax industry) fractionation on the microstructure and mechanical performance of injection-moulded polypropylene composites. A multi-scale experimental approach, including 2D particle size and shape analysis, X-ray tomography, tensile testing, and micromechanical modelling, was employed. Our results show that optimizing flax shives size and shape distribution via fractionation improves their aspect ratio and orientation in the composite, thus enhancing the composite stiffness and strength by up to 17.9 % (95 % CI: 9.6–26.2 %) and 25.2 % (95 % CI: 24.0–26.5 %), respectively, when comparing composites with the lowest and highest properties. To understand the origin of the reinforcement mechanisms, the stiffness of flax shives was back-calculated based on different micromechanical models using both analytical (Halpin-Tsai, Mori-Tanaka) and numerical finite element modelling. The findings highlight that model selection and orientation assumptions strongly influence the estimated stiffness. Based on realistic microstructural inputs, the flax shives stiffness was estimated to be in the range of 21.84–36.45 GPa using Mori-Tanaka model and 11.83–23.0 GPa using Finite Element modelling depending on the tested flax shives fractions. This integrated approach demonstrates the importance of tailoring plant-based filler morphology to optimize their reinforcing capacity while offering a transferable methodology for valorising agricultural co-products in composite materials. • Flax shives (FS) particle size strongly governs composite microstructural features. • 200–315 µm FS yield the highest aspect ratio (2.61) in composites. • FS fractionation boosts tensile strength (25.2 %) and stiffness (17.9 %) of composites. • 2D/3D imaging maps FS size influence on tensile performance. • FS modulus (11.8–36.5 GPa) predicted via Mori-Tanaka and FE modelling.