Effect of flax yarn surface treatment on the fibre-matrix interfacial performance of flax-PLA 3D printed structures

The combination of continuous natural fibre and fused filament fabrication 3D printing enables the manufacturing of environmentally friendly and lightweight composites with a high design freedom. However, the mechanical properties of natural fibre are sensitive to the quality of fibre-matrix interface because of the incompatibility between these two kinds of materials. A two‑step surface treatment approach is proposed to improve the interfacial compatibility for continuous flax yarns as reinforcement in a polylactic acid matrix. First, an alkaline treatment is performed to clean fibres. Second, the deposition of polyethylene glycol as a coupling agent between fibres and matrix is carried out. This coating improves the flax yarn tensile strength compared to the yarn after alkaline treatment. The effect of surface treatment on flax properties and yarn structure is evaluated. A two-step process is developed to manufacture specimens. A composite filament is first manufactured by an extrusion coating process. Then this filament is introduced into a 3D printer to manufacture unidirectional composite plates. The mechanical properties of the composite filaments and plates are evaluated before and after surface treatment. Full treatment (after both alkaline treatment and PEG deposition) leads to composite filaments and plates displaying a significant 13–15% increment in strength.