Flame-retardant treatments of continuous flax fibres and rayon rovings: A novel approach for 3D-printing applications

• For the first time, flax and rayon yarns were treated with flame-retardants prior to application in 3D-printing. • Treated yarns were co-extruded with flame-retarded PLA using a die extrusion process. • Printed parts exhibited a high level of flame retardancy. • Non-burning dripping was significantly reduced by yarn-treatment. This study reports, for the first time, the treatment of continuous flax fibres and rayon rovings with flame-retardant (FR) formulations prior to application in fused filament fabrication (FFF) processes. The FR solutions consisted of inorganic salts, namely ammonium sulphamidate and sulfamic acid, combined with an organic phosphorus-nitrogen compound. Thermogravimetric analysis (TGA) and microscale combustion calorimetry (MCC) confirmed the enhanced thermal stability of the treated yarns, along with a significant reduction in non-burning melt dripping. The FR yarns were co-extruded with polylactic acid (PLA), which was also flame-retarded using ammonium polyphosphate (APP), to produce filaments embedding the natural fibres in the core. These composite filaments were successfully 3D-printed into test specimens that achieved V-0 classification in UL-94 tests at 4 mm thickness and exhibited a limiting oxygen index (LOI) of up to 34%. The peak heat release rate (pHRR) was reduced by 45-50% compared to non-flame-retarded PLA-flax or PLA-rayon composites. The mechanical properties of the individual yarns, extruded filaments and 3D-printed parts were evaluated. Ongoing work aims to increase the fibre volume fraction to further enhance the mechanical performance of printed composites. This work introduces a viable pathway for the development of flame-retarded, bio-based, continuous natural fibre composites suitable for reliable 3D printing.