CC BY-NC 4.0Schirp, ArneArneSchirpHenzler, MatthiasMatthiasHenzlerHohenhoff, GerritGerritHohenhoffAli, WaelWaelAliMayer-Gall, ThomasThomasMayer-Gall2025-11-172025-11-172026https://publica.fraunhofer.de/handle/publica/499406https://doi.org/10.24406/publica-643110.1016/j.polymdegradstab.2025.11174610.24406/publica-64312-s2.0-105020663608This 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.entrue3d-printingContinuous fibresFilamentsFlame retardantFlaxMechanical propertiesRayonjournal article