Two-step polyelectrolyte complex coating for flame retardant flax

Flax fabric textile is of great interest as bio-fiber reinforcement in composites, which has gained popularity due to the high demand for sustainable materials. Despite its sustainability, flax is inherently flammable. This study demonstrates a two-step polyelectrolyte complex (PEC) coating that renders flax fabric self-extinguishing. The PEC coating consists of polyethylenimine (PEI) and sodium hexametaphosphate (PSP), cured with a citric acid buffer. Sonication employed during the curing process improves the infiltration of the coating into the flax fibers and maintains the fabric’s hand. When comparing non-sonicated and sonicated flax samples, the latter achieves self-extinguishing behavior and passes the vertical flame test. Pyrolysis-combustion flow calorimetry reveals that this PEI/PSP coating (with sonication) reduces the peak heat release rate and total heat release by 40% and 67%, respectively. The intumescent mechanism is predominantly observed in the condensed phase, as suggested by differential scanning calorimetry and X-ray photoelectron spectroscopy. The results demonstrate that the two-step PEC treatment with sonication effectively reduces the flammability of flax fabric, making it a promising option for the development of functional fibers, particularly for bio-composite reinforcement.