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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. [ABSTRACT FROM AUTHOR]

Nylon–cotton blended fabric (NYCO), commonly used in military uniforms and workwear for its comfort and resilience, presents an inherent fire risk due to its high flammability and high heat release. There are few effective flame retardant treatments for NYCO that are environmentally benign and maintain the hand of the fabric. To solve this problem, a layer‐by‐layer (LbL)‐assembled 15 quadlayer (QL) coating, consisting of chitosan (CH), phytic acid (PA), and tannic acid (TA), is applied to NYCO. Individually pairing TA or PA with CH is not self‐extinguishing, but they work synergistically in the QL system to create an effective intumescent coating. This completely bio‐sourced, water‐based coating self‐extinguishes in a vertical flame test (ASTM D6413), with only 16.6% weight added to NYCO, yielding 91% postburn residue and a 51% decrease in peak heat release rate. This novel coating provides an opportunity to replace environmentally questionable flame retardant treatments with a fully sustainable system. [ABSTRACT FROM AUTHOR]