Unexpected flame retardancy mechanism of sulfuric acid treated and pyrolyzed waste cotton fabrics

Cotton fabrics are widely employed in the textile sector. However, despite being biodegradable, the extensive production of cotton textiles results in significant textile waste, highlighting the imperative for advanced research into recycling and reuse strategies. Herein, we prepared a flame retardant using waste cotton fabrics treated with sulfuric acid followed by a short pyrolysis process. The 20 % sulfuric acid treated cotton fabric achieved a high carbonization yield of 29.9 %, nearly three times higher than untreated cotton fabric, and retained its textural shape after pyrolysis at 500–800°C whereas the filaments were fused together by further concentrated H₂SO₄ treatments (40 % and 60 %). Limiting oxygen index (LOI) and cone calorimeter tests were conducted to evaluate the flame-retardant properties of the pyrolyzed cotton fabric (CS20), which demonstrated a significant enhancement with LOI and total smoke production (TSP) values of 38 % and 0.00078 m², respectively, compared to the 19 % and 0.02412 m² observed for pure cotton fabric. Those results showed that H₂SO₄ treatment and pyrolysis transformed cotton fabrics into non-flammable materials with highly thermal-resistant chemical structures. However, conventional analytical techniques such as X-ray diffraction, Raman spectroscopy, and element analysis did not reveal significant structural differences between untreated cotton and CS20. In stark contrast, laser desorption/ionization time-of-flight mass spectrometry clearly revealed the delicate structural differences in immature sp² carbon domains formed in cotton fabric and CS20 during pyrolysis. This study confirms that waste cotton fabric can be converted into flame retardant fabric by simple H₂SO₄ and pyrolysis treatment, which was supported by detailed structural characterization and a plausible mechanism.