Fabrication of phosphorus-free flame-retardant regenerated cellulose fibers via amidoximation and catechol functionalization of waste recycled polyacrylonitrile

The development of flame retardant technologies are essential for regenerated cellulose fibers, while the resource recycling of waste fibers remains a challenging issue. Herein, a novel and sustainable strategy was proposed to simultaneously realize Lyocell fiber flame-retardant modification and waste polyacrylonitrile (PAN) resource reuse. Two phosphorus-free flame retardants (A-PAN and A-PAN-Y) were successfully synthesized from recycled waste PAN filaments through hydrolysis, amidoximation and Schiff base reaction, and then applied for the modification of Lyocell fibers. Under similar weight gain rate, the prepared Lyocell@A-PAN (10.8 wt%) and Lyocell@A-PAN-Y (11.3 wt%) presented outstanding fire resistance with limiting oxygen index values (LOI) up to 32.3 % and 34.7 %, respectively. In comparison with pristine Lyocell fiber, the peak heat release rate (PHRR) was reduced by 38.9 % and 45 %, while the total heat release (THR) decreased by 55.8 % and 60 %, respectively. No obvious flame spread was observed during vertical burning tests. Mechanism analysis confirmed a typical gas-condensed phase synergistic flame-retardant effect: amidoxime groups released inert nitrogen-containing gases to dilute combustible volatiles, and the introduced catechol structure further promoted the formation of highly graphitized compact char layer. Distinct from common single modified systems, this work realized the high-value recycling of waste PAN fibers and phosphorus free flame retardation of Lyocell fibers, providing a feasible and sustainable strategy for the circular recycling of PAN waste and high-performance functional cellulose materials.