Fabrication of regenerated cellulose fibers with superior flame retardancy and mechanical properties based on melamine cross-linking and TEMPO oxidation of cellulose

Improving the flame retardancy of Lyocell fibers without compromising their mechanical properties is the key to expanding their high-end applications. To achieve this goal, an innovative phosphorus-free flame-retardant modification strategy of "crosslinking protection first and then directional oxidation" was proposed. Melamine-crosslinked Lyocell fibers (MC@Lyocell) were constructed through the treatment of hydroxymethylated melamine, which were then directionally oxidized by a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) mediated NaClO oxidation system to in-situ generate sodium carboxylate (-COO - Na⁺) groups of cellulose fibers to obtain high flame-retardant oxidized MC@Lyocell fibers (OMC@Lyocell). The modified fibers exhibited significantly enhanced flame retardancy: Limiting oxygen index (LOI) increased from 17.0% to 30.5%, the peak heat release rate (PHRR) and total heat release (THR) decreased by 79.6% and 50%, respectively. Meanwhile, the structural integrity and mechanical properties were well maintained. Compared with the original fibers, the crystallinity only slightly decreased by 8%∼15%, with slower mass loss and a decline of polymerization degree. The melamine crosslinked network confines oxidation to cellulose crystal surfaces and amorphous regions, resolving the contradiction between carboxyl generation and mechanical property decrease. This environmentally friendly and efficient phosphorus-free strategy provides a new direction for preparing high-performance, structurally stable flame-retardant cellulose fibers.