Durable Flame-Retardant and UV-Protective Viscose Fibers Enabled by Blend Wet Spinning with a Triazole-Functionalized Cyclotriphosphazene Additive

The modified viscose fibers show durable flame retardancy. • They also exhibit significantly enhanced ultraviolet resistance. • A phosphorus-nitrogen synergy promotes char formation. • The modification retains mechanical strength and whiteness. • This offers a cleaner route for multifunctional fibers. Viscose fiber is inherently highly flammable, and conventional post-finishing flame-retardant treatments often suffer from limited washing durability. To overcome these drawbacks, a triazole-containing cyclotriphosphazene derivative (HTDA) was synthesized and incorporated to fabricate flame-retardant viscose fiber as a multifunctional additive via blend wet spinning. This incorporation does not impair spinnability, and the resulting fibers simultaneously achieve durable flame retardancy and markedly enhanced ultraviolet resistance. At an HTDA loading of 20 wt.%, the limiting oxygen index reaches 30.4% and remains as high as 30.3% even after 50 laundering cycles, evidencing excellent wash durability. Cone calorimetry further confirms outstanding fire safety: compared with neat viscose fiber, the modified fiber shows a 42% reduction in peak heat release rate and a 39% decrease in total heat release. In addition, the ultraviolet protection factor significantly increases from 11 to 68, while the retention of single-fiber breaking strength exceeds 80%, meeting the basic requirements for textile applications. Mechanistic investigations reveal that HTDA acts through a phosphorus–nitrogen synergistic pathway in both the condensed and gaseous phases, promoting char formation and effectively suppressing combustion. Overall, this work provides a sulfur-free, post-finishing-free strategy for producing multifunctional high-performance viscose fibers with durable flame retardancy and enhanced UV protection.