Lignin-based flame retardant containing nitrogen and phosphorus coordinated by Fe³⁺ for TPU

Thermoplastic polyurethane (TPU) exhibits high flammability which affect its wide application. Lignin, a natural and renewable polymer, possesses inherent charring capabilities attributed to its three-dimensional macromolecular benzene ring network structure. Derived from non-grain biomass, lignin aligns with the principles of green chemistry. It serves as a promising raw material for intumescent flame retardant charring agents. To address this issue, a lignin-based intumescent flame retardant incorporated with Fe³⁺ was developed for enhancing the flame retardancy of TPU. The lignin was epoxidated and aminated, followed by its reaction with phosphorus monomer and iron salt to produce the lignin-based intumescent flame retardant (LEAD@Fe) containing Fe³⁺. The presence of Fe³⁺ facilitated the catalytic conversion of lignin into robust intumescent carbon layer with phosphorus and nitrogen, which effectively insulated against heat. Moreover, Fe³⁺ acted as a catalyst in reducing the generation of flammable gases during TPU combustion. And the iron oxides produced during the thermal decomposition of LEAD@Fe significantly enhanced the thermal stability of TPU. Reductions of 45.2 % in peak heat release rate (PHRR), 54.5 % in peak smoke production rate (PSPR) and 59.0 % in toxic gas emissions were observed with 7 % LEAD@Fe for TPU, which suggested that LEAD@Fe can effectively reduce toxic gas emissions and heat release during combustion. Furthermore, it highlighted the robust performance achieved through strong hydrogen bond interactions between -OH and -NH₂ groups of LEAD@Fe and TPU chains. This approach ensured the compliance with flame retardancy performance while preserving the mechanical properties of TPU, offering an eco-friendly and efficient strategy for developing wide applied TPU.