Flame retardant epoxy resins enabled by mechanochemical phosphorylation of lignin for long-term outdoor applications

Epoxy resin (EP) is widely used outdoors due to its high bonding strength and good chemical resistance. However, its inherent flammability limits its application scope, and exposure under ultraviolet (UV) irradiation will severely destroy the crosslinking network, resulting in bond failure. In this work, a solvent-free mechanochemical strategy was employed to phosphorylate lignin, yielding a multifunctional filler (PL). At the loading of 10% PL, the cured EP achieved a UL-94 V-0 rating, and its limiting oxygen index (LOI) was 35.7%. In the cone calorimetry test, the peak heat release rate (pHRR) and total smoke production (TSP) were reduced by 70.9% and 62.7%, respectively. In addition, PL exhibited strong free radical scavenging ability, effectively inhibiting chain scission during UV aging. As a result, the EP maintained a smooth and intact surface after 150 h of UV exposure. This study not only provides a sustainable and efficient strategy for enhancing the flame retardancy and weatherability of EP, but also offers a high-value utilization pathway for lignin.