Thiol-ene click chemistry for core-shell flame retardants with tunable phosphorus oxidation states: Enhanced fire safety and anti-aging performance in EVA composites

The flammability of ethylene-vinyl acetate copolymer (EVA), caused by its thermal-oxidative aging, poses a serious fire hazard in the cable industry. To address the poor compatibility, easy migration, and complex modification processes of intumescent flame retardants (IFR) in EVA, a highly efficient and mild thiol-ene click chemistry strategy is developed to construct a series of core-shell flame retardants (APP@Si@PS and CFA@Si@PS) with tunable phosphorus oxidation states (-1, +1, +3, +5) in the shell layer. The click reaction rapidly grafts functional shells onto both APP and CFA cores under UV (30min, r.t.), thereby improving the compatibility of the flame retardant. At a low addition amount of 25 wt%, the obtained EVA composites (EVA-3 to EVA-6) achieve UL-94 V-0 rating, with a limiting oxygen index (LOI) as high as 27.5%. Among them, the EVA-5 containing +3 phosphorus oxidation state shell has the best comprehensive performance: its peak heat release rate (PHRR) and carbon monoxide release rate (PCOPR) are reduced by 80% and 59% respectively compared to pure EVA, and the retention of elongation at break was 90.0% after aging at 180 °C for 7 days. This work highlights the power of click chemistry for the rapid, mild, and scalable fabrication of multifunctional halogen-free flame retardants, and demonstrates the multifunctional advantages of APP@Si@PS for EVA cable materials, including enhanced fire safety, improved mechanical properties and anti-aging performance.