Ligand-Exchange-Induced Growth of NiCo-LDH on Halloysite for Enhanced Flame Retardancy of Unsaturated Polyester Resin

Unsaturated polyester resin (UPR) is a long-chain, hydrocarbon-rich thermoset that is intrinsically highly flammable, which severely limits its engineering applications. Herein, NiCo-layered double hydroxide (NiCo-LDH) nanosheets were covalently grafted onto halloysite nanotubes (HNTs) via an in situ substitution route that employed ZIF-67 as a sacrificial bridging agent, yielding a hierarchical HNTs@NiCo-LDH hybrid. Upon incorporation into UPR, the hybrid formed a dense metal oxide shield during combustion that suppressed the release of combustible volatiles and attenuated radiative heat feedback, thereby imparting superior flame retardancy. Cone calorimetry revealed that 6 wt % HNTs@NiCo-LDH (UPR-H@L) reduced the peak heat release rate by 37.0% and the peak smoke production rate by 23.9% relative to neat UPR. Transition-metal-catalyzed charring generated a compact, continuous carbonaceous barrier that further inhibited mass and energy transfer. Simultaneously, the tensile and impact strengths of UPR-H@L increased, demonstrating a synergistic reinforcement effect. This work establishes a facile physical-barrier/chemical-catalytic strategy for simultaneously enhancing the fire safety and mechanical performance of UPR composites.