Construction of a Furan-Based Core–Shell APP Flame Retardant for High-Performance Polyurethane Composites

Abstract Polyurethane (PU) with both high flame retardancy and excellent mechanical performance is highly desired for cutting-edge applications. Herein, a novel furan-based core-shell flame retardant was synthesized through in situ formation of a crosslinked furfuryl alcohol-maleic anhydride-divinylbenzene copolymer (FMD) shell layer on the surface of ammonium polyphosphate (APP) by self-stabilized precipitation polymerization. The applicability of APP@FMD in thermoplastic PU (TPU) and rigid PU foam (RPUF) with distinct structures and combustion behaviors was investigated systematically. With the incorporation of 8 wt % APP@FMD0.3, TPU/8%APP@FMD0.3 achieved a UL-94 V-0 rating with a limiting oxygen index (LOI) of 26.7%, and melt dripping was also effectively suppressed. Similarly, a UL-94 V-0 rating with an LOI of 26.6% was reached for RPUF composite containing 7 wt % APP@FMD0.3. Cone calorimetry tests showed that the peak heat release rate and total heat release of TPU/8%APP@FMD0.3 were reduced by 69.7% and 37.9%, respectively, compared with neat TPU. Meanwhile, these values of RPUF/8%APP@FMD0.3 were reduced by 14.5% and 16.7%, respectively, compared with neat RPUF. The highly enhanced flame retardancy was mainly attributed to the formation of a denser and more intact char layer, together with the gas-phase dilution and radical-trapping effects derived from APP decomposition. Moreover, the tensile strength of TPU and compressive performance of RPUF were perfectly retained, owing to highly improved filler dispersion and interfacial compatibility induced by the FMD shell layer of APP@FMD. The present work provides a facile and effective strategy for designing and fabricating furan-based core-shell flame retardants toward mechanically robust and fire-safe PU composites.