Strong yet tough epoxy with superior fire suppression enabled by bio-based phosphaphenanthrene towards in-situ formed Diels-Alder network

The synchronized improvement of strength, toughness and fire suppression poses to be a critical trade-off issue towards high-performance epoxy resin. Aiming to impart epoxy with balanced multifunctional improvement, the Schiff base-derived bio-based phosphaphenanthrene small-molecule reinforcer towards in-situ constructed Diels-Alder network within existing 3D network was proposed. The incorporation of 3.5 wt% reinforcer (CQ-DOPO) concurrently enhanced the tensile and impact strength of epoxy by 26.6% and 48.4% without visible deterioration of transparency. The increment of glass transition temperature by 14 °C via dynamic mechanical analysis was observed. The underpinning experimental and simulation investigation verified the proof of concept of in-situ formed Diels-Alder sacrificing network towards higher crosslinking density and strong-weak dual network. Raman spectra witnessed the breaking of sacrificing bond during fracture. Moreover, 3.5 wt% CQ-DOPO with an ultralow phosphorous loading of 0.245 wt% enabled to impart epoxy with UL-94 V-0 rating as well as limiting oxygen index of 32.0%. The multi-scale dual-phase analysis unveiled the synchronized suppression of fire reactions in vapor phase via PO· quenching as well as in condensed phase via char microstructure optimization. Hence, the rational strategy via in-situ construction of Diels-Alder sacrificing bond toward dual network exploits a novel roadmap for high-performance epoxy.