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 degrees 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 re-actions in vapor phase via PO center dot 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.