Renewable vanillin-based flame retardant toughening agent with ultra-low phosphorus loading for the fabrication of high-performance epoxy thermoset

The poor impact toughness and flame retardant performance have greatly restricted the engineering application of epoxy thermoset. To obtain the high-performance epoxy composites, the renewable vanillin-based flame retardant toughening agent (PVSi) was synthesized and incorporated into epoxy. The use of PVSi macromolecules can significantly enhance the impact toughness of epoxy. With 5 wt% of PVSi, the impact strength of the epoxy was maximally raised by 189.69%, from 12.42 kJ/m2 of the neat EP to 35.98 kJ/m2 of EP/PVSi5 composites. The toughening effect of PVSi macromolecules on epoxy was closely linked to its structural features, such as the flexible phenylsiloxane, active imine and polar phosphaphenanthrene groups. Simultaneously, the EP/PVSi5 composites reached up to the V-0 rating in vertical burning test (UL-94) and 29.5% in limiting oxygen index (LOI), with only 0.27 wt% ultra-low phosphorus loading. Additionally, the suppressed heat release, the evidently reduced toxic pyrolytic volatiles, and the promoted charring capability of EP/PVSi composites can be obtained, with phosphaphenanthrene, phenylsiloxane and diaminodiphenylsulfone groups in PVSi macromolecules jointly playing a role. These results indicated the improved fire safety of epoxy. Furthermore, the free radical scavenging effect of P· and PO·, the fuel dilution effect of nonflammable NH3 and SO2, the catalytic charring effect of the pyrophosphoric acid and metaphosphoric acid, the charring-stability effect of phenylsiloxane group and the suppression effect of high-quality carbon layers were analyzed and summarized. It was expected that PVSi would pave the way for the development of more highly efficient flame retardant toughening agents and high-performance epoxy thermoset.