Synthesis of a bio-based in-suit reinforcing flame retardant containing DOPO group and its application in EP

It was an enormous challenge to endow flame retardant for epoxy resin (EP) with high efficiency, high compatibility, low addition and environmental protection. Herein, a bio-based flame retardant (PHAO) was synthesized using protocatechualdehyde, p-aminobenzonitrile, and 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) as precursors via a facile approach. PHAO was applied to EP to prepare EP/PHAO composites. Furthermore, PHAO demonstrated outstanding compatibility with the EP matrix, the interface free energy (IFE) value among PHAO and EP was 0.9 mJ/m². For EP/3PHAO, the tensile strength enhanced from 55.4 MPa of pure EP to 67.4 MPa. EP/5PHAO, obtained by adding 5 wt% PHAO into EP, had a phosphorus content of only 0.34%, reaching the V-0 level in UL-94 test, and the limiting oxygen index was 32.5%. In comparison to pure EP, the peak heat release rate, total heat release and total smoke production of EP/5PHAO declined by 25.5%, 15.4%, and 15.3%, respectively. The flame retardancy of EP/5PHAO in gaseous and condensed phases was confirmed by investigating the residual carbon and pyrolysis behavior of EP/5PHAO. The synthesis of PHAO offered a solution for designing bio-based flame retardant and advancing the flame retardancy and mechanical properties of EP.