Abstract A bio‐based flame retardant (PDE) was synthesized through a facile one‐pot reaction of 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) with protocatechualdehyde and ethanol. PDE was fully characterized by fourier transform infrared spectroscopy (FTIR), 1H NMR, 31P NMR and ESI‐MS. Epoxy resin (EP) with 9 wt PDE (EP/9PDE) passed UL94 V‐0 rating and possessed the limited oxygen index (LOI) value of 33.2, the curved EP thermosets kept transparent. When chitosan (CS) was used to replace one‐third of PDE in EP/9 PDE, the modified EP also passed UL94 V‐0 rating with the LOI value of 32.3, the curved thermosets kept transparent and owned better absorption effect toward ultraviolet light. Cone calorimeter test and TG‐FTIR analysis were performed on the EP samples, the chars after burning were characterized by scanning electron microscope (SEM), energy‐dispersive X‐Ray spectroscopy (EDX), Raman spectrum and FTIR. The results showed that the synergistic effect of PDE‐CS on flame retardant was ascribed to the protective effect of denser char layer, the release of some phosphorus containing free radicals and the dilution effect of released nonflammable gases such as H2O and CO2.
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