One‐step Synthesis of a Phosphaphenanthrene–Imidazole Phosphorus–Nitrogen Flame Retardant for Epoxy Thermosets: Degradation Pathway and Fire‐Safety Mechanism

ABSTRACT Epoxy resins (EPs) possess excellent mechanical strength, chemical resistance, and adhesion, yet their inherent flammability limits their use in fire‐sensitive applications. Under increasingly stringent safety requirements, developing simple and scalable halogen‐free flame retardants remains a significant challenge. In this work, a phosphorus–nitrogen flame retardant, 6‐(2‐(1H‐imidazol‐1‐yl)ethyl)dibenzo[c,e][1,2]oxaphosphinine 6‐oxide (DIM), was synthesized via a one‐step nucleophilic addition reaction between 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) and 1‐vinylimidazole (VIM), and subsequently incorporated into epoxy thermosets, either alone or in combination with melamine (MEL). Fourier‐transform infrared spectroscopy (FTIR) was used to evaluate the formation of DIM. The thermal degradation, fire performance, char structure, phosphorus retention, and evolved products of the resulting thermosets were investigated using thermogravimetric analysis (TGA), cone calorimetry, scanning electron microscopy (SEM), Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectroscopy (ICP‐OES), TGA coupled with Fourier‐transform infrared spectroscopy (TG‐FTIR), and pyrolysis‐gas chromatography/mass spectrometry (Py‐GC/MS). Changes in the characteristic FTIR bands of the PH and vinyl groups, together with the retention of the DOPO and imidazole frameworks, supported the formation of the proposed addition product. Among the investigated formulations, EP/DIM‐MEL4 exhibited the greatest improvement in fire performance, with reductions of 31.9% and 36.0% in peak heat release rate and total heat release, respectively, relative to neat EP. Microscopic and spectroscopic analyses indicated the formation of a more expanded, continuous, and structurally ordered protective char. ICP‐OES analysis showed that approximately 31.1% of the initial phosphorus was recovered in the solid residue. TG‐FTIR and Py‐GC/MS further suggested that volatile phosphorus‐containing species contributed to gas‐phase radical inhibition, whereas nitrogen‐containing volatiles derived from the imidazole moiety and MEL contributed to flame‐zone dilution. Overall, the improved fire safety of the DIM‐containing epoxy thermosets is attributed to the combined effects of gas‐phase inhibition and condensed‐phase barrier formation. This work provides a simple strategy for developing DOPO‐based phosphorus–nitrogen flame‐retardant systems for epoxy thermosets.