Three commercialized flame retardants, 1,2-bis(diphenylphosphinoyl)ethane (EDPO), 6,6-(1,2-phenethyl)bis-6H-dibenz[c,e][1,2]oxaphosphorin-6,6-dioxide (HTP-6123), and hexa-phenoxy-cyclotriphosphazene (HPCTP), were used to prepare the flame retardant diglycidyl ether of bisphenol A (DGEBA) epoxy resin (EP) under the same experimental conditions. The effects of T g , thermal stability, and water absorption properties of EP caused by the three flame retardants were investigated and compared, together with their flame retardant efficiency. Results showed that the introduction of the three flame retardants improved the flame retardant performance of EP but led to decreases in T g and decomposition temperature. EDPO showed higher flame retardant efficiency than the other two flame retardants. EP/EDPO showed higher thermal stability, better flame retardant performance, higher T g value, and lower water absorption than EP/HTP-6123 and EP/HPCTP. The study discovered that EDPO and HTP-6123 primarily act through the gas phase flame retardant mechanism, while HPCTP is primarily driven by the condensed phase mechanism. K E Y W O R D S diphenylphosphine oxide, DOPO, epoxy resin, flame retardant, hexa-phenoxycyclotriphosphazene

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Summary By curing the reaction mixture of diphenylphosphine oxide (DPO) and diglycidyl ether of bisphenol A with 4,4′‐diaminodiphenylsulfone, flame‐retardant epoxy resins (EP/DPO) were prepared. Flame‐retardant epoxy resins modified with 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) were similarly prepared (denoted as EP/DOPO). The limiting oxygen index value of pure epoxy resin, EP/DPO–P‐0.9 (with a phosphorus content of 0.9 wt%), and EP/DOPO–P‐0.9 are 23.0, 30.5, and 29.4%, respectively. EP/DPO–P‐0.9 reached a UL‐94 vertical burning test V‐0 rating, while EP/DOPO–P‐0.9 failed. The results of the cone calorimetry test, thermo‐oxidative degradation behavior study, and pyrolysis‐gas chromatography/mass spectrometry analysis indicated that both flame retardants mainly act through the gas‐phase activity mechanism. Together, the results of this study suggest that EP/DPO are high performance resins with good thermal stability, high glass transition temperature, and low water absorptivity for practical applications.