Performance comparison of flame retardant epoxy resins modified by DPO-PHE and DOPO-PHE

Two phosphorus-containing flame retardants, (bis(4-hydroxyphenyl)methyl)diphenyl phosphine oxide (DPO-PHE) and 1-(bis(4-hydroxyphenyl)methyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-PHE) were synthesized and characterized. The two target compounds both were used as a flame retardant and a co-curing agent of 4,4′-diaminodiphenylsulfone of bisphenol A diglycidyl ether to prepare flame-retardant epoxy resins (EP/DPO-PHE and EP/DOPO-PHE). The thermal and flame-retardant properties of the thermosets were investigated by thermogravimetric analysis (TGA), limited oxygen index (LOI) measurement, and UL-94 vertical burning testing (UL-94). The results indicated that both flame retardants can improve the flame retardancy of epoxy thermosets. TGA results indicated that the char yields of EP/DPO-PHE-P-0.9 and EP/DOPO-PHE-P-0.9 thermosets were increased from 19.1% of EP/P-0 to 25.3% and 27.8%, respectively. Compared with that of EP/P-0, the LOI of the EP/DPO-PHE-P-0.5 thermoset increased from 23.7% to 30.1%, and the sample reached UL-94 V-0. Meanwhile, the LOI of the EP/DOPO-PHE-P-0.7 thermoset increased from 23.7% to 28.9%, and the sample reached UL-94 V-0. Cone calorimetry test, scanning electron microscopy of burning residues, and pyrolysis-gas chromatography/mass spectrometry indicated that both flame retardants mainly act through the gas-phase mechanism. In addition, no condensed-phase activity of DPO-PHE was observed, whereas the condensed-phase activity of DOPO-PHE was negligible. Immersion experiments showed that the water absorption activity of flame retardant epoxy resins fluctuates. Compared with DOPO-PHE, DPO-PHE was more effective as a higher flame-retardant and exhibited better characteristics for improving the Tg, thermal stability, and water absorption of modified resins.