Comparison of the performances of epoxy resin thermosets cured by two P-containing anhydrides

Two P-containing anhydrides, ethyl (2,5-dioxotetrahydrofuran-3-yl) (methyl) phosphinate (MTA-MA) and 3-(5-oxido-6H-phosphanthridin-5-yl) dihydrofuran-2,5‑dione (DOPO-MA), were synthesized and used along with methyl tetrahydrophthalic anhydride as co-curing agents for curing epoxy resin (EP, mark E51). Via the cross-linking reaction during the solidifying process, P element was introduced into EP matrix via coherent bond. The results showed that both flame-retarding curing agents improved the flame retardancy of EP and MTA-MA exhibited a better performance than DOPO-MA. When the P content in the EP matrix reached 1.0 wt%, the EP/MTA-MA-1.0 thermoset passed UL-94 V0 rating and the limiting oxygen index (LOI) was 32.6%. In addition, EP/DOPO-MA-1.0 achieved UL-94 V1 rating and the LOI value was 30.2%. Cone calorimeter tests, scanning electron microscopy-energy dispersive x-ray spectroscopy, and pyrolysis gas/mass spectrometry results indicated that EP/MTA-MA exhibited better flame-retardancy performance in both the gas and condense phases. In addition, the thermal stability, mechanical properties, and glass transition temperature of EP/MTA-MA and EP/DOPO-MA were studied.