Multi-element hybrid flame retardants balance flame retardancy and mechanical performance of epoxy coatings

A phosphorus‑silicon‑boron ternary hybrid flame retardant (BSiP) was synthesized using 9,10-dihydro-9-oxa-10-phosphaphenantrene-10-oxide, p-phthalaldehyde, (3-aminopropyl) triethoxysilane and boric acid. The BSiP was applied to modify the performance of epoxy (EP) coatings. A phosphorus‑boron binary hybrid flame retardant BP was incorporated into this system to balance the performance of EP coatings. Using BSiP and BP alone requires 3 wt% and 6 wt% for EP coating, respectively, to achieve the vertical burning UL-94 V0 rating, while 4 wt% BSiP/BP (1/1) is enough for EP coating to achieve the same UL-94 rating. In addition, the impact strength of EP/BSiP/BP is 11.8 KJ/m2, which is 131 % and 103 % more than that of EP and EP/BSiP, respectively. The BSiP and BP work in both the gas phase and the condensed phase, and they have a good synergistic effect on flame retardancy of EP. The blend of BSiP and BP increases the free volume of EP and thus toughens EP coatings. The hybridization of multi-element covalent bonds and physical blending effectively balance the flame retardancy and impact strength of EP coatings.