An intrinsic flame retardant epoxy resin with high transparency and strengthened mechanical property

In order to search for multifunctional epoxy thermosets (EP) with low flammability, high transparency and satisfied mechanical performance, DOPO‐based phosphonate ammonium salt (DOA) was synthesized from 10‐hydroxy‐9,10‐dihydro‐9‐oza‐10‐phosphaphenanthrene‐10‐oxide (DOPO‐OH) and 2‐amino‐2‐methyl‐1,3‐propanediol (AMPD). Under the influence of DOA, the flame‐retardant and mechanical performances of the resulting EP were obviously improved. On account of the enhanced interaction and the incorporated flexible fragments in epoxy macromolecular chains, the tensile strength, elongation at break, and impact toughness of EP/5.0 wt% DOA significantly increased from 65.4 ± 1.2 MPa, 6.7 ± 0.6%, and 12.1 ± 1.3 kJ m−2 of EP to 81.4 ± 2.8 MPa, 10.6 ± 0.5%, and 18.0 ± 1.1 kJ m−2, respectively. In the presence of DOA, the limiting oxygen index (LOI) value of EP/5.0 wt% DOA increased to 35.5% and it passed the underwriter laboratories‐94 vertical burning tests (UL‐94 V) and got a V‐1 rating. Moreover, the peak value of heat release rate (PHRR) was decreased by 38.0%. The analyses of char residues and volatile products showed that the activities of DOA on reducing the flammability of EP were ascribed to the protective effect of the char, the release of incombustible gases, and the radical‐capture action of phosphorus‐containing free radicals. Moreover, the modified epoxy thermosets still retained a high transparency. DOPO‐based phosphonate ammonium salt modified epoxy thermosets exhibit excellent flame retardancy, strengthened mechanical properties and high transparency. Meanwhile, the relevant mechanisms are also disclosed through comprehensive studies. Generally, the as‐obtained epoxy thermosets would fulfill the needs of diversified material functions in the future.