Boron-nitrogen proton transfer reaction: Further exploration of DOPO-based flame retardants for enhanced flame resistance, mechanical properties, and adhesion in epoxy resins

A novel flame retardant of epoxy resin (EP) containing P/N/B flame-retardant elements, DTA-B, was synthesized by introducing element B into the P/N flame-retardant system through a proton transfer reaction. DTA-B has a high residual carbon content of up to 55.8 wt% at 800 °C. The addition of 5 wt% DTA-B to EP significantly enhanced its flame retardancy, with the limiting oxygen index of the EP/5DTA-B composite increasing to 33.2 %, achieving the UL-94 V-0 rating. Cone calorimeter tests showed that, compared to pure EP, the peak heat release rate, total heat release, and total smoke production of EP/5DTA-B were reduced by 40.4 %, 20.7 %, and 27.4 %, respectively. Analysis of the pyrolysis behavior and char formation revealed the flame-retardant mechanism of DTA-B in both the gas and condensed phases. The flexural strength and tensile strength of EP/5DTA-B were 120.4 MPa and 62.3 MPa, respectively, which represent improvements of 18.7 % and 9.9 % compared to pure EP (101.4 MPa and 56.7 MPa). Furthermore, the introduction of DTA-B also improved the adhesive performance of EP, with the shear strength of EP/5DTA-B adhesive bonding steel and wooden bars being 14.38 MPa and 9.29 MPa, respectively, both higher than those of pure EP (7.23 MPa and 5.63 MPa). Compared to EP/5DTA and EP/5DOPO, EP/5DTA-B has the lowest phosphorus content but the best flame retardancy, reflecting the synergistic effect of the P/N/B flame-retardant elements. This study provides new insights into the introduction of element B into the P/N flame-retardant system and the development of DOPO-based flame retardants.