Synchronously enhanced flame-retardance and mechanical properties of epoxy resin via a P/N-containing active curing agent

The inherent flammability of epoxy resin remains a critical challenge in material applications. To address this issue while overcoming the common drawbacks of flame-retardant additives-including poor miscibility with epoxy matrices, deterioration of mechanical properties, and long-term leaching - recent research has focused on developing efficient reactive flame retardants capable of participating in epoxy curing reactions. In this study, a novel multifunctional reactive flame retardant (TD-DOPO) synthesized through a one-step reaction of tyramine, 1,4-benzaldehyde, and 9,10-dihydro-9-oxo-10-phosphanthracene-10-oxide (DOPO) was presented. In the epoxy system, TD-DOPO demonstrated exceptional flame-retardant performance via a biphasic synergistic mechanism, achieving UL-94 V-0 rating at a remarkably low phosphorus content of merely 0.2 wt%, and the peak heat release rate (PHRR) and total heat release (THR) were also reduced by 31.5 % and 22.4 %, respectively. Concurrently, the limiting oxygen index (LOI) increased significantly from 22.2 % to 33.1 %. Notably, the multiple reactive sites of TD-DOPO not only synergistically modulated the epoxy ring-opening kinetics but also facilitated the formation of a hierarchical crosslinked network due to its inherent rigid molecular structure. This unique molecular architecture simultaneously enhanced the mechanical properties of the composite, resulting in a 29.6 % improvement in toughness. Furthermore, the tensile and flexural modulus increased to 2183.5 MPa and 3909.8 MPa, respectively, while excellent thermal properties were maintained. This work demonstrates that the design and synthesis of novel multi-reactive center flame retardants represents a straightforward yet effective strategy for developing flame retardant epoxy resin with good comprehensive performances.