A Phosphate-Imidazoline-Boron Ester Reactive Flame Retardant Facilitating B → N Coordination to Enhance Epoxy Resin

A ubiquitous challenge exists in the pursuit of high-efficiency flame retardancy for flame retardants, which is the inability of these agents to effectively maintain the mechanical stability of epoxy resin (EP). To address this issue, a phosphate-imidazoline-boron ester flame retardant (PDDBS) was designed and synthesized. The hydroxyl groups within PDDBS were capable of participating in the curing reaction of EP. At a 1 wt % addition level, the T_g of EP/1PDDBS was increased to 148.2 °C. Mechanical property tests from bending and tension experiments indicated that the addition of PDDBS enhanced the mechanical performance of EP. When 5 wt % PDDBS was added, the tensile and flexural strength reached 69.9 and 123.5 MPa, respectively. Furthermore, PDDBS exhibited certain flame-retardant capabilities: compared with neat EP, the peak heat release rate, total heat release, and total smoke production of EP/5PDDBS were decreased by 48.6, 15.4, and 28.4%, respectively. Analysis of residual carbon revealed that thermal decomposition of boron-based groups triggers the formation of boron oxide, which facilitates the construction of a carbonaceous layer and thereby enhances the thermal stability of EP composites. Developing PDDBS through synthesis offers an approach to reconciling the compromise between flame-retardant capability and structural strength.