Chemical structure construction of DOPO-containing compounds for flame retardancy of epoxy resin: A review

Fire-safety epoxy resin with excellent comprehensive performance becomes the requirement for further application in the industry. As a commercially available compound, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) exhibits strong potential to develop effective flame retardants for epoxy resin due to its reactive group of P[sbnd]H and dominant function in gas phase. In order to overcome the adverse effect on mechanical properties and improve flame-retardant effectiveness, DOPO has been used as an initial reactant to synthesize various compounds for epoxy resin. In terms of the different substituents at phosphorus atom in DOPO including P[sbnd]C, P[sbnd]O/P[sbnd]N, and P-ring structure, this work reviews the research progress of these derivatives with the phosphorus element only from DOPO structure in the last decade. Around the relationship between chemical structure and properties, the effect of DOPO-containing compounds on the properties of epoxy resin is discussed, mainly involving flame-retarded performance, heat resistance, and mechanical properties. Although a large number of DOPO derivatives has been designed and used for epoxy resin, some issues still need to be improved, such as migration of flame retardants, deterioration of mechanical properties, processibility, economic competitiveness, and so on. Therefore, an efficient flame retardant can not only improve the flame retardation at low loading but also maintain original outstanding properties of epoxy resin. Through tailoring the chemical structure, to endow epoxy resin with outstanding comprehensive performance in an eco-friendly and economic manner is the further development tendency for DOPO-based flame retardants.