Esterification Oligomerization of Phosphaphenanthrene Biphenol Caused Efficient Flame Retardant Effect of Group Aggregation in Epoxy Resin

The efficient and simple synthetic method prepared several linear phosphaphenanthrene biphenol oligomers DQPC-n (n = 1, 2, 4) to explore the effects of phosphaphenanthrene groups with different aggregation amounts on flame retardant behavior in epoxy resin. The aggregated phosphaphenanthrene groups in DQPC-n bring epoxy resins with better flame retardancy than phenol compound DOPO-HQ. The limited oxygen index values of epoxy resins with DQPC-n were more than 35.8% and obtained a V-0 rating in the UL 94 test, while that with DOPO-HQ only passed a V-1 rating at the same phosphorous content. Simultaneously, DQPC-n showed efficient flame retardancy in the cone calorimeter test. Especially, the DQPC-1 with moderate aggregated phosphaphenanthrene groups exerted a better flame inhibition effect in epoxy resin, and the values of THR, TSP, and av-EHC of the DQPC-1/EP were lower than that of DOPO-HQ/EP. The mechanism results demonstrated that the pyrolysis of DQPC-1 concentrated released more phosphorus-containing fragments to prevent the combustion chain reaction process. In addition, DQPC-1 can facilitate the aromatic residue formation in the condensed phase. Therefore, the aggregation effect of the phosphaphenanthrene groups in this thesis provides a method for developing efficient epoxy resin additives.