The synergistic effect of maleimide and phosphaphenanthrene groups on a reactive flame-retarded epoxy resin system

A novel reactive flame-retarded epoxy resin system was prepared by copolymerizing diglycidyl ether of bisphenol-A (DGEBA) with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), N,N'-bismaleimide-4,4'-diphenylmethane (BDM) and 4,4'-diamino-diphenyl sulfone (DDS). Curing behavior, thermal and flame-retardant properties of the cured epoxy resins were investigated by differential scanning calorimeter (DSC), thermogravimeric analysis (TGA), limited oxygen index (LOI) measurement, UL94 test and cone calorimeter. The results indicated that phosphaphenanthrene group was introduced into the multicomponent system by addition reaction of DOPO with BDM. Compared with traditional DOPO-DGEBA systems, the EP/DDS/BDM/DOPO thermosets showed greatly improved glass transition temperatures (210-223 °C). The results of combustion tests indicated that the addition of BDM or DOPO into DGEBA could improve the flame resistance of the thermosets. Most importantly, the flame-retardant property was further improved when BDM and DOPO coexisted in the epoxy resin systems. For example, compared to the control samples, the EP/DDS/BDM/DOPO-15 thermoset displayed better flame retardancy with higher LOI value and UL94 rating, lower peak of heat release rate (pk-HRR) and average of effective heat of combustion (av-EHC) under the same content of BDM and phosphorus, strongly confirming the synergistic effect of BDM and DOPO. In addition, in a particular proportion, BDM and DOPO synergistically functioned in the condensed-phase and gaseous-phase at the same time. The flame retardant mechanism was studied by TGA and cone calorimeter coupled with the analysis of the char residues.