Behavior and mechanism of flame retardant epoxy resins with intrinsic phosphorus-nitrogen phenolic resin curing agents

In this paper, two phosphorus-nitrogen phenolic resins (DPO-PR, DOPO-PR) were synthesized from phenol, paraformaldehyde, dicyandiamide, DOPO, and diphenylphosphine oxide (DPO) as raw materials as curing agents and were applied in epoxy resins. The structures of DPO-PR and DOPO-PR were characterized by FTIR, 1H-NMR and 31P-NMR. In the TGA test, the decomposition temperature of DPO-PR/EP and DOPO-PR/EP decreased, but the residue yield increased substantially. DPO-PR/EP had a higher LOI value than DOPO-PR/EP at the same addition amount. Compared with PR/EP, DPO-PR/EP showed that pk-HRR decreased by 41.5 %, THR decreased by 29.4 %, and a slight increase in residue yield. DPO-PR/EP and DOPO-PR/EP were analyzed by TG-MS, and the results showed that both DPO-PR/EP and DOPO-PR/EP can decompose phosphorus free radicals to exert quenching effects. Characterization of the char layer by SEM and XPS showed that DPO-PR and DOPO-PR were able to produce phosphoric acid substances to promote the dehydration of the epoxy resin into char layer, enhance the integrity of the char layer of the epoxy resin, and effectively exert the shielding and barrier effect.