Flame retardancy and mechanical properties of epoxy thermosets modified with a novel DOPO-based oligomer

A novel 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO)-based oligomer indicated as PDAP was synthesized via the nucleophilic addition reaction between DOPO and imine obtained from the condensation reaction of terephthalaldehyde and p-phenylenediamine. The chemical structure of PDAP was characterized by FTIR, 1H-NMR and 31P-NMR. The PDAP serving as co-curing agent of 4, 4′-diaminodiphenylmethane was employed to develop epoxy resins with highly improved flame retardancy. The thermal stabilities, flame-retardant and mechanical properties of epoxy thermosets were studied by thermogravimetric analysis (TGA), limiting oxygen index (LOI) measurement, UL-94 test, dynamic mechanical analysis (DMA) and tensile test. The flame-retardant mechanism of epoxy thermoset modified with PDAP was investigated by FTIR, Py-GC/MS, SEM and laser Raman spectroscopy. The results showed that with the incorporation of 7 wt% PDAP, the modified epoxy thermoset achieved a LOI value of 35.3% and V-0 rating in UL-94 test. Interestingly, all the modified thermosets showed blowing-out effects during UL-94 test. The flame-retardant mechanism was attributed to the quenching effect of phosphorus-containing free radicals and diluting effect of nonflammable gases in the gas phase, and the formation of phosphorus-rich char layers in the condensed phase. Furthermore, the results of DMA and tensile test revealed that the incorporation of PDAP decreased the glass transition temperature slightly, and meanwhile improved the tensile strength of epoxy thermoset.