Harnessing metal-free click chemistry: phosphorus and triazole synergy for improved flame retardant epoxy thermosets

The synthesis of novel and effective flame retardants for epoxy resins is an attractive area of research. The use of modern chemical tools for this purpose is highly intriguing. In this work, three novel flame retardants (FRs) containing phosphorus and triazole units were synthesized using metal-free azide-alkyne click chemistry. The FRs were designed to have different N/P ratios. The synthesized FRs were characterized using conventional spectroscopic methods. The FRs were added at various ratios to an epoxy resin, and the obtained resins were thermally cured. The thermosets' thermal and flame retardancy properties were characterized. The thermal degradation temperatures of the FR-containing epoxy thermosets were reduced to lower temperatures, but the char yields increased compared to the neat resin. Combining the triazole units and the phosphorus-containing groups led to superior flame retardancy. The limiting oxygen index (LOI) value of the thermosets was found to be as high as 42.0 ± 0.3 %. Micro combustion calorimeter (MCC) test results of the FR-containing samples showed significant reductions in the peak heat release rate values (pHRR) compared to that of the neat epoxy resin. All the FR-containing thermosets were rated as V-0 according to the UL-94 test. Furthermore, owing to the presence of N and P, intumescent behavior was observed. The significant flame retardant action of the synthesized FRs is attributed to the formation of dense intumescent chars upon combustion and to the synergistic effect of the P- and N-containing species in the gas phase, which capture free radicals and dilute the gas phase.