Synthesis of solid reactive organophosphorus‐nitrogen flame retardant and its application in epoxy resin

Abstract A solid reactive organophosphorus‐nitrogen flame retardant 4N‐BDP was devised and synthesized in order to address the flaws of the widely used organophosphorus flame retardants BDP as well as the issue of flame retardant modification of epoxy resin (EP). A series of flame‐retardant EP composites were prepared by adding 4N‐BDP and BDP to EP in a specific proportion. The flame‐retardant properties of the two were compared, and the flame‐retardant mechanism of 4N‐BDP was deeply analyzed. The results of the UL‐94 test revealed that sample 4N‐BDP‐6 obtained the greatest V‐0 level, while BDP‐6 had no level, when the addition ratio of flame retardant was 8.03%. In the cone calorimeter test, the peak smoke production rate (pSPR) and total smoke production (TSP) of BDP‐6 were not significantly different from those of pure EP. In contrast, TSP and pSPR of 4N‐BDP‐6 decreased by 39.50% and 50.00% respectively compared with EP. Compared with BDP, 4N‐BDP shows better flame retardancy and smoke suppression performance, which can significantly improve the fire resistance of EP. The flame‐retardant mechanism of 4N‐BDP was resolved by scanning electron microscopy, XPS, and thermogravimetric analysis/infrared spectrometry. 4N‐BDP can increase the residual carbon rate of the composites, improve the residual carbon strength, play an excellent condensed phase flame retardant effect, and avoid further decomposition of the materials. In the gas phase, the decomposition of 4N‐BDP produces PO·, PO2· and NH3, which play a good quenching effect and dilution effect. The escape of NH3 will leave bubbles in the residual carbon, making the residual carbon more fluffy, further improving the ability of heat insulation and oxygen insulation, and enhancing the flame retardant effect of the condensed phase. In general, the flame‐retardant mechanism of 4N‐BDP is a synergistic combination of condensed phase and gas phase flame retardant mechanism. This work provides a new approach to improve the defects of organophosphorus flame retardant BDP commonly used in the market and enhance the fire resistance of EP.