P/N/S flame retardant based on DOPS-triazine groups for improving the flame retardancy, char formation properties and mechanical properties of epoxy resin

As epoxy resins (EPs) are widely used in adhesives, electronic packaging materials and aerospace, it is essential to construct new and efficient flame-retardant EP systems to reduce the risk of fire caused by their flammability. In this study, the condensation reaction of melamine (MEL) and 4-hydroxybenzaldehyde (PHBA) produced the intermediate melamine Schiff base (MSB), which was then successfully synthesized with 9,10-dihydro-9-oxa-10-phospha- phenanthrene-10-sulfide (DOPS) by addition reaction to produce a DOPS-triazine intumescent flame retardant MPD containing a novel structure of P/N/S, and applied to fabricate high-performance EP/MPD. As expected, the MPD has good flame-retardant properties due to the synergistic effect of DOPS groups and triazine groups. The thermoset resin EP/MPD passes the UL-94 V-0 test with the addition of only 7.5 % MPD and has a high LOI value of 32.7 %, which is 47.3 % higher than that of pure EP. In addition, the peak heat release rate (PHRR), total heat release (THR) and fire growth rate (FIGRA) of EP/MPD-12.5 % decreased by 33.9 %, 67.7 %, and 65.8 %, respectively, compared with that of pure EP. From the viewpoint of the char formation performance, the expansion height of the char residue of EP/MPD after combustion increased significantly up to 5.6 cm, which was over three times greater than the pure EP value. Moreover, the appropriate amount of MPD can effectively improve the flexural and impact strengths of EP thermosets. This study provides a new idea for the preparation of high-performance flame retardant MPD without affecting the mechanical properties of EP.