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In order to further improve the flame‐retardant properties of epoxy resins (EPs), two hydroxyl‐containing phospha‐phenanthrene derivatives, DOPO‐PHBA and DOPS‐PHBA, were prepared from the molecular structure design by using 9,10‐dihydro‐9‐oxa‐10‐phospha‐phenanthrene‐10‐oxide (DOPO), 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐sulfide (DOPS), and 4‐hydroxybenzaldehyde (PHBA) as the raw materials. These composites of DOPO‐PHBA/EP and DOPS‐PHBA/EP were prepared by melt blending. The results showed that UL‐94 V‐0 was achieved with an LOI of 31.9% when just 7.5% DOPS‐PHBA was added to EP, while a limited oxygen index of 35.2% was achieved with 10% DOPO‐PHBA/EP. Moreover, in comparison with 7.5% DOPO‐PHBA/EP, 7.5% DOPS‐PHBA/EP has the lower peak of heat release rates, total heat release values, and fire growth rate, demonstrating that the flame‐retardant DOPS‐PHBA is more effective at reducing heat release, enhancing the material's flame retardancy, and lessening the intensity of EP burning. The Thermogravimetric‐infraredspectrometry (TG‐IR) results showed that the phosphorus‐containing radicals generated by DOPS‐PHBA/EP pyrolysis trapped the free radicals, such as H˙, O˙, or HO˙, thus providing free radical quenching. Furthermore, combining the macro and micro Scanning electron microscopy (SEM) test results, DOPS‐PHBA/EP has a better char formation ability and a denser char layer, which can play a better role in heat and oxygen insulation. [ABSTRACT FROM AUTHOR]

Fire dangers have grown to be a serious potential issue that could endanger people's health and safety in recent years. A lot of emphasis has been paid to the employment of straightforward techniques to create multifunctional synergistic flame retardants in order to create multifunctional flame retardant epoxy curing compounds. In this paper, a flame retardant TFD with DOPS and phenoxytriazine as flame retardant core groups was synthesized by molecular structure design using cyanuric chloride, p-hydroxybenzaldehyde, and DOPS as the raw materials, and it was melt blended with EP to construct a flame retardant thermoset resin EP/TFD. The results show that the addition of only 10% TFD enables EP to pass the UL-94 V-0 test with an LOI of up to 35.3%. Moreover, TFD was able to effectively reduce the peak heat release rate, total heat release, and fire growth index of epoxy curing compounds, whereas the fire performance index was significantly improved compared with that of pure EP, indicating that TFD could confer higher fire safety to EP. Moreover, TFD can significantly enhance the mechanical toughness of EP with appropriate additive amount. That is, the maximum rise compared with the pure EP enhancement of 46.3%. This program serves as a valuable resource for the advancement of halogen-free flame retardants with superior efficiency. [Display omitted] • Homemade sulfurized DOPO flame retardant. • Synthesis of bifunctional group synergistic flame-retardant epoxy thermoset resins based on sulfurized DOPO flame retardants. • Low dosage, excellent flame retardant performance. • EP/TFD has good thermal stability; fire safety. • Excellent char formation and mechanical toughness. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synthesis of a P/N/S-containing efficient flame retardant MPD based on DOPS-triazine groups. • Excellent compatibility with EP. • Low dosage, excellent flame retardant performance. • EP/MPD has good thermal stability; fire safety. • Significantly improves EP's char formation and mechanical properties. 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. [ABSTRACT FROM AUTHOR]