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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]

[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]

• In this paper, DOPS and triazine-trione groups are introduced into the same molecular to construct a new efficient flame retardant system through the synergistic effect of double functional groups. • Adding DOPS-THEIC to EP, we found that when DOPS-THEIC was added at 12.5 wt%, the LOI value of EP/DOPS-THEIC increased to 32.8%, achieving a UL-94 V-0 rating. • The PHRR, THR and MLR values of EP/DOPS-THEIC are significantly lower than pure EP, indicating that DOPS-THEIC can effectively inhibit EP's heat release. The char layer produced during combustion reduces substrate mass loss, proving that DOPS-THEIC has good char formation ability. • The flame retardant DOPS-THEIC participated in the curing process of EP composites, forming a variety of cross-linked network structures and improving EP's flexural strength. • The introduction of the S element in DOPO allows the EP to release more sulphur-containing compounds upon combustion, which combine with some water to form strong acids such as sulphuric acid, promoting the dehydration of the substrate into char. To reduce the risk of fire due to the flammability of epoxy resins (EP), the use of low-cost, non-toxic raw materials for the synthesis of efficient flame retardants is becoming an important research direction in the field of flame retardant technology. This research with tris (2-hydroxyethyl) isocyanurate (THEIC), maleic anhydride (MAH) and 9,10-dihydro-9-oxa-10- phospha-phenanthrene-10-sulfide (DOPS) as raw materials, through esterification reaction successfully synthesised a novel structure of DOPS-triazine-trione flame retardant tris (3-(9,10-dihydro-9-oxa-10-phospha-phenant hrene-10-sulfide)-butanedioic acid monoethyl ester-based) triazine-trione (DOPS-THEIC) and apply it to EP to prepare EP/DOPS-THEIC. The results demonstrate that the incorporation of 12.5 wt% DOPS-THEIC in EP leads to a significant increase in the Limiting Oxygen Index (LOI) value, reaching 32.8 %, which corresponds to achieving a UL-94 V-0 rating. Moreover, the CONE test revealed notable reductions in the peak heat release rate (PHRR), total heat release (THR), and mass loss rate (MLR) values by 74.1 %, 32.4 %, and 52.6 %, respectively, compared to pure EP. These findings indicate that the addition of DOPS-THEIC effectively suppresses the heat release during combustion, attributed to the formation of a char layer that minimizes mass loss and acts as an improved flame retardant in the condensed phase. Furthermore, as the DOPS-THEIC content increased, the EP composites exhibited remarkable retention of their excellent mechanical properties. This observation reinforces the promising potential of DOPS-THEIC as an effective flame retardant for enhancing the fire resistance of EP materials. [Display omitted] [ABSTRACT FROM AUTHOR]