Synthesis of phosphaphenanthrene derivatives as flame retardants and their properties in epoxy resin composites

Abstract 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.