Multi-element heterocyclic compound derived from DOPO and thiadiazole toward flame-retardant epoxy resin with satisfactory mechanical properties

In order to comply with the environmentally friendly flame retardant requirements of epoxy resin and strive to maintain the comprehensive performance of the epoxy resin matrix, a P/N/S-containing heterocyclic halogen-free reactive flame retardant (VAP) was constructed. Vanillin (VA), 2-amino-5-mercapto-1,3,4-thiadiazole, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene10-oxide (DOPO) were chosen as raw materials to prepare VAP through a simple one-pot reaction. The addition of VAP slightly decreased the curing activity of the system while the transparency of the samples was maintained well. Moreover, the maximum degradation rate of VAP modified samples dropped by 47.26% and the amount of char residue at 800°C increased. When 10 wt% VAP (0.66 wt% P) was added, EP-10 passed UL-94 V-0 rating and the limited oxygen index value reached 30.3%. In contrast to pure EP, the peak heat release rate, total heat release, and total smoke production of EP-10 were reduced by 36.87%, 25.61%, and 29.07%, representing good smoke and heat suppression effect. Besides, the glass transition temperature ( T g ) of the samples remained above 165°C, showing a slight drop. Due to the existence of heteroatoms and the introduction of heterocyclic structure, the enhancement of intermolecular force in the system counteracted the deterioration of mechanical properties caused by the reduction of crosslinking density, maintaining the excellent mechanical properties of epoxy resin.