A reactive flame retardant based on thiadiazole and phosphaphenanthrene for improving the flame retardant and smoke inhibition properties of epoxy resins

Abstract It is difficult to effectively manage the emission of harmful smoke from flame‐retardant modified epoxy resins (EP). In order to solve this difficulty, a highly efficient flame‐resistant PAD containing phosphorus, nitrogen, and sulfur was synthesized using a simply one‐pot method employing 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO), 2‐amino‐5‐mercapto‐1,3,4‐thiadiazole (AMT), and terephthalaldehyde for not only enhancing its flame retardant properties but also effectively suppressing the emission of fumes. Results have confirmed that the incorporation of 5 wt% flame retardant into the EP successfully passed the UL‐94 V‐0 rating, besides 24.2% reduction in the peak of smoke produce rate (PSPR), with outstanding fire‐resistant properties and effectively reduce smoke emissions. In addition, incorporating 7.5% of PAD into EP increased its flexural strength and modulus, from 78.4 and 2917.5 to 86.6 and 3162.9 MPa, respectively. Due to the abundance of reactive groups including sulfhydryl and secondary amine in PAD, as well as the nitrogen atoms presented in the thiadiazole and benzene rings of the phosphaphenanthrene structure, which could create hydrogen bonds with the matrix and participated in π‐π interactions, therefore, the mechanical properties of flame retardant epoxy resin (FREP) were greatly enhanced.