Effect of sulfur-based bifunctional flame retardants on pyrolysis behaviour of epoxy thermoset resins

•An efficient condensation reaction to build high yield flame retardants is reported.•Low dosage, excellent flame retardant performance.•TP has good synergy with DOPS.•DSTP releases abundant pyrolysis products and has good char formation properties.•Maintain good tensile and flexural strength. Enhancing fire safety and meeting the need for it to be adapted to various fields is made possible in large part by flame-retardant modified epoxy resin (EP). Reaction conditions, cost, and yield are some of the main constraints on the widespread development and application of flame retardants. For this reason, in this study, EP modifiers with dual flame-retardant group structures were constructed with the goal of mild reaction conditions, facile synthesis paths, and high yields. First, isocyanate triazine-trione derivative TP was synthesized by a addition reaction. Then, DOPS was grafted onto TP by addition reaction to construct DSTP with double flame retardant groups (DOPS/TP). It was added to EP in order to create highly fire-resistant EP/DSTP material. A thorough analysis was conducted on the effect of DSTP on the flame retardancy, pyrolysis behaviour, and flame retardancy mechanism of EP. The findings demonstrated that EP can pass the UL-94 V-0 test with a LOI value of 35.1% and a considerable decrease in heat and smoke release values when using 10 wt% (P: 0.68 wt%; S: 0.71 wt%) DSTP. The behaviour of pyrolysis demonstrates that DSTP can prevent combustion in the gas-phase by releasing inert and phosphorous free radicals, as well as non-combustible gas. It can also capture and quench active free radicals, such as ·H and ·OH. At the same time, the flexible branch chain and benzene ring structure contained in DSTP promoted the EP curing, and improved the tensile and flexural strength of EP. This work pioneered a strategy for constructing high-performance flame retardant epoxy thermoset resins through simple and mild reaction conditions.