Functional Group Effect on Char Formation, Flame Retardancy and Mechanical Properties of Phosphonate–Triazine-based Compound as Flame Retardant in Epoxy Resin

A series of novel flame-retardant thermosets were prepared by melt blending of phosphonate–triazine-based compound TNTP, triazine-based compound TN, and phosphonate-based compound TP, respectively. The curing systems were consisted of diglycidyl ether of bisphenol A (DGEBA) and 4,4′-diamino-diphenyl sulfone (DDS). The thermal behaviors and flame retardancy of these flame-retardant thermosets were investigated in terms of thermogravimetric analysis (TGA), limiting oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter tests. TGA results showed that the char formation of flame-retardant thermosets could be significantly improved due to the presence of phosphonate moiety rather than triazine unit. It was found that the excellent flame retardant effect of TNTP was not contributed by either single group of phosphonate or triazine. An obvious synergic-effect on flame retardant produced by a combination of phosphonate and triazine moiety. The LOI value of TNTP-3/DGEBA/DDS could achieve 32.4% and reach UL-94 V-0 rating, while that of TN-3/DGEBA/DDS was 29.0% and failed in UL-94 test, and TP-3/DGEBA/DDS with a LOI value of 31.8% just reach UL-94 V-1 rating. Moreover, cone calorimeter test revealed that the incorporation of TNTP to epoxy thermoset with 1.5 wt % phosphorus content could result in a decrease of peak heat release rate (PHRR), total heat release (THR), average mass loss rate (AMLR), total smoke release (TSR), average CO yield (ACOY), and average CO₂ yield (ACO₂Y) compared with DGEBA/DDS control. The results from TGA data, scanning electronic microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) indicated TNTP modified thermosets had a comprehensive flame retardant mechanism, including the gas phase, condensed phase and phosphorus–nitrogen synergism mechanism. Furthermore, the mechanical properties of all the thermosets were also investigated by Izod impact strength and flexural property tests.