Improvement of the flame retardant and thermomechanical properties of epoxy resins by a vanillin-derived cyclotriphosphazene-cored triazole compound

Abstract

A vanillin-derived cyclotriphosphazene-cored triazole compound (HHCTP) was successfully synthesized as a flame retardant for epoxy thermosets with high efficiency while preserving their thermomechanical properties. 1H- and 31P nuclear magnetic resonance (NMR), as well as Fourier Transform infrared (FT-IR), were applied to confirm the molecular structure of HHCTP. The dynamic mechanical analysis (DMA) measurements demonstrated that the introduction of HHCTP improved the storage modulus of the epoxy composites but decreased the glass transition temperature. The epoxy thermoset containing 7.5 wt% HHCTP (EP/HHCTP-7.5) achieved a UL-94 V-0 classification and an LOI of 31.5%, manifesting its outstanding flame-retardant efficiency. In addition, the cone calorimeter results showed that the total heat release (THR) and the peak value of the heat release rate (PHRR) of EP/HHCTP-7.5 were decreased by 35.6% and 50.0%, respectively, in comparison with those of the unmodified epoxy. To further study the flame retardant mechanism of HHCTP, pyrolytic gas species were detected utilizing thermogravimetric-infrared spectroscopy (TG-FTIR). In addition, the microstructure of the char residues was analyzed via X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM) techniques. The phosphazene groups in HHTCP clearly stimulated the production of an intumescent, compact, and strong char layer, which improved the flame retardancy of the DGEBA matrix during burning, according to the SEM data. As a result, the underlying components were protected from further degradation and combustion, resulting in effective flame retardancy of the DGEBA thermoset.