Synergistic flame retardant mechanism of MoO₃ and intumescent flame retardants: Insights from real-time char layer resistance monitoring

Abstract

This study explores the synergistic flame retardant mechanism of molybdenum trioxide (MoO₃) and intumescent flame retardants (IFRs) in a styrene-butadiene-styrene (SBS) matrix using real-time resistance monitoring. By tracking the char layer resistance during combustion and combining multiple characterization techniques, we examined MoO₃’s impact on the char layer morphology. The results demonstrate that the incorporation of MoO₃ enhances the flame-retardant performance of IFR. At 1 wt % MoO₃, the UL-94 rating improves from non-classified to V-1. Thermogravime tric Analysis reveals that MoO₃ lowers the initial decomposition temperature of the material from 240 °C to 231 °C while increasing the char residue. X-ray Photoelectron Spectroscopy and Fourier transform infrared analyses confirm that MoO₃ acts as a catalyst, promoting ammonium polyphosphate decomposition and facilitating the formation of phosphate esters, which enhance the char layer's viscosity and compactness. While MoO₃ does not directly participate in char crosslinking, it accelerates char formation, improves expansion, and strengthens flame retardancy. Real-time resistance data further indicate that MoO₃ reduces char formation time and slows char layer cracking, thereby enhancing flame resistance. This study provides new insights into the MoO₃/IFR synergy and introduces a novel approach for monitoring char layer changes during combustion.