Boron–Silicon Hybrid-Coated Magnesium Hydroxide with Gas-Phase and Condensed-Phase Flame Retardancy for EVA Prepared by a One-Step Method

Abstract The toxic smoke produced during the combustion of flame-retardant polymers poses a severe threat to human safety in fire scenarios. Magnesium hydroxide (MH) is widely adopted as a halogen-free, low-toxicity, and eco-friendly flame-retardant filler. Nevertheless, MH suffers from low flame-retardant efficiency stemming from its sole gas-phase flame-retardant mechanism, as well as poor compatibility with the polymer matrix. In this work, boron–silicon hybrid-coated magnesium hydroxide (SiBMH) was fabricated via a one-step method, endowing SiBMH with outstanding flame-retardant efficiency in both the gas phase and the condensed phase. The resultant ethylene-vinyl acetate copolymer (EVA)/48SiBMH composite achieved a V-0 rating in the vertical burning test, with a limiting oxygen index (LOI) of 29.0%. By contrast, the EVA/48MH composite only reached an nonrated (NR) rating with an LOI of 26.0%. Furthermore, the peak heat release rate (PHRR) and total heat release (THR) of EVA/48SiBMH decreased by 12.7 and 15.4%, respectively, compared to the EVA/48MH composite. The superior flame-retardant performance of SiBMH originates from multiple mechanisms revealed by thermogravimetric analysis, thermogravimetry-infrared spectrometer, and scanning electron microscopy characterizations. The boron–silicon hybrid coating layer delayed the thermal decomposition of MH, prolonging the water-vapor release duration to dilute combustible gas. Moreover, the boron–silicon hybrid coating layer together with MgO derived after combustion constructed a continuous and dense inorganic barrier layer that hindered heat and oxygen transport. In addition, an organic hydrophobic segment introduced by KH5775 suppressed the agglomeration of SiBMH particles and greatly enhanced the interfacial force between SiBMH particles and the EVA matrix. Compared with EVA/48MH, the tensile strength of EVA/48SiBMH increased by 37.2% and the elongation at break increased by 22.7%. Therefore, this boron–silicon coating strategy represents a green, efficient, and multifunctional approach to simultaneously improve both the flame retardancy and mechanical properties of polymer-based composites.