Synthesis of highly dispersed magnesium hydroxide and its application in flame-retardant EVA composites

Magnesium hydroxide (MH) is widely recognized as an environmentally friendly, halogen-free flame retardant and has been extensively applied in ethylene-vinyl acetate (EVA) copolymers. However, its poor dispersion in the polymer matrix and low flame retardant efficiency remain significant challenges. In this study, hexagonal magnesium hydroxide (MHz) with well-defined morphology, high specific surface area, and excellent dispersion was inventively synthesized via hydrothermal synthesis using sodium hydroxide (NaOH) and sodium carbonate (Na₂CO₃) as the hydrothermal medium. Three EVA composites filled with MHz and two different commercial magnesium hydroxides (MHx and MHy) were prepared via melt-blending, and their flame retardant and mechanical properties were systematically compared. The results indicate that the EVA composite containing 60 wt% MHz (EVA/MHz60) exhibits superior flame retardant performance, achieving a limiting oxygen index (LOI) of 49.2%, which is 53.8% and 9.1% higher than that of MHx and MHy, respectively. The UL-94 rating reached V-0. Furthermore, the peak heat release rate (PHRR) of EVA/MHz60 is significantly reduced to 150.6 kW m^-2, marking a 33.2% and 29.9% reduction compared to the EVA composite containing 60 wt% MHx (EVA/MHx60) and EVA composite containing 60 wt% MHy (EVA/MHy60), respectively. Additionally, the tensile strength of EVA/MHz60 is improved by 101.6% and 76.4% compared to that of EVA/MHx60 and EVA/MHy60, respectively. The improvement in tensile strength of the EVA/MHz60 composite can be attributed to the nanoscale and uniform dispersion of MHz with hexagonal morphology, which can enhance the interfacial adhesion with the EVA matrix, thus the flame retardant and mechanical properties of the composite can be improved simultaneously.