Synthesis of Core/Shell Structured Zinc Borate/Silica and Its Surface Charring for Enhanced Flame Retardant Properties

An efficient zinc borate/silica composite flame retardant (CFR), with accumulating nano-silica (SiO2) on the surface of grown zinc borate (ZnB) flakes to form a core/shell structure, was synthesized via a facile precipitation route. When mixed with methyl vinyl silicone rubber (VMQ), this combined structure of CFR brought advantages in both flame retardancy and mechanical properties. During a process of combustion, these CFR particles migrated and assembled on the surface of combustion zone, increasing the local concentration and generating a tight and intumescent surface char layer even at a lower loading amount. In comparison with the VMQ/physical mixture (PM, with the same proportion of SiO2 and ZnB), the EVA/CFR formulation passed a UL94 V–0 rating and got a LOI value of 29.5, when used as an alternative of SiO2 filler. Meanwhile, CFR played an effective role in inhibiting the spread of fire; VMQ/CFR showed a fire growth index (FGI) of 0.56 much lower than 0.92 of VMQ/PM. In this case, the heat release rate (HRR), total heat released (THR) and smoke production rate (SPR) were considerably reduced. Moreover, the mechanical properties of VMQ/CFR blends, including both tensile strength (TS) and elongation at break (EAB), were significantly enhanced.