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. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The zinc borate/silica composite flame retardant showed enhanced flame retardancy and mechanical properties by increasing local concentration and generating an intumescent char layer. Compared to physical mixtures, it exhibited superior performance in both flame retardancy and mechanical properties.