Flame-retardant properties of magnesium hydroxystannate and strontium hydroxystannate coated calcium carbonate on soft poly(vinyl chloride)

The flame-retardant and smoke-suppressant properties of soft poly(vinyl chloride) (PVC) treated with zinc hydroxystannate (ZHS), calcium carbonate (CaCO₃), magnesium hydroxystannate [MgSn(OH)₆], strontium hydroxystannate [SrSn(OH)₆], ZHS-MgSn(OH)₆, ZHS-SrSn(OH)₆, MgSn(OH)₆-coated CaCO₃, SrSn(OH)₆-coated CaCO₃, ZHS-MgSn(OH)₆-coated CaCO₃, and ZHS-SrSn(OH)₆-coated CaCO₃ were studied with the limited oxygen index, char yield, and smoke density rating methods; the mechanical properties were also studied. The results showed that, with the equivalent addition of the corresponding hydroxystannate, the soft PVC treated with hydroxystannate-coated CaCO₃ had a higher limited oxygen index than the corresponding hydroxystannate, and the soft PVC treated with the agents containing magnesium had a higher limited oxygen index than the soft PVC treated with the agents containing strontium, except for ZHS-MgSn(OH)₆-coated CaCO₃. The improvement in the char formation of the hydroxystannate-coated CaCO₃ was better than that of the corresponding hydroxystannate in most cases, and the aforementioned agents reduced the smoke density rating, decreased the tensile strength, and increased the elongation and impact strength basically. Thermal analysis showed that the additives promoted the evolution of hydrogen chloride, early crosslinking, and rapid charring through the strong catalyzing effect of Lewis acids.