Zinc hydroxystannate-coated metal hydroxide fire retardants: Fire performance and substrate-coating interactions

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Fire retardant (FR) properties, including limiting oxygen index, peak rate of heat release and smoke parameter have been measured and compared for unfilled and filled polyvinyl-chloride (PVC) based cable formulations, containing various amounts of uncoated and zinc-hydroxystannate (ZHS)-coated alumina trihydrate (ATH) and magnesium hydroxide (MH) fillers. Uncoated ATH or MH proved to be efficient FR additives, but when coated with ZHS, further improvements were observed. ATH was more effective than MH in both uncoated and ZHS-coated forms. For a halogen-free ethylene-vinyl acetate (EVA) cable formulation, the content of vinyl acetate and loading of ZHS-coated ATH required to yield optimum FR properties was determined. The interaction between ATH substrate and ZHS coating was also studied by X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier-transform spectroscopy, using samples with ZHS-contents ranging from 1 to 15 wt%. An increase in the binding energies of the Zn 2p3/2 and Sn 3d5/2 peaks was found, together with alterations in the positions of the (Sn)O—H stretching bands. There was no evidence for condensation and the formation of Al—O—Sn bonds.

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