Influence of sodium bromide on the thermal decomposition of tetraphenylphosphonium montmorillonite

Tetraphenylphosphonium montmorillonite (TPP-MMT) was prepared from an aqueous solution of tetraphenylphosphonium bromide and a bentonite consisting primarily of sodium rich montmorillonite (Na-MMT) together with minor amounts of cristobalite, quartz and nontronite. The original bentonite, the TPP-MMT and mixtures of TPP-MMT with various amounts of NaBr were investigated by in-situ high-temperature X-ray diffraction and thermogravimetric analysis. The decomposition of TPP-MMT was strongly dependent on the presence of NaBr. NaBr took part in a retro-reaction to Na-MMT + TPP-Br, of which the latter decomposed into gaseous products. The decomposition temperature of pure TPP-MMT was about 100 °C higher than mixtures of TPP-MMT with NaBr. The TPP-ions in pure TPP-MMT decomposed continuously within the MMT interlayer space, so that there was probably nothing left but H+. Since NaBr is a by-product of the TPP-MMT synthesis from Na-MMT and since halogen ions strongly influence fire retardant properties of polymer-based composites, this reaction is a critical aspect in the evaluation of such materials. Cristobalite and quartz were inert during the reactions observed up to temperatures of 1000 °C.

► Tetraphenylphosphonium montmorillonite (TPP-MMT) & TPP-MMT + NaBr by HT-XRD.
► NaBr as impurity strongly changed the decomposition of TPP-MMT.
► Proposed reaction at high temperature: TPP-MMT + NaBr → Na-MMT + TPP-Br.
► Results may be crucial for thermal stability of other clay/polymer nanocomposites.