Synthesis of a Solid Superacid and Its Application in Flame-Retardant Poly(vinyl chloride) Material

TiO₂/PO₄ ^3- solid superacid was synthesized by using the precipitation immersion method and characterized by means of X-ray diffraction (XRD) and an energy-dispersive spectrometer; it was added into flexible poly(vinyl chloride) (PVC) composites as a flame retardant. The smoke suppression and flame retardance of TiO₂/PO₄ ^3- accompanied by Sb₂O₃ were investigated through the limiting oxygen index (LOI), cone calorimetry test, and thermogravimetry analysis (TGA). The results indicated that the LOI value of PVC/Sb₂O₃/(TiO₂/PO₄ ^3-) reached to 32.3%, which is higher by 6% than that of neat PVC (26.3%). In addition, Sb₂O₃/(TiO₂/PO₄ ^3-) greatly reduced the peak heat release rate and total heat release simultaneously in comparison to using Sb₂O₃ separately, suggesting a significant synergistic effect between TiO₂/PO₄ ^3- and Sb₂O₃ on improving the flame retardancy of the PVC. Further, results of TGA and differential thermal analysis showed that the thermal stability of the composites was greatly improved. For the Fourier transform infrared analysis, Sb₂O₃/(TiO₂/PO₄ ^3-) leads to a large amount of functional group surplus, which specifically indicates that its carbon residue increases. The surface of the char formed after combusting of the PVC compounds was observed through scanning electron microscopy. It is found that the solid superacid can promote decomposition, pyrolysis, and cross-linking of PVC into the compact and continuous char layer on the surface of the material, which improved the flame retardancy and smoke suppression of PVC.