Influencing mechanism of transition metal oxide on thermal decomposition of ammonium polyphosphate

Transition metal oxide (MO) can obviously influence the thermal decomposition process of ammonium polyphosphate (APP) to improve the flame retardant efficiency of intumescent flame retardant composites based on APP in polymer. ZnO, Fe₂O₃ and TiO₂, in same amount were added into APP to study the influence of MO on thermal decomposition behavior of APP, and to analyze the evolution of chemical state of metallic atoms and phosphorus atom and crystal structure in the interaction processes by thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD) respectively. TGA and XPS spectra showed that MO could catalyze the releasing of NH₃ and H₂O of APP in the earlier period, and increase the high temperature residue in the later period due to the formation of metallic phosphate. The sequence of catalytic activity for the process of releasing of NH₃ and H₂O was as follows: ZnO>Fe₂O₃>TiO₂, and that of cross-linking ability for thermal decomposition product P-O of APP was as follows: Fe₂O₃ >ZnO >TiO₂. XRD showed that APP could react with ZnO, Fe₂O₃ and TiO₂ to produce Zn(PO₃)₂, Fe₄(P₂O₇)₃ and TiP₂O₇, respectively, at high temperature.