Thermal degradation mechanism of Ni₂O₃ synergistic intumescent flame retardant polypropylenes

Using Ni₂O₃ as synergist, the imtumescent flame retardant system consisted of polypropylene (PP), ammonium polyphosphate (APP) and di-pentaerythritol (DPER) was prepared. The results of oxygen index (OI) and UL-94 test show that with the addition of 1% Ni₂O₃, LOI is 28.2% and the flame retardant PP can reach V-0 level. The results of thermogravimetric analyses (TGA), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) demonstrate that the addition of Ni₂O₃ makes the mass loss rate of APP decrease and the high temperature residue increase. At higher temperature, Ni₂O₃ is decomposed to NiO. NiO interacts with polyphosphorus acid (decomposed product of APP) to form nickel metaphosphate, which makes the thermal stability of polyphosphoric acid increase. NiO is helpful to form the compact intumescent char layer and enhance flame retardant properties of material since it can inhibit polyphosphoric acid to decompose into P₂O₅, and make more polyphosphoric acid participate in esterification reaction and char formation.