Thermal degradation mechanism of Bi₂O₃ synergistic intumescent flame retardant polypropylene

The intumescent flame retardant polypropylene(PP) with excellent flame retardancy was prepared by using Bi₂O₃ as synergist in the flame retardant system consisted of ammonium polyphosphate (APP) and di-pentaerythritol. The intumescent flame retardant PP had limited oxygen index and tensile strength up to 25.2% and 28.2 MPa, respectively when the content of Bi₂O₃ was 0.1 phr. Adding Bi₂O₃ obviously decreased the mass loss rate of APP and significantly enhanced the mass retention rate of APP at high temperature. At high temperature, Bi₂O₃ could react with polysphosphorus acid decomposed from APP to form Bi-O-P, which increased the thermal stability of polysphosphorus acid. The study indicates that the interaction of Bi₂O₃ and APP during combustion generates bismuth phosphate and inhibits polyphosphorus acid to decompose to produce P₂O₅, and thus more polyphosphorus acid will be available for esterification reaction and for char formation. Consequently, more compact intumescent char layer are formed and the flame retardancy of PP is improved.