An insight into gas phase flame retardant mechanisms of AHP versus AlPi in PBT: Online pyrolysis vacuum ultraviolet photoionization time-of-flight mass spectrometry

As is well known, gas phase flame retardant mechanisms are very hard to be understood for the extremely intricate burning process but they are crucial for developing highly efficient flame retardants. Pyrolysis provides a valid way for this purpose because combustion proceeds after it. In this work, the pyrolysis behaviors of aluminum hypophosphite (AHP), aluminum diethylphosphinate (AlPi), poly(1,4-butylene terephthalate) (PBT) and PBT composites (with 10 wt% AHP or AlPi) were investigated by the fragment-free pyrolysis vacuum ultraviolet photoionization time of flight mass spectrometry (PY-PI-TOFMS) at different temperatures in real time. The gas phase flame retardant mechanism of AHP in PBT is proposed to be the flame inhibition effect of PH₃, H₃PO₂ and P₄ pyrolyzed from AHP. That of AlPi in PBT is speculated to be phosphorus-containing compounds and a kind of phosphorus radical pyrolyzed from AlPi as flame inhibitors and the promoted pyrolysis of AlPi by PBT matrix to be synchronous with it. The eight-membered cyclic non-covalent adduct of C₂H₅–(O=)P•–OH radical and a molecule of diethylphosphinic acid via intermolecular double hydrogen-bond interaction can produce PO type radicals more effectively in flame. Thermogravimetric analysis results are in accordance with those of pyrolysis. And the much better gas phase flame retardant effect of AlPi than AHP in PBT concluded from combustion tests are interpreted by pyrolysis results, which provides powerful guidance for developing highly efficient flame retardants.