Using TG–FTIR and TG–MS to study thermal degradation of metal hypophosphites

Three typical metal hypophosphite flame retardants La(H2PO2)3·H2O (LHP), Ce(H2PO2)3·H2O (CHP), and Al(H2PO2)3 (AHP) were synthesized and characterized by Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction, scanning electron microscopy, thermogravimetric analysis (TG), derivative thermogravimetric analysis, and differential thermal analysis. The thermal degradation products from the synthesized metal hypophosphites were also investigated using thermogravimetry coupled with Fourier transform infrared spectroscopy (TG–FTIR) and thermogravimetry coupled with mass spectrometry (TG–MS). The synthesized metal hypophosphites were also used as flame retardants for poly (1,4-butylene terephthalate) (PBT), and the combustion properties of flame-retarded PBT were evaluated using the limiting oxygen index and UL-94 tests. The results showed that the metal hypophosphites LHP, CHP, and AHP can be used as effective flame retardants for PBT, and these compounds can be obtained through a simple precipitation method. TG–FTIR and TG–MS results showed that the degradation process of AHP involves two steps, corresponding to the removal PH3 reaction and the further dehydration reaction of the hydrogen phosphate aluminum. While LHP and CHP have three degradation steps, the additional step is due to that LHP and CHP which will loss the crystal water at lower temperature.