Study on the thermal degradation of mixtures of ammonium polyphosphate and a novel caged bicyclic phosphate and their flame retardant effect in polypropylene

The thermal degradation behaviour of a novel caged bicyclic phosphate (Trimer) and its mixture with ammonium polyphosphate (APP) was studied by TG, FTIR and TG-FTIR. The flame retardant effect of Trimer/APP (IFR) in polypropylene (PP) was evaluated by limiting oxygen index (LOI) and vertical burning test (UL-94) at different mass ratio of Trimer and APP. Obvious synergistic effect can be observed between Trimer and APP, which remarkably improves the flame retardant properties of PP/IFR system. A maximum LOI value of 28.8% and UL-94 V-0 rating were achieved when the mass ratio of Trimer and APP was 2:1 at a total IFR loading of 25 wt%. Different degradation behaviour of Trimer/APP (with the mass ratio of 2:1) from APP or Trimer was observed by TG. The experimental TG curve of Trimer/APP, compared with the theoretically calculated one, showed lower initial degradation temperature and higher char residue at 800 °C, demonstrating interactions of Trimer and APP during thermal degradation. Such interactions were shown by gaseous degradation products analyzed by TG-FTIR. APP can chemically interact with Trimer, which changes the degradation process of Trimer. In the condensed phase, dehydration, cross-linking and char formation dominated the Trimer/APP interactions.