Correlation between the initial degradation temperature of organophosphorus-based flame retardants and flame retardancy of polypropylene composites

The efficiencies of three organophosphorus-based flame retardants (AlHPi with 31% P content, AlPi with 23% P content, and AlPo with 26% P content) on polypropylene (PP) were investigated. The initial degradation temperature (T_id) and the temperature of maximum mass loss rate T_max values of PP closely aligned with those of AlHPi and AlPo, while AlPi exhibits a significantly higher T_id than that of PP. This disparity in thermal stability is reflected in the UL-94 vertical burning (VB) test results. The PP/AlPi composite failed the UL-94 VB test, and the PP/AlHPi composite achieved a V-2 rating, inferior to the V-0 rating obtained by the PP/AlPo composite. The cone calorimetry results showed that the PHRR of pristine PP was notably reduced from 1274 kW/m² to 267 kW/m² in the PP/AlHPi composite, 622 kW/m² in the PP/AlPi composite, and 252 kW/m² in the PP/AlPo composite. The THR of pristine PP decreased from 487 MJ/m² to 212 MJ/m² in the PP/AlHPi composite, 405 MJ/m² in the PP/AlPi composite, and reached a minimum value of 183 MJ/m² in the PP/AlPo composite. The TSP of pristine PP decreased from 135 m² to 18 m² in the PP/AlHPi composite, to 25 m² in the PP/AlPi composite, and to a minimum value of 9 m² in the PP/AlPo composite.