Different flame retardancy effects and mechanisms of aluminium phosphinate in PPO, TPU and PP

The effects of aluminium phosphinate (AlPi) on the flame retardancy of three different polymers, namely, poly(2, 6-dimethyl-1, 4-phenylene oxide) (PPO), thermoplastic polyurethane (TPU) and polypropylene (PP), were investigated through cone calorimeter tests (CONE), vertical burning tests (UL-94), limiting oxygen index (LOI), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) observation. The results showed that the amounts of AlPi needed to reach V-0 UL-94 rating in the PPO, TPU and PP matrices were 0 wt%, 30 wt% and 50 wt%, respectively. Moreover, the addition of AlPi significantly increased the LOI values of PPO and PP, but had almost no effect on that of TPU. With the addition of AlPi, the peak of heat release rate (PHRR) of PPO and TPU decreased whereas that of PP increased. The total heat evolved (THE) of all polymers decreased by adding AlPi into the polymers. With the addition of AlPi, PPO composite formed a dense char layer and showed the best flame retardancy, TPU composite formed a thinner char layer and PP composite did not form char layer during combustion. Based on the thermal degradation behaviour and the char residue of these composites, the flame retardancy mechanisms of the three systems were discussed. For AlPi/PPO composites, the flame retardancy mechanism was condensed phase charring mechanism because of the formation of continuous char layer during combustion. For AlPi/TPU composites, AlPi acted mainly in gas phase, and a synergistic reaction of phosphorus and nitrogen compounds occurred and diethylphosphic acid providing flame inhibition was released during the burning process. As a result, the addition of AlPi successfully reduced the melt-dripping and enhanced flame retardancy of TPU matrix. For AlPi/PP composites, AlPi acted as a flame inhibitor in gas phase, decreasing the effective heat of combustion of the volatiles (THE/TML) and increasing the amount of CO. (C) 2014 Elsevier Ltd. All rights reserved.