Influence of small difference in structure of polyamide charring agents on their flame-retardant efficiency in EVA

Two kinds of polyamide charring agents, poly(piperazinyl malonamide) (PPMA) and poly(piperazinyl succinamide) (PPSA), were synthesized, and their only difference is that PPSA increases one methylene in the main chain compared with PPMA, which were confirmed via Fourier transform infrared spectroscopy (FTIR), Elemental analysis (EA), 1H nuclear magnetic resonance (1H-NMR) spectrum and 13C solid NMR spectrum. Thermogravimetric (TG) analysis showed that the residue of PPMA was remarkably higher than that of PPSA during the thermal decomposition process. When they were used to flame retard EVA, PPMA showed higher efficiency than PPSA for passing the UL-94 V-0 rating, increasing the oxygen index (OI), and decreasing the total heat release (THR) and total smoke production (TSP); while the PPSA was more efficient in decreasing the peak of heat release rate (PHRR) and the peak of smoke production rate (peak SPR). The investigation for the pyrolysis products of PPMA and PPSA demonstrated that various aromatic structures were produced due to the dehydrogenation of piperazine ring during the decomposition of PPMA and PPSA, which endowed them with excellent charring capability, and the charring efficiency of PPMA was higher than that of PPSA. However, the PPSA with lower charring capability tended to decompose and cyclize to the imides and volatilize into the gas phase, which resulted in the more ideal microstructure of PPSA system than that of PPMA system, finally led to the lower PHRR and peak SPR of PPSA system in cone calorimeter test.