Contribution of Phenylimide to Flame Retardancy, Anti-dripping and Smoke Suppression of PET

A phenylimide-containing monomer was first introduced into PET backbone by bulk copolymerization, and a series of inherently flame-retardant, anti-dripping and smoke-suppressed copolyesters, P(ET-co-BN)(n), where n denotes the molar part of BN per hundred mole of dimethyl terephthalate (DMT) (namely, BN: DMT = n:100), were successfully synthesized. Thermogravimetric analysis (TGA) indicated that the introduction of the phenylimide group not only increased the char residue of the copolyesters at high temperatures, but also greatly decreased the thermal decomposition rate of the copolyesters. The initial decomposing temperature (T-5%) of the copolyesters showed a slight increase compared with that of PET, indicating that the original thermal stability was maintained. With increasing the proportion of phenylimide, the copolyesters exhibited higher glass transition temperature (T-g). Further more, the higher was the phenylimide content, the higher was the limiting oxygen index (LOI). When P(ET-co-BN)(20) was tested at a high oxygen concentration of 29%, flame propagation was obviously slowed and extinguished within 52 s. After LOI test, the top zone of P(ET-co-BN)(20) specimen was covered by intumescent char layer, and no dripping was observed. Compared with those of neat PET, the peak heat release rate (p-HRR), peak release smoke rate (p-RSR) and total smoke release (TSR) of P(ET-co-BN)(20), obtained from cone calorimetry, were reduced by 51.5%, 44.7% and 31.1%, respectively, indicating its inherently low flammability and smoke suppression. Raman spectroscopy indicated that the formation of a thermally stable graphitic structure was promoted by the phenylimide at high temperatures. It could also be proved by SEM that the char layer of the copolyester was compact and coherent, which took an active part in isolating the unburned polymer from heat source, cutting off oxygen and preventing the volatilization of flammable fragments, resulting in excellent inherent flame retardancy and smoke suppression. Without traditional flame retardant elements, such as chloride, bromine and phosphorus, this high-performance copolyester was only composed of carbon, nitrogen, hydrogen and oxygen, which would be really eco-friendly.