The high-temperature self-crosslinking contribution of azobenzene groups to the flame retardance and anti-dripping of copolyesters

This manuscript reports a new method to solve the conflict between the flame retardance and anti-dripping of poly(ethylene terephthalate) by introducing azobenzene groups into the main-chain. This novel polyester containing azobenzene units in the main chain (PEAT) was synthesized via esterification and copolycondensation of terephthalic acid, ethylene glycol and azobenzene-4,4'-dicarboxylic acid. TG-DSC, rheological and UV-Vis results proved the occurrence of the thermal and chemical crosslinking reaction by adjacent azobenzene groups. In the TG test, the residue and thermooxidative stability were obviously increased by the high-temperature crosslinking of the azobenzene groups. The cone calorimetric analysis and LOI tests further confirmed the self-extinguishing properties and inhibition of melt-dripping. Elemental analysis results indicated that besides partial decomposition of azobenzene groups into N-2, most nitrogen was retained in the residue after high-temperature crosslinking. Besides, the char became more compact with an increase of azobenzene-4,4'-dicarboxylic acid. All the results demonstrated that through the high-temperature crosslinking of the azobenzene group, PEAT exhibited a good balance between flame retardance and anti-dripping properties.