Inherent flame retardation of semi-aromatic polyesters via binding small-molecule free radicals and charring

Inherent flame-retardant semi-aromatic polyesters, containing special aryl ether and/or ketone structures (Ar-CO-Ar, Ar-O-Ar, Ar-O-Ar-O-Ar or Ar-O-Ar-CO-Ar-O-Ar) were synthesized successfully. Interestingly, these polyesters show different flame retardancy beyond our traditional knowledge that more benzene rings are beneficial to flame retardancy. The polyester containing Ar-O-Ar-O-Ar shows excellent flame retardancy, whose LOI value reaches 34.1% and the UL-94 rating is V-0. Meanwhile, the polyester with the Ar-O-Ar-CO-Ar-O-Ar structure does not perform expectedly well (31.6% and V-2 rating respectively). In order to make clear the effect of aryl ether and/or ketone structure units on the flame retardancy, the pyrolysis behaviours and the char residue are investigated by Py-GC/MS, TGA, and SEM. In the TGA test, the char residues of polyesters containing Ar-CO-Ar, Ar-O-Ar Ar-O-Ar-O-Ar or Ar-O-Ar-CO-Ar-O-Ar are 31.6%, 22.5%, 30.6% or 38.7%, respectively. These values do not match with the calculated results, which indicate that some special reactions occur during combustion. Furthermore, these polyesters show a common initial pyrolysis pathway and subsequent unique processes in the Py-GC/MS test. Their pyrolysis intermediate products can bind small-molecule free radicals, and eventually form different conjugated aromatic structures. In this way, inherent flame-retardant polyesters are obtained even without any traditional flame-retardant elements. And their flame retardant performance has great relationship with the amount of char formation, the microstructure of char, and the chemical structure of pyrolysis products.