In situ EPR spin-trapping tracking of macroradical formation and the role of a hindered phenol anti-oxidant in reducing thermo-oxidative phenomena of poly(ethylene terephthalate)

One of the major issues concerning poly(ethylene terephthalate) (PET) produced with titanium-based catalysts is its sensitivity to high-temperature oxidation, which results in performance loss and discoloration. Here, the thermo-oxidative phenomena of titanium-catalyst-produced PET during its synthesis and post-processing in the presence of hindered phenol anti-oxidant Irganox 1425 has been investigated by a spin-trapping technique. Three types of spin adducts have been determined and quantified; –O−•CH− is the dominant radical intermediate, while •CH2− and –O−•CH− are mainly produced by the decomposition of diethylene glycol (DEG) units. An excessively high DEG content leads to instability of PET chains at high temperatures and so is not conducive to suppressing oxidation. An anti-oxidant can effectively eliminate radicals induced by thermo-oxidative reactions and improve the thermal stability of PET, especially in the post-processing stage. A comprehensive thermal oxidation mechanism is proposed, in which PET thermolysis starts with hydrogen abstraction to produce –O−•CH−, and then auto-oxidation ensues by radical intermediates attacking other PET chains. The cyclic reaction is reasonably terminated by the anti-oxidant serving as a hydrogen donor to quench radical intermediates.