Thermo-oxidative Degradation of PET

Thermo-oxidation occurs at elevated temperature in the presence of oxygen. The thermo-oxidative degradation of PET is a chemical process leading to formation of initiators (hydroperoxides) 

The labile nature of the hydrogens a to the ester groups towards abstraction by oxygen provides the initiation reaction. In the presence of oxygen, the alkyl radical formed rapidly react with oxygen to form a peroxy radical. Then, the peroxy radical abstracts hydrogen atom for polymer chain to form a hydroperoxide (18). 

The next step is thermal scission of the hydroperoxide to form an alkoxy macroradical (and HO° species) that may abstract hydrogen atom to form a hydroxyl group. This species can undergo chain scission to form an acid and aldehyde chains ends. 

Thermo-oxidative degradation of PET is a complex phenomenon that gives formation of many species. Cross-linking and gel formation could occur. 

In industrial applications, PET materials are commonly treated in air at elevated temperature and thermo-oxidative degradation occurs. 

---

Table 2.7 Kinetic data obtained for the thermal and thermo-oxidative degradation of PET... 

Both this study and later work by Botelho and co-workers [13] showed that, unlike polyolefins, the thermo-oxidative degradation of PET involves non-oxidative thermal degradation processes, especially in the early stages. The overall thermo-oxidative process in PET is therefore extremely complex. Recent studies [19,20] using state-of-the-art mass spectroscopic techniques have provided further evidence for the role of hydroxylated terephthalate fragments in PET discoloration. 

From a practical standpoint, the other problem with oxidative degradation of PET besides evolution of tainting species such as acetaldehyde is polymer discoloration. As already noted, Buxbaum [3] suggested that discoloration could be due to unsaturated polymeric species such as vinyls and aldol derivatives, at least in the case of thermal degradation. An extensive study of the yellowing of PET under thermal and thermo-oxidative conditions was carred out by Edge and co-workers [11]. They concluded that coloured species were due to hydroxylation of the terephthalate ring, followed... 

Few studies have been carried out specifically on the thermo-oxidative degradation of PBT [12, 13, 21], and it is generally considered that the oxidation pathway is very similar to that described in the section on PET. 

PTT shares several similar thermo-oxidation degradation mechanisms with PET [32], Some of the more important ones are as follows ... 

A particularly relevant thermo-oxidative study on PET degradation and PBT reported the degradation products observed for ethylene dibenzoate [39], The products observed paralleled those of the photolysis and photo-oxidation reports discussed above with benzoic acid, vinyl benzoate, 2-hydroxyethylene dibenzoate, 2-carboxymethoxy benzoate and the coupling product, 1,4-butylene dibenzoate, being reported. The 2-hydroxyethylene dibenzoate and 2-carboxymethoxy... 

---