Degradation, thermo-oxidative

The mechanism developed originally by Holland and Gee [1] to explain the thermal oxidation of rubbers and polyolefins has been successfully applied to the same situation in various polymers. A generalised form of the reaction sequence may be set out as follows  

Reaction (3.4) is fixation of an oxygen molecule onto an alkyl radical. It is generally a very fast reaction if the concentration of oxygen in the polymer is moderate-to-high this rapidly transforms alkyl radicals into peroxy radicals. It is essentially reaction (3.5) that will determine the rate of oxidation of the polymer, the rate being a function of the bond strength of the C-H bond broken and the stability of the alkyl 

The chain-breaking reactions consist of the monomolecular reaction (3.6a), the pseudo-monomolecular reaction (3.6b) and the bimolecular reaction (3.7). They show the decomposition of hydroperoxide groups. The pure thermal decomposition of hydroperoxide involves high activation energies, especially the monomolecular reaction (3.6a). In the case of molten PET, the temperature is more than sufficient to bring such reactions into play. Decomposition of hydroperoxide is efficiently catalysed by various metal ions which may be present in the polymer as catalyst residues, or as part of additive packages. This is especially the case with metal ions that exhibit more than one stable oxidation state. Such catalysed reactions are equivalent to reaction (3.7) but are much faster. 

The radicals formed in the initiation, propagation and chainbranching steps can not only fix oxygen and abstract hydrogen, they may also be subject to monomolecular decomposition processes. This type of reaction leads to chain scission and, as a consequence, to a decrease in the molecular weight of the polymer. 

Studies of the thermal oxidation of PET have been carried out by various authors over several years [2-20]. 

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There have been many studies on the thermal and thermo-oxidative degradation of PMMA.23 24 It is well established that the polymer formed by radical polymerization can be substantially less stable than predicted by consideration of the idealized structure and that the kinetics of polymer degradation are dependent on the conditions used for its preparation. There is still some controversy surrounding the details of thermal degradation mechanisms and, in particular, the initiation of degradation.31... 

In the presence of oxygen, thermo-oxidative degradation takes place, which is much faster than thermal degradation in an inert atmosphere. 

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

To minimize the thermo-oxidative degradation of the pellets, characterized by colour or yellowness . 

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

Phosphite processing stabilizers are used in PET to maintain the IV, suppress yellowing and overall to reduce thermo-oxidative degradation. 

Nonstoichiometric mixtures were also concocted from BCB and K-353. All were completely compatible, as evidenced by a single initial Tg, except the BCB/dicyanate blends, which showed small exothermic transitions attributable to crystallization phenomenon. The results from the thermal analyses of these blend systems, in particular the K353/BCB system, lend credence to our belief that curing via Diels-Alder cycloaddition may predominate. As such, the blend systems were more stable toward thermo-oxidative degradation than their pure bisdienophile components. 

Mattson B (1993) Thermo-oxidative degradation and stabilization of rubber materials. PhD Thesis, Royal Inst of Technology, Stockholm... 

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