Mechanisms of chemical ageing oxidation processes

Oxidation processes are especially important in hydrocarbon polymer matrices. These processes result from a radical chain reaction established 

23 Osmotic cracking in a polyester for boat hulls, as revealed by the kinetic curve of weight changes after immersion in boiling water (according to Mortaigne et al., 1992). 

Oxygen addition to alkyl radicals. This is a very fast process and thus practically structure and temperature independent. The corresponding rate constant is very high = 10 -10 l.mor. s (Kamiya and Niki, 

Peroxyl radical reaction with the polymer. This is a generally much slower process, which is structure dependent. In saturated hydrocarbon polymers, e g. polyethylene (PE) and polypropylene (PP), it is exclusively a hydrogen atom abstraction. In this case. Per is a hydroperoxide group (POOH). The corresponding rate constant is very low = 10 -10 l.mor. s at ambient temperature (see Table 12.7). In polyenic elastomers, e g. polybutadiene (PBD) and polyisoprene (PIP), step 3 can also be an addition to double bonds. In this case. Per is a peroxide bridge (POOP). The corresponding rate constant is also very low typically = 10 -10 l.mor. s at ambient temperature for an intramolecular addition (see Table 12.7). 

In the absence of antioxidants, radicals terminate according to bimolecular processes (steps 4, 5 and 6). At relatively low temperature, close to ambient temperature, the corresponding termination rate constants classify in the following order (Gillen et al., 1995)  

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