Autoxidation oxygen levels

Several more complex species may also result from addition of H- to the polymer, especially on the aromatic rings. The presence of oxygen will lead to an autoxidation cycle to be established. The relative importance of chain scission versus crosslinking in PET will be very dependent on conditions, e.g., humidity, oxygen levels, crystallinity, dosage, dosage rate and temperature. 

It has been proposed that 2 can be generated in chemical reactions (so-called dark biochemistry) by unstable oxygen adducts, endoperoxides, metal complexes (101, 102), and peroxyl radical recombinations (103-105), and that the low levels of internal hydroperoxides produced thereby initiate lipid autoxidation chains (102, 106, 107, respectively). 

The radical chain mechanism outlined here avoids the ineffective direct reaction of molecular oxygen with the substrate hydrocarbon. The fast propagation reactions produce ROOH that in turn can initiate new radical chains. As the primary product of the reaction initiates new reactions, one ends up with an autocatalytic acceleration. The propagating peroxyl radicals can also mutually terminate and yield one molecule of alcohol and ketone in a one-to-one stoichiometry. The ratio between the rate of propagation and the rate of termination is referred to as the chain length and is of the order of 50-1000. As the desired chain products are more susceptible to oxidation, autoxidations are normally carried out at low conversions in order to keep the selectivity to an economically acceptable level. 

Exposure of polyolefins to UV radiation in the presence of air leads to autoxidative reactions with the incorporation of oxygenated groups into the polymer. The increase in UV absorbance (measured at 310 mn) of irradiated polypropylene is reported to be too low to measure for wavelengths of irradiation in the range of 260-600 nm. Even at 260 nm, only a minimal increase was observed [121], For polypropylene containing flame retardants, either DBDE (decabromodiphenyl oxide) or TBA (tetrabromobisphenol A) at a 2% level, the maximum efficiency of chain scission was observed at X. = 280 and 260 nm, respectively. 

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