Oxidation of polyamides differs from polyolefins one in several aspects:- The presence of a nitrogen attractive heteroatom on the main chain changes the chemistry of oxidation, as for example the rate at which radicals combine and hence the ratio chain scission over crosslink,- The nature of end groups implies post condensation reactions that counterbalance chain scission effects.For the different phenomena can overlap with opposite influences, it is compulsory to have at our disposal a reliable model predicting degradation kinetics in the use conditions different from accelerated ageing and a kinetic model gives better insights in the experimental data. Literature on amide model compounds was first reviewed in order to extract a suitable set of rate constants to incorporate in a classical model describing auto-oxidation of polymers. When compared to polyolefin rate constants, it was shown that polyamide oxidation is characterized by:- a lower chain scission over carbonyl ratio,- a weaker stability of the hydroperoxydes formed,- a higher propagation rate constant.These data were used as constraints in order to limit the number of adjustable parameters in the model. The rate constants remaining unknown were estimated from hydroperoxide kinetics and carbonyl build up curves. Thus, a scheme was elaborated for pure polyamide :- able to simulate the changes in molar mass,- easily adapted to describe the even most complex case of phenols stabilized polyamides.The heuristic properties of this model were compared with other data (hydroperoxides titrations, carbonyl build up and chains scissions) of ageing of pure PA11 thermally oxidized at several temperatures under various oxygen pressures.

Kinetics of the oxidation of polyamide 11

248th National Meeting (2014)

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