Polyolefin photo-stabilisation mechanisms. Reactions of tetramethylpiperidine derivatives in model systems

The relative rate of cyclohexyl radical scavenging by oxygen or 4-oxo-2,2,6,6-tetramethylpiperidino-N-oxyl has been measured at 25°C, together with the relative rates of cyclohexyl peroxy radical attack on cyclohexane or substituted hydroxylamines derived from 2,2,6,6-tetramethylpiperidine. These competitive processes are important in the stabilisation of polymers against photo-oxidative destruction by piperidine derivatives. Peroxy radical scavenging by the substituted hydroxylamine appears to be considerably more important than alkyl radical scavenging by the nitroxide, although both processes are essential for prolonged ultra violet (uv) stabilisation of a polymer. However, a comparison of experimental photo-protection with that predicted by the measured rate constant ratios shows that other processes are needed to account entirely for the observed stabilisation. Other factors which may be involved in piperidine photo-protection of polymers include thermal decomposition of the substituted hydroxylamine to reform nitroxide in polar(oxidised) zones and especially the association of nitroxides with hydroperoxide groups (the dominant photo-initiator in degrading polyolefins).