Photo-stabilisation of commercial polypropylene by piperidine compounds: The rôle of stable free radicals

The photo-stabilisation of commercial polypropylene by a hindered piperidine stabiliser, bis 2,2,6,6-tetramethyl-4-piperidinyl] sebacate, a model piperidine compound, 2,2,6,6-tetramethyl-4-piperidinol, and its corresponding N-oxy derivative is examined using luminescence and ESR spectroscopy. All three compounds inhibit the photolysis of the luminescent α,β-unsaturated carbonyl impurity groups present in the polymer. ESR spectroscopy shows that the hindered piperidine stabiliser operates by the same mechanism as that of the stable hindered piperidino-N-oxy radical compound. Possible mechanisms whereby the piperidine compounds inhibit the photolysis of the α,β-unsaturated carbonyl groups are also discussed.