Jean Marchal

Gamma-radiation-induced oxidation—a very convenient tool to unravel the mechanisms of polyolefin aging and stabilization

Abstract The production of ketones during the oxidative degradation, aging and weathering of polyethylenes is explained by two primary reactions: the well-known termination reaction and a competitive chain reaction which must be introduced in the usual reaction scheme. Experimental conditions are found in which antioxidant (AO) properties of N -oxyl or nitroxyl (>NO . ) radicals are observed and can be studied without having to assume that they depend on the unreasonable condition that > NO . , even produced at very low concentration during the natural aging and weathering of polymer materials, would be able to compete with O 2 to scavenge alkyl and other organic free radicals. Therefore, the classical scheme of the AO efficiency of >NO . generated by the oxidation of so-called hindered amine light stabilizers (HALS) is challenged. Also, the usual scheme of the oxidative degradation of 2,4-dimethylpentane is shown to be incorrect. The intramolecular propagation reaction favoured does not involve ter -alkylperoxyl but rather ter -alkoxyl free radicals, while the reverse is found for intermolecular propagation by these species. It will be necessary to take into account these new conclusions in understanding the oxidative degradation of the whole series of polypropylenes.

Antioxidant activity of hindered aliphatic amines and derivatives; polyolefin stabilization at 25°C—II. Theoretical treatment and application to oxidation of 2,4-dimethylpentane by radiation and thermolysis

Abstract It was shown in Part I of this series (Crouzet and Marchal, 1992) that the overall effect of nitroxyl radical > NO derivatives of hindered aliphatic amines (HA) on the kinetics of the γ-radiation-induced autoxidation of 2,4-dimethylpentane (DMP) samples in open glass test tubes at 25°C with I = 72 rad min-1 is similar to the effects, at the same concentration (ca 3 × 10-2 M) of BHT (Ionol) which is a typical chain-breaking phenolic antioxidant even though the mechanisms of action of the two kinds of inhibitors are quite different. This result must originate from the competition of the trapping of alkyl radicals and H atoms by O2 and > NO . To get a quantitative answer to this problem, the modelling of the kinetics of the decrease of the oxygen concentration [O2] in liquid samples undergoing γ-radiation-induced autoxidation is established and applied to our DMP samples. The conclusion of this modelling is that [O2] reached a constant value very close to the initial equilibrium value [O2]0. Thus the study of the competition itself is possible. The modelling is also done for liquid samples undergoing chemically-induced autoxidation by AIBN thermolysis at 60°C and applied to DMP. It appears that analysis of chemically-induced autoxidation as is usually done is not appropriate for such a study though interesting because H atoms are not produced.