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

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.