Antioxidant consumption in squalane and polyethylene exposed to chlorinated aqueous media

Squalane stabilized with 0.2 wt.% of Irganox 1010 and a medium-density polyethylene containing 0.1 wt.% of the same antioxidant were exposed to two different aqueous media (water solutions containing either 10 ppm Cl2 or 10 ppm ClO2, both buffered to pH = 6.8) at different temperatures between 30 and 70 °C. The squalane phase was characterized by differential scanning calorimetry (oxidation induction time, OIT) and infrared spectroscopy, and the aqueous media were analysed after concentrating the analytes using liquid–liquid extraction by liquid chromatography, mass spectrometry and infrared spectroscopy. OIT measurements were carried out on the polyethylene samples after exposure to the chlorinated aqueous media. Exposure of the squalane systems to water containing ClO2 resulted in discolouration by the formation of quinoid structures and a faster depletion of the antioxidant than exposure to water containing Cl2. The activation energy for the loss of antioxidant activity on exposure to ClO2-water was very low (<10 kJ mol−1) in the squalane test (no diffusion control) and 21 ± 2 kJ mol−1 at a depth of 1–2 mm from the surface of polyethylene plaques (diffusion control). Calculation from earlier published OIT data from a HDPE exposed to Cl2-water yielded an activation energy for the loss antioxidant activity of 68 kJ mol−1. The antioxidant degradation products obtained from the exposure to the ClO2 aqueous medium were found at a higher concentration, were more polar and exhibited a higher proportion of low molar mass species than those obtained after exposure to the Cl2 aqueous medium. The important chemical difference between ClO2 and Cl2 is that the former is a one-electron oxidant whereas the latter preferentially reacts by hydrogen substitution. Possible further reactions, in agreement with the observations made, are proposed.