Organochalcogen stabilizers efficiently protect model polyolefins exposed to chlorinated media

Abstract

The small amounts of chlorine dioxide that are routinely supplemented to drinking water as a disinfectant also cause a degradation of the polyolefin pipes that are used for distribution of the water. Commonly used phenolic antioxidants can extend the service life of the polymer but the expected lifetime is still much shorter than desired (50 years) due to depletion of the antioxidant in the surface zone exposed to the aqueous solution. In search for better stabilizers for the pipes, we have tested an organotellurium compound, 4-(N,N-dimethylamino)phenyl 3-phenoxypropyl telluride (1), as well as its corresponding selenium and sulphur analogues and a series of organotellurium compounds where the electron density at the heteroatom was varied. Stabilizers were dissolved in squalane, which is a liquid hydrocarbon that could serve as a model for a polyolefin. The oxidation induction time (OIT), determined after exposure of the squalane solution to an aqueous solution of 10 ppm of chlorine dioxide for various times was determined by DSC to indicate the loss of antioxidant protection. Whereas Irganox 1010 was only effective as a stabilizer for a few hours, many of the organochalcogen compounds were considerably more resistant (>91 h for compound 1) towards chlorine dioxide.Thermogravimetric analyses of antioxidants indicated insignificant decomposition below 200 °C and increasing stability for the lighter chalcogen compounds (telluride < selenide < sulfide). Among organotelluriums, stability increases with increasing electron density at the heteroatom. Oxidation potentials of stabilizers as determined by cyclic voltammetry correlated fairly well with their protective effect in squalane (OIT-values). We therefore hypothesize that these compounds act primarily as electron donors towards peroxyl radicals. As determined by 125Te NMR-spectroscopy, organotellurium compound 1 in the presence of an excess of chlorine dioxide failed to produce an oxidation product. This may be the clue to its long-lasting protective effect in the squalane-assay.