Degradation of polyethylene pipes distributing chlorinated water : depletion of stabilizers, release of degraded products and polymer degradation

Abstract: This thesis presents the study of antioxidants depletions in chlorinated media (10 ppm Cl2 - and ClO2 - aqueous media), polyethylene pipes degradations scenarios and migration of the degraded species to aqueous phase. Pressure testing on pipes and squalane testing with controlled pH aqueous media (6.8 ± 0.2) containing either Cl2 or ClO2 were used for assessing the degradation products. Though squalane test used a liquid hydrocarbon instead of real plastic, it provided reliable data with an efficient and readily way of sampling. Medium density polyethylene pipes stabilized with hindered phenol and phosphite antioxidants were pressure tested with water containing 4 ppm chlorine dioxide (ClO2) at 90 °C as internal medium. The stabilizers were rapidly consumed towards the inner pipe wall; the rate of consumption was four times greater than in chlorinated (Cl2) aqueous (4 ppm, pH = 6.8) at the same temperature. The depletion of stabilizers occurred far into the pipe wall. The subsequent polymer degradation was an immediate surface reaction. It was confirmed by differential scanning calorimetry, infrared spectroscopy and size exclusion chromatography that in the surface layer which came into contact with the oxidizing medium, the amorphous component of the polymer was heavily oxidized leaving a highly crystalline powder with many carboxylic acid chain ends in extended and once-folded chains. The depletion rate of the antioxidant exposed to ClO2 - aqueous media was also four times faster than that exposed to Cl2 - aqueous media by squalane test. The aqueous media was extracted to condense the remaining antioxidants and the degraded species from the squalane solution. The absorption spectra from infrared spectroscopy illustrated that carbonyl groups exist in degraded species from both ClO2 - and Cl2 - aqueous, and chlorine-carbon bond presented only in ClO2 - aqueous. It was proved by liquid chromatography that the peaks of oxidizing species formed in ClO2 aqueous media were more intense and they were different from the ones degraded compounds in Cl2.