Deterioration of Polyethylene Exposed to Chlorinated Species in Aqueous Phases : Test Methods, Antioxidants Consumption and Polymer Degradation

Sammanfattning: This thesis presents a study of antioxidant depletion in water containing chlorinated species (water containing 10 ppm either Cl2 or ClO2, buffered to pH = 6.8), the degradation products in the aqueous phase, and polyethylene pipe degradation scenarios. A low molecular weight hydrocarbon analogue (squalane) was used instead of solid polyethylene as the host material for the antioxidants, and the depletion of antioxidants has been studied. The phenolic antioxidant Irganox 1010 was consumed ca. 4 times faster in water containing 10 ppm ClO2 than in water containing 10 ppm Cl2. The different degradation products in extracts from the aqueous phase identified by infrared, liquid chromatography and mass spectrometry revealed the different degradation mechanisms between ClO2 (" cleavage) and Cl2 (hydrogen substitution). The squalane test shows no energy barrier between 30 and 70 °C, and the activation energy of the antioxidant in solid PE was found to be ca. 21 kJ mol-1. A linear relationship has been established between the time to reach antioxidant depletion in the polyethylene tape samples and the time to reach depletion in samples based on squalane containing the same antioxidants. The surface oxidation and surface embrittlement of PE tape on long time exposure have been studied by IR and SEM. Pressure testing on medium density PE pipes with a controlled pH aqueous media (6.8 ± 0.2) containing 4 ppm either ClO2 or at 90 °C showed that the stabilizers were rapidly consumed towards the inner pipe wall and the rate of consumption in ClO2 was 4 times greater than in Cl2 solution. 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.