Dominating reactions in the degradation of HDPE during long term ageing in water

A Phillips type high density polyethylene was extruded six times without additives and the compression molded plates prepared from the granules were stored in distilled water for 12 months. Specimens withdrawn from the containers at regular intervals were thoroughly characterized with various methods including the determination of weight changes, color, MFI, functional group content (FTIR), molecular weight (GPC), thermal (DSC) and mechanical (tensile) properties. The results proved that all reactions taking place during the storage of HDPE in distilled water are related to each other; the correlation of all functional groups formed or consumed in them is surprisingly close. The amount of oxygen present determines the direction of reactions, larger oxygen content leads to chain scission, to an increase of methyl content and to the formation of carbonyl groups. Most of these reactions go through double bonds, their number decreases during storage. In spite of the large number of reactions proposed in the literature, only one or two dominating reactions determine the changes in the chain structure of the polymer and thus the properties of the final product under the conditions of this study. Any variation in the conditions of storage is reflected in the properties of the polymer. Stabilizers used under extractive conditions must be stable against hydrolysis and should trap oxygen centered radicals.