Data are reported from oven aging in air for more than 13 years (!) and from water immersion tests for 6 years at 80 °C on various high-density polyethylene geomembranes (GM) used in landfill lining. The mechanical properties and oxidative induction times (OIT) of the samples were monitored during the long-term testing. Aging behavior in hot air is different from that in hot water. During oven aging a slow, exponential decrease of OIT is observed. Even after 13.6 years there is no indication of an oxidative degradation of the mechanical properties. During immersion in water a strong reduction in OIT occurs within the first year, after which time the curve levels off. Oxidation starts when very low OIT-values are reached after about 5 years at which time the mechanical strength rapidly falls to values below the yield point. We conclude from these data that the service life of HDPE GM's is essentially determined by the slow loss of stabilizers due to migration. The oxidation starts only after the depletion of antioxidants and then quickly leads to brittleness of the sample. However, no complete oxidative deterioration has been observed to date. We estimate by the van't Hoff rule that under normal ambient conditions many centuries will have to pass before the functional mechanical properties of state-of-the-art stabilized HDPE bulk material will be reduced below acceptable limits by oxidative degradation.
Plastic products used in geotechnical engineering, so-called geosynthetics, are often made of polyolefins (polyethylene or polypropylene) and stabilized against oxidative degradation by antioxidants (AO). Three types of AO “packages” are used: (P1) phenols and phosphites/sulfides, (P2) hindered amines (HAS) with a marginal stabilization P1 of the basic resin, (P3) a combination of packages P1 and P2 with comparable amounts of phenol and HAS. We report about long-lasting oven aging and water immersion tests at 80 °C of 29 different polyolefin based products (geomembranes, geonets and geotextiles). Pronounced differences in the degradation behavior were found depending solely on the type of AO package. In case of P1, the oxidative degradation is prevented by the AO. Autocatalytic oxidation only begins after depletion of AO and may lead to catastrophic failure depending on the environmental conditions. In case of P2, degradation starts right from the beginning. However, it is decelerated and proceeds gradually. In case of P3, the degradation behavior seems to be a superposition of the characteristics obtained for P1 and P2. Plotting the fractional loss of the AO in P1 and P2 as function of the square root of aging time, we identified two regimes of AO depletion. We suggest to attribute this observation to the different mechanisms of AO depletion.