Effect of aging on the stress crack resistance of an HDPE geomembrane

The changes in the stress crack resistance, SCR (measured by the single point notched constant tensile load; ASTM D5397 - appendix) for a 1.0 mm high-density polyethylene (HDPE) geomembrane aged in synthetic leachate at six temperatures (25, 40, 55, 70, 85 and 95 °C) and in air and water at 55 °C were investigated for almost five years. There are observed changes in the SCR before the geomembrane chemically degraded (as manifest by melt index and tensile properties) and even before the antioxidants depletion at temperatures below 70 °C (as manifest by the standard and high pressure oxidative induction times). This change is attributed to morphological changes during aging that affected the inter-lamellar connections due to: (a) annealing that increased the strength of the inter-lamellar connections (over the temperature ranges examined), and (b) the proposed chain disentanglement mechanism. The annealing and chain disentanglement mechanisms have a counteracting effect on the inter-lamellar connections and hence on the SCR. Chain disentanglement dominated over the annealing effect at 25, 40, 55 and 70 °C, resulting, respectively, in a decrease in SCR to an equilibrium SCRm of 0.83, 0.65, 0.26 and 0.43 of the initial stress crack resistance of the geomembrane (SCRo). The maximum decrease in SCR is observed when the geomembrane is aged in leachate at 55 °C with the SCR decreasing to 0.26 SCRo. The SCR of the geomembrane aged in air and water at 55 °C decreased to about 0.5 SCRo, implying that the surfactant used in leachate may have enhanced the rate and the extent of chain disentanglement (i.e., decreasing the time to reach a lower SCRm) due to plasticization of the amorphous zone.