Antioxidant depletion from five geomembranes of same resin but of different thicknesses immersed in leachate

Abstract

The effect of thickness on the antioxidants depletion time for five high density polyethylene geomembranes (with nominal thicknesses of 0.5, 1.0, 1.5, 2.0, and 2.4 mm) immersed in synthetic leachate at six different temperatures (25, 40, 55, 70, 85 and 95 °C) is investigated. The geomembranes were manufactured from the same geomembrane resin (i.e., polymer resin, antioxidant/stabilizer package, and carbon-black batch). Four (1.0, 1.5, 2.0, and 2.4 mm) of the five geomembranes were manufactured during the same production run by changing the pulling speed of the geomembrane from the extrusion die. The depletion of antioxidants/stabilizers inferred from both standard (Std-) and high pressure (HP-) oxidative induction times (OIT)s show that increasing geomembrane thickness resulted in longer depletion times for temperatures above 40 °C but little to negligible difference in projected depletion times below 40 °C for this antioxidant package. The increase in depletion times was not proportional to the square of the geomembrane thickness as theoretically predicted if depletion was diffusion controlled and the diffusion coefficient was the same for each GMB. Thus, the depletion of antioxidants is not fully governed by diffusion and/or the GMB's diffusion coefficients are different. Regardless of the GMB thickness, the time for Std-OIT depletion was shorter than the time to residual HP-OIT above 70 °C but was longer at or below 55 °C. For example, for 2.4 mm GMB, the inferred time for Std-OIT depletion was 0.2–0.3 (at 85 °C), 2–3.5 (at 55 °C) and 9.5 (at 40 °C) times that for the HP-OIT to deplete to residual. The projected Std-OIT depletion times for 1.0 and 2.4 mm GMBs were: 2.7 and 4 years, respectively, at 60 °C; 23 and 26 years, respectively, at 40 °C; and about 330 years at 20 °C.