Method for degrading polyethylene sheet samples in an oxidative environment

Stress cracking is a failure mode that must be considered when evaluating the lifetime of polymer components. In modeling this failure mode, mechanical properties as a function of the extent of degradation must be evaluated. This paper describes an apparatus and method for degrading thin polymer sheets in an oxidative environment. A model system of PE in hot chlorinated water with an oxidation reduction potential of approximately 825 mV is considered. The oxidative environment is achieved by a water bath with temperature, pH and free chlorine controlled to 60-65°C, 5.2 +/- 0.5 ppm free chlorine, and 6.3 +0.5/-0.0 pH. Under these conditions, the molecular weight of 50-60 micron thick PE sheets was reduced by 60% and the strain at break for the sheets was reduced by 90% after 1000 hours exposure.