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Polymer Mechanics Laboratory

Research > Lifetime prediction

Antioxidant loss in polyethylene exposed to chlorinated water

Hot chlorinated water environments can affect the mechanical performance of polymers. Through degradation by oxidation, these environments can decrease the molecular weight of the polymer and lead to changes in strength and flexibility. To prevent degradation, antioxidants are combined with the polymer resin during manufacturing. Phenolic antioxidants, which react with radicals generated during oxidation, are effective at preventing oxidation but have a tendency to migrate out of the polymer as well as be chemically consumed when the polymer is exposed to chlorinated water. When the antioxidant is depleted, the polymer is vulnerable to oxidation. The goal of this study is to characterize both antioxidant loss and mechanical degradation in polyethylene samples exposed to hot water containing chlorine concentrations found in potable water. Unstressed samples were exposed to both RO and 5 ppm chlorinated water baths at 80C and a pH of 6.8. The samples initially contained 0, 500, 1000 and 2000 ppm of a single phenolic antioxidant and were exposed for up to 8 weeks. Oxidation Induction Time (OIT) results showed that antioxidant loss was much faster in samples exposed to chlorinated water. Time scales for chlorine diffusion and reaction indicate that antioxidant depletion was largely dependant on chlorine migration. In samples exposed to chlorinated water, strain at break and molecular weight results showed that the samples became brittle once the antioxidant was depleted. As a result, the time for antioxidant depletion may be used as a conservative estimate for the lifetime of the polymers in these environments.

Fig. 1: Process and instrumentation diagram for chlorinated water bath