Environmental Aging of Polymers to Evaluate Their Potential for Remediating Natural Gas Pipelines

We assess the aging of polymeric materials in natural gas environments for potential applications in pipeline remediation as liner materials. Three commercially available polymers─high-density polyethylene (HDPE), Nylon-6/6 polyamide (PA), and polyvinylidene fluoride (PVDF)─are aged under accelerated conditions in a model natural gas environment of pure alkanes at 250 PSI and 90 °C for up to 30 days. The Young’s modulus and yield stress of these polymers remain unchanged within experimental error after exposure to the natural gas environment. Dynamic mechanical analysis, however, reveals drastic changes to polymer chain dynamics, with the activation energy for segmental relaxations reduced by up to 50%, corresponding to accelerated molecular motion. The environmental aging is confirmed through FTIR, which found an increase in the density of natural gas molecules within the polymer matrices. Additionally, these changes in the dynamics within polymeric solids are reversible; prolonged removal from the gas atmosphere resulted in the activation energies returning to near-initial values within 2 weeks. These observations suggest that this aging response is dominated by physical processes in which the polymers absorb natural gas molecules due to the increase in partial pressures, as opposed to a chemical mechanism in which the natural gas reacts irreversibly with the polymer chains. In applications for pipeline remediation, our results indicate that polymeric liners will provide sufficient mechanical rigidity but may suffer from accelerated rates of creep facilitated by the increase in local polymer dynamics.