Prediction of polymer tube life for solar hot water systems: A model of antioxidant loss

The successful use of polymer components in solar hot water systems requires an understanding and method of predicting material degradation in water. The relevant degradation mechanism is oxidation. Degradation from oxidation can be delayed through the use of antioxidant additives. However, once the antioxidants have been depleted, oxidation of the polymer and subsequent loss of mechanical integrity occur rapidly. In this study, antioxidant loss from polymer tubes is modeled. Dimensional analysis and results of the model show the rate of antioxidant loss is controlled by diffusion through the polymer. The diffusion time scale is dictated by the tube wall thickness. Antioxidant concentration profiles and depletion rates are presented for three representative tube geometries and temperatures of 293 and 333 K. The time to deplete 90% of the antioxidant is on the order of 1000–10,000 h.