Mechanical performance of polymer tubes intended for use in solar heat exchangers

Polymer materials considered for use in domestic hot water heating systems must maintain mechanical properties in the working fluid over their target lifetimes. In potable water, chlorine and pH combine to create an oxidative environment, commonly characterized by the oxidative reduction potential (ORP), that can chemically attack a polymer, resulting in permanent loss of mechanical strength and stiffness. Water absorption and hydrolysis can also impact polymer properties. In the present study, experiments were conducted to evaluate the mechanical performance of polysulfone, polybutylene, and nylon 6,6 immersed for up to 1,100 hours in water at ORP levels of 550 and 825 mV at 60 and 80°C. Mechanical performance was evaluated by measuring creep compliance and the change in tensile strength and molecular weight after exposure. Surface morphology of the exposed materials was examined using scanning electron microscopy. Nylon 6,6 showed significant degradation in strength and creep compliance in all environments. Despite some variability in measured properties, the blend of polybutylene, which has additives to prolong life, did not degrade. Polysulfone performed the best of the three materials with no discernable change in properties over the duration of the experiments.