In the last two decades a lot of efforts have been made to improve solar-thermal systems in terms of costs, weight, and overall performance by providing better alternatives than conventional materials. In order to replace conventional materials by polymeric materials, the envisaged materials must withstand the environmental and the mechanical loading conditions of this application and maintain a service life of 20 years. Solar-thermal systems are constantly exposed to different media (e.g., air and heat carrier fluid) and operate at elevated temperatures. Therefore, it is of utmost importance to investigate the different aging mechanisms of polymeric materials under service relevant conditions. Moreover, to provide drinking water that fulfills the safety standards, chlorine is used as a water disinfectant in many regions of the world. Hence, polymeric materials must have a sufficient chlorine resistance for the application in solar-thermal collectors. ^The objective of this thesis is to investigate the effects of chlorinated water with varying chlorine contents at an elevated temperature on the properties of polymeric materials used for solar-thermal systems.
For this particular purpose, three grades of polypropylene and three grades of glass fiber reinforced polyamide were characterized. Polypropylene included a commercial black-pigmented block copolymer pipe grade (PP_C) and two homopolymer-based PP grades stabilized with two different stabilizers packages. For the polyamide, a commercial aliphatic PA grade with glass fiber content of 30% (PAGF30_C) was used as benchmark and as base material for the other two material grades. Both material grades were stabilized with antioxidant packages. ^In order to characterize the fatigue crack growth (FCG) resistance in chlorinated water with varying chlorine contents at 80 C, both materials were characterized under superimposed mechanical-environmental loading using a specific test setup. For the PP grades cracked round bar (CRB) specimens and for the PAGF30 compact type (CT) specimens were used. While for the PP the tests were performed at 1, 2.5, 5 and 10 ppm, for PAGF30 they were performed at 1, 5, and 10 ppm. On the one hand, both materials showed that the crack initiation is affected by the environment. However, only the PAGF30 grades showed a clear correlation between the number of cycles to failure and the chlorine content. On the other hand, the FCG kinetics curves of the PP grades, unexpectedly, showed a superior FCG resistance at higher chlorine contents. In contrast, the PAGF30 grades revealed a different behavior, as the FCG resistance of PAGF30_C and the PAGF30_P was reduced at higher chlorine contents. ^While the FCG rate of PAGF30_C was enhanced by a factor of 4.8 at 10 ppm compared to 1 ppm, it was enhanced only by a factor of 1.8 for PAGF30_P. This can be attributed to the addition of the phenolic stabilizer package to the material. The FCG resistance of PAGF30_A remained stable at various chlorine contents and only a slight decrease in the slope was observed at higher chlorine contents.