The degradation behavior of four commercially available engineering plastics – two amorphous [poly(phenylene ether)-polystyrene blend (PPE + PS) and polycarbonate (PC)] and two semicrystalline [polyamide 12 (PA12-1 and PA12-2)] plastic grades – for solar thermal applications was investigated. According to northern climate conditions and based on the experiences with the Solarnor® collector , maximum stagnation temperatures in air of 140 °C for up to 500 h and maximum operation temperatures in water of 80 °C for up to 16000 h were considered as aging conditions. To characterize the aging behavior, analytical tests, including DSC and SEC, were performed and a tensile test was carried out. For interpretation of physical aging effects, DSC measurements and modulus and yield stress values delivered the most sensitive parameters. While for the amorphous plastics physical aging was characterized as enthalpy relaxation around the glass transition, for the semicrystalline plastics mainly changes in crystallinity were obtained. To interpret chemical degradation, changes in weight average molecular mass and in strain-to-break values turned out to be most appropriate.
Among the investigated materials, most significant degradation was obtained already for PPE + PS after exposure to hot air within the first 125 h and after 2000 h in hot water (Figure 13.1.3). While hot water aging led to physical and chemical aging for PC determined with mechanical properties, DSC and particularly Mw measurements
Figure 13.1.3 Relative strain-to-break versus aging time in air at 140°C and in water at 80°C for PPE + PS and for PC.
did not show any indication for chemical aging after hot air exposure. Owing to embrittlement and the strong scatter found with strain-to-break after aging in hot air, further investigations need to be carried out. In contrast to the results for PPE + PS and PC, hot water treatment was more severe to PA12 than hot air. Interestingly, significant degradation was obtained only for PA12-HT after hot water exposure. However, the significant drop in oxidation temperature (Tox) and physically detected aging by means of DSC of PA12-HI after hot water aging was presumably outweighed by the various physical (post – and recrystallization) and chemical aging effects (crosslinking, molecular degradation). Hence, the required performance as a black absorber material for northern climates seems to be met.