For this part of the research work, four different commercially available so-called “commodity” polymers – two random copolymer polypropylenes (PP-1 and PP-2) and two silane crosslinked polyethylenes (PE-X1 and PE-X2) – were investigated as to their aging behavior for application as solar thermal absorber materials. Based on experience with northern climates, stagnation at 140 °C in air up to 500 h and operation at 80 °C in water up to 16000h were considered as aging conditions. To study the aging behavior, DSC, SEC, and tensile tests in the small – and large-strain regime were part of the experimental program.
The most significant degradation was obtained for PE-X1, as indicated by a substantial decrease in strain-to-break within the first measurement interval in hot air (125 h) and after 6000 h in hot water (Figure 13.1.4). While both PE-X types exhibited a decrease in crystallinity after exposure to air at 140 °C, probably caused by crosslinking, only PE-X2 revealed a decrease in crystallinity together with a strong
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decrease in the oxidation temperature (Tox) (probably due to leaching of stabilizers) during hot water immersion, also indicating crosslinking. The phenomena observed with DSC due to changes in the morphology and the network were not reflected by mechanical properties such as modulus and yield stress, presumably due to competing effects ofcrosslinking and reduction in crystallinity. The two PP types, on the other hand, both exhibited an increase in crystallinity after aging in hot air and hot water. For PP-1 a decrease in Mw was also found, enhancing the tendency for post – and recrystallization. Moreover, for both PPtypes an increase in melting temperature due to recrystallization was determined after hot air exposure. On the other hand, the broadening of the melting peak range after hot water exposure was related to postcrystallization. Nevertheless, mechanical properties in the small-strain and large – strain regime turned out to be nearly independent of the aging conditions, implying that the aging mechanisms found by analytical methods may perhaps cancel each other out in terms of their effect on the mechanical performance parameters.
From the commodity type plastics, PE-X2 and PP-2 are the most promising candidates for solar black absorber materials. Owing to the high stagnation temperatures for northern climates being in the range of the melting temperature for these plastics, suitable overheating protection devices are needed, however, to avoid dimensional instability.
13.1.3.4
