Effect of carbon nanotubes on the global aging behavior of β-nucleated polypropylene random copolymers for absorbers of solar-thermal collectors

This paper deals with the effect of carbon nanotubes (CNT) on the basic characteristics and the global aging behaviour of β-nucleated polypropylene random copolymer absorber material for overheating protected solar thermal collectors. Hot air aging experiments at an elevated temperature of 135 °C were carried out using micro-sized specimens with a thickness of 100 µm. The unaged specimens were characterized as to the antioxidant content and the mechanical and thermal properties. The CNT pigment affected the semi-crystalline morphology (less β-content) and increased the stiffness significantly. The aging indicators strain-at-break, average molar mass, primary antioxidant content and oxidation temperature were monitored for up to 4000 h. A more pronounced degradation was deduced for the compounds with CNT. Compared to carbon black modified grades a higher deterioration rate was ascertained for the CNT compounds. The amount of CNT should be kept at low levels (<0.1 wt%) just needed to achieve sufficient solar absorbance (>90%) of the absorber sheet.