Black-pigmented polypropylene materials for solar thermal absorbers - Effect of carbon black concentration on morphology and performance properties

Black-pigmented polyolefin compounds are widely used for unglazed collectors and recently also for glazed flat-plate collectors with overheating protection. In this paper the effects of pigment concentration (carbon black (CB) - masterbatch) on morphology and performance properties of a polypropylene grade for elevated temperature applications (PP-RCT) are investigated. Compounds with varying black pigment concentrations (from 0.1 to 1.0 wt.%) were prepared and moulded to 2 mm thick sheets. Morphological characteristics, optical and mechanical properties, as well as long-term properties were characterized on specimen level by optical microscopy, Differential Scanning Calorimetry (DSC), UV/Vis/NIR- and IR-spectrometry and tensile testing. The investigated black compounds exhibited integral solar absorbance values of about 95% and integral infrared emittance values from 93% to 95%. Hence, the effect of black pigment concentration on the solar- and infrared-optical properties was small. Regarding the mechanical properties, the elastic modulus and the ultimate properties were slightly affected by the black pigment content. A more pronounced effect was deduced after long-term exposure in hot air at 135 °C. With increasing carbon black concentration a significant reduction of the embrittlement time from approximately 4000 h to 3200 h was obtained.