The influence of carbon black on properties of orientated polypropylene 2. Thermal and photodegradation

This paper extends previously reported work and describes the effect of carbon black on thermal and photo-oxidative degradation of oriented polypropylene (PP) geotextile tapes. PP tapes containing carbon black with different particle size, structure (or aggregate shape), specific surface volatile content and concentrations have been exposed separately to circulatory air ovens at 130 degrees C and UVB lamps at 60 degrees C respectively. The exposed tapes were studied using tensile tensiometric, IR spectrophotometric and thermal analytical (DSC) techniques. Thermal stabilities of PP tapes filled with carbon blacks having low volatile contents are comparable to those of unfilled PP tapes. Ar given oven-exposure times, carbon blacks with varying particle sizes and structures, except for volatile content, show only slight influence on thermal stability of the PP tapes. The thermal stability was not significantly affected in tapes containing increased carbon black concentrations from 2.5 to 5% w/w The carbon black with the highest volatile content adversely affected thermal stability possibly because the surface oxygenated groups present act as sensitisers. Dynamic thermal analysis (DSC) shows that the magnitude of shifts to lower temperatures of the post-fusion oxidative exotherm (T-on) reflect respective losses in tensile properties and supports previous evidence that T-on is a sensitive indicator of the degree of polymer oxidation. However, the presence of carbon black significantly increases UV durability of these filled tapes. The UV resistance in terms of tensile properties is inversely proportional to carbon black particle diameter. The carbon black sample with a high concentration of reactive oxygenated groups, measured as volatile content, shows relative inferior UV resistance to tapes containing low volatile content blacks with similar particle size. No effect of carbon black structure on UV stability was observed, which agreed with the previous results for carbon black-containing polyethylene films under UVA and UVB exposures. The overall evidence suggests that carbon blacks having smallest particle size, possibly high structure and low volatile contents will yield PP tapes having optimum thermal and UV stabilization behaviours. (C) 1999 Elsevier Science Ltd. All rights reserved.