Effect of Carbon Black on UV stability of LLDPE films under artificial weathering conditions

Abstract

Carbon Black is recognised for its ability to stabilise polyolefins against UV degradation. Linear low density polyethylene (LLDPE) 75 μm extruded films containing a variety of carbon black with different particle size, structure and concentrations were exposed to two accelerated artificial weathering devices, a xenon arc source, e.g. Xenotest 150S, and fluorescent tube sources with UVA and UVB lamps, under controlled temperature and humidity. The changes in physicochemical properties during exposures were studied using tensile testing, Fourier transform infrared (FTIR) spectroscopic and differential scanning calorimetric (DSC) methods. Presence of each carbon black shows significant improvement in UV stabilisation compared to clear films, especially for those with small particle sizes as expected. There is no consistent effect of carbon black structure on UV stabilisation for various particle-sized carbon blacks. An increase in carbon black concentration from 1.5 to 3.5% w/w also improved UV stabilisation. For UVA and UVB sources, presence of carbon black, while increasing carbonyl group generation with respect to unit loss in tensile property with respect to unfilled LLDPE, also appears to suppress Norrish Type II scissions at photochemically generated carbonyl centers in polymer chains. This is especially the case for the smallest (20 nm) particle sizes. Thus the photostabilising efficiency of carbon black is based on both physical surface-area-dependent UV absorption and photochemical activity. Under xenon arc exposure, however, this latter is minimal.