Entrapment of stabilisers in silica .1. Controlled release of additives during polypropylene degradation

In this study antioxidants (AOs) and hindered amine Light stabilisers (HALS) were adsorbed to silicas prior to blending and processing in polypropylene films. In the absence of AOs and HALS, polymer stability was found to be highly dependent on the active metal (Ti, Al, Fe) contents of the silicas (as demonstrated by carbonyl index and embrittlement data). Laser light-scattering has been employed to demonstrate that silica enhances polymer crystallinity (and hence reduces clarity) during ageing by acting as a nucleating agent, but that this is moderated when the stabilisers are bound to the silica. The distinct differences arising in stabilisation performance of AOs and HALS when prebound to silica were attributed to the ability of stabilisers to selectively desorb, depending on environmental conditions. On thermal ageing the majority of HALS exhibited increased activity, whereas during photo-ageing the majority showed reduced activity. All physical mixtures of preadsorbed antioxidants demonstrated reduced activity on photo-ageing, as did many on thermal ageing. The reduction in performance was greater than expected, even when the additive effects of single stabilisers were accounted for. (C) 1997 Elsevier Science Limited.