Antagonism between transition metal pro-oxidants in polyethylene films

Abstract

The oxidation of linear low density polyethylene (LLDPE) films containing the combination of pro-oxidants titanium (IV) dioxide (TiO2) with either cobalt (II) stearate (CoSt) or iron (II) stearate (FeSt) have been evaluated under accelerated photo- and thermo-oxidative conditions as well as on outdoor weathering. LLDPE containing only surface-compatibilised nano-TiO2 rapidly photo-whitens and embrittles at a low apparent extent of oxidation (as measured by carbonyl index) due to formation of microscopic voids of ∼150 nm. When CoSt was also included in the film, antagonism occurred shown by embrittlement times longer by ∼90%, higher carbonyl index and absence of film whitening. In contrast, films containing TiO2/FeSt whitened during photo-oxidation and exhibited lower antagonism with only 44% longer times to embrittlement and lower carbonyl index. Antagonism between pro-oxidants was not observed under dark thermo-oxidative conditions. X-ray Fluorescence Microspectroscopy elemental maps revealed that the TiO2 nanoparticles were spatially correlated with iron and cobalt metal ions allowing scavenging of electrons and holes through cycling of the redox states of the metal without producing radical species to initiate polymer oxidation. It is suggested that the antagonism differences between TiO2/CoSt and TiO2/FeSt pro-oxidants is related to the respective reduction potentials of Co3+/2+ and Fe3+/2+ and their effect on the UV conduction and valence band edges of the TiO2 particle. In these ways the photochemistry of TiO2 is suppressed and the photo-oxidative lifetime is governed by the chemistry of the transition metal pro-oxidant.