Hindered amine stabilizers as sources of markers of the heterogeneous photooxidation / photostabilization of carbon chain polymers pp.1027-1042
Authors: (J. Pilař, J. Pospíšil, Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic)
Abstract: Application of Hindered Amine Stabilizers (HAS) is the state-of-the-art approach to protection of carbon-chain polymers such as polyolefins and polystyrene or blends containing these against weathering. During outdoor exposure, the polymers loose their material properties due to the solar radiation-triggered photooxidation. The complex mechanism of the stabilization involving cyclic oxidation-triggered transformation of HAS is outlined. Monitoring of the formation of the HAS-developed key transformation products responsible within the regenerative mechanism for the effective stabilization, HAS-related nitroxides, was used to confirm the heterogeneous character of photooxidation of two carbon-chain polymers, polypropylene and a specific polyethylene copolymer. Depth profiles of nitroxides were monitored in a long-term photooxidation regime using Electron Spin Resonance Imaging (ESRI) technique. The shape of concentration profiles of the nitroxides accumulated in the stationary state upon filtered Xenon lamp-equipped weatherometer exposure was interpreted in term of the oxygen diffusion limited oxidation and radiation penetration in oxidation-stressed polymer surfaces. The data indicate differences in the response of the polymer matrix and of the used stabilizer system based on secondary HAS and O-alkyl hydroxylamine HAS and/or HAS combination with UV absorbers. Imaging of nitroxides is a precise tool for marking heterogeneous oxidation of polyolefins.