A molecular study towards the interaction of phenolic anti-oxidants, aromatic amines and HALS stabilizers in a thermo-oxidative ageing process

The molecular interactions of phenolic anti-oxidants, aromatic amines and HALS stabilizers are successfully studied by heating the stabilizers under oxidative conditions in polar and non-polar solvents. The polar solvent bis(2-methoxyethyl)ether is used to mimick polar engineering plastics like e.g. TPE-U's, whereas the non-polar solvent squalane or 2,6,10,15,19,23-hexamethyl-tetracosane is used to mimick polypropylene. The oxidation rate is followed by the analysis of samples taken in time using various analytical techniques as e.g. IR, HPLC-PDA, GC-FID, GC-MS and LC-MS. A general occuring interaction between sterically hindered phenols and aromatic amines, i.e. regeneration of the aromatic amine by the sterically hindered phenol, is demonstrated by varying the molecular structure of the phenol as well as the polarity of the system. Studies using mixtures of a HALS with structurally different sterically hindered phenols visualized a general antagonistic effect between the phenols and the HALS, in which the HALS consumes the phenol. In all stabilizer combinations using the sterically hindered phenols studied here, a stabilization of the resulting quinone form is observed which can, dependent on the molecular structure of this quinone, lead to solubility issues in polymers. (C) 2002 Elsevier Science Ltd. All rights reserved.