Thermo-oxidative degradation of isotactic polypropylene at high temperatures: phenolic antioxidants versus HAS

The influence of a phenolic antioxidant and two hindered amine stabilizers (HAS) on the degradation behaviour of isotactic polypropylene was investigated for temperatures in the range 120-149 degrees C. Consistent with earlier reports, phenolic-stabilized specimens were found to decompose in a catastrophic manner at the end of an induction period, whereas the mechanical properties of HAS-stabilized samples deteriorated more gradually. These differences are due to fundamental differences in the mode of action of the stabilizers. Phenolic antioxidants act as inhibitors, hindering the formation of hydroperoxide sequences and the scission of primary bonds, provided that their concentration in the polymer remains above a critical value. On the other hand, gel permeation chromatography and viscosity measurements show that HAS do not inhibit the steady cleavage of polymer chains. This has been ascribed primarily to the observation that nitroxyl radicals and hydroxylamine ethers, considered to be the most important stabilizing species associated with HAS, are formed during aging. HAS are thought, therefore, only to retard the degradation of the polymer. (C) 2000 Elsevier Science Ltd. All rights reserved.