Accelerated ageing of polypropylene stabilized by phenolic antioxidants under high oxygen pressure

Polypropylene (PP) samples stabilized by a hindered phenol (Irganox 1010) were submitted to thermal ageing at 80 � C in air at atmospheric pressure or in pure oxygen at 5.0 MPa pressure. Both the polymer oxidation and the stabilizer consumption were monitored by Infra- red spectrometry and thermal analysis. The stabilizer effi- ciency, as assessed by the ratio induction time/stabilizer concentration is almost constant at atmospheric pressure even when the stabilizer concentration is higher than its solubility limit in PP (0.4% or 24 � 10 � 3 mol L � 1 ). In con- trast, at high pressure, the efficiency decreases almost hyperbolically with the stabilizer concentration when this latter is higher than 6.0 � 10 � 3 mol L � 1 . The results indi- cate the existence of a direct phenol-oxygen reaction negli- gible at low oxygen pressure but significant at 5.0 MPa pressure. The reality of this reaction has been proved on the basis of a study of the thermal oxidation of a phenol solution in a nonoxidizable solvent. A kinetic model of PP oxidation in which stabilization involves three reactions has been proposed. It simulates correctly the effect of oxy- gen pressure and stabilizer concentration on carbonyl build-up and stabilizer consumption. V C 2008 Wiley Periodi-