Abstract
The effective stabilisation of polypropylene (PP) plastics for durable applications is often achieved by compounding with a combination of hindered phenolic (SHPh) and amine (HAS) antioxidants. Hence, to assess the lifetime of these materials it is useful and often necessary to test the status of each type of stabiliser used during or after exposure. In this paper, we discuss the measurement of the steady-state rate of post-induction oxygen consumption during the model reaction of initiated cumene oxidation. This is in order to determine antioxidants of the HAS type derived from 2,2.6,6-tetramethylpiperidine. As already published, the measurement of the induction period can be used for the simultaneous determination of the SHPh content. The rate of the post-induction oxidation reaction depends linearly on the reciprocal square root of the concentration of this HAS-antioxidant type, for a sufficiently wide concentration range. The presence of sterically hindered phenolic stabilizers (SHPh) and organophosphites usually of the kind used has no influence on this post-induction phase. Thus the dependence established offers a remarkable possibility of determining simultaneously the effective stabiliser concentrations of the HAS and SHPh antioxidants by means of one model reaction. We present the results of the successful determination of a HAS and an SHPh antioxidant in PP granules. They have to be ground down to particles with mean dimensions smaller than 250 μm. We found perfect coincidence with the data given by the granule producer.
Thus the model reaction of initiated cumene oxidation is a useful method for the simultaneous quantitative study of SHPh and 2,2.6,6-tetramethylpiperidine derived HAS-antioxidants, even in the presence of organophospites in PP, and possibly other plastics.