Standard Tests and Comparative Methods

The OIT test is used to assess the relative stability of a series of related polymer samples. It does however suffer from several limitations, which can include poor reproducibility and precision [1,5-7]. Some of the issues relate to  

In a study on the thermal and UV ageing of two commercial polyfoxymethy-lene) (POM) samples, one of which was a copolymer (see related study discussed later under Section 4.3, thermogravimetric analysis (TGA)), used in car interior applications, involving both DSC and TGA, isothermal OIT measurements were made at several different temperatures [8]. One conclusion from this study was that extrapolation of the OIT data from high temperatures (molten state) to ambient temperatures in the solid state does not reflect effective antioxidant performance at room temperature , and thus measurements close to the melting point are not appropriate for reliable lifetime estimations. 

In OIT determinations, the test samples are flushed continuously with oxygen during the course of the measurement [1], This is in contrast to oxygen-uptake measurements that are mostly undertaken in a static oxygen atmosphere (see below). Effects such as dynamic removal of volatile antioxidants in the OIT method can lead to these two methods yielding very different assessments of a polymer formulation s stability [1]. 

Gijsman and Hamskog [12] reported recently on a study of stabilised and unstabilised PP in apparatus that enabled the simultaneous measurement of oxygen update (OU) and imaging CL, which enabled them to obtain comparable information on an oxidising area. Some conclusions from the study were (see Ref. [12] for full details)  

The carbonyl index is not a standard technique, but is a widely used convenient measurement for comparing the relative extent and rate of oxidation in series of related polymer samples. The carbonyl index is determined using mid-infrared spectroscopy. The method is based on determining the absorbance ratio of a carbonyl (vC = 0) band generated as a consequence of oxidation normalised normally to the intensity of an absorption band in the polymer spectrum that is invariant with respect to polymer oxidation. (In an analogous manner, a hydroxyl index may be determined from a determination of the absorbance intensity of a vOH band normalised against an absorbance band that is invariant to the extent of oxidation.) In the text following, two examples of multi-technique studies of polymer oxidation will be discussed briefly each includes a measure of a carbonyl index. 

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