Analysis of Antioxidants and Light Stabilisers in Polymers by Modern Liquid Chromatography

Abstract

The identification and quantification of antioxidants, light stabilisers and other polymer additives is necessary for quality-assurance purposes but it is also useful for other reasons such as screening of unknown or competitors’ products and in polymer degradation studies. Polyolefins, the largest-volume family of commercially important high-tonnage thermoplastic polymers, dominate the world market in terms of antioxidant and light-stabiliser usage. Since there is a wide variety and complexity of polyolefin stabilisers, many of them differing only slightly in structure, coupled with the fact that the additive package usually contains more than one component, a technique capable of separating these compounds for qualitative or quantitative analysis is essential. Most polymer additives, and especially today’s stabilisers, are compounds of insufficient volatility to be analysed by gas chromatography or they are thermally unstable and decompose under the conditions of gas Chromatographic separation. Liquid chromatography (LC) eliminates this disadvantage and provides additional advantages with respect to gas chromatography for the separation, detection and identification of polymer additives. Although the classical LC techniques, i.e. column, paper and thin-layer chromatography, have been used for the analysis of antioxidants, light stabilisers and other polymer additives, they are today being replaced by modern liquid chromatography, i.e. high-performance liquid chromatography (HPLC) and size-exclusion chromatography (SEC). The preliminary step of additive extraction from polymers drastically influences the success of the Chromatographic analysis. Normal versus reversed-phase and isocratic versus gradient-elution HPLC separation of synthetic mixtures of additives and of solvent extracts from polymers are discussed. Some SEC analysis of additives in polymers may be performed without the time-consuming extraction procedure. Recent developments such as high-resolution capillary gas chromatography with cold on-column injection, high-speed LC, microcolumn LC and supercritical fluid chromatography are presented in terms of their application to the analysis of polymer additives. In view of the fact that monitoring of polymer additives has become increasingly important because of stricter laws and regulations, the need for fast and reliable analytical methods has increased.