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Determination of antioxidants in polypropylene by liquid chromatography

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Abstract

Polyolefins, such as polypropylene, are subject to thermal and oxidative degradation and are not used in practical applications unless protected by several antioxidants. Liquid exclusion (LEC) and liquid adsorption (LAC) chromatography have been used to determine the antioxidants in commercial polypropylenes and molded parts. Although a LEC analysis takes as long as 3 hr, LAC separations can be obtained in less than 15 min. Prior to the chromatographic measurements, the antioxidants were extracted from the talc-filled polypropylene using tetrahydrofuran (THF) and Methylene chloride (CH2Cl2) for the LEC and LAC separations, respectively. Room-temperature extraction of the additives as a function of time showed that 24 hr was sufficient for THF. A comparison of the LEC chromatogram for a freshly molded part to that for a part which was heat treated showed not only the depletion of certain antloxidants but also showed that the antioxidant distearyl 3,3′-thiodipropionate (DSTDP) thermally decomposed to give stearyl propionate. Mass spectrometry was used to confirm the identity of the additives in the polypropylene without extraction. The “life” af several polypropylenes was also measured by thermogravimetric analysis and compared to the concentration of DSTDP.

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