FOURIER-TRANSFORM INFRARED SPECTROMETRIC DETERMINATION OF STEARIC-ACID IN STYRENIC POLYMERS

The conventional polymer dissolution/re-precipitation method, in which stearic acid is extracted from the bulk and from the surface of styrenic polymers, fails to establish whether stearic acid is used as an external or internal lubricant, whereas an ethanol wash of the resin selectively removes external stearic acid. These simple additive isolation procedures, followed by infrared spectrometry, were used to show whether stearic acid was applied on the surface of the pellets or blended throughout the polymer. A rapid, precise and accurate method to determine both external and internal stearic acid by using Fourier transform infrared spectrometry is described. The accuracy of the method was determined to be within 6%. The detection limit was ca. 0.01% m/m in the resin.