Gas Chromatographic Monitoring of Hydroxyl Components in Oxidized Turbine Engine Lubricants

Abstract

Changes in levels of hydroxyl compounds in synthetic ester based turbine engine lubricants were measured by derivatization and detection by gas chromatography with an atomic emission detector. Conversion of hydroxyl groups to trifluoracetates were made with the acetylating agent N-methylbis[trifluoroacetamide]. Although individual hydroxyl compounds could be quantified by gas chromatography, total hydroxyl content was most accurately measured by the quantity of the reaction byproduct, N-methyl trifluoroacetamide. Analyses of field samples showed a good relationship between elevations in hydroxyl numbers and deposition problems and were more reliable than COBRA, TAN and conductivity values. A good correlation was also noted between deposition in a dynamic coking apparatus and hydroxyl number increases. Analyses of corrosion and oxidation tested lubricants reveals relatively smaller increases in hydroxyl number despite large increases in degradation levels.