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The current turbine oil oxidation bench tests have been in place for many years. Recently, however, the basestocks used to formulate these lubricants have changed significantly. Traditional basestocks, containing high levels of aromatics and sulfur, have been substantially displaced by more highly refined basestocks, which have very low levels of aromatics and almost no sulfur. Over time it has become clear that the oxidation performance of the different basestock classes is quite different. One key question, however, remains unanswered. “What modifications are required in current bench tests to adequately mimic oxidation processes in modern turbine lubricants?” In order to adequately address this critical question, the fundamentals of lubricant oxidation and stabilization will be discussed. Data will be presented on the oxidation of different basestocks under varying conditions of temperature, metal catalysts and antioxidant type, using model bulk oil oxidation tests. The data highlights the fundamental oxidation mechanisms that can occur in turbine oil systems and new test methods are proposed.

Using simple turbine oil formulations, we studied the effects of different antioxidant systems on oxidation behaviour. Fourier transform infrared was used as a sensitive probe to monitor the depletion of different antioxidant types present in turbine oils based on Group II, III and IV base stocks. It was found that the depletion of alkylated diphenylamine antioxidants from turbine oil systems is generally slower when hindered phenolics are present in the formulation. Furthermore, the type of hindered phenolic used in the formulation is critical for preserving this amine activity, with the bis-phenolic 4,4-methylenebis(2,6-di-tert-butylphenol) showing superior performance over a phenolic ester of approximately the same molecular weight. This effect was explained by the higher hindered phenolic activity of the bis-phenolic relative to the phenolic ester. The ability of the bis-phenolic to preserve or protect the amine antioxidant, combined with its higher hindered phenolic activity, was shown to improve the oxidation resistance of turbine oils. The results also showed a surprisingly high oxidation stability for turbine oils formulated with Group III base stock, with the oxidation resistance of the Group III systems exceeding that of the equivalent Group II and Group IV systems.

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A review of current oxidation and deposit bench tests used for the evaluation of engine oil performance will be presented. Some of the more meaningful tests will be utilized to evaluate a number of antioxidant systems for oxidation and deposit control capabilities in engine oils formulated with 470 ppm of ZDDP-derived phosphorus. The antioxidant components are selected from a series of commonly used and commercially available materials plus one new developmental component. These components include an organo-molybdenum compound (MoDTC), an alkylated diphenylamine (NDPA), a conventional hindered phenolic (HPE), a high performance hindered phenolic (MBDTBP), and a new multi-functional boronated MBDTBP. The performance of these fully formulated engine oils will be ranked in the selected bench tests in order to highlight the benefits of each antioxidant system under evaluation. The results point to significant benefits with the molybdenum- and boronated-systems, or mixed molybdenum-/boronated-systems, for oxidation control, while systems containing NDPA and MBDTBP are favored more for deposit control. Unique and superior performing antioxidant systems will be recommended for screening in fired engine and bench wear tests.