Engine Oil Antioxidant Selection for Delivering Superior Oxidation and Deposit Control Protection

Abstract

Engine oil formulators face a number of challenges when developing modern passenger car motor oils. In recent years greater performance demands have been placed on engine oils to deliver superior oxidation and deposit control protection. This has occurred concurrently with the mandated reductions of phosphorus driven by concerns to protect engine catalyst systems. This has forced the use of lower levels of zinc dialkyldithiophosphate (ZDDP) in modern engine oils. ZDDP is known to be one of the most cost effective antioxidants available. Reductions in its use must be compensated for by the use of other phosphorus-free antioxidants. A challenge exists for the engine oil formulators to identify the most cost effective alternatives to ZDDP while at the same time utilizing inexpensive and rapid, yet meaningful bench test techniques. This paper looks at the use of a bulk oil oxidation test (AlbOT), pressurized differential scanning calorimetry (PDSC), the Caterpillar Micro-Oxidation Test (CMOT) and the Thermo-Oxidation Engine Oil Simulation Test (TEOST-MHT) to evaluate and rank the robustness of passenger car engine oil performance for both oxidation protection and deposit control capabilities. These tests are utilized to screen a number of antioxidant systems in engine oils formulated with 500 ppm of phosphorus derived from ZDDP. The antioxidant components are selected from a series of commonly used and commercially available materials plus some developmental materials. These components include a molybdenum compound, alkylated diphenylamines, hindered phenolics, and new developmental and experimental multi-functional antioxidants. The performance of these fully formulated engine oils are ranked in the selected bench tests in order to highlight the benefits of each antioxidant system under evaluation. Structure activity studies are carried out on a select group of tests in order to gain further insight as to how antioxidant chemical structure impacts oxidation and deposit control performance. The results point to significant performance benefits when multi-functional antioxidants are employed or when the optimum molecular weight antioxidant is selected.

F2012-A03-023