Oil CompositionRefined oil is derived, as one would expect, from crude oil. Its composition varies due to the source of the crude as well as the processing needed to create the finished product. Oils are viscous, non-polar organic substances of biotic and/or abiotic origin. They do not readily adhere to other surfaces. That being the case, they afford a high degree of lubricity. In addition, motor oil serves as a coolant and a compression seal.
Chemically, motor oils are mostly saturated hydrocarbons of about 15 to 50 carbon atoms. These hydrocarbons may include not only straight and branched chain structures, but also cyclic components. However, petroleum used for motor oil must have certain components removed. These include alkenes and certain nitrogen and sulfur compounds. Such compounds oxidize with heat to produce corrosive acid compounds that attack metal. Additives are formulated in, not only to lower friction, but also to increase detergency and dispersancy.
Grease CompositionGrease, logically, is not that much different. Grease serves best when it is applied to areas that don’t require as much or as frequent lubrication, or where oil cannot accomplish the task. Grease has a thick initial characteristic, but on use thins out to oil-like consistency due to shear. Grease also impedes the introduction of water and clings to the structure to which it is applied.
Typically, grease consists of an oil emulsified with a thickening soap. As a result, greases are said to be thixotropic. The soap may be metallic stearates such as calcium or lithium stearate or stearate derivatives. Perhaps the reader has heard of lithium grease? Other thickeners may also be added to the oil, such as clays or mica. Other ingredients may be added to greases, such as graphite or molybdenum sulfide.
Motor Oil and Grease in ConclusionAs can be readily discerned from the discourse above, motor oil and grease are closely related. Additives are the main difference. For oil, they usually are many and varied to “fine-tune” the product and its viscosity. The main additive in grease is a thickening agent such as a metallic soap, which is itself slippery. It is used as lubricant in applications where heavy viscosity is needed and oil is impractical to use.
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- National Park Service: Environmental Contaminants Encyclopedia – Oil, New (Unused Motor Oil) Entry
- Used Mineral-Based Crankcase Oil: Chemical Identity
- NASA: Real Time Oil Reservoir Evaluation Using Nanotechnology