Everybody knows what "greasy" means, even if only from that coating on your hands after downing a bucket of the Colonel’s finest. But rendered animal fat is a different kind of grease from that used for lubrication.
Lubricating grease is a type of semisolid lubricant used in applications that cannot retain a continuous supply of oil, such as in open bearings or gears. Although most lubricating greases start out as petroleum-based oil, the manufacturing process adds thickeners and performance additives specific to the intended application.
Most grease thickeners are not so different from bath soap. Soap is an emulsifier that does more than merely make the oil thicker, and acts differently from soap and water.
Soap is an emulsifier that allows oil and water to mix. As a cleanser, soap dissolves oil when added to water because of the shape of the soap molecule, which has a positively charged, single valence metallic ion such as sodium, potassium or lithium attached to one end of a long carbon/hydrogen chain. Chemically speaking, the single valence ion replaces one of the hydrogen ions in a molecule of stearic acid or other monoglyceride. Normally the stearic acid molecule is nonpolar, but the addition of the positive ion makes the soap molecule polar in an unusually way, part water soluble and part oil soluble.
When it is dissolved in water the positively charged "head" of the soap molecule (the end with the metallic ion) clings to polar water molecules while the nonpolar tail grabs oil molecules, thereby bonding oil and water together like a piece of double-stick tape. The result is a homogeneous mixture that can be washed away. When soap is dissolved in oil something completely different happens. The oil molecules are so repulsive to the positive heads of soap molecules that the latter stick their heads together forming a hollow circle of protection called an inverse micelle. This is like a circle that wagon trains might form with all the horses pointed towards the campfire in the center.
The hollow micelles thicken the grease mixture, but also create interesting physical properties.
The result is a shear-thinning thixotropic or pseudo-plastic fluid, so named because the viscosity of the fluid is reduced under shear. The application of sufficient shear force, such as a wheel turning against an axle, causes the micelles to break down.
This reduces the viscosity approaching that of the base oil, but this thixotropic behavior is reversible, meaning that when the shear force is reduced the micelles reform and the grease thickens back to its original viscosity.
The application of grease under high pressure with a grease gun creates a shear force as the pressure forces the grease through the small opening in the gun. It thins enough to fill spaces in the part being lubricated, but then returns to its original sticky consistency.
There are many different types of grease that are manufactured for specific applications, which determines the selection of the type of soap and other additives. This influences the properties of the grease in terms of temperature, water resistance, chemical stability and shear resistance.
Lubrication is required for any and all machines with moving parts. The vast majority use grease of one form or another. Ever since the invention of the wheel, civilization has slid on its grease.
Richard Brill is a professor of science at Honolulu Community College. Email questions and comments to rickb@hcc.hawaii.edu.
