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"Why is aluminum-complex grease not as popular in industry as lithium or lithium-complex grease?"
The main reason is cost. Performance-wise, aluminum-complex grease performs just as well as lithium-complex grease in many aspects and may even surpass it in a few key areas, but it is more expensive to produce. It also takes quite a bit longer to manufacture aluminum-complex grease. This means the manufacturer is producing less product than it could if it were manufacturing lithium-complex grease.
Keep in mind that grease is composed of base oil, a thickener and additives. Typical amounts would be 70 to 90 percent base oil, 3 to 30 percent thickener and up to 10 percent additives. The amounts will depend on the specific use for which the grease is designed.
The thickener is formed in the base oil by a chemical reaction. The byproducts are then removed and more oil and additives are mixed. Then, the mixture is homogenized. The more thickener that is added, the stiffer the finished product. These thickeners are sometimes called soaps.
Grease thickeners can be simple metal soaps, complex metal soaps or non-soap thickeners. The difference between simple and complex metal soaps is chemical and relates to whether long and short chain fatty acids are used in the grease production.
Many thickener types exist, although some are more popular and more widely used. Some of the more popular thickener types include lithium complex, aluminum complex, calcium complex and polyurea.
As you can see from the table below, many properties of lithium and aluminum are comparable. Typically, aluminum-complex grease will be a little better at resisting water washout and have a better oxidation stability, but this is not an absolute. It will depend on the formulation and the quality of the constituents used by the manufacturer to make the grease.
There are also some user conveniences that are enhanced with lithium as opposed to aluminum. For example, aluminum grease is a bit tougher to clean up if any finds its way onto your skin or clothing.
The one drawback that aluminum-complex grease has when pitted against lithium-complex grease is its work or shear stability. The shear stability of grease is its ability to withstand repeated working with minimum change in its structure or consistency. This means that aluminum-complex grease may break down more easily and not retain its consistency.
As with many lubricant properties, there is always a tradeoff that must be weighed. In a situation where water washout is expected, the aluminum-complex grease would likely be the best option. However, for most general-purpose applications, the lithium grease's lower cost and overall performance would win out.