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According to a recent NLGI Grease Production Survey, approximately 70 percent of the grease sold worldwide is based on either simple lithium soap or lithium complex thickener. You might ask, “Why are those thickener types so popular?” and “How do I decide which one is best for my application?” This article will provide answers to these and a few other questions.
First, let us take a historical perspective. Clarence E. Earle, an American chemical engineer, was granted U.S. Patent No. 2,274,675 on March 3, 1942, for an invention called “Lubricant Containing Lithium Salts.” This is the first description in the patent literature of a grease based on simple lithium soaps. Although the soaps described in the patent are the types typically used to produce lithium-soap-based greases today, the lithium greases described by Earle ushered in a new era in the lubricating grease industry.
Greases based on lithium soap possess many improved properties compared to the other alkali metal soaps that existed in 1942. They have better water resistance properties compared to sodium soap greases, better high-temperature properties compared to calcium soap greases, and excellent mechanical properties (both resistance to shearing and good ability to be pumped). Although they are more expensive to manufacture than other grease types, lithium soap greases offer so many advantages compared to sodium and calcium soap thickeners that extra cost is offset by improved performance.
Lithium complex greases were developed in the late 1940s. U.S. Patent 2,417,428 was granted to Lester W. McClennan on March 18, 1947. This is one of the first patents to describe complex soap greases. However, it wasn’t until the early 1980s that lithium complex greases entered the market in large volumes and started to displace the simple lithium soap greases that had been the mainstay of the industry since the 1950s.
Lithium complex greases possess many of the properties of simple lithium soap greases and also have higher dropping points, allowing the greases to be used at higher temperatures. A comparison of the thickener-related properties of the products is given in Table 1.
The dropping point of lithium complex greases is higher than that of simple lithium soap greases due to the presence of a second thickener component, known as the complexing agent. Modern lithium complex greases typically use a shorter chain-length difunctional carboxylic acid, such as azelaic acid or adipic acid. The lithium salt of these materials is typically present in a significantly lower proportion compared to the simple lithium soap thickener. An alternate material used as a complexing agent is boric acid. The use of this material also results in an elevated dropping point.
Mechanical stability, also known as shear stability, is the ability of grease to maintain consistency when subjected to mechanical shear forces. Simple lithium greases have good resistance to breakdown due to shear, and lithium complex greases also exhibit good resistance to shear. This property makes both simple lithium and lithium complex greases popular for use in a wide range of applications.
The water resistance of simple lithium and lithium complex greases is related to the solubility of the thickener. Lithium hydroxide has limited solubility in water (about 10 percent), and the thickeners based on it also have limited solubility. This provides good resistance to both washing by water and the absorption of water. Although other thickener types (calcium, barium) have better inherent water resistance compared to lithium and lithium complex thickeners, those products have negative aspects that make them less desirable for many applications. In addition, the water resistance properties of simple lithium and lithium complex greases can be enhanced by the addition of polymer additives in small concentrations.
The oil separation properties of a grease relate to both the product’s lubrication ability and storage stability. The grease must release enough oil in the contact zone of the application (bearings, gears), while not releasing so much oil during storage to cause the product to become unusable. If the oil separates excessively during storage, the grease may not be able to be remixed and used.
Table 1. Properties of Lithium and Lithium Complex Greases
Now, to answer the questions posed at the start: Why are the lithium and lithium complex thickener types so popular? It’s the versatility of the thickeners, making greases containing them suitable for use in a broad range of applications. Which one is best for your application? The thickeners are similar in many properties, so the best way to determine which to specify is the operating temperature of the application. Many prefer lithium complex greases as general-purpose lubricants, and that is a good approach, since they can be used in a wide range of applications and over the widest range of temperatures. However, for specific applications, simple lithium soap greases are often the most economical choice.