Types of Grease Thickeners

Grease Thickeners and Features

Grease is used for lubricating a wide range of machinery, from manufacturing equipment in factories to heavy machinery operating at construction sites. A simple search for grease on an online store yields a variety of products. However, in many workplaces, the choice of grease is often based not on understanding the differences between products, but rather on reasons such as “we’ve always used this one,” “it’s cheap,” or ” the manufacturer recommends it.”

In reality, by correctly understanding the differences in properties between various types of grease and selecting the right one for the application, you can reduce grease consumption and decrease the frequency of parts replacement. This article explains what grease is made of and how changes in its components affect its characteristics.

Components of Grease

Like grease, there are oil-based lubricants such as gear oil and hydraulic oil that are used for lubrication purposes. So, what is the difference between grease and lubricating oil?

Anyone who has used both would immediately recognize that grease is a semi-solid lubricant, similar to mayonnaise or butter, while lubricating oil is a liquid lubricant, comparable to salad oil or honey. But what exactly creates this difference?

The answer lies in the thickener. Lubricating oils, such as gear oil or hydraulic oil, are typically composed of base oil (such as mineral oil) and various additives to enhance performance. When a thickener is added to this mixture, it becomes a semi-solid grease.

In summary:

• Lubricating oil = Base oil + Additives
• Grease = Thickener + Base oil + Additives

Therefore, the thickener is the defining component that makes grease what it is. Changing the type of thickener significantly alters the characteristics of the grease.

So what types of thickeners are used in grease, and what properties do each of them have?

Is the Thickener Just “Soap”?

The most common thickener used in grease is lithium soap. Grease that uses lithium soap as a thickener is generally referred to as lithium grease. When we hear the word “soap,” many of us might think of bar soap used in bathrooms or at sinks. However, in a broader chemical classification, thickeners and bar soaps fall into the same category.

Surprisingly, the broad definition of soap is “an alkali salt of a fatty acid,” and this applies to both bar soap and the thickener in lithium grease—they are both composed of alkali salts of higher fatty acids.

Since “alkali salt of a fatty acid” may be challenging to visualize, let’s use a concrete example. If you’ve ever made handmade soap, you’ll know it involves reacting oils or fats (such as cooking oil or beef tallow) with an alkali, like sodium hydroxide. This reaction produces soap by combining the fatty acid molecules in the oils with the alkali. (While the actual chemistry is more complex, this simplification is sufficient for our purposes.)

The properties of the soap produced depend significantly on the combination of the fatty acid and the alkali used.

For example:

• Reacting stearic acid (a fatty acid) with sodium hydroxide yields sodium stearate, a key ingredient in bar soap.
• Reacting stearic acid with potassium hydroxide yields potassium stearate, which is used in liquid soaps, such as hand soap.

These examples demonstrate how different combinations of fatty acids and alkalis yield soaps with varying properties.

So, what is the main component of lithium soap used in lithium grease? It is made by reacting lithium hydroxide with fatty acids such as stearic acid. While the specific fatty acids may vary by product, all lithium greases share the common trait of using lithium hydroxide as the alkali.

This raises two critical questions:

• Why is lithium grease so widely used?
• What other types of thickeners exist, and how do their properties differ from lithium soap?

Types of Soap-Based Thickeners: Their Advantages and Disadvantages

The most commonly used grease is lithium grease, and its biggest advantage is its low cost. The cheapest greases found in online stores or hardware shops are almost always lithium-based.

However, lithium grease has notable disadvantages. One of the main drawbacks is its poor water resistance. You may have seen lithium grease leaking from the pins of heavy machinery used outdoors, turning white after exposure to rain. Similarly, in heavy machinery used for ground improvement that is frequently exposed to muddy water, users often report that the grease quickly washes away, no matter how often it is applied. This is because the grease reacts with water, causing it to lose its original performance and fail to stay on the lubricated area.

Another disadvantage is limited heat resistance. Lithium grease is suitable for applications that don’t get too hot; however, in most cases, it melts and drips off at temperatures around 150°C, leaving the lubricated parts unprotected. This makes it unsuitable for steel-related equipment exposed to high temperatures or for heavy machinery that generates frictional heat under high loads, where maintaining lubrication is critical.

To overcome these weaknesses, a modified version known as lithium complex grease was developed by partially replacing the fatty acids used in lithium soap with other types of acids. This results in a thickener that is less prone to emulsification and can withstand temperatures above 200°C, significantly improving both water and heat resistance.

It’s worth noting that both lithium soap grease and lithium complex grease use lithium hydroxide as the alkali to produce the thickener. However, other types of alkalis can also be used. For example:

• Replacing lithium hydroxide with calcium hydroxide produces calcium soap grease, also known as calcium complex grease.
• Using aluminum hydroxide results in aluminum soap grease or aluminum complex grease.

Among these, aluminum complex grease is especially notable. Like lithium complex grease, it offers excellent water and heat resistance, and it is less likely to harden under high temperatures. This makes it suitable for use in applications exposed to both heat and moisture, and it is widely adopted for such purposes.

Non-Soap-Based Grease

In addition to greases made by combining fatty acids and alkalis, some greases use completely different substances as thickeners. The most notable examples are bentonite grease and urea grease.

Bentonite grease uses a clay-based substance called bentonite as its thickener. Its most distinctive feature is that it has no dropping point—unlike soap-based greases, which, even if heat-resistant, eventually drip when exposed to high temperatures, bentonite grease does not melt and drip.

This may sound like a highly advantageous property, but bentonite grease has a major weakness: it tends to harden under high temperatures. Although it doesn’t drip, you must still be cautious when using it in high-temperature equipment. In some cases, it may be easier to deal with a grease that drips than one that solidifies, especially when it comes to equipment overhauls.

Urea grease, on the other hand, uses organic compounds containing urea groups as its thickener. It offers excellent heat and water resistance, making it a common alternative in applications where lithium grease is insufficient.

Like aluminum complex grease and lithium complex grease, urea grease has few weaknesses. However, one drawback is its poor compatibility with soap-based greases, which are more commonly used. In general, mixing different types of grease should be avoided, but it is essential to note that soap-based and urea greases are particularly incompatible when mixed.

Therefore, in environments where maintaining grease performance is critical, it may be necessary to implement operational controls, such as strict grease application management or even standardizing all greases to urea grease, to avoid performance degradation.

The Benefits of Choosing the Right Grease

We’ve now explored grease thickeners and their characteristics. While the discussion has been somewhat technical, what are the actual benefits of understanding the properties of grease?

The primary benefit is that you can make more informed, cost-saving decisions. In many sites where grease is used today, lithium grease is often chosen due to its price appeal, but as you can see, that choice doesn’t always lead to lower total costs.

For example, in one case, switching the grease used on heavy machinery exposed to water from lithium grease to aluminum complex grease resulted in a reduction in grease consumption to approximately one-fourth. As a result, not only were grease purchase costs reduced, but also less time was required for re-greasing, allowing that time to be spent on more productive work.

In another case, a rolling mill production line reduced its annual grease consumption by over 4,000 kg simply by reviewing its grease selection. Since heavy grease usage places a significant burden on the environment, reducing grease consumption contributes not only to cost savings but also to a lower environmental impact.

Conclusion

As we’ve seen, there are various types of grease, each with its advantages and disadvantages. By understanding the properties of grease, you can make choices that lead to reduced costs and lower environmental impact.

At NCH, we offer a wide range of solutions to support our customers as partners in lubrication management, helping you achieve both cost efficiency and environmental sustainability.