A Guide to Grease Thickeners

Used for over 3000 years, grease is a key lubricant used to operate a variety of machines and bearings. Over 80% of the world’s bearings are lubricated with grease. Grease is an excellent lubricant to use when liquid lubricants fail to do the job. Greases are made of three main components: base oil (70-95%), thickener (3-30%), and additives (up to 10%). We are going to examine the second component: thickeners. Thickeners are essential as they are the “sponge” that holds the base oil and additives.

What are Thickeners?

When combined with the base oil and additives, the thickener forms a semi-fluid structure. Conventional thinking suggests the structure indicates the grease is mainly thickener, however, the thickener is a material that holds the lubricant until it is dispersed. As mentioned above, the overwhelming majority of any grease is composed of base oil. There are many types of compounds that can be used as thickeners.

Greases are classified into two major families: soap and non-soap thickeners. Over 90% of greases worldwide are classified as soap thickeners. Soap-based thickeners are produced via an acid-base reaction known as saponification. The end-result is a soap and water mixture. The water is removed and the remaining soap is used as a thickener for grease. The type of soap thickener will depend on which acids and bases are used in saponification. Some common compounds used are:

  • High molecular weight fatty acids: Stearic and 12 Hydroxy Stearic Acid (12 HSA)
  • Short chain complexing acids: Tallow, Azelaic, and Sebacic Acid
  • Most bases are a metallic hydroxide compound (i.e. lithium, calcium, etc.).

Types of Soap Thickeners

Simple Soap: This results from the reaction of one fatty acid and a metallic hydroxide. The most common soap, lithium soap, is produced

Types of Soap Thickeners – Source: NYE Lubricants

with 12 HSA and lithium hydroxide. The metallic hydroxide defines the thickener and other types besides lithium can be used.

Mixed Soap: Less common than simple soap, mixed soap is created in similar fashion as simple soap. However, the “mixed” characteristic is derived from mixing multiple metallic hydroxide compounds with a fatty acid. A common mixed soap is Ca/Li soap, which is made with calcium hydroxide and lithium hydroxide.

Complex Soap: Like simple soaps, complex soaps use a single metallic hydroxide. In order to create the complex-thickened grease, a fatty acid is combined with a short chain complexing acid. The acid mixture is then combined with a metallic hydroxide to for a complex thickener. Lithium complex grease, the most popular in North America, is made with lithium hydroxide, 12 HSA, and azelaic acid. These thickener types have an advantage over simple soap because of better high-temperature properties.

Types of Non-Soap Thickeners

Urea: Also known as polyurea, these thickeners are a reaction product of di-isocyanate combined with mono and/or diamines. The ratios of the ingredients will determine the characteristics of the thickener. This classification includes diurea, tetraurea, urea-urethane and others. Since there are no metallic elements in polyurea grease, the grease is ashless and subsequently more oxidatively stable. Polyurea greases are the most popular non-soap grease today.

Organophilic Clay: Also referred to as organo clay or clay thickeners, these thickeners are mineral based usually made from bentonite, hectorite, or montmorillonite. The minerals are purified into a clay and treated to be compatible with organic chemicals. The clay is dispersed in a lubricant to form a grease. Clay greases have no melting point and are traditionally used in high-temperate greases (however the oil will oxidize quickly at elevated temperatures).

Other: Polyurea and clay thickeners are the most used non-soap greases, but there are some other specialty thickeners that are used. These include:

  • Teflon
  • Mica and silica gel
  • Calcium sulfonate
  • Polytetrafluoroethylene (PTFE)
  • Carbon blacks
NLGI Grades – Source: Noria

NLGI Classifications

In addition to composition, the other key classification for grease is quite obvious: thickness. Defined as consistency, a grease’s consistency is its resistance to deformation by applied force. This is measured by penetration. A standard test, specifically ASTM D217, measures cone penetration after five (5) seconds for a grease at 77 F (25 C). The unit of measure is tenths of a millimeter and the NLGI classifies grease based on its penetration. The range of grades is 000 to 6. See the chart to the left for a full breakdown NLGI grades.

Most greases today fall in between the 1 and 3 grades with NLGI 2 being the most common. High penetration greases such as 00 and 0 can be used in central systems and colder environments.

Selecting the Appropriate Thickener and Grade

The right grease could vary greatly depending on your application, operating environment, and other factors. High temperature environments may require firmer (higher NLGI grade) greases and certain thickeners with high-temperature properties. It is best to consult OEM guides or speak with your grease manufacturer or distributor to get a recommendation.

Switching and mixing greases could either prove to be extremely costly. Most thickeners do not mix together and there are specific greases that are not compatible with others. It is recommended to match “like-for-like.” If you plan to make a switch, it is best to completely drain your equipment before applying new grease.

What is Viscosity Index?

While researching lubricants, there are many factors to consider in selecting a lubricant: viscosity, flash point, pour point, and oxidation stability. Viscosity is the most important parameter since the viscosity grade can be the difference between optimal performance and machine breakdown. However, the ISO Viscosity Grade (VG) is determined at 40⁰C and will fluctuate depending on operating temperature. Viscosity index is a measure of how much the viscosity will change as temperature rises or falls.

Viscosity Index Explained

Viscosity requirements are based on things such as: component design, loads, and speed. Machine recommendations do not account for operating temperatures and temperature ranges. Therefore, it is imperative to take into account average operating temperature when selecting a viscosity. To account for changing temperatures, the viscosity index was developed to measure viscosity stability as temperatures change. Viscosity index is a unit-less number that is derived by measuring a fluid’s viscosity from 40⁰C to 100⁰C.

The higher the viscosity index, the greater the stability of the lubricants viscosity. As shown in the chart below, the difference in viscosity index could greatly affect lubricant viscosity and performance:

Source: Machinery Lubrication, Noria

As temperatures move towards extreme highs and lows, the difference in Oil A and Oil B is magnified. Oil B, which has a VI of 150, maintains a viscosity closer to its ISO VG of 150 as temperatures rise and fall. On the other hand, Oil A fluctuates much more and could adversely affect performance at extreme temperatures. If your operation will have fluctuating loads, speeds, temperatures, etc., it is imperative to select a lubricant with a higher viscosity index.

Viscosity indexes, which can be found on most product data sheets, typically range from 90 to 160, but can exceed 400 and be as low as -60. The viscosity index can also give insight into the type of base oil and its quality. More refined mineral oils and synthetics will have higher VIs than lower quality base oils. Some products may include viscosity-index improver additives to help stabilize the lubricant in extreme conditions. VI-improver additive molecules adopt a coil shape in cold temperatures and have little effect on viscosity. In higher temperatures, the molecules uncoil and thicken the oil to stabilize viscosity. However, it is important to note that oils with VI-improvers will see permanent loss of VI and viscosity over time.

When Should You Opt for Higher VI

If your operations are going to have variable loads, variable temperatures, variable speeds, and other environmental variables, it is important to select a lubricant with a higher viscosity index. As these variables change, so will the lubricants viscosity. Therefore, it is crucial to invest in a lubricant that will maintain an optimal viscosity across different operating conditions. Conversely, if your operation is fairly consistent, it may suit you to select a lubricant with a lower viscosity index in order to save money.

Some machines may not possess data to identify the optimum viscosity, which could be problematic as ISO viscosity grades are separated by 50% increments between grades (e.g. 46 → 68, 100 → 150). With such large increments, finding the precise optimal viscosity becomes even more difficult. This problem is magnified at lower temperatures, where differences in lubricant viscosity are much larger (as shown in the chart above).

Calculating VI

If you are unsure of a lubricants viscosity or viscosity index, there are online calculators available to help you. If you are unsure of a viscosity index, simply enter the viscosity at two different temperatures and it will return the viscosity index. If you are unsure of a viscosity at a given temperature, enter a known viscosity, known temperature and viscosity index to find the desired temperature to find the new viscosity.

Key Takeaways

In conclusion some of the key reasons to have a lubricant with a higher viscosity index include:

  • Optimal operating viscosity is unknown
  • Varying operating temperatures and/or extreme operating temperatures
  • Other operating variables such as speed and load
  • You want to increase energy efficiency
  • You want to extend oil service and machine service life

Most of these involve improving performance that may be adversely affected by operating uncertainties. In these instances, it is ideal to opt for lubricants with higher VI. In the following instances, using cost-effective lower VI lubricants may prove beneficial to your bottom line:

  • Constant speeds and loads
  • Operating temperature remains the same
  • Optimal viscosity is known and can be consistently reached

If there is more certainty with your operating process, it may not be necessary to invest in a lubricant with a higher VI. It is important to evaluate your operating processes and consult machine manuals to understand your operating conditions. If you are faced with uncertainty and variance in your operations, a higher viscosity index will help smooth operations and increase performance across different loads, speeds, and temperatures.