Thickners Used For Formulating Grease

Thickeners transform the oil into grease & control many of the important properties & performance characteristics of the grease. Developments in thickeners have been fundamental to the advances in grease technology. The contribution of thickeners has been so central to developments that many greases are often classified by the type of thickener used to give the structured matrix and consistency.
The two principal groups of thickeners are metal soaps and inorganics. Soap based greases being by far the most widespread.

Simple Soaps The earliest greases were made by reacting lime with vegetable oils, or animal fats, in the presence of water, to produce a Calcium soap of the natural fatty acid. The resulting thickened oil was adequate for simple lubrication tasks such as cartwheel and waterwheel shafts and bearings. Sodium soaps, which are very similar to domestic soap used for washing, were found to have higher melting points than calcium soaps. Greases based on sodium soaps were fundamental in the lubrication of steam engines and the early machinery of the industrial revolution. Aluminium soap greases were developed at the same time as sodium greases as engineers searched for improved lubrication for steam engines.

Lithium Soap
Catalysed principally by developments in the aircraft industry during the late 1930's, the introduction of lithium based greases in the 1940’s came as a very significant step forward. Early lithium soaps were made from simple stearic acid, derived principally from beef tallow. Today, almost all lithium greases are based on the castor oil derivative, 12-hydroxy stearic acid. The capabilities of lithium grease include excellent mechanical stability, good water resistance and reasonably good high temperature performance, up to 120ºC. During the second half of the 20th century, lithium greases replaced the earlier greases in the great majority of applications.

Complex Soaps
Derivatives of a single metal reacted with a combination of different types of acids can be crystallised into the same fibrous thickener structure. The principle advantage of complex greases is their ability to withstand high operating temperatures. Aluminium Complex greases have dropping points above 240ºC, are highly water resistant & offer good lubricating properties at high temperatures. Furthermore, a low thickener concentration can provide the required grease consistency and this leads to excellent pumpability.
Calcium Complex found favour due to their load carrying and extreme pressure capabilities.Their high temperature performance was also found to be similar to aluminium complex greases with dropping points above 250ºC. Their extreme pressure capability is provided by integral components of the co-crystallised soap structure; calcium acetate, calcium sulphonate or similar calcium derivatives are crystallised into the thickener structure. In contrast to aluminium complexes, a very high soap content is required to provide a given grease consistency and this can detract from pumpability. More recent Calcium sulphonate complexes also offer excellent water resistance, coupled with good high temperature performance and load carrying capacity.

Lithium Complex
Over the past decade, lithium complex greases have been finding increasing popularity. With dropping points above 300ºC, this type of grease can withstand peak operating temperatures of up to 240ºC for short periods of time. Continuous temperatures of 150ºC should not present a problem for grease formulated with mineral oils; service temperatures of up to and in some cases exceeding 200ºC are possible using synthetic base stocks. This excellent high temperature performance results from a high melting point coupled with the very dense fibre structure of the soap matrix. Very small polar fibres strongly bind the base lubricant providing excellent protection against oxidation due to low heat transfer throughout the matrix. Li & Li-complex greases account for over 70% of grease produced.

Inorganic and Other Thickeners
In non-soap type greases inorganic, organic & synthetic materials are used as thickeners. Inorganic thickeners are in the form of very fine powders which have enough porosity & surface area to absorb oil. The powders must be evenly dispersed in the grease so either high-shear mechanical mixing or some special dispersing additives are required for the grease formulation.

Clays
Fine clays, particularly bentonite clays, were used in grease formulations from the beginning of the 20th century, primarily in an attempt to improve high temperature performance. The use of clay as the gelling agent results in a grease that does not melt or drop at high temperatures. However, the lack of a fibrous matrix structure does limit the stability of clay based greases. Furthermore, the deleterious effects of oxidation can still occur in the base oil at elevated temperatures. Oil oxidation and separation can result in a residue of abrasive clay being deposited on the machine surfaces. Silica Gel imparts properties similar to a clay, in grease formulations.

PTFE
Inert base fluids, such as perfluoropolyethers, are used as lubricants in aggressive environments and require a PTFE thickener that is equally inert. Applications for this type of grease include contact with aggressive solvents and strong acids and alkalis. PTFE greases operate well under low pressure, such as in vacuum pumps and high speed bearings in vacuum environments (space).

Polyurea
This special polymer thickener system is normally a reaction product between different types of iso-cyanates and amines. Polyurea greases exhibit extremely good high temperature performance and have, in many cases, become the preferred choice for filled-for-life applications in both bearings and joints. On the other hand, drawbacks such as poorer performance at ambient temperatures and the toxic nature of raw materials have limited their development into a more multipurpose product.

Polyethylene
Although not very common, polyethylene and other similar polymers and waxes are used in very specific applications. In circumstances where very high centrifugal force or very rapid acceleration prevail, a traditional matrix grease fails. Separation of the thickener and base fluid components occurs rapidly, immediately reducing lubricating ability. Polyethylene can be produced with a density very close to that of the chosen mineral base oil and separation does not occur. High speed flexible couplings are lubricated with this type of grease.

Polypropylene
Another innovative type of grease introduced into the market is based on a polymer (polypropylene) thickener with an optimised crystalline-amorphous balance. This type of grease offers many advantages over standard multipurpose greases: Controlled oil bleed, extended service life, resistance to water and aggressive chemicals, enhanced additive response and not least a high film thickness in the track (efficient surface separator).