Selecting High-Temperature Greases

High-temperature greases, as the name describes, are designed to maintain their lubricating properties at high operating temperatures. By virtue of the extreme conditions they operate under, these greases are very complex mixtures. Fortunately, identifying the product that best suits the application is fairly straightforward.

The selection of greases that can withstand extreme temperature conditions poses some challenging lubrication engineering decisions. In addition to heat resistance, the greases in use often require additional properties like exceptional load-carrying capability, oxidation stability, water washout resistance and good pumpability.

The first step towards selecting the right hightemperature grease is to identify the possible causes of product degradation during operation. Though the primary cause of degradation is thermal stress, contact with air leads to oxidative failure, which also contributes to the degradation.

Once the sources of degradation have been determined, aspects such as viscosity, viscosity index, stability of the composition and the additive package properties also need to be considered.

To a very large extent, the properties of a grease are determined by its three components, the base oil, the thickener and the additive package.

Base oils: Base oils can be subdivided into mineral and synthetic base oils. Mineral oils are the most widely used base oil components and represent approximately 95% of the grease manufactured. Solvent-refined mineral oils have a high percentage of unstable, unsaturated molecules that tend to promote oxidation. Mineral oils that have undergone extensive processing to remove the reactive molecules or saturate the molecules (with hydrogen) exhibit improved stability (oxidative and thermal).

Combining two or more smaller hydrocarbons to larger molecules produces synthetic hydrocarbon base oils. These base oils provide the best thermal and oxidative stability. Highly refined mineral and synthetic hydrocarbon base oils typically start to degrade at between 110C and 150C. For operation at still higher temperatures, ester silicones or fluorosilicones or more highly fluorinated base oils such as PFDF are recommended.

Thickeners: Three different types of thickeners can be used in a grease formulation: Organic (polyurea), inorganic (bentonite, silica) and soap/complex soap (lithium, calcium, sodium, etc.) thickeners. However, the usefulness of the grease over time depends on the stability of the whole formulation package and not just the stability of the thickener.

Polyureas have additional inherent antioxidant and anti-wear properties. Lithium complex thickeners have maximum temperature limits superior to those of simple lithium soap thickened greases. Overall, metal ion soap thickeners have thermal degradation limits that range roughly from 120C to 200C.

Additives: The additive must be capable of working synergistically with the thickener and the oil, resulting in a balanced, stable mixture of the three components. The additive package also introduces desirable properties like oxidation resistance, corrosion protection and wear resistance to the grease.

A critical issue that must be addressed prior to the selection and application of greases is the possibility of different greases mixing, and more importantly, their compatibility.

Grease compatibility

Compatibility or incompatibility between hightemperature greases must be addressed prior to their selection and application. Since the total grease formulation is a complex but balanced mixture of its components, any addition of unplanned chemicals can upset this well-defined balance and lead to degradation of performance levels.

Mixing incompatible greases results in an increase or decrease in grease consistency. Therefore it is very important that specially designed high-temperature grease products not be mixed.

This article is adapted from Dow Corning’s Molykote Smart Lubrication Series. For more information, send an e-mail to industrial@dowcorning.com,or use the reader service number below.

Reader Service Card No. 411

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Criteria for selecting a high-temperature grease

1. Is operation of the equipment requiring lubrication continuous or intermittent? If it is continuous, a premium grade product that meets the operational requirements should be selected.

2. What is the real temperature range at which the equipment will operate? Does it exceed 150C?

3. Does the equipment go through heating and cooling cycles at operating and non-operating intervals?

4. Is the lubricant exposed to moisture during operation? Moisture upsets the thermal stability of the grease formulation.

5. How long are relubrication intervals and how hard is relubrication? If relubrication is difficult, a premium-grade grease product should be chosen to achieve lower maintenance costs (generally, lower maintenance costs more than offset the higher price of the upgraded lubricant).