The selection of greases that can withstand extreme temperature conditions poses challenging lubrication engineering decisions, according to Molykote, a business unit of Dow Corning Corp. in Midland, Mich.

Molykote’s Smart Lubrication Series of enewsletters gives advice for anyone requiring greases in their industrial process. The fall enewsletter notes that in addition to heat resistance, greases in an application often require additional properties like exceptional load-carrying capability, oxidation stability, water wash-out resistance and good pumpability.

The first step towards selecting the proper high temperature grease is to identify the possible causes of product degradation during operation. Though the primary cause of degradation is thermal stress, contact with air, if it occurs, 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, according to Molykote.

To a large extent, the properties of a grease are determined by its three components: base oil, thickener and additive package:
  • Base Oils. These can be subdivided into mineral and synthetic. Mineral oils are the most widely used base-oil components, representing approximately 95 percent of the greases 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, which provide the best thermal and oxidative stability. Highly refined mineral and synthetic hydrocarbon base oils typically start to degrade between 230 and 302°F (110 and 150°C). For operation at still higher temperatures, ester silicones, fluorosilicones or more highly fluorinated base oils such as PFPE are recommended.

  • Thickeners. Three different types of thickeners can be used in a grease formulation. They are organic (polyurea), inorganic (bentonite, silica) and soap/complex soap (lithium, calcium, sodium etc.). 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 230 to 392°F (120 to 200°C).

  • 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 such as 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.
Compatibility or incompatibility between high temperature greases must be addressed prior to their selection and application. Because the total grease formulation is a complex but balanced mixture of its components, any addition of unplanned chemicals can upset the well-defined balance and lead to degradation of performance levels. The lack of grease compatibility is revealed as grease consistency changes - higher or lower. With specially designed high temperature grease products, the importance of not mixing greases becomes more pronounced.

The answers to specific questions about your application will help you determine which grease is the one for your job. Ask yourself:
  • 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.

  • What is the real temperature range at which the equipment will operate? Does it exceed 302°F (150°C)?

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

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

  • How long are re-lubrication intervals and how hard is re-lubrication? If re-lubrication is difficult, a premium grade grease product should be chosen to achieve lower maintenance costs, which generally offsets the higher price of the premium, upgraded lubricant.

Grease formulation is a complex but balanced mixture of components, and any addition of unplanned chemicals can upset it, resulting in performance degradation, according to Molykote. 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. And to select the appropriate grease for any application, the end-user should ask himself equipment-use questions designed to point him to the right lubricant.