T here are many criteria to consider when selecting a high-temperature grease for hot, grease-lubricated equipment. The selection must include consideration of oil type and viscosity, oil viscosity index, thickener type, stability of the composition formed by the oil and the thickener), additive composition and properties, ambient temperature, operating temperature, atmospheric contamination, loading, speed, relubrication intervals, etc. With the variety of details to resolve, the selection of greases that must accommodate extreme temperature conditions poses some of the more challenging lubrication engineering decisions.
Given the variety of options, the potential for incompatibility problems and high prices for a given high-temperature product, the lubrication engineer must be selective and discriminating when sourcing products to meet high-temperature requirements.
‘High’ is relative when characterizing temperature conditions. Bearings running in a steel mill roll-out table application may be exposed to process temperatures of several hundreds of degrees, and may experience sustained temperatures of 250ºF to 300ºF (120ºC to ±150ºC). Automotive assemblers hang painted metal parts on long conveyors and weave them through large drying ovens to dry painted metal surfaces. Operating temperatures for these gas-fired ovens are maintained around 400ºF (205ºC).
In these two cases, the selection criteria differ appreciably. In addition to heat resistance, the grease to be used in a hot steel mill application may require exceptional load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a price suitable for large-volume consumption. With all of the important factors to consider, it is useful to have a grease selection strategy.
A reasonable starting point for selecting a high-temperature grease is to consider the nature of the temperatures and the causes of product degradation. Greases could be divided by temperatures along the lines in Table 1.
There is general correlation between a grease’s useful temperature range and the expected price per pound. For instance, a fluorinated hydrocarbon-based (type of synthetic oil) grease may work effectively as high as 570ºF (300ºC) in space applications but may also cost hundreds of dollars per pound. The grease’s long-term behavior is influenced by the causes of degradation, three of which are particularly important: mechanical (shear and stress) stability, oxidative stability and thermal stability. Oxidative and thermal stresses are interrelated. High-temperature applications will generally degrade the grease through thermal stress, in conjunction with oxidative failure occurring if the product is in contact with air. This is similar to what is to be expected with most industrial oil-lubricated applications.
Recipe for a High-Temperature Grease
When selecting lubricants for oil-lubricated applications, one often begins with the consideration of base oil performance properties. This is also a good starting point for grease products. Grease is composed of three components: the base oil, the thickener and the additive package. There is a variety of options from which the manufacturer creates the final product. Table 2 includes some of these options. 1
Base oils can be subdivided into mineral and synthetic types. Mineral oils are the most widely used base oil component, representing approximately 95 percent of the greases manufactured. Synthetic esters and PAO (synthetic hydrocarbons) are next, followed by silicones and a few other exotic synthetic oils. 2
The American Petroleum Institute divides base oils into five categories that are useful in initially selecting base oil by performance limits.
The Group I products are naphthenic and solvent-refined paraffinic petroleum stocks with a high percentage of unstable ‘unsaturated’ molecules that tend to promote oxidation. Additionally, there are polar products that remain in the Group I base oils called heterocycles (nitrogen, sulfur and oxygen- containing molecules). Although the polar products are reactive, they help to dissolve or disperse additives to produce the final product.
The Group II and Group III are mineral oils that experience extensive processing to remove the reactive molecules and saturate (with hydrogen) the molecules to improve stability. In a sense, these base oils are more like the Group IV synthetic hydrocarbons (PAOs) than the Group I mineral oils. The oxidative and thermal properties can be very good as a consequence of the removal of the reactive heterocyclic molecules.
The Group IV synthetic hydrocarbons (SHC fluids) are produced by combining two or more smaller hydrocarbons to synthesize larger molecules. These fluids may have slightly better stability, but command a higher price. The Group V base oils have a defined but different degradation path (not primarily thermal or oxidative).
Mineral and synthetic base oils degrade thermally in conjunction with oxidative degradation if the product is in contact with air. The break point at which the individual oil molecules in a highly refined (Group II+, Group III) mineral oil and synthetic hydrocarbons will begin to unravel, releasing carbon atoms from the molecular chain, is about 536ºF to 608ºF (280ºC to 320ºC). 3,4 The grease manufacturer will select materials given their familiarity, and perhaps availability, of the raw materials. If the manufacturer makes a particular type of synthetic base fluid and is intimately familiar with the various destruction mechanisms of that fluid, then it is likely that this type of synthetic base will often be selected for new product development.
The materials selected as the grease thickeners may be organic, such as polyurea; inorganic, such as clay or fumed silica; or a soap/complex soap, such as lithium, aluminum or calcium sulfonate complex. The usefulness of the grease over time depends on the package, not just the thickening system or the type of base oil. For instance, silica has a dropping point of 2,732ºF (1,500ºC) as one extreme example. 5 However, because grease performance depends on a combination of materials, this does not represent the useful temperature range. Some clay-thickened (bentonite) greases may similarly have very high melting points, with dropping points noted on the product data sheets as 500ºC or greater. For these nonmelting products, the lubricating oil burns off at high temperatures, leaving behind hydrocarbon and thickener residues.
The organic polyurea thickener system offers temperature range limits similar to the metal soap-thickened grease, but additionally it has antioxidation and antiwear properties that come from the thickener itself. Polyurea thickeners might become more popular but they are difficult to manufacture, requiring the handling of several toxic materials. While the thickener has a high dropping point, the composition begins to thermally degrade at temperatures which limit its usefulness over time at high temperatures. However, it does not have the pro-oxidant tendencies of the metal soap-thickened greases. The exception is the calcium sulfonate complex thickener system. Similar to the polyurea, it possesses inherent antioxidant, rust-inhibiting properties, but in addition has inherent high dropping points and EP/antiwear properties.
The third category option is the metal soap or complex soap thickener system. Lithium complex-thickened grease has maximum temperature limits superior to that of simple lithium grease, because the thickener offers higher thermal degradation limits. Collectively, metal soap thickeners have thermal degradation limits that range between 250ºF to 430ºF (120ºC and 220ºC). 6 However, unless the grease composition is properly fortified against oxidation and thermal degradation, the end product showing a dropping point of 500ºF (260ºC) or greater would not be any more useful for long-term service than a grease with a low dropping point.
The additives selected for grease manufacture must likewise be viewed as parts of the whole rather than simply discrete parts that must withstand set test limits. The additives tend to provide properties for greases in fashion similar to lubricating oils: oxidation stability, corrosion resistance, wear resistance, low temperature flow characteristics, water resistance, etc. The additive must be capable of working synergistically with the thickener and the oil to lead to a balanced, stable mixture of the three distinct components.
High-Temperature Grease Compatibility
Compatibility, or incompatibility, between high-temperature greases must be addressed prior to selection. Because greases represent a complex mixture of chemicals with a well-defined and engineered balance, the addition of unplanned chemicals tends to upset the balance and degrade performance levels. Following the Arrhenius rate rule, chemical reactivity doubles for each 10ºC rise in temperatures, incompatibility issues are more pronounced at elevated temperatures. The lack of compatibility shows up as grease thinning. If thinning occurs, the user may relubricate to flush out the original product until the problem ceases. Alternatively, the user has a more difficult choice to make, requiring dismantling the equipment to remove the original product and cleaning the system. The thickeners, additives and base oils may each have problems at differing temperature ranges and time limits in use. Before converting major systems to a new grease, exhaustive testing may be warranted to prevent significant cost and time delay due to long-term maintenance problems.
While testing is warranted when changing between classes of thickeners, there is relatively less potential for problems occurring when switching within families of metal soap or complex soap-thickened products (lithium to lithium, lithium complex to lithium complex, aluminum complex to aluminum complex, etc.). Greases will generally soften when critical limits are reached (however hardening is also possible), a consequence of the matrix between the additive, oil and thickener becoming unstable and decomposing. It is difficult to determine exactly when the decomposition will occur, considering temperature and time line. When variables are introduced, such as a new mixture of chemicals (a result of grease mixing), it becomes more difficult to predict the outcome. This points to the importance of not mixing greases. With specially designed high-temperature grease products, these issues can become more pronounced. Many of the exotic fluids used in very high-temperature greases (fluorinated polyethers, perfluro-polyethers, phenal-polyethers, silicones, etc.) will last longer than their thickening systems.
If a particular grease component is sensitive to moisture, then regardless of the grease’s ability to withstand the heat alone, the use of the product must be weighed against the risk of process moisture degradation of the grease. It could be unwise to use a water-soluble glycol oil type of grease in an application that is subject to high moisture, such as a conveyor wash system. Even though the fluid may be capable of resisting thermal breakdown from the heat of the drying system, the moisture poses a performance risk that may not be entirely eliminated.
How does one know if an application warrants a special-performance, high-temperature product?
Because oils, additives and bases will react at various rates, there is something good to be said about using simpler products. Consider whether the application is intermittent or continuous high-temperature. If it is continuous - constant 392ºF (200ºC) or greater - then go with the higher-tier product after appropriate testing. If the temperature is intermittent, then a middle-tier product may be equally useful with appropriately adjusted relubrication intervals.
Selecting a High-Temperature Grease