High-Temperature Greases
Noria Corporation
Tags: greases, grease compatibility
T here are many criteria to consider when selecting a hightemperature 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-Temperature
‘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.
Selection Strategies
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
Base Oils
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.
Thickeners
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 soapthickened 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.
Additives
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 welldefined 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 hightemperature grease products, these issues can become more pronounced. Many of the
exotic fluids used in very high-temperature greases (fluorinated polyethers, perfluropolyethers, 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 watersoluble 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
1. Determine the real temperature range. The operating temperature may be less
than what it seems. Use a contact or noncontact sensor to measure the operating
temperature of the grease. Does it exceed 392ºF (200ºC)?
2. Is it intermittent or continuous? If it is continuous, then look for a top-tier product
that meets the operational requirements.
3. Do heating and cooling cycles accompany machinery operating and nonoperating
intervals? Consider if moisture may be induced through either atmosphere or
impingement.
4. What is the reasonable relubrication interval or opportunity? If relubrication is
going to be difficult, then consider a top-tier product to achieve a lower use cost
even though it’s more expensive.
5. Consider any cosmetic issues. Can the product drip onto a component in process?
Relubrication frequency and volume must be balanced against product
contamination issues.