The basics of lubricating grease and in-use testing
Written by Amy Rishell and John Sander
Grease
Grease is used in many different industries, including automotive, chemical, aircraft, manufacturing plants, power generation, and water purification plants. It is used in a variety of applications, including couplings, chains, bearings, gears and sliding surfaces. Grease has to withstand a lot of abuse and many different environments.
As defined by ASTM International, grease is a semifluid-to-solid dispersion of a thickener in a liquid lubricant. The dispersion of the thickener forms a two-phase system and immobilizes the liquid lubricant by surface tension and other physical forces. Other ingredients imparting special properties are often included (1).
Grease is usually characterized by its thickener composition, base fluid type and additive content. The following will provide a brief overview of the common base fluids, thickeners and additives used in grease formulations.
Base Fluids
Grease is often described as synthetic or nonsynthetic. This description generally refers to the type of base fluid used as a component of the grease. Lubricating grease base fluids are usually categorized into two broad categories – mineral oil or synthetic. Each of these categories can be broken down into subcategories.
Subcategories of mineral oils are paraffinic and naphthenic. Both of these types of mineral oil are derived through refining crude oil. The major difference is the level of hydrocarbon saturation. Paraffinic oils are more saturated than naphthenic base oils. This means that they are composed of more straight chain hydrocarbons and less aromatic and naphthenic ring structures in their molecular make-up. Many have found the solvency properties of the naphthenic oils to be superior to those of paraffinic oils, resulting in more stable grease with less base fluid bleed. However, paraffinic oils have generally been found to provide superior oxidation resistance and seal compatibility than naphthenic oils and are often used in the formulation of greases for which these properties are important
Numerous categories of synthetic base fluids exist, but some of the most common types used in grease formulas are polyalphaolefins (PAO), polyisobutylenes (PIB), esters, polydimethylsiloxanes, and perfluoroethers. PAO and esters are the most commonly used of these synthetic base fluids. These fluids are generally used to provide increased performance in applications that are exposed to the extremes of temperature or highly oxidative environments. The downside to the synthetic base fluids as compared to the mineral oils is their higher cost. For many applications, the extra cost may not be necessary as the mineral oil performance would suffice. The higher cost must be balanced against the performance.
Thickeners
Three broad categories of thickener systems used in grease formulations are insoluble solids, polymers and soaps. The first category uses insoluble powders that are introduced into the base fluid under high shear conditions until they are thoroughly dispersed and thickened into grease without any real chemical reaction. These formulations could be likened to the childhood favorite, the "mud pie". Common solids include organo-clay, fumed silica, carbon black, and different types of pigments (2).
The next thickener category is polymers, such as polyurea and polytetrafluoroethylene. Polymer-thickened greases are composed of a base fluid and a polymeric material with gel-forming capability properly dispersed to produce the desired thickness of grease. The thickener is a low-molecular weight organic polymer that is usually formed in situ in the base fluid and yields no byproducts that must be removed (such as the water or alcohols produced in the formation of soap greases) (2).
The final, but most common, category of grease thickener is soap. The term "soap" is derived from the saponification reaction that results in the thickener. Saponification is depicted in the following general reaction:
MOH + ROOH → MOOR + HOH
In this reaction, a metal hydroxide (base) is reacted with carboxylic acid to form a metal carboxylate salt (soap) and water. When this reaction is carried out in a lubricating fluid, it results in the formation of a crystalline soap dispersed in the fluid (2). From this soap base, performance additives and the appropriate amount of additional oil are added to form a semi-solid or gel-like material.
Most soap-type greases are sheared, or "milled", to form a consistent thickness throughout the material. The soap category of greases, along with the base fluid selection, will be discussed further in the section "Applications by Thickener Type".
Additives
In most cases, the grease base fluid and thickener do not possess all of the properties necessary to provide maximum performance in the applications in which they will be used. To enhance their properties, additives are incorporated. Many specific additives can be employed, but for the purpose of this text, general classes of additives will be discussed. They are:
• Antioxidants – As the name would suggest, antioxidants are incorporated to protect the grease from oxidative breakdown. Antioxidants allow the grease to last longer in use by preventing degradation of the fluid over time.
• Anti-wear and extreme pressure (EP) additives– Anti-wear additives are added to decrease the rate of wear by formation of sacrificial film to prevent direct metal-to-metal contact under light to moderate loads. Like anti-wear additives, EP additives also form a protective coating on a metal surface, but the coating chemistry can prevent damage to contacting surfaces under pressure conditions where anti-wear additives are inadequate.
• Corrosion inhibitors – These additives are added to enhance the grease's ability to protect metal surfaces against corrosive attacks by rust or surface active additives that are incorporated into the grease and can be aggressive toward certain metal types.
• Tackifiers – Another common additive is a tackifier, which makes grease stickier, thus providing increased adhesion to metal surfaces under conditions of high impact and water contact.
The components that make up a grease formulation play an important role in how it is characterized. The previous discussion should provide a guide to help a user better understand grease types and how to choose the proper grease for a given application.
