Grease Technology - Part I


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Iowegia - USA
Grease Technology - Part I by MolaKule History A Lubricant's main purpose is to reduce the frictional burden that is found in machines. Ancient engineers recognized that certain natural oils and fats reduced friction and rolling resistance when applied to them. This ancient practice is found in the archeological record and indicates that the first lubricants were olive oils or animal fats mixed with various inorganic fillers to reduce the friction of rolling stones over planks, and for reducing the frictional forces of chariot wheels. In fact, the first calcium soap grease was a mixture of animal fats (today we know them as mixed esters of glycerol), calcium oxide, and lime. Frictional heating would cause a chemical reaction between the lime and calcium oxide producing an in situ saponification action. In even earlier times, spear points were greased to allow further penetration into wooly mammoth hides. How many times have you seen a carpenter apply a soap or a grease to a nail or screw to reduce driving friction? Here we address greases for automotives applications. General Any oil that is mixed with either organic or inorganic fillers is called a grease. A grease is a semisolid lubricant that stays put where other methods of lubrication would fail. Modern greases contain one or more fillers, a base oil, additives, and an optional tackifier, the latter makes the grease sticky and increases adherence. The filler holds the oil in its interstitial spaces, but it is the oil in the grease that actually lubricates. Oils for Greases Oils for greases may be either mineral or synthetic and of various viscosities, depending on the target application. Any synthetic oil from diesters to alykylated cyclopentanes may be used, with poly-alpha-olefins being the most used oil today for synthetic greases. In reality, most oils for greases are mixtures of mineral oils, synthesized hydrocarbons such as PAO or alkylated cyclopentanes, and esters. Some of the same oil types and oil mixture formulas used for gear lubes are also used for greases, since many of the EP additives are the same. Thickeners While it is the oil that characterizes the type, it is the thickener that determines the identity of the grease. Most thickeners are made of metallic soaps or alkali metals of Calcium, Lithium, Sodium, or Aluminum. For example, a lubricating grease prepared from a PAO oil that is thickened with lithium 12-hydrostearate (LiOH.H2O) would be referred to as a lithium-soap thickened synthetic hydrocarbon. An ester thickened with an organomodified clay would be described as a clay-based synthetic ester*. A PAO-based grease thickened with an Aluminum complex thickener of aluminum hydroxide [Al(OH)3] would be called an "Aluminum Complex" synthetic grease. Some greases are known by the additives in the grease. For example, if one adds molybdenum sulfide powder (MoS2) to a PAO/ester or PAO/alkylated cyclopentane oil, and the thickener for the grease is aluminum hydroxide, the grease might be known as a "Synthetic-based Moly Fortified Aluminum Complex Grease." Thickener types are Metallic Soap Thickeners, Complexed Metallic Soap Thickeners, and Non-Soap Thickeners. The Non-Soap thickeners are organo-clay, polyurea, and PTFE. The Metallic Soap Thickeners are: - Aluminum Soap - Hydrated Calcium Soap - Sodium Soap - Lithium Soap Complex Metallic Soap Thickeners gives the grease a higher temperature boost with better oxidation and high drop points. The Metallic Soap Thickeners are: - Aluminum Complex - Calcium Complex - Barium Complex - Lithium Complex The Non-Soap thickeners are organo-clay, polyurea, and PTFE. The organo-clay greases requires thorough dispersion of the oil in the clay and require a polar dispersant to force the grease to gel. Organo-clay greases are used where temperatures hit 500 F, such as ovens, steel rolling mills, etc. Of course, the base oil in this case should be a synthetic oil of viscosity between ISO 220 and 680. Polyurea Greases are made by mixing oils with suitable amines and isocyanates or diisocynanates. Polurea greases are also good high temperatures greases. These greases are NOT currently compatible with any other grease. They do perform well in low shear applications such as ball bearings, but not as well in high-shear applications such as roller bearings. PTFE (Teflon) powder does made a good grease thickener. In part two we will examine the advantages and disadvantages of the general grease types, illustrate how greases are made, tested, and classified according to the National Lubricating Grease Institute (NLGI). [ September 30, 2003, 01:37 PM: Message edited by: MolaKule ]
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