Brief Introduction of Lubricant Additive Detergent
Modern equipment must be lubricated to prolong its lifetime. A lubricant performs a number of critical functions. These include lubrication, cooling, cleaning and suspending, and protecting metal surfaces against corrosive damage. Lubricant comprises a base fluid and an Additive Package . The primary function of the base fluid is to lubricate and act as a carrier of additives. The function of additives is either to enhance an already-existing property of the base fluid or to add a new property. The examples of already-existing properties include viscosity, viscosity index, pour point, and oxidation resistance. The examples of new properties include cleaning and suspending ability, antiwear performance, and corrosion control. The extent of the desirability of various properties differs from lubricant to lubricant and largely depends on the conditions of use. Automotive use, for example, requires lubricants with good oxidation resistance, suitable low and high temperature viscosities, high-viscosity index (i.e., minimum loss in viscosity with an increase in temperature), and good cleaning and suspending ability. Conversely, the use as nonautomotive lubricants, such as industrial and metalworking lubricants, emphasizes oxidation resistance, antiwear performance, corrosion control, and cooling ability.
One of the most critical properties of the automotive lubricants, especially engine oils, is their ability to suspend undesirable products from thermal and oxidative degradation of the lubricant. Such products form when the by-products of fuel combustion, such as hydroperoxides and free radicals, go past piston rings into the lubricant and, being reactive species, initiate lubricant oxidation. The resulting oxidation products are thermally labile and decompose to highly polar materials with a tendency to separate from the bulk lubricant and form surface deposits and clog small openings. The former will lead to malfunctioning of the closely ftted surfaces, such as those between pistons and cylinder walls, and the latter will impair oil flow to parts needing lubrication. The separation tendency of these products relates to their high polar to nonpoplar ratio, which makes them less soluble in largely nonpolar Base Oil . A lubricant with high-oxidation resistance, due to the quality of the base fluid or the presence of a good oxidation inhibitor additive package, will slow down the formation of these undesirables.
Oxidation inhibitors, detergents, and dispersants make up the general class of additives called stabilizers and deposit control agents. These additives are designed to control deposit formation, either by inhibiting the oxidative breakdown of the lubricant or by suspending the harmful products already formed in the bulk lubricant. Oxidation inhibitors intercept the oxidation mechanism, and dispersants and detergents perform the suspending part. Detergents are the topic of this chap ter, and dispersants are the topic of the subsequent chapter. Detergents are metal salts of organic acids that frequently contain associated excess base, usually in the form of carbonate. Dispersants are metal-free and are of higher molecular weights than detergents. The two types of additives work in conjunction with one another.
The fnal products of combustion and lubricant decomposition include organic and inor ganic acids, aldehydes, ketones, and other oxygenated materials. The acids have the propensity to attack metal surfaces and cause corrosive wear. Detergents, especially basic deter gents, contain reserve base that will neutralize the acids to form salts. Although this decreases the corrosive tendency of the acids, the solubility of the salts in the bulk lubricant is still low. The organic portion of the detergent, commonly called soap, has the ability to associate with the salts to keep them suspended in the bulk lubricant. However, in this regard, detergents are not as effective as dispersants because of their lower molecular weight. The soap in detergents and dispersants also has the ability to suspend nonacidic oxygenated products such as alcohols, aldehydes, and resinous oxygenates. The mechanism by which this occurs is depicted in Figure.
Dispersants and detergents together make up the bulk, ∼45 to 50%, of the total volume of the lubricant additives manufactured. This is a consequence of their major use in engine oils, transmis sion fluids, and tractor hydraulic fluids; all of which are high-volume lubricants. As mentioned earlier, detergents neutralize oxidation-derived acids as well as help suspend polar oxidation products in the bulk lubricant. Because of this, these additives control rust, corro sion, and resinous buildup in the engine. Like most additives, detergents contain a surface-active polar functionality and an oleophilic hydrocarbon group, with an appropriate number of carbon atoms to ensure good oil solubility. Sulfonate, phenate, and carboxylate are the common polar groups present in detergent molecules. However, additives containing salicylate and thiophos phonate functional groups are also sometimes used.
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