Lubricant Properties and the Role of Additives

Lubricant Properties and the Role of Additives
Polymeric Dispersants
Oxidation and Bearing Corrosion Inhibitors
Anti-wear Additives
Viscosity Modifiers
Pour Point Depressants
Miscellaneous Additives

Lubricant Properties and the Role of Additives:

Some of the most important properties necessary for satisfactory lubricant performance are:

1. Low volatility under operating conditions. Volatility characteristics are essentially inherent in the choice of base oil for a particular type of service and cannot be improved by the use of additive materials.

2. Satisfactory flow characteristics in the temperature range of use. Flow characteristics largely depend on the choice of base oil; however, they can be improved modifiers. The former improve low-temperature flow properties, while the latter enhance high-temperature viscosity characteristics.

3. Superior stability or ability to maintain desirable characteristics for a reasonable period of use. While these characteristics depend to some extent on the base materials, which enhance base fluid properties in this area. Lubricant stability is affected by the environment in which it operates. Such factors as temperature, oxidation potential and contamination with water, unburned fuel fragments, and corrosive acids limit the useful life of a lubricant. This is the area where additives have made a major contribution in improving the performance characteristics and extending the useful life of lubricants.

4. Compatibility with other materials in the system. Compatibility of lubricants with seals, bearings, clutch plates, etc., may also be partially associated with the base oil. However, additive chemistry can have a major influence on such characteristics. Additives can be classified as materials that impart new properties to or enhance existing properties of the lubricant or fuel into which they are incorporated. (Lubrizol)

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Although these compounds are commonly called detergents, their function appears to be the dispersing of particulate matter rather than cleaning up existing dirt and debris. Therefore, it is more appropriate to categorize them as dispersants. The following is a brief outline of the representative types of metallic dispersants.

Sulfonates: Sulfonates are the products of the neutralization of a sulfonic acid with a metallic base. Commercially available sulfonates are of two types: petroleum sulfonates and synthetic sulfonates. Petroleum (or natural) sulfonates are metal salts of sulfonic acids that were formerly byproducts of the sulfuric acid treatment of oil fractions in the manufacture of white oils. Currently, with the high demand for detergent oils, sulfonates rather than white oils have become the principal product. The structure of the organic portion of petroleum sulfonates is not completely known. Depending on the crude oil source, the structure can have varying proportions of aliphatic, naphthenic, and aromatic hydrocarbon groups.

Synthetic sulfonates are metal salts of acids produced from the sulfonation of alkylated aromatics by reaction with sulfur trioxide. Most metallic cations of sulfonate detergents are calcium, magnesium, and sodium. Oil-soluble sulfonates containing metal in excess of the stoichiometric amount are called basic sulfonates. Among the advantages of basic sulfonates is a greater ability to neutralize acidic bodies in addition to serving as a dispersant for contaminants.

Salicylates: Salicylates are generally prepared from alkyl phenols by a chemical scheme known as the Kolbe reaction. The potassium salicylate may be metathesized with calcium chloride or magnesium chloride. The resulting salts are then overbased to form highly basic detergents that have proven effective in diesel engine oil formulations.

Phenates and Phenol Sulfide Salts: The broad class of metal phenates includes the salts of alkylphenols, alkylphenol sulfides, and alkylphenol aldehyde products. Oil solubility is provided by alkylating the phenol with olefins that generally contain seven or more carbon atoms. Sulfur is incorporated into the phenates by reacting the alkylphenol with sulfur chloride or elemental sulfur. The introduction of sulfur and the presence of a methylene bridge lowers the corrosivity of the products toward bearing materials and improves their antioxidant characteristics.

Thiophosphonates: Commercial products of this type are generally derived from acidic components produced by the reaction from acidic components produced by the reaction between polybutene (500 to 1000 molecular weight range) and phosphorus pentasulfide. However, these materials have almost vanished from use. (Lubrizol)

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A major development in the additive field was the discovery and use of ashless dispersants. These materials may be categorized into two broad types: high-molecular weight polymeric dispersants used to formulate multigrade oils and lower molecular weight additives for use where viscosity modification is not necessary. These additives are much more effective than the metallic types in controlling sludge and varnish deposits that result from intermittent and low-temperature gasoline engine operation. Compounds useful for this purpose are again characterized by a polar group attached to a relatively high molecular weight hydrocarbon chain. The polar group generally contains one or more of the elements nitrogen, oxygen and phosphorus. The solubilizing chains are generally higher in molecular weight than those in detergents; however, they may be quite similar in some instances. (Lubrizol)

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Polymeric Dispersants:

These ashless dispersants may serve the dual function of dispersant and viscosity modifier. They have two different structural features: those that are similar to materials employed as viscosity modifiers and those of polar compounds (which impart dispersancy). (Lubrizol)

Oxidation and Bearing Corrosion Inhibitors:

The function of an oxidation inhibitor is to prevent deterioration of the lubricant associated with oxygen attack. These inhibitors either destroy free radicals (chain breaking) or interact with peroxides involved in the oxidation mechanism. Among the widely used antioxidants are phenolic types and zinc dithiophosphates. The former are considered to be of the chain-breaking variety, whereas the latter are believed to be peroxide destroyers. The corrosion of bearing metal is generally considered of the bearing metal. In engine operation, these acids to be due largely to reaction of the acid with the oxides either originate from products of incomplete fuel combustion that find their way into the lubricant as blowby gases or are produced from lubricant oxidation. Oxidation inhibitors can significantly reduce this tendency. Detergents can reduce bearing corrosion by neutralizing the corrosive acids. Other inhibitors such as zinc dithiophosphates and phosphosulfurized olefins not only inhibit oxidation but also form a protective film on the bearing surface, making it impervious to acid attack.

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Phenolic Inhibitors (Chain-breaking):

The inhibitor efficiency of phenol is markedly increased by substituting alkyl groups in the two ortho and para positions. It is particularly enhanced when the ortho para substituent is a primarily alkyl group. A variety of hindered phenols are produced commercially and employed as inhibitors in transformer, turbine, and engine oils.

Zinc Dithiophosphates (Peroxide-destroying):

Of greatest commercial importance in engine lubricants are the zinc dithiophosphates, which not only serve as antioxidants but also provide both antiwear and bearing corrosion protection. Stability increases with the length of the alkyl chain and is lower for secondary alkyl groups with the same number of carbon atoms. It should be noted, however, that the overall performance characteristics of ZDPs are not related to the decomposition temperature. (Lubrizol)

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Antiwear Additives:

Wear is the loss of metal with subsequent change in clearance between surfaces moving relative to each other. If continued, particulate matter, and attack of corrosive acids can occur. Metal-to-metal contact can be prevented by adding film-forming compounds that protect the surface either by physical absorption or chemical reaction. Dithiophosphates are widely used for this purpose and are particularly effective in reducing wear in valvetrain mechanisms. These compounds are described under oxidation and bearing corrosion inhibitors. Other effective additives contain phosphorus, sulfur, or sulfur, or abrasive wear can be prevented by effective removal of particulate matter by filtration of both the air entering the engine and the lubricant during engine operation. Corrosive wear is largely the result of acidic blowby products formed during fuel combustion. This type of wear can be controlled by using alkaline additives such as basic phenates and sulfonates. (Lubrizol)

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Viscosity Modifiers:

Viscosity modifiers, or viscosity index improvers as they were formerly known, comprise a class of materials that improves the viscosity/temperature characteristics of the lubricant. This modification of rheological properties results in increased viscosity at all temperatures. The viscosity increase is more pronounced at high temperatures which significantly improves the viscosity index of the lubricant.

Viscosity modifiers are generally oil-soluble organic polymers. The polymer molecule in solution is swollen by the lubricant, and the volume of the swollen entity determines the degree to which the polymer increases viscosity. The higher the temperature, the larger the volume and the greater the "thickening" effect of the polymer. Hence, the oil tends to "thin" less due to increased temperature. In addition to viscosity improvement, the performance of these polymers also depends on shear stability or resistance to mechanical shear and on their chemical and thermal stability.

With a given polymer system, shear stability decreases with an increase in molecular weight. The loss due to shear is reflected in a loss in lubricant viscosity. On the other hand, the "thickening power" of the viscosity modifier increases with an increase in molecular weight for a given polymer type. A performance balance must then be established which takes into consideration shear stability and viscosity needs as well as thermal and oxidative stability in actual engine operation. (Lubrizol)

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Pour Point Depressants:

Pour point depressants prevent the congelation of oil at low temperature. This phenomenon is associated with crystallization of the paraffin wax that is present in mineral oil fractions. To provide low pour points, the refiner removes wax constituents, which solidify at relatively high temperatures, in a process known as dewaxing. Complete dewaxing would reduce the yield of lube oil to an uneconomical level. Therefore, the dewaxing process is supplemented by using additives that lower the pour point of the oil. Pour point depressants do not prevent wax from crystallizing in the oil. Rather, they are absorbed on the wax crystals and, thus, reduce the amount of oil occluded on the crystal. Reducing the crystal volume permits lubricant flow. (Lubrizol)

Miscellaneous Additives:

This category includes antirust compounds and foam inhibitors. Chemicals employed as rust inhibitors include sulfonates, alkenyl succinic acids, substituted imidazolines, amines, and amine phosphates. A considerable amount of information on these additives is contained in patent literature. Antifoam agents include silicones and miscellaneous organic copolymers.

Additive Use:

While the general nature of additives may be the same for various types of lubricants, the specific types of additives chosen will depend upon the service in which the lubricant will be used and the characteristics of the base fluid. (Lubrizol)

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