Lubricating oils for internal combustion engines of automobiles or trucks are subjected to a demanding environment during use. This environment results in the oil suffering from oxidation. The oxidation is catalyzed by the presence of impurities in the oil, such as iron compounds. Oxidation is also promoted by the elevated temperatures of the oil during use. This oxidation of lubricating oils during use is typically controlled, at least to some extent, by the use of antioxidant additives that may extend the useful life of the oil, for example by reducing or preventing unacceptable viscosity increases.
There have been a number of attempts to employ lubricants to reduce the friction in an internal combustion engine so as to reduce the fuel consumption of the engine. Numerous classes of lubricant additives have been suggested for use as friction modifiers and to increase the energy efficiency imparted to an engine by a lubricant.
Molybdenum containing additives are known to deliver a variety of beneficial properties to lubricants. Non-limiting examples of lubricants that benefit from the addition of molybdenum include passenger car motor oils, natural gas engine oils, heavy-duty diesel oils, and railroad oils. Over the years molybdenum, when used properly, has been shown to deliver improved anti-wear protection, improved oxidation control, improved deposit control, and improved friction modification for fuel economy.
U.S. Pat. No. 4,692,256 discloses a molybdenum-containing lubricant composition comprising a sulfur compound and an oil-soluble molybdenum compound. The oil-soluble molybdenum compound is prepared by reacting a hexavalent molybdenum compound with an alkyl amine.
U.S. Pat. No. 4,259,195 is directed to antioxidant additives for lubricating oils. The additives are prepared by combining a polar promoter, an acidic molybdenum compound, and a basic nitrogen compound to form a molybdenum-containing composition.
U.S. Pat. No. 4,266,945 discloses molybdenum-containing compositions prepared by the reaction of a molybdenum acid with a phenol, or an aldehyde condensation product of the phenol, and a primary or secondary amine.
The performance of molybdenum compounds as friction modifiers may be determined by a number of factors, including but not limited to:    1. The system in which the additive is being tested. The design of the bench test, rig test, or engine test by which the molybdenum additive is being evaluated affects performance.    2. The other types of additives present in the oil. It is well-known that molybdenum additives exhibit synergistic responses with additives such as sulfurized antioxidants and EP additives, ZDDP, and alkylated diphenylamines.    3. The chemical structure of the oil-soluble molybdenum compound. It is well-known that structurally different molybdenum additives respond differently as friction modifiers under a given set of test conditions.
Existing molybdenum technology, however, suffers from a number of problems that have limited its widespread use in lubricants. These problems include at least one of color, oil solubility, cost and corrosion.
Color Imparted by Molybdenum Compounds
Many molybdenum technologies deliver high levels of color when used even at moderate levels in crankcase oils. The tendency of molybdenum additives to color finished crankcase oils has resulted in the commercialization of a limited number of oil soluble molybdenum products. The contribution of molybdenum additives to finished oil color is purely an aesthetic issue, and is unrelated to any performance property or product compatibility of the molybdenum additives. However, in certain lubricants, a light oil is viewed as a clean oil. Alternatively, a dark oil may sometimes be viewed as a used oil. From a marketing standpoint, it may be undesirable for the consumer to perceive that a specific motor oil brand contains used oil since used oil represents low performance. Light oils may therefore be desired for marketing purposes.
When highly colored molybdenum sources are used at low levels, e.g. 100-150 ppm-delivered molybdenum as is typically required for oxidation, deposit and wear control, discoloration is not substantial but may still be visible. However, when these highly colored molybdenum compounds are used at high levels, e.g. 400-1000 ppm-delivered molybdenum as is generally required for friction modification, discoloration is often significant.
Traditionally, the color of fully formulated crankcase oils has been determined using the ASTM 0 1500 color scale. Two types of unacceptable colors are possible. The first type of discoloration results in a dark rating on the D 1500 scale. The amount of acceptable finished lubricant darkening depends on the customer and application. There are no set standards for the amount of discoloration or darkening that is allowed. Generally, D 1500 ratings equal to or greater than 5.0 are considered unacceptable for a finished crankcase oil. Certain customers may find it difficult to market and sell such dark crankcase oils. The second type of discoloration produces “no match” on the D 1500 color scale. These finished crankcase oils, in addition to showing no match, are also very dark. Again, certain customers may find it difficult to market and sell such dark crankcase oils.
Oil Solubility of Molybdenum Compounds
Many commercially available molybdenum additives designed for use in lubricants exhibit limited solubility in the finished lubricant product. For widespread use of a molybdenum product in lubricant applications, the product will generally not only be soluble at friction modifier treat-levels in the finished lubricant, it will generally also be soluble in the additive concentrates used to prepare the finished lubricant. An acceptable degree of solubility has been difficult to obtain in the past.
Cost of Molybdenum Compounds
Molybdenum has long been viewed as an expensive additive for crankcase applications. Part of the reason for the high cost stems from the fact that many of the commercial molybdenum products have low levels, e.g. less than 5% by weight, of molybdenum in the additive. In some cases, expensive organic ligands or expensive manufacturing processes are used to produce the commercial molybdenum compounds. There is a need for products with higher molybdenum, contents prepared from lower cost raw materials.
Corrosive Effects of Certain Molybdenum Compounds
Many molybdenum technologies contain sulfur. The presence of sulfur in various crankcase applications may be detrimental because certain types of sulfur are incompatible with elastomer seals, and may be corrosive. Even the less aggressive forms of sulfur can be corrosive in very high temperature crankcase environments where significant amounts of oxygen and water are present. There are also trends to reduce the amount of sulfur present in finished crankcase lubricants. As these trends start to become a reality, additives containing sulfur will become less desirable.
It is also well-known that certain molybdenum-containing friction modifiers function by a decomposition mechanism that results in the formation of a mixed molybdenum sulfide/molybdenum oxide layer on the metal surface of the engine. The molybdenum species that form on the metal surface may vary significantly, and their composition is affected by the types of additives in the lubricant and the engine or test design. For example, it is known that molybdenum dithiocarbamates decompose when heated to produce products that include free amine and carbon disulfide. Both such products are aggressive towards any copper that may be present in the engine bearings. Furthermore, free amines are known to be aggressive towards certain types of elastomer seals present in a wide variety of engines. It is therefore desirable from a compatibility standpoint to develop new additives that are low in sulfur and free amines.
It has unexpectedly been found that the molybdenum additives of the present invention may provide benefits to compositions, including lubricating compositions, without the problems commonly associated with molybdenum additives. The properties of the organic molybdenum complexes disclosed herein may include at least one of improved effectiveness as friction modifiers, reduced tendency to color finished crankcase oils, low cost of raw materials, and a simplified production process.