Limited oil resources and rapidly increasing crude oil prices have created a need for lubricants with longer useful lives. Also, longer intervals between crankcase oil changes will reduce the volume of used oil for disposal. For these and other reasons, the efficiency and useful lives of oil-based lubricants, particularly crankcase lubricants, must be improved.
Oxidation of the oil component in the lubricant substantially shortens its useful life. Oxidation yields corrosive acids and an undesirable increase in viscosity. While high quality basestocks tend to be relatively resistant to oxidation, contaminants (e.g., iron) and common additives can greatly accelerate oxidation. Inclusion of detergents (e.g. calcium or magnesium detergents) and dispersants (e.g. polyamine or polyester derivatives of alkenyl succinic acids or anhydrides) is desirable for oil performance, but these additives accelerate oxidation to such an extent that oxidation is a major cause of reduced useful life.
With depletion of high quality basestock oil reserves, resort to lower quality basestocks has become necessary. These lower quality basestocks have a greater tendency to oxidize than do higher quality basestocks.
If the life of a crankcase lubricant is to be maximized, oxidation must be minimized. Over the years various oxidation inhibitors, or antioxidants, have been proposed. Examples of antioxidants which have been proposed for use in crankcase lubricants include zinc dihydrocarbyl dithiophosphates which are primarily used as antiwear agents but also act as antioxidants, aromatic amines (e.g. alkylated phenylamines and phenyl-.alpha.-naphthylamines), hindered phenols, alkaline earth metal salts or sulfurized alkyl phenols in which the alkyl groups have 5 to 12 carbon atoms (e.g., calcium nonylphenyl sulfide and barium octylphenyl sulfide), phosphosulfurized or sulfurized hydrocarbons, and oil soluble copper compounds.
Some of the above mentioned antioxidants are very effective. Thus, European Patent No. 24 146 B teaches lubricating compositions comprising a major amount of lubricating oil, from 1 to 10 wt % of certain ashless dispersants or from 0.3 to 10 wt % of certain nitrogen- or ester- containing polymeric viscosity index improver dispersants, or mixtures of dispersants and viscosity index improver dispersants; 0.01 to 5 wt % of zinc dihydrocarbyl dithiophosphate (ZDDP); and 5 to 500 parts per million (ppm) by weight of added copper in the form of oil-soluble copper compound. The patent notes that the inexpensive copper antioxidants are effective at low concentrations and therefore do not add much to the products cost. In the amounts employed, the copper compounds do not interfere with the performance of other components of the lubricating system. In many instances, completely satisfactory results are obtained when the copper compound is the sole antioxidant in addition to ZDDP. Alternatively, the patent teaches that for particularly severe conditions where a supplementary antioxidant may be desirable, the amount of supplementary antioxidant required is small, often far less than the amount required in the absence of the copper compound. Supplementary antioxidants mentioned include phenols, hindered phenols, bis-phenols, sulfurized phenols, catechol, alkylated catechols, sulfurized alkyl catechols, diphenylamine, alkylated diphenylamines, phenyl-l-naphthylamine and its alkylated derivatives, alkyl borates, aryl borates, alkyl phosphates, aryl phosphites, aryl phosphates, O,O,S-trialkyl dithiophosphates, O,O,S-triaryl dithiophosphates, and O,O,S-trisubstituted dithiophosphates containing both alkyl and aryl groups.
A specific copper-containing lubricant described in U.S. Pat. No. 4,705,641 comprises (A) a basestock and (3) a copper salt and a molybdenum salt wherein the total concentration of the copper and molybdenum metal or metal ions in solution ranges between about 0.006 and about 0.5 wt % of the basestock (60 to 5000 ppm by weight). The stated preferred concentration of copper and molybdenum ranges from about 0.009 to about 0.1 wt % of the basestock (90 to 1000 ppm by weight).
EP 280,579 A and EP 280,580 A each mention in comparative Example 4 lubricating compositions containing copper oleate and molybdenum oleate.
Despite the beneficial effect of using soluble copper described in EP 24,146 B, the antioxidant literature contains many suggestions that copper should be avoided. For example, an antioxidant system comprising in association (a) a particular sulfur-containing molybdenum complex and (b) an aromatic amine is described in UK patent 2,097,422. The recited sulfur containing molybdenum complex is prepared by reacting an acidic molybdenum compound with a basic nitrogen-containing substance and a sulfur source. Preferably the reaction is carried out in the presence of a polar promoter. Example 10 describes a test for oxidation stability wherein copper is used as an oxidation catalyst.
Another patent teaching that copper is an oxidation catalyst is EP 404,650 A. That application discloses oil-soluble overbased additives comprising an alkali or alkaline earth metal carbonate in combination with a substantially hydrocarbon insoluble organic molybdenum derivative. The application states that the molybdenum derivative is solubilized during the overbasing reaction, perhaps by incorporating the derivative into the micelles of the metal carbonate colloid. The molybdenum complex may be a molybdenum-amine complex formed by reacting an acidic molybdenum compound with an amine, e.g. a primary aliphatic amine. Also taught are oxygen containing molybdenum complexes formed by reacting a molybdenum compound with an oxygen containing compound, e.g. a glycol. Example 16 describes a TFOUT (thin film oxygen uptake test) using a naphthenate of lead, copper, iron, manganese and tin as catalyst.
Despite the breadth of the antioxidant literature, highly effective antioxidants for lubricating compositions, particularly crankcase lubricants, in which the proportion of metal containing antioxidants can, if desired, be kept low, are still needed.