This invention concerns lubricating compositions for use in industrial equipment requiting antiwear control.  More particularly this invention concerns lubricating compositions providing load-carrying (antiwear) control for industrial equipment without sacrificing oxidation resistance. 
The art of lubricating oil formulation has become increasingly complex with ever more stringent industry standards dictated by developing industrial equipment technology.  One requirement for industrial lubricants is to provide wear control.  At the same time the lubricant formulation should provide resistance to oxidation and sludge formation in order to achieve operation life of adequate length.  Experience has shown that the incorporation of one type of additive in a lubricant composition can have a negative impact on the function of another type of additive in that composition.  Indeed, the presence of antiwear additives in lubricants often reduces the oxidation stability and increases sludge formation compared to a similar oil where the antiwear additive is absent.  Thus, there is a need for industrial lubricants that provide improved antiwear performance without sacrificing oxidation resistance and deposit control. 
According to the invention, a lubricant composition especially suitable for use in industrial equipment requiring antiwear properties and oxidation resistance is provided, comprising a major portion of a base oil, an effective amount of both a sulfur and phosphorous containing antiwear additive, and an antioxidant or a mixture of antioxidants. 
The lubricant composition described herein comprises a major amount of a base oil of lubricating viscosity, a sulfur and phosphorus containing anti-wear additive, and a mixture of one or more antioxidant additives.  Compressor, hydraulic, turbine or other industrial lubricating compositions can be formulated using this combination of components. 
The lubricating oil base stock is any natural or synthetic lubricating base oil stock fraction typically having a kinematic viscosity at 40xc2x0 C. of about 14 to 220 cSt, more preferably about 20 to 150 cSt, most preferably about 32 to 68 cSt. 
The lubricating oil basestock can be derived from natural lubricating oils, synthetic lubricating oils or mixtures thereof.  Suitable lubricating oil basestocks include basestocks obtained by isomerization of synthetic wax and slack wax, as well as hydrocrackate basestocks produced by hydrocracking (rather than solvent extracting) the aromatic and polar components of the crude.  Suitable basestocks include those in API categories I, II and III, where saturates level and Viscosity Index are:
Group Ixe2x80x94less than 90% and 80-120, respectively;
Group IIxe2x80x94greater than 90% and 80-120, respectively; and
Group IIIxe2x80x94greater than 90% and greater than 120, respectively. 
Natural lubricating oils include petroleum oils, mineral oils, and oils derived from coal or shale. 
Synthetic oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as polymerized and inter-polymerized olefins, alkylbenzenes, polyphenyls, alkylated diphenyl ethers, alkylated diphenyl sulfides, as well as their derivatives, analogs and homologs thereof, and the like.  Synthetic lubricating oils also include alkylene oxide polymers, interpolymers, copolymers and derivatives thereof wherein the terminal hydroxyl groups have been modified by esterification, etherification, etc.  Another suitable class of synthetic lubricating oils comprises the esters of dicarboxylic acids with variety of alcohols.  Esters useful as synthetic oils also include those made from C5 to C12 monocarboxylic acids and polyols and polyol ethers. 
The lubricating oil may be derived from unrefined, refined, rerefined oils, or mixtures thereof.  Unrefined oils are obtained directly from a natural source or synthetic source (e.g., coal, shale, or tar sand bitumen) without further purification or treatment.  Examples of unrefined oils include a shale oil obtained directly from a retorting operation, a petroleum oil obtained directly from distillation, or an ester oil obtained directly from an esterification process, each of which is then used without further treatment.  Refined oils are similar to the unrefined oils except that refined oils have been treated in one or more purification steps to improve one or more properties.  Suitable purification techniques include distillation, hydrotreating, dewaxing, solvent extraction, acid or base extraction, filtration, and percolation, all of which are known to those skilled in the art.  Rerefined oils are obtained by treating refined oils in processes similar to those used to obtain the refined oils.  These rerefined oils are also known as reclaimed or reprocessed oils and often are additionally processed by techniques for removal of spent additives and oil breakdown products. 
Lubricating oil base stocks derived from the hydroisomerization of wax may also be used, either alone or in combination with the aforesaid natural and/or synthetic base stocks.  Such wax isomerate oil is produced by the hydro-isomerization of natural or synthetic waxes or mixtures thereof over a hydro-isomerization catalyst. 
Natural waxes are typically the slack waxes recovered by the solvent dewaxing of mineral oils; synthetic waxes are typically the wax produced by the Fischer-Tropsch process. 
The resulting isomerate product is typically subjected to solvent dewaxing and fractionation to recover various fractions of specific viscosity range.  Wax isomerate is also characterized by possessing very high viscosity indices, generally having a VI of at least 130, preferably at least 135 and higher and following dewaxing, a pour point of about xe2x88x9220xc2x0 C. and lower. 
The production of wax isomerate oil meeting the requirements of the present invention is disclosed and claimed in U.S. Pat.  Nos.  5,049,299 and 5,158,671 which are incorporated herein by reference. 
The compositions of the invention include an effective amount of a sulfur and phosphorus containing antiwear compound or additive.  A preferred additive is an alkylated ester derivative of a sulfurized phosphite or phosphate.  A more preferred additive compound has the formula I: 
where R1, R2, R3 and R4 may be the same or different hydrocarbyl groups of from about 1 to about 18 carbon atoms.  Preferably R1 and R2 are the same and are branched alkyl groups of from about 6 to about 10 carbon atoms, R3 is an alkyl group of from about 1 to about 4 carbon atoms, and R4 is an alkyl group of from about 6 to about 10 carbon atoms.  Typically the antiwear additive will comprise from about 0.05 to about 2.5 wt %, based on the total weight of the composition. 
The lubricant composition of the invention also includes an effective amount of an antioxidant or mixtures of antioxidants, such as aryl amines, phenylene diamines, hindered phenolics and thiocarbamates, or derivatives thereof, which may or may not be sulfurized.  A preferred embodiment of the is invention incorporates an effective amount of aromatic amine anti-oxidant or mixture of aromatic amine antioxidants.  Aromatic amine antioxidants useful in the present invention are selected from the aromatic amines and mixtures thereof represented by formulae II and III.  
where R1 and R2 are independently hydrogen or C1 to C18 alkyl.  Typically the amine or mixture of amines will constitute from about 0.05 to about 2.5 wt %, based on the weight of the composition.  An especially preferred composition includes amines of formula II and III in the weight ratio of about 0.2 to about 4.0.  Indeed, a most preferred composition includes amine II in which R1 and R2 are hydrogen, and amine III in which R1 and R2 are C4 to C8 alkyl. 
Fully formulated industrial oils typically may also contain additional additives, known to those skilled in the industry, used on an as-received basis.  The lubricating oils of the present invention may contain, in addition to the aforesaid antioxidant and antiwear additives, other additives typically used in lubricating oils, such as pour depressants, rust inhibitors, thickeners, metal passivators, demulsifiers and antifoamants. 
Pour depressant additives for lubricating oils are typically polymers or co-polymers, with polymethacrylates and poly-vinlyacetate alkylfumarate as commonly used examples.  Rust inhibitor additives can be of a variety of chemical types, with ester and amide derivatives of alkylated succinic acid being among the most commonly used in lubricating oils.  Thickeners may be any oligomer, polymer or co-polymer which can be employed to increase the viscosity of the oil in a controlled manner.  Such materials include hydrocarbons, such as polybutenes, olefin copolymers and high viscosity poly-alpha olefins, and polyalkyacrylates, such as polymethacrylates and olefin-acrylate co-polymers. 
Metal passivators can be of a wide variety of chemical types which interact with metals typically present in lube systems to prevent such metals from exercising a deleterious effect on the functionality of the lubricant.  Commonly used metal passivators include thiazines, triazoles, benzotriazoles and dimercaptothiadiazoles, including alkyl and other derivatives, and mixtures thereof.  Demulsifiers are employed to enhance the rapid separation of oil and water in lube systems, and most often consist of poly-oxyalkylated derivatives of multi-hydroxyl substrates such as sugars.  Poly-acrylates and poly-alkylsiloxanes, and their derivatives, are widely employed in lubricants as antifoamants. 
Materials such as these are well known in the art.  Lubricating oil additives are described generally in xe2x80x9cLubricants and Related Productsxe2x80x9d by Dieter Klamann, Verlag Chemie, Deerfield, Fla., 1984, and also in xe2x80x9cLubricant Additivesxe2x80x9d by C. V. Smalheer and R. Kennedy Smith, 1967, pages 1-11. 
The following non-limiting Examples and Comparative Examples illustrate the invention.