Antioxidants and antiwear additives for lubricants
FIELD OF THE INVENTION 
     The invention relates to lubricants. More specifically, the invention relates to lubricant additives which have multifunctional antioxidant/antiwear/antifriction properties which comprise the reaction product of an organotin compound and a phosphoric acid compound. 
     BACKGROUND OF THE INVENTION 
     Under normal operating and storage conditions, lubricants are subject to high temperatures and oxygen which leads to their oxidation and decomposition. Oxidation of lubricants causes the build-up of oil-soluble acids, lacquers and sludge which contribute to serious damage to engines and other lubricated systems. Varnish and lacquer deposits form on hot metal surfaces that are exposed to the lubricant. These deposits are further oxidized to hard carbonacious materials. Antioxidant additives for lubricants have been described as improving lubricant thermal and oxidative stability which thereby enhances the ability of the lubricant to resist oxidation. 
     Additionally, the metal parts of mechanical systems under heavy loads and working under high performance conditions such as high speeds and temperatures will deteriorate due to the frictional forces created by relatively moving and bearing metal surfaces. Often, lubricants for such operations, i.e. high performance lubricants, do not prevent wear of the metal and as a result the performance of the system is adversely affected. It is desirable to blend additive packages containing antiwear/antifriction additives with lubricants in order to prevent wear and increase the service and operating life of the machinery. 
     Zinc dithiophosphates are often described as having antioxidant and antiwear properties when used with various oleagenous compositions. However, zinc-containing additives are undesirable because of high costs associated with the production of zinc and undesirable environmental effects of zinc derivatives. 
     Organotin compounds are described as high temperature stabilizers in polyvinylchloride (PVC) plastics especially rigid PVC as high temperature stabilizers in Kirk-Othmer&#39;s Encyclopedia of Chemical Technology, Vol. 16, pp 573-579, John Wiley and Sons Publishers (1981). 
     In U.S. Pat. No. 4,551,258 to Ikeda et al, dated Nov. 5, 1985, grease compositions containing the reaction products of zinc dithiophosphates, phosphoric acid esters and organotin compounds are disclosed as additives for greases. 
     SUMMARY OF THE INVENTION 
     An improved multifunctional lubricant additive has now been found. The use of additive amounts of phosphoric acid derived organometallic reaction products in automotive and industrial lubricants significantly enhances the thermal and oxidative stability, corrosion inhibiting, antirust, load-carrying, antifriction and/or antiwear properties of lubricants, extends serviceable engine life and offers a zinc-free alternative to traditionally used zinc dithiophosphates. The additive also has the additional properties of friction reduction, cleanliness/detergency, antifatiguing, antiscuffing, antiscoring and antistaining. The invention is directed to a lubricant or fuel additive having multifunctional antioxidant/antiwear properties comprising the reaction product of an organotin compound and a phosphoric acid, lubricant and fuel compositions containing the additive and methods of making the same. 
     The organometallic compound is an organotin made by known processes which are described in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 16, p. p. 573-579, John Wiley and Sons Publishers (1981) which is incorporated herein by reference. Essentially the process of making the organotin compound involves alkylation of tin tetrachloride with a suitable alkylating agent such as the Grignard Reagent, represented by the formula R 1  MgCl, where R 1  is a hydrocarbyl or the direct reaction of tin with alkyl halides. 
     The organotin compound is a substituted organotin represented by the structural formula: 
     
         R.sub.a Sn(Y).sub.4-a 
    
     where R is a hydrocarbyl group, a is an integer ranging from 1 to 3 and Y is an oxygen atom, a sulfur atom, a halogen atom, hydroxide group, or a hydrocarbyl group containing 2 to 60 carbon atoms and containing at least one heteroatom which is sulfur or oxygen. Suitable halogens include those elements from group VIIA of the Periodic Table of the Elements such as fluorine, chlorine, bromine, iodine and astatine. The hydrocarbyl group designated by R is an alkyl group which includes any paraffinic hydrocarbon radical, represented by the general formula C n  H 2n+1  where n ranges from 2 to 20, preferably 4 to 18. The hydrocarbyl group can also be an aryl group which is a ring structure characteristic of the phenyl, naphthyl, phenanthryl and anthryl groups. R can also be a combination of alkyl and aryl such as aralkyl or alkaryl. Non-limiting examples of suitable organotin compounds include tributyltin chloride, dibutyltin oxide, dimethyltin dichloride, dimethyltin oxide, dibutyltin dichloride, tributyltin fluoride, tributyltin oxide and triphenyltin oxide. Other possible organotin compounds, as mentioned above, are those in which Y is a hydroxide group or a hydrocarbyl group containing at least one oxygen or sulfur atom: Non-limiting examples of these include di-n-butyltin diacetate, di-n-butyltin dilaurate, di-n-butyltin bis (isooctyl mercaptoacetate), di-n-butyltin bis (laurylmercaptide), di-n-octyltin bis (isooctyl mercaptoacetate) and triphenyltin hydroxide. 
     The phosphoric acid is represented by the structural formula: ##STR1## where R&#39; is a hydrocarbyl group i.e., alkyl or aryl or a combination thereof such as aralkyl or alkaryl which contains 1 to 60 carbon atoms, preferably 1 to 20 carbon atoms, or R&#39; is a hydrocarbyl group which contain 2 to 60 carbon atoms, preferably 2 to 20. Optionally, the hydrocarbyl group can contain 1 heteroatom, at most 20. Heteroatoms are oxygen, sulfur or nitrogen. Z is a sulfur or oxygen atom. Phosphorus compounds contemplated include the phosphoric acid esters and dithiophosphoric acid esters such as di-(2-ethylhexyl) dithiophosphoric acid, dibutyl dithiophosphoric acid, diisobutyl dithiophosphoric acid, dioctyl dithiophosphoric acid, diamylphosphoric acid, dimethylphosphoric acid, diethyl phosphoric acid, di-n-propyl phosphoric acid, di-n-butyl phosphoric acid and dihexyl phosphoric acid. The dithiophosphoric acids are made by known methods, usually by treating P 4  S 10  with alcohols, phenols or naphtols, or any other suitable hydroxyhydrocarbyl, hydroxyaliphatic, hydroxyaryl or hydroxyalkylaryl compounds. 
     The additives of the invention are prepared in the presence of a base when the organotin halide is used. The base is helpful in facilitating the reaction by deprotonating the phosphoric acid. A precipitate salt forms which is readily removed by filtration. Inorganic bases such as hydroxides and metal oxides of group 1A of the Periodic Table of the Elements are useful. Non-limiting examples include lithium, sodium and potassium hydroxides and oxides. Nitrogen-containing bases such as ammonium hydroxide and organic bases, particularly the tertiary amines, such as triethylamine and tributylamine, are also suitable, although the primary and secondary amines are also used. To prevent side reactions, the tertiary amines are preferred because, unlike the primary and secondary amines, they are less likely to act as a nucleophile. 
     The reactants combine in stoichiometric proportions such that one equivalent amount of the organotin compound reacts with one equivalent amount of the the dithiophosphoric acid. An excess of one reactant over another can be used. Thus, the resulting products are varied in structure, and although there is no intent to be limited by any one reaction mechanism, it is believed that the reaction can be generalized by the illustrative mechanisms which follow. 
     Where the organotin halide is used, one mole of the organotin compound reacts with up to 3 moles of acid: ##STR2## 
     Where R, R&#39; and Z are as defined above, a is an integer ranging from 1 to 3, c=4-a, X is a halide and B designates the base. 
     Alternatively, where the organotin oxide is used, one mole of organotin reacts with 2 moles of the acid. ##STR3## 
     Where R, R&#39; and Z are as defined above. The reactants can be contacted at ambient pressure and temperatures ranging from at least 0° C. to 15° C., at most from 110° C. to 150° C. As mentioned above, the reactants combine stoichiometrically, thus, an excess of one reactant, over the other can be used. Generally, the sequence of reaction involves adding the phosphoric acid which is optionally combined with the base to the organotin compound. However, any sequence is used; thus, the reactants can be combined in one step. The reactants are contacted for at least 15 minutes to 1 hour and up to 4 hours to 24 hours or more. A solvent or diluent inert to the reactants can be optionally used to facilitate the reaction. Examples of suitable solvents include toluene, xylenes or hexane. 
     An important feature of the invention is the ability of the additive to improve the oxidation resistance of the lubricant. It is believed that the effectiveness of the reaction products of the present invention is due to the synergistic activity of the phosphorous compound which has known additive properties and the organotin compound which has known temperature stabilizing ability. The reactants combine into an effective composition which retains the beneficial additive characteristics of the starting materials and also acquires new antioxidant properties. 
     The additives are most effective in industrial lubricating applications where large charges of oil are expected to last the lifetime of the machinery without being replaced. The antioxidant additives of the present invention are particularly necessary in this respect because throughout the serviceable life, the oil is exposed to oxidizing conditions such as circulating air, water and metal oxidation products resulting from the wear of metal surfaces. 
     Gas turbine engines require multifunctional lubricant additives of the type herein described. The temperature ranges, contamination and oxidation conditions to which the lubricant is exposed can be such that oil deterioration can be rapid. 
     The additives are also useful in diesel engine oils, i.e., those used in marine diesel engines, locomotives, power plants and high speed automotive diesel engines. These multifunctional antioxidants can be particularly useful in diesel engines because the engines do not combust as cleanly or completely as gasoline engines. Oil degradation in service is a consequence of oxidative breakdown from blow-by gases containing high concentrations of oxides of nitrogen which promote oil oxidation and accelerate oil thickening. Additionally, metallic contaminants from metals commonly present in these engines such as copper, lead and iron catalyze oxidation. 
     Gasoline burning engines also benefit from the additives of the invention, although they can be, under certain circumstances, easier to lubricate than comparable diesel engines, because of cleaner combustion and, occasionally, less demanding operating conditions. However, since engine efficiency is ever-increasing, in order to conserve scarce resources, the need for multifunctional engine lubricant additives which improve resistance to corrosion, oxidation and wear predominates. 
     Automatic transmission fluids are another class of lubricants which benefit from the additives of the present invention. These fluids represent a careful balance of properties needed to meet the unique requirements of automatic transmissions. Improved oxidation stability, extra corrosion protection and antiwear properties are particularly important and necessary properties of these fluids. Hydraulic fluids in industrial equipment and air compressors have similar requirements and, thus, these additives are also beneficial in these fluids. 
     Gear oils are another class of fluids which would benefit from the additives of the present invention. Typical of such oils are automotive spiral-bevel and worm-gear axle oils which operate under extreme pressures, load and temperature conditions which require antiwear additives. Additionally, hypoid gear oils operating under both high speed, low-torque and low-speed, high torque conditions require lubricants that contain the multifunctional antiwear additives of the present invention. When these gear oils are in service they are in intimate contact with air, making them prone to oxidation which leads to decomposition and polymerization products. 
     In general, the mineral oils, both paraffinic and naphthenic and mixtures thereof can be employed as a lubricating oil or as the grease vehicle. The lubricating oils can be of any suitable lubrication viscosity range, for example, from about 45 SSU at 100° F. to about 6000 SSU at 100° F., and preferably from about 50 to 250 SSU at 210° F. Viscosity indexes from about 70 to 95 being preferred. The average molecular weights of these oils can range from about 250 to about 800. 
     Where synthetic oils, or synthetic oils employed as the vehicle for the grease, are desired in preference to mineral oils, or in mixtures of mineral and synthetic oils, various synthetic oils may be used. Typical synthetic oils include polyisobutylenes, polybutenes, hydrogenated polydecenes, polypropylene glycol, polyethylene glycol, esters, hydrogenated synthetic oils, polyphenyls, siloxanes and silicones (polysiloxanes) and alkyl-substituted diphenyl ethers. 
     Where the lubricant is employed as a grease, the lubricant is generally used in an amount sufficient to balance the total grease composition, after accounting for the desired quantity of the thickening agent, and other additive components included in the grease formulation. A wide variety of materials can be employed as thickening or gelling agents. These can include any of the conventional metal salts or soaps, such as calcium, or lithium stearates or hydroxystearates, which are dispersed in the lubricating vehicle in grease-forming quantities in an amount sufficient to impart to the resulting grease composition the desired consistency. Other thickening agents that can be employed in the grease formulation comprise the non-soap thickeners, such as surface-modified clays and silicas, aryl ureas, calcium complexes and similar materials. In general, grease thickeners can be employed which do not melt or dissolve when used at the required temperature within a particular environment; however, in all other respects, any material which is normally employed for thickening or gelling hydrocarbon fluids for forming greases can be used in the present invention. 
     The additives are particularly useful in greases used in automobile chassis lubrication. Chassis lubricants need the multifunctional additives primarily because the machinery is exposed to many environments and extreme conditions, i.e., high and low temperatures, rain, mud, dust, snow and other conditions such as road salts and road conditions. The additives of the invention are necessary to provide improved rust protection, better oxidation and mechanical stability, reduced fretting and corrosion and improved load carrying capability. 
     The lubricating oils and greases contemplated for blending with the reaction product can also contain other additive materials such as corrosion inhibitors, detergents, extreme pressure agents, viscosity index improvers, co-friction reducers, co-antiwear agents, co-antioxidants and the like. 
     The additives can be blended in a concentration from at least 0.01 wt. % to about 0.3 wt. % to at most 5 wt. % to about 10 wt. % based on the total weight of the lubricant composition. 
     The additives are also useful in fuels. When the additives are utilized in fuels, the fuels contemplated are liquid hydrocarbon and liquid oxygenated fuels such as alcohols and ethers. The additives are blended in a concentration from about 25 to about 500 pounds of additive per 1000 barrels of fuel. 
     Specifically, the fuel compositions contemplated include gasoline base stocks such as a mixture of hydrocarbons boiling in the gasoline boiling range which is from about 90° F. to about 450° F. This base fuel may consist of straight chains or branched chains or paraffins, cycloparaffins, olefins, aromatic hydrocarbons, or mixtures thereof. The base fuel is derived from among others, straight run naphtha, polymer gasoline, natural gasoline or from catalytically cracked or thermally cracked hydrocarbons and catalytically cracked reformed stock. The composition and octane level of the base fuel are not critical and any conventional motor fuel base is employed in the practice of this invention. Further examples of fuels of this type are petroleum distillate fuels having an initial boiling point from about 75° F. to about 135° F. and an end boiling point from about 250° F. to about 750° F. It should be noted in this respect that the term distillate fuels is not intended to be restricted to straight-run distillate fractions. These distillate fuel oils are straight-run distillate fuel oils catalytically or thermally cracked (including hydrocracked) distillate fuel oils etc. Moreover, such fuel oils are treated in accordance with well-known commercial methods, such as acid or caustic treatment, dehydrogenation, solvent refining, clay treatment and the like. 
     Particularly contemplated among the fuel oils are Nos. 1, 2 and 3 fuel oils used in heating and as Diesel fuel oils, gasoline, turbine fuels and jet combustion fuels. 
     The fuels can contain alcohols and/or gasoline in amounts of 0 to 50 volumes per volume of alcohol or the fuel can be an alcohol-type fuel containing little or no hydrocarbon. Typical of such fuels are ethers and alcohols such as methanol, ethanol and mixtures of methanol and ethanol. The fuels which are treated with the additive include gasohols which are formed by mixing 90 to 95 volumes of gasoline with 5-10 volumes of ethanol or methanol. A typical gasohol contains 90 volumes of gasoline and 10 volumes of absolute ethanol. 
     The fuel compositions of the instant invention additionally comprise any of the additives generally employed in fuel compositions. Thus, compositions of the instant invention additionally contain conventional carburetor detergents, anti-knock compounds such as tetraethyl lead, anti-icing additives, upper cylinder and fuel pump lubricity additives and the like. 
     The foregoing examples illustrate the two possible alternative routes for preparing the additive product of the invention. For instance, in Example 1, the product was prepared from an organotin halide and a base. Example 2 illustrated preparation of the additive using an organotin oxide which did not require the presence of a base. However, other routes to prepare the product of this invention can also be used. 
    
    
     EXAMPLE 1 
     Approximately 32.6 g, 0.1 mol, of tributyltin chloride was added to a stirred solution of di-(2-ethylhexyl) dithiophosphoric acid (35.4 g, 0.1 mol) and triethylamine (12.1 g, 0.12 mol) in toluene at 25° C. The mixture was stirred at a temperature of 25° C. for 1 hour and brought to reflux for 1 hour. The solvent was evaporated under reduced pressure to leave a brownish oil (63 g, 98%). 
     EXAMPLE 2 
     Approximately 34.1 g, 0.096 mol of di-(2-ethylhexyl)dithiophosphoric acid was added to a suspension of dibutyltin oxide (12 g, 0.048 mol) in toluene at 25° C. The mixture was refluxed for 2 hours and cooled to ambient temperature. The solvent was evaporated under reduced pressure to afford a brownish oil (43 g, 96%). 
     The organotin phosphoric acids exhibit very good multipurpose antioxidant and antiwear properties in mineral based lubricants under both the mild and severe conditions which exist under a variety of mechanical conditions. These properties not only enhance the thermal and oxidative stability of premium quality automotive and industrial lubricants which extends their service life but they also offer a zinc-free alternative to zinc dithiophosphates. 
     EVALUATION OF THE PRODUCT 
     The reaction product of Example 2 was blended in a concentration of 1 wt. % in a 200 second solvent refined, paraffinic, neutral mineral oil and evaluated for antioxidant performance in the Catalytic Oxidation Test at 325° F. for 40 hours (Table 1) and in the Catalytic Oxidation Test at 325° F. for 72 hours (Table 2). For comparative purposes, the oxidation-inhibiting test results of commercial, traditional phenolic and arylamine antioxidants were reported in Tables 1 and 2 along with the superlative test results achieved by the additive of the invention. 
     The Catalytic Oxidation Test procedures consisted of subjecting a volume of the test lubricant to a stream of air which was bubbled through the test composition at a rate of about 5 liters per hour for the specified number of hours and at the specified temperature. Present in the test composition were metals frequently found in engines, namely: 
     1) 15.5 square inches of a sand-blasted iron wire; 
     2) 0.78 square inches of a polished copper wire; 
     3) 0.87 square inches of a polished aluminum wire; and 
     4) 0.107 square inches of a polished lead surface. 
     The results of the tests were presented in terms of percent change in kinematic viscosity (Delta KV), change in neutralization number (Delta TAN) and lead loss (mg). Essentially, the small percentage of change in KV meant that the lubricant maintained its internal resistance to oxidative degradation under high temperatures, the small change in TAN indicated that the oil maintained its low acidity level under oxidizing conditions and the small change in lead loss indicated that the lubricant was not corrosive to lead under corrosive conditions. 
     The ability of the oil containing the additives of the present invention, to prevent the wearing down of metal parts under severe operating conditions, was tested in the ASTM D2266 Shell 4-Ball Wear Test. The results of the test were presented in Table 3. Following the standard ASTM testing procedure, the test apparatus was a device comprising four 1/2 inch AISI-C-52100 steel balls, three of which were in contact with each other in one plane in a fixed triangular position in a reservoir containing the test sample which was 1.0 wt. % by weight of the total composition of the product of Example 2 blended into a base oil which was a mixture of an 80% solvent paraffinic bright (a high viscosity, fully refined and dewaxed lubricating oil traditionally used for blending with lower viscosity oils) and a 20% solvent paraffinic neutral lubricant oil (a purified, dewaxed lubricating oil stock traditionally blended with a bright stock to make a high quality lubricating oil). The fourth ball was above and in contact with the other three. The fourth ball was rotated at 2000 rpm while under an applied load of 60 kg, which pressed it against the three balls with pressure applied by weight and lever arms. The tests were conducted at 200° F. for 30 minutes. The diameter of the scar on the other three lower balls was measured with a low power microscope and the average diameter measured in two directions on each of the three lower balls was taken as a measure of the antiwear characteristics of the test composition. Table 3 shows the marked decrease in wear scar diameter obtained with respect to the test composition containing the product of Example 2. 
     
                       TABLE 1                                                     
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Catalytic oxidation Test                                                  
40 hours at 325° F.                                                
          Additive Change      % Change Lead                              
          Conc.    in Acid Number                                         
                               in Viscosity                               
                                        Loss                              
Item      (wt. %)  Delta TAN   Delta KV, %                                
                                        (mg)                              
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Base Oil (200                                                             
          none     16.68       326      23.7                              
second, solvent                                                           
refined,                                                                  
paraffinic                                                                
neutral,                                                                  
mineral oil)                                                              
Commercially                                                              
          1.0      7.08        36.9     20.0                              
Obtained                                                                  
Arylamine                                                                 
Antioxidant                                                               
(Irganox L-57)                                                            
(in above oil)                                                            
Commercially                                                              
          1.0      5.33        49.6     14.0                              
Obtained                                                                  
Phenolic                                                                  
Antioxidant                                                               
(Ethyl 702)                                                               
(in above oil)                                                            
Example 2 1.0      3.39        17.5      0.4                              
(in above oil)                                                            
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                       TABLE 2                                                     
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Catalytic Oxidation Test                                                  
72 hours at 325° F.                                                
          Additive Change      % Change                                   
                                       Lead                               
          Conc.    in Acid Number                                         
                               in Viscosity                               
                                       Loss                               
Item      (wt. %)  Delta TAN   Delta KV                                   
                                       (mg)                               
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Base Oil (200                                                             
          none     22          410     47.3                               
second, solvent                                                           
refined,                                                                  
paraffinic                                                                
neutral,                                                                  
mineral oil                                                               
Commercially                                                              
          1.0      8.75        65.3    12.6                               
Obtained                                                                  
Arylamine                                                                 
Antioxidant                                                               
(Irganox L-57)                                                            
(in above oil)                                                            
Commercially                                                              
          1.0      9.36        82.3    29                                 
Obtained                                                                  
Phenolic                                                                  
Antioxidant                                                               
(Ethyl 702)                                                               
(in above oil)                                                            
Example 2 1.0      3.72        37      0.4                                
(in above oil)                                                            
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                       TABLE 3                                                     
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Four Ball Wear Test                                                       
60 kg/2000 rpm/30 min/200° F.                                      
            Additive                                                      
Item        Concentration (%)                                             
                          Wear Scar (mm)                                  
______________________________________                                    
Base Oil (80%                                                             
            none          3.549                                           
solvent                                                                   
paraffinic                                                                
bright, 20%                                                               
neutral                                                                   
lubricant oils)                                                           
Example 2   1.0           0.642                                           
(in above oil)                                                            
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     From the data presented in Tables 1, 2 &amp; 3, it was clear that the compound of the invention was both an excellent antioxidant and antiwear additive.