Antioxidant Combinations of Molybdenum Complexes and Organic Sulfur Compounds for Lubricating Oils

This invention relates to new lubricating oil additives and lubricating oil compositions prepared therefrom. More specifically, it relates to new lubricating oil compositions containing an antioxidant additive combination of a sulfur containing molybdenum compound and an organic sulfur compound.

Molybdenum disulfide has long been known as a desirable additive for use in lubricating oil compositions. However, one of its major detriments is its lack of oil solubility. Molybdenum disulfide is ordinarily finely ground and then dispersed in the lubricating oil composition to impart friction modifying and antiwear properties. Finely ground molybdenum disulfide is not an effective oxidation inhibitor in lubricating oils.

As an alternative to finely grinding the molybdenum disulfide, a number of different approaches involving preparing salts of molybdenum compounds have been tried. One type of compound which has been prepared is molybdenum dithiocarbamates. Representative compositions are described in U.S. Pat. No. 3,419,589, which teaches molybdenum (VI) dioxide dialkyldithiocarbamates; U.S. Pat. No. 3,509,051, which teaches sulfurized oxymolybdenum dithiocarbamates; and U.S. Pat. No. 4,098,705, which teaches sulfur containing molybdenum dihydrocarbyl dithiocarbamate compositions.

An alternative approach is to form dithiophosphates instead of dithiocarbamates. Representative of this type of molybdenum compound are the compositions described in U.S. Pat. No. 3,494,866, such as oxymolybdenum diisopropylphosphorodithioate.

U.S. Pat. No. 3,184,410 describes certain dithiomolybdenyl acetylacetonates for use in lubricating oils.

Braithwaite and Greene in Wear, 46 (1978) 405432 describe various molybdenum containing compositions for use in motor oils.

U.S. Pat. No. 3,349,108 teaches a molybdenum trioxide complex with diethylenetriamine for use as an additive for molten steel.

Russian Pat. No. 533,625 teaches lube oil additives prepared from ammonium molybdate and alkenylated polyamines.

British patent application No. 2,037,317A teaches molybdenum succinimide complexes combined with an active sulfur donor to improve sludge dispersancy and antifriction properties of the oil.

Another way to incorporate molybdenum compounds in oil is to prepare a colloidal complex of molybdenum disulfide or oxysulfides dispersed using known dispersants. U.S. Pat. No. 3,223,625 describes a procedure in which an acidic aqueous solution of certain molybdenum compounds is prepared and then extracted with a hydrocarbon ether dispersed with an oil soluble dispersant and then freed of the ether. U.S. Pat. No. 3,281,355 teaches the preparation of a dispersion of molybdenum disulfide by preparing a mixture of lubricating oil, dispersant, and a molybdenum compound in water or C.sub.1-4 aliphatic alcohol, contacting this with a sulfide ion generator and then removing the solvent. Dispersants said to be effective in this procedure are petroleum sulfonates, phenates, alkylphenate sulfides, phosphosulfurized olefins and combinations thereof.

It has now been found that a lubricating oil additive which effectively stabilizes a lubricating oil against oxidation can be prepared by combining (a) a sulfur containing molybdenum compound prepared by reacting an ammonium tetrathiomolybdate, and a basic nitrogen compound, preferably in the presence of a polar promoter, with (b) an organic sulfur compound.

More specifically, this invention is directed to a lubricating oil additive comprising a combination of

(a) an oil soluble sulfur containing molybdenum complex prepared by (1) reacting ammonium tetrathiomolybdate and a basic nitrogen compound selected from the group consisting of a Mannich base, phosphonamide, thiophosphonamide, phosphoramide, dispersant viscosity index improvers, or mixtures thereof to form a molybdenum complex wherein from 0.01 to 2 atoms of molybdenum are present per basic nitrogen atom, and

(b) an oil soluble organic sulfur compound or mixture thereof, wherein the organic sulfur compound of component (b) is present in an amount of from 0.02 to 10 parts by weight per part by weight of the sulfur containing molybdenum complex.

In U.S. Pat. Nos. 4,259,194 and 4,283,295, and of common inventive entity and assignee to this application, there is a teaching of a class of oil soluble sulfur containing molybdenum complexes prepared by reacting ammonium tetrathiomolybdate, and a basic nitrogen composition in the absence or presence of a polar promoter, respectively, to form molybdenum and sulfur containing complexes which are reported therein as useful for inhibiting oxidation, imparting antiwear and extreme pressure properties, and/or modifying the friction properties of a lubricating oil. It has now been discovered that lubricating oils are more effectively stabilized against oxidation when said complexes are used in combination with an organic sulfur compound. Lubricating oil compositions containing the additive combination prepared as disclosed herein are effective as either fluid and grease compositions (depending upon the specific additive or additives employed) for inhibiting oxidation, imparting antiwear and extreme pressure properties, and/or modifying the friction properties of the oil which may, when used as a crankcase lubricant, lead to improved mileage.

The precise molecular formula of the molybdenum compositions of component (a) of the combination is not known with certainty; however, they are believed to be compounds in which molybdenum, whose valences are satisfied with atoms of sulfur, is either complexed by or the salt of one or more nitrogen atoms of the basic nitrogen containing composition used in the preparation of these compositions. These molybdenum complexes which are described in U.S. application Ser. Nos. 52,695 and 52,700, both filed June 28, 1979 are incorporated herein by reference.

The molybdenum compound used to prepare the sulfur containing molybdenum compounds of component (a) of this invention is ammonium tetrathiomolybdate, (NH.sub.4).sub.2 MoS.sub.4.

The basic nitrogen compound must have a basic nitrogen content as measured by ASTM D-664 or D-2896. It is preferably oil-soluble. Typical of such compositions are Mannich bases, phosphonamides, thiophosphonamides, phosphoramides, dispersant viscosity index improvers, and mixtures thereof. These basic nitrogen containing compounds are described below (keeping in mind the reservation that each must have at least one basic nitrogen). Any of the nitrogen containing compositions may be after treated with e.g., boron, using procedures well known in the art so long as the compositions continue to contain basic nitrogen. These after treatments are particularly applicable to the Mannich base compositions.

A class of compounds useful for supplying basic nitrogen are the Mannich base compositions. These compositions are prepared from a phenol or C.sub.9-200 alkylphenol, an aldehyde, such as formaldehyde or formaldehyde precursor such as paraformaldehyde, and an amine compound. The amine may be a mono or polyamine and typical compositions are prepared from an alkylamine, such as methylamine or an ethylene amine, such as, diethylene triamine, or tetraethylene pentaamine and the like. The phenolic material may be sulfurized and preferably is a C.sub.80-100 alkylphenol, dodecylphenol or a C.sub.8-10 alkylphenol. Typical Mannich bases which can be used in this invention are disclosed in U.S. Pat. Nos. 4,157,309 and 3,649,229; 3,368,972; and 3,539,663, the disclosures of which are hereby incorporated by reference. The last application discloses Mannich bases prepared by reacting an alkylphenol having at least 50 carbon atoms, preferably 50 to 200 carbon atoms with formaldehyde and an alkylene polyamine HN(ANH).sub.n H where A is a saturated divalent alkyl hydrocarbon of 2 to 6 carbon atoms and n is 1-10 and where the condensation product of said alkylene polyamine may be further reacted with urea or thiourea. The utility of these Mannich bases as starting materials for preparing lubricating oil additives can often be significantly improved by treating the Mannich base using conventional techniques to introduce boron into the composition.

Another class of composition useful for preparing the additives of this invention are the phosphoramides and phosphonamides such as those disclosed in U.S. Pat. Nos. 3,909,430 and 3,968,157 the disclosures of which are hereby incorporated by reference. These compositions may be prepared by forming a phosphorus compound having at least one P-N bond. They can be prepared, for example, by reacting phosphorus oxychloride with a hydrocarbyl diol in the presence of a monoamine or by reacting phosphorus oxychloride with a difunctional secondary amine and a monofunctional amine. Thiophosphoramides can be prepared by reacting an unsaturated hydrocarbon compound containing from 2 to 450 or more carbon atoms, such as polyethylene, polyisobutylene, polypropylene, ethylene, 1 hexene, 1,3-hexadiene, isobutylene, 4-methyl-1-pentene, and the like, with phosphorus pentasulfide and nitrogen containing compound as defined above, particularly an alkylamine, alkyldiamine, alkylpolyamine, or an alkyleneamine, such as ethylene diamine, diethylene triamine, triethylene tetraamine, tetraethylene pentaamine, and the like.

Another class of nitrogen containing compositions useful in preparing the molybdenum compositions of this invention includes the so-called dispersant viscosity index improvers (VI improvers). These VI improvers are commonly prepared by functionalizing a hydrocarbon polymer, especially a polymer derived from ethylene and/or propylene, optionally containing additional units derived from one or more comonomers such as alicyclic or aliphatic olefins or diolefins. The functionalization may be carried out by a variety of processes which introduce a reactive site or sites which usually has at least one oxygen atom on the polymer. The polymer is then contacted with a nitrogen containing source to introduce nitrogen containing functional groups on the polymer backbone. Commonly used nitrogen sources include any basic nitrogen compound especially those nitrogen containing compounds and compositions described herein. Preferred nitrogen sources are alkylene amines, such as ethylene amines, alkyl amines, and Mannich bases.

Preferred basic nitrogen compounds for use in this invention are carboxylic acid amides, and Mannich bases.

The polar promoter which is preferably used to prepare the molybdenum complex of component (a) of this invention is one which facilitates the interaction between the ammonium tetrathiomolybdate and the basic nitrogen compound. A wide variety of such promoters are well known to those skilled in the art. Typical promoters are 1,3-propanediol, 1,4-butanediol, diethyleneglycol, butyl cellosolve, propylene glycol, 1,4-butyleneglycol, methyl carbitol, ethanolamine, diethanolamine, N-methyl-di-ethanol-amine, dimethyl formamide, N-methyl acetamide, dimethyl acetamide, methanol, ethylene glycol, dimethyl sulfoxide, hexamethyl phosphoramide, tetrahydrofuran and water. Preferred are water and ethylene glycol. Particularly preferred is water.

A method for preparing the molybdenum complex of component (a) of this invention is to react ammonium tetrathiomolybdate and a basic nitrogen-containing compound preferably in the presence of a polar promoter with or without diluent. The diluent is used, if necessary, to provide a suitable viscosity for each stirring and desirably is one which does not react with the ammonium tetrathiomolybdate. Typical diluents are lubricating oil and liquid compounds containing only carbon and hydrogen. The reaction is carried out at a temperature from the melting point of the mixture to reflux temperature. It is ordinarily carried out at atmospheric pressure although higher or lower pressures may be used if desired.

In the reaction mixture, the ratio of molybdenum compound to basic nitrogen compound is not critical; however, as the amount of molybdenum with respect to basic nitrogen increases, the filtration of the product becomes more difficult. Since the molybdenum component probably oligomerizes, it is advantageous to add as much molybdenum as can easily be maintained in the composition. Usually, the reaction mixture will have charged to it from 0.01 to 2.00 atoms of molybdenum per basic nitrogen atom. Preferably from 0.4 to 1.0, and most preferably from 0.4 to 0.7, atoms of molybdenum per atom of basic nitrogen is added to the reaction mixture.

The polar promoter, which is optionally and preferably used, is ordinarily present in the ratio of 0.1 to 50 mols of promoter per mol of molybdenum compound. Preferably from 0.5 to 25 and most preferably 1.0 to 15 mols of the promoter is present per mol of molybdenum compound.

Representative of the organic sulfur compounds of component (b) which may be used in combination with the molybdenum complex of component (a) include hydrocarbyl sulfides and polysulfides of the formula R.sub.2 S.sub.x where R is hydrocarbyl, preferably C.sub.1-40 alkyl and aryl, preferably phenyl, and x is 1 to 6, are preferably 1 or 2, thioacetamide, thiourea, and mercaptans of the formula RSH where R is as defined above. Also useful as organic sulfur compounds are traditional sulfur-containing antioxidants such as wax sulfides and polysulfides, sulfurized olefins, sulfurized carboxylic acid esters, sulfurized ester-olefins, sulfurized alkylphenols and the metal salts thereof, and the reaction product of an olefin and sulfurized alkylphenol, metal dihydrocarbyl dithiophosphates metal dithiocarbamates, and phosphosulfurized terpenes.

The sulfurized carboxylic acid esters are prepared by reacting sulfur, sulfur monochloride, and/or sulfur dichloride with an unsaturated ester under elevated temperatures. Typical esters include C.sub.1 -C.sub.20 alkyl esters of C.sub.3 -C.sub.24 unsaturated acids, such as palmitoleic, oleic, ricinoleic, petroselinic, vaccenic, linoleic, linolenic, oleostearic, licanic, paranaric, tariric, gadoleic, arachidonic, cetoleic, fatty acids, as well as the other unsaturated acids such as acrylic, crotonic, etc. Particularly good results have been obtained with mixed unsaturated fatty acid esters, such as are obtained from animal fats and vegetable oils, such as tall oil, linseed oil, olive oil, caster oil, peanut oil, grape oil, fish oil, sperm oil, and so forth.

Exemplary esters include lauryl tallate, methyl oleate, ethyl oleate, lauryl oleate, cetyl oleate, cetyl linoleate, lauryl ricinoleate, oleyl linoleate, lauryl acrylate, styryl acrylate, 2-ethylhexyl acrylate, oleyl stearate, and alkyl glycerides.

Cross-sulfurized ester olefins, such as a sulfurized mixture of C.sub.10 -C.sub.25 olefins with fatty acid esters of C.sub.10 -C.sub.25 fatty acids and C.sub.1 -C.sub.25 alkyl or alkenyl alcohols, wherein the fatty acid and/or the alcohol is unsaturated may also be used.

Sulfurized olefins are prepared by the reaction of the C.sub.3 -C.sub.6 olefins or a low-molecular-weight polyolefin derived therefrom or C.sub.8 -C.sub.24 olefins with a sulfur-containing compound such as sulfur, sulfur monochloride, and/or sulfur dichloride. Particularly preferred are the sulfurized olefins described in U.S. Pat. No. 4,132,659 which is incorporated herein by reference.

Particularly useful are the diparaffin wax sulfides and polysulfides, cracked wax-olefin sulfides and so forth. They can be prepared by treating the starting material, e.g., olefinically unsaturated compounds, with sulfur, sulfur monochloride, and sulfur dichloride. Most particularly preferred are the paraffin wax thiomers described in U.S. Pat. No. 2,346,156.

Sulfurized alkylphenols and the metals salts thereof include compositions such as sulfurized dodecylphenol and the calcium salts thereof. The alkyl group ordinarily containing from 9-300 carbon atoms. The metal salt may be preferably, a group I or group II salt, especially sodium, calcium, magnesium, or barium.

The reaction product of a sulfurized alkylphenol and cracked wax olefin is described in U.S. Pat. No. 4,228,022 which is incorporated herein by reference. The alkyl group present in the alkylphenol preferably contains from 8 to 35 carbon atoms and preferably the olefin contains from 10 to 30 carbon atoms.

The metal hydrocarbyl dithiophosphates may be represented generally by the formula ##STR1## wherein R.sub.1 and R.sub.2 may be the same or different hydrocarbyl radicals containing from 1 to 18 carbon atoms and preferably 2 to 12 carbon atoms including radicals such as alkyl, alkenyl, aryl, aralkyl, alkaryl and cycloaliphatic radicals. Thus, the radicals R.sub.1 and R.sub.2 may, for example, be ethyl, n-propyl, i-propyl, n-butyl, i-butyl, sec-butyl, n-hexyl, 2-ethylhexyl, octadecyl, phenyl, benzyl, butylphenyl, cyclohexyl, propenyl, butenyl, etc.

M is a Group I metal, a Group II metal, aluminum, tin, cobalt, lead, molybdenum, manganese or nickel, and m is an integer which is equal to the valence of the metal M. Preferably M is zinc.

These compounds can be prepared by the reaction of a suitable alcohol or mixture of alcohols with phosphorus pentasulfide followed by rection with the appropriate metal compound. Methods to prepare these compounds are described in U.S. Pat. Nos. 3,083,850; 3,102,096; 3,293,181; and 3,489,682 and the disclosures thereof are incorporated herein by reference.

The phosphorosulfurized terpenes as represented by pinene, dipenene, allo-ocimene, etc., are another group of dithiophosphate diesters which are active sulfur donors. Of the terpenes, the bicyclic pinene is preferred. The phosphosulfurized terpene is readily obtained by reaction of about one mole of diester of thiophosphoric acid and one mole of pinene at a temperature of at least 100.degree. C., e.g. 100.degree. C. to 200.degree. C. The preferred active sulfur donor can be characterized as the bornyl ester of dihydrocarbyl (C.sub.2 -C.sub.20) dithiophosphoric acids (as shown in U.S. Pat. No. 2,689,258).

The metal dithiocarbamates made by methods well known in the art have the following general formula ##STR2## wherein R.sub.3 and R.sub.4 may be the same or different hydrocarbyl radicals containing 1 to 30 carbon atoms and preferably 1 to 12 carbon atoms, including such radicals as alkyl, alkenyl, aryl, aralkyl, and alkaryl, M is a metal of the group consisting of alkali and alkaline earth metals, aluminum, nickel, lead, cobalt, molybdenum, manganese and tin, and n is a subscript of corresponding to the valance M.

Preferred organic sulfur compounds which may be used in combination with the molybdenum complex of component (a) are metal dihydrocarbyl dithiophosphates, metal dithiocarbamates, sulfurized olefins, alkyl and aryl sulfides, alkyl and aryl polysulfides, sulfurized fatty acids, sulfurized alkylphenols, the reaction product of an olefin and sulfurized alkylphenol and phosphosulfurized terpenes.

The most preferred organic sulfur compounds are the hydrocarbyl mono and disulfides which may be represented generally by the formula

wherein R.sub.5 and R.sub.6 are the same or different hydrocarbyl radical each containing from 1 to 40 carbon atoms and preferably 1 to 20 carbon atoms, including radicals such as alkyl, alkenyl, aryl, aralkyl, alkaryl. Thus, the radicals R.sub.5 and R.sub.6 may, for example, be ethyl, propyl, n-hexyl, decyl, dodecyl, octadecyl, eicosyl, phenyl, benzyl phenylethyl, butylphenyl, propenyl, butenyl, etc. and y is 1 to 2.

Also included within the scope of hydrocarbyl sulfides are the C.sub.1 -C.sub.20 dialkyl esters of thiodipropionic acid such as dilaurylthiodipropionate and distearylthiodipropionate.

The lubricating oil compositions containing the additives of this invention can be prepared by admixing, by conventional techniques, the appropriate amount of the sulfur containing molybdenum complex of component (a) and the organic sulfur compound of component (b) with a lubricating oil. The selection of the particular base oil depends on the contemplated application of the lubricant and the presence of other additives. Generally, the amount of the combined additives of components (a) and (b) will vary from 0.05 to 15% by weight and preferably from b 0.2 to 10% by weight.

The lubricating oil which may be used in this invention includes a wide variety of hydrocarbon oils, such as naphthenic bases, paraffin bases and mixed base oils as well as synthetic oils such as esters and the like. The lubricating oils may be used individually or in combination and generally have a viscosity which ranges from 50 to 5,000 SUS and usually from 100 to 15,000 SUS at 38.degree. C.

In many instances it may be advantageous to form concentrates of the combination of additives within a carrier liquid. These concentrates provide a convenient method of handling and transporting the additives before their subsequent dilution and use. The concentration of the additive combination within the concentrate may vary from 15 to 90% by weight although it is preferred to maintain a concentration between 15 and 50% by weight. The final application of the lubricating oil compositions of this invention may be in marine cylinder lubricants as in crosshead diesel engines, cranckcase lubricants as in automobiles and railroads, lubricants for heavy machinery such as steel mills and the like, or as greases for bearings and the like. Whether the lubricant is fluid or a solid will ordinarily depend on whether a thickening agent is present. Typical thickening agents include polyurea acetates, lithium stearate and the like.

If desired, other additives may be included in the lubricating oil compositions of this invention. These additives include antioxidants or oxidation inhibitors, dispersants, rust inhibitors, anticorrosion agents and so forth. Also antifoam agents stabilizers, antistain agents, tackiness agents, antichatter agents, dropping point improvers, antisquawk agents, extreme pressure agents, odor control agents and the like may be included.

The following examples are presented to illustrate the operation of the invention and are not intended to be a limitation upon the scope of the claims.

To a 1-liter flask are added 290 grams of a Mannich base prepared from dodecylphenol, methylamine and formaldehyde and having an alkalinity value of 110 and containing 2.7% nitrogen, an 200 ml of a hydrocarbon thinner. The mixture is heated to 65.degree. C. and 50 ml water and 29 grams of (NH.sub.4).sub.2 MoS.sub.4 are added. The mixture is stirred at reflux, 104.degree. to 110.degree. C., for 41/2 hours. The temperature is increased to 155.degree. C. and held at this temperature for 1 hour. The temperature is then increased to 180.degree. C. over a period of 10 minutes and held for 2 hours. The mixture is filtered through diatomaceous earth and then stripped to 180.degree. C. at 20 mm Hg to yield the product.

Neutral lubricating oil formulations are prepared by adding each of the molybdenum compound (a) and the sulfur compound (b) separately into the oil with stirring according to standard methods known in the art.

Formulation Oil I

6 m moles/kg molybdenum complex of Example 1

0.5% by weight didodecyl sulfide

Formulation Oil II

3 m moles/kg molybdenum complex of Example 1

8 m moles/kg zinc dithiophosphate from sec.-butanol and methylisobutyl carbinol

Formulation Oil III

6 m moles/kg molybdenum complex of Example 1

0.5% by weight diphenyl sulfide.

The oxidation stability of lubricating oil compositions containing the additive combination according to this invention may be tested in an Oxidator B Test. According to this test, the stability of the oil is measured by the time in hours required for the consumption of 1 liter of oxygen by 100 grams of the test oil at 340.degree. F. In actual test, 25 grams of oil is used and the results are corrected to 100-gram samples. The catalyst which is used at a rate of 1.38 cc per 100 cc oil contains a mixture of soluble salts providing 95 ppm copper, 80 ppm iron, 4.8 ppm manganese, 1100 ppm lead and 49 ppm tin. The results of this test are reported as hours to consumption of 1 liter of oxygen and is a measure of the oxidative stability of the oil.

The oxidation stability of lubricating oils containing the combinations of this invention are enhanced as compared to oil formulations not containing the additive combination.

In a similar manner, when each of a sulfurized cracked wax olefin (C.sub.15-18), didecyldisulfide, dilaurylthiodipropionate or the reaction product of sulfurized cracked wax olefin (C.sub.15-18) and sulfurized tetrapropylenephenol are substituted for the sulfur compounds used in the above formulations, the oxidative stability for the oil is enhanced as compared to the oil not containing the additive combination.