More particularly, the invention relates to lubricating oil compositions which comprise a combination of a trinuclear molybdenum compound and an ester of a dithiocarboxylic acid to improve its oxidation stability.

The use of molybdenum compounds as friction reducing agents or as additives to improve the oxidation stability of lubricating oils is well known in the art and is described for example in US-A-4, 501, 678 and in US-A-4, 479, 883 in both of which dimeric molybdenum compounds are used in combination with dithiocarbamates for this purpose.

Again, US-A-4, 395, 343 describes the use of an antioxidant combination of a sulphur containing molybdenum compound, eg a molybdenum dithiocarbamate, and an organic sulphur compound which may be a sulphurised ester of a carboxylic acid for lubricating oils.

However, no trinuclear molybdenum compounds are disclosed.

More recently, our prior published WO 98/26030 describes lubricating oil compositions comprising a trinuclear molybdenum compound optionally in combination conventional additives including inter alia antioxidants.

Although a high level of oxidation stability is a key requirement for current lubricant formulations, the increased drain intervals demanded by future specifications are likely to increase these demands further.

It has now been found that by using a trinuclear organomolybdenum compound with a combination of antioxidants including a specific type of ester of a dithiocarboxylic acid can significantly improve the oxidation stability of lubricating oils when comapred with use of either dinuclear organomolydenum compounds, or trinuclear organomolybdenum compounds alone or when the latter is used in combination with conventional organic antioxidants.

Accordingly, the present invention comprises a lubricating oil composition comprising a major amount of an oil of lubricating viscosity and a minor amount of (a) a trinuclear organomolybdenum compound of the generic formula : Mo3Sx-(Q) (I) wherein x is from 4 to 10, preferably 7, Q is a core group, which may be a ligand, (b) a sulphur compound which is a hydrocarbyl ester of a thiodicarboxylic acid of the formula (III)

R'-O-C (O)- (RZ)"-S- (R3) m C (O)-O-R4 (III) wherein Rl and R4 are each H or the same or different hydrocarbyl groups provided that both Rl and R4 are not H, and R2 and R3 are each the same or different hydrocarbylene groups and each of n and m is an integer from 1-10, (c) a phenolic antioxidant and (d) an aminic antioxidant.

The oil of lubricating viscosity present in the compositions of the present invention are suitably selected from synthetic and mineral oils. These include the GroupI, Group II and Group III basestocks sourced from natural/mineral oils and the Group IV and GroupV basestocks sourced from synthetic oils.

Thus, the Groups I-III basestocks may be derived from : i) conventionally processed components such as paraffinic, napthenic and mixed paraffinic and naphthenic components from petroleum crudes using extraction, dewaxing and hydrofining or hydrocracked, hydrotreated and/or hydroisomerised petroleum fractions, ii) non-conventionally processed components such as distillates, raffinates and waxes followed by solvent dewaxing, catalytic dewaxing and/or iso- dewaxing, and iii) syn-crudes from a process such as the Fischer-Tropsch process which generated waxes and which may in turn be hydroisomerised and then subjected to catalytic dewaxing and/or iso-dewaxing.

The Groups IV and V basestocks being synthetic lubricating oils which can be used in the compositions of the present invention include esters of saturated or unsaturated dicarboxylic acids (eg phthalic acid, succinic acid, alkyl succinic acids, alkenyl succinic acids, maleic acid, azelaic acid, suberic acid, sebacic acid, fumaric acid, adipic acid, linoleic acid dimer, malinic acid, alkylmalonic acid and alkenylmalonic acids) with a variety of alcohols or glycols (eg butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoether and propylene glycol). Specific examples of these esters include dibutyl adipate, di (2-ethylhexyl) sebacate, di-n-hexyl fumarate, dicotyl sebacate, diisooctyl azleate, diisodecyl azleate, dioctyl phthalate, didecyl phthalate, dieicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, and the complex ester formed by reacting one mole of sebacic acid with two moles of tetraethylene glycol and two moles of 2- ethylhexanoic acid. Esters usable as synthetic oils also include those made from C5-CI2 monocarboxylic acids and polyols and polyol ethers such as neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol and tripentaerythritol.

The oils, whether they are of synthetic or natural origin, are suitable for use in an engine crankcase, particularly a gasoline engine crankcase. Thus, mineral or synthetic lubricating oils having a kinematic viscosity at 100°C (KVIoo) of 3. 5 to 25 cSt are particularly preferred for this purpose. Especially preferred are lubricating oil compositions in which the oil component is a mixture of a mineral oil with a saturates level of between 90 and 99%, having : i. a viscosity index suitably of at least 90, preferably from 90-150 and more preferably from 120-150 ; ii. a sulphur level suitably below 0. 3% by weight, preferably below 0. 1 % by weight and more preferably below 0. 03% by weight ; and iii. in the finished formulation a KVloo in the range from 3. 5-20.

The process is particularly effective on formulations having a viscosity characteristics from OW-20 to 20W-50 and are preferably the so called 5W-30, 5W-20 and 10W-30 formulations.

The components (a) to (d) in the lubricating oil compositions of the present invention are suitably either soluble or dispersible in oil and are preferably oil soluble.

The trinuclear molybdenum compounds (a) are of formula (I) Mo3Sx-(Q) (I) wherein x is from 4 to 10, preferably 7, Q is a core group. Such trinuclear organomolybdenum compounds are relatively new and are claimed and described in our prior published US-A-5, 906, 968. The matter disclosed in this prior US patent on the structure, preparation and properties of the trinuclear molybdenum compounds is incorporated herein by reference. In these compounds the core group (Q) may be a ligand capable of rendering the organomolybdenum compound of formula (I) oil soluble and to ensure that said molybdenum compound is substantially charge neutral. The core group (Q) is generally associated with suitable ligands such as Ly wherein L is the ligand and y is of a sufficient number, type and charge to render the compound of formula (I) oil soluble and to neutralise the charge on the compound of formula (I) as a whole. Thus, more specifically, the trinuclear molybdenum compound used in the compositions of the present invention may be represented by the formula (II) : M03S,, Ly (11) The ligands"L"are suitably dihydrocarbyl dithiocarbamates of the structure (-S2CNR5) wherein the dihydrocarbyl groups, R impart oil solubility to the trinuclear molybdenum compound. In this instance, the term"hydrocarbyl"denotes a substituent having carbon atoms directly attached to the remainder of the ligand and is predominantly

hydrocarbyl in character within the context of this invention. Such substituents include the following : (1) hydrocarbon substituents, ie, aliphatic (for example alkyl or alkenyl), alicyclic (for example cycloalkyl or cycloalkenyl), aromatic-, aliphatic-and alicyclic-substituted aromatic nuclei and the like, as well as cyclic substituents wherein the ring is completed through another portion of the ligand (that is, any two indicated substituents may together form an alicyclic group) ; (2) substituted hydrocarbon substituents, ie, those containing nonhydrocarbon groups which, in the context of this invention, do not alter the predominantly hydrocarbyl character of the substituent. Those skilled in the art will be aware of suitable groups (eg halo (especially chloro), amino, alkoxyl, mercapto, alkylmercapto, nitro, nitroso, sulphoxy etc.) ; and (3) hetero substituents, ie, substituents which, while predominantly hydrocarbon in character within the context of this invention, contain atoms other than carbon present in a chain or ring otherwise composed of carbon atoms.

The hydrocarbyl groups are preferably alkyl (e. g, in which the carbon atom attached to the remainder of the ligand"L"is primary, secondary or tertiary), aryl, substituted aryl and ether groups.

Importantly, the hydrocarbyl groups of the ligands should be such that they have a sufficient number of carbon atoms to render the compound (I) soluble or dispersible in the oil to which it is added. The total number of carbon atoms present among all of the hydrocarbyl groups associated with the ligands of the trinuclear organomolybdenum compound is suitably at least 21, preferably at least 25, more preferably at least 30 and even more preferably at least 35, typically e. g., 21 to 800. For instance, the number of carbon atoms in each hydrocarbyl group will generally range from 1 to 100, preferably from 1 to 40 and more preferably from 3 to 20.

The hydrocarbyl ester (b) of a thiodicarboxylic acid is of the formula (III) R'-O-C (O)- (RZ) nS- (R3) n,-C (O)-O-R4 (III) wherein R'and R4 are each H or the same or different hydrocarbyl groups provided that both Rl and R4 are not H, and R and R are each the same or different hydrocarbylene groups wherein each of n and m is an integer from 1-10.

Thus, the hydrocarbyl groups represented by each of R'and R4 may be same or different and are suitably saturated or unsaturated hydrocarbyl groups, preferably saturated groups. Where the hydrocarbyl groups are saturated groups, these may be selected from

alkyl, aryl, alkaryl or aralkyl group, the aromatic nuclei being considered as saturated in respect of the thiodicarboxylic ester (b) for the purposes of this invention. The hydrocarbyl group suitably contains on average from 1 to 20 carbon atoms, preferably from 1 to 16 carbon atoms and more preferably from 1-12 carbon atoms. The hydrocarbyl group is preferably a saturated alkyl group having from 1-16 carbon atoms. Of these a dodecyl group is most preferred. It should be noted that whilst each of Rl and R4 can be H, in a given molecule at least one of R'and R4 should be a hydrocarbyl group. It is preferable that both Rl and R4 are hydrocarbyl groups.

The hydrocarbylene groups-R2-and-R3-each signify a divalent group which may be an alkylen, an arylene group or combinations thereof having from 1 to 12 carbon atoms, preferably from 2-4 carbon atoms and is free of olefinic unsaturation. The hydrocarbylene group may be straight chain, branched chain group and in the case of the arylene group, it may carry nuclear substituents. Typical examples of such groups may be represented by the following structures : -CH2- -CH (R6) CH (R7)- -CH-Ar-or -Ar-CH- groups, wherein R6, R7 and R8 are the same or different H or alkyl groups having from 1 to 4 carbon atoms and Ar is a phenyl group. The hyrdocarbyl and hydrocarbylene groups in the thiodicarboxylic ester (b) should be so chosen that the final product is oil soluble. Typical examples of compounds (b) include 3, 3'-di (decyl) thiodipropionate and 3, 3'-di (dodecyl) thiodipropionate which may be represented respectively by the formulae (IV) and (V) : [H2iCio-0-C (0)-CH2-CH2-] 2-S (IV) [H25Cl2-0-C (O)-CH2-CH2-] 2-S (V) The thiodicarboxylic esters are suitably prepared reacting sulphur or a sulphur compound such as eg sulphur monochloride or sulphur dichloride with a corresponding unsaturated ester. under elevated temperatures. In the case of (IV) and (V) above the reactant ester is a propionate ester. Compound (V), for instance, is also commercially available as Irganox PS 800 (ex Ciba Speciality Chemicals) and has a CAS No. 123-28-4.

The compositions of the present invention also include at least two other antioxidant compounds which are respectively a phenolic compound (c) and an aminic compound (d).

Among the phenolic compounds, hindered phenols are preferred.

Examples of such phenolic compounds include inter alia : 4, 4'-methylene bis (2, 6-di-tert-butylphenol) 4, 4'-bis (2, 6-di-tert-butylphenol) 4, 4'-bis (2-methyl-6-tert-butylphenol) 2, 2'-methylene bis (4-ethyl-6-tert-butylphenol) 2, 2'-methylene bis (4-methyl-6-tert-butylphenol) 4, 4'-butylidene bis (3-methyl-6-tert-butylphenol) 4, 4'-isopropylidene bis (2, 6-di-tert-butylphenol) 2, 2'-methylene bis (4-methyl-6-nonylphenol) 2, 2'-isobutylidene bis (4, 6-dimethyl phenol) 2, 2'-methylene bis (4-methyl-6-cyclohexylphenol) 2, 6-di-tert-butyl-4-methylphenol 2, 6-di-tert-butyl-4-ethylphenol and 2, 4-dimethyl-6-tert-butylphenol Specific hindered phenols which are preferred as the antioxidants may be represented by the generic formulae (VI)- (IX) below in which Rs, Rlo, and Rl are the same or different alkyl groups from 3-9 carbon atoms and x and y are integers from 1 to 4.

(VI) (VII)

Suitable aminic compounds which may be used in the compositions of the present invention are diaryl amines, aryl naphthyl amines and alkyl derivatives of diaryl amines and the aryl naphthyl amines. Preferred aminic antioxidants are represented by the formulae (X) and (XI) wherein each off) 2 and R13 is a hydrogen atom or represents the same or different alkyl groups from 1-8 carbon atoms.

Specific examples of the aminic compounds that may be used in the compositions of the present invention include inter alia : Monoalkyldiphenyl amines such as eg monooctyldiphenyl amine and monononyl diphenyl amine ; dialkyldiphenyl amines such as eg 4, 4'-dibutyldiphenyl amine, 4, 4'-dipentyldiphenyl amine, 4, 4'-dihexyldiphenyl amine, 4, 4'-diheptyldiphenyl amine, 4, 4'-dioctyldiphenyl amine and 4, 4'-dinonyldiphenyl amine ; polyalkyldiphenyl amines such as eg tetra-butyldiphenyl amine, tetra-hexyldiphenyl amine, tetra-octyldiphenyl amine and tetra-nonyldiphenyl amine ; the naphthylamines such as eg a-naphthylamine and phenyl-a-naphthylamine ; butylpheny-a-naphthylamine, pentylphenyl-a-naphthylamine,

hexylphenyl-a-naphthylamine, heptylphenyl-a-naphthylamine, octylphenyl-a-naphthylamine and nonylphenyl-a-naphthylamine. Of these, dialkyldiphenyl amine and naphthylamines are preferable.

In general, the trinuclear organomolybdenum compound (a) and the antioxidants comprising the hydrocarbyl thioester (b), a phenolic compound (c) and an aminic compound (d) will form a minor component of the total lubricant composition. For example, the trinuclear organomolybdenum compound typically will comprise about 0. 05 to about 5. 00 wt %, preferably from 0. 1-2. 0% by weight of the total composition. Typically, the concentration of molybdenum in such compositions is suitably up to 3000 ppm, preferably from 25-3000 ppm and more preferably from 50-2000 ppm. The combined thioester, phenolic and aminic antioxidants in such compositions is suitably from about 0. 10 to about 3. 0 wt % of the total composition.

It has also been found that if the weight ratio of trinuclear organomolybdenum compound to the combined antioxidants (b), (c) and (d) is in the range of about 10 : 90 to about 90 : 10, optimum dispersancy retention can be achieved by these combined antioxidants of the present invention.

It is particularly preferred that the lubricating oil composition comprises in addition to the trinuclear organomolybdenum compound (a), a mixture of antioxidants comprising the hydrocarbyl ester (b) the phenolic compound (c) and the aminic compound (d) described above in a weight ratio of the trinuclear molybdenum compound to these antioxidants ranging from about 80 : 20 to about 20 : 80, preferably typically 50 : 50.

Optionally, the trinuclear organomolybdenum compound (a) and the combined antioxidants (b), (c) and (d) may be admixed with a carrier liquid in the form of a concentrate. The combined concentration of these four components in the concentrate may vary from 1 to 80% by weight, and will preferably be in the range of 5 to 10% by weight.

The trinuclear organomolybdenum compound and antioxidants combination of the present invention can be used with any of the conventional dispersants used hitherto in the lubricating compositions. Examples of such dispersants include inter alia the polyalkylene succinimides, Mannich condensation products of polylalkylphenol-formaldehyde polyamine and boronated derivatives thereof. However, it is preferable to use ashless dispersants such as the ashless succinimides, especially the polyisobutenyl succinimides of a polyamine such as eg tetraethylenepentamine or its homologues, benzylamine ashless dispersants, and ester ashless dispersants. The dispersants are generally used in the compositions of the present

invention in an amount ranging from about 2-10% by weight based on the total weight of the lubricant composition, preferably from about 4-8% by weight.

In general, these lubricating compositions may include additives commonly used in lubricating oils especially crankcase lubricants, such as antiwear agents, detergents, rust inhibitors, viscosity index improvers, extreme-pressure agents, friction modifiers, corrosion inhibitors, emulsifying aids, pour point depressants, anti-foams and the like.

A feature of the present invention is that lubricant compositions comprising high saturates base oils and trinuclear organomolybdenum compounds in combination with the antioxidants (b), (c) and (d) provide unexpected improvement in oxidation control and significant benefits in fuel economy. In the case of lubricants compositions comprising high saturates base oils for diesel engine oils, the present invention confers the added benefits of viscosity increase control and dispersancy retention over compositions which contain only one of these antioxidants used alone.

Examples : General Procedure : There are a number of engine and bench tests that are aimed at evaluating the oxidation stability of lubricant compositions. One of the most severe engine tests applied is the double length Sequence IIIE test (IIIE2) which can be expensive and time consuming.

We have now developed a screener oxidation (ERCOT Test) which measures viscosity increase (KV4o) as a function of time instead of the IIIE2 test and a strong correlation has been demonstrated between the ERCOT test and the IIIE2 test, which is used to test potential candidates.

The lubricant compositions below (each of which contained the same conventional detergent inihibitor package in the same amount and is hereafter identified as"DI") were tested in the extended ERCOT test which is carried out at atmospheric pressure, a temperature of 165°C using an iron acetyl acetonate catalyst and the test was conducted over a duration of 64-168 hours. Air was bubbled through a lubricant composition to be tested at the rate of 1. 7 litres per minute. The results are tabulated below. In the Tables the following abbreviations have been used : PDN 5203-Trinuclear Molybdenum dithiocarbamate (ex Infineum, containing 5. 1% by weight molybdenum) IrganoxOPS180-didodecyl-3, 3'-thiodipropionate (CAS No. 123-28-4)

EHC45-Hydrocracked oil 45, Group II basestock (ex Exxon, also known as IOL-120X) PTN8464-Linear olefin copolymer-VI improver Parabar 10100-Corrosion inhibitor and S scavenger (ex Amoco) Irganox@ L57-Octylated/butylated diphenylamine (ex Ciba S C) Irganox@ L135-Di-tertiary butyl phenolic ester (ex Ciba S C) Irganox L101-High molecular weight phenolic antioxidant (ex Ciba S C) Irganoxt) L115-Bis (ditertiary butyl phenolic ester derivative) of a thiol (ex Ciba S C) Irganox0 L150-A mixed aryl amine, high molecular weight phenolic resin and bis (ditert.-butyl phenolic ester) of a thiol (ex Ciba S C) The results in the Table below show extended ERCOT Test data expressed as % viscosity increase. Tests 5 and 9 (according to the invention) demonstrate the lowest % viscosity increase under these conditions. Component Basecase DI Basecase + Basecase + Basecase + Basecase + Basecase + Basecase + Basecase + Basecase + alone Mo trimer Mo trimer + Mo trimer + Mo trimer + Irg L150 Irg PS 800 Irg L 150 + Mo trimer + Irg PS 800 Irg L150 Irg L150 + Irg PS 800 L 57 + L 135 Irg PS 800 + PS 800 Test No. 1 2 3 4 5* 6 7 8 9* EHC 45 84.25 83.25 82.75 92.75 82.25 83.75 83.75 83.25 78.40 DI 9.65 9.65 9.65 9.65 9.65 9.65 9.65 9.65 9.65 PTN 8464 6.10 6.10 6.10 6.10 6.10 6.10 6.10 6.10 9.90 Pdn 5203 - 1.00 1.00 1.00 1.00 - - - 1.00 Irganox# PS 800 - - 0.50 - 0.50 - 0.50 0.50 0.50 Irganox# L 150 - - - 0.50 0.50 0.50 - 0.50 - Irganox# L 57 - - - - - - - - 0.35 Irganox# L 135 - - - - - - - - 0.15 Pbr 10100 -- - - - - - 0.05 Extended ERCOT performance - relative viscosity increase / % 136 hrs (%) > 1000 > 1000 > 1000 213.72 145.16 > 1000 382.64 161.44 148.20 144 hrs (%) > 1000 > 1000 > 1000 513.77 278.39 > 1000 > 1000 346.92 285.09 *Tests according to the Invention