This invention relates to a lubricant composition containing a combination of additives providing antiwear properties, and to the antiwear additive combination contained therein.

It is well-known to include an antiwear additive in lubricating oils such as engine oils.

Wear results mainly from the rubbing together of two metal surfaces, i.e. in boundary lubrication regimes, such as is found in valve trains in internal combustion engines. It is believed that the antiwear additive acts to provide a protective film over the metal surfaces. One well-known class of antiwear additives is the metal alkylphosphate, especially zinc dialkyldithiophosphate ("ZDDP"). Generally ZDDP is employed at treat levels of 1 to 2 wt.% based on the total weight of the lubricant, which gives a phosphorus level in the lubricant typically in the range of from 0.05 to 0.15 wt.%.

With the increase in the use of catalytic converters in recent years to convert the exhaust fumes from internal combustion engines to less noxious substances, there is increasing concern that the phosphorus from the lubricating oil will tend to poison the catalysts, thus preventing them from functioning to full effect. Consequently there is a desire to reduce phosphorus levels, i.e. ZDDP levels, in the lubricant. However, any reduction will result in a corresponding reduction in antiwear performance. There is therefore a need for an effective reduced-phosphorus antiwear additive.

We have found surprisingly that a combination of an organo-molybdenum compound and a zinc thiophosphate compound such as ZDDP act synergistically to provide better wear protection than that achieved using a comparable amount of the zinc thiophosphate compound alone.

US-A-4178258 describes the use as a lubricating oil additive of molybdenum bis(dialkyldithiocarbamate) to reduce wear in diesel engines caused by the blow-by of soot, when such wear is aggravated by the presence in the oil of ZDDP.

US-A-4812246 describes the use in lubricating oils for internal combustion engines of molybdenum dithiocarbamate (MoDTC) or molybdenum dithiophosphate (MoDTP) to aid stability against the effects of NO _x gases. The additive package may include ZDDP.

EP-A-0113045 describes an antifriction additive combination for small, high power engines comprising a sulphurised oxymetal organophosphorodithionate and/or dithiocarbamate in combination with ZDDP, calcium alkylbenzene (or petroleum) sulphonate and an alkenyl succinic acid imide (or boron derivative).

However it is known that MoDTC decomposes when heated in use to decomposition products which include free amine and carbon disulphide. Both such products are aggressive towards copper which is present in the engine bearings. Wear in bearings is particularly objectionable since repair involves complete dismantling of the engine. Carbon disulphide tends to boil off fairly rapidly and does not constitute a special problem. However even small amounts of free amine can cause damage.

We have found that the synergism described above extends not only to dithiocarbamates of molybdenum but also to molybdenum compounds which are nitrogen free and which therefore do not decompose to free amine. A preferred compound is also free from sulphur.

Accordingly, in one aspect the present invention provides a lubricant composition comprising a base oil of viscosity from 3 to 26 cSt (mrn-^/s) at 100°C and an antiwear additive combination comprising (a) an oil soluble or oil dispersible, nitrogen and phosphorus free organo-molybdenum compound or mixture of compounds and (b) a zinc thiophosphate compound or mixture of compounds selected from zinc dialkyldithiophosphate, zinc diaryldithiophosphate, zinc alkylaryldithiophosphate, zinc arylalkyldithiophosphate and mixtures thereof.

In another aspect the present invention provides an antiwear additive combination comprising (a) an organo-molybdenum compound or mixture of compounds as defined above and (b) a zinc thiophosphate compound or mixture of compounds as defined above.

In a further aspect the present invention provides the use in a lubricant composition of a combination of additives as an antiwear agent, the combination of additives comprising (a) an

organo-molybdenum compound or mixture of compounds as defined above and (b) a zinc thiophosphate compound or mixture of compounds as defined above.

The preferred organo-molybdenum compound comprises a carboxylate. Not only do such compounds contain no nitrogen or phosphorus but they are relatively cheap and contain a higher proportion of molybdenum than the more complex compounds. As will be described in more detail, it is believed that the effective additive content is governed by the metal content. Thus only about one third of the amount e.g. of molybdenum 2-ethyl hexanoate need be used by comparison with molybdenum dithiocarbamate.

In a further aspect the present invention provides the use in a lubricant composition of one or more molybdenum carboxylate compounds as an antiwear agent.

Thus the present invention has the advantage that the organo-molybdenum compound can replace some of the zinc thiophosphate compound used in lubricant compositions with the effect that the phosphorus level in the lubricant is reduced without substantially reducing the antiwear performance of the lubricant or increasing the tendency towards bearing corrosion.

The organo-molybdenum compound is preferably selected from a carboxylate and a xanthate and mixtures thereof, the organo groups of which may be substituted with a hydrocarbyl group, with the proviso that the organo group selected results in an organo- molybdenum compound that is oil-soluble or oil-dispersible, preferably oil-soluble.

Where the organo group of the organo-molybdenum compound is a carboxylate, this is preferably a Ci to C50, more preferably a Cg to CJS, carboxylate group. Examples of suitable carboxylates include octanoate, e.g. 2-ethyl hexanoate, naphthenate and stearate. These compounds may be prepared, for example, by reacting molybdenum trioxide with the alkali metal salt of the appropriate carboxylic acid under suitable conditions.

Where the organo group of the organo-molybdenum compound is a xanthate, the compound preferably has the formula :

Mo ₂ ROCS2) ₄ (I)

where R is a Ci to C30 hydrocarbyl group, preferably an alkyl group. Examples of suitable molybdenum xanthate compounds and their method of preparation are described in European patent application EP-A-433025, the disclosure of which is incorporated herein by reference.

The zinc thiophosphate compound (b) has the general formula

where R5, Rg, R7 and Rg each independently represent a hydrogen atom, a Ci to C20 alkyl group, a Cg to C ₂ g cycloalkyl, aryl, alkylaryl or aralkyl group, or a C3 to C ₂ Q hydrocarbyl group containing an ester, ether, alcohol or carboxyl group. Preferably each of R5 to Rg is a C ₂ to Cig, more preferably C3 to Cg, alkyl group which may be straight-chain or branched. Such compounds are commercially available and are supplied by, for example, Exxon Chemical Company.

The amounts of each of the antiwear additives (a) and (b) to be included in the lubricant composition according to the invention are the amounts that are effective in providing the desired level of antiwear performance, whilst reducing the amount of phosphorus to an acceptable level.

Whilst not being limited to any particular theory, it is believed that the antiwear properties of the organo-molybdenum compound (a) are generally attributable to the presence of the molybdenum. Thus when determining the amount of organo-molybdenum compound to be incorporated into the lubricant composition, one first needs to determine the desired amount of molybdenum. Preferably the amount of molybdenum contained in the lubricant composition is from 0.001 to 0.5 wt.%, more preferably 0.005 to 0.2 wt.%, and most preferably 0.01 to 0.05 wt.%, based on the total weight of the lubricant composition. The amount of organo- molybdenum compound that this corresponds to depends upon the type of compound selected.

Where the organo-molybdenum compound is a carboxylate, the amount of compound used depends upon the molecular weight of the carboxylate group selected. For example, where

the carboxylate is 2-ethyl hexanoate, the amount of molybdenum carboxylate used is preferably from 0.005 to 2.5 wt.%, more preferably from 0.025 to 1.0 wt.%, and most preferably from 0.05 to 0.25 wt.%, based on the total weight of the lubricant composition.

Where the organo-molybdenum compound is a xanthate, the amount of compound used depends upon the molecular weight of the hydrocarbyl, e.g. alkyl, groups contained in the xanthate groups. Typically, however, the amount of molybdenum xanthate used is preferably from 0.003 to 2.0 wt.%, more preferably from 0.01 to 0.7 wt.% and most preferably from 0.03 to

0.2 wt.%, based on the total weight of the lubricant composition.

Similarly, it is believed that the antiwear properties of the zinc thiophosphate compound (b) are generally attributable to the presence of the phosphorus. Thus, when determining the amount of zinc thiophosphate to incorporate, one first needs to determine the desired amount of phosphorus in the lubricant composition due to the zinc thiophosphate compound. Preferably this amount is from 0.001 to 0.3 wt.%, more preferably from 0.01 to 0.2 wt.%, and most preferably from 0.02 to 0.1 wt.%, based on the total weight of the lubricant composition. The amount of zinc thiophosphate this corresponds to depends on the molecular weight of the R groups as defined in formula (III) above. Typically, however, the amount of zinc thiophosphate compound incorporated into the lubricant composition is preferably from 0.01 to 3.0 wt.%, more preferably from 0.1 to 2.0 wt.%, and most preferably from 0.2 to 1.0 wt.%, based on the total weight of the lubricant composition.

The ratio of organo-molybdenum compound (a) to zinc thiophosphate compound (b) is preferably such that the weight ratio of molybdenum to phosphorus in the lubricant composition, due to the presence of compounds (a) and (b), is from 1:50 to 100:1, more preferably from 1:10 to 20: 1, and most preferably from 1:1 to 10:1. In a preferred embodiment of the invention, the molybdenum replaces at least 50% by weight of the phosphorus that would otherwise be contained in the lubricant composition if the sole antiwear additive were ZDDP.

The base oil employed in the lubricant composition according to the invention may be any base oil of viscosity suitable for use of the lubricant in an engine e.g. as a crankcase oil or gear oil. Thus the base oil may be, for example, a conventionally refined mineral oil, an oil derived from coal tar or shale, a vegetable oil, an animal oil, a hydrocracked oil, a hydroisomerised oil, or a synthetic oil, or a mixture of two or more of these types of oils.

Examples of synthetic oils include polyalphaolefins, polybutene, alkylbenzenes, polyglycols, esters such as polyol esters or dibasic carboxylic acid esters, alkylene oxide polymers, and silicone oils. The viscosity of the base oil depends upon the intended use, but generally is in the range of from 3 to 26 cSt (mm ² /s) at 100°C, preferably from 3 to 20 cSt (mm ² /s) at 100°C.

The antiwear additive compounds (a) and (b) may be mixed directly with the base oil, but, for ease of handling and introduction of the compounds to the base oil, are preferably in the form of additive concentrate comprising the additive compound, or mixture of both compounds, contained in a carrier fluid. Thus in a further aspect the present invention provides an additive concentrate comprising (a) an organo-molybdenum compound as defined above, (b) a zinc thiophosphate compound as defined above, and (c) a carrier fluid. The carrier fluid is typically an oil and may be, for example, any of the oils mentioned above in the description of the base oil. Alternatively, it may be an organic solvent, for example naphtha, benzene, toluene, xylene and the like. The carrier fluid should be compatible with the base oil of the lubricant composition, but otherwise is preferably inert. Generally the concentrate will comprise from 10 to 90 wt.% of - the additive(s), preferably from 30 to 70 wt.%, the balance being the carrier fluid.

The lubricant composition according to the invention may also contain other additives, which may be added directly to the base oil, as a separate additive concentrate, or included in the concentrate of the antiwear additives. For example, where the lubricant is an engine oil, other additives that may be incorporated include one or more of a detergent, dispersant, antioxidant, corrosion inhibitor, extreme pressure agent, antifoaming agent, pour point depressant and viscosity index improver. Such additives are well-known and the selection of appropriate additives could readily be determined by a person skilled in the art of lubricant formulating.

The lubricant composition may find use in any application where the parts to be lubricated are subject to wear. It is especially suitable for use as an engine oil (i.e. crankcase oil) for internal combustion engines.

The invention is illustrated by the following Examples in which two levels of molybdenum dithiocarbamate are compared to molybdenum 2-ethyl hexanoate at the lower level, and to ZDDP alone, sufficient ZDDP being added to maintain the phosphorus level constant throughout.

Example 1 (Comparative)

A number of engine oils were formulated as listed below using as a basecase oil a conventional engine oil formulation based on a conventionally refined mineral oil and containing standard engine oil additives except that zinc dialkyldithiophosphate (ZDDP) was omitted. The engine oil had a viscosity of 14 cSt at 100°C.

The ZDDP used was PARANOX 14 (trade name) supplied by Exxon Chemical Company Ltd. A treat level of 1.0 wt.% of this ZDDP provides 0.08 wt.% phosphorus measured using X-ray fluorescent analysis, standard test AMS 86.002.

The organo-molybdenum compound used in the comparative examples was MOLYVA 822 (trade name), a sulphurised oxy-molybdenum dithiocarbamate C'MoDTC") supplied by R.T. Vanderbilt Company Ltd. A treat level of 1.0 wt.% of this MoDTC provides 0.05 wt.% molybdenum measured using ICP (inductively-coupled plasma) analysis.

All percentages are by weight based on the weight of the fully formulated engine oil.

Engine Oil 1A : Basecase Oil containing no ZDDP and no MoDTC

Engine Oil IB : Basecase Oil containing 0.63% ZDDP (= 0.05% P) and no MoDTC

Engine Oil 1C : Basecase Oil containing no ZDDP and 1.0% MoDTC (= 0.05% Mo)

Engine Oil ID : Basecase Oil containing 0.63% ZDDP (= 0.05% P) and 1.0% MoDTC (= 0.05% Mo)

Engine Oil IE : Basecase Oil containing 0.63% ZDDP (= 0.05% P) and 0.2% MoDTC (= 0.01% Mo)

The resulting engine oils were tested for valve train wear by measuring camshaft wear and tappet scuffing using a motored cylinder head test rig which is equivalent to the industry standard TU-3 engine test CEC L-38-T-87, which test procedure is available from the CEC Secretariat, 61 New Cavendish Street. London Wl 8AB. The results are given in Table 1 below.

Example 2

An engine oil was formulated using the same basecase engine oil as in Example 1 and containing 0.63 wt.% of the ZDDP compound PARANOX 14, and 0.07 wt.% of MOLYHEXCEM (trade name), molybdenum 2-ethyl hexanoate available from Mooney Chemicals, which provides 0.01 wt.% molybdenum measured using IPC analysis.

The resulting engine oil, labelled Engine Oil 2, was tested for valve train wear as described in Example 1, and the results are given in Table 1 below.

Table 1

Example %Mo %P Camshaft wear (μm) Tappet Scuffing (merit)

1A 36.9 5.0

IB 0 0..0055 7.4 5.6

1C 0 0..0055 32.6 5.6

ID 0 0..0055 0 0..0055 2.1 7.6

IE 00..0011 00..0055 4.6 7.5

0.01 0.05 5.4 7.5

Good results are indicated by a low value for camshaft wear and a high value for tappet scuffing. Thus from Table 1 it can be seen that the addition of ZDDP as the sole antiwear additive (Example IB) provides some improvement in camshaft wear and a small improvement in tappet scuffing. The addition of MoDTC as the sole antiwear additive (Example 1C) provides a negligible improvement in camshaft wear and a small improvement in tappet scuffing. The addition of a combination of ZDDP and MoDTC or molybdenum carboxylate as antiwear additives (Examples ID, IE and 2) provides a significant improvement in both camshaft wear and tappet scuffing. Although (with ZDDP) addition of MoDTC at the 0.05 wt.%

level produces the best result for camshaft wear there is no significant improvement over the 0.1 wt.% level in tappet scuffing, and at this level there is Uttle difference in performance between MoDTC and Mo 2-ethyl hexanoate, both being at an acceptable level and representing an improvement over the use of ZDDP alone.