This invention relates to the use of certain additives in lubricating oils, especially in crankcase lubricating oils, which make possible a reduction in wear when an engine is operating at low temperatures.

Crankcase lubricating oils normally contain one or more antiwear agents, that is, agents which reduce the wear of metal parts. Commonly used antiwear agents include dihydrocarbyl dithiophosphate metal salts, the zinc salts being the most widely used. Such zinc salts, known as ZDDPs, may be prepared by, for example, first forming a dithiophosphoric acid, usually by reaction of an alcohol or a phenol with P ₂ ^S _5/ ^and then neutralising the dithiophosphoric acid with a zinc compound, for example, zinc oxide, zinc hydroxide or zinc carbonate, optionally in the presence of a promoter. Typically ZDDPs used as antiwear agents in crankcase lubricants have the formula Zn[SP(S)(OR) (OR ¹ ) ] ₂ where R and R ¹ may be the same or different hydrocarbyl radicals containing from 1 to about 18 carbon atoms.

Although ZDDPs have proved to be very satisfactory antiweaf agents for many purposes, there still remains a need to impart adequate low temperature antiwear properties to crankcase lubricants because lubricants containing ZDDPs have been found to fail tests in which the lubricants are evaluated at low temperatures and low speeds. Tests carried out under such conditions simulate those in an engine while it is warming up.

An automotive engine will typically operate, when at its normal running temperature, at a lubricant temperature in the range of from 80 to 150°C. At the beginning of the. warming-up period, however, the lubricant temperature will be lower than 80°C and, on a cold day, may be as low as 0°C or below. Tests for evaluating engine wear at low temperatures, therefore.

are typically run at least in part at lubricant temperatures in the range of from 20 to less than 80 ^° C, these temperatures normally being measured in the main oil gallery.

Low temperature wear tests generally evaluate the ability of an oil to protect the valve train of the engine. Although test details and conditions vary from manufacturer to manufacturer, most tests of this kind measure cam wear (measured directly from a notional line extending longitudinally along the surface of the cam at the cam nose) and some also measure follower (rocker pad) scuffing (which is usually given a visual merit rating) .

The applicants have now surprisingly found that the use together in a lubricating oil of certain ZDDPs and certain metal-containing detergents makes it possible to provide an oil having particularly advantageous low temperature properties. In particular, the invention makes it possible to provide lubricating oils which perform well in one or more low temperature wear tests.

There have been prior proposals to formulate lubricating oils containing both metal-containing detergents and ZDDPs. US-A-4375418 is concerned with such lubricating compositions having reduced friction properties in medium and high speed marine diesel engines. US-A-4387033 relates to an improved process for the production of highly basic calcium sulphonates and discloses formulations containing such sulphonates and ZDDPs. EP-A-113045 relates to the reduction of the mechanical friction loss of small high power four-cycle engines, and discloses lubricating oil compositions containing metal-containing detergents and ZDDPs. EP-A-317348, which relates to lubricating oil compositions which exhibit improvements in, among other things, low temperature cylinder wear and ring wear, discloses lubricating oil compositions which comprise an oil of lubricating viscosity as the major component and

as the minor component (A) a mixture of (1) at least one calcium overbased detergent inhibitor and (2) at least one magnesium overbased detergent inhibitor, and (B) a mixture of at least one zinc di-(primary hydrocarbyl) dithiophosphate and (2) at least one zinc di-(secondary hydrocarbyl) dithiophosphate, the ratios of the weights of (A) to (B) , A(l) to A(2) , and B(l) to B(2) being within specified ranges.

The invention provides the use of at least one metal-containing detergent and at least one zinc dihydrocarbyl dithiophosphate as lubricating oil additives in a crankcase lubricating oil for reducing wear in an engine while operating below normal running temperatures (i.e. typically below 80°C) , at least 50 mass % of the total metal-containing detergent being overbased calcium detergent having a total base number of at least 250, at least 50 mass % of the total metal in the metal-containing detergent being calcium, at least 35 mole % of the zinc dihydrocarbyl dithiophosphate hydrocarbyl groups being secondary hydrocarbyl groups containing up to 5 carbon atoms, and 0 to 25 mole % of the zinc dihydrocarbyl dithiophosphate hydrocarbyl groups being primary hydrocarbyl groups.

The invention also provides a method of reducing wear in an engine during warming up of the engine to normal running temperatures, in which the engine is lubricated by a lubricating composition comprising at least one metal-containing detergent and at least one zinc dihydrocarbyl dithiophosphate, at least 50 mass % of the total metal-containing detergent being overbased calcium detergent having a total base number of at least 250, at least 50 mass % of the total metal in the metal- containing detergent being calcium, at least 35 mole % of the zinc dihydrocarbyl dithiophosphate hydrocarbyl groups being secondary hydrocarbyl groups containing up to '5 carbon atoms, and 0 to 25 mole % of the zinc dihydrocarbyl dithiophosphate hydrocarbyl groups

being primary hydrocarbyl groups.

The term "hydrocarbyl" as used herein means that the group concerned is primarily composed of hydrogen and carbon atoms but does not exclude the presence of other atoms or groups in a proportion insufficient to detract from the substantially hydrocarbon characteristics of the group concerned.

Total base numbers given in this specification are measured in accordance with ASTM D 2896.

It is to be understood that, where appropriate, references herein to the detergent and ZDDP in the singular include the use of mixtures of detergents or mixtures of ZDDPs.

Metal-containing detergents may be present, for example, as the metal salts of sulphonic acids, alkyl phenols, sulphurised alkyl phenols, thiophosphonates, alkyl salicylates, naphthenates and other oil-soluble mono- and di-carboxylic acids. Mixtures of two or more of these metal salts may be used. The detergents have a polar head (the salt-forming group) and one or more groups of relatively high molecular weight so that the substance has detergent (surfactant) properties.

Tlϊe metals are usually alkali or alkaline earth metals. The most commonly used are Ca or Mg, or mixtures of Ca and Mg or mixtures of Ca and/or Mg with Na.

If desired a mixture of at least one calcium detergent having a TBN below 250 and at least one calcium detergent having a TBN greater than 250 may be used provided that the overall TBN of the calcium detergent component is at least 250. The overbased calcium detergent may be the only metal-containing detergent present, although minor amounts (less than 50 mass % of the total metal-containing detergent) of other metal- containing detergents, for example, an overbased magnesium detergent or even magnesium and sodium detergents together, may be used. The calcium detergent

component used in accordance with the invention advantageously has a TBN of at least 300, preferably 400. The TBN of any additional detergent is advantageously at least 20, preferably at least 250, more preferably at least 400. The TBN of the final lubricating oil is advantageously in the range of from 2 to 20.

Preferred calcium-containing detergents for use in accordance with the invention are overbased (or highly basic) calcium sulphonates, while preferred additional detergents are overbased magnesium sulphonates. Overbased metal sulphonates may be produced, for example, by heating a mixture comprising an oil-soluble alkaryl sulphonic acid or sulphonate and an excess of metal compound above that required for complete neutralization of any sulphonic acid present, and thereafter forming a dispersed carbonate complex by reacting the excess metal with carbon dioxide to provide the desired overbasing.

Sulphonic acids suitable for use in the preparation of overbased sulphonates are typically obtained by the sulphonation of alkyl-substituted aromatic hydrocarbons. The alkaryl sulphonates usually contain from 9 to 70 carbon atoms, preferably from 16 to 50 carbon atoms, per alkyl-substituted aromatic moiety.

The metal compounds which may be used in neutralizing these alkaryl sulphonic acids to provide the sulphonates include the oxides and hydroxides, alkoxides, carbonates, carboxylates, sulphides, hydrosulphides, nitrates, borates and ethers of sodium, magnesium, calcium, strontium and barium. Examples are calcium oxide, calcium hydroxide, magnesium oxide, magnesium acetate and magnesium borate.

Overbased phenates, thiophosphonateε, salicylates and naphthenates may be prepared by methods well known in the art.

Overbased calcium detergents, and any other overbased metal-containing detergents, used in accordance with the invention are preferably used as oil

concentrates containing 10 to 80, preferably 40 to 70, mass % active ingredient. The final oil advantageously contains 0.2 to 2.0, preferably 0.5 to 1.6, mass % metal- containing detergent on an active ingredient basis.

The ZDDP component used in accordance with the invention may be derived from an alcohol of the formula R- _j ^CHOH, wherein each of R and R ₂ represents a hydrocarbyl group, advantageously an aliphatic hydrocarbyl group, preferably an alkyl group, having 1 to 3 carbon atoms and the total number of carbon atoms in R*L and R ₂ is no more than 4, or from an alcohol mixture containing such an alcohol. In any such alcohol mixture at least 35 mole % of the mixture must comprise at least one alcohol of the formula R- _j ^CHOH, and the proportion of any primary alcohol(s) present must not exceed 25 mole %. Advantageously, such an alcohol mixture does not contain tertiary alcohols.

The ZDDP may be prepared by reacting phosphorus pentasulphide with the appropriate alcohol or alcohol mixture at a temperature of about 20 to 200°C to give the corresponding dihydrocarbyl dithiophosphoric acid(s) and the acid(s) is/are reacted with a basically reacting zinc compound, advantageously ZnO, to give the ZDDP(s). The preparation of ZDDPs by a method of this type is well known to those skilled in the art, and need not be described in detail.

It will be appreciated that although the preparation of a ZDDP from a single secondary alcohol of the formula R- _j ^CHOH will give rise to a ZDDP in which both hydrocarbyl groups are R- _j ^CH- groups, the preparation of a ZDDP from a mixture of alcohols R _a OH and R^OH will give rise to a statistical mixture of ZDDPs in which the two hydrocarbyl groups are R _a R _a , R _a ^R b' ^{and R} b ^R b respectively. Such a mixture can be used in the present invention without separation of the individual components.

Preferably, the ZDDP component comprises at least one ZDDP containing both an R _a group and an R _j -, group (that is, groups of different reactivities are present in the same molecule) rather than merely being a mixture of a more reactive compound and a less reactive compound.

Especially preferred secondary alcohols from which ZDDPs used in accordance with the invention may be derived are isopropyl alcohol and sec-butyl alcohol. The ZDDP component is advantageously the mixture of ZDDPs derived from an alcohol mixture comprising 30 mass % (42 mole %) isopropyl alcohol and 70 mass % (58 mole %) methyl sec-butyl methanol or the mixture of ZDDPs derived from an alcohol mixture containing 85 mass % (91 mole %) sec-butyl alcohol and 15 mass % (9 mole %) isooctyl alcohol.

In an oil containing the additives in accordance with the invention, the total ZDDP content is preferably in the range of from 0.4 to 1.8, preferably 0.7 to 1.4, mass % active ingredient, based on the final oil.

The metal-containing detergent component and the ZDDP component are preferably used in mass proportions, on an active ingredient basis, in the range of 5:1 to 1:9, preferably 2:1 to 1:3.

The additives used in accordance with the invention are oil-soluble or (in common with certain of the other additives referred to below) are dissolvable in oil with the aid of a suitable solvent, or are stably dispersible materials. Oil-soluble, dissolvable, or stably dispersible as that terminology is used herein does not necessarily indicate that the materials are soluble, dissolvable, miscible, or capable of being suspended in oil in all proportions. It does mean, however, that the additives are, for instance, soluble or stably dispersible in oil to an extent sufficient to exert their intended effect in the environment in which the oil is

employed. Moreover, the additional incorporation of other additives may also permit incorporation of higher levels of a particular additive, if desired.

Base oils with which the additives may be used include those suitable for use as crankcase lubricating oils for spark-ignited and compression-ignited internal combustion engines, for example, automobile and truck engines, marine and railroad diesel engines, including synthetic and natural base oils.

The lubricating oil base stock conveniently has a viscosity of about 2.5 to about 12 cSt (about 2.5 x 10 ^~6 to about 12 x 10 ^~6 m ² /s) and preferably about 2.5 to about 9 cSt. (about 2.5 x 10~ ⁶ to about 9 x 10 ^"6 m ² /s) at 100°C.

The additives used in accordance with the present invention may be employed in a lubricating oil composition which comprises lubricating oil, typically in a major proportion, and the additives, typically in a minor proportion, for example, in a proportion as indicated above. Additional additives may be incorporated in the composition to enable it to meet particular requirements. Examples of additives which may be included in lubricating oil compositions are viscosity index improvers, corrosion inhibitors, oxidation inhibitors, friction modifiers, dispersants, anti-foaming agents, other anti-wear agents, pour point depressants, and rust inhibitors.

When lubricating compositions contain one or more of the above-mentioned additives, each additive is typically blended into the base oil in an amount which enables the additive to provide its desired function. Representative effective amounts of such additives, when used in

crankcase lubricants, are as follows:

Additive Mass % a.i.* Mass % a.i.*

(Broad) (Preferred)

Viscosity Modifier 0.01-6 0.01-4

Corrosion Inhibitor 0.01-5 0.01-1.5

Oxidation Inhibitor 0.01-5 0.01-1.5

Dispersant 0.1-20 0.1-8

Pour Point Depressant 0.01-5 0.01-1.5

Anti-Foaming Agent 0.001-3 0.001-0.15

Friction Modifier 0.01-5 0.01-1.5

Mineral or Synthetic Oil Base Balance Balance

* Mass % active ingredient based on the final oil.

When a plurality of additives are employed it may be desirable, although not essential, to prepare additive concentrates comprising the additives (the concentrate being referred to herein as an additive package) whereby several additives can be added simultaneously to the base oil to form the lubricating oil composition. Dissolution of the additive concentrate into the lubricating oil may be facilitated by solvents and by mixing accompanied with mild heating, but this is not essential. The concentrate or additive package will typically be formulated to contain the additive(s) in proper amounts to provide the desired concentration in the final formulation when the additive package is combined with a predetermined amount of base lubricant. Thus, one or more additives used in accordance with the present invention can be added to small amounts of base oil or other compatible solvents along with other desirable additives to form additive packages containing active ingredients in an amount, based on the additive package, of, for example, from 2.5 to 90 mass %, and preferably from 5 to 75 mass %, and most preferably from 8 to 50 mass % by weight, additives in the appropriate proportions with the remainder being

base oil .

The final formulations may employ typically about 10 mass % of the additive-package with the remainder being base oil.

The following Examples illustrate the invention.

Examples 1 to 8 and Comparative Examples 1 to 6

A series of fully formulated lubricating oil compositions were prepared containing the detergents and ZDDPs specified in Tables I to IV below in addition to viscosity modifier, corrosion inhibitor, oxidation inhibitor, dispersant, pour point dispersant, anti-foam and friction modifier additives used in conventional proportions (which were the same for each composition) .

Each composition was tested in one or more low temperature wear tests and the results obtained are summarized in Tables I to IV. The substances and tests identified in the Tables by abbreviations or symbols were:

300 TBN Ca: an overbased calcium sulphonate, containing 53 % active ingredient, having a TBN of approximately 300 and containing 11.9 mass % calcium, the percentages being calculated on the total mass of the active ingredient plus diluent oil.

400 TBN Ca: an overbased calcium sulphonate, containing 55 mass % active ingredient, having a TBN of approximately 400 and containing 15.5 % mass % calcium, the percentages being calculated on the total mass of the active ingredient plus diluent oil.

400 TBN Mg: an overbased magnesium sulphonate, containing 55 % active ingredient, having a TBN of approximately 400 and containing 9.2 mass % of magnesium, the percentages being calculated on the total mass of the active ingredient plus diluent oil.

A : a ZDDP derived from an alcohol mixture containing 30 mass % (42 mole %) isopropyl alcohol and 70 mass % (58 mole %) 4 methyl- pentan-2-ol.

B : a ZDDP derived from an alcohol mixture containing 85 mass % (91 mole %) sec-butyl alcohol and 15 mass % (9 mole %) isooctyl alcohol.

C : a ZDDP derived from an alcohol mixture containing 65 mass % (69 mole %) isobutyl alcohol and 35 mass % (31 mole %) isoamyl alcohol.

D : a ZDDP derived from isooctyl alcohol

Peugeot TU3: Test Method CEC L-38-T-87

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TABLE 1

Formulation Ex. 1 Ex. 2 Comp. 1 Camp. 2

Avg. follower wear 1.9 2.0 0.5 8.0 (μm/100 hrs)

*mass % active ingredient based on the final oil

[l m t 7.5 mn]

* mass % active ingredient based on the final oil Data in brackets are from repeated tests.

TABLE III

* mass % active ingredient based on the final oil

TABLE IV

Avg.cam wear, μm 14.8 3.1 27.0 [limit 25 max]

Max.follower wear, μm 1.6 2.3 4.9 [limit 3.5 max]

Avg.follower wear, μm 0.9 1.4 2.9 [limit 2.0 max] mass % active ingredient based on the final oil

The results summarized in Tables I to IV indicate that the use of detergents and ZDDPs in accordance with the invention gives an improvement in low temperature wear properties and, in particular, makes it possible to formulate lubricating oils capable of performing well in low temperature wear tests.