LUBRICATING COMPOSITIONS

Abstract: A lubricating composition which comprises a base oil of mineral and/or synthetic origin and, as a friction reducing additive, a combination of molybdenum disulphide, one or more zinc dithiocarbamates and one or more metal dithiophosphates, which composition contains no substantial amount of zinc naphthenate.

Description: LUBRICATING COMPOSITIONS The present invention relates to lubricating compositions, particularly to lubricating greases containing such compositions, and more particularly to lubricating greases for use in constant velocity joints of all types, e.g. to constant velocity fixed ball joints and to the plunging type constant velocity joint. The primary purpose of lubrication is separation of solid surfaces moving relative to one another, to minimise friction and wear. The materials most frequently used for this purpose are oils and greases. The choice of lubricant is mostly determined by the particular application. Lubricating greases are employed where heavy pressures exist, where oil drip from the bearings is undesirable or where the motion of the contacting surfaces is discontinuous so that it is difficult to maintain a separating film in the bearing. Because of design simplicity, decreased sealing requirements and less need for maintenance, greases are almost universally given first consideration for lubricating ball and roller bearings in electric motors, household appliances, automotive wheel bearings, machine tools or aircraft accessories. Greases are also used for the lubrication of small gear drives and for many slow- speed sliding applications. Lubricating greases consist primarily of a fluid lubricant, such as an oil, and a thickener. Essentially, the same type of oil is employed in compounding a grease as would normally be selected for oil lubrication. Fatty acid soaps of lithium, calcium, sodium, aluminium and barium are most commonly used as thickeners. However, thickeners may be one of a variety of solid materials, including clays, soaps and complexes such as those of lithium, and urea-type compounds . The base oil may be of mineral or synthetic origin. Base oils of mineral origin may be mineral oils, for example produced by solvent refining or hydro- processing. Base oils of synthetic origin may typically be mixtures of hydrocarbon polymers, for example liquid polymers of alpha-olefins . They may also be conventional esters, for example polyol esters. The base oil may also be a mixture of these oils. Preferably the base oil is that of mineral origin sold by the Royal Dutch/Shell Group of Companies under the designations "HVI" or "MVIN", is a polyalphaolefin, or is a mixture of the two. Synthetic hydrocarbon base oils, for example those sold by the Royal Dutch/Shell Group of Companies under the designation "XHVI" (trade mark) may also be used. A lubricating grease preferably contains 5 to 20% by weight of thickener. Lithium soap thickened greases have been known for many years. Typically, the lithium soaps are derived from Cη_o-2 / preferably Ci5_ι_g, saturated or unsaturated fatty acids or derivatives thereof. One particular derivative is hydrogenated castor oil, which is the glyceride of 12-hydroxystearic acid. 12-Hydroxystearic acid is a particularly preferred fatty acid. Greases thickened with complex thickeners are well known. In addition to a fatty acid salt, a complexing agent is incorporated into the thickener, which is commonly a low to medium molecular weight acid or dibasic acid or one of its salts, such as benzoic acid or boric acid or a lithium borate. Urea compounds used as thickeners in greases include the urea group (-NHCONH-) in their molecular structure. These compounds include mono-, di- or polyurea compounds, depending upon the number of urea linkages. Various conventional grease additives may be incorporated into the lubricating greases, in amounts normally used in this field of application, to impart certain desirable characteristics to the grease, such as oxidation stability, tackiness, extreme pressure properties and corrosion inhibition. Suitable additives include one or more extreme pressure/antiwear agents, for example zinc salts such as zinc dialkyl or diaryl dithiophosphates, borates, substituted thiadiazoles, polymeric nitrogen/phosphorus compounds made, for example, by reacting a dialkoxy amine with a substituted organic phosphate, amine phosphates, sulphurised sperm oils of natural or synthetic origin, sulphurised lard, sulphurised esters, sulphurised fatty acid esters, and similar sulphurised materials, organo- phosphates for example according to the formula (OR) 3P=0 where R is an alkyl, aryl or aralkyl group, and triphenyl phosphorothionate; one or more overbased metal-containing detergents, such as calcium or magnesium alkyl salicylates or alkylarylsulphonates; one or more ashless dispersant additives, such as reaction products of polyisobutenyl succinic anhydride and an amine or ester; one or more antioxidants, such as hindered phenols or amines, for example phenyl alpha naphthylamine; one or more antirust additives; one or more friction-modifying additives; one or more viscosity-index improving agents; one or more pour point depressing additives; and one or more tackiness agents. Solid materials such as graphite, finely divided molybdenum disulphide, talc, metal powders, and various polymers such as polyethylene wax may also be added to impart special properties. Zinc dithiocarbamates are sometimes used as antioxidants and also as corrosion inhibitors in some lubricants such as diesel and gasoline engine oils and industrial oils. In lubricants generally, the reduction of friction (i.e. the increase of slideability) combined with the reduction of wear (i.e. the reduction of surface damage caused by mechanical and/or corrosive action) is highly desirable . There have been a number of proposals to reduce friction levels using a variety of additives, usually incorporating organic molybdenum-based formulations. Often the friction reducing effect is thickener dependent, i.e. an additive will work well with one thickener type but not with another. Molybdenum disulphide (a non-organic molybdenum compound) is a known highly wear resistant solid lubricant and is used in a wide variety of lubricants. Equally common is the use of metal (dialkyl or diaryl) dithiophophates in lubricants as extreme pressure or anti wear agents. Tribology Transactions, vol 33, no. 3, (1990) pages 345 to 354 reviews the effects of organic molybdenum compounds (such as molybdenum dialkyl dithiocarbamates (MoDTC) and molybdenum dithiophosphates (MoDTP) ) on wear and friction with zinc dialkyl dithiophosphate (ZnDTP) -containing lubricant blends. In the early part of the review, tests on various components in a lubricating oil establishes inter alia that zinc compounds, including a zinc dithiocarbamate, exhibited similar high load antiwear performance to a molybdenum dithiocarbamate. Later in the review, a number of products commonly speculated to be decomposition products of MoDTC or MoDTP and ZnDTP additives, were suspended into a reference grease for independent evaluation to determine whether indeed any of those products gave good friction and antiwear properties. Molybdenum disulphide, thought by some to be such a decomposition product, was tested in a reference grease thickened with a lithium 12-hydroxy stearate soap and containing 0.5% mass zinc dibutyl dithiocarbamate as an oxidation inhibitor. The resulting grease exhibited high (i.e. undesirable) friction and only moderate anti-wear properties; zinc compounds tested separately in the same reference grease also showed no reduced friction. In WO 97/03152, a friction reducing additive combination has been described comprising molybdenum disulphide, zinc naphthenate and metal dithiophosphate optionally in combination with metal dithiocarbamates. In Example 22 it is shown that the presence of zinc naphthenate is essential. It has now surprisingly been found that molybdenum disulphide, zinc dithiocarbamate and a metal dithiophosphate in combination work synergistically as a friction reducing agent in lubricating compositions, especially greases, whilst retaining good, low anti- wear properties. It has been found that zinc naphthenate does not need to be present in this combination. Furthermore such combination is not thickener dependent. Tested against the use of molybdenum disulphide alone or in combination with one of the two other components, the friction reduction is shown to be quite unexpected. The use of lubricating oil compositions containing molybdenum disulphide, molybdenum dithiocarbamate and metal dithiophosphate has been described in WO 94/11470 and GB-A-2 255 103. WO 94/11470 or GB-A-2 255 103 neither teach nor hint that the molybdenum dithiocarbamate might be replaced by another metal dithiocarbamate . The present invention accordingly provides a lubricating composition which comprises a base oil of mineral and/or synthetic origin and a combination of molybdenum disulphide, one or more zinc dithiocarbamates and one or more metal dithiophosphates which composition contains no substantial amount of zinc naphthenate. The composition of the present invention contains no substantial amount of zinc naphthenate. This means that the composition contains less than 0.3% by weight of zinc naphthenate, based on total weight of lubricating composition. The amount of zinc naphthenate is the amount of compound per se, excluding further compounds which may be present in a commercial product e.g. mineral oil. The composition preferably contains less than 0.05% by weight of zinc naphthenate, more preferably less than 0.01% wt . Most preferably, the composition does not contain zinc naphthenate. Preferred is the use of the friction reducing additive combination in a lubricating grease which comprises a base oil of mineral and/or synthetic origin and a thickener, which is preferably a lithium soap, either simple or complex, or a urea compound. Such a lubricating grease preferably contains molybdenum disulphide in an amount of from 0.5 to 10% by weight, more preferably 1 to 4% by weight. Independently, the grease preferably contains zinc dithiocarbamate in an amount of from 0.01 to 5% by weight, more preferably 0.3 to 2.4% by weight, for a urea thickened grease and a complex lithium soap thickened greases, and an amount of from 0.5 to 10% by weight, more preferably 1 to 4% by weight, for a simple lithium soap thickened grease. A lubricating grease of the invention further preferably contains said one or more metal dithiophosphates in a total amount of from 0.15 to 10% by weight; more preferably 1 to 3% by weight. All amounts are based on total weight of the grease composition. It is especially useful if the three components are present in an interrelated or 3-way proportion such that, for a urea thickened grease and for a complex lithium soap thickened grease the weight ratio of molybdenum disulphide to metal dithiophosphate is in the range of from 1:0.15 to 1:1, the weight ratio of metal dithiophosphate to zinc dithiocarbamate is in the range of from 1:0.25 to 1:1.5, and the weight ratio of molybdenum disulphide to zinc dithiocarbamate is in the range of from 1:0.4 to 1:1.5. For a simple lithium soap thickened grease the weight ratio of molybdenum disulphide to metal dithiophosphate preferably is in the range of from 1:0.15 to 1:1, the weight ratio of metal dithiophosphate to zinc dithiocarbamate is in the range of from 1:1 to 1:4.5 and the weight ratio of molybdenum disulphide to zinc dithiocarbamate is in the range of from 1:0.4 to 1:1.5. Preferably, the one or more metal dithiophosphates is/are selected from zinc dialkyl-, diaryl- or alkylaryl-dithiophosphates, and the one or more zinc dithiocarbamates is/are selected from zinc dialkyl-, diaryl- or alkylaryl-dithiocarbamates, in which dithiophosphates and/or dithiocarbamates usefully any alkyl moiety is straight chain or branched and preferably contains from 1 to 12 carbon atoms. The thickener of lubricating greases, as mentioned hereinbefore, preferably comprises a urea compound, a simple lithium soap or a complex lithium soap. A preferred urea compound is a polyurea compound. Such thickeners are well known in lubricant grease technology. Surprisingly effective compositions of the invention use simple lithium soaps as thickeners. Lubricating compositions of the invention may be prepared by incorporating the additive combination into a base oil in conventional manner. For greases this may be via hot or cold mixing followed by homogenisation to ensure uniform dispersion of the additive components. Other additives, e.g. antioxidants, may be included if necessary or desired. In accordance with the present invention there is further provided a method of lubricating a constant velocity joint comprising packing it with a lubricating grease according to the present invention. In accordance with the present invention there is still further provided a constant velocity joint packed with a lubricating grease according to the present invention. The present invention will now be described by reference to the following Examples, in which all percentages are given by weight. Example 1 A lithium soap grease was prepared by adding a slurry of 1.12% w LiOH.F^O and water in the proportions of 1 part LiOH.I^O to 5 parts water to 9.15% w hydrogenated castor oil fatty acid in cold base oil (a blend of MVIN and HVI oils) and heating the mix in a sealed autoclave to 150°C. The steam was vented off and heating continued to 220°C before the reaction mass was quick cooled (at a rate of 6 to 7°C per minute) and the product homogenised. Additives were incorporated into the grease. In all cases, as antioxidant, 0.5% w of an aromatic amine was used. A grease of the present invention and a number of comparison greases were prepared and tested. For each grease the amount of molybdenum disulphide, ZnDTC (zinc diamyl dithiocarbamate) and ZnDTP (4-methyl-2-pentyl zinc dithiophosphate) was varied as shown in Table 1. The friction coefficient and wear scar diameter were evaluated for each grease; the results are also given in Table 1. The friction and wear measurements were made using an oscillating SRV friction tester. For friction measurements the oscillating SRV friction tester (from Optimol Instruments) was used with a 10 mm ball on a flat lapped surface as test geometry. An oscillation frequency of 50 Hertz and a stroke of 1.5 mm was used throughout. The friction coefficient was recorded after two hours of operation under the test conditions of a load of 300 Newtons at a temperature of 100°C. Wear was assessed by measuring the diameter of the wear scar on the ball at the end of the two hour test using an optical graticule. A grease of the present invention was compared with lithium greases having only molybdenum disulphide or molybdenum disulphide plus zinc dithiocarbamate or molybdenum disulphide plus zinc dithiophosphate all with the same base grease plus thickener plus antioxidant. It can be clearly seen from the results that the grease of the present invention has a significantly lower friction coefficient and wear scar diameter than any of the comparison greases. Example 2 A urea grease C was prepared by heating 5% w of 4, 4 '-diphenylmethane diisocyanate in base oil (a mixture of 75% w HVI 160B and 25% w HVI 650) to 70°C and then adding 10.8% w stearylamine . The mixture was then further heated to 150°C before being cooled to 80°C. The other additives to be included in the formulation were then added. The formulated grease was then homogenised at ambient temperature. A grease of the present invention and a number of comparison greases were prepared and tested. Again in all cases, an antioxidant, 0.5% w of an aromatic amine, was added and for each grease the amount of molybdenum disulphide, ZnDTC (zinc diamyl dithiocarbamate) and ZnDTP (4-methyl-2-pentyl zinc dithiphosphate) were varied. The friction coefficient and wear for each grease was evaluated as in Example 1. Results are given in Table 2. It can be seen that also for a urea-thickened grease a significantly low friction coupled with low wear is exhibited, but with a lower proportion of ZnDTC than for the lithium soap thickened grease of Example 1. Table 1 Grease of Example Molybdenum Friction Wear Scar No. Disulphide ZnDTC ZnDTP 3ef f icient Diameter (%w) (%w) (%w) (nm) 1 3 2 1 0. ,055 0. 50 Comparison A - 0. ,143 0. 85 Comparison B 3 - - 0. .138 0. 84 Comparison C 3 2 - 0. ,075 0. 58 Comparison D 3 - 1 0. 075 0. 65 Table 2 Grease of Example Molybdenum Friction Wear Scar No. Disulphi .de ZnDTC ZnDTP Coefficient Diameter (%w) (%w) (%w) (nm) 2 3 0.5 1 0.055 0.57 Comparison E - - - 0.100 0.46 Comparison F 3 - - 0.153 0.78 Comparison G 3 - 1 0.080 0.67 Comparison H 3 2 1 0.093 0.53 Exampl e 3 A lithium complex grease was prepared by adding a 50%w slurry of LiOH.H0 and boric acid in water to hydrogenated castor oil fatty acid, calcium alkyl salicylate, and calcium octoate, in conventional proportions, in base oil, (a mixture of HVI oils) together with conventional antioxidants, and then heating the charge to 210°C with stirring. After slowly cooling the mixture to 160°C a quinoline and a hydroxyphenyl were added as additional antioxidants. The whole was then slowly cooled to ambient temperature and molybdenum disulphide, ZnDTC (zinc diamyl dithiocarbamate and ZnDTP ( -methyl-2-pentyl zinc dithiophosphate) were added; the resulting grease was then homogenised. The grease obtained contained 3% w molybdenum disulphide, 1.5% w ZnDTC and 1.5% w ZnDTP as friction reducing additive. Using the same test methods as described in Example 1 above, the friction coefficient and wear for the grease were measured at 300N and 100°C as 0.058 and 0.48 nm respectively. Thus the surprising low friction coefficient coupled with low wear properties are also given when using a lithium complex thickened grease.

Claims: C L A I M S 1. A lubricating composition which comprises a base oil of mineral and/or synthetic origin and a combination of molybdenum disulphide, one or more zinc dithiocarbamates and one or more metal dithiophosphates, which composition contains no substantial amount of zinc naphthenate. 2. A composition as claimed in claim 1, wherein the one or more zinc dithiocarbamates are selected from zinc dialkyl-, diaryl-, and alkylaryl dithiocarbamates. 3. A composition as claimed in claim 1 or claims 2, wherein the one or more metal dithiophosphates are selected from zinc dialkyl-, diaryl- and alkylaryl dithiophosphates . 4. A composition as claimed in any one of claims 1 to 3, which is a grease and further contains a thickener. 5. A composition as claimed in claim 4, wherein the thickener is a simple lithium soap. 6. A composition as claimed in claim 5, wherein the weight ratio of molybdenum disulphide to metal dithiophosphate is in the range of from 1:0.15 to 1:1, the weight ratio of metal dithiophosphate to zinc dithiocarbamate is in the range of from 1:1 to 1:4.5 and the weight ratio of molybdenum disulphide to zinc dithiocarbamate is in the range of from 1:04 to 1:1.5. 7. A composition as claimed in claim 4, wherein the thickener is a urea compound or a complex lithium soap and wherein the weight ratio of molybdenum disulphide to metal dithiophosphate is in the range of from 1:0.15 to 1:1, the weight ratio of metal dithiophosphate to zinc dithiocarbamate is in the range of from 1:0.25 to 1:1.5, and the weight ratio of molybdenum disulphide to zinc dithiocarbamate is in the range of from 1:0.4 to 1:1.5. 8. A method of lubricating a constant velocity joint comprising packing it with a lubricating grease as claimed in any one of claims 4 to 7. 9. A constant velocity joint packed with a lubricating grease as claimed in any one of claims 4 to 7.

Inventors: FLETCHER ROBERT ANTHONY, FLETCHER, ROBERT, ANTHONY
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1998
Main classification: C10M141
IPC codes: C10M133, C10M139, C10M141, C10M169, C10N10, C10N30, C10N40, C10N50