IMPROVED LOAD-CARRYING GREASE

Abstract: A grease composition has improved load-carrying capability by incorporation of a polyhydric alcohol having at least three hydroxy groups and at least two metal hydrocarbylthiophosphate compounds in which the metal is different in at least two compounds. Glycerol, antimony dialkyldithiophosphate and zinc dialkyldithiophosphate are preferred compounds.

Description: The present invention relates to a grease composition having improved load-carrying properties and a method of its preparation. The use of polyhydric alcohols such as glycerol as a grease additive is known. For example, U.S. Patent 2,858,273 discloses incorporating a polyhydric alcohol, a metal organodithiocarbamate and an alkanol amine into calcium soap greases to impart extreme pressure properties. Patentee teaches that the alkanol amine is essential to provide good mechanical stability. He further teaches that alcohols containing 2 to 4 hydroxyl groups are operative in his compositions. As another example, U.S Patent 3,133,020 discloses a lithium based grease having improved extreme pressure properties due to the incorporation therein of an aliphatic polyhydric alcohol and a metal organodithiocarbamate. U.S. Patent 3,158,574 describes a grease composition containing an aliphatic polyhydroxy additive as a structure modifier. The grease may also contain, amongst other compounds, a metal salt of a dialkyldithiophosphate, for example cadmium or zinc salts thereof, as an anti-oxidant compound. U.S. Patent 3,203,897 describes a grease composition which may include glycerin to increase the dropping point, and a metal salt of a dithiophosphoric acid ester, for example a calcium, barium or zinc salt thereof, to extend the operating life of the grease. European published patent application 155,131A describes a grease composition with high dropping point which optionally includes a metal phosphorodithioate additive. U.K. published patent application 2,185,492A describes a grease composition containing as an extreme pressure and friction reducing additive an organic molybdenum compound, for example molybdenum dialkyl dithiocarbamate or molybdenum dialkyl dithiophosphate. The grease may also contain an organic zinc compound, for example zinc dialkyl dithiophosphate. There is no disclosure of incorporating a polyhydric alcohol into the grease composition. However, none of the prior art references teach or suggest that the load-carrying capability of a grease composition can be significantly improved by incorporating therein the particular combination of additives claimed herein. Now according to the present invention, it has been discovered that the load-carrying capability of greases can be significantly enhanced by incorporating therein a polyhydric alcohol having at least three hydroxy groups and at least two metal hydrocarbylthiophosphate compounds in which the metal is different in at least two compounds. Preferred additives are glycerol, antimony dialkyldithiophosphate and zinc dialkyldithiophosphate. According to the present invention, there is provided a grease composition comprising: (a) a lubricating oil, (b) a thickener, (c) a polyhydric alcohol having at least three hydroxy groups, and (d) at least two metal hydrocarbylthiophosphate compounds in which the metal is different in at least two compounds. The present invention also contemplates a method for preparing said composition. A wide variety of lubricating oils can be employed in preparing the grease compositions of the present invention. Accordingly, the lubricating oil base can be any of the conventionally used mineral oils, synthetic hydrocarbon oils or synthetic ester oils, depending upon the particular grease being prepared. In general these lubricating oils will have a viscosity in the range of about 5 × 10⁻⁶ to about 0.01 m²/s (about 5 to about 10,000 cSt) at 40°C, although typical applications will require an oil having a viscosity ranging from about 1 × 10⁻⁵ to about 1 × 10⁻³ m²/s (about 10 to about 1,000 cSt) at 40°C. Mineral lubricating oil base stocks used in preparing the greases can be any conventionally refined base stocks derived from paraffinic, naphthenic and mixed base crudes. Synthetic lubricating oils that can be used include esters of dibasic acids, such as di-2-ethylhexyl sebacate, esters of glycols such as a C₁₃ oxo acid diester of tetraethylene glycol, or complex esters such as one formed from 1 mole of sebacic acid and 2 moles of tetraethylene glycol and 2 moles of 2-ethylhexanoic acid. Other synthetic oils that can be used include synthetic hydrocarbons such as polyalphaolefins; alkyl benzenes, e.g. alkylate bottoms from the alkylation of benzene with tetrapropylene, or the copolymers of ethylene and propylene; silicon oils, e.g. ethyl phenyl polysiloxanes, methyl polysiloxanes, etc.; polyglycol oils, e.g. those obtained by condensing butyl alcohol with propylene oxide; carbonate esters, e.g. the product of reacting C₈ oxo alcohol with ethyl carbonate to form a half ester followed by reaction of the latter with tetraethylene glycol, etc. Other suitable synthetic oils include the polyphenyl ethers, e.g. those having from about 3 to 7 ether linkages and about 4 to 8 phenyl groups. (See U.S. Patent 3,424,678, column 3.) The amount of lubricating oil in the grease can also vary broadly, but, typically, will range from about 50 to about 98 wt.%, preferably from about 75 to about 95 wt.%, of the grease. The grease will also contain a thickener dispersed in said lubricating oil. However, the particular thickener employed is not critical and can vary broadly. Accordingly, a thickener based on aluminum, barium, calcium, lithium and sodium soaps or their mixtures may be suitably employed. Clays, dyes, polyureas and other organic thickeners may be used as well. Pyrrolidone based thickeners can also be used. Preferred thickeners are lithium, calcium, their complexes or mixtures thereof. A lithium complex based thickener is particularly useful in the grease of the present invention. The amount of thickener in the grease will typically range from about 1 to about 30 wt.% of the grease, depending upon the physical characteristics desired in the finished grease. For most purposes, between about 5 to about 20 wt.%, preferably from about 10 to about 15 wt.%, of said thickener will be present in the grease. Usually, the greases of the present invention are prepared by dispersing or mixing the thickener in the lubricating oil followed by heating at elevated temperatures (e.g. from about 40° to about 260°C depending upon the particular thickener used) until the mixture thickens. Subsequently, the hot grease is cooled and additional oil and additives are incorporated to provide the final product. More specific information on grease preparation techniques may be found in C. J. Boner, Manufacture and Application of Lubricating Greases, Reinhold Publishing Corp., New York (1954) and NLGI Lubricating, Grease Guide, First Edition, Published by NLGI, Kansas City, Missouri (1980), the disclosures of which are incorporated herein by reference. Any of the aliphatic polyhydric alcohols having at least 3 hydroxy groups can be employed in the grease of the present invention. Alcohols containing 3 or 4 hydroxy groups are preferred. Specific examples of alcohols which may be used include glycerol, pentaerythritol and the like. Alcohols containing 3 hydroxy groups are more preferred, glycerol being particularly preferred. However, the other alcohols may also provide a substantial improvement in load carrying capability of the grease. The metal hydrocarbylthiohosphate compounds added to the grease may be represented by the formula: wherein n is 1-6, preferably 2-4 and more preferably 2-3; and x is 1-3, preferably 1-2 and preferably 1. R₁ and R₂ may each individually be a hydrocarbyl group and more particularly an alkyl, aryl, alkaryl or aralkyl group and the unsaturated counterparts thereof of 1 to 30, preferably 1 to 20 and more preferably 1 to 10 carbon atoms and one of R₁ or R₂ may be hydrogen. The groups as defined for R₁ and R₂ may include heterooxygen, nitrogen, sulfur or phosphorus atoms interspersed therein. Preferred hydrocarbylthiophosphate compounds are the alkylthiophosphates, with dialkyldithiophosphates being most preferred. Preferably, the metal alkylthiophosphate compounds comprise two metal dialkyldithiophosphate compounds. M can he any metal selected from the group of aluminum, antimony, cadmium, copper, lead, tin and zinc. Preferred metals are antimony, lead and zinc, with antimony and zinc being most preferred. Thus, preferred metal hydrocarbylthiophosphates include antimony dialkyldithiophosphates, lead dialkyldithiophosphates and zinc dialkyldithiophosphates. Accordingly, preferred metal hydrocarbylthiophosphate groups added to the present invention include antimony dialkyldithiophosphates in combination with lead dialkyldithiophosphates, zinc dialkyldithiophosphates or their mixtures, with antimony dialkyldithiophosphates and zinc dialkylkithiophosphates being most preferred. Although very minor amounts of the polyhydric alcohol and the metal hydrocarbylthiophosphate compounds provide improvement in the load-carrying capability of greases, these additives will normally be employed within certain ranges. In the case of the polyhydric alcohol, normally 0.1 to about 5 wt.%, preferably from about 0.2 to 1.0 wt.%, based on weight of the grease, will be employed. The total amount of metal hydrocarbylthiophosphate used can range from about 0.2 to about 10 wt.%, preferably from about 0.5 to about 4 wt.%, based on weight of the grease. Methods of preparing the polyhydric alcohol and metal hydrocarbylthiophosphate compounds used herein are well known to one skilled in the art. The polyhydric alcohol and metal hydro carbylthiophosphate compounds are preferably added to the grease during the final steps of its preparation. The grease may also contain small amounts of conventional additives which include, but are not limited to, anticorrosive agents, pour point depressants, tackiness agents, viscosity improvers, oxidation inhibitors, dyes and the like, which are incorporated for specific purposes. The multipurpose grease of the present invention has a variety of uses and may be suitably employed in essentially any application requiring a grease, including use in wheel bearings, industrial equipment and the like. The present invention will be further understood by reference to the following Examples which are not intended to restrict the scope of the claims appended hereto. In the Examples, the load-carrying characteristics of the various greases were determined by means of tests performed on a Timken test machine. In the Timken test, a hardened steel ring or "cup" is rotated against a hardened steel test block while being lubricated with the grease under test. A test grease is deemed a "failure" if the test block is damaged during the test, and a "pass" if the test block is not damaged. Further details regarding the Timken test procedure may be found in ASTM method D-2782, Example 1 ― Effect of Polyhydric Alcohol and Metal Dialkyldithiophosphates on the Timken Response of Lithium Complex Greases Timken tests were performed on several lithium complex base greases containing various amounts of antimony dialkyldithiophosphate, zinc dialkyldithiophosphate and glycerol. The results of these tests are shown in Table 1. The data in Table 1 show that the load-carrying capability of a lithium complex grease is significantly enhanced when glycerol and two metal dialkyldithiophosphate compounds each having a different metal component are added to said grease. Example 2 ― Effect of Auxiliary Additives on the Timken Response of a Fully Formulated Lithium Complex Grease Timken tests were performed on two samples of the same fully formulated lithium complex grease which contain antimony dialkyldithiophosphate, zinc dialkyldithiophosphate and other additives. One sample also contained glycerol. The results of these tests are shown in Table 2. The data in Table 2 show that the addition of glycerol, antimony dialkyldithiophosphate and zinc dialkyldithiophosphate to a fully formulated lithium complex grease results in a significant increase in the load-carrying capability of the grease. The data also show that other additives do not affect the Timken response of the grease. Example 3 ― Effect of Metal Dialkyldithiophosphate Concentration on the Timken Response of a Lithium Complex Grease Timken tests were performed on two samples of the same lithium complex base grease having an increased concentration of zinc dialkyldithiophosphate in one sample and an increased concentration of antimony dialkyldithiophosphate in the other. A polyhydric alcohol was not present in either sample. The results of these tests are shown in Table 3. Test sample K in Table 3 shows that the addition of 3.0 wt.% of zinc dialkyldithiophosphate to a lithium complex base grease does not enhance the load-carrying capability of said grease. This is consistent with test sample C in Table 1 which used 1.5 wt.% zinc dialkyldithiophosphate. Thus, the load-carrying capability of a grease is not improved by increased concentrations of zinc dialkyldithiophosphates. In contrast, test sample L in Table 3 shows that the addition of 1.5 wt.% antimony dialkyldithiophosphate to a lithium complex base grease dramatically increases the load-carrying capability of the grease (for comparison, see test sample B in Table 1 which used 0.25 wt.% antimony dialkyldithiophosphate). However, copper corrosion performance of test sample L was unacceptable as a value of 4 (heavy corrosion) was obtained using ASTM D 130/IP 154. When test sample B in Table 1 was subjected to ASTM D 130/IP 154, the sample passed with an acceptable value of 1. Thus, use of the additive system of the present invention allows the grease manufacturer to obtain the load-carrying benefits of antimony dialkyldithiophosphate, but at concentrations which do not result in unacceptable corrosion. Example 4 ― Effect of Glycerol on the Timken Response of a Simple Lithium Grease Timken tests were performed on two samples of a fully formulated simple lithium grease containing antimony dialkyldithiophosphate and zinc dialkyldithiophosphate in which glycerol was added to one sample. The results of these tests are shown in Table 4. The data in Table 4 show that the load-carrying capability of a simple lithium grease is also enhanced when a polyhydric alcohol and two dialkyldithiophosphate compounds having different metals are added. Example 5 ― Effect of Polyhydric Alcohols on the Timken Response of a Lithium Complex Grease Timken tests were performed on two samples of the same lithium complex base grease containing antimony dialkyldithiophosphate and zinc dialkyldithiophosphate in which glycerol was added to one sample and propylene glycol to the other. The results of these tests are shown in Table 5. Test sample H in Table 1 and test sample 0 in Table 5 show that increasing the amount of glycerol from 0.3 to 1.0 wt.% does not increase the load-carrying capability of a grease. Test sample P in Table 5 shows that using 0.3 wt.% propylene glycol (a polyhydric alcohol having two hydroxy groups) instead of glycerol does not increase the load-carrying capability of said grease.

Claims: 1. A grease composition having improved load-carrying capability which comprises: (a) a lubricating oil, (b) a thickener, (c) a polyhydric alcohol having at least three hydroxy groups, and (d) at least two metal hydrocarbylthiophosphate compounds in which the metal is different in at least two compounds. 2. The composition of claim 1 wherein said polyhydric alcohol is glycerol or pentaerythritol or a mixture thereof. 3. The composition of claim 1 or claim 2, wherein the metal hydrocarbylthiophosphate compounds have the formula: wherein n is 1-6; x is 1-3; R₁ and R₂ are each hydrogen or a hydrocarbyl group having from 1 to 30 carbon atoms; and M is a metal selected from the group consisting of aluminum, antimony, cadmium, copper, lead, tin, zinc and mixtures thereof. 4. The composition of claim 3 wherein said hydrocarbyl group is an alkyl, aryl, alkaryl, aralkyl group or mixtures thereof. 5. The composition of claim 3 wherein said metal hydrocarbylthiophosphate compounds comprise at least two metal alkylthiophosphate compounds. 6. The composition of claim 5 wherein said metal alkylthiophosphate compounds comprise two metal dialkyldithiophosphate compounds. 7. The composition of claim 6 wherein said metal dialkyldithiophosphate compounds are antimony dialkyldithiophosphate and zinc dialkyldithiophosphate. 8. The composition of any preceding claim, wherein said thickener is based on a fatty acid soap of calcium, lithium, their complexes or mixtures thereof. 9. The composition of any preceding claim, wherein said polyhydric alcohol comprises glycerol. 10. A method for improving the load-carrying capability of a lubricating oil based grease which comprises adding to said grease: (a) a polyhydric alcohol having at least three hydroxy groups, and (b) at least two metal hydrocarbylthiophosphate compounds in which the metal is different in at least two compounds. 11. The method of claim 10 wherein said polyhydric alcohol is as defined in claim 2 or claim 9. 12. The method of claim 10, wherein the metal hydrocarbylthiophosphate compound is as defined in any one of claims 3 to 7. 13. The method of any one of claims 10 to 12, wherein said grease is one thickened with a fatty acid soap of calcium, lithium, their complexes or mixtures thereof.

Inventors: ALEXANDER, ALBERT GORDON
Applicants: EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1988
Main classification: C10M169
IPC codes: C10M141, C10N10, C10N30, C10M169, C10N50, C10N40