EP0523064B1

EP0523064B1 - Lubricating grease composition

Info

Publication number: EP0523064B1
Application number: EP91904931A
Authority: EP
Inventors: Wolfgang Mader; Georg Grun
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1990-03-07
Filing date: 1991-03-05
Publication date: 1994-03-09
Anticipated expiration: 2011-03-05
Also published as: DE69101376D1; JPH05504369A; GB9005104D0; CA2077555A1; WO1991013955A1; JP2964165B2; GB2241707A; DE69101376T2; EP0523064A1

Abstract

A lubricating grease composition comprising a major proportion of a lubricating base oil, a lithium soap complex and an organic metal compound. The metal of the organic metal compound is selected from molybdenum, tungsten, vanadium, niobium and tantalum, and is preferably molybdenum. The grease can be used to lubricate mechanical parts such as antifriction bearings, and exhibits a long lifetime at high temperatures.

Description

This invention relates to a lubricating grease composition and, more especially, to grease compositions thickened with lithium soap.
Lithium soap greases are well known and are used in the lubrication of many automotive and industrial applications, for example bearings, gears and couplings. Whilst lithium soap greases, and particularly lithium soap complex greases, are often favoured for higher temperature applications, the greases have the disadvantage that they have a limited lifetime at temperatures exceeding about 120°C as the oil tends to separate ("bleed") from the grease, and thus frequent reapplication of the grease is required.
The present invention provides a lithium complex grease with prolonged lifetime at high temperatures. More specifically the invention provides a lubricating grease composition comprising:

(i) a major proportion of a lubricating base oil;
(ii) from 2 to 30 weight percent of a lithium complex thickener;
(iii) from 0.1 to 10 weight percent of a thiocarbamate or xanthate compound containing a metal selected from molybdenum, tungsten, vanadium, niobium and tantalum; and
(iv) from 0.1 to 5 weight percent of an amine antioxidant compound;

The grease according to the invention has the advantage that it has a significantly improved lifetime at high temperatures, the improvement being particularly evident at temperatures between about 120 and 160°C. The grease has a lower tendency to bleed oil compared with other lithium soap-containing greases. Thus by employing the grease, the number of reapplications necessary to maintain satisfactory lubrication of the mechanical part to which the grease is applied can be considerably reduced. In some instances, where the mechanical part is enclosed in a sealed container, the grease can provide "lifetime" lubrication, i.e. it need not be renewed for the lifetime of the mechanical part. Examples of such applications include automotive antifriction bearings and sliding bearings.
The amount of lithium soap complex present in the grease is preferably from 2 to 30 wt %, preferably 5 to 20 wt % based on the total weight of the composition. It is preferred that the lithium complex soap comprises a lithium soap of a C₁₂ to C₂₄ hydroxy fatty acid and at least one other lithium compound. This other lithium compound is advantageously either a lithium soap of a C₂ to C₁₂ aliphatic dicarboxylic acid or a lithium salt of boric acid, or a mixture of both.
More preferably the lithium soap of the hydroxy fatty acid is a C₁₆ to C₂₀ hydroxy fatty acid. A particularly preferred hydroxy fatty acid is hydroxystearic acid, for example 9-hydroxy-, 10-hydroxy-, or 12-hydroxystearic acid, more preferably the latter. Ricinoleic acid, which is an unsaturated form of 12-hydroxystearic acid having a double bond in the 9-10 position, can also be used. Other suitable hydroxy fatty acids include 12-hydroxybehenic acid and 10-hydroxypalmitic acid.
Where the other lithium-containing compound is an aliphatic dicarboxylic acid, the proportion of dicarboxylic acid to hydroxy fatty acid employed is generally a weight ratio of between 0.05 and 1, usually between 0.1 and 0.8, parts dicarboxylic acid per part hydroxy fatty acid. The dicarboxylic acid is preferably a C₄ to C₁₂, more preferably C₆ to C₁₀, aliphatic dicarboxylic acid. Examples of suitable acids include oxalic, malonic, succinic, glutaric, adipic, suberic, pimelic, azelaic, dodecanedioic and sebacic acids. Azelaic and sebacic acids are especially preferred.
Where the other lithium-containing compound is the lithium salt of boric acid, the proportion of hydroxy fatty acid to boric acid employed is generally a weight ratio of between 3 and 100 parts, usually between 5 and 80 parts, hydroxy fatty acid per part boric acid.
The lithium soap complex may also include a second hydroxycarboxylic acid. This generally has 3 to 14 carbon atoms and an -OH group not more than 6 carbon at atoms away from the carboxyl group. This acid can be either an aliphatic acid such as lactic acid, 6-hydroxydecanoic acid, 3-hydroxybutanoic acid, 1-hydroxycaproic acid, 4-hydroxybutanoic acid, 6-hydroxy-alpha-hydroxystearic acid, etc. or an aromatic acid such as parahydroxy-benzoic acid, salicylic acid, 2-hydroxy-4-hexylbenzoic acid, metahydroxybenzoic acid, 2,5-dihydroxybenzoic acid (gentisic acid); 2,6-dihydroxybenzoic acid (gamma resorcyclic acid); 4-hydroxy-4-methoxybenzoic acid, etc. or a hydroxyaromatic aliphatic acid such as orthohydroxyphenyl, metahydroxyphenyl, or parahydroxyphenyl acetic acid. A cycloaliphatic hydroxy acid such as hydroxycyclopentyl carboxylic acid or hydroxynaphthenic acid could also be used. Particularly useful hydroxy acids are lactic acid, salicylic acid, and parahydroxybenzoic acid.
Alternatively a lower alkyl ester, usually, a C₁ to C₄ alkyl ester of the above second hydroxycarboxylic acid can be used instead of the free acid. This is especially beneficial when boric acid is employed as the ester aids dispersion of the insoluble salt. As example of a preferred ester is methyl salicylate. When a second hydroxycarboxylic acid is employed in the grease composition, the proportion is generally a weight ratio of 0.1 to 10 parts, usually 0.5 to 5 parts, second hydroxycarboxylic acid per part of the acid component of the said other lithium-containing compound, for example boric acid.
The metal of the organic metal compound is preferably molybdenum. The organic component of this compound is preferably a thiocarbamate or xanthate. Thus preferred organic metal compounds are molybdenum thiocarbamate and molybdenum xanthate compounds, for example, molybdenum dialkyldithiocarbamate and molybdenum oxysulphide dialkyldithiocarbamate.
The preferred amount of the organic metal compound present in the grease composition is from 0.1 to 10 wt %, more preferably 0.5 to 5 wt %, based on the total weight of the composition. The organic metal compound acts as an antiwear agent and prolongs the high temperature lifetime of the grease.
In a preferred embodiment, the grease according to the invention also contains an antioxidant, which is generally present in an amount from 0.1 to 5 wt %, preferably 0.5 to 3 wt %, based on the total weight of the composition. The antioxidant employed is preferably an amine based antioxidant compound, an example being alkyldiphenylamine.
The lubricating base oil employed in the grease composition can be any of the conventionally used lubricating oils and is preferably a mineral oil or a blend of mineral and synthetic oils. In general these lubricating oils have a viscosity in the range of 2,62 to 64,2 mm²/s (35 to 300 SUS) at 99°C (210°F), and a viscosity index in the range of 30 to 170, preferably 30 to 140. 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 a di-2-ethylhexyl sebacate, esters of glycols such as C₁₃ oxo acid diester or 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 alkyl benzenes, e.g. alkylate bottoms from the alkylation of benzene with tetrapropylene, or the polymers and copolymers of alpha olefins; silicone 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 ethylcarbonate 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 US Patent 3,424,678, column 3).
The grease according to the invention also has the advantage that the usual benefits associated with lithium complex greases may be retained, together with the additional benefit of prolonged lifetime at high temperatures. Usual benefits associated with lithium soap greases include good water resistance, high dropping point, and good mechanical and chemical stability. The grease according to the invention can also be employed in low temperature, as well as high temperature, applications.
The invention will now be illustrated by the following Examples. All the greases of these Examples were made t a consistency of NLGI grade 2, i.e. all the greases have a 60-stroke work penetration at 25°C of 265-295, as determined by ASTM Method D217 or D2665.

Examples 1-4

Examples 1-3 demonstrate grease composition according to the invention based on lithium complexes comprising a lithium soap of a hydroxy fatty acid and a lithium soap of an aliphatic discarboxylic acid. Example 4 is comparative, using the same lithium soap complex.
The test results of Examples 1-4 are given collectively in Table 1.

Example 1

A lithium complex grease was formulated with the following components:
The base oil and lithium soap complex components were formulated as described in Example 1 of GB Patent 1,384,904. The molybdenum oxysulphide dialkyldithiocarbamate (MDDC) and the alkyldiphenylamine additives were then added to the grease, and the grease was milled using a conventional grease mill.
The lifetime of the grease at high temperature was measured according to the standard test DIN 51821 T2 A/1500/6000. In this test a set of five bearings is run under specified conditions of temperature, load and speed. When the required driving momentum becomes too high due to lubricant starvation, the alternator shuts down automatically. Five lubricant lifetimes are thereby obtained and the following data is calculated:

F₁₀: which represents the time after which the first ten percent of the bearings are starved of lubricant; and
F₅₀: which represents the time after which fifty percent of the bearings are starved of lubricant.

A grease which has an F₅₀ value of at least 100 hours at a given temperature is regarded as a lifetime lubricant at that temperature.
The results of the test are given in Table 1 below.

Example 2

A grease composition was formulated as in Example 1 except that MDDC employed was supplied as Molyvan A from Vanderbilt Company Inc.
The grease was tested as described in Example 1 and the results are given in Table 1 below.

Example 3

A grease composition was formulated as in Example 1 except that the alkyldiphenylamine was omitted and the amount of MDDC increased to 3 wt %. The balance was made up by increasing the amount of base oil to 81.5 wt %.
The grease was tested as described in Example 1 and the results are given in Table 1 below.

Example 4

The greases of Examples 1-3 were compared with a conventional lithium complex soap grease. This grease contained the same base oil and lithium complex as the above Examples, but contained no molybdenum compound nor alkyldiphenylamine.
This comparative grease was tested as described in Example 1 and the results are given in Table 1 below.
The results show that grease compositions according to the invention have significantly longer lifetime than comparative greases using the same base oil and lithium complex. For example, at 140°C Grease 1 has a lifetime over five times as great as comparative Grease 4.

Examples 5 & 6

These two examples demonstrate a grease according to the invention based on a lithium complex comprising a lithium soap of a hydroxy fatty acid and the lithium salt of boric acid, and compare this grease with a conventional grease containing the same lithium soap complex. The results are given collectively in Table 2.

Example 5

A grease composition was formulated with the following components:
The base oil and lithium soap complex components were formulated as described in Example 2 of US Patent 3,758,497. The MDDC and alkyldiphenylamine additives were then added, and the grease was milled using a conventional grease mill.
The grease was tested as described in Example 1 and the results are given in Table 2 below.

Example 6

A conventional lithium complex grease containing the same lithium soap complex as the grease of Example 5, but omitting the MDDC and alkyldiphenylamine was tested as described in Example 1, for comparative purposes. The results are given in Table 2 below.
These results show that, although the F₁₀ lifetime at 160°C are comparable at approximately 60 hours, the F₅₀ lifetime of Grease 5, a grease according to the invention, is 1.7 times greater than the F₅₀ lifetime of comparative Grease 6.

Example 7

A grease composition was formulated as in Example 2 except that the paraffinic base oil was replaced with a base oil blend of 28.5 wt %, based on the total grease composition, of a poly-alpha-olefin (V₄₀∼48mm²/s, V₁₀₀ _∼ 7.9mm²/s) and 52 wt %, based on the total grease composition, of a paraffinic oil base (V₄₀∼110mm²/s, V_100∼ 12mm ²/s). The blended base oil yielded the following viscosities:
V₄₀∼80 mm²/s, V₁₀₀∼10.3mm²/s, VI∼111.
The grease was tested as described in Example 1 at 160°C. The F₁₀ result was 101 hours and the F₅₀ result was 134 hours. Thus this grease also demonstrated long lifetime at high temperature.

Claims

A lubricating grease composition comprising:
(i) a major proportion of a lubricating base oil;

(ii) from 2 to 30 weight percent of a lithium complex thickener;

(iii) from 0.1 to 10 weight percent of a thiocarbamate or xanthate compound containing a metal selected from molybdenum, tungsten, vanadium, niobium and tantalum; and

(iv) from 0.1 to 5 weight percent of an amine antioxidant compound;
the weight percents being based on the total weight of the composition.
A grease composition according to Claim 1 wherein the metal of the thiocarbamate or xanthate compound is molybdenum.
A grease composition according to Claim 1 wherein component (iii) is molybdenum dialkyldithiocarbamate or molybdenum oxysulphide dialkyldithiocarbamate.
A grease composition according to any preceding claim containing 0.1 to 3 weight percent of component (iv).
A grease composition according to any preceding claim wherein the antioxidant is an alkyldiphenylamine.
A grease composition according to any preceding claim wherein the lithium complex thickener comprises a lithium soap of a C₁₂ to C₂₄ hydroxy fatty acid and at least one other lithium-containing compound.
A grease composition according to Claim 6 wherein the other lithium-containing compound is a lithium soap of a C₂ to C₁₂ aliphatic dicarboxylic acid.
A grease composition according to Claim 6 wherein the other lithium-containing compound is a lithium salt of boric acid.
A grease composition according to any of Claims 6 to 8 wherein the lithium complex thickener also comprises a lithium soap of C₃ to C₁₄ hydroxycarboxylic acid having an -OH group not more than 6 carbon atoms away from the carboxyl group.
A grease composition according to any preceding claim wherein the base oil is a mineral oil or a blend of mineral and synthetic oils.