POLYMER THICKENED GREASE COMPOSITIONS AND THEIR USE

Abstract

The invention provides a grease composition comprising: (a) a polymer thickened grease; (b) an acid having a pKa in the range of 2-7 wherein the pKa is measured in water at 25° C.; and (c) a metal salt. The invention further provides a mechanical component having a metal surface onto which a layer of the grease composition according to the invention has been applied. In addition, the invention relates to the use of the present grease composition for reconditioning the lubrication of a mechanical component having a metal surface; and the use of the present grease composition for protecting a mechanical component having a metal surface against corrosion, wear or anti-fretting. The invention also relates to a method for preparing the present grease composition.

Description

FIELD OF THE INVENTION

The present invention relates to a grease composition; a bearing, bearing component or bearing application system having a metallic surface onto which a layer of the grease composition has been applied; the use of the present grease composition for reconditioning the lubrication in mechanical components; the use of the present grease composition for protecting mechanical components against corrosion, wear or fretting, and a method for preparing the present grease composition.

BACKGROUND OF THE INVENTION

Polymer thickened lubricating greases and their preparation are known in the art. In U.S. Pat. No. 3,392,119, a grease is described comprising a white mineral oil that has been thickened by means of a copolymer of ethylene and a homopolymer of polypropylene. In U.S. Pat. No. 3,850,828, a lubricating grease composition has been described which is thickened with a polymeric mixture comprising a polyethylene and an atactic polypropylene. In EP 0700986 A2, a polymeric thickener for lubricating grease compositions is disclosed comprising containing a mixture of copolymers or homopolymers of polypropylene having a high molecular weight and a copolymer or homopolymer of propylene having a low molecular weight. Further, In EP 0942063, a lubricating grease composition is described comprising at least one polyolefin component, at least one base oil component and at least one rubber component.

These known polymer thickened lubricating grease compositions still leave room for improvement in respect of various applications, such as for instance off-shore applications and wind turbine applications.

SUMMARY OF THE INVENTION

Object of the present invention is to provide a new set of polymer thickened grease compositions for protecting mechanical components against corrosion, wear or fretting.

Another object is to provide new polymer thickened grease compositions for reconditioning the lubrication in mechanical components.

Surprisingly, it has now been found that grease compositions can be prepared that form a highly attractive protection layer on mechanical components such as bearings by mixing a polymer thickened grease with an acid and a metal salt.

Accordingly, the present invention relates to a grease composition comprising:

(a) a polymer thickened grease;
(b) an acid having a pKa in the range of 2-7 wherein the pKa is measured in water at 25° C.; and
(c) a metal salt.

The present invention also relates to a mechanical component such as a bearing, bearing component or bearing application system having a metallic surface onto which a layer of the grease composition has been applied.

In addition, the present invention relates to the use of the present grease composition for reconditioning the lubrication in mechanical components, and to the use of the present grease composition for protecting mechanical components against corrosion, wear or fretting.

Further, the present invention provides a method for preparing the present grease composition.

DETAILED DESCRIPTION OF THE INVENTION

The present invention also provides a grease composition comprising: (a) a polymer thickened grease; (b) an acid having a pKa in the range of 2-7 wherein the pKa is measured in water at 25° C.; and (c) a metal salt, wherein upon applying the grease composition to a metal surface of a mechanical component, the metal salt reacts with the acid and the acid or the acid so obtained etches the metal surface, thereby forming on the metal surface a layer comprising the metal of the metal salt.

The metals of the metal salt to be used in accordance with the present invention can be chosen from a wide variety of metals. Suitably, the metal of the metal salt is selected from the group consisting of Group 1A, 1B, 2A, 2B, 4A, 5A, 7B and 8B elements of the Periodic Table. Preferably, the metal in the metal salt is selected from the group consisting of bismuth, zinc, sodium, calcium, magnesium, antimony, barium, manganese, silver, gold, copper and lead. More preferably, the metal in the metal salt is bismuth or zinc.

Suitably, the metal salt is selected from the group consisting of naphtenates, octoate, carbamate, carboxamides, sulfonamides, stearates, hydroxystearates and any mixture thereof. Preferably, the metal salt is a stearate, a hydroxystearate or a mixture thereof.

In the grease composition according to the present invention use can be made of a variety of acids, provided that they have at least one pKa in the range of 2-7 wherein the pKa is measured in water at 25° C. Preferably, the acid to be used is a polyprotic acid. The polyprotic acid to be used in accordance with the present invention has suitably two pKa values in the range of from 2-10, preferably two pKa values in the range of from 2-8, wherein the pKa is measured in water at 25° C. More preferably the polyprotic acid is a triprotic acid having three pKa values in the range of from 2-13 wherein the pKa is measured in water at 25° C. Most preferably the triprotic acid is phosphoric acid.

The polymer thickened grease to be used in accordance with the present invention comprises a lubricating base oil and a polymeric thickener. In the present grease composition preferably only use is made of a polymeric thickener, not a conventional soap-metal salt thickener. However, such a conventional thickener may be incorporated in the present grease composition as long as it does not adversely affect the overall properties of the grease composition.

It is observed that the metal salt to be used in accordance with the present invention does not act as a thickener. The metal salt to be used in accordance with the present invention is added to a grease composition that has already adequately been thickened by means of a polymer. The metal salt serves a different purpose, namely to react with the acid to form on the metal surface a layer comprising the metal of the metal salt.

The present grease composition suitably comprises:

(a) 1-99 wt % of the lubricating base oil;
(b) 1-30 wt % of the polymeric thickener;
(c) 0.01-10 wt % of the acid; and
(d) 0.01-40 wt % of the metal containing compound, all weight percentages based on the total weight of the grease composition.

The weight percentage of the acid is based on a 85% concentration of the acid concerned.

Preferably, the present grease composition comprises:

(a) 10-95 wt % of the lubricating base oil;
(b) 5-15 wt % of the polymeric thickener;
(c) 0.5-5 wt % of the acid; and
(d) 1-10 wt % of the metal containing compound, all weight percentages based on the total weight of the grease composition.

In one embodiment of the present invention, the grease composition preferably comprises in addition a polymeric ester in an amount in the range of from 0.1-40 wt %, based on the total weight of the grease composition. Polymeric esters are copolymers of olefins and dicarboxylic acids which are esterified with various kinds of alcohols. Their viscosities suitably range from 5 mm²/s up to 700 mm²/s. Suitably, the average molecular weights (Mw) do not exceed 7,000 Dalton, ensuring that the polymeric esters are shear stable. Suitable examples of polymeric esters that can be used in accordance with the present invention include di-esters, polyol esters, mono esters, glycol esters and PEG esters. The polymeric ester is preferably high polar or low polar polymer ester

The polymeric thickener to be used in the polymer thickened grease suitably comprises at least one component selected from the group consisting of natural rubber, polypropylene, polyisoprene, polybutadiene, poly(styrene-butadiene), poly(ethylene-propylene-diene), polyurethane, polymethacrylate, polyisobutylene, poly(isobutylene-succinic acid), poly(isobutylene-succinic acid-polyacrylamide), polyurea and polyethylene.

The polymeric thickener preferably comprises a first component and a second component, with the first component having a higher weight average molecular weight than the second component. Preferably, the polymeric thickener comprises a high molecular weight component and a low molecular weight component, characterized in that the thickener comprises a mixture of (1) a (co- or homo-)polymer of propylene with a weight average molecular weight of more than 200.000 and (2) a (co- or homo-)polymer of propylene with a weight average molecular weight of less than 200.000.

The polymeric thickener to be used in accordance with the present invention contains a high molecular weight component comprising a (co- or homo-)polymer of propylene with a weight average molecular weight of more 200.000, preferably 200.000-350.000 and a low molecular weight component comprising a (co- or homo-) polymer of propylene with a weight average molecular weight of less than 100.000, preferably 50.000-100.000.

The weight ratio between the high molecular weight component and the low molecular weight component in the polymeric thickener can be 1:40-3:1, suitably 1:40-1:1, preferably 1:40-1:5, more preferably 1:25-1:15, and most preferably 1:18-1:20. Outside this preferred range for the weight ratio between the high and low molecular weight components the final lubricating grease composition will generally not have desired application properties, in particular mechanical stability and consistency, i.e. be too “rubbery/elastic” and/or too “buttery”. However, as the properties of the final composition are also dependent on the lubricant base oil and additives incorporated in grease compositions, as well as on the way the composition is prepared, other ratios may also be used for obtaining the desired properties of the final composition, as is well known to a man skilled in the art.

According to the present invention, the low molecular weight component is preferably a polypropylene homopolymer, more preferably a polypropylene homopolymer with a melt flow rate of 500-1500 dg/min, especially 750-1250 dg/min as determined by test ASTM D 1238 L.

The high molecular weight component preferably has a melt flow rate (ASTM D-1238) of 1.5-15, more preferably 1.5-7, especially 3-5. The low molecular weight component is preferably a polypropylene homopolymer. Preferably, the high molecular weight component is a polypropylene homopolymer or a propylene/ethylene-copolymer.

The polymer thickener according to the invention is generally used in the lubricating grease composition in conventional amounts, i.e. from 1-20, preferably 5-15, especially about 10 percent by weight of the total grease composition. Other amounts can be used if desired.

Apart from the polymeric thickeners mentioned in detail hereinabove, the present grease composition may also contain other polymeric thickeners.

As the lubricating base oil any lubricating oil known per se may be used, such as mineral oils, synthetic hydrocarbons, ester oils, vegetable oils and mixtures thereof, of different viscosity. The type of base oil and viscosity can be selected to suit specific applications.

Furthermore, additives known per se may be incorporated in the lubricant grease composition, as long as they do not have a detrimental effect on the thickener composition, the base oil and/or the final grease composition. The grease composition may additionally comprise at least one additive component which is selected from the group consisting of antioxidants, corrosion inhibitors, anti-wear agents and pressure tolerance-increasing additives, and wherein the total content of the additive component(s) is in the range between 0.2 and 15% by weight, and preferably between 1 and 8% by weight, based on the total weight of the grease composition.

In one embodiment of the present invention the present grease composition additionally comprises an ester oil in an amount in the range of from 1-40 wt %, based on total weight of the grease composition.

Suitably, the present grease composition may additionally comprises an inert powder, preferably diamond powder, in an amount in the range of from 0.01-10 wt %, based on total weight of the grease composition.

The present invention also relates to a bearing, bearing component or bearing application system having a metallic surface onto which a layer of the grease composition in accordance with the present invention has been applied.

The present invention further provides a bearing, bearing component or bearing application system having a metallic surface onto which a layer has formed comprising the metal of the metal salt.

The bearing, bearing components or bearing application system may be pre-treated with the grease composition ensuring that it carries a protective layer against wear, fretting or corrosion before use, or such a protective metal layer will be formed on the bearing, bearing component or bearing application system during operation when the present grease composition is used for lubricating purposes.

It is also an object of the invention is to provide grease compositions which prevent fretting, wear and corrosion. Another object of the invention is to provide a polymer-thickened grease composition for reconditioning the lubrication of a mechanical component having a metal surface.

Hence, the present invention also relates to the use of the grease composition according to the present invention for reconditioning the lubrication of a mechanical component having a metal surface. Preferably, the present invention relates to the use of the grease composition according to the present invention for reconditioning the lubrication of a bearing, bearing component or bearing application system. When the grease composition is used for reconditioning the lubrication of a mechanical component having a metal surface, the metal of the metal salt is preferably bismuth. The present grease composition will be used for this purpose in emergency and accidental circumstances before operation of the device in question is interrupted. In that case the grease composition can suitably be injected by means of a device involving pyrotechnic means. Such devices are as such known. In US 2010.0101442 A1 such a device has for instance been described. Said document is hereby incorporated by reference.

The present invention further relates to the use of the grease composition according to the present for protecting a mechanical component having a metal surface against corrosion. Preferably, the present invention relates to the use of the grease composition according to the present for protecting a bearing, bearing component or bearing application system.

The present invention in addition relates to the use of the grease composition according to the present invention for protecting a mechanical component having a metal surface against wear. Preferably, the present invention relates to the use of the grease composition according to the present for protecting a bearing, bearing component or bearing application system against wear.

Moreover, the present invention relates to the use of the grease composition according to the present invention for protecting a mechanical component having a metal surface against fretting. Preferably, the present invention relates to the use of the grease composition according to the present invention for protecting a bearing, bearing component or bearing application system against fretting.

When the grease composition is used for protecting a mechanical component having a metal surface against wear, the metal of the metal salt is preferably bismuth. In case the grease composition is used for protecting a mechanical component having a metal surface against wear and fretting, the metal of the metal salt is preferably bismuth and/or zinc. In case the grease composition is used for protecting a mechanical component having a metal surface against corrosion the metal of the metal salt is preferably zinc, whereas the acid applied is preferably an acid having a pKa in the range of 2-7, preferably three pKa values in the range of from 2-13. Preferably, the acid then used is phosphoric acid. For anti-corrosion purposes such a grease composition can suitably be applied in corrosion sensitive applications, transport and storage.

In addition, the present invention relates to a method for preparing a grease composition according to the present invention comprising the following steps:

(a) mixing the polymer thickened grease with the metal salt; and
(b) adding to the mixture obtained in step (a) the acid.

Preferably, in step (a) the metal salt is heated above its melting point.

Suitably, during step (a) one or more anti-wear additives, anti-fretting additives, anti-corrosion additives, anti-oxidants, inert powder(s) and/or other additives are additionally mixed with the other components.

Preferably, a final mixing is carried out in step (a) using a three-roll mill or a ball-mill

The polymeric thickener composition according to the invention can be prepared by mixing the polymers in a manner known per se, which can optionally involve heating and/or the use of suitable solvents.

The polymers of the subject invention are mixed with a lubricating base oil and optional additives by means of conventional techniques known per se resulting in the grease composition according to the invention.

The preparation of the grease composition is preferably be carried out under a protective atmosphere, such as a nitrogen gasflow, for avoiding oxidation of the oils during heating.

According to a preferred embodiment the solid polymeric components and the lubricating base oil are heated together (preferably under stirring) to a temperature above the melting point of polypropylene, preferably 190-210° C., although other temperatures may be used if required.

After the polymers are dissolved in the lubricating base oil and optionally additives have been added, the grease is cooled from the mixing temperature to room temperature.

According to an especially preferred embodiment of the invention this cooling is carried out in a period of time between 1 sec.-3 min, preferably 10 sec.-1 min, more preferably about 25-35 sec. This rapid cooling process, which forms an important aspect of the invention, will be indicated hereinbelow as “quenching”. The quenching of the lubricant grease composition can be carried out, for instance, by pouring the grease composition on a metal plate, although any other suitable rapid cooling method may also be used, such as spraying.

The quenching process according to the invention has a major influence on the grease structure, giving significant improvement of the properties of the final grease compositions compared to both conventional lubricating greases, as well as lubricating greases according to the invention which are cooled slowly, e.g. in approximately 1 degree per minute by the use of conventional cooling methods, such as simply keeping the grease in the reaction vessel with external/internal cooling. This results, for the polymer grease, in a lubricant lacking any mechanical stability.

In the grease composition according to the invention, the polymeric thickener forms a sponge-like structure, which gives the grease its appearance and structure. The lubricating base oil is kept within the pore-like spaces within the thickener structure, and bleeds out during service of the grease. The thickener-structure is very irregular with large pores as well as very small pores. The above indicated quenching of the lubricant grease composition provides a grease according to the invention with a smoother and more uniform structure of the polymeric thickener, with more uniformly distributed spaces for keeping the lubricant oil.

During service of the lubricant grease, the oil bleeds out of the oil/thickener-structure onto the surfaces of the bearing, thereby providing the lubricating action. The oil bleeding characteristics at the service temperature of the lubricant grease composition (i.e. the running temperature of the bearing, as well as the “start up” temperature) are therefore critical for obtaining the lubricating action of the composition.

The mechanical stability of the grease is dependent on the polymer thickener used, the lubricating base oil used, as well as the additives used. Further, the mechanical properties of the grease can be influenced by “working” the grease after the thickener is mixed with the lubricating base oil, as is well known to a man skilled in the art of lubricants. Preferably, the grease is “worked” to a consistency desired and/or required for its intended use.

The mechanical stability of the grease can be ascertained by means of tests known in the art, such as the Shell roll stability test. Preferably, the grease will have a penetration after the Shell roll stability test (24 hrs at 80° C., 165 rpm), of max. 430.

The consistency of the grease can be classified by means of the NLGI-class. According to the present invention the grease can usually be prepared to a NLGI-class range 00 to 4. An NLGI-class of 0 or 00 can be made, resulting in a lubricant composition. Also, the viscosity of the separated oil must be acceptable, and preferably be constant.

The polymer-thickened grease composition according to the present invention can be used for all conventional applications for lubricant grease compositions, so long as these are compatible with the components of the lubricant grease composition. The present grease composition can be used for lubricating bearings, couplings, toothed transmission gears, chutes or other instruments.

The mechanical component having a metal surface to be treated with the grease composition according to the present invention is preferably a bearing, bearing component or a bearing application system. The bearing component may be inner rings, outer rings, cages, rollers, balls and seal-counter faces. The bearing application system in accordance with the present invention comprises bearing housings, mounting axles, shafts, bearing joints and shields.

Further uses of the lubricant grease compositions according to the present invention are e.g. agricultural machinery, bearings in dam-gates, low noise electric motors, large size electric motors, fans for cooling units, machine tool spindles, screw conveyor, and off-shore and wind turbine applications.

The present invention further relates to a grease composition obtainable according to the method of the invention.

The grease composition in accordance with the present invention may have the form of a lubricant composition having an oily character or a paste, depending on the viscosity of the final grease composition.

The invention will now be further illustrated by the following Examples, which do not limit the invention in any way.

EXAMPLES

Example 1

A number of greases were used in 4-ball wear scar test experiments according to DIN 51350/5 (1 minute at 1400 N). The grease compositions used in these test experiments are shown in Table 1. The results of these experiments are shown in Table 2, whereby it is observed that in experiment No. 5 use is made of grease No. 4 but on the balls a pre-coating of grease No. 3 was applied. The pre-coating was formed by degreasing the balls with a solvent; immersing the balls in the grease No. 3; heating the balls at 85° C. for 2 hours; cooling and storing the balls for 16 hours at ambient temperature; and cleaning/washing the balls with a solvent. From the test results shown in Table 2 it is evident that the use of the grease compositions according to the present invention (Grease Nos. 3 and 4) brings about considerably improved wear properties when compared with conventional greases (Grease Nos. 1, 2, and 4).

TABLE 1
Grease
composition   Composition of grease
Grease No. 1  11% polymer, 89% lubricating oil
Grease No. 2  11% polymer, 82% lubricating oil, 6%
              EP/anticorrosion additives, 1% antioxidant
              additive
Grease No. 3  41.5% polymer, 1% anti-oxidant, 6%
              anticorrosion/EP additives, 20% polymer
              ester, 20% ester-oil, 1.5% phosphoric acid
              (conc. 85%), 10% bismuth hydroxystearate
Grease No. 4* SKF factory fill grease MT33 with metal-
              based thickener
*commercially available grease

TABLE 2
		Wear scar
		(load 1400 N,
Experiment No.	Grease composition	1 minute) in mm
Experiment No. 1	Grease No. 1	2.5
Experiment No. 2	Grease No. 2	1.8
Experiment No. 3	Grease No. 3	0.4
Experiment No. 4	Grease No. 4	2.3
Experiment No. 5	Grease Nos. 3 (pre-coating) and 4	0.85

Example 2

In this example greases were subjected to the Fafnir test in accordance with ASTM D4170 wherein the balls in a trust bearing are oscillated for 22 hrs and the wear in the ball-ring contact point is a measure of false brinelling (friction oxidation). The test results are shown in Table 3. In experiment No. 6 a grease No. 5 is used having the following composition: 69% polymer grease (9% polypropylene and 91% base lubricating oil), 20% anti-friction additive, 5% anti-corrosion, 5% anti-wear additive, 1% interne further standard additives.

In experiment No. 7 use is the same grease No. 5 but on the rings a pre-coating of grease No. 3 (see Table 1) was applied. The pre-coating was formed by degreasing the rings with a solvent; immersing the rings in the grease No. 3; heating the rings at 85° C. for 2 hours; cooling and storing the rings for 16 hours at ambient temperature; and cleaning/washing the rings with a solvent. From Table 3 it is clear that the pre-treatment of the test bearing rings in accordance with the present invention (experiment No. 7) brings about much less friction oxidation weight loss when compared with the grease without the pre-treament with the layer building grease No. 3 (experiment No. 6).

TABLE 3
Experiment No.   Weight loss of rings (mg)
Experiment No. 6 3
Experiment No. 7 1

Example 3

A number of greases as specified in Table 4 were used in reconditioning experiments. In this respect it is first noted that in some applications the failure of a bearing can result in high costs due to damaging of vital parts of machine, e.g. paper mills. Also the failure of a lubricated bearings can results in fatal accidents, one can think of a helicopter which will come down very fast and will crash. Hence, it could be life and costs saving when the bearing and lubrication can be reconditioned in case of an imminent failure for a certain period during which a helicopter can land/touchdown safely, or in case of a paper mill which is about to fail to allow the operator enough time to stop the production in a controlled procedure.

The greases were tested in a laboratory aircraft simulation test set-up. The aim is to recondition the bearing for at least 60 minutes. The following conditions were applied:

• • Bearing speed: 10 000 rpm (inner ring)
  • Load: 400 daN; balls/IR: 2000 MPa; balls/OR: 1600 MPa
  • Initial grease (running in): Thin film of running-in grease, <0.1 g
  • Injection grease for reconditioning at failure: Manual at approximately 150° C. (reading from recorder). Upon an indication of a failure the grease in question was added to the lubricant system initially present.

The results of the experiments are shown in Table 5. From Table 5 it is clear that the grease composition according to the present invention (experiment No. 10) is able to recondition the original lubricant system in a considerably improved manner when compared to conventional greases (experiments Nos. 8 and 9) because it is allows the lubricant system to survive for a much longer time, whereas at the same time a much less amount of grease is required.

TABLE 4
Grease
composition   Composition of grease
Grease No. 6* Shell Aeroshell G14 with metal-based thickener
Grease No. 7* Klueber Isoflex NBU15 with metal-based thickener
Grease No. 8  42% polymer, 1% anti-oxidant, 6% anticorrosion/EP
              additives, 20% polymer ester, 20% ester-oil, 1.0%
              phosphoric acid (conc. 85%), 10% bismuth
              hydroxystearate
*commercially available grease

TABLE 5
		Survival time after
	Grease	moment of failure	Grease fill
Experiment No.	composition	(minutes)	(mg)
Experiment No. 8	Grease No. 6	51	2
Experiment No. 9	Grease No. 7	35	4
Experiment No. 10	Grease No. 8	82	0.9

Claims

1. A grease composition comprising:

(a) a polymer thickened grease;

(b) an acid having a pKa in the range of 2 7 with the pKa being measured in water at 25° C.; and

(c) a metal salt.

2. The grease composition according to claim 1, further comprising the step of applying the grease composition to a metal surface of a mechanical component, the metal salt reacting with the acid and the acid or the acid so obtained etches the metal surface, thereby forming on the metal surface a layer including the metal of the metal salt.

3. The grease composition according to claim 2, wherein the metal of the metal salt is selected from the group consisting of Group 1A, 1B, 2A, 2B, 4A, 5A, 7B and 8B elements of the Periodic Table.

4. The grease composition according to claim 3 comprising: all weight percentages are based on the total weight of the grease composition.

(a) 1-99 wt % of the lubricating base oil;

(b) 1-20 wt % of the polymeric thickener;

(c) 0.01-10 wt % of the acid; and

(d) 0.01-40 wt % of the metal containing compound, wherein

5. The grease composition according to claim 4 further comprising a polymeric ester in an amount in the range of from 0.1-40 wt %, based on the total weight of the grease composition.

6. The grease composition according to claim 5, wherein

the acid is a polyprotic acid having two pKa values in the range of from 2-10.

7. The grease composition according to claim 6, wherein the acid is phosphoric acid.

8. The grease composition according to claim 7, wherein the polymeric thickener includes at least one component selected from the group consisting of natural rubber, polypropylene, polyisoprene, polybutadiene, poly(styrene-butadiene), poly(ethylene-propylene-diene), polyurethane, polymethacrylate, polyisobutylene, poly(isobutylene-succinic acid), poly(isobutylene-succinic acid-polyacrylamide), polyurea and polyethylene.

9. The grease composition according to claim 8, wherein the polymeric thickener comprises a first component and a second component, with the first component having a higher weight average molecular weight than the second component.

10. The grease composition according to claim 9, wherein the polymeric thickener includes a high molecular weight component and a low molecular weight component, and wherein

the thickener further comprises a mixture of (1) a (co- or homo-)polymer of propylene with a weight average molecular weight of more than 200.000 and (2) a (co- or homo-)polymer of propylene with a weight average molecular weight of less than 200.000.

11. The grease composition according to claim 10, wherein the weight ratio between the high molecular weight component and the low molecular weight component is 1:40-1:5, preferably 1:25-1:15, more preferably 1:18-1:20.

12. The grease composition according to claim 11, wherein the low molecular weight component is a polypropylene homopolymer.

13. The grease composition according to claim 12, wherein the low molecular weight component has an average molecular weight between 20.000 and 100.000 and a melt flow rate of 500-1500, preferably 750-1250.

14. The grease composition according to claim 13, wherein the high molecular weight component is a polypropylene homopolymer or a propylene/ethylene-copolymer.

15. The grease composition according to claim 14, wherein the high molecular weight component has an average molecular weight of 200.000-350.000 and a melt flow rate of 1.5-15, preferably 1.5-7.

16. The grease composition according to claim 15, wherein the metal salt is selected from the group consisting of naphtenates, octoate, carbamate, carboxamides, sulfonamides, stearates, hydroxystearates and any mixture thereof.

17. The grease composition according to claim 16, wherein the metal salt is a stearate, a hydroxystearate or a mixture thereof.

18. The grease composition according to claim 17, wherein the metal in the metal salt is selected from the group consisting of bismuth, zinc, sodium, calcium, magnesium, antimony, barium, manganese, silver, gold, copper and lead.

19. The grease composition according to claim 18, wherein the metal in the metal salt is bismuth or zinc.

20. The grease composition according to claim 19, further comprising an ester oil in an amount in the range of from 1-40 wt %, based on the total weight of the grease composition.

21. The grease composition according to claim 20 further comprising an inert powder in an amount in the range of from 1-10 wt %, based on the total weight of the grease composition.

22. The grease composition according to claim 21 further comprising at least one additive component which is selected from the group consisting of antioxidants, corrosion inhibitors, anti-wear agents and pressure tolerance-increasing additives, and

wherein the total content of the additive component(s) is in the range between 0.2 and 15% by weight, and preferably between 1 and 8% by weight, based on the total weight of the grease composition.

23. A mechanical component having a metal surface onto which a layer of a grease composition has been applied, the grease composition comprising:

(a) a polymer thickened grease;

(b) an acid having a pKa in the range of 2-7 with the pKa being measured in water at 25° C.; and

(c) a metal salt.

24. The mechanical component according to claim 23, further comprising providing a metal surface onto which a layer has formed comprising the metal of the metal salt.

25. The mechanical component according to claim 23 further comprising a bearing, bearing component or bearing application system.

26. Use of a grease composition for reconditioning the lubrication of a mechanical component having a metal surface, the grease composition comprising:

(a) a polymer thickened grease;

(b) an acid having a pKa in the range of 2 7 with the pKa being measured in water at 25° C.; and

(c) a metal salt.

27. Use of the grease composition according to claim 26, further comprising for protecting a mechanical component having a metal surface against corrosion.

28. Use of the grease composition according to claim 26, further comprising for protecting a mechanical component having a metal surface against wear.

29. Use of the grease composition according to claim 26, further comprising for protecting a mechanical component having a metal surface against fretting.

30. Use of the grease composition according to claim 29, wherein the mechanical component comprises a bearing, bearing component or bearing application system.

31. A method for preparing a grease composition having a polymer thickened grease; an acid having a pKa in the range of 2-7 with the pKa is measured in water at 25° C.; and a metal salt, the method comprising the following steps:

a) mixing the polymer thickened grease with the metal salt; and

b) adding to the mixture obtained in step (a) the acid.

32. The method for preparing a grease composition according to claim 31, wherein in step (a) the metal salt is heated above its melting point.

33. The method according to claim 31, further comprising additionally mixing during step (a) one or more anti-wear additives, anti-corrosion additives, anti-oxidants, inert powder(s) and/or other additives.

34. The method according to claim 33, wherein a final mixing in step (a) is carried out using a three-roll mill or ball-mill.