WO2011015210A1

WO2011015210A1 - A method for manufacturing a grease composition

Info

Publication number: WO2011015210A1
Application number: PCT/EP2009/005663
Authority: WO
Inventors: Franciscus Catherina Martinus Fiddelaers; Sébastien DAVID
Original assignee: Skf B.V.
Priority date: 2009-08-05
Filing date: 2009-08-05
Publication date: 2011-02-10

Abstract

The present invention relates a method for manufacturing a grease composition comprising as components a base oil, amorphous silicon oxide particles and at least one metal salt of an organic acid, which method comprises mixing the components in any possible order of sequence and subjecting before or after mixing at least one of the components to a mechanical treatment, a thermal treatment or to both a mechanical treatment and a thermal treatment.

Description

A METHOD FOR MANUFACTURING A GREASE COMPOSITION

FIELD OF THE INVENTION

The present invention relates to a method for the manufacture of a grease composition.

BACKGROUND OF THE INVENTION

Grease compositions are widely used for lubricating beaπngs and other structural components. A grease is an essential product to reduce, for example, wear, fπction, running temperatures and energy losses.

Greases are mateπals which compπse a base oil that is thickened with a metal soap, and they are usually prepared by reacting a metal hydroxide with a fatty acid in the presence of the base oil. Conventional metal soap greases require an energy intensive grease cooking and milling process in order to achieve proper thermo-mechanical stability. Conventional metal soap greases can still be sensitive to poor thermo-mechanical stability and can require additional treatments It is known to improve further the stability, and thus the lubricating capacity, of conventional greases by adding solid additives during the thickening process. Examples of such solid additives are, for example, molybdenum disulfide, graphite, zinc oxide and/or a silica gel. The process of grease cooking and milling and additional treatments is relatively expensive because it is earned out at an elevated temperature over a relatively long period of time. Moreover, the greases so prepared are still unsuitable for a variety of applications, and not all conventional greases are suitable for food and beverage processing applications. Consequently, there is a need for greases which can easily be manufactured at low costs, which are stable and show highly attractive lubricating properties In addition, there is a need for greases that are biodegradable, environmentally benign and food compatible.

SUMMARY OF THE INVENTION

Object of the present invention is to provide a method for preparing a grease composition which shows excellent lubricating properties, which can easily be manufactured at low costs and which are more environmental friendly. Surprisingly, it has now been found that this can be established when a base oil, amorphous silicon oxide particles and at least one metal salt of an organic acid are mixed in any possible order of sequence, and one or more of these components is subjected before or after mixing to a mechanical treatment, a thermal treatment or to both a mechanical treatment and a thermal treatment

Accordingly, the present invention relates to a method for manufacturing a grease composition compπsing as components a base oil, amorphous silicon oxide particles and at least one metal salt of an organic acid, which method comprises mixing the components in any possible order of sequence and subjecting before or after mixing at least one of the components to a mechanical treatment, a thermal treatment or to both a mechanical treatment and a thermal treatment

DETAILED DESCRIPTION OF THE INVENTION

The base oil

The nature of the base oil to be used in accordance with the present invention is not essential The base oil may be selected from the group consisting of mineral base oils and synthetic base oils Mineral base oils are derived from crude oils and are either formulated on the basis of aromatic, paraffinic and/or naphthenic base oils. Further, a wide range of synthetic base oils are known and they include esters, poly-α-olefins, polysiloxanes and the like.

The base oil to be used in accordance with the present invention may comprise a base oil blend

Suitably, blends of mineral base oils and synthetic base oils may be used. Preferably, the base oil or the base oil blend to be used in accordance with the present invention has a kinematic viscosity in the range of 1 to 60 000 cSt at a temperature of 40⁰C according to DIN 51562/1. The silicon oxide particles

The silicon oxide particles to be used in accordance with the present invention are derived from an amorphous silicon oxide The amorphous silicon oxide may contain various amounts of water, implying that it may compπse silicic acid. In this respect it is noted that silicic acid is a general name for a group of chemical compounds, oligomers and polymers consisting of silicon, hydrogen, and oxygen.

According to a preferred embodiment of the present invention, the amorphous silicon oxide particles are derived from an amorphous hydrophilic silicon oxide More preferably, they are deπved from an amorphous fumed silicon oxide. Most preferably, the silicon oxide particles are derived from an amorphous hydrophilic fumed silicon oxide Fumed silicon oxide is an exceptionally pure form of silicon oxide made from silica tetrachloride as a starting material, as is well known in the art Suitable sources for the fumed silicon oxide are Aerosil® which is commercially available from Evonik Industries (formerly known as Degussa) or Cap-o-Sil® which is commercially available from Cabbot Corporation.

Preferably, the amorphous silicon oxide particles are derived from amorphous silicon oxide having a BET specific surface area of at least 50 m²/g; more preferably at least 75 m²/g, yet even more preferably at least 100 m²/g, even yet more preferably at least 125 m²/g and most preferably at least 150 m²/g. Although is it preferred that the BET specific surface area is as high as possible, it will usually not be higher than 500 m²/g Methods for determining the BET specific surface area are well known in the art. According to the present invention, it is also preferred that at least 90% of the amorphous silicon oxide particles have a mean particle size of 5-50 nm, preferably of 5-40 nm, more preferably of 10-40 nm and most preferably of 20-40 nm. The total mean particle size distribution of the amorphous silicon oxide particles is preferably in the range of 1-50 nm. The metal salts of the organic acids

According to the present invention, use is made of at least one metal salt of an organic acid. The organic acid is preferably selected from unbranched, branched, saturated or unsaturated organic acids. The organic acids may be aliphatic monocarboxylic acids or aliphatic dicarboxylic acids. The organic acid is preferably an unbranched, branched, saturated or unsaturated organic acid. However, it is preferred that the organic acid is saturated since lubricating compositions and grease compositions including unsaturated organic acids have a reduced oxidation stability. Most preferably the organic acid is an unbranched organic acid. Preferably, the organic acid to be used in accordance with the present invention is a fatty acid. It will be understood that a fatty acid is an aliphatic monocarboxylic acid derived from, or contained in esterified form in an animal or vegetable fat, oil or wax. Suitably, the organic acid of the at least one metal salt comprises 2-24 carbon atoms.

Suitably, the amorphous silicon oxide particles and the base oil are mixed with two or more different metal salts of similar or different organic acids. When use is made of at least two different metal salts of organic acids, the first metal and the second metal may be the same metal or they may differ from each other. In addition, the organic acid of the first metal salt and the organic acid of the second metal salt may be the same organic acid or they may differ from each other. However, since at least two different metal salts of organic acids are used, the at least first and second metals will not the same metal when the organic acid of the first metal salt and the organic and the organic acid of the second metal salt are the same organic acid, and vice versa. For example, the amorphous silicon oxide particles and the base oil can suitably be mixed with a first metal salt of an organic acid, preferably a fatty acid, comprising 2-16 carbon atoms, preferably 6-8 carbon atoms, a second metal salt of an organic acid, preferably fatty acid, comprising 20-24 carbon atoms and/or a third metal salt of an organic acid, preferably a fatty acid comprising 18 carbon atoms, or any mixture of such metal salts.

When use is made of a mixture of metal salts of different organic acids, the organic acids suitably comprise a different number of carbon atoms. Preferably, use is made of two metal salts of organic acids, a first metal of an organic acid, preferably a fatty acid, comprising 2-16 carbon atoms, preferably 6-8 carbon atoms, and a second metal salt of an organic acid, preferably a fatty acid, comprising 20-24 carbon atoms. Alternatively, the organic acid, preferably fatty acid, of the first metal salt may comprise 2- 16, preferably 6-8 carbon atoms, or 20-24 carbon atoms and the organic acid, preferably fatty acid, of the second metal salt may comprise 18 carbon atoms. In one embodiment of the present invention the base oil and amorphous silicon oxide particles are mixed with a first metal salt of an organic acid, preferably a fatty acid, comprising 2-16 carbon atoms, preferably 6-8 carbon atoms, and a second metal salt of an organic acid, preferably fatty acid, comprising 18 carbon atoms. In another embodiment of the present invention, the base oil and amorphous silicon oxide particles are mixed with a first metal salt of an organic acid, preferably a fatty acid, comprising 20-24 carbon atoms and a second metal salt of an organic acid, preferably a fatty acid, comprising 18 carbon atoms. In yet another embodiment of the present invention, the base oil and amorphous silicon particles are mixed with a first metal salt of an organic acid, preferably a fatty acid, comprising 2-16 carbon atoms, preferably 6-8 carbon atoms, a second metal salt of an organic acid, preferably a fatty acid, comprising 20-24 carbon atoms and a third metal salt of an organic acid, preferably a fatty acid, comprising 18 carbon atoms.

The organic acid that comprises 18 carbon is stearic acid, 12-hydroxy stearic acid or oleic acid. Also a mixture of a metals salt of stearic acid and a metal salt of 12-hydroxy stearic acid may be used.

The metal in the metal salt is preferably an alkali metal or an alkaline earth metal of Groups 1 and 2 of the Periodic System of Elements. Suitable examples of metals include lithium, potassium, sodium, calcium, aluminium, rubidium, cesium, francium, beryllium, strontium, barium, radium and magnesium. In addition it is noted that the metal in the metal salt to be used can be a semi-metal such as borium. According to a preferred embodiment according to the present invention, the metal is an alkaline earth metal, most preferably calcium. Further additives

The grease compositions may additionally comprise other thickening components, e.g. polymers. The grease compositions according to the present invention may comprise other additives to tailor its suitability to a certain use as is well known in the art. Such additives include anti-wear agents, anti-corrosion agents, rust inhibitors, friction modifiers, antioxidants, VI-improvers and the like as is well known by the person skilled in the art

The method for manufacturing the grease composition

A common disadvantage of conventionally manufactuπng methods is that it requires a multiple number of hours for blending the various components, gelling and cooling of the grease composition At a batch scale of about 1-5 metric tons, the total cooking (gelling) and cooling can take about four hours or more, whereas grease milling can require two or more hours. Usually, the total manufacturing time takes about eight hours However, the method according to the present invention can be performed in a very short manufacturing process, wherein blending, gelling and cooling is preferably performed within one hour, more preferably within half an hour period. The mechanical treatment, preferably grease milling, in accordance with the present invention for a 5 metric ton volume can require about two or two and a half hours

In accordance with the present invention the components of the grease compositions can be mixed in any possible order of sequence and before or after mixing at least one of the components, preferably the at least one metal salt of an organic acid and/or the amorphous silicon particles, is subjected to a mechanical treatment, a thermal treatment or to both a mechanical treatment and a thermal treatment Hence, (a) one of the components, preferably the amorphous silicon oxide particles or the at least one metal salt of an organic acid; (b) a mixture of two of the components, preferably a mixture of the at least one metal salt of an organic acid and the amorphous silicon particles, or (c) a mixture of the three components is subjected to a mechanical treatment and/or thermal treatment

Preferably, the amorphous silicon oxide particles, the at least one metal salt of an organic acid, or a mixture thereof is before or after mixing with the other component(s) subjected to of an organic acid a mechanical treatment, a thermal treatment or to both a mechanical treatment and a thermal treatment In accordance with the present invention the entire amount of base oil to be used or parts of the base oil can, for example, be added at one or more stages of the process. Suitable embodiments of the present invention include:

Subjecting a mixture of the at least one metal salt of an organic acid and the amorphous silicon oxide particles to a mechanical and/or thermal treatment, followed by adding to the mixture so obtained the base oil and optionally any further additives, and subjecting the grease composition so obtained to a mechanical and/or thermal treatment.

Subjecting a mixture of the at least one metal salt of an organic acid, the amorphous silicon oxide particles and a part of the base oil to a mechanical and/or thermal treatment, followed by adding to the mixture so obtained the remaining part of the base oil and optionally any further additives, and subjecting the grease composition so obtained to a mechanical and/or thermal treatment.

Subjecting a mixture of the at least one metal salt of an organic acid, the amorphous silicon oxide particles and the base oil to a mechanical and/or thermal treatment, followed by adding to the mixture so obtained any further additives, and subjecting the grease composition so obtained to a mechanical and/or thermal treatment.

Subjecting a mixture of the base oil, the amorphous silicon oxide particles and the at least one metal salt of an organic acid, and optionally any further additives, to a mechanical and/or thermal treatment.

Subjecting the amorphous silicon oxide particles to a mechanical and/or thermal treatment of, followed by adding the at least one metal salt of an organic acid to the thermally treated silicon particles and subjecting the mixture so obtained subsequently to a mechanical and/or thermal treatment. The base oil and optionally any further additives are then added to the mechanically and/or thermally treated mixture and the grease composition so obtained is then subjected to a mechanical and/or thermal treatment.

Subjecting the amorphous silicon oxide particles to a mechanical and/or thermal treatment, followed by adding the base oil, the at least one metal salt of an organic acid and optionally any further additives, to the mechanically and/or thermally treated silicon oxide particles, and subjecting the grease composition so obtained to a mechanical and/or thermal treatment. Subjecting the at least one metal salt of an organic acid to a mechanical and/or thermal treatment, followed by adding the base oil, the amorphous silicon oxide particles and optionally any further additives to the at least one metal salt so obtained, and subjecting the grease composition so obtained to a mechanical and/or thermal treatment.

Subjecting the at least one metal salt of an organic acid to a mechanical and/or thermal treatment of, followed by adding the amorphous silicon oxide particles to the mechanically and/or thermally treated metal salt(s) and subjecting the mixture so obtained subsequently to a mechanical and/or thermal treatment. The base oil and optionally any further additives are then added to the mechanically and/or thermally treated mixture and the grease composition so obtained is then subjected to a mechanical and/or thermal treatment.

As indicated above, the grease composition may comprise any further additives. Such further additives can be added to one or more of the components at any stage of the preparation process of the grease composition.

In the process according to the present invention each of the components or any mixture of the components can be subjected to a mechanical and/or thermal treatment in any possible order of sequence. For example, all components can be added together after which the mechanical and/or thermal treatment is applied. One of the components (e.g. the amorphous silicon particles) can first be subjected to a mechanical and/or thermal treatment after which one other component (e.g. the metal salt of an organic acid) or the two other components (i.e. the metal salt of an organic acid and the base oil) can be added to the mechanically and/or thermally treated component, followed by subjecting the grease composition so obtained to a mechanical and/or thermal treatment. Alternatively, one of the components (e.g. the amorphous silicon particles) can first be subjected to a mechanical and/or thermal treatment after which one other component (e.g. the metal salt of an organic acid) can be added to the mechanically and/or thermally treated component, subjecting the mixture so obtained to a mechanical and/or thermal treatment, followed by adding yet another component (e.g. the base oil) to the mechanically and/or thermally treated mixture adding, and subjecting the grease composition so obtained to a mechanical and/or thermal treatment. In a suitable embodiment of the present invention, the method for manufacturing the grease comprises the following sequential steps"

(a) subjecting the amorphous silicon oxide particles to a mechanical treatment, a thermal treatment or to both a mechanical treatment and a thermal treatment; and

(b) mixing the amorphous silicon oxide particles so obtained with the base oil and the at least one metal salt of an organic acid to form a grease composition.

In another suitable embodiment of the present invention, the method for manufacturing the grease comprises the following sequential steps.

(a) mixing the amorphous silicon oxide particles with the base oil and the at least one metal salt of an organic acid to form a grease composition; and

(b) subjecting the grease composition so formed to a mechanical treatment, a thermal treatment or to both a mechanical treatment and a thermal treatment.

In yet another suitable embodiment of the present invention, the method for manufacturing the grease comprises the following sequential steps:

(a) subjecting a mixture of the amorphous silicon oxide particles and the at least one metal salt of an organic acid to a mechanical treatment, a thermal treatment or to both a mechanical treatment and a thermal treatment; and

(b) mixing the mixture particles so obtained with the base oil to form a grease composition.

In a preferred embodiment of the present invention, the method for manufacturing the grease comprises the following sequential steps.

(a) subjecting the amorphous silicon oxide particles to a thermal treatment, and

(b) mixing the amorphous silicon oxide particles so obtained with the base oil and the at least one metal salt of an organic acid to form a grease composition; and.

(c) subjecting the grease composition so formed to a mechanical treatment. Preferably, the grease composition is manufactured by optionally subjecting the amorphous silicon oxide particles to a thermal treatment, preferably a heating step, to reduce the water content of the amorphous silicon oxide particles, followed by mixing the amorphous silicon oxide particles with the base oil and the metal salt of an organic acid to form a grease composition, whereafter the grease composition so formed is subjected to a mechanical treatment, preferably a milling step

The mechanical treatment is preferably a milling step which can be performed in any suitable milling apparatus, e.g. a high pressure homogeniser, a colloid mill, a roller mill (e.g a three-roller mill) or a worm gear mill. Preferably, the milling apparatus is a worm gear milling apparatus. The milling step can be performed under inert conditions, i e. in the absence of air or oxygen and/or in the absence of water (vapour). The thermal treatment is preferably a heating step. The heating step preferably involves heating at a temperature in the range of 30-150⁰C, more preferably 40-120⁰C and in particular 45-110°C. In this heating step, the water content of the amorphous silicon oxide particles is reduced, preferably to a water content of the silicon oxide particles of less than 5% by weight, more preferably less than 1% by weight, even more preferably less than 0.5% by weight, yet even more preferably less than 0 25% by weight, based on the total weight of the silicon oxide particles. The water content of the amorphous silicon oxide particles is usually more than 0.01% by weight, based on the total weight of the silicon oxide particles. Composition of the grease composition

The grease composition to be prepared in accordance with the present invention suitably comprises a base oil, a silicon oxide and at least one metal salt of an organic acid, wherein the total amount of the silicon oxide and the metal salt of the organic acid is 0 1-40% by weight, based on the total weight of the grease composition

Suitably, the amount of the amorphous silicon oxide particles will be 0.1-20% by weight, preferably 1-5% by weight, based on the total weight of the amorphous silicon oxide particles and the at least one metal salt of the organic acid. The skilled person will understand how such amounts can be realised starting from the respective starting materials.

According to a preferred embodiment of the present invention, the total amount of the metal salt of the organic acid, based on the total amount of the amorphous silicon oxide particles and the metal salt of the organic acid, is within a range of 0.1-30% by weight, preferably within the range of 0.1-20% by weight, based on the total weight of the grease composition. Applications

The grease composition according to the present invention can be used in many applications. However, it is in particular useful for lubricating a bearing, preferably a rolling element bearing, e.g. a spherical roller bearing, a taper roller bearing, a cylindrical roller bearing, a needle roller bearing, a ball bearing, and may also be used to lubπcate a sliding or plain bearing. It is furthermore very useful in coupling and gearing applications.

The grease compositions according to the present invention encompass NLGI (National Lubricating Grease Institute) grades ranging from NLGI grade 000 to NLGI grade 6. Preferably, the grease compositions according to the present invention have a dropping point of at least 70⁰C up to 200⁰C according to ASTM D-2265.

When used in low loading geaπngs, the grease composition has preferably a NLGI grade of 000 to 1. When used in high loading geaπngs, the grease composition has preferably a NLGI grade of 0 to 2. When used in bearings, the grease composition has preferably a NLGI grade of 1 to 4, more preferably a NLGI grade of 2 or 3 and most preferably a NLGI grade of 2.

EXAMPLES Example 1

5 kg of a grease composition comprising 5.0% by weight of Aerosol® 200, 77% by weight of a poly-α-olefin (40 cSt. at 40⁰Cj, 15% by weight of calcium stearate, 0.3% by weight of Ciba Irgalub 349 (mono- and di-alkylphosphateamines), 1.0% by weight of Rhein Chemie 2410, 0.2% by weight of Ciba Irgamed 39, 0.5% by weight of Rhein Chemie 3760 and 1.0% by weight of Rhein Chemie 3560, based on the total weight of the final grease composition (100% by weight), was prepared by mixing all ingredients at room temperature during 10 minutes. The grease composition so obtained was then subjected to milling in a three-roller mill during 30 minutes at room temperature. The grease composition so prepared was subjected to a full bearing test. The bearing test was run under the conditions of medium speed, low bearing load, and medium to high bearing temperatures. The bearing test was run under the specification of FE 8 DIN 51819. The Fe8 test is used to perform a mechanical-dynamical test for lubricants and greases. The test reveals the capability of lubricants and greases to provide lubricating properties and wear protection to the roller bearing under the specific mechanical and dynamical loading conditions. The Fe8 test-bench equipped with two angular contact ball bearings, 7312. The two bearings are spring loaded and consequently apply a test load of 10 kN. The test bearings are equipped on the outer ring of the test bearings with thermocouples to measure the running temperature of the bearings. The friction torque is measured through a sensor. The test bearings are mounted on a rotating shaft driven by an electromotor through a reduction gear unit which realizes a test speed of 1500 rpm. The test bearing 7312 has the dimensions shown in Table 1.

Table 1

The full beaπng test was performed under the operating conditions as specified in Table 2.

Table 2

The test revealed a very low friction coefficient. The test was suspended after 750 h of running. The grease composition passed the test with a very good wear result: less than 10 mg of wear was measured. The bearings did not show any pittings.

The grease composition was also subjected to a second full bearing test The second beaπng test was run under conditions of low to very low bearing speed, high bearing load, and medium bearing temperature. The bearing test was run under the specification of FE 8 DIN 51819. The Fe8 test-bench is equipped with two tapered roller bearings. The two test bearings are spring loaded and consequently apply a test load of 80 kN. The self-regulating temperature through fan-cooling is about 80 ⁰C. The test bearings are equipped on the outer πng with thermocouples to measure the running temperature of the bearings. The friction torque is measured through a sensor. The test bearings are mounted on a rotating shaft driven by an electromotor through a reduction gear unit which realizes a test speed of 75 rpm. The test bearing 31312 has the dimensions. As shown in Table 3. Table 3

The test was performed under the operating conditions as specified in Table 4.

Table 4

The test revealed that the friction torque was remaining on a low level despite of a small increase around 350 testing hours The grease passed the test with a low beanng friction torque and moderate wear.

Example 2

5 kg of a grease composition compπsing 3.5% by weight of Aerosil® 200, 79% by weight of mineral oil of ExxonMobil, 68 cSt at 40 ⁰C, 12% by weight of calcium-12- hydroxystearate, and 0.5% by weight benzotπazole of Ciba, 0.5% by weight irgalub 349 (mono- and dialkylphosphateamines) of Ciba, 3% by weight tπphenylphosphorothionate of Ciba, and 1.5% by weight sodium sebacacid, based on the total weight of the final grease composition (100% by weight), was prepared by mixing all ingredients duπng 10 minutes at room temperature. Subsequently, the grease composition so obtained was heated at 80 °C for 3 hours, followed by milling by means of a three-roller-mill during 30 minutes at room temperature. The performance of this grease composition in various tests is shown in Table 5.

Table 5

Example 3

A grease composition comprising 2.0% by weight of Aerosil® 200, 50% by weight of mineral oil of ExxonMobil, 68 cSt at 40 ⁰C, 4% by weight of calcium- 12-hydroxystearate, and 4% by weight of calcium-stearate, based on the total weight of the final grease composition (100% by weight), was prepared by mixing all ingredients during 10 minutes at room temperature. The mixture so obtained was then milled during 30 minutes at room temperature using a three-roller-mill. Subsequently, the mixture so obtained was heated at 80 ⁰C for 6 hours. Subsequently, the following additives were added at room temperature to the grease composition so obtained: (a) 0 5% by weight benzotπazole of Ciba, (b) 0.5% by weight irgalub 349 (mono- and dialkylphosphateamines) of Ciba, (c) 0.5% by weight triphenylphosphorothionate of Ciba, (d) 2% by weight sodium sebacacid and 36 5% by weight of the mineral oil of ExxonMobil, 68 cSt at 40 ⁰C, based on total weight of the final composition. Subsequently, the grease composition so obtained was milled during 30 minutes at room temperature using a three-roller mill.

The performance of this grease composition in various tests is shown in Table 6.

Table 6

From the results shown in the above Tables it will be clear that the grease compositions prepared in accordance with the present invention are highly attractive, and that they can be produced in a very easy and thus attractive manner.

Claims

1. A method for manufacturing a grease composition comprising as components a base oil, amorphous silicon oxide particles and at least one metal salt of an organic acid, which method comprises mixing the components in any possible order of sequence and subjecting before or after mixing at least one of the components to a mechanical treatment, a thermal treatment or to both a mechanical treatment and a thermal treatment.

2. A method according to claim 1, wherein the amorphous silicon oxide particles, the at least one metal salt of an organic acid, or a mixture thereof is before or after mixing with the other component(s) subjected to a mechanical treatment, a thermal treatment or to both a mechanical treatment and a thermal treatment.

3. A method according to claim 1 comprising the following sequential steps:

4. A method according to claim 1 comprising the following sequential steps:

5. A method according to claim 1 comprising the following sequential steps:

(a) subjecting the amorphous silicon oxide particles to a thermal treatment; and (b) mixing the amorphous silicon oxide particles so obtained with the base oil and the at least one metal salt of an organic acid to form a grease composition; and.

(c) subjecting the grease composition so formed to a mechanical treatment.

6. A method according to claim 1 comprising the following sequential steps:

7. A method according to any one of claims 1-6, wherein the thermal treatment is a heating step

8 A method according to claim 7, wherein the heating step involves heating at a temperature in the range of from 40- 100⁰C

9 A method according to claim 7 or 8, wherein in the heating step, the water content of the amorphous silicon oxide particles is reduced to less than 5 % by weight, based on total weight of the silicon oxide particles

10 A method according to any one of claims 1-9, wherein the mechanical treatment is a milling step

11 A method according to claim 10, wherein the milling step is a worm gear milling step.

12. A method according to any one of claims 1-11, wherein the amorphous silicon oxide particles are deπved from amorphous hydrophilic silicon oxide.

13 A method according to any one of claims 1-12, wherein the amorphous silicon oxide particles are deπved from amorphous fumed silicon oxide

14 A method according to any one of claims 1-13, wherein the amorphous silicon oxide particles are deπved from amorphous silicon oxide having a BET specific surface area of at least 50 m /g

15 A method according to any one of claims 1-14, wherein at least 80% of the silicon oxide particles have a mean particle size of 5-50 nm.

16. A method according to any one of claims 1-15, wherein the organic acid comprises 2- 24 carbon atoms

17. A method according to any one of claims 1-16, wherein the amorphous silicon oxide particles and/or the base oil are mixed with two or more metal salts of different organic acids to form a grease composition

18 A method according to any one of claims 1-17, wherein the amorphous silicon oxide particles and/or the base oil are mixed with a first metal salt of an organic acid comprising 2-16 carbon atoms, a second metal salt of an organic acid comprising 20-

24 carbon atoms and/or a third metal salt of an organic acid comprising 18 carbon atoms, or any mixture of these metal salts.

19. A method according to any one of claims 1-18, wherein the organic acid is an unbranched organic acid

20. A method according to any one of claims 1-19, wherein the organic acid is a fatty acid.

21. A method according to any one of claims 1-20, wherein the base oil comprises a base oil blend.

22. A method according to any one of claims 1-21, wherein the base oil or the base oil blend has a kinematic viscosity in the range of 1 to 60.000 cSt at a temperature of 4O⁰C according to DIN 51562/1.

23. A method according to any one of claims 1-22, wherein the metal is an alkali metal or an alkaline earth metal.

24. A method according to claim 24, wherein the metal is calcium.

25. A method according to any one of claims 1-24, wherein the total amount of the silicon oxide particles and the at least one metal salt in the grease composition formed is 0.1-40% by weight, based on the total weight of the grease composition.