EP2334769B1

EP2334769B1 - A method for preparing a grease composition

Info

Publication number: EP2334769B1
Application number: EP09777668.6A
Authority: EP
Inventors: Dick Meijer
Original assignee: SKF AB
Current assignee: SKF AB
Priority date: 2008-08-15
Filing date: 2009-08-05
Publication date: 2015-11-04
Anticipated expiration: 2029-08-05
Also published as: EP2334769A1; WO2010017909A1; BRPI0917870A2; ES2559755T3

Description

Field of the invention

The present invention relates to a method for producing a grease composition, the grease composition obtainable by said method, and the use of the grease composition for lubricating a bearing, a gearing system or a coupling.

Background of the invention

Greases are widely used for lubricating bearings and other structural components. A grease is an essential product to reduce, for example, wear, friction, running temperatures and energy losses.
Lubricating greases comprise a lubricating base oil and a thickener. During service of the lubricant grease, the oil bleeds out of the oil/thickener-structure onto the surface of the bearing, thereby establishing a steady-state oil film on the surface of the bearing which provides 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.
In current lubricating greases, such as soap-thickened greases, oil bleeding characteristics are strongly dependent on temperature. In low temperature applications (such as in windmills) the bleeding of the oil of conventional greases is often so low that oil starvation occurs, i.e. there is not a sufficient amount of oil released from the grease composition.
It is further observed that not all grease compositions show an acceptable mechanical stability and consistency. Poor mechanical stability leads to a collapse of the grease structure upon shearing, resulting in an undue grease leakage and undesirable reduction of grease life. Furthermore, some greases appear to be very "noisy", which means that during service, the lubricated bearing produces a lot of noise.
Moreover, it is noted that in conventional greases the metal-containing soap has an adverse effect on the oil film on the surface of the bearing.
In EP 0700986 A2 grease compositions have been described that most preferably only contain a polymeric thickener or a mixture of polymeric thickeners, i.e. non-polar thickeners. The use of metal soap thickeners, i.e. polar thickeners is discouraged. The non-polar thickeners are prepared by a rapid cooling process.

Summary of the invention

An object of the invention is to provide a method for producing grease compositions that provide improved and multifunctional steady-state oil films on the surfaces of bearings, gearing systems or couplings.
Surprisingly, it has now been found that this can be established when use is made of a multiple thickener system and a rapid cooling step.
Accordingly, the present invention relates to a method for producing a grease composition comprising a base oil and a multiple thickener system which comprises a non-polar thickener component and a polar thickener component, which method involves the steps of:

preparing a first mixture which comprises a polar thickener and a base oil;
preparing a second mixture which comprises a non-polar thickener and a base oil;
preparing a third mixture which comprises the first mixture or the polar thickener which has been separated from the first mixture and the second mixture;
cooling at least the second mixture to room temperature by means of a rapid cooling process; and
subjecting the third mixture to a mechanical shearing treatment.

Detailed description of the invention

The method in accordance with the present invention can be carried out in number of ways.
In a first embodiment the present method comprises the steps of:

mixing a polar thickener into a base oil at a temperature above the melting point of the polar thickener to prepare the first mixture;
mixing a non-polar thickener into a base oil at a temperature above the melting point of the non-polar thickener to prepare the second mixture;
mixing the first mixture into the second mixture to prepare the third mixture;
cooling the third mixture to room temperature by means of a rapid cooling process, and
subjecting the third mixture to a mechanical shearing treatment.

In a second embodiment the present method comprises the steps of:

mixing a polar thickener into a base oil at a temperature above the melting point of the polar thickener to prepare the first mixture;
cooling the first mixture to room temperature by means of a rapid cooling process,
mixing a non-polar thickener into a base oil at a temperature above the melting point of the non-polar thickener to prepare the second mixture;
cooling the second mixture to room temperature by means of a rapid cooling process;
mixing the cooled first mixture into the cooled second mixture to prepare the third mixture; and
subjecting the third mixture to a mechanical shearing treatment.

In a third embodiment the present method comprises the steps of:

preparing at an elevated temperature the first mixture which comprises a polar thickener and a base oil;
cooling the first mixture to room temperature, preferably by means of a rapid cooling process,
separating the polar thickener from the first mixture;
mixing a non-polar thickener into a base oil at a temperature above the melting point of the non-polar thickener to prepare the second mixture;
mixing the polar thickener into the second mixture at an elevated temperature to prepare the third mixture;
cooling the third mixture to room temperature by means of a rapid cooling process; and
subjecting the third mixture to a mechanical shearing treatment.

In a fourth embodiment the present method comprises the steps of:

preparing at an elevated temperature the first mixture comprising a polar thickener and a base oil;
cooling the first mixture to room temperature, preferably by means of a rapid cooling process,
separating the polar thickener from the first mixture;
mixing a non-polar thickener into a base oil at a temperature above the melting point of the non-polar thickener to prepare the second mixture;
cooling the second mixture to room temperature by means of a rapid cooling process;
mixing the polar thickener into the cooled second mixture to prepare the third mixture; and
subjecting the third mixture to a mechanical shearing treatment.

In accordance with the present invention the non-polar thickener can suitably be mixed into the base oil at a temperature in the range of from 90-250°C, preferably in the range of from 140-230°C, and more preferably in the range of from 160-210°C, to prepare the second mixture.
In the present invention use is made of a rapid cooling process. Suitably, the rapid cooling process is performed in less than 30 seconds. Preferably, the rapid cooling process is performed at a cooling rate of between 5 and 20°C per second. More preferably, the rapid cooling process is performed at a cooling rate of between 6 and 15°C per second.
The elevated temperature at which the first mixture can be prepared is suitably in the range of from 90-250°C, preferably in the range of from 140-230 °C, more preferably in the range of from 160-210 °C. The elevated temperature to be used depends on the type of polar thickener to be used, as will be understood by the skilled person.
In accordance with the third and fourth embodiment of the present method, the polar thickener is suitably formed during the preparation of the first mixture. The polar thickener can, for example, be formed by heating a metal hydroxide and a stearic acid in a base oil at a temperature of 120°C. After formation the polar thickener obtained can, for instance, be separated from the base oil by way of washing.
The elevated temperature at which the polar thickener is mixed into the second mixture to form the third mixture is preferably in the range of from 90-250°C, more preferably in the range of from 140-210°C. The elevated temperature to be used depends on the type of polar thickener to be used, as will be understood by the skilled person.
The polar thickener to be used in accordance with the present invention is suitably a metal based soap or a metal based complex soap. Preferably, the polar thickener is a metal salt of a fatty acid or a hydroxy-fatty acid.
The fatty acid to be used in accordance with the present invention is preferably stearic acid or hydroxyl-stearic acid.
The base oil to be used in accordance with the present invention can suitably comprise one or more of mineral oils, polyalphaolefins, ester oils, and/or vegetable oils.
The non-polar thickener to be used in accordance with the present invention can suitably comprise one or more polymers, polymer-like rubbers, synthetic rubbers and/or natural rubbers. Suitably examples of polymers include polypropylenes.
Suitably, the non-polar thickener substantially consists of a mixture of (1) a (co- or homo-) polymer of propylene with an average molecular weight equal to or larger than 200,000 and (2) a (co- or homo-) polymer of propylene with an average molecular weight equal to or smaller than 100,000.
Preferably, the non-polar thickener to be used in accordance with the present invention comprises a high molecular weight component comprising a (co- or homo-) polymer of propylene with a weight average molecular weight equal to or larger than 200,000, preferably in the range of from 200,000-500,000 and a low molecular weight component comprising a (co- or homo-) polymer of propylene with a weight average molecular weight equal to or smaller than 100,000, preferably in the range of from 10,000-100,000.
The weight ratio between the high molecular weight component and the low molecular weight component in the non-polar thickener is preferably 1:40-1:5, more preferably 1:25-1:15, more preferably about 1:19.
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 500-2000 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 about 3.5.
The metal in the metal based soap or the metal based complex soap is preferably selected from the group consisting of lithium, potassium, sodium, calcium, magnesium, bismuth, aluminium, antimony, barium, lead and zinc. More preferably, metal is selected from the group consisting of bismuth, zinc, calcium and lithium. Most preferably, the metal is bismuth.
The preparation of the grease composition should preferably be carried out under a protective atmosphere, such as a nitrogen gasflow, for avoiding oxidation of the oils during heating.
The rapid cooling process, which forms an important aspect of the invention, will be indicated herein below 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. Preferably, the cooling process takes place in the absence of any additional water because of the condensation that will take place in and on the base oil and, if applicable, the metal plate.
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 structure and will become dispersion-like (particles in oil).
In the polymer-thickened lubricating grease 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.
As can be seen from scanning electron micrographs (SEM) photographs, in greases which are slowly cooled during their preparation, 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.
Although in its broadest sense the invention is not restricted to any method for preparing the grease nor to any explanation as to how the improved properties of the grease composition according to the invention are obtained, it is believed that the smoother and more uniform thickener structure obtained by quenching has a beneficial influence on the final properties of the grease composition, such as the mechanical properties, the oil bleeding characteristics, the noise characteristics, as well as the transport of the oil within the grease structure, so that the properties of the polymer-thickened grease compositions obtained by the use of the polymeric thickener according to the invention are improved even further.
The third mixture in accordance with the present invention is subjected to a mechanical shearing treatment. For this purpose a three-roll mil, a grease worker or any conventional mechanical shearing equipment can be used. During the working of the grease, further additives can be added as is well known to a man skilled in the art.
Suitably, the polar thickener is present in an amount in the range of from 1-25% by weight and the base oil is present in an amount in the range of from 74-99% by weight, based on the total amount of the first mixture.
Suitably, the non-polar thickener is present in an amount in the range of from 1-25% by weight and the base oil is present in an amount in the range of from 74-99% by weight, based on the total amount of the second mixture.
Suitably, the first mixture is present in an amount in the range of from 1-99% by weight and the second mixture is present in an amount in the range of from 1-99% by weight, based on the total amount of the third mixture.
The grease composition produced in accordance with the present invention displays unique properties by providing an improved multifunctional steady-state oil film on surfaces of bearings, gearing systems and couplings. The multifunctional properties of the steady-state oil film include conductivity, anti-wear, anti-fretting and anti-corrosion properties. Moreover, grease compositions produced in accordance with the present invention give a significant higher bleeding of the oil from the grease composition at low temperatures (ambient temperature or less), providing good mechanical properties and excellent quiet running characteristics.
The grease compositions that are produced in accordance with the present invention show in addition the following advantages:

oil bleeding characteristics that are less temperature-dependent than the characteristics of lubricant grease compositions known in the state of the art; better transport of the oil within the grease structure, which leads to improved grease service life;
good lubricating ability at low temperatures (below 100°C); good mechanical stability, i.e. "roll" stability/shear stability;
improved grease noise characteristics, i.e. a lower noise level of the lubricated bearing in the SKF BEQUIET-test; long relubrication intervals.

A further advantage of the grease composition according to the present invention is, as indicated earlier, the improved transport of the lubricating base oil within the grease structure. Hence, the present invention also relates to a grease composition obtainable by a method according to the present invention.
During service of the grease, the oil bleeds out at the surface of the grease structure. The oil separated at the surface should be replenished with oil from the "inside" of the grease structure through oil transport within the grease structure itself. In conventional greases, this oil transport is often very poor, resulting in a reduction of the amount of oil available for lubrication at the surface of the grease structure, even though the grease structure as such still contains enough oil. This effect contributes to a reduction of grease life and even starvation, so that frequent relubrication is required. In greases which are thickened with the polymeric thickener according to the invention the oil transport within the grease structure is improved as can be seen from oil bleeding tests according to DIN 51817 which shows a three-dimensional shrinkage of the grease sample.
The mechanical stability of the grease is dependent on the 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 or 48 hrs at 80°C., 165 rpm), of max. 350.
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 1 to 3. An NLGI-class of 0 can be made, however, will usually give undue grease leakage.
It should be understood that the grease composition according to the present invention can also suitably be in the form of a paste.
The oil bleeding characteristics or oil separation properties of the grease compositions according to the present invention should be such that continuously an effective amount of oil is provided at the running temperatures of the bearing, which are influenced by the ambient temperature. The grease composition according to the present invention provides acceptable oil separation at temperatures as low as 0°C.
Methods for the determination of oil bleeding characteristics are well known to a man skilled in the art, see for example DIN 51817.
Also, the viscosity of the separated oil must be acceptable, and preferably be constant.
For practical applications, the amount of noise produced by the lubricated bearing during service should obviously be as low as possible. Also, the noise produced by a bearing gives an indication of efficiency of the lubrication process and the amount of damage by particle overrolling occurring in the bearing.
The noise level produced by a bearing is not only dependent on the properties of the bearing itself and on the conditions under which the bearing is operated, but also on the noise characteristics of the grease composition used. Grease noise characteristics can be determined by means of the SKF BEQUIET grease noise tester, which is described in the SKF publication E4147.
The above properties of the final grease composition are of course also dependent on the properties of the base oil and additives used in the final grease composition, as is well known to a man skilled in the art. For optimizing the grease compositions the following parameters are also important: the ratio of the polymeric components in the thickener mixture, and the cooling speed during preparation and pre-working procedures.
Apart from the base oil and the multiple thickener system, grease composition according to the present invention may also contain other grease additives that are known in the art, as long as they do not have a detrimental effect on the multiple thickener system, the base oil and/or the final grease composition.
Further, the present invention relates to the use of a grease composition prepared in accordance with the present method for lubricating a bearing, a gearing system or a coupling. 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, and a screw conveyor.
The invention will now be further illustrated by the following figures and examples, which do not limit the invention in any way.

Examples

Example 1

A grease composition A was prepared in accordance with the present invention, having the following composition: 10.9%wt polypropylene, 0.9%wt anti-oxidant 5.5%wt anti-corrosion and anti-wear additives, 73.6%wt mineral oil, and 9.1%wt of bismuth hydroxyl stearate, based on total weight of grease composition. This grease composition was produced by mixing the polypropylene, and the mineral oil at a temperature of 195°C, and quenching the mixture so obtained in 20 seconds to a temperature of 23°C (ambient temperature). The bismuth hydroxyl stearate, anti-oxidant, anti-corrosion, and anti-wear additives were added to the base grease so obtained during the milling which was carried out with a 3-roll mill. The performance of this grease composition in terms of mechanical stability and anti-wear properties is shown in Tables 1 and 2.

Example 2

A grease composition B was prepared in accordance with the present invention, having the following composition: 11%wt of polypropylene, 79%wt mineral oil, and 10%wt of the same metal-based soap as used in Example 1, based on total weight of the grease composition. This grease composition was produced by mixing the polypropylene and the mineral oil at a temperature of 195°C, and quenching the mixture so obtained in 20 seconds to a temperature of 23°C (ambient temperature). The bismuth hydroxyl stearate was added to the base grease so obtained during the milling which was carried out with a 3-roll mill. The performance of this grease composition in terms of mechanical stability and anti-wear properties is shown in Tables 1 and 2.

Example 3 (Comparative example)

A grease composition C was prepared having the following composition: 12%wt polypropylene, 1%wt anti-oxidant, 6%wt anti-corrosion and anti-wear additives, and 81%wt mineral oil, based on total weight of the grease composition. This grease composition was produced by mixing the polypropylene, and the mineral oil at a temperature of 195°C, and quenching the mixture so obtained in 20 seconds to a temperature of 23°C (room temperature). The anti-oxidant, anti-corrosion, and anti-wear additives were added to the base grease so obtained during the milling which was carried out with a 3-roll mill.
The performance of this grease composition in terms of mechanical stability and anti-wear properties is shown in Tables 1 and 2.

Example 4 (Comparative example)

A grease composition D was prepared having the following composition: 11%wt polypropylene and 89%wt mineral oil, based on total weight of the grease composition. This grease composition was produced by mixing the polypropylene and the mineral oil at a temperature of 195°C, and quenching the mixture so obtained in 20 seconds to a temperature of 23°C (ambient temperature).
The performance of this grease composition in terms of mechanical stability and anti-wear properties is shown in Tables 1 and 2.
From the results shown in Tables 1 and 2, it will be clear that the grease compositions produced in accordance with the present invention (grease compositions A and B) display an improved performance when compared with grease compositions that do not contain a multiple thickener system (grease compositions C and D).
Table 2

Grease Anti Wear Properties by Wear Scar (1400 N, 60 sec) in SHELL 4-Ball Tester [mm]

C 1.85

A 1.45

D 2.6

B 1.65

Claims

A method for producing a grease composition comprising a base oil and a multiple thickener system which comprises a non-polar thickener component and a polar thickener component, which method involves the steps of:
- preparing a first mixture which comprises a polar thickener and a base oil, which polar thickener is a metal based soap or a metal based complex soap, and wherein the polar thickener is present in an amount in the range of from 1-25% by weight and the base oil is present in an amount in the range of from 74-99% by weight, based on the total amount of the first mixture;

- preparing at a temperature in the range of from 90-250°C a second mixture which comprises a non-polar thickener and a base oil, which non-polar thickener comprises a high molecular weight component comprising a (co- or homo-) polymer of propylene with a weight average molecular weight equal to or larger than 200,000 and a low molecular weight component comprising a (co- or homo-) polymer of propylene with a weight average molecular weight equal to or smaller than 100,000, and wherein the non-polar thickener is present in an amount in the range of from 1-25% by weight and the base oil is present in an amount in the range of from 74-99% by weight, based on the total amount of the second mixture;

- preparing a third mixture which comprises the first mixture or the polar thickener which has been separated from the first mixture and the second mixture;

- cooling at least the second mixture to room temperature by means of a rapid cooling process which is performed in less than 30 seconds; and

- subjecting the third mixture to a mechanical shearing treatment.
A method according to claim 1, which method comprises the steps of:
- mixing a polar thickener into a base oil at a temperature above the melting point of the polar thickener to prepare the first mixture;

- mixing a non-polar thickener into a base oil at a temperature above the melting point of the non-polar thickener to prepare the second mixture;

- mixing the first mixture into the second mixture to prepare the third mixture;

- cooling the third mixture to room temperature by means of a rapid cooling process, and

- subjecting the third mixture to a mechanical shearing treatment.
A method according to claim 1, which method comprises the steps of:
- mixing a polar thickener into a base oil at a temperature above the melting point of the polar thickener to prepare the first mixture;

- cooling the first mixture to room temperature by means of a rapid cooling process,

- mixing a non-polar thickener into a base oil at a temperature above the melting point of the non-polar thickener to prepare the second mixture;

- cooling the second mixture to room temperature by means of a rapid cooling process;

- mixing the cooled first mixture into the cooled second mixture to prepare the third mixture; and

- subjecting the third mixture to a mechanical shearing treatment.
A method according to claim 1, which method comprises the steps of:
- preparing at a temperature in the range of from 90-250°C the first mixture which comprises a polar thickener and a base oil;

- cooling the first mixture to room temperature, preferably by means of a rapid cooling process,

- separating the polar thickener from the first mixture;

- mixing a non-polar thickener into a base oil at a temperature above the melting point of the non-polar thickener to prepare the second mixture;

- mixing the polar thickener into the second mixture at an elevated temperature to prepare the third mixture;

- cooling the third mixture to room temperature by means of a rapid cooling process; and

- subjecting the third mixture to a mechanical shearing treatment.
A method according to claim 1, which method comprises the steps of:
- preparing at a temperature in the range of from 90-250°C the first mixture comprising a polar thickener and a base oil;

- cooling the first mixture to room temperature, preferably by means of a rapid cooling process,

- separating the polar thickener from the first mixture;

- mixing a non-polar thickener into a base oil at a temperature above the melting point of the non-polar thickener to prepare the second mixture;

- cooling the second mixture to room temperature by means of a rapid cooling process;

- mixing the polar thickener into the cooled second mixture to prepare the third mixture; and

- subjecting the third mixture to a mechanical shearing treatment.
A method according to claim 4 or 5, wherein the polar thickener is formed during the preparation of the first mixture.
A method according to any one of claims 1-6, wherein the temperature at which the polar thickener is mixed into the second mixture to form the third mixture is in the range of from 90-250 °C.
A method according to any one of claims 1-7, wherein the polar thickener is a metal salt of a fatty acid or a hydroxy-fatty acid.
A method according to claim 8, wherein the fatty acid is stearic acid or hydroxyl stearic acid.
A method according to any one of claims 1-9, wherein the base oil comprises one or more of mineral oils, polyalphaolefins, ester oils, and/or vegetable oils.
A method according to any one of claims 1-10, wherein the metal is selected from the group consisting of lithium, potassium, sodium, calcium, magnesium, bismuth, aluminium, antimony, barium, lead and zinc.