FIELD OF THE INVENTION

The present invention relates to a liquid lubricating oil composition; a method for preparing the liquid lubricating oil composition. The invention also relates to the use of the liquid lubricating oil composition for lubricating a mechanical component having a metal surface; and the use of the liquid lubricating oil for protecting a mechanical component having a metal surface against corrosion, wear and/or fretting. BACKGROUND OF THE INVENTION

Lubricants such as greases and oils are generally composed of a majority of one or more base oils plus a variety o additives to reduce friction between surfaces o components within a mechanical system, thereby preventing wear which results from surface-to-surface contacts between the components. Examples of such mechanical components include for instance bearings, bearing components and gear box components.

Greases are semi- solid lubricants that generally consist of a soap emulsified with a mineral or vegetable oil which possess a high viscosity, whereas oils generally have a viscosity which is much lower. Due to this difference in viscosity, greases can be more suitable than oils for particular applications, and vice versa. For example, greases are mostly not suitable for use in gearbox applications because of the insufficient lubricant circulation within the respective systems, whereas lubricating oils may be better suited for such applications because they provide a much better lubricant circulation. The performance of lubricating oils in many applications, for example in low speed applications or wind turbine gearboxes, requires the use of high base oil viscosities to ensure sufficient build-up of a lubricant film. The performance of lubricating oils, especially those with lower base oil viscosity, still leaves room for improvement because fretting, wear, or micro-pitting may occur due to insufficient build-up of a lubricant film, which may result in an early breakdown of the mechanical system in question. SUMMARY OF THE INVENTION

Object of the present invention is to provide a lubricating oil composition with improved lubricating properties, especially in low speed applications such as for instance wind turbine applications and industrial gear box applications. A further object of the invention is to provide a lubricating oil composition that provides equal or better lubricant-film formation while having a viscosity that is much lower compared to existing lubricating oils used in such applications. It has now been found that this can be established when a lubricating oil comprises a dispersion of grease particles. Accordingly, the present invention relates to a liquid lubricating oil composition comprising a dispersion of grease particles in a lubricating oil, which grease particles comprise a grease composition which comprises a mixture of a polymer and a base oil.

The lubricating oil compositions according to the present invention have the advantage that they have the ability to suppress surface fatigue and enhance lubrication properties because the dispersion of grease particles advantageously promotes an improved lubricant film formation on mechanical components. The present invention enables the formation of permanent lubricant films of higher thickness when compared with conventional lubricating oils of similar viscosity. In particular, the present invention enables the formation of permanent lubricant films of higher thickness in low speed applications when the risk of a mechanical breakdown at an early stage is considerable. In this respect one can think for instance of gear box applications and wind turbine applications. In accordance with the present invention the properties of the grease particles are very attractively transferred into the lubricating oil, resulting in the formation of a porous, oil- retaining layer on the surface of the mechanical component. In turn, this layer promotes the formation of a thick lubricant film on the mechanical component. Under conditions of low speed operation or conditions where normally a thin lubricant film is expected, for example, under boundary lubrication conditions, this reduces the risk of surface fatigue, and leads to reduced wear. In addition, as a result of the improved lubricating performance of the present lubricating oil composition the amount of aggressive additives otherwise needed can suitably be decreased or even be avoided. DETAILED DESCRIPTION OF THE INVENTION

The liquid lubricating oil composition of the present invention has suitably a kinematic viscosity at 40°C in the range of from 5-1000 cSt. Preferably, the present liquid lubricating oil composition has a kinematic viscosity at 40°C in the range of from 10-400 cSt, more preferably 20-200 cSt, and most preferably in the range of from 32-100 cSt.

In accordance with the present invention the liquid lubricating oil composition comprises a dispersion of grease particles in a lubricating oil, which grease particles comprise a grease composition which comprises a mixture of a polymer and a base oil. The nature of the lubricating oil to be used in accordance with the present invention is not essential, provided it has a kinematic viscosity at 40°C in the range of from 5-400 cSt, preferably in the range of from 20- 200 cSt.

The lubricating 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 can be used and include for instance esters, poly-alpha-olefins and polysiloxanes.

The lubricating oil to be used in accordance with the present invention may also comprise a base oil blend. Suitably, blends of mineral base oils and/or synthetic base oils may be used.

The base oil to be used in the grease composition of the invention is one which may ordinarily be used as the base oil of a lubricating oil or as the base oil of a grease, but will suitably have a kinematic viscosity at 40°C in the range of from 5-1000 cSt, preferably in the range of from 10- 400 cSt.

Also the base oils may be any lubricating oils known per se, such as mineral oils, synthetic hydrocarbons, ester oils, vegetable oils and mixtures thereof. In the context of the present application the kinematic viscosity measurements at 40°C are carried out in accordance with DIN 51562/1. The base oil to be used in the grease composition may be similar to the lubricating oil or it may differ from the lubricating oil. Preferably, the lubricating oil and the base oil to be used in the grease composition are a similar type of oil.

Suitably, the grease particles are present in an amount in the range of from 0.5-50 wt , based on the total weight of the liquid lubricating oil composition. Preferably, the grease particles are present in an amount in the range of from 1-20 wt , more preferably in an amount in the range of from 5-15 wt , based on the total weight of the liquid lubricating oil composition.

The grease composition suitably comprises the polymer in an amount in the range of from 1- 50wt , based on the total weight of the grease composition. Preferably, the grease composition comprises the polymer in an amount in the range of from 5-17wt , more preferably in an amount in the range of from 9-15 wt , based on the total weight of the grease composition. The grease composition suitably comprises the base oil in an amount in the range of from 81- 99wt , based on the total weight of the grease composition. Preferably, the grease composition comprises the polymer in an amount in the range of from 83-95wt , more preferably in an amount in the range of from 85-91 wt , based on the total weight of the grease composition. Suitably, the grease composition will contain mainly the polymer and the base oil. Suitably, the grease composition will consist for more than 80wt of the polymer and the base oil, based on the total weight of the grease composition. It will be understood that the grease composition may contain other components such as lubricant additives known per se. Preferably, the grease composition will consist for more than 90wt of the polymer and the base oil, based on the total weight of the grease composition. More preferably, the grease composition will consist for more than 95wt of the polymer and the base oil, based on the total weight of the grease composition.

Suitably, the polymer is selected from the group consisting of polyethylene, polypropylene, natural rubber, polypropylene, polyisoprene, polyamide, polybutadiene, poly(styrene- butadiene), poly(ethylene-propylene-diene), polyurethane, polymethacrylate, polyisobutylene, poly(isobutylene- succinic acid), poly(isobutylene-succinic acid-polyacrylamide), polyurea and polyethylene. Mixtures of two or more of these polymers can be used.

Preferably, the polymer is a polymer of propylene. The polymer of propylene 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 polymer of propylene comprises a high molecular weight component and a low molecular weight component, characterized in that the polymer of propylene comprises a mixture of (1) a (co- or homo-)polymer of propylene with a weight average molecular weight of more than 200.000, preferably 200.000-350.000, and (2) a (co- or homo-)polymer of propylene with a weight average molecular weight of less than 200.000, preferably less than 100.000, more 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. Preferably, the (co- or homo- polymer of propylene with a weight average molecular weight of less than 100.000 is a polypropylene homopolymer. Preferably, the (co- or homo-)polymer of propylene with a weight average molecular weight of more than 200.000 high molecular weight component is a polypropylene homopolymer or a propylene/ethylene-copolymer.

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. In another suitable embodiment of the present invention, the grease composition may be based on the low molecular weight component only. In this case, the high molecular weight component is omitted.

As indicated before, the grease composition of which the grease particles to be used in accordance with the present invention are formed may comprise one or more different polymers. For instance, the grease composition may contain a polymer of ethylene and a polymer of propylene. Preferably, the polymer in the grease composition consists mainly of a polymer of propylene, i.e. the polymer consists of more than 50wt% of a polymer of propylene, preferably more than 80wt%, and more preferably more than 95wt% of a polymer of propylene, based on the total amount of polymer in the grease composition.

Suitably, the grease particles are in the form of fibers. The grease fibers suitably have an average diameter in the range of 0.05-10 micrometer. Preferably, the grease fibers have an average diameter in the range of 0.1-5 micrometer, more preferably a diameter in the range of 0.5-1 micrometer.

Furthermore, additives known per se may be incorporated in the lubricant 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 0.1 and 15% by weight, and preferably between 0.5 and 10% by weight, based on the total weight of the grease composition. An advantage of the present liquid lubricating oil composition is, however, that the amount of such additives, in particular aggressive additives, can be decreased or even be avoided, whilst still very attractive lubricating properties are established.

The present invention further relates to a method for preparing the liquid lubricating oil composition according to the present invention, comprising the steps of:

(a) mixing the polymer and the base oil at a temperature above the melting point of the polymer; (b) cooling the mixture as obtained in step (a) to a temperature in the range of from 0-120 °C in less than 3 minutes;

(c) mixing the grease composition as obtained in step (b) with the lubricating oil to obtain a dispersion of grease particles in the lubricating oil; and

(d) recovering the liquid lubricating oil composition.

Step (a) can suitably be carried out at a temperature in the range of from 150-250°C, preferably in the range of from 170-230°C, more preferably in the range of from 190-210°C. Step (a) can be carried out by mixing the polymer(s) and the base oil in a manner known per se, which can optionally involve the use of suitable solvents. The polymer(s) is (are) are mixed with the base oil and optionally one or more additives. After the polymer(s) is (are) dissolved in the base oil and optionally additives have been added, the mixture so obtained is cooled from the mixing temperature to a temperature in the range of 0-120°C in less than 3 minutes. Preferably, the mixture obtained in step (b) is cooled in step (b) to a temperature in the range of from 10-100 °C, more preferably in the range of from 15-35 °C, even more preferably to room temperature. Suitably, the cooling in step (b) is carried out in a period of time between 1 sec. - 2 min., preferably 10 sec. - 1 min., more preferably about 5- 15 sec. This rapid cooling process, which forms an important aspect of forming the grease composition will be indicated hereinafter as "quenching". The quenching of the mixture as obtained in step (a) 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 has a major influence on the structure of grease composition, giving significant improvement of the properties of the final liquid lubricating oil compositions compared to both conventional lubricating oil compositions. The mixing process is preferably carried out under a protective atmosphere, such as a nitrogen gas flow, in order to avoid oxidation of the polymer components and the oils during heating.

In step (c), the grease composition as obtained in step (b) is mixed with the lubricating oil to obtain a dispersion of grease particles in the lubricating oil. Step (c) can suitably be carried out at a temperature in the range of from 10-100°C, preferably in the range of from 15-50°C, more preferably in the range of from 20-40°C. Step (c) can be carried out by mixing the grease composition and the lubricating oil in a manner known per se, which can optionally involve the use of suitable solvents. The polymer(s) is (are) are mixed with the lubricating oil and optionally one or more additives. The preparation of the liquid lubricating oil composition is preferably carried out under a protective atmosphere, such as a nitrogen gas flow, in order to avoid oxidation of the oils during heating.

In the liquid lubricating oil composition according to the invention the grease composition has a sponge-like structure, which gives the grease composition its appearance and structure. The base oil is kept within the pore-like spaces within the sponge-like structure. As can be seen from scanning electron micrographs (SEM) photographs, sponge-like-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 composition with a smoother and more uniform structure.

The present invention further relates to the use of a liquid lubricating oil composition according to the present invention for lubricating a mechanical component having a metal surface. The present invention particularly relates to the use of the present liquid lubricating oil composition for lubricating a mechanical component with a metal surface in low speed applications. The skilled person will understand that low speed applications are defined as those applications where the operating speed is close to zero; the operating speed is below zero during to start/stop sequences; the combination of operating speed and lubricant oil viscosity does not result in a lubricant film providing low friction and/or surface protection; or the combination of operating speed and lubricant oil viscosity results in boundary or mixed lubrication conditions. Depending entirely on the rolling and sliding speed in the lubricated contact, the rotational speed in low speed applications may vary, for example, between 0-3000 rpm.

Suitable low speed applications include wind turbine applications, gear box applications, and industrial gear box applications. The present invention is of particular interest in wind turbine applications and gear box applications. In addition, the present invention relates to the use of the present liquid lubricating oil composition for protecting a mechanical component having a metal surface against corrosion, wear and/or fretting. Suitably, the mechanical component comprises a bearing, bearing component or gear box component. The present invention further relates to a grease composition obtainable according to the method of the invention. The invention will now be further illustrated by the following Examples, which do not limit the invention in any way.

Example 1

A liquid lubricating oil composition in accordance with the present invention was prepared as follows. A grease composition was firstly prepared by mixing a polypropylene in an amount of 15wt with 85wt of a PAO base oil at a temperature of 180 °C to form a grease composition. The polypropylene was a low-molecular weight polypropylene polymer having an average molecular weight of 82.000 g/mol, and the PAO base oil was a Durasyn 168 oil obtained from Ineos Oligomers having a viscosity of 46 cSt at 40°C. The mixture so obtained was then cooled to a temperature of 20 °C in 15 seconds. 6.7wt of the cooled grease composition was than mixed with 93.3 wt of the same PAO base oil as described hereinbefore to form a dispersion of grease fibers in a lubricating oil. The grease fibers, not taking into account the base oil of the grease composition, were present in an amount of 1.0 wt , based on total weight of the lubricating oil. The grease fibers had an average diameter of 3 micrometer, and the liquid lubricating oil composition so obtained had a viscosity of 48 cSt at 40°C.

The roller surface appearance of 81102TN bearings (commercially available from SKF) was tested using the liquid lubricating composition so prepared and a liquid lubricating oil composition consisting of the PAO base oil only respectively. The tests were carried out under lubricating conditions for the duration of 4 hours under application conditions of 1200 rpm, 80 °C and a maximum contact pressure of 1.5 GPa. The surface of the 81102TN bearing which was tested and lubricated with the liquid lubricating oil composition consisting only of the PAO base oil is shown in Figure 1, whereas the surface of the 81102TN bearing which was tested and lubricated with the liquid lubricating oil composition in accordance with the present invention is shown in Figure 2. From these Figures it will be clear that the surface bearing which was lubricated with the lubricating oil in accordance with the present invention remained completely intact, whereas the surface of the bearing lubricated with the PAO base oil is heavily damaged and shows extensive wear and micro-pitting.

Example 2

In addition to the two lubricating oils as used in Example 1, and for additional comparison purposes, a polymer grease composition was prepared by mixing at a temperature of 180 °C, 15wt% of the same polypropylene as used in Example 1 with 85 wt% of the same PAO base oil as used in Example 1. The roller surface appearance of 81102TN bearings (commercially available from SKF) was tested using respectively the grease composition so obtained, the liquid lubricating composition of Example 1 consisting of the grease composition dispersed in PAO oil having a viscosity of 46cSt, and a PAO base oil having a viscosity of 400cSt. The PAO base oil was a Durasyn 1741 obtained from Ineos Oligomers. The tests were carried out under lubricating conditions for the duration of 1 hour under application conditions of 0-1200 rmp, 80 °C and a maximum contact pressure of 1.5 GPa. In Figure 3, the film-buiding properties of these three materials are shown. The lubricant film is evaluated qualitatively from 0 - 100% using Lubcheck measurements of the electrical capacitance in the rolling contact, as a function speed. The Lubcheck value may vary between 0% indicating metallic contact, and 100% indicating full separation of the metallic counterparts due to the presence of a lubricant film. Figure 3 shows that the polymer grease resulted in the formation of a permanent thick film which, from friction prespective, is clearly not desirable. Moreover, the consistency of the polymer grease makes it unsuitable for use in gearbox applications. The use of the PAO oil of 400cSt resulted in film-formation of which the thickness is dependent on the speed applied, whereby the lubricant film collapses completely at low speed. However, the liquid lubricating oil in accordance with the present invention showed strongly improved film-building properties when compared to the use of the PAO oil of 400cSt only. This finding indicates that the film- building properties derived from the grease fibers are transferred into the lubricating oil. In addition, due to the impact of the dispersion of the grease fibers, the lubricating oil composition according to the invention has obtained better film-forming properties than the PAO oil of 400cSt, making present lubricating oil composition much more attractive in low speed applications.