Technical Field of the Invention

This invention relates to a grease composition and to use of the grease composition in a sealed bearing. Background of the Invention

Sealed bearings are used in many different types of machinery. The seals prevent the ingress of dirt and water that could damage the bearing and impair the functioning of the machinery. Typical seal materials include acrylonitrile-butadiene rubber (NBR) and

fluoroelastomers (FKM) . A grease is provided at the seal contact in order to prolong the life of the seal. There can be significant friction at the seal-metal interface which can lead to heat generation and a loss of

efficiency. The present inventors have sought to provide grease compositions that can act to reduce the friction at the seal-metal interface and thereby improve the efficiency of sealed bearings.

Summary of the Invention

The present inventors have surprisingly found that the combination of a low viscosity polyalphaolefin base oil with a calcium thickener can provide a grease generating particularly low friction at a seal-metal interface. Accordingly the present invention provides a grease composition comprising from 40 to 90wt% of a first base oil component which is a polyalphaolefin and having a kinematic viscosity at 100°C of between 1 and 5mm ² /s, wherein the weight percentage is based upon the weight of the grease composition, and comprising a thickener which is a C13-C21 fatty acid salt of calcium.

In a further aspect the present invention provides the use of a grease composition at a seal-metal interface in a sealed bearing wherein the grease composition comprises a base oil and a thickener, wherein the base oil comprises from 40 to 90wt% of a first base oil which is a polyalphaolefin and has a kinematic viscosity at 100°C of between 1 and 5mm ² /s, wherein the weight

percentage is based upon the weight of the grease composition, and comprises a thickener which is a C13-C21 fatty acid salt of calcium.

Detailed Description of the Invention

The grease composition comprises from 40 to 90wt% of a first base oil component which is a polyalphaolefin and has a kinematic viscosity at 100°C of between 1 and

5mm ² /s. The grease composition may comprise further base oils, but preferably the first base oil component makes up at least 50wt%, and preferably at least 80wt% of the total base oil (based upon the total weight of base oil in the grease composition) .

The thickener in the grease composition is a C13-C21 fatty acid salt of calcium, and preferably is a

hydrogenated castor oil fatty acid salt of calcium. The amount of thickener is preferably from 2 to 18wt%, based upon the weight of the grease composition, more

preferably from 2 to 14wt%.

The grease composition of this invention may also comprise anti-oxidants, rust preventatives, oiliness agents, extreme pressure additives, anti-wear agents, solid lubricants, metal deactivators, polymers,

detergents and other additives.

The anti-oxidants include, for example, 2,6-di-t- butyl-4-methylphenol, 2 , 6-di-t-butyl-paracresol, Ρ,Ρ'- dioctyldiphenylamine, N-phenyl-a-naphthylamine and phenothiazines .

The rust preventatives include paraffin oxide, carboxylic acid metal salts, sulphonic acid metal salts, carboxylic acid esters, sulphonic acid esters, succinic acid esters, sorbitan esters and various amine salts.

The oiliness agents, extreme pressure additives and anti-wear agents include, for example, sulphurised zinc dialkyl dithiophosphates, sulphurised zinc diaryl dithiophosphates , sulphurised zinc dialkyl

dithiocarbamates , sulphurised zinc diaryl

dithiocarbamates , sulphurised molybdenum dialkyl dithiophosphates, sulphurised molybdenum diaryl

dithiophosphates, sulphurised molybdenum dialkyl dithiocarbamates, sulphurised molybdenum diaryl

dithiocarbamates, organic molybdenum complexes,

sulphurised olefins, triphenylphosphates ,

triphenylphosphorothionates, tricresylphosphates , other phosphate esters and sulphurised fats and oils.

The solid lubricants include, for example,

molybdenum disulphide, graphite, boron nitride, melamine cyanurate, PTFE (polytetrafluoroethylene ) , tungsten disulphide, mica, graphite fluoride and so on.

The metal deactivators include, for example, Ν,Ν'- disalicylidene-1 , 2-diaminopropane, benzotriazole , benzoimidazole, benzothiazole and thiadiazole.

The grease compositions may be manufactured using standard grease manufacturing routes .

The grease compositions of the invention are suitably used at a seal-metal interface in a sealed bearing. The seal is suitably made from a material such as acrylonitrile-butadiene rubber (NBR) , hydrotreated acrylonitrile-butadiene rubber (HNBR) or a fluoroelastomer (FKM) . The bearing will contain a lubricant and the grease composition of the invention should be applied such that the grease composition is not contaminated by the lubricant .

Examples

The invention is further explained in detail below by means of examples and comparative examples, but the invention is in no way limited by these examples .

Greases were prepared by adding base oil and fatty acid to a kettle, heating to 85-90°C (at 3°C/min) and stirring at lOOrpm. Calcium hydroxide was added as a powder and then water was added. The kettle was closed and heated to 135°C (at 2°C/min) . The contents were stirred at lOOrpm and the wall temperature limit of the kettle was set at 140°C. The kettle was vented

approximately 40 minutes after reaching 135°C. Additional base oil was heated to 80°C and then added via a pump to the kettle. The contents were stirred at 150rpm for 5 minutes and the product temperature was kept between 130 and 135°C. A nitrogen tube was connected the kettle, and the contents were exposed to nitrogen flow for 15 minutes, thereby dehydrating the sample. The contents were cooled to 100°C (at 2°C/min) with stirring at lOOrpm. Additives were added, using additional base oil for dilution, at 80°C or lower. After addition of an additive, the contents were stirred for 1 minute at lOOrpm. The contents were then cooled to 50-60°C

(controlled by wall temperature setting: 20°C at 5°C/min and stirring at lOOrpm) . De-aeration was begun at 70°C, using a vacuum of -150mbar for 10 minutes and stirring at

50rpm. The product was homogenised at 1 x 300bar. All the formulations are summarised in Table 1 below, with the quantities given as wt% based upon the total weight of the formulation:

Table 1

The PAO 2.0 was a polyalphaolefin and had a

viscosity at 100°C of 2.0cSt. The XHVI 3.0 and XHVI 4.0 were base oils made by a Fischer-Tropsch process and were obtained from Shell. The XHVI 3.0 had a viscosity at 100°C of 3.0cSt and the XHVI 4.0 had a viscosity at 100°C of 4.0cSt. HCOFA is hydrogenated castor oil fatty acid. The additive package was the same in each of the example and comparative examples.

The greases were applied to NBR seals. A sealed bearing was run for 24 hours and then the friction was measured .

The results are given in table 2:

Table 2

The grease composition of the invention (wherein the base oil is a polyalphaolefin) shows considerably lower friction than the grease compositions of the comparative examples (wherein the base oils have similar viscosities but are made by Fischer-Tropsch processes) .