IMPROVED ROLL STABILITY IN A GREASE COMPOSITION

Technical Field of the Invention

This invention relates to a grease composition having improved roll stability.

Background of the Invention

In use, greases are subjected to mechanical shearing which can lead to changes in the size and dispersion of thickener particles and resulting degradation of the grease. Roll stability is a measure of the stability of a grease when subjected to mechanical shearing. In the roll stability test according to ASTM D 1831, grease is placed in a cylinder containing a steel roller and the cylinder is rotated. The penetration of the grease is measured before and after the test, and the difference in

penetration values indicates the roll stability.

The present inventors have sought to provide grease compositions having improved roll stability.

Summary of the Invention

The present inventors have surprisingly found that adding calcium salicylate to grease compositions can improve roll stability. Accordingly the present invention provides the use of calcium salicylate to improve roll stability in a grease composition comprising a base oil and a fatty acid salt of lithium and/or calcium.

In a further aspect the present invention provides a grease composition comprising a base oil, a fatty acid salt of lithium and/or calcium and from 0.3 to 0.6wt% calcium salicylate, wherein the weight percentage is based upon the weight of the grease composition. Detailed Description of the Invention

The present invention provides the use of calcium salicylate to improve roll stability in a grease

composition. A method for measuring roll stability is defined in ASTM D 1831. The duration of the cylinder rotation may be varied but is preferably from 1 to 200 hours, more preferably from 10 to 50 hours and most preferably is 18 hours. The temperature during the cylinder rotation may be varied, e.g. from room

temperature to 100°C, but is preferably 65°C. Suitably water is mixed with the grease before the roll stability is measured, e.g. 10wt% water based upon the weight of the grease composition. The roll stability is determined by the difference in penetration before and after the roll stability test. Penetration is suitably measured by the method defined in ASTM D 1403 and is typically reported in 0.1mm. A small difference in penetration (particularly below 100 0.1mm) is indicative of good roll stability, whereas a large difference in penetration (particularly above 100 0.1mm) is indicative of poor roll stability .

In the present invention, calcium salicylate is incorporated in a grease composition to improve roll stability. This means that the roll stability of the grease composition is better than the roll stability of an otherwise identical grease composition that does not contain calcium salicylate (i.e. the penetration

difference is lower for the grease composition than for an otherwise identical grease composition that does not contain calcium salicylate) .

The amount of calcium salicylate that is suitably incorporated into the grease composition to improve roll stability is suitably from 0.3 to 0.6wt% based upon the weight of the grease composition. The inventors have found that using less than 0.3wt% calcium salicylate does not give the desired improvement in roll stability and using more than 0.6wt% provides no further improvement.

The calcium salicylate is suitably overbased calcium salicylate .

The base oil in the grease composition is one which may ordinarily be used as the base oil of a grease composition and there are no special restrictions. The base oil is suitably chosen from mineral oils, synthetic oils, animal and plant oils, and mixtures thereof.

In particular it is possible to use, singly or as mixtures, base oils which belong to Group I, Group II, Group III, Group IV and so on of the API (American

Petroleum Institute) base oil categories.

Group I base oils include, for example, paraffinic mineral oils obtained by a suitable combination of refining processes such as solvent refining,

hydrorefining, and dewaxing in respect of lubricating oil fractions obtained by atmospheric distillation of crude oil .

Group II base oils include, for example, paraffinic mineral oils obtained by a suitable combination of refining processes such as hydrorefining and dewaxing in respect of lubricating oil fractions obtained by

atmospheric distillation of crude oil.

Group III base oils and Group 11+ base oils include paraffinic mineral oils manufactured by a high degree of hydrorefining in respect of lubricating oil fractions obtained by atmospheric distillation of crude oil, base oils refined by the Isodewax process which dewaxes and substitutes the wax produced by the dewaxing process with isoparaf fins, and base oils refined by the Mobil wax isomerisation process.

Examples of synthetic oils include polyolefins, polyoxyalkylene glycols such as polyethylene glycol or polypropylene glycol, esters such as di-2-ethylhexyl sebacate or di-2-ethylhexyl adipate, polyol esters such as trimethylolpropane esters or pentaerythritol esters, per fluoroalkyl ethers, silicone oils and polyphenyl ethers .

Polyolefins include polymers of various olefins or hydrides thereof. Any olefin may be used, and as examples mention may be made of ethylene, propylene, butene and a- olefins with five or more carbons. In the manufacture of polyolefins, olefins may be used singly or two or more kinds may be used in combination. Particularly suitable are the polyolefins called poly-a-olefins (PAO) . These are base oils of Group IV.

GTL (gas to liquid) base oils synthesised by the Fischer-Tropsch method of converting natural gas to liquid fuel have a very low sulphur content and aromatic content compared with mineral oil base oils refined from crude oil and have a very high paraffin constituent ratio, and so have excellent oxidative stability.

As typical examples of animal and plant oils mention may be made of castor oil and rapeseed oil.

The various base oils may be used singly or in mixtures .

The thickener in the grease composition is a fatty acid salt of lithium and/or calcium. The fatty acid is suitably a Ci ₃ -C ₂ i fatty acid. Suitably the thickener is a fatty acid salt of lithium and preferably the thickener is lithium-12-hydroxystearate . The grease composition suitably comprises from 2 to 20wt% of fatty acid salt of lithium and/or calcium, based upon the weight of the grease composition.

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 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. Preferably the calcium salicylate is the first additive added to the reacted combination of thickener and base oil.

The grease compositions of the invention may be used for general industrial lubrication, including use in heavy duty bearings. They may also be used in heavy duty plain and rolling element bearings operating under harsh conditions including shock loading in wet environments. Furthermore they may be used in automotive applications such as wheel bearings (taper and rolling) , chassis points, universal steering, suspension and transmission joints, water pump and generating bearings, and shackles and linkages .

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 combining 12- hydroxystearate with base oil in a kettle and heating to 90°C (heat rate of 3°C/min) whilst stirring at 150rpm. A mixture of base oil, water and lithium hydroxide was prepared and was heated to 60°C with stirring. The lithium hydroxide mixture was added to the kettle at 90°C and the kettle was closed. The kettle was then heated to 200°C (heat rate of 3°C/min) and the mixture was stirred at 200rpm. When the temperature reached 200°C, the kettle was vented and stirring rate was reduced to lOOrpm. After venting the kettle was heated to 215°C (heat rate of 4°C/min) and the mixture was stirred at 150rpm. The mixture was then cooled to 160°C (heat rate of 3°C/min) with stirring at 200rpm. At 160°C further preheated base oil was added by pump, and then the kettle was stirred at 150rpm for 15 minutes. The kettle was cooled to 100°C (heat rate of 5°C/min) with stirring at lOOrpm. After cooling, calcium salicylate was added and then an additive package was added after 10 minutes stirring. Further base oil was added, followed by stirring at 150rpm. The product was mixed and de-aerated for 20 minutes, filtered, homogenised and de-aerated.

The formulations of the grease compositions are shown in Table 1 :

Example 4 is a repeat of Example 3, and Comparative Example 4 is a repeat of Comparative Example 3. The additive package is a standard package of grease

additives and was the same in all examples and

comparative examples. The base oil was a paraffinic base oil with a viscosity of 220cSt.

Roll stability tests were carried out according to ASTM D 1831. In a first series of tests, the cylinder was rotated for 18 hours at 65°C. In a second series of tests, the cylinder was rotated for 100 hours at 65°C. Scanning electron microscope pictures were taken after the Roll Stability tests. The pictures were assessed to determine whether the thickener remained intact (fibre ^¬ like structures are visible) or had been destroyed

(fibre-like structures are generally not visible) .

The results of the roll stability tests are

summarised in Table 2:

The examples of the invention (all containing calcium salicylate) exhibited significantly improved roll stability compared to the comparative examples (not containing calcium salicylate) .