Patent Application
20130267447
This application is a National Phase Entry of International Application No. PCT/IB2011/055622, filed on Dec. 12, 2011, which claims priority to French Patent Application Serial No. 1060442, filed on Dec. 13, 2010, both of which are incorporated by reference herein.
The present invention relates to grease compositions which can be used in particular in industrial applications or public works, requiring very good resistance to high loads and impacts.
There are numerous applications where liquid lubricants are unsuitable because they “drift” with respect to the lubrication point. These are in particular rolling-contact bearings and slider bearings, open gear sets, metal cables and chain drives, and more generally applications with no sealing system. For these applications, lubricating greases are used, which are solid or semi-fluid substances resulting from the dispersion of a thickener in a liquid lubricant, incorporating additives which confer particular properties upon them.
The vast majority of lubricating greases are prepared with thickeners of fatty acid metal salt types. The fatty acid is dissolved in the base oil at a relatively high temperature, and then the appropriate metal hydroxide is added. After having evaporated by boiling the water formed during the reaction, cooling is carried out for a specific period of time in order to form the soap lattice.
Lithium, sodium, calcium, barium, titanium or aluminium hydroxides, or certain aluminium trimers, are suitable for example as metal compounds for manufacturing grease. Long-chain fatty acids, of the order of C14 to C28, mainly C18, generally originate from vegetable (castor oil for example), or animal (for example tallow) oils. They can be hydrogenated. The best-known derivative is 12-hydroxystearic acid originating from ricinoleic acid. In combination with the long-chain fatty acids, it is also possible to use short-chain acids, typically comprising between 6 and 12 carbon atoms, such as for example azelaic acid, benzoic acid. So-called complex greases are then formed.
The metal soaps form a fibrous structure, except for the aluminium soaps, which exhibit a spherical gel structure. Other inorganic thickeners such as, for example, bentonite, silica gel can be used. The thickeners also include polycarbamides (polyureas). For applications where the grease is located in an unconfined enclosure (for example open gear sets in cement works etc.), greases thickened with metal soaps, and in particular with simple or complex metal soaps of aluminium are greatly superior to the other greases.
In fact, the greases thickened with polyureas do not have sufficient mechanical stability, in particular due to their thixotropic nature, which leads to their becoming destructured under mechanical stresses. The inorganic thickeners also present problems of mechanical resistance and water resistance, which has a detrimental effect in open systems.
As for the metal soaps, they exhibit very good mechanical resistance. In particular, the greases thickened with simple or complex aluminium soaps exhibit very good mechanical resistance, are highly water-resistant (in particular the complex aluminium greases), and exhibit excellent adhesion to metal surfaces.
Patent FR 1 048 670 describes for example a grease comprising an aluminium soap, allowing the lubrication of heavy parts in industry, which operate under stringent temperature and load conditions. The presence of aluminium soap prevents the separation of the base oil from the grease, and therefore the destructuring of the grease. Application FR 2 172 080 also describes particularly stable greases thickened with aluminium soaps, as does application FR 2 012 238, which also stresses their water resistance.
Application EP 0661378 discloses examples of greases thickened with complex aluminium soaps containing molybdenum dithiocarbamate (MoDTC), alone or in combination with zinc dithiophosphate (ZnDTP). These compositions contain no graphite. Their EP performances are mediocre, particularly those of the grease containing only MoDTC.
Greases thickened with complex aluminium soaps, and comprising anti-wear, extreme pressure additives, as well as a friction modifier, molybdenum bisulphide (MoS2), are marketed under the name of COPAL MS 2 for application to open gear sets in heavy industry such as the cement industry, in sugar refineries (grinding mills, kiln girth gear drives, stewing rings), or for heavy construction machinery. The presence of MoS2 guarantees good lubrication even in the event of accidental overheating and avoids any binding or jamming of the lubricated components. However, these greases have a high molybdenum (Mo) content, that it would be useful to reduce for environmental reasons, and extreme-pressure properties which could be improved, in particular in order to withstand impacts and jolts, for example in grinding mills and heavy-construction machinery. The greases can contain various solid lubricants, known for their friction properties and their anti-wear properties, such as for example molybdenum or tungsten bisulphide, graphite or polytetrafluoroethylene (PTFE).
Application FR 2 723 747 discloses high-temperature greases for constant-velocity joints comprising mineral and/or synthetic base oils, polyurea thickeners and MoS2 as solid lubricant, as well as graphite and at least one organic molybdenum compound, preferentially molybdenum dithiocarbamate. The solid lubricants graphite, MoDTC, PTFE, make it possible to reduce the content of expensive solid lubricant MoS2, but without however replacing it completely. The molybdenum content of the compositions disclosed is at least of the order of 5000 ppm.
This application does not relate to the field of greases for open systems, in industry or in public works, and does not describe greases thickened with simple or complex aluminium soaps. The problem is to produce greases for vehicle constant-velocity joints with high-temperature resistance, and having an improved lifespan, lubricity, and constant load resistance. No mention is made of the improvement in the extreme-pressure properties.
The publication “Effect of graphite on friction and wear characteristics of molybdenum dithiocarbamate”, Y. Yamamoto et al., Tribology Letters, Vol 17, No. 1, July 2004, discloses the booster effect of graphite on the anti-wear and friction performances of MoDTC, in a specific lubricating oil, squalane. This improvement does not relate to the extreme-pressure properties, and is especially appreciable in the presence of succinimide dispersant. This study is limited to a single natural lubricating oil and does not deal with the field of greases. No specific combination of base oil, graphite and MoDTC with other components necessary for the formulation of greases, in particular thickeners, is disclosed. It is therefore absolutely impossible to conclude from this publication whether the booster effect of the graphite on the friction and wear performances of the MoDTC, and even less on the extreme-pressure properties, will be reproduced in a grease comprising a particular thickener.
Surprisingly, the applicant found that greases thickened with aluminium soaps, and comprising, as friction modifier, a molybdenum dithiocarbamate combined with graphite, exhibited improved extreme-pressure properties. The compositions according to the invention can exhibit these improved properties with a reduced molybdenum content.
The present invention relates to grease compositions comprising:
Preferentially, the compositions according to the invention comprise, as base oils (a), one or more mineral base oils, alone or in a mixture, and optionally one or more synthetic base oils. According to an embodiment, the compositions according to the invention comprise at least one complex aluminium soap as thickener (b). Preferentially, in the compositions according to the invention, the simple or complex aluminium soap(s) constitute at least 80% by weight of the thickener (b) in said compositions.
According to an embodiment, the grease compositions according to the invention also comprise one or more additives chosen from the anti-wear and/or extreme-pressure additives, preferentially containing phosphorus and sulphur, preferentially the dithiophosphates. According to a preferred embodiment, the compositions according to one of claims 1 to 5 comprise moreover one or more polymers, preferentially chosen from the polyisobutenes. Preferentially, in the compositions according to the invention, the molybdenum content is comprised between 500 and 5000 ppm, preferentially between 1000 and 4800 ppm.
Preferentially, in the compositions according to the invention, the content by mass of graphite is comprised between 0.5 and 3%, preferentially between 0.7 and 2%.
The present invention also relates to the use of a grease composition as described above for the lubrication of open systems, preferentially open gear sets, metal cables, chain drives. Preferentially, this use takes place in the field of industry, preferentially the cement, sugar or steel industry, or in the field of public works. The present invention also relates to the use of an additive composition comprising molybdenum dithiocarbamate and graphite in order to increase the welding load, measured according to the standard DIN 51350/4, of a grease thickened with simple or complex aluminium soaps. Finally, the present invention also relates to an open system, preferentially open gear sets, metal cable, chain drive, to which a grease composition as described above is applied.
The or the base oil(s) used in the compositions according to the present invention can be oils of mineral or synthetic origin of Groups I to V according to the classes defined in the API (American Petroleum Institute) classification. The mineral base oils according to the invention include all types of bases obtained by atmospheric and vacuum distillation of crude oil, followed by refining operations such as extraction with a solvent, deasphalting, dewaxing with a solvent, hydrotreatment, hydrocracking and hydroisomerization, hydrofinishing. The base oils of the grease compositions according to the present invention can also be synthetic oils, such as certain esters, silicones, glycols, polybutene, polyalphaolefins (PAO), alkylbenzene, alkylnaphthalene. The base oils can also be oils of natural origin, for example alcohol and carboxylic acid esters, which can be obtained from natural resources such as sunflower, rapeseed, palm, soya oil etc.
According to an embodiment, the grease compositions according to the invention contain a mixture of mineral oils, for example paraffinic mineral oils and naphthenic mineral oils, as base oils (a). According to another embodiment, they contain a mixture of mineral oil(s), for example paraffinic mineral oils and naphthenic mineral oils, and of synthetic oils, for example polyalphaolefins. The mixture of base oils (a) is set so that its viscosity at 40° C. according to the standard ASTM D 445 is comprised between 100 and 500 cSt, preferentially between 110 and 300 cSt, preferentially between 150 and 250 cSt.
The greases according to the present invention are thickened with simple or complex aluminium soaps which exhibit superior mechanical properties and adhesion to surfaces, as well as excellent water resistance. The aluminium and fatty acid soaps can be prepared separately, or in situ during the manufacture of the grease (in the latter case, the fatty acid(s) are dissolved in the base oil, then the appropriate metal compound is added). The simple aluminium soaps are for example prepared from aluminium hydroxide Al(OH)3 and one or more long-chain fatty acids, typically comprising from 10 to 28 carbon atoms, saturated or unsaturated, for example stearic acid.
The complex aluminium soaps are for example prepared from stearic acid, benzoic acid, and aluminium trimers corresponding to formula (I):
Where R is a hydrocarbon radical, preferentially alkyl, for example isopropyl.
Compared with the simple soaps, the complex aluminium soaps have the advantage of a better high-temperature performance. The aluminium soaps are preferentially used in contents of the order of from 5 to 30% by weight, preferentially from 10 to 25% by weight, preferentially from 105 to 20% by weight, typically 12% by weight in the greases according to the invention. In the compositions according to the invention, the quantity of metal soap(s) is generally adjusted so as to obtain greases of grade 00, of grade 0, of grade 1 or of grade 2 according to the NLGI classification.
The greases according to the invention mainly contain simple or complex aluminium soaps as thickener. This means that these soaps together represent the highest percentage by weight in the greases according to the invention, compared with the percentage by weight of the other thickening materials. Preferentially, the quantity of the simple or complex aluminium soap(s), constitutes at least 50%, even more preferentially at least 80% by weight with respect to the total weight of thickening materials, in the grease compositions according to the invention.
According to an embodiment, the greases according to the invention can contain simple or complex metal soaps of fatty acids as the main thickener, and smaller quantities of other thickeners, such as other simple or complex metal soaps, polyureas, or inorganic thickeners, of the bentonite or aluminosilicate type. Preferentially, the greases according to the invention are free of polyurea-type thickeners, which are technically more complicated to manufacture, in particular because the components used in their manufacture, such as the isocyanates and amines, are very toxic and not very stable during storage. Even more preferentially, the greases according to the invention exclusively contain simple or complex metal soaps of aluminium as thickener.
The grease compositions according to the invention contain graphite and molybdenum dithiocarbamate. It was surprisingly found, even though graphite is known for its low resistance under a high load, that this combination makes it possible to obtain greases thickened with aluminium soaps having very good extreme-pressure properties, with a reduced level of molybdenum. It is not desirable to completely remove the molybdenum compounds as the greases thickened with aluminium soaps which contain none, or few, of these show no improvement in extreme-pressure properties. For example, graphite is known to present problems of resistance under a high load, and a grease containing only graphite has mediocre extreme-pressure properties.
Moreover, the greases containing only molybdenum dithiocarbamate as additives do not guarantee adequate protection of parts because MoDTC requires a certain activation temperature in order to be effective, and it is generally used in combination with other additives, in particular additives containing phosphorus and sulphur. The greases thickened with aluminium soaps (complex) containing only MoDTC as additive have mediocre extreme-pressure performances.
The compositions according to the invention contain molybdenum dithiocarbamates, friction-modifying additives well known to a person skilled in the art. These molybdenum dithiocarbamate organometallic friction modifiers can for example be molybdenum dialkyldithiocarbamates corresponding to formula (II):
Where X1, X2, X3, X4 are alkyl chains, preferentially comprising from 2 to 13 carbon atoms, preferentially from 2 to 6 carbon atoms.
Preferentially, the quantity of MoDTC in the compositions according to the invention is adjusted so that their molybdenum content is comprised between 500 and 5000, preferentially between 1000 and 4800 ppm, preferentially between 1500 and 4500 ppm, preferentially between 2000 and 4000, preferentially between 2500 and 3000 ppm. Too low an MoDTC content leads to mediocre extreme-pressure properties, too high a content is damaging to the environment. This content can be measured according to the usual techniques, plasma, atomic absorption, X-ray fluorescence.
Typically, the content by mass of dialkyldithiocarbamate of the compositions according to the invention is comprised between 0.3 and 2%, preferentially between 0.5 and 1.7%, preferentially between 0.7 and 1.5%, preferentially between 0.8 and 1.2%, typically equal to 1%. According to a preferred embodiment, in the compositions according to the invention, the Mo/[graphite] ratio between the molybdenum content, in ppm, and the percentage by mass of graphite in said compositions, is comprised between 1000 and 4000, preferentially between 1500 and 3500, preferentially between 2000 and 3000, preferentially between 2500 and 2900. This Mo/[graphite] ratio allows an optimization of the extreme-pressure properties for a given quantity of MoDTC.
The grease compositions according to the invention contain graphite, in any form that can be incorporated in the greases. For example, the graphite used in the compositions according to the invention can be a powder of micrometric size, with particle sizes comprised approximately between 1 and 15 μm, and for example a size distribution characterized by a diameter D50 comprised between 3 and 8 μm, preferentially between 5 and 7 μm. Preferentially, the content by mass of graphite in the compositions according to the invention is comprised between 0.5 and 3%, preferentially between 0.7 and 2%, preferentially between 0.8 and 1.5%, preferentially between 0.9 and 1.2%.
The greases according to the invention optionally contain anti-wear and extreme-pressure additives containing phosphorus and sulphur, commonly used in the formulation of greases and lubricants. These are for example and non-limitatively thiophosphoric acid, thiophosphorous acid, the esters of these acids, the salts thereof and the dithiophosphates, preferentially the dithiophosphates, in particular zinc dithiophosphates (ZnDTP).
The zinc dithiophosphates of formula (III) are in particular preferred:
where
R1, R2, R3, R4 are, independently of each other, linear or branched alkyl groups comprising from 1 to 24, preferentially from 3 to 14 carbon atoms or optionally substituted aryl groups comprising from 6 to 30, preferentially from 8 to 18 carbon atoms.
These different compounds can be used alone or in a mixture in the grease compositions according to the invention. Their percentage by mass is preferentially comprised between 0.5 and 5% by weight, preferentially between 0.7 and 2% by weight, or also between 0.8 and 1.5% by weight with respect to the total weight of the composition. The lubricating compositions according to the present invention can also contain phosphorus-containing anti-wear and extreme-pressure additives, such as for example the alkyl phosphates or alkyl phosphonates, phosphoric acid, phosphorous acid, the mono, di and triesters of phosphorous acid and phosphoric acid, and salts thereof. The lubricating compositions according to the present invention can also contain sulphur-containing anti-wear and extreme-pressure additives, for example the dithiocarbamates, thiadiazoles and benzothiazoles, sulphurized olefins.
The greases according to the invention can also contain any types of additives suited to their use, for example antioxidants, such as aminated or phenolic antioxidants, anti-rust additives which can be oxygenated compounds such as esters, for example sorbitan monoleate, oxidized waxes, copper passivators etc. These different compounds are generally present in contents less than 1%, or even 0.5% by mass in the greases.
The greases according to the invention can also contain polymers, for example polyolefins, polyisobutene (PIB), polyethylenes, polypropylene, heavy PAOs, olefin copolymers (OCP) for example hydrogenated diene/styrene, polymethacrylates (PMA), in contents generally comprised between 1 and 35%. These polymers are used to improve the cohesiveness of the greases, which thus withstand centrifugation better. These polymers (in particular the PIBs) also result in better adhesion of the grease to surfaces, and increase the viscosity of the base oil fraction and therefore the thickness of the film of oil between parts subject to friction. The quantity incorporated varies as a function of the molar mass, viscosity, and sought effect. For example, PIBs with a molar mass comprised between 15,000 and 25,000 daltons at contents comprised between 1 and 10% by mass, are used in order to increase the adhesion of the grease to the metal surfaces.
The greases according to the invention are preferentially manufactured by forming the metallic soap in situ. One or more fatty acids are dissolved in a fraction of the base oil or the base oil mixture at room temperature. This fraction is generally of the order of 50% of the total quantity of oil contained in the final grease. The fatty acids can be long acids, comprising from 14 to 28 carbon atoms, in order to form a simple soap, optionally combined with short fatty acids, comprising from 6 to 12 carbon atoms, in order to form complex soaps.
The compounds containing aluminium, preferentially of the aluminium trimer type as described above, are added at a temperature of approximately 60 to 80° C. The reaction of the fatty acids with the aluminium-containing compound(s) is allowed to proceed while heating to approximately 200° C. The grease is then cooled down, in particular by the remaining base oil fraction. The additives are then incorporated at approximately 80° C. followed by mixing for a sufficient time to obtain a homogeneous grease composition.
The consistency of a grease measures its hardness or its fluidity at rest. It is quantified by the penetration depth of a cone with given dimensions and mass. The grease is subjected to mixing beforehand. The conditions for measuring the consistency of a grease are defined by the standard ASTM D 217.
According to their consistency, the greases are divided into 9 NLGI (National Lubricating Grease Institute) classes or grades commonly used in the field of greases. These grades are shown in the table below. The greases according to the invention are preferentially greases of consistency comprised between and 265 and 430, preferentially between 310 and 430, preferentially between 265 and 340 tenths of a millimetre according to ASTM D217. Preferentially, they are of grade NLGI 00, 0, 1 or 2, i.e. their consistency is respectively comprised between 400 and 430, or 355 and 385, or 310 and 340, or 265 and 295 tenths of a millimetre according to ASTM D217.
The greases intended for open gear sets must adhere to the surfaces, but should not be too consistent as they are applied by bubbling of the gears in a tank or by spraying. The grades 0, 00, or 1 are therefore preferred, i.e. greases with a consistency respectively comprised between 400 and 430, or 355 and 385, or 310 and 340 tenths of a millimetre according to ASTM D217.
Grease compositions were prepared with a base grease prepared from base mineral oil, and a complex aluminium soap obtained by the reaction of benzoic acid, glycerol tristearate (stearin), and a source of aluminium (aluminium trimer corresponding to formula (I) above). The base oil represents 82.50% by weight of the base grease, and the complex aluminium soap 17.50% by weight.
These are grade 2 greases according to the NLGI classification.
This base grease is used in the composition of a commercial grease comprising 2.5% MoS2, and various additives: an anti-wear additive (ZnDTP), an extreme pressure additive as well as an anti-rust additive, which is taken as reference (reference 1). The grease compositions were prepared with the same base grease and with the same additives as the commercial reference grease, but with the MoS2 completely or partially replaced by other solid lubricants:
The extreme-pressure performances of all these greases were measured according to the standard DIN 51350/4, by measuring the welding load in the 4 ball EP test (measurement of the load that needs to be applied in order to weld together 4 balls arranged according to a specific arrangement and between which a specific quantity of grease has been introduced). Table 1 summarizes the compositions and performances of greases A to E in comparison with reference 1. The greases D and E according to the invention have significantly improved extreme-pressure properties (EP) compared with the reference, and a much lower molybdenum content. Grease A, which contains no molybdenum, shows no improvement in the extreme-pressure properties. Greases B and C show a slight improvement in the EP properties (less than that observed with greases D and E), but their molybdenum content remains very high.
Grease compositions were prepared with a base grease prepared from mineral base oil, and a simple lithium soap obtained by reaction of 12-hydroxystearic acid and lithium hydroxide (LiOH, H2O) The base oil represents 90.00% by weight of the base grease, and the simple lithium soap 10.00% by weight. These are grade 2 greases according to the NLGI classification.
This base grease is used in the composition of a grease taken as reference (reference 2) comprising moreover 1% MoS2, and various additives: an anti-wear additive (ZnDTP), an extreme pressure additive, as well as an anti-rust additive, which is taken as reference (reference 2). Grease compositions F and G were prepared with the same base grease and the same additives as the reference 2 grease, but the MoS2 was replaced by graphite and MoDTC (identical to those of example 1). The extreme pressure performances of the reference 2 grease and greases F and G were measured according to the standard DIN 51350/4, by measuring the welding load in the 4 ball EP test.
Table 1 summarizes the compositions and the performances of greases F and G in comparison with reference 2 grease. No improvement in the extreme-pressure properties was found between reference 2 grease and grease F or grease G. In this context of thickening with lithium soap, the combination of MoDTC+graphite does not act in the same way as in the context of thickening with aluminium.
Without wishing to be bound by any theory, it seems that the spherical gel structure of the aluminium soaps, which is distinguished from the fibrous structure of the other metal soaps of fatty acids (in particular lithium), used as thickeners, has an influence on the mode of action and the performances of the additives.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 60 442 | Dec 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2011/055622 | 12/12/2011 | WO | 00 | 6/11/2013 |
