Use of glycerol ester as antiwear agent

Abstract

The present invention relates to the use of at least one glycerol ester as antiwear agent in a lubricant composition comprising at least one base oil. The present invention relates to a method for reducing aluminum wear for motorcycle engine comprising the lubrication of said motorcycle engine with a lubricant composition comprising at least one base oil and also comprising at least one glycerol ester as antiwear agent.

Description

[0001] USE OF GLYCEROL ESTER AS ANTIWEAR AGENT

[0002] The present invention concerns the use of one glycerol ester as antiwear agent in a lubricant composition, in particular for the lubrication of motorcycle engines.

[0003] Lubricant compositions, also called “lubricants”, are commonly employed in engines with the main aim of reducing the forces of friction between the various moving metallic components in engines. They are in addition effective for preventing premature wear or even damage of these components, and in particular of their surface.

[0004] For this purpose, a lubricant composition is made up conventionally of a base oil, which is generally combined with several additives specifically for enhancing the lubricating performance of the base oil, but also for providing additional performance.

[0005] More particularly, additives known as “antiwear” additives are contemplated in order to reduce the wear of the components in the propulsion system, especially of the mechanical components of the motor, and thereby to prevent deterioration in the durability of the motor. There are a wide variety of antiwear additives, examples including dimercaptothiadiazoles, polysulfides, especially sulfur-containing olefins, amine phosphates, or else phosphosulfur additives, such as metal alkylthiophosphates, more particularly zinc alkylthiophosphates, and more specifically zinc dialkyldithiophosphates or ZnDTP.

[0006] The antiwear additives presently employed in lubricants for vehicle motors, such as dimercaptothiadiazoles, zinc dithiophosphate or else polysulfides, have the disadvantage, unfortunately, of being corrosive.

[0007] Unlike passenger car engine oil, motorcycle engine oil lubricates both engine and transmission part. In motorcycle clutch basket, the friction plate rubs with the clutch hub which is made of aluminum. Thus higher aluminum wear is observed in case of motorcycle engine oil.

[0008] Several products with variation of viscosity were already proposed but without any improvement of the aluminum wear.

[0009] The aim of the present invention is thus to provide an antiwear agent, especially for improving the aluminum wear. The aim of the present invention is also to provide an improved method for reducing aluminum wear in particular for motorcycle engine.

[0010] Therefore, the present invention relates to the use of at least one glycerol ester as antiwear agent in a lubricant composition comprising at least one base oil.

[0011] The antiwear agent according to the invention is advantageous in that it allows in particular to reduce the aluminum loss during the use of the engine, in particular of the motorcycle engine.

[0012] Glycerol ester

[0013] As mentioned above, the present invention is based on glycerol ester or glycerol ester(s) as antiwear agent.

[0014] According to an embodiment, the glycerol ester according to the invention is an ester of glycerol with at least one carboxylic acid comprising a hydroxyphenyl function. In other words, the glycerol ester used according to the invention is preferably an ester obtained from the reaction of glycerol with at least one carboxylic acid comprising a hydroxyphenyl function.

[0015] According to the invention, the term “carboxylic acid” refers to a RCOOH group, R being for example an alkyl group, preferably a (C8-Cso)alkyl group., said alkyl group being optionally substituted

[0016] Within the present invention, the expression "Ct-Cz" means a carbon-based chain which can have from t to z carbon atoms, for example C1-C3 means a carbonbased chain which can have from 1 to 3 carbon atoms.

[0017] Within the present invention, the term "an alkyl group" means: a linear or branched, saturated, hydrocarbon-based aliphatic group.

[0018] Preferably, the glycerol ester of the invention includes at least one hydroxyphenyl (or phenol) function. According to an embodiment, the glycerol ester includes a phenyl group carrying a hydroxyl substituent in para position, said phenyl group being optionally substituted with other groups in meta position(s).

[0019] According to a preferred embodiment, the glycerol ester used according to the invention is obtained from the reaction of glycerol with one carboxylic acid comprising a hydroxyphenyl function and optionally with at least one other carboxylic acid other than a carboxylic acid comprising a hydroxyphenyl function. According to an embodiment, the glycerol ester used according to the invention is obtained from the reaction of glycerol with one carboxylic acid comprising a hydroxyphenyl function.

[0020] According to another embodiment, the glycerol ester used according to the invention is obtained from the reaction of glycerol with one carboxylic acid comprising a hydroxyphenyl function and with at least one other carboxylic acid other than a carboxylic acid comprising a hydroxyphenyl function.

[0021] According to an embodiment, the glycerol ester is a mixed ester of glycerol with at least one fatty acid having 8 to 24 carbon atoms and at least one carboxylic acid also comprising a hydroxyphenyl function. According to this embodiment, the glycerol ester is a mixed ester obtained from the reaction of glycerol with at least one fatty acid having 8 to 24 carbon atoms and with at least one carboxylic acid also comprising a hydroxyphenyl function.

[0022] According to an embodiment, the glycerol ester used as antiwear agent in the present invention has the following general formula: wherein:

[0023] - X is H or a linear or branched, saturated or unsaturated or aromatic (C2- C4o)alkylcarbonyl group;

[0024] - R1is a linear or branched (Ci-C2o)alkylene group; and

[0025] - R2and R3, identical or different, represent a (Ci-C2o)alkyl group.

[0026] According to the invention, the term “alkylcarbonyl” means a -CO-alkyl group, the alkyl group being as defined above.

[0027] According to the invention, the term “alkylene” means a bivalent radical corresponding to an alkyl group without a terminal hydrogen atom. It can be represented by the CnH2n formula. Unless otherwise specified, this group comprises from 1 to 6 carbon atoms. Said radical may be represented by the formula (CH2)nwherein n is an integer varying from 1 to 6. The antiwear agent according to the invention may also be in the form of a mixture, and thus comprises several compounds of formula (I) as defined above.

[0028] Preferably, in formula (I) as mentioned above, R2and R3, identical or different, represent a (Ci-C )alkyl group, preferably a (Ci-C5)alkyl group.

[0029] Preferably, in formula (I) as mentioned above, R2and R3are identical and represent a methyl or t-butyl group.

[0030] According to an embodiment, the glycerol ester has the formula (I) wherein X is H or a -C(=O)-R4-CH3group, R4 being a linear or branched (Ci-Cso)alkylene group, preferably a (C5-Ci5)alkylene group, and more preferably a C10H20 group.

[0031] According to an embodiment, the glycerol ester has the formula (I) wherein R1 is a linear or branched (Ci-C )alkylene group, preferably a (C2-C4)alkylene group, and more preferably a C2H4 group.

[0032] Preferably, the glycerol ester has at least one of the following formulae:

[0033] The antiwear agent according to the invention is either a glycerol ester as defined above, used alone, or is a mixture of several different glycerol esters as defined above.

[0034] According to an embodiment, the glycerol ester is a mixture of several mono-, di- and / or triesters of glycerol.

[0035] According to an embodiment, the glycerol ester used as antiwear agent according to the invention is a mixture of the compound (1-1 ) and of the compound (I-3).

[0036] According to an embodiment, the glycerol ester used as antiwear agent according to the invention is a mixture of the compound (I-2) and of the compound (I-3).

[0037] According to an embodiment, the glycerol ester used according to the invention is a mixture of several mono- and / or diesters of glycerol, having the formula (I) as defined above.

[0038] According to an embodiment, the amount of glycerol ester is comprised from 0.01% to 3%, preferably from 0.1 % to 0.5%, and more preferably from 0.2% to 0.3%, by weight in relation to the total weight of the lubricant composition.

[0039] Base oil

[0040] As mentioned above, the lubricant composition used within the present invention also comprises a base oil.

[0041] A lubricant composition as considered according to the invention more particularly comprises one or more base oils and, optionally, other additives conventionally considered in lubricant compositions.

[0042] Conventionally, a lubricant composition comprises one or more base oils. These base oils may be selected from base oils conventionally used in the field of lubricant oils, such as mineral, synthetic or natural, animal or vegetable oils or mixtures thereof.

[0043] It may be a mixture of several base oils, for example a mixture of two, three or four base oils.

[0044] The base oils of the lubricant compositions according to the invention may in particular be oils of mineral or synthetic origin belonging to groups I to V according to the classes defined in the API classification (or their equivalents according to the ATIEL classification) and presented in Table 1 below or mixtures thereof.

[0045] Table 1

[0046] Mineral base oils include all types of base oils obtained by atmospheric and vacuum distillation of crude oil, followed by refining operations such as solvent extraction, de-alkalization, solvent dewaxing, hydrotreating, hydrocracking, hydroisomerization and hydrofining.

[0047] Synthetic base oils can be esters of carboxylic acids and alcohols, polyalphaolefins or polyalkylene glycol (PAG) obtained by polymerization or copolymerization of alkylene oxides comprising from 2 to 8 carbon atoms, in particular from 2 to 4 carbon atoms. The polyalphaolefins used as base oils are for example obtained from monomers comprising 4 to 32 carbon atoms, for example from decene, octene or dodecene, and whose viscosity at 100°C is between 1.5 and 15 mm2.s-1 according to ASTM D445. Their average molecular weight is generally between 250 and 3000 according to ASTM D5296. Mixtures of synthetic and mineral oils, which may be biobased, can also be used.

[0048] There is generally no limitation on the use of different base oils in the lubricant composition, except that they must have properties, such as viscosity, viscosity index, sulfur content, or oxidation resistance, suitable for use in engine systems, especially vehicle engines.

[0049] Preferably, the lubricant composition under consideration according to the invention comprises at least one base oil selected from group II, III and IV oils of the API classification, and mixtures thereof.

[0050] According to one embodiment, the base oil is selected from the group consisting of: base oils of Group I, base oils of Group II, base oils of Group III, and mixtures thereof, and is preferably a mixture of two or three base oils.

[0051] Preferably, the lubricant composition used within the invention comprises a mixture of two base oils, in particular of one base oil of Group I and one base oil of Group III.

[0052] According to an embodiment, the amount of base oil(s) is comprised from 70% to 90%, preferably from 75% to 85%, by weight in relation to the total weight of the lubricant composition.

[0053] According to an embodiment, the amount of base oil(s) of Group I is comprised from 5% to 30%, preferably from 10% to 20%, by weight in relation to the total weight of the lubricant composition.

[0054] According to an embodiment, the amount of base oil(s) of Group III is comprised from 40% to 80%, preferably from 50% to 70%, by weight in relation to the total weight of the lubricant composition.

[0055] Additives

[0056] A lubricant composition used according to the invention may comprise any type of additives suitable for the intended use of the lubricant, as detailed hereafter, for example for use in engine systems for light or heavy vehicles.

[0057] These additives can be introduced alone and / or in the form of a mixture, or "additive package", like those already available for sale in commercial lubricant formulations for vehicle engines, with performance levels as defined by the ACEA (“Association des Constructeurs Europeens d'Automobiles”) and / or the API (American Petroleum Institute), which are well known to the man of the trade. According to an embodiment, the lubricant composition used according to the invention further comprises at least one additive selected from the group consisting of: friction modifying additives, anti-wear additives different from the anti-wear agent according to the invention, and thus being not selected from glycerol esters, extreme pressure additives, antioxidants, viscosity index improvers, pour point depressant additives, dispersants, antifoam agents, thickeners, corrosion inhibitors, and mixtures thereof.

[0058] According to an embodiment, the lubricant composition used according to the invention further comprises at least one additive selected from the group consisting of: antioxidants, viscosity index improvers, pour point depressant additives, and mixtures thereof.

[0059] A lubricant composition used according to the invention may comprise at least one friction modifying additive.

[0060] The friction modifying additives may be selected from compounds providing metallic elements and ash-free compounds, preferably from ash-free compounds.

[0061] Among the compounds providing metallic elements, mention may be made of transition metal complexes such as Mo, Sb, Sn, Fe, Cu, Zn whose ligands may be hydrocarbon compounds comprising oxygen, nitrogen, sulfur or phosphorus atoms.

[0062] Advantageously, the friction-modifying additives are selected from ash-free compounds, generally of organic origin and which may be more particularly selected from monoesters of fatty acids and polyols, alkoxylated amines, alkoxylated fatty amines, fatty epoxides, borate fatty epoxides, fatty amines or fatty acid glycerol esters. According to the invention, the fatty compounds comprise at least one hydrocarbon group comprising from 10 to 24 carbon atoms.

[0063] According to an advantageous embodiment, a lubricant composition comprises at least one friction-modifying additive, in particular molybdenum-based.

[0064] In particular, the molybdenum-based compounds may be selected from molybdenum dithiocarbamates (Mo-DTC), molybdenum dithiophosphates (Mo-DTP), and mixtures thereof.

[0065] Advantageously, a lubricant composition considered according to the invention may comprise from 0.01 to 5% by weight, preferably from 0.01 to 5% by weight, more preferably from 0.1 to 2% by weight or even more preferably from 0.1 to 1.5% by weight, based on the total weight of the lubricant composition, of friction modifying additives. A lubricant composition according to the invention may comprise at least one anti-wear and / or extreme pressure additive.

[0066] Anti-wear additives and extreme pressure additives protect rubbing surfaces by forming a protective film adsorbed on these surfaces.

[0067] There is a wide variety of anti-wear additives. Preferably for the lubricant composition according to the invention, the anti-wear additives are selected from phospho-sulphur additives such as metal alkylthiophosphates, in particular zinc alkylthiophosphates, and more specifically zinc dialkyldithiophosphates or ZnDTP. Preferred compounds are of the formula Zn((SP(S)(OR3)(OR4))2, wherein R3and R4, which may be the same or different, independently represent an alkyl group, preferably an alkyl group having from 1 to 18 carbon atoms.

[0068] Amine phosphates are also anti-wear additives which can be employed in the lubricant composition according to the invention. However, the phosphorus provided by these additives can act as a poison to automotive catalytic systems because these additives are ash generators. These effects can be minimized by partially substituting the amine phosphates with additives that do not contribute phosphorus, such as, for example, polysulfides, in particular sulfur-containing olefins. Advantageously, the extreme-pressure and / or anti-wear additive(s) may be present in a lubricant composition according to the invention in a content ranging from 0.01% to 6% by weight, preferably from 0.05% to 4% by weight, more preferably from 0.1% to 2% by weight relative to the total mass of lubricant composition.

[0069] A lubricant composition considered according to the invention may comprise at least one antioxidant additive. The antioxidant additives are essentially dedicated to delaying the degradation of the lubricant composition in service. This degradation may result in particular in the formation of deposits, in the presence of sludge or in an increase in the viscosity of the lubricating composition. They act in particular as radical inhibitors or hydroperoxide destroyers.

[0070] Among the antioxidant additives commonly used, we can mention the phenolic type antioxidant additives, the amino type antioxidant additives, the phosphosulphur antioxidant additives. Some of these antioxidant additives, for example phosphosulfur antioxidant additives, can be ash generating. Phenolic antioxidant additives may be ashless or may be in the form of neutral or basic metal salts. In particular, the antioxidant additives may be selected from sterically hindered phenols, sterically hindered phenol esters and sterically hindered phenols comprising a thioether bridge, diphenylamines, diphenylamines substituted with at least one C1-C12 alkyl group, N,N'-dialkyl-aryl-diamines and mixtures thereof.

[0071] Preferably, the sterically hindered phenols are selected from compounds comprising a phenol group in which at least one vicinal carbon of the carbon bearing the alcohol function is substituted by at least one C1-C10 alkyl group, preferably a C1- Ce alkyl group, preferably a C4 alkyl group, preferably by the ter-butyl group.

[0072] Amino compounds are another class of antioxidant additives that can be used, optionally in combination with phenolic antioxidant additives. Examples of amine compounds are aromatic amines, for example aromatic amines of the formula NR5R6R7wherein R5represents an aliphatic group or an aromatic group, optionally substituted, R6represents an aromatic group, optionally substituted, R7is a hydrogen atom, an alkyl group, an aryl group or a group of the formula R8S(O)ZR9in which R8is an alkylene group or an alkenylene group, R9is an alkyl group, an alkenyl group or an aryl group and z is 0, 1 or 2. Sulfurized alkyl phenols or alkali and alkaline earth metal salts thereof may also be used as antioxidant additives.

[0073] A lubricant composition considered according to the invention may contain any type of antioxidant additives known to the skilled person. Advantageously, the lubricant composition comprises at least one ash-free antioxidant additive.

[0074] Also advantageously, a lubricant composition considered according to the invention may comprise from 0.1% to 2% by weight, relative to the total weight of the composition, of at least one antioxidant additive.

[0075] A lubricant composition used according to the invention may also comprise at least one viscosity index improver (VI). Viscosity index improvers (VI), in particular viscosity index improver polymers, ensure good cold-holding properties and minimum viscosity at high temperatures.

[0076] Examples of viscosity index improvers include polymeric esters, hydrogenated or non-hydrogenated homopolymers or copolymers of styrene, butadiene and isoprene, olefin homopolymers or copolymers, such as ethylene or propylene, polyacrylates and polymethacrylates (PMA).

[0077] Advantageously, a lubricant composition used according to the invention comprises at least one viscosity index improver selected from linear, grafted, comb or star, preferably star, polymethacrylates (PMA) and hydrogenated polyisoprenestyrene (PISH).

[0078] In particular, the viscosity index improving additive(s) may be present in a lubricant composition according to the invention in a content ranging from 1 to 20% by weight, in particular from 10 to 15% by weight, based on the total weight of the lubricant composition.

[0079] According to one embodiment, a lubricant composition according to the invention is free of viscosity index improving additive.

[0080] A lubricant composition used according to the invention may comprise at least one pour point depressant additive (also known as "PPD" agents). By slowing down the formation of kerosene crystals, pour point depressant additives generally improve the cold behavior of the lubricant composition. Examples of pour point depressants include alkyl polymethacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes and alkylated polystyrenes.

[0081] In particular, the pour point depressant additive(s) may be present in a lubricant composition according to the invention in a content ranging from 0.01% to 1% by weight, in particular from 0.1% to 0.5% by weight, based on the total weight of the lubricant composition.

[0082] A lubricating composition used according to the invention may also include at least one dispersing agent. The dispersing agents ensure the suspension and removal of insoluble solid contaminants consisting of oxidation by-products that are formed when the lubricant composition is in use. They may be selected from Mannich bases, succinimides and derivatives thereof.

[0083] In particular, a lubricant composition considered according to the invention may comprise from 0.2% to 10% by weight of dispersing agent(s), based on the total weight of the composition.

[0084] A lubricating composition used according to the invention may also comprise at least one antifoam additive. The antifoam additives may be selected from polar polymers such as polymethylsiloxanes or polyacrylates.

[0085] In particular, a lubricant composition considered according to the invention may comprise from 0.01 % to 3% by weight of antifoam additive(s), relative to the total weight of the lubricant composition.

[0086] As mentioned above, all of the additives detailed above can be introduced in the form of a mixture or "package" of additives.

[0087] According to this embodiment, the additive package may represent from 1% to 30% by mass with respect to the total mass of the composition, in particular from 1% to 20% by mass. The present invention also relates to the use as defined above, wherein the lubricant composition is intended for the lubrication of motorcycle engines.

[0088] The antiwear agent as defined above, in particular having the formula (I), (1-1), (I-2) or (I-3), is preferably used in lubricant compositions intended for the lubrication of motorcycle engines.

[0089] The present invention also relates to a method for reducing aluminum wear for motorcycle engine comprising the lubrication of said motorcycle engine with a lubricant composition comprising at least one base oil and also comprising at least one glycerol ester as antiwear agent, said glycerol ester being preferably as defined above.

[0090] According to an embodiment, the base oil is as defined above.

[0091] Preferably, in the method of the invention, the glycerol ester is as defined above.

[0092] The present invention also relates to a method for reducing aluminum wear for motorcycle engine, said method comprising the lubrication of said motorcycle engine with a lubricant composition comprising at least one base oil and also comprising at least one glycerol ester as antiwear agent, said glycerol ester being a compound having the formula (I), or mixtures of such compounds.

[0093] The present invention also relates to a method for reducing aluminum wear for motorcycle engine, said method comprising the lubrication of said motorcycle engine with a lubricant composition comprising at least one base oil and also comprising at least one glycerol ester as antiwear agent, said glycerol ester being a compound having the formula (1-1 ), or mixtures of such compounds.

[0094] The present invention also relates to a method for reducing aluminum wear for motorcycle engine, said method comprising the lubrication of said motorcycle engine with a lubricant composition comprising at least one base oil and also comprising at least one glycerol ester as antiwear agent, said glycerol ester being a compound having the formula (I-2), or mixtures of such compounds.

[0095] The present invention also relates to a method for reducing aluminum wear for motorcycle engine, said method comprising the lubrication of said motorcycle engine with a lubricant composition comprising at least one base oil and also comprising at least one glycerol ester as antiwear agent, said glycerol ester being a mixture of a compound having the formula (1-1 ) and a mixture of a compound having the formula (I-3).

[0096] The present invention also relates to a method for reducing aluminum wear for motorcycle engine, said method comprising the lubrication of said motorcycle engine with a lubricant composition comprising at least one base oil and also comprising at least one amine compound as antiwear agent, said glycerol ester being a mixture of a compound having the formula (1-1 ) and a mixture of a compound having the formula (I-3).

[0097] EXAMPLES

[0098] Example 1 : Preparation of the lubricant compositions

[0099] The below lubricant compositions (CL composition according to the invention and CC comparative composition) are prepared according to the usual protocols that are well-known from the skilled person.

[0100] The compositions are mentioned in table 2:

[0101] The glycerol ester of CL1 is a mixture of a compound of formula (I-2) and of a compound of formula (I-3). The CAS number of the glycerol ester is 909801-83-8.

[0102] The additive package comprises in particular a dispersant, a detergent, an antioxidant, and / or an anti-foam additive.

[0103] The physicochemical parameters of the compositions are indicated in table 3, here below: KV40 is the kinematic viscosity at 40°C of the lubricant composition, that is measured according to the ASTM D445 standard.

[0104] KV100 is the kinematic viscosity at 100°C of the lubricant composition, that is measured according to the ASTM D445 standard.

[0105] The viscosity index (VI) is calculated from the measures of the kinematic viscosity at 40°C and 100°C. These values are then compared to the results of two reference oils. The calculation is made according to the ASTM D2270 standard.

[0106] CCS at -20°C is determined according to the ASTM D5293-20 standard.

[0107] Example 2: Evaluation of aluminum wear

[0108] This evaluation is made with the LVFA test (Low Velocity Friction Apparatus).

[0109] LVFA test is used to evaluate the lubricant’s frictional characteristics for clutch applications for Motorcycle, Industrial, Off-Road and Marine Application.

[0110] It determines the friction coefficient between components in rotating-sliding contact at various speeds, loads, and temperatures. Test Fluid temperature is indirectly controlled by heat medium oil and load is applied by servo actuator with hydraulic system, rotates the friction plate by servo motor via gearbox for high rigidity driveline.

[0111] LVFA method is designed to measure frictional characteristics in a continuously changing velocity.

[0112] LVFA test was performed using the cellulose friction plate from Royal Enfield J Series engine and steel plate was replaced using aluminum plate which was prepared as per the dimension of the friction plate.

[0113] The Al plate is made of Aluminium Alloy HE30 / 6082, composition is given below:

[0114] The test conditions are as follows:

[0115] Temperature: 130°C

[0116] Surface Pressure: 0.5 Mpa

[0117] Sliding Speed: 1.0 m / s.

[0118] Weight of the aluminum plate was measured before and after the test, as mentioned in the below table: It can thus be seen that the aluminum weight loss is strongly reduced with a composition comprising the antiwear agent according to the invention, in comparison with the comparative composition (without any antiwear agent according to the invention).

Claims

CLAIMS1. The use of at least one glycerol ester as antiwear agent in a lubricant composition comprising at least one base oil.

2. The use of claim 1 , wherein the glycerol ester is an ester of glycerol with at least one carboxylic acid comprising a hydroxyphenyl function.

3. The use of claim 1 or 2, wherein the glycerol ester is obtained from the reaction of glycerol with one carboxylic acid comprising a hydroxyphenyl function and optionally with at least one other carboxylic acid other than a carboxylic acid comprising a hydroxyphenyl function.

4. The use of any one of claims 1 to 3, wherein the glycerol ester is a mixed ester of glycerol with at least one fatty acid having 8 to 24 carbon atoms and at least one carboxylic acid also comprising a hydroxyphenyl function.

5. The use of any one of claims 1 to 4, wherein the glycerol ester has the following general formula:wherein:- X is H or a linear or branched, saturated or unsaturated or aromatic (C2- C4o)alkylcarbonyl group;- R1is a linear or branched (Ci-C2o)alkylene group; and- R2and R3, identical or different, represent a (Ci-C2o)alkyl group.

6. The use of claim 5, wherein the glycerol ester has the formula (I) wherein R2and R3, identical or different, represent a (Ci-C )alkyl group, preferably a (C1- Cs)alkyl group.

7. The use of claim 5 or 6, wherein the glycerol ester has the formula (I) wherein R2and R3are identical and represent a methyl or t-butyl group.

8. The use of any one of claims 5 to 7, wherein the glycerol ester has the formula (I) wherein X is H or a -C(=O)-R4-CH3group, R4 being a linear or branched (Ci-Cso)alkylene group, preferably a (C5-Ci5)alkylene group, and more preferably a C10H20 group.

9. The use of any one of claims 5 to 8, wherein the glycerol ester has the formula (I) wherein Ri is a linear or branched (Ci-C )alkylene group, preferably a (C2- C4)alkylene group, and more preferably a C2H4 group.

10. The use of any one of claims 1 to 9, wherein the glycerol ester has at least one of the following formulae:

11. The use of any one of claims 1 to 10, wherein the glycerol ester is a mixture of several mono-, di- and / or triesters of glycerol.

12. The use of any one of claims 1 to 11 , wherein the amount of glycerol ester is comprised from 0.01% to 3%, preferably from 0.1% to 0.5%, and more preferably from 0.2% to 0.3%, by weight in relation to the total weight of the lubricant composition.

13. The use of any one of claims 1 to 12, wherein the lubricant composition further comprises at least one additive selected from the group consisting of: friction modifying additives, anti-wear additives being not selected from glycerol esters, extreme pressure additives, antioxidants, viscosity index improvers, pour point depressant additives, dispersants, antifoam agents, thickeners, corrosion inhibitors, and mixtures thereof.

14. The use of any one of claims 1 to 13, wherein the lubricant composition is intended for the lubrication of motorcycle engines.

15. A method for reducing aluminum wear for motorcycle engine comprising the lubrication of said motorcycle engine with a lubricant composition comprising at least one base oil and also comprising at least one glycerol ester as antiwear agent, said glycerol ester being preferably as defined in any one of claims 2 to 12.

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