EP4073211B1 - Lubricant composition for limiting friction - Google Patents

Description

The present application relates to novel lubricating compositions, in particular for reducing friction between mechanical parts, preferably between two parts of an engine, such as a vehicle engine. For example, the lubricating compositions according to the invention can be used to lubricate an internal combustion engine, in particular a vehicle engine, in particular a motor vehicle engine.

The purpose of lubricants is to reduce friction and wear in mechanical parts, particularly in vehicle engines, and more specifically in motor vehicles.

To reduce these friction phenomena, it is known to incorporate friction modifiers into lubricants.

Among friction modifiers, organomolybdenum compounds represent a family of compounds whose friction-reducing properties have been widely described. However, it is known to those skilled in the art that the use of organomolybdenum compounds, in particular organomolybdenum compounds comprising a dithiocarbamate group, can cause an aggravation of the wear phenomena of mechanical parts. Other solutions have then been proposed to reduce friction between two mechanical parts.

Among these other solutions, polymeric organic friction modifiers are currently sometimes used. As polymeric organic friction modifiers, mention may be made of the reaction product of an optionally functionalized polyolefin, a polyether, a polyol and a carboxylic acid terminal group. The document WO2011/107739 describes polymeric organic friction modifiers.

This type of polymeric friction modifier makes it possible to achieve coefficients of friction between mechanical parts which sometimes remain too high for the intended applications. Other types of polymeric friction modifiers incorporated in lubricating compositions are described in the documents US 2017/0275555 , FR 2992 655 And US 2019/0292483 .

There is therefore a particular interest in providing lubricating compositions to reduce friction between mechanical parts.

An objective of the present application is to provide lubricating compositions for reducing friction between mechanical parts.

Still other objectives will become apparent from reading the description of the invention which follows.

• au moins une huile de base ;
• de 0,005 à 10% en poids d'au moins un modificateur de frottement organique polymérique ; et
• de 0,005 à 10% en poids d'au moins un ester qui est un triester d'acide citrique et d'un monoalcool présentant de 1 et 10 atomes de carbone,

ledit modificateur de frottement organique polymérique étant le produit de réaction entre :

1. a) au moins une sous-unité polymère hydrophobe qui comprend un polymère hydrophobe choisi parmi les polyoléfines, les polyacryliques et les polystyrènes;
2. b) au moins une sous-unité polymère hydrophile qui comprend un polymère hydrophile choisi parmi les polyéthers, les polyesters et les polyamides ;
3. c) éventuellement au moins un fragment de squelette capable de lier ensemble des sous-unités polymères; et
4. d) éventuellement un groupe de terminaison de chaîne.

These objectives are fulfilled by the present invention which provides a lubricating composition comprising, relative to the total weight of the lubricating composition:

• at least one base oil;
• from 0.005 to 10% by weight of at least one polymeric organic friction modifier; and
• from 0.005 to 10% by weight of at least one ester which is a triester of citric acid and a monoalcohol having 1 and 10 carbon atoms,

said polymeric organic friction modifier being the reaction product between:

1. (a) at least one hydrophobic polymer subunit which comprises a hydrophobic polymer selected from polyolefins, polyacrylics and polystyrenes;
2. b) at least one hydrophilic polymer subunit which comprises a hydrophilic polymer selected from polyethers, polyesters and polyamides;
3. (c) optionally at least one backbone fragment capable of linking together polymer subunits; and
4. d) optionally a chain terminating group.

More particularly, the inventors have surprisingly discovered that the combination of a polymeric organic friction modifier of the aforementioned type and an ester of the aforementioned type makes it possible to significantly improve the coefficient of friction between mechanical parts.

Indeed, it was discovered by the inventors that the ester chosen from the triesters of citric acid and monoalcohols having 1 to 10 carbon atoms makes it possible to surprisingly boost the effect of the polymeric organic friction modifier.

Preferably, the polymeric organic friction modifier has a weight average molecular weight ranging from 1000 to 30000 Daltons, preferably ranging from 1500 to 25000 Daltons, advantageously ranging from 2000 to 20000 Daltons. Weight average molecular mass can be measured by size exclusion chromatography.

According to one embodiment of the invention, the polymeric organic friction modifier is as defined in the application WO2011/107739 .

Preferably, the hydrophobic polymer subunit of the polymeric organic friction modifier comprises a hydrophobic polymer derived from a polymer of a mono-olefin comprising 2 to 6 carbon atoms, said mono-olefin preferably being selected from ethylene, propylene, butene and isobutene, more preferably being isobutene, and said polymer of a mono-olefin containing a chain of 15 to 500, preferably 50 to 200 carbon atoms.

The hydrophilic polymer subunit of the polymeric organic friction modifier comprises a hydrophilic polymer selected from polyethers, polyamides and polyesters, preferably selected from polyethers.

The hydrophilic polymer subunit is preferably linear or branched.

The polyester is for example chosen from polyethylene terephthalate, polylactide and polycaprolactone.

The polyether is for example chosen from polyglycerols and polyalkylene glycols.

According to a particularly preferred embodiment, the hydrophilic polymer subunit comprises a hydrophilic polymer which is a polymer of a water-soluble alkylene glycol. Preferably, the hydrophilic polymer subunit comprises a hydrophilic polymer which is a polyethylene glycol (PEG), preferably a PEG having a molecular weight ranging from 300 to 5,000 Daltons, advantageously ranging from 400 to 1,000 Daltons, in particular ranging from 400 to 800 Daltons.

For example, the hydrophilic polymer subunit comprises a hydrophilic polymer selected from PEG ₄₀₀ , PEG ₆₀₀ and PEG ₁₀₀₀ .

According to another embodiment, the hydrophilic polymer subunit comprises a poly(ethylene-propylene glycol) or poly(ethylene-butylene glycol) copolymer.

Alternatively, the hydrophilic polymer subunit comprises a hydrophilic polymer selected from polyethers and polyamides derived from diols and diamines, respectively, said diols and diamines containing at least one functional group selected from acid groups, for example a carboxylic acid group, sulfonyl groups, for example a sulfonylstyrene group, amines, for example tetraethylenepentamine (TEPA) or polyethyleneimine (PEI), and hydroxyl groups, for example, mono- or copolymers based on sugars.

In one embodiment, some of the hydrophobic polymer subunits and the hydrophilic polymer subunits may be linked to form block copolymer units. The hydrophobic polymer subunits and the hydrophilic polymer subunits capable of forming block copolymer units include functional groups that enable them to link to each other.

Preferably, the hydrophobic polymer subunit comprises at least one diacid or anhydride group, said diacid or anhydride group being introduced into the hydrophobic polymer subunit by reaction with an unsaturated diacid or anhydride, for example maleic anhydride. The hydrophobic polymer subunit comprising at least one diacid or anhydride group is capable of reacting by esterification with a hydrophilic polymer subunit with a hydroxyl end, for example a polyalkylene glycol.

Alternatively, the hydrophobic polymer subunit comprises at least one epoxide group, said epoxide group being introduced into the hydrophobic polymer subunit by an epoxidation reaction with a peracid, for example perbenzoic or peracetic acid. The hydrophobic polymer subunit comprising at least one epoxide group is capable of reacting with a hydrophilic polymer subunit with a hydroxyl and/or acid termination.

In yet another variant, the hydrophobic polymer subunit comprises at least one unsaturated end, said unsaturated end being by esterification reaction between at least one hydroxyl group of the hydrophilic polymer subunit and an unsaturated monocarboxylic acid, for example vinyl acids, in particular acrylic or methacrylic acid. The hydrophobic polymer subunit comprises at least one unsaturated end and is capable of reacting by copolymerization of free radicals with a hydrophobic polymer subunit of polyolefin type.

Preferably, the hydrophobic polymer subunit comprises a polyisobutylene succinic anhydride (PIBSA), having a molecular weight ranging from 300 to 5000 Daltons, preferably ranging from 500 to 1500 Daltons, in particular ranging from 800 to 1200 Daltons. The polyisobutylene succinic anhydride is derived from the maleinization of a polyisobutylene, in particular derived from the reaction between a polyisobutylene having an unsaturated terminal group and maleic anhydride.

The block copolymer units as described above are directly linked to each other.

Alternatively, the block copolymer units, units as described above are linked by said at least one backbone fragment.

The backbone fragment is selected from polyols and polycarboxylic acids, preferably from polyols.

The polyol is selected from diols, triols, tetraols, dimers or trimers of diol, triol or tetraol and chain-extended polymers of diol, triol or tetraol.

Preferably, the polyol is selected from glycerol, neopentyl glycol, trimethylolethane, trimethylolpropane, trimethylolbutane, pentaerthyritol, dipentaerthyritol, tripentaerthyritol and sorbitol.

Advantageously, the polyol is glycerol.

The polycarboxylic acid is selected from dicarboxylic acids and tricarboxylic acids. Preferably, the polycarboxylic acid is a dicarboxylic acid, preferably a straight-chain dicarboxylic acid, more preferably a dicarboxylic acid having a chain length of between 2 and 10 carbon atoms.

Advantageously, the polycarboxylic acid is chosen from oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid and maleic acid.

Advantageously, the polycarboxylic acid is adipic acid.

Alternatively, the backbone fragment is selected from low molecular weight alkenyl succinic anhydrides (ASA), preferably from C18 alkenyl succinic anhydrides.

The choice of the backbone fragment capable of linking together the block copolymer units is adapted according to the case where the linkage of the units is between two hydrophobic polymer subunits, between two hydrophilic polymer subunits or between a hydrophobic polymer subunit and a hydrophobic polymer subunit.

The skeletal fragments included in the polymeric organic friction modifier are of the same or different nature.

The number of block copolymer units included in the polymeric organic friction modifier varies from 1 to 20, preferably from 1 to 15, preferentially from 1 to 10, advantageously from 1 to 7.

When the product of the reaction of producing the polymeric organic friction modifier terminates with a reactive group (e.g. the groups hydroxyls of PEG), it may be desirable or useful in certain circumstances to introduce a chain terminating group at the end of the reaction product.

Preferably, the chain terminating group is selected from fatty carboxylic acids, preferably from C12 to C22 acids, linear or branched, saturated or unsaturated, more preferably from lauric acid, erucic acid, isostearic acid, palmitic acid, oleic acid and linoleic acid, advantageously from palmitic acid, oleic acid and linoleic acid.

Preferably, the polymeric organic friction modifier used in the present invention has an acid number of less than 20, preferably less than 15.

The reaction by which the polymeric organic friction modifier used in the present invention is produced is a one-step or multi-step process.

According to a specific embodiment, the polymeric organic friction modifier used in the present invention is the reaction product of a functionalized polyolefin, a polyether, a polyol and a carboxylic acid terminal group.

The lubricating composition according to the invention comprises from 0.005 to 10% by weight, preferably from 0.05 to 5% by weight, preferably from 0.1 to 3% by weight, more preferably from 0.2 to 2% by weight, of polymeric organic friction modifier(s) as defined above, relative to the total weight of the lubricating composition.

The lubricating composition according to the invention comprises from 0.005 to 10% by weight, preferably from 0.05 to 5% by weight, preferably from 0.1 to 3% by weight, more preferably from 0.2 to 2% by weight, of at least one ester of the aforementioned type, relative to the total weight of the lubricating composition.

According to a preferred embodiment, the citric acid ester is a triester of citric acid and a monoalcohol having from 2 to 8 carbon atoms.

The alcohols used to prepare citric acid ester or tartaric acid ester are saturated or unsaturated, linear, cyclic or branched alcohols, optionally substituted by acid and/or epoxide groups.

Preferably, the alcohol used to prepare the citric acid ester is linear and saturated and has a hydrocarbon chain consisting of carbon and hydrogen atoms. In other words, according to a particular embodiment, the alcohol used to prepare the citric acid ester or the tartaric acid ester does not comprise heteroatoms other than those of the hydroxyl function.

The citric acid esters used in the invention can be obtained by methods well known to those skilled in the art, for example by reacting citric acid with one or more alcohols. These chemical reactions, well known to those skilled in the art, can take place with or without a catalyst, with or without a solvent.

According to one embodiment, the citric acid ester is chosen from triethylcitrate, tributylcitrate and their mixture.

• leur mélange.

According to a preferred embodiment of the invention, the ester of the lubricating composition is a triester of citric acid and a monoalcohol having from 2 to 8 carbon atoms; and

• their mixture.

According to a preferred embodiment of the invention, the ester of the lubricating composition is chosen from triethylcitrate, tributylcitrate and their mixture.

The lubricating composition according to the invention comprises one or more base oils, preferably in an amount of at least 50% by weight, more preferably at least 60% by weight, or even at least 70% by weight, relative to the total weight of the lubricating composition.

The base oil(s) may be chosen from mineral, synthetic or natural, animal or vegetable lubricating base oils known to those skilled in the art.

The base oils used in the lubricating compositions according to the invention may 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) (table 1) or their mixtures.

[Table 1]

Table 1 Saturated content Sulfur content Viscosity index (VI) Group I < 90% > 0.03% 80 ≤ VI < 120 Mineral oils Group II ≥ 90% ≤ 0.03% 80 ≤ VI < 120 Hydrocracked oils Group III ≥ 90% ≤ 0.03% ≥ 120 Hydrocracked or hydroisomerized oils Group IV Polyalphaolefins (PAO) Group V Esters, PAG and other bases not included in groups I to IV

The mineral base oils according to the invention include all types of base oils obtained by atmospheric and vacuum distillation of crude oil, followed by refining operations such as solvent extraction, de-alphatation, solvent dewaxing, hydrotreating, hydrocracking, hydroisomerization and hydrofinishing.

Mixtures of synthetic and mineral oils can also be used.

There is generally no limitation on the use of different lubricating bases to produce the lubricating compositions according to the invention, except that they must have properties, in particular viscosity, viscosity index, sulfur content, resistance to oxidation, adapted to their use.

The base oils of the lubricating compositions according to the invention may also be chosen from synthetic oils, such as certain esters of carboxylic acids and alcohols, and from polyalphaolefins. The polyalphaolefins used as base oils are for example obtained from monomers comprising from 4 to 32 carbon atoms, for example from octene or decene, and whose viscosity at 100 °C is between 1.5 and 15 mm ² .s ^-1 according to the ASTM D445 standard. Their average molecular mass is generally between 250 and 3,000 according to the ASTM D5296 standard.

According to a particular embodiment, the lubricating composition according to the invention comprises from 60 to 99.5% by weight of base oils, preferably from 70 to 99.5% by weight of base oils, relative to the total weight of the composition.

Many additional additives can be used for this lubricating composition according to the invention.

The preferred additional additives for the lubricating composition according to the invention are chosen from detergent additives, anti-wear additives other than phospho-sulfur additives, friction modifying additives other than the polymeric organic friction modifiers defined above, extreme pressure additives, dispersants, pour point improvers, anti-foaming agents, thickeners and mixtures thereof.

Amine phosphates are anti-wear additives that can be used in the lubricating composition according to the invention. However, the phosphorus provided by these additives can act as a poison for automobile catalytic systems because these additives generate ash. These effects can be minimized by partially substituting the amine phosphates with additives that do not provide phosphorus, such as, for example, polysulfides, in particular sulfur-containing olefins.

Advantageously, the lubricating composition according to the invention may comprise from 0.01 to 6% by mass, preferably from 0.05 to 4% by mass, more preferably from 0.1 to 2% by mass relative to the total mass of lubricating composition, of anti-wear additives and extreme pressure additives.

• au moins 50% en poids, de préférence au moins 60% en poids, de préférence encore au moins 70% en poids, d'une ou plusieurs huiles de base ;
• de 0,005 à 10% en poids, de préférence de 0,05 à 5% en poids, de préférence de 0,1 à 3% en poids, de préférence encore de 0,2 à 2% en poids, d'un ou plusieurs modificateurs de frottement organiques polymériques ;
• de 0,005 à 10% en poids, de préférence de 0,05 à 5% en poids, de préférence de 0,1 à 3% en poids, de préférence encore de 0,2 à 2% en poids, d'un ou plusieurs esters choisis parmi les esters de glycérol, les ester d'acide citrique, les esters d'acide tartrique et leur mélange ;
• éventuellement de 0,005 à 30% en poids, de préférence de 0,1 à 25% en poids, de préférence encore de 1 à 20% en poids, d'un ou plusieurs additifs fonctionnels différents des modificateurs de frottement organiques polymériques et des esters de glycérol des esters d'acide citrique et des esters d'acide tartrique, de préférence choisis parmi les additifs détergents, les additifs anti-usure différents des additifs phospho-soufrés, les additifs modificateurs de frottement, les additifs extrême pression, les dispersants, les améliorants du point d'écoulement, les agents anti-mousse, les épaississants et leurs mélanges.

Preferably, the lubricating composition according to the invention comprises, relative to the total weight of lubricating composition:

• at least 50% by weight, preferably at least 60% by weight, more preferably at least 70% by weight, of one or more base oils;
• from 0.005 to 10% by weight, preferably from 0.05 to 5% by weight, preferably from 0.1 to 3% by weight, more preferably from 0.2 to 2% by weight, of one or more polymeric organic friction modifiers;
• from 0.005 to 10% by weight, preferably from 0.05 to 5% by weight, preferably from 0.1 to 3% by weight, more preferably from 0.2 to 2% by weight, of one or more esters chosen from glycerol esters, citric acid esters, tartaric acid esters and their mixture;
• optionally from 0.005 to 30% by weight, preferably from 0.1 to 25% by weight, more preferably from 1 to 20% by weight, of one or more functional additives other than polymeric organic friction modifiers and glycerol esters, citric acid esters and tartaric acid esters, preferably chosen from detergent additives, anti-wear additives other than phospho-sulfur additives, friction modifier additives, extreme pressure additives, dispersants, pour point improvers, anti-foaming agents, thickeners and mixtures thereof.

Advantageously, the lubricating composition according to the invention may comprise at least one additional friction modifier additive different from the polymeric organic friction modifiers defined above. The additional friction modifier additive may be chosen from a compound providing metallic elements and an ash-free compound. Among the compounds providing metallic elements, mention may be made of transition metal complexes such as Sb, Sn, Fe, Cu, Zn, Mo, the ligands of which may be hydrocarbon compounds comprising oxygen, nitrogen, sulfur or phosphorus atoms.

Advantageously, the lubricating composition according to the invention can comprise at least one antioxidant additive.

The antioxidant additive generally makes it possible to delay the degradation of the lubricating composition in service. This degradation can notably result in the formation of deposits, the presence of sludge or an increase in the viscosity of the lubricating composition.

Antioxidant additives act in particular as radical inhibitors or hydroperoxide destroyers. Among the commonly used antioxidant additives, mention may be made of phenolic antioxidant additives, amine antioxidant additives, and phosphosulfur antioxidant additives. Some of these antioxidant additives, for example phosphosulfur antioxidant additives, may be ash-generating. Phenolic antioxidant additives may be ash-free or in the form of neutral or basic metal salts. Antioxidant additives may in particular be chosen from sterically hindered phenols, sterically hindered phenol esters and sterically hindered phenols comprising a thioether bridge, diphenylamines, diphenylamines substituted by at least one C ₁ -C ₁₂ alkyl group, N,N'-dialkyl-aryl-diamines and mixtures thereof.

Preferably according to the invention, the sterically hindered phenols are chosen from compounds comprising a phenol group of which at least one vicinal carbon of the carbon carrying the alcohol function is substituted by at least one C ₁ - C ₁₀ alkyl group, preferably a C ₁ - C ₆ alkyl group, preferably a C ₄ alkyl group, preferably by the tert-butyl group.

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 NR ⁷ R ⁸ R ⁹ in which R ⁷ represents an aliphatic group or an aromatic group, optionally substituted, R ⁸ represents an aromatic group, optionally substituted, R ⁹ represents a hydrogen atom, an alkyl group, an aryl group or a group of the formula R ¹⁰ S(O) _z R ¹¹ in which R ¹⁰ represents an alkylene group or an alkenylene group, R ¹¹ represents an alkyl group, an alkenyl group or an aryl group and z represents 0, 1 or 2.

Sulfurized alkyl phenols or their alkali and alkaline earth metal salts can also be used as antioxidant additives.

Another class of antioxidant additives is that of copper compounds, for example copper thio- or dithio-phosphates, copper salts of carboxylic acids, dithiocarbamates, sulphonates, phenates, acetylacetonates copper. Copper I and II salts, succinic acid or anhydride salts can also be used.

The lubricating composition according to the invention may contain all types of antioxidant additives known to those skilled in the art.

Advantageously, the lubricating composition comprises at least one ash-free antioxidant additive.

Also advantageously, the lubricating composition according to the invention comprises from 0.5 to 2% by weight relative to the total mass of the composition, of at least one antioxidant additive.

The lubricating composition according to the invention may also comprise at least one detergent additive.

Detergent additives generally reduce the formation of deposits on the surface of metal parts by dissolving secondary oxidation and combustion products.

The detergent additives that can be used in the lubricating composition according to the invention are generally known to those skilled in the art. The detergent additives may be anionic compounds comprising a long lipophilic hydrocarbon chain and a hydrophilic head. The associated cation may be a metal cation of an alkali or alkaline-earth metal.

The detergent additives are preferably chosen from alkali metal or alkaline earth metal salts of carboxylic acids, sulfonates, salicylates, naphthenates, as well as phenate salts. The alkali and alkaline earth metals are preferably calcium, magnesium, sodium or barium.

These metal salts generally comprise the metal in stoichiometric quantity or in excess, therefore in a quantity greater than the stoichiometric quantity. These are then overbased detergent additives; the excess metal providing the overbased character to the detergent additive is then generally in the form of a metal salt insoluble in oil, for example a carbonate, a hydroxide, an oxalate, an acetate, a glutamate, preferably a carbonate.

Advantageously, the lubricating composition according to the invention can comprise from 2 to 4% by weight of detergent additive relative to the total mass of the lubricating composition.

Also advantageously, the lubricating composition according to the invention may also comprise at least one pour point lowering additive.

By slowing down the formation of paraffin crystals, pour point depressant additives generally improve the cold behavior of the lubricating composition according to the invention.

Examples of pour point depressant additives include polyalkyl methacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes, alkylated polystyrenes.

Advantageously, the lubricating composition according to the invention can also comprise at least one dispersing agent.

The dispersing agent may be chosen from Mannich bases, succinimides and their derivatives.

Also advantageously, the lubricating composition according to the invention may comprise from 0.2 to 10% by mass of dispersing agent relative to the total mass of the lubricating composition.

The lubricating composition of the present invention may also comprise at least one additional polymer capable of improving the viscosity index. Examples of additional polymers improving the viscosity index include polymeric esters, homopolymers or copolymers, hydrogenated or non-hydrogenated, of styrene, butadiene and isoprene, polymethacrylates (PMA).

The present invention also relates to the use of the lubricating composition as defined above for the lubrication of metal parts, in particular for the lubrication of engines, in particular internal combustion engines, for example vehicle engines.

Advantageously, the lubricating composition according to the invention makes it possible to reduce friction, in particular between two mechanical parts, for example two parts of an engine, in particular an internal combustion engine, for example a vehicle engine.

Thus, the invention relates to the use of the lubricating composition according to the invention for reducing the wear of mechanical parts, for example parts of an engine, in particular of a vehicle engine.

The present application also relates to a method for lubricating mechanical parts, in particular in an engine, such as an internal combustion engine, comprising at least one step of bringing at least one part into contact with a lubricating composition according to the invention.

The present invention will now be described with the aid of non-limiting examples.

Example 1 : Lubricating compositions

The compositions of Table 2 (CL: lubricating composition according to the invention; CC: comparative composition; apart from composition CL4 which is a comparative composition) were prepared by mixing at 60°C the ester and/or the polymeric friction modifier in a composition comprising the base oil, the viscosity index improver and the additive package, to obtain the proportions indicated in Table 2. The percentages indicated are related to 100% by weight of lubricating composition including the ester and/or the polymeric friction modifier.

[Table 2]

Table 2. Lubricating compositions according to the invention and comparative compositions Lubricating composition CL1 (% by weight) CL2 (% by weight) CL3 (% by weight) CL4 (% by weight) CL5 (% by weight) CC1 (% by weight) CC2 (% by weight) triethylcitrate 1 2 0.5 - - - 1 tributylcitrate - - - 1 - - Glyceryl triheptanoate - - - 1 - - - Organic friction modifier according to the invention* 0.5 0.5 0.25 0.5 0.5 0.5 - Additive Pack 1** 13.3 12.3 14.05 13.3 13.3 14.3 13.8 Viscosity index improver (olefin copolymer) 6.1 6.1 6.1 6.1 6.1 6.1 6.1 Group III base oil 79.1 79.1 79.1 79.1 79.1 79.1 79.1 *Organic friction modifier according to the invention which is the reaction product between:
(a) a hydrophobic polymer subunit which comprises a hydrophobic polymer selected from polyolefins, polyacrylics and polystyrenes;
(b) a hydrophilic polymer subunit which comprises a hydrophilic polymer selected from polyethers, polyesters and polyamides;
(c) optionally at least one backbone fragment capable of linking together polymer subunits; and
d) optionally a chain terminating group.
** including detergents, dispersants, antioxidants and anti-wear agents

The compositions of Table 3 (CL: lubricating composition according to the invention; CC: comparative composition) were prepared by mixing at 60°C the ester and/or the polymeric friction modifier in a composition comprising the base oil, the viscosity index improver and the additive package, to obtain the proportions indicated in Table 3. The percentages indicated are related to 100% by weight of lubricating composition including the ester and/or the polymeric friction modifier.

[Table 3]

Table 3. Lubricating compositions according to the invention and comparative compositions Lubricating composition CL7 (% by weight) CC3 (% by weight) CC4 (% by weight) triethylcitrate 1 - - Organic friction modifier according to the invention 0.5 - 0.5 Additive Pack 2* 10.7 10.9 10.8 Viscosity index improver (acrylic polymers) 5.8 5.9 5.9 Group III and IV base oils 82 83.2 82.8 * including detergents, dispersants, antioxidants and anti-wear agents.

Example 2 : Tribological test results

The tribological tests were carried out under the following conditions:

[Table 4]

Table 4. Tribological test conditions Continuous sliding Charge 30N (0.96 Gpa) Temperature 100 °C SRR 50% Speed training 0.1 m/s Time (min) 5 15 30 60 120 Stribeck Charge 30N (0.96 Gpa) Temperature 100 °C SRR 50% Speed training 3 to 0.007 m/s

The coefficient of friction of the lubricating compositions tested is determined at 100°C using an MTM device (Traction Machine or Mini Traction Machine) using a 2 cm diameter hardened steel ball on a hardened steel plane.

The MTM device may be a device from PCS Instruments. This device allows a steel ball and a steel plane to be set in relative motion in order to determine the coefficients of friction for a given lubricant composition while varying various properties such as speed, load, and temperature.

The hardened steel plane is of reference AISI 52100 with a mirror finish (Ra less than 0.01 µm) and the ball is also of reference AISI 52100 made of hardened steel.

The applied load is 30 N (0.96 Gpa) and the rotation speed varies from 0.007 m/s to 3 m/s.

Approximately 50 ml of the tested lubricating composition was introduced into the device. The ball is engaged face against plane, said ball and said plane being actuated independently so as to create a mixed rolling/sliding contact.

The coefficient of friction is measured and recorded using a force sensor.

The test is carried out for a duration of 121 minutes (alternating between periods known as continuous sliding and "stribeck"). The speed is initially kept constant at 0.1 m/s and at each interval defined in the table, the speed changes from 3 to 0.007 m/s for one minute before returning to a speed of 0.1 m/s at the end of said defined period.

The coefficient of friction is thus measured as a function of the defined speed.

Table 5 gives the results for the compositions of Table 2, expressed in terms of friction coefficient as a function of sliding speed. [Table 5] Speed of 0.01 m/s Speed of 0.1 m/s Friction coefficient CC1 0.068 0.070 Friction coefficient CC2 0.133 0.112 Friction coefficient CL1 0.040 0.040 Friction coefficient CL2 0.022 0.021 Friction coefficient CL3 0.050 0.061 Friction coefficient CL4 0.048 0.047 Friction coefficient CL5 0.040 0.041

Table 6 gives the results for the compositions in Table 3, expressed in terms of friction coefficient as a function of sliding speed. [Table 6] Speed of 0.01 mm/s Speed of 0.1 mm/s Friction coefficient CL7 0.073 0.065 Friction coefficient CC3 0.131 0.115 Friction coefficient CC4 0.142 0.122

• la valeur du coefficient de friction ne dépend pas de manière significative de la teneur en modificateur de frottement polymérique et/ou en ester.
• l'ester n'a pas d'effet notable sur le coefficient de friction lorsqu'il est utilisé seul, sans modificateur de frottement polymérique.
• existe une synergie entre l'ester défini dans la présente invention et les modificateurs de frottement polymérique au sein de la composition lubrifiante pour diminuer significativement le coefficient de friction et par conséquent pour limiter les frottements entre les pièces mécaniques.

The results thus demonstrate that:

• The value of the friction coefficient does not depend significantly on the content of polymeric friction modifier and/or ester.
• The ester has no noticeable effect on the coefficient of friction when used alone, without a polymeric friction modifier.
• There is a synergy between the ester defined in the present invention and the polymeric friction modifiers within the lubricating composition to reduce significantly the coefficient of friction and therefore to limit friction between mechanical parts.

Claims (9)

1. A lubricant composition comprising, based on the total weight of lubricant composition:

   - at least one base oil;

   - 0.005 to 10% by weight of at least one polymeric organic friction modifier; and

   - 0.005 to 10% by weight of at least one ester which is a triester of citric acid and of a monohydric alcohol having from 1 to 10 carbon atoms,

   said polymeric organic friction modifier being the reaction product of:

   a) at least one hydrophobic polymer subunit which comprises a hydrophobic polymer selected from polyolefins, polyacrylics and polystyrenes;

   b) at least one hydrophilic polymer subunit which comprises a hydrophilic polymer selected from polyethers, polyesters and polyamides;

   c) optionally at least one backbone moiety capable of linking polymeric subunits together; and

   d) optionally a chain termination group.
2. The composition according to claim 1, wherein the polymeric organic friction modifier has a weight average molecular weight of from 1000 to 30,000 Daltons.
3. The composition according to one of claims 1 or 2, wherein the organic friction modifier is the reaction product of a functionalised polyolefin, a polyether, a polyol and a carboxylic acid end group.
4. The composition according to any one of claims 1 to 3, wherein the ester is a triester of citric acid and of a monohydric alcohol having from 2 to 8 carbon atoms.
5. The composition according to any one of claims 1 to 4, wherein the ester is selected from triethylcitrate, tributylcitrate and mixtures thereof.
6. The composition according to any one of claims 1 to 3, comprising, based on the total weight of lubricant composition:

   - at least 50% by weight, preferably at least 60% by weight, more preferably 70% by weight, of one or more base oils;

   - from 0.005 à 10% by weight, preferably 0.05 to 5% by weight, preferably from 0.1 to 3% by weight, more preferably from 0.2 to 2% by weight, of one or more polymeric organic friction modifiers;

   - from 0.005 to 10% by weight, preferably from 0.05 to 5% by weight, preferably from 0.1 to 3% by weight, more preferably from 0.2 to 2% by weight, of one or more esters selected from triesters of citric acid and of a monohydric alcohol having from 1 to 10 carbon atoms;

   - optionally from 0.005 to 30% by weight, preferably from 0.1 to 25% by weight, more preferably from 1 to 20% by weight, of one or more functional additives other than polymeric organic friction modifiers and glycerol esters, citric acid esters and tartaric acid esters preferably selected from detergent additives, anti-wear additives other than phospho-sulphur additives, friction modifying additives, extreme pressure additives, dispersants, pour point depressants, anti-foaming agents, thickeners and mixtures thereof.
7. Use of the composition according to any one of claims 1 to 6 for reducing friction between two mechanical parts, preferably between two parts of an engine, such as a vehicle engine.
8. The use according to claim 7 for reducing the wear of parts, in particular engine parts.
9. A method for lubricating mechanical parts, particularly in an engine, notably an internal combustion engine, comprising at least one step of bringing a part into contact with the lubricating composition according to any one of claims 1 to 6.