LUBRICANT COMPOSITION WITH IMPROVED OXIDATION RESISTANCE PROPERTIES

Abstract

LUBRICANT COMPOSITION WITH IMPROVED OXIDATION RESISTANCE PROPERTIES The present invention relates to the use, in a lubricating composition intended for a gas engine, of at least one phosphate ester type compound, for improving the oxidation resistance of said lubricating composition, in which said phosphate ester type compound corresponds to the following formula (I): (I) in which at least one of R1, R2 and R3 represents a saturated or unsaturated, linear or branched aliphatic chain, comprising from 2 to 50 carbon atoms, and optionally containing one or more heteroatoms, in particular one or more oxygen atoms; the others of R1, R2 and R3, identical or different, being chosen from a hydrogen atom and an aliphatic chain, saturated or unsaturated, linear or branched, comprising from 2 to 50 carbon atoms, and optionally containing one or more heteroatoms, in particular one or more oxygen atoms.Figure for the abstract: none.

Description

Title of the invention: LUBRICATING COMPOSITION WITH IMPROVED OXIDATION RESISTANCE PROPERTIES

[0001] The present invention relates to the field of gas engines. Its subject more particularly is a lubricating composition, in particular for gas engines, having improved oxidation resistance properties.

[0002] Lubricants developed specifically for the lubrication of gas engines have been known for several years.

[0003] The major characteristic of gas applications is the higher combustion temperatures than in Diesel applications. Oxidation is a natural chemical process during which the lubricant is altered and oxidation can be promoted by the combined effect of contact with oxygen and high temperature conditions. During the oxidation process, the molecules of hydrocarbons and synthetic fluids oxidize, transforming into a series of more or less polymerized oxygenated products, leading to more or less acidic products.

[0004] Following a reaction of the oil with the oxidized products, at the high temperature conditions and the pressure to which the oil is subjected during its use, it begins to saturate with the presence of both soluble and insoluble oxidized compounds.

[0005] Oxidation of the lubricating composition leads to degradation of the 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 composition.

[0006] The first lubricants dedicated to gas engines contained group I base oils according to the API classification. These lubricants exhibited good behavior at high temperatures, in particular they formed little or no deposits but had limited resistance to oxidation. However, as indicated above, these phenomena have a significant negative impact on the life of the lubricant and also on the oil change interval.

[0007] There is currently a need to provide lubricating compositions for gas engines having improved oxidation resistance properties.

[0008] The present invention therefore aims to provide lubricating compositions for gas engines having improved oxidation resistance properties.

[0009] Another object of the invention is to provide lubricating compositions for gas engines having improved oxidation resistance properties, allowing to increase the life of the lubricant but also to increase the oil change intervals.

[0010] Thus, the present invention relates to the use, in a lubricating composition intended for a gas engine, of at least one compound of phosphate ester type, to improve the resistance to oxidation of said lubricating composition.

[0011] As indicated above, by "oxidation" is meant here the contact of the lubricating composition with oxygen (02).

[0012] According to the invention, the phosphate ester type compound corresponds to the following formula (I): O (I) "'OR I . -V OR

[0013] in which at least one of R1, R2 and R3 represents an aliphatic chain, saturated or unsaturated, linear or branched, comprising from 2 to 50 carbon atoms, in particular from 10 to 40 carbon atoms and more particularly from 15 to 30 carbon atoms, and optionally containing one or more heteroatoms, in particular one or more oxygen atoms;

[0014] the others of R1, R2 and R3, identical or different, being chosen from a hydrogen atom and an aliphatic chain, saturated or unsaturated, linear or branched, comprising from 2 to 50 carbon atoms, in particular from 10 to 40 carbon atoms and more particularly from 15 to 30 carbon atoms, and optionally containing one or more heteroatoms, in particular one or more oxygen atoms.

[0015] Phosphate ester type compounds are described in more detail in the rest of the text.

[0016] Preferably, a phosphate ester used according to the invention comprises at least one phosphate monoester.

[0017] As mentioned above, the oil in a gas engine must have a high resistance to degradation. This is mainly due to the high temperatures and corrosiveness of the gases produced during the combustion process and the contact between oxygen and the lubricant. This oxidation process must therefore be monitored regularly.

[0018] As is apparent from the examples which follow, the inventors have found that the use of a compound of the phosphate ester type according to the invention, in a lubricating composition advantageously, and surprisingly, makes it possible to significantly improve the resistance to oxidation of said lubricating composition (or lubricant).

[0019] In the context of the present invention, the improvement in the oxidation resistance of a lubricant is defined by a reduction in the quantity of oxidized compounds present in said lubricant.

[0020] The amount of oxidized compounds present in the lubricant can be evaluated by an infrared method as described later in the present application, which makes it possible to assess the level of oxidation of engine oils. This parameter is quantified by means of the infrared spectrometry technique (FT-IR), by means of measuring the changes in the concentration of the constituents of the oxidized compounds in the appropriate spectral range, as described later.

[0021] The improvement in resistance to oxidation is understood in relation to what is observed in the absence of a phosphate ester type compound according to the invention in the lubricating composition.

[0022] The lubricating composition according to the invention is intended for a gas engine.

[0023] The gas engines according to the invention include:

[0024] - stationary gas engines;

[0025] - mobile gas engines, in particular gas engines for vehicles including including heavy goods vehicles or public transport vehicles such as buses.

[0026] Advantageously, the lubricating composition used according to the invention is intended for a gas engine in a motor vehicle, in particular for heavy goods vehicles, or even for a stationary gas engine.

[0027] By “gas engine” is meant an engine running on natural gas, including liquefied natural gas (LNG) or compressed natural gas (CNG), but also biogas.

[0028] Other characteristics and variants of the use of phosphate ester compounds according to the invention to improve resistance to oxidation will become more apparent on reading the description and examples which follow, given by way of illustration and not limitation of the invention.

[0029] In the rest of the text, the expressions “between ... and ...”, “ranging ... to ...” and “varying from ... to ...” are equivalent and are intended to mean that the limits are included, unless otherwise stated.

[0030] Unless otherwise indicated, the expression “comprising a(n)” must be understood as “comprising at least one(n)”.

[0031] As indicated above, the present invention relates to the use, in a lubricating composition intended for a gas engine, of at least one compound of phosphate ester type as defined above, to improve the oxidation resistance of said lubricating composition.

[0032] The present invention also relates to the use of a lubricating composition intended for a gas engine comprising a phosphate ester type compound such as as defined above, to improve the oxidation resistance of said lubricating composition.

[0033] The present invention also relates to a method for lubricating a gas engine by using a lubricating composition comprising a compound of phosphate ester type as defined above, to improve the resistance to oxidation of said lubricating composition.

[0034] The present invention also relates to a method for improving the oxidation resistance of a lubricating composition intended for a gas engine, comprising a step of lubricating said gas engine with a lubricating composition comprising a compound of phosphate ester type as defined above.

[0035] In particular, the use of a lubricating composition comprising at least one compound of phosphate ester type according to the invention makes it possible to reduce by at least 15%, in particular by at least 20%, more particularly by at least 25%, even more particularly by at least 30%, for example more than 40%, in particular more than 60% the quantity of oxidized compounds present in said lubricating composition, in comparison with the quantity measured for the same lubricating composition without phosphate ester compound.

[0036] The lubricating composition used in the context of the present invention may comprise one or more compounds of phosphate ester type(s) as defined above and in particular as detailed in the remainder of the text.

[0037] It is understood that a lubricating composition according to the invention may comprise a mixture of two or more phosphate ester type compounds.

[0038] By "phosphate ester" according to the invention, it is thus understood to denote a single phosphate ester or a mixture of at least two phosphate esters as defined above.

[0039] As indicated above, the phosphate ester type compound used according to the invention, alone or as a mixture, corresponds to the following formula (I): ÔR2

[0040] in which at least one of R1, R2 and R3 represents an aliphatic chain, saturated or unsaturated, linear or branched, comprising from 2 to 50 carbon atoms, in particular from 10 to 40 carbon atoms and more particularly from 15 to 30 carbon atoms, and optionally containing one or more heteroatoms, in particular one or more oxygen atoms;

[0041] the others of R1, R2 and R3, identical or different, representing a hydrogen atom or an aliphatic chain, saturated or unsaturated, linear or branched, comprising from 2 to 50 carbon atoms, in particular from 10 to 40 carbon atoms and more particularly from 15 to 30 carbon atoms, and optionally containing one or more heteroatoms, in particular one or more oxygen atoms.

[0042] In the context of the invention, the following terms are understood to mean:

[0043] - “aliphatic chain”, a hydrocarbon group comprising carbon atoms and hydrogen, linear or branched, saturated or unsaturated, non-aromatic, and possibly comprising one or more heteroatoms.

[0044] An aliphatic hydrocarbon chain is an aliphatic chain consisting only of carbon and hydrogen atoms. The aliphatic hydrocarbon chain can be saturated (in which case it is called an alkyl chain or group); it can alternatively be unsaturated (in which case it is called an alkenyl chain or group).

[0045] - “alkyl”, a saturated, linear or branched, aliphatic hydrocarbon group;

[0046] - “alkenyl” a linear or branched, unsaturated, aliphatic hydrocarbon group, in other words carrying one or more double bonds, in particular a single double bond.

[0047] The phosphate ester type compound used according to the invention can therefore be a mono-, di- or triester of phosphate, depending on the nature of the groups R1, R2 and R3.

[0048] Preferably, the phosphate ester type compound is chosen from monoesters and diesters of formula (I), and mixtures thereof.

[0049] In particular, the phosphate ester type compound used according to the invention comprises at least one phosphate monoester of formula (I) above. In other words, a lubricant according to the invention comprises at least one monoester of formula (I) in which R1 represents a saturated or unsaturated, linear or branched aliphatic chain, comprising from 2 to 50 carbon atoms, in particular from 10 to 40 carbon atoms and more particularly from 15 to 30 carbon atoms, and optionally containing one or more heteroatoms, in particular one or more oxygen atoms; and R2 and R3 represent hydrogen atoms.

[0050] Preferably, in the abovementioned formula (I), said aliphatic chain(s) are aliphatic chains having from 5 to 50 carbon atoms, in particular from 10 to 40 carbon atoms and more particularly from 15 to 30 carbon atoms, comprising one or more oxygen atoms and one or more unsaturations -CH=CH-, preferably one unsaturation -CH=CH-.

[0051] In particular, said aliphatic chain may comprise a C2 to C4 (poly)oxyalkylene group. The (poly)oxyalkylene group is preferably linear. For example, it may be a (poly)oxyethylene, (poly)oxypropylene group. Preferably, the (poly)oxyalkylene group is a (poly)oxyethylene group.

[0052] The number of oxyalkylene units, in particular oxyethylene, of the (poly)oxyalkylene group can range from 1 to 15, in particular from 1 to 8 and more particularly from 1 to 5, in particular from 1 to 3.

[0053] In a particular embodiment, said aliphatic chain is of the following formula (G):

[0054] A-[O-CnH2n]x-« (G)

[0055] where A is an aliphatic hydrocarbon chain, preferably linear, preferably unsaturated, comprising from 5 to 25 carbon atoms, in particular from 10 to 20 carbon atoms, in particular from 15 to 20 carbon atoms, for example 18 carbon atoms;

[0056] n is an integer between 2 and 4, in particular n is 2 (oxyethylene group),

[0057] x is an integer ranging from 1 to 15, in particular from 1 to 8 and more particularly from 1 to 5, especially from 1 to 3; and

[0058] • represents the bond to an oxygen atom of the phosphate group.

[0059] In a particular embodiment, in the aliphatic chain of formula (G), A is a linear aliphatic hydrocarbon chain of 15 to 20 carbon atoms, for example 18 carbon atoms, and having unsaturation. For example, A is a chain derived from oleyl alcohol.

[0060] Thus, preferably, the phosphate ester type compound used according to the invention may be chosen from the compounds of the following formula (II):

[0061] in which:

[0062] - m is 1 or 2;

[0063] - n is an integer between 2 and 4, in particular n is 2 (oxyethylene group);

[0064] - x is an integer ranging from 1 to 15, in particular from 1 to 8, and more particularly from 1 to 5, especially 1 to 3;

[0065] - A is an aliphatic hydrocarbon chain, preferably linear, preferably unsaturated, containing from 5 to 25 carbon atoms, in particular from 10 to 20 carbon atoms, in particular from 15 to 20 carbon atoms, for example 18 carbon atoms. For example, A is a chain derived from oleyl alcohol;

[0066] and mixtures thereof.

[0067] Preferably, the phosphate ester compound used according to the invention is a mixture of monoester and diester of formula (II) mentioned above.

[0068] In one embodiment, the phosphate ester compound used according to the invention comprises at least one phosphate monoester of the following formula (II-a): O II ■P—OH OH

[0069] where A, n and x are as defined previously.

[0070] Preferably, in formula (II-a), A is a linear aliphatic hydrocarbon chain of 15 to 20 carbon atoms, for example of 18 carbon atoms, and having unsaturation.

[0071] In particular, the phosphate monoester compound may correspond to the following formula (III): O (HD Q--P—QH Y OH

[0072] in which:

[0073] - R5 represents an alkyl group, linear or branched, preferably linear, comprising from 2 to 12, preferably from 6 to 10, carbon atoms, for example an octyl group;

[0074] - X2 represents an alkylene radical, linear or branched, preferably linear, comprising from 2 to 12, preferably from 6 to 10, carbon atoms, for example 8 carbon atoms, and

[0075] - x represents an integer from 1 to 15, in particular from 2 to 15, of preferably from 3 to 10.

[0076] In a particular embodiment, the phosphate monoester compound is of formula (II-a) above, in which A is derived from oleic alcohol (Cl8 alcohol).

[0077] According to one embodiment, a phosphate monoester compound corresponds to the following formula (III-a): Q (Ill-a)

[0078] x being as defined above, in particular x is from 1 to 3.

[0079] According to one embodiment, a phosphate ester compound used according to the invention is n-octyl polyethylene glycol ether phosphate (CAS number 39464-69-2).

[0080] In a particular embodiment, a phosphate ester compound according to the invention comprises at least one phosphate diester of the following formula (II-b): O (Il-b) —O--P- OH S

[0081]

[0082]

[0083]

[0084]

[0085]

[0086]

[0087]

[0088]

[0089]

[0090] where Ai and A2, identical or different, are as defined for the hydrocarbon chain A, in particular are aliphatic hydrocarbon chains, preferably linear, preferably unsaturated, comprising from 5 to 25 carbon atoms, in particular from 10 to 20 carbon atoms, in particular from 15 to 20 carbon atoms, for example 18 carbon atoms; Xi and x2, identical or different, are chosen from zero and integers, xi+x2 is an integer ranging from 1 to 15, in particular from 1 to 12, and more particularly from 2 to 10, for example from 3 to 5; ni and n2 are integers between 2 and 4, in particular ni and n2 are 2 (oxyethylene group). In particular, a phosphate diester compound may be of formula (II-b) above, in which Ai and A2, preferably identical, are linear aliphatic hydrocarbon chains of 15 to 20 carbon atoms, for example of 18 carbon atoms, and having unsaturation. In particular, a phosphate diester compound may be of formula (II-b) above, in which Ai and A2 are chains derived from oleyl alcohol. In particular, the phosphate diester compound may have the following formula (IV): O (V) R—QH=CH-X^O-^ -P—O^C^-O^X'—Œ=QH^ *• OH in which: - R5 and R'5, identical or different, are as defined for the group R5 in formula (III) above, - X2 and X'2, identical or different, are as defined for group X2 in formula (III) above, and - Xi and x2 are as defined in formula (Il-b). According to one embodiment, a phosphate diester compound corresponds to the following formula (IV-a): has)

[0091] Xi and x2 being as defined above, in particular xi+x2 is between 1 and 12, in particular between 2 and 10, and more particularly between 3 and 5.

[0092] In one embodiment, a lubricating composition used according to the invention comprises a mixture of at least two phosphate esters corresponding to the abovementioned formula (I), in particular of the abovementioned formula (II).

[0093] In particular, a lubricating composition used according to the invention may comprise a mixture of phosphate ester compounds corresponding to the above-mentioned formula (I), in particular (II), said mixture comprising at least one phosphate monoester compound of formula (II-a), in particular of formula (III), in particular of formula (III-a) as defined above.

[0094] A lubricating composition according to the invention may comprise in particular a mixture of at least two different phosphate monoester compounds as defined above, in particular of formula (II-a), in particular of formula (III), in particular of formula (III-a), said phosphate monoesters differing for example by the number of oxyalkylene units, in particular oxyethylene, of the aliphatic chain.

[0095] A lubricating composition according to the invention may also comprise a mixture of one or more phosphate monoesters according to the invention, in particular of formula (II), in particular of formula (II-a), in particular of formula (III), in particular of formula (III-a), with one or more phosphate diesters according to the invention, in particular of formula (II), in particular of formula (II-b), for example of formula (IV), in particular of formula (IV-a), as defined above.

[0096] Preferably, said monoester(s) and said diester(s) are of formulae (II-a) and (II-b), in which the aliphatic chains A, A1 and A2 are identical.

[0097] The phosphate ester compounds of formula (I) according to the invention may be prepared by methods known to those skilled in the art. They may, for example, be obtained by reaction of a phosphating agent, such as phosphatic acid, with one or more (poly)alkoxylated fatty alcohols. Alternatively, they may be commercially available.

[0098] According to one embodiment, the lubricating composition used according to the invention contains from 0.1% to 10%, preferably from 0.5% to 5%, and preferentially from 0.7% to 3% by mass of phosphonate compound as defined above, relative to the total mass of said lubricating composition.

[0099] A lubricating composition used according to the invention may comprise, in addition to said phosphate ester type compound according to the invention as defined above, one or more base oils, as well as additives, in particular as defined in the remainder of the text.

[0100] These base oils can be chosen from base oils conventionally used in the field of lubricating oils, such as mineral, synthetic or natural, animal or vegetable oils or their mixtures.

[0101] The base oils used in the lubricating 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 (table 1), or their mixtures.

[0102] [Tables 1] Saturates content Sulphur content Viscosity index (VI) Group I Mineral oils <90% > 0.03% 80 <VI < 120 Groupement II Huiles hydrocraquées >90% <0.03% 80 <VI < 120 Groupement III Huiles hydrocraquées ou hydro-isomérisées >90% <0.03% >120 Group IV Polyalphaolefins (PAO) Group V Esters and other bases not included in groups I to IV

[0103] 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-alphatting, solvent dewaxing, hydrotreating, hydrocracking, hydroisomerization and hydrofinishing.

[0104] Mixtures of synthetic and mineral oils, which can be bio-sourced, can also be used.

[0105] In particular, a lubricating composition formulated according to the invention based on a phosphate compound as described above, may further comprise one or more base oils chosen from group I, II, III and IV oils of the APL classification. In particular, a lubricating composition formulated according to the invention may comprise one or more group II and / or III base oils, in particular several group II base oils.

[0106] According to a particular embodiment, the lubricating composition used according to the invention comprises at least 50% by mass, in particular from 60% to 98% by mass, and more particularly from 70% to 95% by mass, and preferably from 75% to 90% by mass of one or more base oils, relative to the total mass of said lubricating composition, said base oil(s) being more particularly chosen from group II and / or group III base oils.

[0107] Preferably, the lubricating composition used according to the invention contains one or more group II base oils.

[0108] According to a particular embodiment, the lubricating composition used according to the invention comprises at least 50% by mass, in particular from 60% to 98% by mass, and more particularly from 70% to 95% by mass, and preferentially from 75% to 90% by mass of one or more, preferably several, group II base oils, relative to the total mass of said lubricating composition.

[0109] A lubricating composition used according to the invention may further comprise all types of additives suitable for use in an engine lubricant, in particular for a gas engine.

[0110] These additives can be introduced in isolation and / or in the form of a mixture like those already available for sale for formulations of commercial lubricants for vehicle engines, with performance levels as defined by the ACEA (Association of European Automobile Manufacturers) and / or the API (American Petroleum Institute), well known to those skilled in the art.

[0111] Such additives may be chosen from friction modifying additives, anti-wear additives, extreme pressure additives, detergents, antioxidants, other than the phosphate ester type compounds according to the invention, viscosity index improvers, pour point lowering additives, dispersants, thickeners, corrosion inhibitors, copper passivating agents, and mixtures thereof.

[0112] According to a particular embodiment, the lubricating composition used according to the invention further comprises one or more viscosity index (VI) improving polymers. Viscosity index improvers, in particular viscosity index improving polymers, make it possible to guarantee good cold resistance and minimal viscosity at high temperature. Examples of viscosity index improving polymers include polymer esters, homopolymers or copolymers, hydrogenated or non-hydrogenated, of styrene, butadiene and isoprene, homopolymers or copolymers of olefin, such as ethylene or propylene, polyacrylates and polymethacrylates (PMA), preferably homopolymers or copolymers of olefin, such as copolymers of ethylene and / or propylene.

[0113] Preferably, the lubricating composition used according to the invention further comprises, as viscosity-improving polymer, one or more olefin copolymers, such as ethylene and / or propylene copolymers.

[0114] These olefin copolymers are traditionally copolymers based on ethylene units and propylene units, or optionally copolymers based on units ethylene, propylene units and diene units (EPDM). Preferably, the viscosity-enhancing polymer is an ethylene-propylene copolymer.

[0115] The amount of olefin copolymer in the lubricating composition according to the invention is for example from 0.01% to 5%, preferably from 0.05% to 3%, preferentially from 0.5% to 1%, by mass relative to the total mass of the lubricating composition. This amount is understood as the amount of dry matter of polymer. Indeed, the olefin copolymer used in the context of the present invention is sometimes found diluted in a mineral or synthetic oil, for example in a group I, II and / or III oil, in particular a group II oil.

[0116] A lubricating composition used according to the invention may also comprise at least one detergent additive.

[0117] Detergent additives generally make it possible to reduce the formation of deposits on the surface of metal parts by dissolving secondary oxidation and combustion products.

[0118] The detergent additives that can be used in a lubricating composition used 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.

[0119] 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.

[0120] These metal salts generally comprise the metal in a 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.

[0121] A lubricating composition used according to the invention may, for example, comprise from 1% to 10%, in particular from 2% to 5%, by mass of detergent additive(s), relative to the total mass of the composition.

[0122] A lubricating composition used according to the invention may comprise at least one additional antioxidant additive, different from the phosphate ester compounds according to the invention.

[0123] Among the commonly used antioxidant additives, mention may be made of phenolic type antioxidant additives, amine type antioxidant additives, phosphosulfur antioxidants. 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. The 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 C1-C12 alkyl group, N,N'-dialkyl-aryl-diamines and mixtures thereof.

[0124] Preferably according to the invention, the sterically hindered phenols are chosen from compounds comprising a phenol group in which at least one vicinal carbon of the carbon carrying the alcohol function is substituted by at least one C1-C10 alkyl group, preferably a C1-C6 alkyl group, preferably a C4 alkyl group, preferably by the tert-butyl group.

[0125] 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 formula NR4R5R6 in which R4 represents an aliphatic group or an aromatic group, optionally substituted, R5 represents an aromatic group, optionally substituted, R6 represents a hydrogen atom, an alkyl group, an aryl group or a group of formula R7S(O)ZR8 in which R7 represents an alkylene group or an alkenylene group, R8 represents an alkyl group, an alkenyl group or an aryl group and z represents 0, 1 or 2.

[0126] Sulfurized alkyl phenols or their alkali and alkaline earth metal salts can also be used as antioxidant additives.

[0127] Another class of antioxidant additives is that of copper compounds, for example copper thio- or dithio-phosphates, copper salts of carboxylic acids, dithiocarbamates, sulfonates, phenates, copper acetylacetonates. Copper I and II salts, succinic acid or anhydride salts can also be used.

[0128] A lubricating composition used according to the invention may comprise from 0.1% to 5% by mass, in particular from 0.5% to 2% by mass of at least one additional antioxidant additive, distinct from the phosphate ester compounds according to the invention, relative to the total mass of the composition.

[0129] According to one embodiment, the lubricating composition according to the invention does not comprise an additional antioxidant, other than the phosphate ester type compounds of formula (I) according to the present invention.

[0130] According to one embodiment, the lubricating composition according to the invention does not comprise an amine antioxidant.

[0131] A lubricating composition used according to the invention may comprise at least one anti-wear additive.

[0132] There is a wide variety of anti-wear additives. Preferably for the composition used according to the invention, the anti-wear additives are chosen from phosphosulfur additives such as metal alkylthiophosphates, in particular zinc alkylthiophosphates, and more specifically zinc dialkyldithiophosphates or ZnDTP. The preferred compounds are of formula Zn((SP(S)(OR2)(OR3)) 2, in which R2 and R3, identical or different, independently represent an alkyl group, preferably an alkyl group comprising from 1 to 18 carbon atoms.

[0133] Amine phosphates are also anti-wear additives that can be used in a composition implemented 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.

[0134] A lubricating composition used 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, of anti-wear additives, by mass relative to the total mass of composition.

[0135] A lubricating composition used according to the invention may further comprise one or more other additives chosen from friction modifying additives, extreme pressure additives, pour point lowering additives, dispersants, thickeners, corrosion inhibitors, copper passivating agents and mixtures thereof.

[0136] The friction modifying 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 Mo, Sb, Sn, Fe, Cu, Zn, the ligands of which may be hydrocarbon compounds comprising oxygen, nitrogen, sulfur or phosphorus atoms. The ash-free friction modifying additives are generally of organic origin and may be chosen from monoesters of fatty acids and polyols, alkoxylated amines, alkoxylated fatty amines, fatty epoxides, fatty borate epoxides; fatty amines or fatty acid glycerol esters, and dithiocarbamates.

[0137] A lubricating composition used according to the invention may comprise from 0.01% to 2% by mass or from 0.01% to 5% by mass, preferably from 0.1% to 1.5% by mass or from 0.1% to 2% by mass of friction modifying additive, relative to the total mass of the composition.

[0138] Advantageously, a lubricating composition used according to the invention is free of friction modifying additive.

[0139] A lubricating composition used according to the invention may also comprise at least one pour point lowering additive.

[0140] By slowing the formation of paraffin crystals, pour point depressant additives generally improve the cold behavior of the composition.

[0141] Examples of pour point depressant additives include polyalkyl methacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes, or alkylated polystyrenes.

[0142] A lubricating composition according to the invention may also comprise at least one dispersing agent. Such dispersing agents ensure the maintenance in suspension and the evacuation of insoluble solid contaminants constituted by the secondary oxidation products which are formed when the lubricating composition is in service. They may be chosen from Mannich bases, succinimides and their derivatives, such as polyisobutylene succinic anhydride derivatives.

[0143] In particular, a lubricating composition according to the invention may comprise from 0.2% to 10% by mass of dispersing agent(s), relative to the total mass of the composition.

[0144] It may also comprise at least one anti-corrosion agent or copper passivating agent, for example compounds such as succinic polyisobutene anhydrides, thiadiazole sulfonates or mercaptobenzothiazoles. They are typically present in a lubricating composition according to the invention at contents of between 0.01% and 1% by mass, relative to the total mass of the composition.

[0145] The lubricating composition used according to the invention and as defined above is intended for a gas engine. It is in particular intended for a gas engine in a motor vehicle, in particular for heavy vehicles and light vehicles, in particular for heavy goods vehicles, or a stationary engine.

[0146] The present invention also describes the use of a lubricating composition as defined above, for lubricating the parts of a gas engine, as explained above.

[0147] The invention will now be described by means of the following examples given of course by way of illustration and not limitation of the invention. EXAMPLES

[0148] Example 1: Preparation of lubricating compositions

[0149] Two lubricating compositions, CL1 and CCI, were prepared by simple mixing, at room temperature, of the following components, in the mass proportions indicated in Table 2.

[0150] [Tables2] Components CL1 CCI Group II base oil (KV100 = 5.2 - 5.6 mm2 / s) 61.7 62.3 Group II base oil (KV100 = 10.0 - 12.0 mm2 / s) 20.8 21 Viscosity improver polymer (Olefin copolymer with 12.5% ​​active matter in a Group II base oil) 5.9 6 Additive package (*) 10.6 10.7 Mixture of mono- and di-phosphate esters of formulae (III-a) and (IV-a) above 1 -

[0151] * includes detergents, antioxidants and anti-wear additives.

[0152] KV100 corresponds to a kinematic viscosity measured at 100°C according to the ASTM D445 standard. Example 2#: Evaluation of oxidation resistance

[0153] Method for evaluating resistance to oxidation

[0154] To evaluate the resistance to oxidation, the lubricant to be tested is subjected to thermal aging by catalyzed oxidation, intended to simulate the evolution of an engine oil subjected to severe conditions (high load, hot sump). The lubricant to be tested is maintained at a temperature above 150°C, in the presence of a ferric catalyst and under air flow for 3 days.

[0155] The level of oxidation of the lubricant is then evaluated by infrared spectrometry according to the ASTM D7214 standard, by calculating the PAI (PAI for Peak Area Increase).

[0156] The area of ​​the infrared peak is linked to the presence of oxidized compounds. Here, we measure the C=O band from 1650 to 1820 cm1. The more oxidized compounds the oil contains, the larger this band is. The area of ​​this band is measured using a subtraction between the spectrum of the oxidized lubricant and that of the new lubricant.

[0157] Results

[0158] Two analyses were carried out for the CCI lubricant and three analyses for the CL1 lubricant. The values ​​obtained concerning the PAI (averages of the analyses) are indicated in Table 3 below.

[0159] [Tables3] CL1 CCI Mean values ​​(PAI C=O) (in A.cm '.mm ') 200 521

[0160] Significant differences are therefore observed when adding 1% of phosphate ester type compound for the composition according to the invention CL1.

[0161] The oxidation values ​​given by the CO band during an infrared analysis are lower for CL1 (in the presence of phosphate ester) in comparison with CCI (without phosphate ester). These results demonstrate the improvement in oxidation resistance performance due to the presence of a phosphate ester compound according to the invention.

Claims

1.

2. Claims Use, in a lubricating composition intended for a gas engine, of at least one compound of phosphate ester type, to improve the oxidation resistance of said lubricating composition, in which said compound of phosphate ester type corresponds to the following formula (I): O (I) , He 2 HO—P—OR ' i ; OR" in which at least one of R1, R2 and R3 represents an aliphatic chain, saturated or unsaturated, linear or branched, comprising from 2 to 50 carbon atoms, in particular from 10 to 40 carbon atoms and more particularly from 15 to 30 carbon atoms, and optionally containing one or more heteroatoms, in particular one or more oxygen atoms; the others of R1, R2 and R3, identical or different, being chosen from a hydrogen atom and an aliphatic chain, saturated or unsaturated, linear or branched, comprising from 2 to 50 carbon atoms, in particular from 10 to 40 carbon atoms and more particularly from 15 to 30 carbon atoms, and optionally containing one or more heteroatoms, in particular one or more oxygen atoms. Use according to claim 1, in which the phosphate ester compound is chosen from the compounds of the following formula (II): in which: - m is 1 or 2; - n is an integer between 2 and 4; - x is an integer ranging from 1 to 15, in particular from 1 to 8, and more particularly from 1 to 5, in particular from 1 to 3; - A is an aliphatic hydrocarbon chain, preferably linear, preferably unsaturated, comprising from 5 to 25 carbon atoms, in particular from 10 to 20 carbon atoms, in particular from 15 to 20 carbon atoms, for example 18 carbon atoms; and mixtures thereof.

3. Use according to claim 2, wherein the phosphate ester compound is a mixture of monoester and diester of formula (II).

4. Use according to any one of claims 2 or 3, wherein the phosphate ester compound comprises at least one phosphate monoester of the following formula (II-a): O (II-a) AC^Hz^O--P—ÛH OH where A, n and x are as defined in claim 2.

5. Use according to claim 4, wherein the phosphate monoester compound corresponds to the following formula (III): o (in) R—CH =CH O-C2 H --P— OH OH in which: - R5 represents a linear or branched, preferably linear, alkyl group comprising from 2 to 12, preferably from 6 to 10, carbon atoms, for example an octyl group; - X2 represents a linear or branched, preferably linear, alkylene radical comprising from 2 to 12, preferably from 6 to 10, carbon atoms, for example 8 carbon atoms, and - x represents an integer from 1 to 15, in particular from 2 to 15, preferably from 3 to 10.

6. Use according to any one of claims 4 or 5, in which the phosphate monoester compound corresponds to the following formula (III-a): O (III-a) [ ' < 1 * OH x being as defined in claim 4, in particular x is comprised from 1 to 3.

7. Use according to any one of claims 2 or 3, wherein the phosphate ester compound comprises at least one phosphate diester of the following formula (II-b): O (Hb) A.| ^o—C„H o--PO 2n“O^-Â2 OH where A1 and A2, identical or different, are as defined for the hydrocarbon chain A, in particular are aliphatic hydrocarbon chains, preferably linear, preferably unsaturated, comprising from 5 to 25 carbon atoms, in particular from 10 to 20 carbon atoms, in particular from 15 to 20 carbon atoms, for example 18 carbon atoms; Xi and x2, identical or different, are chosen from zero and integers, xi+x2 is an integer ranging from 1 to 15, in particular from 1 to 12, and more particularly from 2 to 10, for example from 3 to 5; ni and n2 are integers between 2 and 4, in particular ni and n2 are equal to 2.

8. Use according to claim 7, wherein the phosphate diester compound corresponds to the following formula (IV-a): CoH17 ■ OH X <. | (IV-a) Xi and x2 being as defined in claim 7, in particular xi+x2 is between 1 and 12, in particular between 2 and 10, and more particularly between 3 and 5.

9. Use according to any one of claims 1 to 8, in which the lubricating composition contains from 0.1% to 10%, preferably from 0.5% to 5%, and preferentially from 0.7% to 3% by mass of phosphate compound of formula (I), relative to the total mass of said lubricating composition.

10. Use according to any one of claims 1 to 9, in which the lubricating composition comprises at least 50% by mass, in particular from 60% to 98% by mass, and more particularly from 70% to 95% by mass, and preferably from 75% to 90% by mass of one or more base oils, relative to the total mass of said lubricating composition, said oil(s) base oils being more particularly chosen from group II and / or group III base oils.

11. Use according to any one of claims 1 to 10, wherein the lubricating composition contains one or more Group II base oils.

12. Use according to any one of claims 1 to 11, wherein the lubricating composition further comprises one or more viscosity index improving polymers, preferably one or more olefin copolymers, such as ethylene and / or propylene copolymers.

13. Use according to any one of claims 1 to 12, wherein the lubricating composition further comprises one or more additives selected from friction modifying additives, anti-wear additives, extreme pressure additives, detergents, antioxidants, other than phosphate ester compounds of formula (I), viscosity index improvers, pour point depressant additives, dispersants, thickeners, corrosion inhibitors, copper passivators and mixtures thereof.

14. Use according to any one of claims 1 to 13, wherein the lubricating composition is intended for a gas engine in a motor vehicle, in particular for heavy vehicles and light vehicles, in particular for heavy goods vehicles, or a stationary engine.