Lubricant composition for gas turbines

Abstract

The present invention relates to a lubricant composition, in particular for a gas or steam turbine, comprising: - at least one base oil; - at least one phenylnaphthylamine (PAN) compound, preferably alkylphenyl-α-naphthylamine (APAN ) compound; - at least one diphenylamine (DPA) compound, preferably dialkyl-diphenylamine compound; and - at least one phosphite ester compound, preferably triaryl phosphite ester compound; in which the ratio by weight between said diphenylamine compound(s) and said phosphite ester compound(s) is strictly greater than 1.0.

Description

technical field

[0001] The present invention relates to the field of lubricating compositions, more particularly to the field of turbine lubricating compositions. It relates more specifically to a turbine lubricating composition employing a combination of three types of specific antioxidant additives in specific proportions. Previous technique

[0002] Gas or steam turbines are typically used in the fields of aeronautics, naval, rail transport and electricity production.

[0003] In particular, gas and steam turbines offer advantages such as light weight and high power-to-weight ratios, making them especially well-suited for aeronautical propulsion, particularly for airplanes and helicopters, as well as for naval propulsion, especially for high-speed vessels. Furthermore, modern power generation plants utilize gas turbines that employ high-temperature combustion gases such as liquefied natural gas, or power generation plants that combine a gas turbine and a steam turbine.

[0004] Industrial lubricating compositions, also known as "lubricating oils" or "lubricants," and more specifically lubricants for gas or steam turbines, can be subjected to extreme conditions, particularly operating temperatures exceeding 250°C. This is the case, for example, for lubricating oils for aircraft engine turbines.

[0005] Under such high temperature conditions, lubricating oils can degrade and oxidize. This degradation can result in the formation of deposits, such as varnishes, the presence of sludge, and / or an increase in the viscosity of the composition.

[0006] The oxidation stability of lubricating oils is further reduced by the dissolution of metals within them under the extreme operating conditions described above. Indeed, dissolved metals can catalyze the oxidative degradation of lubricants.

[0007] Such degradation significantly reduces oil lifespan, forcing shorter drain intervals and resulting in substantial operational losses. Consequently, some manufacturers now impose very strict specifications regarding the oxidation stability and high-temperature thermal stability properties of lubricant compositions intended, in particular, for use in gas or steam turbines.

[0008] To increase oxidation stability, most lubricants contain additives designed to inhibit oxidation. Various antioxidant additives have been proposed for use in lubricants, such as sterically hindered phenolic compounds, aromatic organic amines, diphenylamine derivatives (DPAs), and phenylnaphthylamine derivatives (PANs). Document JP 2007 270090 A proposes turbine lubricant compositions comprising a combination of antioxidant additives, exhibiting improved thermal stability and oxidation resistance under high-temperature conditions.

[0009] As an example, document WO 2008 / 009704 proposes a lubricating composition, particularly for turbines, comprising succinate ester and sarcosinic acid as rust inhibitors. The compositions described therein may also include various aromatic amine-type antioxidants, such as phenyl-α-naphthylamine and dialkyl-α-diphenylamine compounds. However, the lubricating compositions proposed in this document do not achieve the required performance levels in terms of thermal stability and oxidation resistance at high temperatures.

[0010] Therefore, there remains a need for a lubricating composition, particularly for turbines, with improved properties in terms of thermal stability and oxidation resistance, while maintaining good corrosion resistance properties, and allowing for the reduction of unwanted deposit formation during lubricant use, especially under high temperature conditions. Summary of the invention

[0011] The present invention is specifically designed to meet this need.

[0012] More specifically, the present invention relates, according to a first aspect, to a lubricating composition, particularly for a gas or steam turbine, comprising: at least one base oil; at least one phenyl-naphthylamine (PAN) compound, preferably alkylphenyl-α-naphthylamine (APAN); at least one diphenylamine (DPA) compound, preferably dialkyl-diphenylamine; and at least one phosphite ester compound, of formula (III-a) as defined below; wherein the DPA / phosphite ester(s) mass ratio is between 2.0 and 3.0 and the PAN / phosphite ester(s) mass ratio is between 2.1 and 3.

[0013] Against all expectations, the inventors discovered that the implementation of a combination of the three specific antioxidant additives mentioned above, at least one phenyl-naphthylamine compound (PAN), at least one diphenylamine compound (DPA) and at least one phosphite ester compound, of formula (III-a) as defined below, with the indicated mass ratios, makes it possible to obtain a lubricating composition exhibiting improved properties in terms of thermal stability, oxidation stability and corrosion resistance, under high temperature conditions.

[0014] In particular, as illustrated in the examples, it is not possible to achieve such performance in terms of thermal stability, oxidation stability and corrosion resistance, by implementing only one, or even two, antioxidants out of the three considered according to the invention.

[0015] Thus, the specific combination of antioxidant additives according to the invention makes it possible to give the lubricant excellent properties for reducing oxidation phenomena and the formation of undesirable deposits that take place during the use of the lubricant, particularly under high temperature conditions and in the presence of oxygen.

[0016] Advantageously, a lubricating composition according to the invention thus exhibits an increased service life.

[0017] As detailed in the examples that follow, corrosion resistance, oxidation stability and thermal stability performance can be evaluated using different tests.

[0018] More specifically, a lubricating composition according to the invention advantageously exhibits oxidation stability, evaluated by the RPVOT (Rotating Pressure Vessel Oxidation Test) according to ASTM D2272, of 2200 minutes or more, advantageously 2500 minutes or more. Advantageously, it exhibits a residual RPVOT, measured according to the "dry TOST - 1000 hours" test adapted from ASTM D7873 and detailed below in the examples, of 70% or more, advantageously 75% or more, and more particularly 80% or more.

[0019] A lubricating composition according to the invention advantageously has an amount of insolubles after 48 hours at 180 °C of less than 30 mg / kg, preferably less than 25 mg / kg, more preferably less than or equal to 20 mg / kg.

[0020] It also exhibits excellent resistance to oxidation and corrosion, evaluated according to the ASTM D4636 test.

[0021] These improved performances in terms of thermal stability, resistance to oxidation and corrosion make it possible to reduce the content, or even to eliminate the presence of additional additives, in particular anti-wear and / or extreme-pressure additives, or pour point depressant (PPD) additives.

[0022] The present invention further relates to the use, in a lubricating composition, in particular for a gas or steam turbine, comprising at least one base oil, of at least one phenyl-naphthylamine (PAN) compound, preferably alkylphenyl-α-naphthylamine (APAN); of at least one diphenylamine (DPA) compound, preferably dialkyl-diphenylamine; and of at least one phosphite ester compound, of formula (III-a) as defined below, said phosphite ester compound(s) being implemented in a DPA / phosphite ester(s) mass ratio of between 2.0 and 3.0 and a PAN / phosphite ester(s) mass ratio of between 2.1 and 3, to improve the thermal stability and oxidation resistance performance of the composition.

[0023] The lubricating composition according to the invention is thus particularly suitable for use as a lubricant for a gas or steam turbine.

[0024] The present invention thus relates, according to another of its aspects, to the use of a composition as described above, as a lubricant for a gas or steam turbine.

[0025] Other characteristics, variations and advantages of the lubricating compositions according to the invention will become clearer from the description and examples that follow, given by way of illustration and not limitation of the invention.

[0026] In the following text, the expressions "between ... and ...", "ranging ... to ..." and "varying from ... to ..." are equivalent and are meant to mean that the limits are included, unless otherwise stated.

[0027] Unless otherwise indicated, the expression "containing one" should be understood as "containing at least one". Detailed description

[0028] For the purposes of this invention, and unless otherwise specified, the following definitions apply: "Alkyl" is a saturated aliphatic group, linear or branched; for example, a Cx to Cz alkyl represents a saturated carbon chain of x to z carbon atoms, linear or branched; "alkenyl" is an unsaturated aliphatic group, linear or branched; for example, a Cx to Cz alkenyl group represents an unsaturated carbon chain of x to z carbon atoms, linear or branched; "cycloalkyl" is a cyclic alkyl group, for example, a Cx to Cz cycloalkyl represents a cyclic carbon group of x to z carbon atoms, for example, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl; "aryl" is a mono- or polycyclic aromatic group, particularly one comprising between 6 and 10 carbon atoms. Examples of aryl groups include phenyl and naphthyl. PHENYL-NAPHTHYLAMINE COMPOUND ( BANG )

[0029] As mentioned previously, according to one of its essential characteristics, a lubricating composition according to the invention comprises at least one phenyl-naphthylamine compound, denoted "PAN".

[0030] The term "phenyl-naphthylamine" refers to a compound selected from N-phenyl-α-naphthylamine, N-phenyl-β-naphthylamine and their derivatives, preferably having one or more alkyl groups as substituents on the phenyl ring.

[0031] More specifically, the phenyl-naphthylamine type compound, preferably alkylphenyl-α-naphthylamine (denoted "APAN") implemented according to the invention corresponds to the following formula (I): in which: n is an integer from 1 to 5; and R1 represents an alkyl group, linear or branched, preferably at C1 to C 22 , particularly in C2 to C12.

[0032] Preferably, the phenyl-naphthylamine type compound, in particular alkylphenyl-α-naphthylamine, has the following formula (Ia): in which R 1 and n are such as defined previously.

[0033] Preferably, the phenyl-naphthylamine compound is an alkylphenyl-α-naphthylamine compound, in particular conforming to the above-mentioned formula (Ia), in which n is an integer from 1 to 3, in particular n equals 1.

[0034] According to a particular embodiment, the phenyl-naphthylamine compound has the following formula (Ib): in which R 1 is as defined previously, preferably R 1 represents an alkyl group at C 2 to C 12.

[0035] Preferably, group R1 is in the para position of the amine group.

[0036] Phenyl-naphthylamine compounds may be commercially available or prepared using synthetic methods known to those skilled in the art.

[0037] Preferably, a lubricating composition according to the invention comprises between 0.05 and 5% by mass of phenyl-naphthylamine compound(s), in particular between 0.1 and 3% by mass, preferably between 0.15 and 1% by mass and more particularly between 0.2 and 0.5% by mass, of phenyl-naphthylamine compound(s), relative to the total mass of the composition. DIPHENYLAMINE COMPOUND ( DPA )

[0038] As previously stated, a lubricating composition according to the invention further comprises at least one diphenylamine compound (denoted DPA).

[0039] The term "diphenylamine compound" refers to diphenylamine and its derivatives, preferably in which at least one, or even both, phenyl groups are substituted by one or more groups selected from linear or branched alkyl groups. More particularly, the diphenylamine-type compound, preferably dialkyldiphenylamine, implemented according to the invention, corresponds to the following formula (II): in which: n2 and n3 are, independently of each other, integers from 1 to 5; and R2 and R3 represent, independently of each other, a linear or branched alkyl group, preferably at C1 to C 22 , particularly in C4 to C8.

[0040] Preferably, the diphenylamine-type compound, preferably dialkyldiphenylamine, implemented according to the invention has the following formula (II-a): in which R2 and R3 are as defined previously.

[0041] Preferably, groups R2 and R3 are in the para position of the amino group.

[0042] Thus, according to a particular embodiment, the diphenylamine compound is chosen from among the p,p'-dialkyldiphenylamine compounds.

[0043] Diphenylamine compounds may be commercially available or prepared by synthetic methods known to those skilled in the art.

[0044] Preferably, a lubricating composition according to the invention comprises between 0.05 and 5% by mass of diphenylamine compound(s), in particular between 0.1 and 3% by mass, preferably between 0.15 and 1% by mass and more particularly between 0.2 and 0.5% by mass, of diphenylamine compound(s) relative to the total mass of the composition.

[0045] According to a particularly preferred embodiment, the phenyl-naphthylamine compound(s) are selected from the compounds of formula (Ib) above, in particular in which R 1 represents an alkyl group in C 2 to C 12; and the diphenylamine compound(s) are selected from the compounds of formula (II-a) above, in particular in which R 2 and R 3 represent, independently of each other, a linear or branched alkyl group, preferably in C 2 to C 12.

[0046] According to a particular embodiment, the phenyl-naphthylamine (PAN) compound(s), in particular as defined above, and the diphenylamine (DPA) compound(s), in particular as defined above, are implemented in a mass ratio of N-phenyl-α-naphthylamine compound(s) / diphenylamine compound(s) of between 0.8 and 1.0, in particular of about 1. PHOSPHITE ESTER

[0047] As previously stated, a lubricating composition according to the invention further comprises at least one phosphite ester compound of the following formula (III-a): in which the R groups represent, independently of each other, alkyl groups, linear or branched at C1 to C10, in particular at C3 to C8, and n represent, independently of each other, 1 or 2.

[0048] Preferably, n equals 2.

[0049] Preferably, n is 2, and R represents groups, especially in ortho and para positions, preferably alkyl groups, preferably branched, in C3 to C6, such as a tert-butyl group.

[0050] Preferably, a phosphite ester compound according to the invention has formula (III-a) in which the -(R) n groups are identical.

[0051] Advantageously, a phosphite ester compound according to the invention is tris(2,4-ditert-butylphenyl)phosphite (CAS 31570-04-4).

[0052] According to a particular embodiment, it is tris(2,4-ditert-butylphenyl)phosphite (CAS 31570-04-4).

[0053] Phosphite ester compounds may be commercially available or prepared by synthetic methods known to those skilled in the art.

[0054] Preferably, a lubricating composition according to the invention comprises between 0.01 and 3% by mass of phosphite ester compound(s), in particular between 0.02 and 1% by mass, preferably between 0.05 and 0.5% by mass and more particularly between 0.1 and 0.3% by mass, of phosphite ester compound(s), relative to the total mass of the composition.

[0055] According to a particular embodiment, the diphenylamine compound(s) are selected from the compounds of formula (II-a) above, in particular in which R 2 and R 3 represent, independently of each other, a linear or branched alkyl group, preferably at C 2 to C 12; and the phosphite ester compound(s) are selected from the compounds of formula (III-a) above, in particular in which n is 2, and the Rs represent groups, in particular in ortho and para positions, preferably alkyl groups, preferably branched, at C 3 to C 6, such as a tert-butyl group. As stated previously, the diphenylamine (DPA) compound(s), in particular as defined above, and the phosphite ester compound(s), as defined above, are implemented in a DPA compound(s) / phosphite ester(s) mass ratio of between 2.0 and 3.0.

[0056] The said phenylnaphthylamine (PAN) compound(s), in particular as defined above, and the said phosphite ester compound(s), as defined above, are implemented in a PAN compound(s) / phosphite ester(s) mass ratio strictly between 2.1 and 3.0.

[0057] It is understood that the various embodiments mentioned above, particularly with regard to the nature of the phenyl-naphthylamine compound(s), the diphenylamine compound(s) and the phosphite ester compound(s), may be combined.

[0058] In particular, a lubricating composition according to the invention advantageously comprises at least the following combination: of one or more phenyl-naphthylamine compounds, in particular alkylphenyl-α-naphthylamine, as defined above, preferably chosen from the compounds of formula (Ib) above, in which R 1 preferably represents an alkyl group in C 2 to C 12; of one or more diphenylamine compounds, in particular dialkyl-diphenylamine as defined above, preferably chosen from the compounds of formula (II-a) above, in which R 2 and R 3 preferably represent, independently of each other, a linear or branched alkyl group, preferably in C 2 to C 12; and one or more phosphite ester compounds, chosen from the compounds of formula (III-a) above, in particular in which n is 2, and the Rs represent groups, especially in ortho and para positions, preferably alkyl groups, preferably branched, in C3 to C6, such as a tert-butyl group;the diphenylamine compound(s) and the phosphite ester compound(s) being implemented in a diphenylamine compound(s) / phosphite ester compound(s) mass ratio between 2.0 and 3.0 and with the PAN / phosphite ester compound(s) mass ratio between 2.1 and 3.

[0059] According to a particular embodiment, a lubricating composition according to the invention comprises: from 0.05 to 5 wt%, in particular from 0.1 to 3 wt%, preferably from 0.15 to 1 wt% and more particularly from 0.2 to 0.5 wt%, of one or more phenyl-naphthylamine compounds, preferably alkylphenyl-α-naphthylamine, in particular as defined above, preferably chosen from among the compounds of formula (Ia) above, in which R 1 preferably represents an alkyl group at C 2 to C 12; from 0.05 to 5 wt%, in particular from 0.1 to 3 wt%, preferably from 0.15 to 1 wt% and more particularly from 0.2 to 0.5 wt%, of one or more diphenylamine compounds, preferably dialkyl-diphenylamine, in particular as defined above, preferably selected from the compounds of formula (II-a) above, in which R 2 and R 3 preferably represent, independently of each other, a linear or branched alkyl group, preferably at C 2 to C 12;and from 0.01 to 3 wt%, in particular from 0.02 to 1 wt%, preferably from 0.05 to 0.5 wt% and more particularly from 0.1 to 0.3 wt%, of one or more phosphite ester compounds, selected from the compounds of formula (III-a) above, in particular where n is 2, and the Rs represent groups, especially in ortho and para positions, preferably alkyl groups, preferably branched, in C3 to C6, such as a tert-butyl group;

[0060] the contents being expressed in relation to the total mass of said lubricating composition. OIL ( S ) BASIC

[0061] As previously stated, a lubricating composition according to the invention comprises at least one base oil.

[0062] The base oil(s) present in a lubricating composition according to the invention are chosen appropriately, in particular with regard to their compatibility with the phenyl-naphthylamine, diphenylamine and phosphite ester compounds implemented according to the invention.

[0063] It can be a mixture of several base oils, for example a mixture of two, three or four base oils.

[0064] These base oils can be chosen from among the base oils conventionally used in the field of lubricating oils, such as mineral, synthetic or natural, animal or vegetable oils or mixtures thereof.

[0065] 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 (Table 1), or their equivalents according to the ATIEL classification, or mixtures thereof. TABLE 1 Saturated fat content Sulfur content Viscosity Index (VI) Group I Mineral Oils < 90% > 0,03% 80 ≤ VI < 120 Group II Hydrocracked Oils ≥ 90% ≤ 0,03% 80 ≤ VI < 120 Group III Hydrocracked or hydro-isomerized oils ≥ 90% ≤ 0,03% ≥ 120 Group IV Polyalphaolefins (PAO) Group V Esters and other bases not included in groups I to IV

[0066] Mineral base oils include all types of bases obtained by atmospheric and vacuum distillation of crude oil, followed by refining operations such as solvent extraction, desalpha removal, solvent dewaxing, hydrotreating, hydrocracking, hydroisomerization and hydrofinishing.

[0067] Synthetic base oils can be esters of carboxylic acids and alcohols, polyalphaolefins (PAOs), or polyalkylene glycols (PAGs) obtained by polymerization or copolymerization of alkylene oxides containing 2 to 8 carbon atoms, particularly 2 to 4 carbon atoms. Polyalphaolefins used as base oils are, for example, obtained from monomers containing 4 to 32 carbon atoms, such as decene, octene, or dodecene, and have a viscosity at 100°C 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.

[0068] Mixtures of synthetic and mineral oils can also be used.

[0069] There are generally no limitations on the use of different lubricating bases to produce the lubricating compositions according to the invention, except that they must have properties, in particular of viscosity, viscosity index, sulfur content, anti-corrosion, suitable for use in gas and / or steam turbines.

[0070] Preferably, the base oil is chosen from API Group II and III oils and their mixtures. Advantageously, Group II and III base oils further improve the oxidation stability properties of the lubricating composition.

[0071] Preferably, the base oil is chosen from group III oils.

[0072] The kinematic viscosity, measured at 40°C according to ASTM D445, of the base oil or mixture of base oils may advantageously be between 20 mm² / s and 100 mm² / s, preferably between 25 mm² / s and 50 mm² / s.

[0073] Advantageously, a lubricating composition according to the invention comprises at least 40% by mass of base oil(s) relative to the total mass of the composition, in particular at least 50% by mass of base oil(s), and especially between 60 and 99.5% by mass and more particularly between 70 and 99% by mass of base oil(s). ADDITIONAL ADDITIVES

[0074] A lubricating composition according to the invention may further comprise additional additives, suitable for use in a lubricant for turbines, such as gas and / or steam turbines.

[0075] Advantageously, a lubricating composition according to the invention comprises one or more additives selected from the antioxidants distinct from the compounds implemented within the framework of the present invention, viscosity index (VI) improvers, pour point depressant (PPD) additives, antifoaming agents, thickeners, corrosion inhibitors, copper passivating agents, and mixtures thereof.

[0076] It is understood that the nature and quantity of additives used are chosen so as not to affect the properties of the lubricating composition, in particular the performance, discussed above, conferred by the combination of the three antioxidants used according to the invention.

[0077] A lubricating composition according to the invention may further comprise one or more corrosion inhibitors.

[0078] Corrosion inhibitors are known to those skilled in the art in the field of lubricants, particularly turbine lubricants.

[0079] They can be more specifically chosen from among organic acid esters, triazole derivatives, N-acyl sarcosines or imidazoline derivatives.

[0080] According to a particular embodiment, a lubricating composition according to the invention comprises one or more corrosion inhibitors selected from triazole-type derivatives and organic acid esters, in particular alkylated organic acid esters.

[0081] Preferably, the triazole-type compound is a benzotriazole or one of its derivatives, preferably a benzotriazole derivative, more preferably a tolyltriazole derivative. Tolyltriazole derivatives may more particularly have the following formula (IV): in which: R 4< and R 5< represent, independently of each other, a hydrogen atom, a linear or branched alkyl group, preferably branched, at C 3 to C 14, preferably at C 6 to C 12; and A- represents an alkylene group, linear or branched, preferably linear, at C 1 to C 6, preferably at C 1 to C 3 and more preferably a methylene group (-CH 2 -), in particular, said tolyltriazole derivative being 2-ethyl-N-(2-ethylhexyl)-N-((4-methylbenzotriazol-1-yl)methyl]hexan-1-amine.

[0082] According to a particular embodiment, the triazole-type compound has formula (IV), in which R4< and R5< represent C6-C12 branched alkyl groups and -A- represents a C1-C3 alkylene group, preferably a methylene group.

[0083] Examples of corrosion inhibitors of the alkylated organic acid ester type include succinic acid esters.

[0084] The corrosion inhibitor additive(s), in particular of the tolyltriazole derivative and / or alkylated organic acid ester type, can be used in a lubricating composition according to the invention, at a rate of 0.01 to 5% by mass, in particular 0.1 to 3% by mass, and more particularly 0.1 to 2% by mass, relative to the total mass of the lubricating composition.

[0085] A lubricating composition according to the invention may further comprise at least one antifoaming additive. Antifoaming additives may, for example, be selected from polar polymers such as polymethylsiloxanes or polyacrylates. In particular, a lubricating composition according to the invention may comprise from 0.01 to 3% by mass of antifoaming additive(s), relative to the total weight of the lubricating composition.

[0086] A lubricating composition according to the invention may further comprise one or more antioxidant additive(s), distinct from the phenyl-naphthylamine, diphenylamine and phosphite ester compounds described above.

[0087] Examples of additional antioxidant additives include sterically hindered phenols, esters of sterically hindered phenols, and sterically hindered phenols containing a thioether bridge. Preferably, 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-C6 alkyl group, preferably a C4 alkyl group, and preferably by the tert-butyl group. Examples of sterically hindered phenol-type antioxidant additives include di-t-butyl-2,6-methyl-4-phenol (BHT), t-butyl hydroquinone (TBHQ), 2,6 and 2,4-dit-butyl phenol, 2,4-dimethyl-6-t-butyl phenol, pyrogallol, and octyl-3,5-di-tert-butyl-4-hydroxy-hydrocinnamate.

[0088] According to a particular embodiment, a lubricating composition according to the invention does not comprise an antioxidant additive distinct from the phenyl-naphthylamine, diphenylamine and phosphite ester compounds described above.

[0089] A lubricating composition according to the invention may also include at least one pour point depressant (PPD) additive. By slowing the formation of paraffin crystals, pour point depressant additives generally improve the cold-weather performance of the lubricating composition according to the invention. Examples of pour point depressants include alkyl polymethacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes, and alkylated polystyrenes. A lubricating composition according to the invention may include from 0.1% to 2%, preferably from 0.2% to 1% by mass of pour point depressant additive(s), relative to the total weight of the composition.

[0090] According to a particular embodiment, a lubricating composition according to the invention comprises less than 200 ppm, in particular less than 100 ppm, especially less than 50 ppm and more particularly less than 10 ppm by mass, or is totally free of pour point lowering additive(s).

[0091] A lubricating composition according to the invention may also include at least one viscosity index improver (VI). Viscosity index improvers, particularly viscosity index improving polymers, ensure good cold-weather performance and minimal viscosity at high temperatures. Examples of viscosity index improving polymers include polymer esters, hydrogenated or non-hydrogenated homopolymers or copolymers of styrene, butadiene, and isoprene, olefin homopolymers or copolymers such as ethylene or propylene, and polyacrylates and polymethacrylates (PMAs), preferably olefin homopolymers or copolymers such as ethylene or propylene.

[0092] In particular, a lubricating composition according to the invention may comprise from 1 to 15% by mass of additive(s) improving the viscosity index, preferably from 5% to 10% by mass, relative to the total weight of the lubricating composition.

[0093] Advantageously, a lubricating composition according to the invention comprises a mass content less than or equal to 200 ppm, in particular less than or equal to 100 ppm, in particular less than or equal to 50 ppm and more particularly less than or equal to 10 ppm, or even is totally free of anti-wear and / or extreme-pressure additives of the phosphate ester type, such as tri(isopropylphenyl) phosphate.

[0094] Advantageously, a lubricating composition according to the invention comprises a mass content less than or equal to 200 ppm, in particular less than or equal to 100 ppm, in particular less than or equal to 50 ppm and more particularly less than or equal to 10 ppm, or even is totally free of anti-wear and / or extreme-pressure additives of the amine phosphate salt type.

[0095] More preferably, a lubricating composition according to the invention comprises less than 200 ppm, in particular less than 100 ppm, especially less than 50 ppm and more particularly less than 10 ppm by mass, or is totally free of anti-wear and / or extreme-pressure phosphorus additives containing sulfur or zinc.

[0096] In fact, the inventors found that the presence of such phosphorus compounds containing sulfur or zinc is likely to induce the formation of unwanted deposits when using the turbine lubricant composition.

[0097] More preferably, a lubricating composition according to the invention comprises less than 200 ppm, in particular less than 100 ppm, especially less than 50 ppm and more particularly less than 10 ppm by mass, or is totally free of anti-wear and / or extreme-pressure phosphorus and / or sulfur additives.

[0098] In terms of formulating a lubricating composition according to the invention, the antioxidants considered according to the invention, phenyl-naphthylamine compounds, diphenylamine and phosphite ester, as described above, can be added to a base oil or mixture of base oils, and then the other complementary additives added.

[0099] Alternatively, the antioxidants considered according to the invention, phenyl-naphthylamine compounds, diphenylamine and phosphite ester, can be added to a pre-existing lubricating formulation, including in particular one or more base oils, and possibly additional additives.

[0100] Alternatively, the antioxidants considered according to the invention, phenyl-naphthylamine compounds, diphenylamine and phosphite ester, can be combined with one or more complementary additives, and the "package" of additives thus formed is added to a base oil or mixture of base oils.

[0101] Advantageously, the implementation of the three antioxidant compounds according to the invention, N-phenyl-naphthylamine compound(s), diphenylamine compound(s) and phosphite ester compound(s), makes it possible to avoid the implementation of anti-wear and / or extreme-pressure additives.

[0102] Thus, according to a particular embodiment, a lubricating composition according to the invention comprises less than 200 ppm, in particular less than 100 ppm, especially less than 50 ppm and more particularly less than 10 ppm by mass, or is totally free of anti-wear and / or extreme-pressure additives.

[0103] According to a particularly preferred embodiment, a lubricating composition according to the invention comprises, or is made up of: a base oil or mixture of base oils, preferably selected from Group II and III base oils, preferably from Group III; one or more phenylnaphthylamine (PAN) compounds, in particular alkylphenyl-α-naphthylamine, as defined above, preferably selected from the compounds of formula (Ia) above, in which R1 preferably represents an alkyl group in C2 to C12; one or more diphenylamine (DPA) compounds, in particular diakyldiphenylamine, as defined above, preferably selected from the compounds of formula (II-a) above, in which R2 and R3 preferably represent, independently of each other, a linear or branched alkyl group, preferably in C2 to C12;one or more phosphite ester compounds, selected from the compounds of formula (III-a) above, in particular where n is 2, and R represent groups, especially in ortho and para positions, preferably alkyl groups, preferably branched, in C3 to C6 positions, such as a tert-butyl group; the mass ratio of DPA compound(s) to phosphite ester(s) being between 2.0 and 3.0 and the mass ratio of PAN to phosphite ester(s) being between 2.1 and 3; and optionally one or more complementary additives, preferably selected from corrosion inhibitor additives, in particular from tolyltriazole derivatives, and alkylated organic acid esters; and antifoaming additives.

[0104] In particular, a lubricating composition according to the invention may comprise, or even be composed of: from 50% to 99.5% by weight, preferably from 70% to 99% by weight of base oil(s), preferably selected from base oils of group II and III, preferably from group III; from 0.05 to 5% by mass, in particular from 0.1 to 3% by mass, preferably from 0.15 to 1% by mass and more particularly from 0.2 to 0.5% by mass, of one or more phenyl-naphthylamine compounds, in particular alkylphenyl-α-naphthylamine, as defined above, preferably selected from compounds of formula (Ia) above, in which R 1 preferably represents an alkyl group at C 2 to C 12;from 0.05 to 5 wt%, in particular from 0.1 to 3 wt%, preferably from 0.15 to 1 wt% and more particularly from 0.2 to 0.5 wt%, of one or more diphenylamine compounds, in particular dialkyl-diphenylamine, as defined above, preferably chosen from the compounds of formula (II-a) above, in which R 2 and R 3 preferably represent, independently of each other, a linear or branched alkyl group, preferably at C 2 to C 12; and from 0.01 to 3 wt%, in particular from 0.02 to 1 wt%, preferably from 0.05 to 0.5 wt% and more particularly from 0.1 to 0.3 wt%, of one or more phosphite ester compounds, selected from the compounds of formula (III-a) above, in particular in which n is 2, and the Rs represent groups, especially in ortho and para positions, preferably alkyl groups, preferably branched, in C3 to C6, such as a tert-butyl group;possibly from 0.01 to 5% by mass, in particular from 0.1 to 3% by mass, and more particularly from 0.1 to 2% by mass, of corrosion inhibitor(s) additive(s), in particular of the tolyltriazole derivative and / or alkylated organic acid ester type; and possibly from 0.01% to 3% by weight of antifoaming additive(s), provided that the mass ratio of diphenylamine compound(s) to phosphite ester compound(s) is between 2.0 and 3.0 and the mass ratio of PAN to phosphite ester(s) is between 2.1 and 3, the contents being expressed in relation to the total mass of said lubricating composition.

[0105] A lubricating composition according to the invention may have a kinematic viscosity, measured at 40°C according to ISO 3104, between 20 mm² / s and 100 mm² / s, in particular between 25 mm² / s and 50 mm² / s.

[0106] A lubricating composition according to the invention advantageously has a viscosity index, measured according to ASTM D2270-93, between 100 and 300, in particular between 100 and 150.

[0107] In particular, it has an acid value, measured according to ASTM D664, of between 0.08 and 0.2.

[0108] As mentioned previously, a lubricating composition according to the invention exhibits excellent properties in terms of oxidation stability, thermal stability and corrosion resistance.

[0109] Advantageously, a lubricating composition according to the invention thus exhibits an increased service life.

[0110] More particularly, a lubricating composition according to the invention advantageously exhibits an oxidation stability "RPVOT", measured according to ASTM D2272, greater than or equal to 2200 minutes, advantageously greater than or equal to 2500 minutes.

[0111] Advantageously, it exhibits a residual RPVOT, measured according to the so-called "dry TOST - 1000 hours" test according to a method adapted from the ASTM D7873 standard, greater than or equal to 70%, advantageously greater than or equal to 75% and more particularly greater than or equal to 80%.

[0112] A lubricating composition according to the invention advantageously has an amount of insolubles after 48 hours at 180 °C of less than 30 mg / kg, preferably less than 25 mg / kg, more preferably less than or equal to 20 mg / kg.

[0113] It also exhibits excellent resistance to oxidation and corrosion, evaluated according to the ASTM D4636 test.

[0114] The invention will now be described by means of the following examples, given by way of illustration and not limitation of the invention. Example Measurement protocols Corrosion resistance assessment

[0115] Corrosion resistance can be quantified by measuring the corrosion of steel in the presence of seawater after 24 hours at 60°C, according to ISO 7120B.

[0116] This method is used to verify the ability of compositions to provide corrosion protection for ferrous metals in the presence of water. It consists of stirring a mixture of 300 ml of test oil with 30 ml of synthetic seawater at a temperature of 60 ± 1°C, in the presence of a cylindrical steel specimen immersed in the oil. After contact with the oil-water mixture, the specimen is examined to verify the presence or absence of rust. The test typically lasts 24 hours. Oxidation resistance evaluation - RPVOT (ASTM D2272)

[0117] A first method for measuring the oxidation stability of a composition is the RPVOT test (for "Rotating Pressure Vessel Oxidation Test" in English) carried out according to the ASTM D2272 standard.

[0118] This method involves placing a 50g sample of oil in a rotating chamber, under oxygen pressure, in the presence of water and a copper catalyst to assess its resistance to oxidation.

[0119] The conditions are as follows: Temperature: 150 °C; Oxygen pressure: 620 kPa; Rotation speed: 100 revolutions per minute; Rotation angle: 30 °.

[0120] The result expresses the lifespan of the tested oil, expressed in minutes.

[0121] The longer the oil's lifespan as determined, the greater its resistance to oxidation. Evaluation of oxidation resistance - residual RPVOT according to the "Dry TOST - 1000" test hours " (adapted from ASTM D7873)

[0122] A second method for measuring the oxidation stability of a composition consists of evaluating the residual RPVOT after artificial aging of the oil in accordance with the dry turbine oxidation stability test, known as "dry TOST - 1000 hours" for "dry Turbine Oxidative Stability Test" in English, adapted from the ASTM D7873 standard, to be carried out over a period of 1000 hours.

[0123] This method consists of heating a tube containing 360 mL of oil sample to 120 °C under oxygen and in the presence of an Fe / Cu catalyst, for a total duration of 1000 hours.

[0124] The tube is then collected for analysis by measuring RPVOT (ASTM D2272 method described above).

[0125] The residual RPVOT is calculated by dividing the RPVOT value of the oil that has undergone dry TOST aging - 1000 hours by that of the so-called "new" oil, i.e., having undergone no aging, and determined according to the first method defined above.

[0126] The higher the residual RPVOT, the more resistant the oil is to oxidation. High-temperature oxidation and corrosion stability (ASTM D 4636-procedure 25 and non-standard test)

[0127] This method of measuring oxidation and corrosion is particularly applicable to hydraulic oils, aviation turbine oils and more generally to oils subjected to high temperatures.

[0128] There is a standardized method for measuring the oxidation and corrosion of lubricating compositions according to ASTM D4636 - procedure 25.

[0129] The method consists of oxidizing a volume of 100 mL of oil for 72 hours at 175°C in the presence of an air flow of 5 L / h and metallic specimens of Copper, Steel, Aluminium, Magnesium and Cadmium.

[0130] Between the beginning and the end of the trial, the following are evaluated: the variation in kinematic viscosity at 40°C, the mass loss of the Cadmium specimen, and the mass loss of the magnesium specimen.

[0131] The smaller the variations thus evaluated, the better the oil's performance with respect to oxidation and corrosion.

[0132] The Applicant has also developed a method for quantifying the oxidation and corrosion of lubricating compositions at high temperatures.

[0133] This non-standard method involves oxidizing a 40 mL volume of oil at 175°C for 24 hours in the presence of a 5 L / h air flow and a copper and cadmium metallic specimen. The change in kinematic viscosity at 40°C is evaluated between the start and end of the test. Thermal stability

[0134] The thermal stability of an oil can be assessed by measuring the amount of insolubles after 48 hours at 180 °C.

[0135] The mass of insoluble matter in an oil is determined gravimetrically by filtering a 100 g sample through a membrane filter having a pore size of 0.45 µm.

[0136] The higher the amount of insolubles, the less thermally stable the oil. Example 1 Preparation of lubricating compositions.

[0137] A lubricating composition according to the invention ( I1), comprising a combination of additives required according to the invention, namely an alkylphenyl-α-naphthylamine, an alkyldiphenylamine and a phosphite ester with an alkyldiphenylamine / phosphite ester ratio strictly greater than 1.0, and comparative compositions ( C1 has C4 ), not including such a specific combination, have been formulated.

[0138] The lubricating compositions were prepared by simple mixing, at room temperature, of the following components, in the mass proportions indicated in Tables 2 and 3 below. TABLE 2 Invention Comparisons I1 C1 C2 C3 C4 Base oil (group III) 98,98 99,13 99,48 99,3 99,38 Alkyl phenylnaphthylamine 0,35 - 0,2 0,2 0,35 Octyl / butyl diphenylamine 0,35 0,35 0,2 0,1 - Tris(di-t-butylphenyl)phosphite 0,15 0,35 - 0,1 - succinate ester 0,05 0,05 0,05 0,05 0,15 Tolyltriazole 0,1 0,1 0,1 0,1 0,1 Polydimethylsiloxane 0,02 0,02 0,02 0,02 0,02

[0139] The physico-chemical characteristics of the compositions thus prepared are summarized in the following table 3. TABLE 3 Composition I1 C1 C2 C3 C4 Viscosity at 40 °C (mm² / s) (1) 31,94 31,95 35,80 31,61 35,81 VI (2)< 131 131 131 131 130 (1)< Kinematic viscosity measured at 40°C according to ISO 3104 (2)< Viscosity index measured according to ASTM D2270-93. Example 2 Evaluation of the properties of lubricant compositions

[0140] The various properties of the lubricating compositions prepared according to example 1 were evaluated in accordance with the measurement protocols detailed above.

[0141] The results are compiled in Table 4 below. TABLE 4 Composition I1 C1 C2 C3 C4 Corrosion resistance No rust No rust No rust No rust No rust RPVOT (mn) 2924 2155 1956 - 1748 residual RPVOT (%) 80 68 - 56 38 Variation of KV40 using the standardized method (%) 4,3 0,8 - 14,1 1,6 Variation of KV40 using non-standardized method (%) 1,5 - 23,43 - - Mass variation on Mg (mg) 0 0 - - 0,2 0 Mass variation on Cd (mg) 0 0 - - 2,2 0 Insoluble at 0.45 µm (mg / kg) 20 48 - 50 20

[0142] It is clear from these examples that the composition according to the invention (composition I1 ) combines excellent properties in terms of oxidation resistance, thermal stability and corrosion resistance at high temperatures, unlike compositions not comprising a combination of the three antioxidants specifically considered according to the invention ( C1 , C2 And C4), and / or not meeting the diphenylamine / phosphite ester mass ratio strictly greater than 1.0 (composition C3 ).

Claims

1. Lubricating composition, more particularly for a gas or steam turbine, comprising: - at least one base oil; - at least one phenylnaphthylamine (PAN), preferably alkylphenyl-α-naphthylamine (APAN), compound; - at least one diphenylamine (DPA), preferably dialkyldiphenylamine, compound; and - at least one phosphite ester compound of formula (III-a) below: in which the groups R independently of one another represent C1 to C10, more particularly C3 to C8, linear or branched alkyl groups and n independently at each occurrence represents 1 or 2; wherein the DPA compound / phosphite ester ratio by mass is between 2.0 and 3.0; and wherein the PAN / phosphite ester ratio by mass is between 2.1 and 3.

2. Lubricating composition according to the preceding claim, wherein said one or more phenylnaphthylamine compounds are of formula (I): in which: n is an integer from 1 to 5; and R1 represents a preferably C1 to C22, more particularly C2 to C12, linear or branched alkyl group; more particularly of formula (I-b) below: in which R1 is as defined above, R1 preferably representing a C2 to C12 alkyl group.

3. Lubricating composition according to any one of the preceding claims, wherein said one or more diphenylamine compounds are of formula (II): in which: n2 and n3 independently of one another are integers from 1 to 5; and R2 and R3 independently of one another represent a preferably C1 to C22, more particularly C4 to C8, linear or branched alkyl group.

4. Composition according to any one of the preceding claims, comprising: - from 0.05% to 5% by mass, more particularly from 0.1% to 3% by mass, preferably from 0.15% to 1% by mass, and very particularly from 0.2% to 0.5% by mass, of one or more phenylnaphthylamine, preferably alkylphenyl-α-naphthylamine, compounds, more particularly as defined in Claim 2; - from 0.05% to 5% by mass, more particularly from 0.1% to 3% by mass, preferably from 0.15% to 1% by mass, and very particularly from 0.2% to 0.5% by mass, of one or more diphenylamine, preferably dialkyldiphenylamine, compounds, more particularly as defined in Claim 3; and - from 0.01% to 3% by mass, more particularly from 0.02% to 1% by mass, preferably from 0.05% to 0.5% by mass, and very particularly from 0.1% to 0.3% by mass, of one or more phosphite ester compounds.

5. Composition according to any one of the preceding claims, further comprising at least one additive selected from antioxidants other than the compounds defined in Claims 1 to 3, viscosity index (VI) improvers, pour point depressants (PPD), antifoams, thickeners, corrosion inhibitors, copper passivators, and mixtures thereof.

6. Use of a composition as defined in any one of Claims 1 to 5 as a lubricant for a gas or steam turbine.

7. Use in a lubricating composition, more particularly for a gas or steam turbine, comprising at least one base oil, of at least one phenylnaphthylamine (PAN), preferably alkylphenyl-α-naphthylamine (APAN), compound; of at least one diphenylamine (DPA), preferably dialkyldiphenylamine, compound; and of at least one phosphite ester compound of formula (III-a) below: in which the groups R independently of one another represent C1 to C10, more particularly C3 to C8, linear or branched alkyl groups and n independently at each occurrence represents 1 or 2, and said one or more phosphite ester compounds being used in a DPA / phosphite ester ratio by mass of between 2.0 and 3.0 and a PAN / phosphite ester ratio by mass of between 2.1 and 3, for enhancing the heat stability performance and oxidation resistance performance of the composition.

8. Use according to the preceding claim, wherein said one or more phenylnaphthylamine compounds are as defined in Claim 2; and / or said one or more diphenylamine compounds are as defined in Claim 3.

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