Improved water-based lubricating fluids

Abstract

The present invention relates to a lubricating fluid, preferably water-based, comprising: a) a sulphur-containing compound or one of the salts thereof, more particularly homocysteine; b) a polyalkylene glycol; c) water; and d) optionally one or more functional additives. The present invention also relates to the use of this lubricating fluid for lubricating and / or cleaning mechanical devices, for metalwork, as a hydraulic fluid, or as a fluid for mining operations. The present invention also relates to a method for lubricating a metal or a composite, the method comprising bringing the metal or the composite into contact with the lubricating fluid.

Description

[0001] Description

[0002] Title: IMPROVED WATER-BASED LUBRICANTS

[0003] FIELD OF INVENTION

[0004] The present invention relates to a lubricating fluid, in particular a water-based fluid, comprising a sulfur additive in combination with at least one polyalkylene glycol. The present invention also relates to the use of this lubricating fluid, in particular a water-based fluid, for the lubrication and / or cleaning of mechanical devices, for metalworking, as a hydraulic fluid, or as a fluid for mining operations. Finally, the present invention relates to a method for lubricating a metal or composite comprising bringing said metal or composite into contact with said lubricating fluid.

[0005] TECHNICAL BACKGROUND

[0006] In many industrial processes, it is common practice to use fluids or greases to lubricate mechanical systems. These lubricating compositions help limit corrosion and wear of mechanical systems, thus keeping them in good condition for as long as possible. Lubricating fluids are therefore used in a variety of fields, including the metallurgical, mining, aerospace, and automotive industries.

[0007] They are particularly used in the metalworking industry to cool and lubricate metal parts during production and / or machining. In this context, they are commonly called metalworking fluids. They reduce heat and friction between the tool and the metal workpiece while preventing surface burn-through and fume generation. These "metalworking fluids" encompass all fluids used in the production, removal, shaping, hardening, and protection of metals. The use of these fluids also improves the quality of metal parts by continuously removing fine particles, chips, and metal filings from both the tool and the workpiece surface.They are therefore essential in this field and are categorized into four main classes: oils, emulsifiable oils, semi-synthetic fluids, and synthetic fluids. These classes are defined in particular in the ASTM D2881-19 standard, Standard Classification for Metalworking Fluids and Related Materials.

[0008] Straight oils typically consist of mineral oils and do not contain water. They may include functional additives and are not emulsifiable in the presence of water. Emulsifiable oils (also called soluble oils) typically contain at least 30% mineral oils by weight. They also include emulsifiers and other types of functional additives, and are then diluted in water to form a macroemulsion (with an average particle size greater than 1 µm).

[0009] Semi-synthetic fluids typically contain less than 50% mineral oils by weight and at least 20% water by weight. They include emulsifiers and other functional additives. When diluted in water, they generally form microemulsions (with an average particle size of less than 1 µm).

[0010] Finally, synthetic fluids generally do not contain mineral oil. Synthetic solution fluids include functional additives and form a solution when mixed with water (they form a single aqueous phase, without micelles).

[0011] Regarding these synthetic fluids, and more generally water-based lubricants, there are currently few extreme pressure and / or anti-wear additives compatible with their water content. Lubricating additives such as water-soluble polymers like polyalkylene glycols (PAGs) are therefore often used. To achieve sufficient tribological properties, their concentration in the fluids can be excessive due to the lack of effective water-soluble extreme pressure and / or anti-wear additives. Their presence can then lead to numerous problems.

[0012] Indeed, these polymers have surfactant properties, which is undesirable for certain applications. Furthermore, polymers such as PAGs can also leave sticky or gummy residues on the machined part and / or an unpleasant odor, making post-processing cleaning more complex. These sticky residues are relatively viscous and insoluble in water. They appear particularly after machining operations: once the equipment is shut down, the fluid can dehydrate and form salts which, combined with the polymers present, can create these sticky or gummy residues. Finally, they can cause compatibility issues with certain materials such as rubber seals.

[0013] There is therefore a need for improved water-based lubricating fluids, in particular those that do not have the disadvantages mentioned above and / or whose polyalkylene glycol content can be controlled according to user needs.

[0014] One objective of the present invention is to provide a lubricating fluid, preferably water-based, with satisfactory or even improved extreme-pressure and / or anti-wear properties. Another objective of the present invention is to provide a lubricating fluid, preferably water-based, with a controlled, preferably reduced, polyalkylene glycol content, while maintaining or even improving its lubricating properties.

[0015] Another objective of the present invention is to provide a lubricating fluid, preferably aqueous-based, in which the amount of polyalkylene glycols can be adapted to the end use.

[0016] SUMMARY OF THE INVENTION

[0017] The present invention meets, in whole or in part, the above objectives.

[0018] The inventors have made a surprising discovery: combining a sulfur compound, as defined below, with a polyalkylene glycol results in improved extreme-pressure and / or anti-wear protection, and more specifically, a synergistic effect. This effectiveness is remarkable, particularly for aqueous-based fluids, which generally have less favorable tribological properties than oil-based fluids.

[0019] This combination also allows for a reduction in the amount of polyalkylene glycol(s) present in the fluid, while maintaining similar or improved tribological properties. This reduction in polyalkylene glycol(s) helps to decrease the sticky residues that form during fluid use and / or to reduce the unpleasant odor of the resulting parts.

[0020] Lubricating fluids according to the invention may also exhibit satisfactory or even improved anti-corrosion properties, which is of particular interest for water-based fluids.

[0021] To determine the anti-wear and extreme-pressure properties of fluids, a person skilled in the art can refer to: ASTM D2783-21, which describes a test method for measuring the extreme-pressure (EP) properties of a lubricating fluid (Standard Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (Four-Ball Method)); or the Reichert Test, for example, as described in "Encyclopedia of Lubricants and Lubrication", 2014, pp. 2142-2144, which describes a test method for measuring the anti-wear properties of a lubricating fluid.

[0022] Thus, the present invention relates to a lubricating fluid comprising: a) a sulfur compound of the following general formula (I), or one of its salts:

[0023] R3-X-CH(NRiR2)-(CH2)n-(S)pZ (I) in which: - Z is a hydrogen atom or a -(CH2) group n -CH(NRiR2)-X-R3;

[0024] - Ri and R2, identical or different, are chosen from: a hydrogen atom or a hydrocarbon chain, saturated or unsaturated, linear, branched or cyclic, aromatic or not, of 1 to 20 carbon atom(s) and which may include one or more heteroatom(s);

[0025] - X is chosen from -C(=O)- , -CH2- or -CN;

[0026] - R3est:

[0027] (i) is zero when X represents -CN,

[0028] (ii) be a hydrogen atom,

[0029] (iii) either - OR a , R a being a hydrogen atom or a hydrocarbon chain, saturated or unsaturated, linear, branched or cyclic, aromatic or not, of 1 to 20 carbon atom(s) and which may include one or more heteroatom(s);

[0030] (iv) i.e. -NRbRc, Rb and Rc , identical or different, being chosen from: a) a hydrogen atom or a hydrocarbon chain, saturated or unsaturated, linear, branched or cyclic, aromatic or non-aromatic, of 1 to 20 carbon atom(s) and which may include one or more heteroatom(s); n is an integer equal to 1 or 2; and p is an integer between 1 and 8, preferably p is equal to 1 or 2; provided that if Z is a hydrogen atom, then p is equal to 1; b) a polyalkylene glycol; c) water; and d) possibly one or more functional additive(s).

[0031] The present invention also relates to the use of a lubricating fluid such as according to the invention, for the lubrication and / or cleaning of mechanical devices, for metalworking, as a hydraulic fluid, or as a fluid for mining.

[0032] The present invention also relates to the use of a sulfur compound of general formula (I) or one of its salts, in combination with a polyalkylene glycol, to improve the tribological properties (in particular extreme-pressure and / or anti-wear properties) of a lubricating fluid, preferably such as according to the invention.

[0033] The present invention relates to a method of lubricating a metal or a composite comprising a step of bringing said metal or composite into contact with a lubricating fluid as per the invention.

[0034] DETAILED DESCRIPTION

[0035] Other features, aspects, objects, and advantages of the present invention will become even clearer upon reading the following description. It is specified that the expressions "from ... to ..." and "between ... and ..." used in this description shall be understood as including each of the limits mentioned.

[0036] The fluid according to the invention is a lubricating fluid that can be used for the lubrication of metals or composites. More specifically, it can be used for the lubrication and / or cleaning of mechanical devices, for metalworking, as a hydraulic fluid, or as a mining fluid, for example, for drilling. The fluid according to the invention can thus be used in numerous industrial sectors, particularly the metallurgical and mining industries.

[0037] Preferably, the fluid is a metalworking fluid, and particularly a synthetic fluid (for example, as defined in ASTM D2881-19, Standard Classification for Metalworking Fluids and Related Materials). It can be used for metal removal processes such as boring, grinding, milling, or cutting; metal forming processes such as stamping, forming, or forging; or metal hardening processes.

[0038] The metals concerned are primarily iron and ferrous metals such as steel (e.g., stainless steel, carbon steel, or low-alloy carbon steel) or cast iron; non-ferrous metals such as copper, zinc, nickel, aluminum, magnesium, zirconium, cobalt, titanium, and their alloys such as brass. Among the composites, one example is nickel-cemented tungsten carbide.

[0039] The fluid according to the invention is preferably in the form of an aqueous solution.

[0040] The fluid can be used as is, as a base for a more complex formulation, or diluted in water before use (it can be described as a "concentrate" before dilution). Preferably, the fluid is diluted in water before use. For such dilution, the fluid (concentrate) can be diluted from 1 to 20% by weight, and preferably from 5 to 10% by weight, in water, relative to the total weight of the diluted fluid.

[0041] In particular, the said fluid is basic, i.e., with a pH strictly greater than 7, preferably between 8 and 12.

[0042] The lubricating fluid according to the invention comprises a sulfur compound of the following general formula (I) or one of its salts:

[0043] R3-X-CH(NRiR2)-(CH2)n-(S)pZ (I) in which:

[0044] - Z is a hydrogen atom or a -(CH2) group n -CH(NRiR2)-X-R3; - Ri and R2, identical or different, are chosen from: a hydrogen atom or a hydrocarbon chain, saturated or unsaturated, linear, branched or cyclic, aromatic or not, of 1 to 20 carbon atom(s) and which may include one or more heteroatom(s);

[0045] X is chosen from -C(=O)- , -CH2- or -CN ; - R3est :

[0046] (i) is zero when X represents -CN,

[0047] (ii) be a hydrogen atom,

[0048] (iii) either - OR a , R a being a hydrogen atom or a hydrocarbon chain, saturated or unsaturated, linear, branched or cyclic, aromatic or not, of 1 to 20 carbon atom(s) and which may include one or more heteroatom(s);

[0049] (iv) i.e. -NRbRc, Rb and R c, identical or different, being chosen from: a hydrogen atom or a hydrocarbon chain, saturated or unsaturated, linear, branched or cyclic, aromatic or non-aromatic, of 1 to 20 carbon atom(s) and which may include one or more heteroatom(s); - n is an integer equal to 1 or 2; and p is an integer between 1 and 8, preferably p is equal to 1 or 2; provided that if Z is a hydrogen atom, then p is equal to 1.

[0050] When Z is a group -(CH2) n -CH(NRiR2)-X-R3, it is understood that n and the different radicals X, Ri, R2 and R3 are respectively identical in the general formula (I): we obtain symmetrical sulfides, disulfides or polysulfides (cf for example cystine and homocystine).

[0051] Heteroatom(s) include, in particular, oxygen, nitrogen and sulfur.

[0052] In particular, Ri and R2 are hydrogen atoms. In particular, n is equal to 2.

[0053] In particular, p is an integer between 1 and 5, preferably p is equal to 1 or 2 or 3.

[0054] Preferably, the said sulfur compound has the following general formula:

[0055] R3-X-CH(NRiR2)-(CH2) n -SH (la) in which Ri, R2, R3, X and n are as defined above.

[0056] We particularly prefer compounds with the general formula (I) or (la), for which X is -C(=O)- and R3 is -OR a , with R a as defined above. R a is preferentially H.

[0057] Preferred compounds include: cysteine, with the formula HS-CH2-CH(NH2)-COOH (CAS 52-90-4 for the L form and CAS 921-01-7 for the D form); cystine, with the formula HOOC-CH(NH2)-CH2-SS-CH2-CH(NH2)-COOH (CAS 56-89-3 for the L form and CAS 349-46-2 for the D form); homocysteine, with the formula HS-(CH2)2-CH(NH2)-COOH (CAS 6027-13-0 for the L form and CAS 6027-14-1 for the D form); homocysteine, with the formula HOOC-CH(NH2)-(CH2)2-SS-(CH2)2-CH(NH2)-COOH (CAS 626-72-2 for the L form and CAS 870-93-9 for the D form); dicysteine ​​trisulfide (HOOC-CH(NH2)-CH2-SSS-CH2-CH(NH2)-COOH), dicysteine ​​tetrasulfide, dicysteine ​​pentasulfide; and dihomocysteine ​​trisulfide (HOOC-CH(NH2)-(CH2)2-SSS-(CH2)2-CH(NH2)-COOH), dihomocysteine ​​tetrasulfide, dihomocysteine ​​pentasulfide.

[0058] The sulfur compounds particularly preferred according to the invention are homocysteine, homocystine, dihomocysteine ​​trisulfide, dihomocysteine ​​tetrasulfide, and dihomocysteine ​​pentasulfide. Homocysteine ​​is a particularly preferred sulfur compound.

[0059] When reference is made in this description to sulfur compounds of general formula (I), the salts of these compounds are included in that expression. When a salt of a sulfur compound of general formula (I) is used, it is preferably a basic salt. In particular, said salt is a metallic salt or a quaternary ammonium salt. Preferably, said salt is selected from alkali metal salts, alkaline earth metal salts, and quaternary ammonium salts.

[0060] The term "metals" used to obtain said metallic salt refers in particular to the metals in columns 1 to 5 (formerly IA to VA) of the periodic table of elements, as well as aluminum, tin, and lead. Specifically, said metal is chosen from the group consisting of: alkali metals, alkaline earth metals, scandium, titanium, zirconium, vanadium, niobium, aluminum, tin, and lead.

[0061] The term "alkali metals" refers to the metals corresponding to group 1 (1 ère column) of the periodic table of elements. Lithium, sodium, and potassium are particularly noteworthy examples. The term "alkaline earth metals" refers to the metals corresponding to group 2 (2 ème column) of the periodic table of elements. Magnesium, calcium, strontium and barium are particularly noteworthy.

[0062] By "quaternary ammoniums", we mean in particular ammoniums of the following general formula (III):

[0063] + N(R5R6R7R8) (III) in which the radicals R5, Re, R? and R8, identical or different, are chosen independently of each other from the group consisting of: the hydrogen atom, a (Ci-Cio)alkyl, a (C6-Cio)aryl, a (Ci-Cio)alkoxy, a polyalkylalkoxy, a (Ci-Cio)alcohol and a polyol; preferably from the hydrogen atom, a (Ci-Cio)alkyl and a (Ci-Cio)alcohol. The ammonium of monoethanolamine ( + NH3(CH2CH2OH)), diethanolamine, triethanolamine and methyldiethanolamine.

[0064] In particular, said basic salt is chosen from among the monoalkaline, dialkaline, alkaline-earth or quaternary ammonium salts of homocysteine ​​or homocystin, preferably from among the dialkaline or alkaline-earth salts of homocysteine ​​or homocystin.

[0065] The basic salt in question is chosen from the following group: monosodium homocysteine, disodium homocysteine; monosodium cysteine, disodium cysteine, disodium cystine, disodium homocystine; monopotassium homocysteine, dipotassium homocysteine; monopotassium cysteine, dipotassium cysteine, dipotassium cystine, and dipotassium homocystine. Disodium or dipotassium homocysteine ​​and disodium or dipotassium homocystine are particularly preferred.

[0066] This sulfur compound can exist in any enantiomeric form, of type L, D, or in racemic form. The L form is particularly preferred, as it is found in nature.

[0067] The sulfur compound with general formula (I) is usually in solid form. For example, cysteine ​​may be commercially available (marketed by Wacker or Showa Denko). Homocysteine ​​may be obtained by the process as described in application WO 2022 / 117951, in particular according to Example 1.

[0068] In the lubricating fluid according to the invention, the quantity of said sulfur compound of general formula (I) may be between 0.1 and 30% by weight, preferably between 0.1 and 20% by weight, and more preferably between 1 and 7% by weight, for example between 1 and 5% by weight relative to the total weight of the fluid. It is understood that said fluid may comprise several sulfur compounds as defined above. In this case, the total quantity of these sulfur compounds may be between 0.1 and 30% by weight, preferably between 0.1 and 20% by weight, and more preferably between 1 and 7% by weight, for example between 1 and 5% by weight relative to the total weight of said fluid.

[0069] Polyalkylene glycol (PAG):

[0070] The fluid according to the invention comprises at least one polyalkylene glycol, in particular a single polyalkylene glycol.

[0071] Polyalkylene glycols are homo- or co-polymers formed from alkylene oxide units. In particular, they can be formed from alkylene oxide units comprising between 1 and 8 carbon atoms, preferably between 2 and 4 carbon atoms.

[0072] Preferably, said PAG is selected from: polyethylene glycol (PEG), polypropylene glycol (PPG), polybutylene glycol, copolymers comprising units of ethylene oxide (EO) and / or propylene oxide (PO) and / or butylene oxide, and mixtures thereof. More preferably, said PAG is selected from: polyethylene glycol (PEG), polypropylene glycol (PPG), and copolymers comprising units of ethylene oxide (EO) and propylene oxide (PO).

[0073] In a most preferred mode, said PAG is a copolymer comprising ethylene oxide (EO) and propylene oxide (PO) units. In particular, said PAG is a copolymer consisting of ethylene oxide (EO) and propylene oxide (PO) units.

[0074] The exact structure of the EO / PO copolymer PAG can vary depending on the sequence and proportion of the ethylene oxide and propylene oxide units. For example, a PAG can be a block copolymer (with distinct EO and PO segments), a statistical copolymer (with randomly alternating EO and PO units), or a grafted copolymer (with EO or PO side chains grafted onto the main chain). Preferably, the PAG according to the invention is an EO / PO block copolymer or a statistical EO / PO copolymer. Examples include Clariant's Genapol PS statistical EO / PO copolymer (CAS No. 58205-99-5) and the Genapol RPF1740 block copolymer, also marketed by Clariant (CAS No. 9003-11-6).

[0075] These block EO / PO copolymers can consist of a central PO block with an EO chain at each end, or a central EO block with a PO chain at each end. They can also be four-block copolymers derived from the sequential addition of EO and PO to ethylenediamine.

[0076] PAGs can have different chain terminations, such as hydroxyl groups (-OH), which can influence their properties. For example, a PAG with hydroxyl terminations is often called a poly(alkylene glycol)diol.

[0077] They are in particular chosen from water-soluble polyalkylene glycols. By "water-soluble polyalkylene glycol", we mean in particular a polyalkylene glycol having a solubility in water of at least 10 g / L, preferably of at least 500 g / L, at room temperature (approximately 25 °C).

[0078] In particular, said PAG has an average molar mass (Mn) between 100 and 25,000 g / mol -1 preferably between 1,000 and 20,000 g / mol -1 preferably still between 1,800 and 18,000 g / mol -1 The number-average molar mass (Mn) can be measured by gel permeation chromatography (GPC).

[0079] In the lubricating fluid according to the invention, the quantity of said polyalkylene glycol may be between 0.1 and 40% by weight, preferably between 1 and 35% by weight, preferably again between 1 and 30% by weight relative to the total weight of the fluid.

[0080] It is understood that the quantity of polyalkylene glycol may vary depending on the use of the fluid according to the invention. Thus, for a synthetic fluid (used for metalworking), the quantity of said polyalkylene glycol may be between 0.1 and 15% by weight, preferably between 0.1 and 10% by weight, and more preferably between 1 and 5% by weight, for example between 2 and 4% by weight, relative to the total weight of the fluid. For a hydraulic fluid, the quantity of said polyalkylene glycol may be between 0.1 and 50% by weight, preferably between 1 and 40% by weight, and more preferably between 5 and 30% by weight, for example between 15 and 30% by weight, relative to the total weight of the fluid.

[0081] It is also understood that the fluid in question may comprise several polyalkylene glycols as defined above. In this case, the total quantity of these polyalkylene glycols may correspond to the quantities mentioned above.

[0082] The said fluid may comprise a lesser quantity of polyalkylene glycol, compared to the same fluid not comprising a sulfur compound as defined above (and in particular retaining or improving its lubricating properties, in particular extreme pressure and / or anti-wear).

[0083] Such polyalkylene glycols may be commercially available or synthesized using methods known to those skilled in the art.

[0084] Preferably, the molar ratio [sulfur compound of general formula (I) / Polyalkylene glycol(s)] is between 0.025 and 20,000, more preferably between 0.1 and 1000.

[0085] Water:

[0086] The lubricating fluid according to the invention is, in particular, an aqueous fluid (i.e., a fluid whose major solvent is water). Preferably, said fluid is an aqueous solution (in particular, said fluid is not an emulsion). More specifically, the various components of the fluid are soluble and / or miscible in water.

[0087] The water used can be more or less hard (hard water contains dissolved calcium and magnesium salts). Preferably, water with a hardness level between 0 and 40°f or higher is used. Demineralized or deionized water is particularly suitable.

[0088] In particular, the amount of water in the fluid according to the invention is at least 20% by weight, preferably at least 30% by weight, relative to the total weight of said fluid. More specifically, the amount of water is between 35% and 90% by weight, more preferably between 40% and 75% by weight, relative to the total weight of said fluid. This includes, in particular, the amount of water in a concentrated fluid, i.e., before final dilution with water prior to use.

[0089] In particular, the fluid comprises less than 5% by weight, preferably less than 1% by weight, of water-insoluble oil(s), relative to the total weight of the fluid. More specifically, it does not comprise any water-insoluble oil.

[0090] The term "water-insoluble oil" refers, in particular, to an oil that does not substantially dissolve in water at room temperature (approximately 25°C). More specifically, a water-insoluble oil has a solubility in water of less than 0.2 g / L at room temperature (approximately 25°C).

[0091] The term "water-insoluble oil" refers specifically to oils belonging to API (American Petroleum Institute) classes I through V. The API classification for lubricating base oils can be found on the NLGI (National Association of Lubricating Grease Manufacturers, Inc.) website: https: / / www.nlqi.org / qrease-qlossarv / api-qroups-iv / . Specifically, mineral oils belong to groups I through III, while synthetic oils belong to groups IV and V. Preferably, the fluid does not contain any oil belonging to groups I through III, and more preferably, to groups I through V. In particular, the fluid does not contain any oil.

[0092] Other possible functional additives:

[0093] The lubricating fluid according to the invention may also comprise one or more other functional additives known to those skilled in the art, in particular selected from among alkali reserve agents, corrosion inhibitors and metal passivating agents, sequestering agents, anti-wear and / or extreme-pressure additives, biocides, anti-foaming additives such as polysiloxanes or acrylate polymers, and antifreeze additives. It is understood that these other functional additives are different from the sulfur compounds of general formula (I) according to the invention. It is also understood that the additives mentioned below may be multifunctional.

[0094] In particular, the total quantity of other functional additives is between 1 and 40%, preferably between 1 and 30%, preferably again between 1 and 20%, by weight relative to the total weight of the fluid.

[0095] The following functional additives can be cited in particular.

[0096] - Bases acting as alkaline reserve agents

[0097] The lubricating fluid according to the invention may comprise at least one base as an alkaline reserve agent. The following are used as alkaline reserve agents: alkanolamines, aminomethylpropanol (AMP-95), diglycolamine (DGA), monoethanolamine (MEA), monoisopropanolamine (MIPA), butylethanolamine (NBEA), dicyclohexylamine (DCHA), diethanolamine (DEA), butyldiethanolamine (NBDEA), triethanolamine (TEA), diisopropanolamine (DIPA), dibutylethanolamine, triisopropanolamine (TIPA), ethylene amines such as ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA) and tetraethylenepentamine (TEPA), methyldiethanolamine (MDEA), methyldiisopropanolamine, dimethylethanolamine, dimethylisopropanolamine, cyclamines such as cyclohexylamine, dicyclohexylamine, 2-amino-2-ethyl-1,3-propanediol, 2-amino-2-methyl-1-propanol, dimethylaminoethoxyethanol,2-(2-aminoethoxy)ethanol, alkali and alkaline earth metal hydroxides (such as lithium, sodium, potassium, or magnesium hydroxides), metal carbonates and bicarbonates (such as sodium or potassium carbonate or bicarbonate), and mixtures thereof. Alkanolamines are preferred.

[0098] The total quantity of base(s) may be between 0.1 and 25%, preferably between 0.1 and 15%, by weight relative to the total weight of the fluid.

[0099] - Corrosion inhibitors and metal passivating agents

[0100] The lubricating fluid according to the invention may include at least one anti-corrosion agent, in particular one that passesivates metals. Metal passivating agents protect metal parts by promoting the formation of metal oxide(s) on their surface.

[0101] Phosphates (especially alkali metal or alkaline earth phosphates), nitrites (especially alkali nitrites), carbonates (especially alkali carbonates), molybdates, boric acids and their salts, sulfonates, sulfates, sulfites, silicates, tolytriazole and its salts, benzotriazoles and their salts, imidazolines and their salts, alkanolamines and amides, alkali and alkanolamine salts of naphthenic acids, amine salts of phosphate esters, carboxylic acids and their salts, alkylsulfonamide carboxylic acids and their salts, arylsulfonamide carboxylic acids and their salts, phenoxy derivatives, alkoxylated amines, and tertiary polyamines such as... pentamethyldipropyltriamine and their salts, alkyl polyalkylene glycol ether phosphate salts and their mixtures.

[0102] In particular, the fluid according to the invention comprises at least one carboxylic acid or one of its salts (in particular selected from alkali salts), as a corrosion inhibitor.

[0103] The carboxylic acids according to the invention can be linear or branched, saturated or unsaturated. In particular, they comprise between 4 and 36 carbon atoms. Examples include the following acids: itaconic acid, citric acid, sebacic acid, caproic / hexanoic acid, enanthic / heptanoic acid, caprylic / octanoic acid, pelargonic / nonionic acid, isononanoic acid, capric / decanoic acid, neodecanoic acid, lauric / dodecanoic acid, stearic / octadecanoic acid, arachidic / eicosanoic acid, palmitic / hexadecanoic acid, erucic acid, oleic acid, arachidonic acid, linoleic acid, linolenic acid, myristic / tetradecanoic acid, behenic / docosanoic acid, alpha-linolenic acid, docosahexaenoic acid, ricinoleic acid, butyric acid, and their salts. Preferably, an aliphatic carboxylic acid or one of its salts is used, and more preferably a monocarboxylic or dicarboxylic acid.In particular, said aliphatic carboxylic acid comprises between 4 and 15 carbon atoms. More specifically, said carboxylic acid is selected from sebacic acid, dodecanoic acid, itaconic acid, and one of their salts.

[0104] One can also choose a corrosion inhibitor known as a metal passivating agent, for example, from triazole derivatives such as tetrahydrobenzotriazole (THBTZ), tolyltryazole (TTZ), benzotriazole (BTZ), or amines substituted with a triazole group, such as N,N-bis(2-ethylhexyl)-1,2,4-triazol-1-yl methanamine, N'-bis(2-ethylhexyl)-4-methyl-1H-benzotriazol-1-methylamine, N,N-bis(heptyl)-ar-methyl-1H-benzotriazol-1-methanamine, N,N-bis(nonyl)-ar-methyl-1H-benzotriazol-1-methanamine, N,N-bis(decyl)-ar-methyl-1H-benzotriazol-1-methanamine, N,N-bis(undecyl)-ar-methyl-1 H-benzotriazole-1-methanamine, N,N-bis(dodecyl)-ar-methyl-1H-benzotriazole-1-methanamine, N,N-bis(2-ethylhexyl)-ar-methyl-1H-benzotriazole-1-methanamine, 1,2,4-triazoles, benzimidazoles, 2-alkyldithiobenzimidazoles, 2-alkyldithiobenzothiazoles, 2-(N,N-dialkyldithiocarbamoyl)benzothiazoles, 2,5-bis(alkyldithio)-1,3,4-thiadiazoles,such as 2,5-bis(tert-octyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-nonyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-decyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-undecyldithio)-1 ,3,4-thiadiazole, 2,5-bis(tert-dodecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-tridecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-tetradecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-pentadecyldithio)-1 3,4-thiadiazole, 2,5-bis(tert-hexadecyldithio)-1, 3,4-thiadiazole, 2,5-bis(tert-heptadecyldithio)-1, 3,4-thiadiazole, 2,5-bis(tert-octadecyldithio)-1, 3,4-thiadiazole, 2,5-bis(tert-nonadecyldithio)-1, 3,4-thiadiazole, 2,5-bis(tert-eicosyldithio)-1, 3,4-thiadiazole, 2,5-bis(N,N-dialkyldithiocarbamoyl)-1, 3,4-thiadiazoles, 2-alkyldithio-5-mercaptothiadiazoles, and mixtures thereof.

[0105] Preferably, the metal passivating agents are chosen from tetrahydrobenzotriazole (THBTZ), tolyltriazole (TTZ), benzotriazole (BTZ), and their salts, taken alone or in mixtures.

[0106] In particular, said anti-corrosion agent is selected from sebacic acid, dodecanoic acid, itaconic acid, tetrahydrobenzotriazole (THBTZ), tolyltriazole (TTZ), benzotriazole (BTZ), and their salts, taken alone or in mixtures.

[0107] The lubricating fluid according to the invention may, in particular, comprise from 0.5 to 25% by weight of corrosion inhibitor(s), preferably from 1 to 15% by weight, more preferably from 2 to 10% by weight of corrosion inhibitor(s), relative to the total weight of said fluid. - Sequestering agents

[0108] The lubricating fluid according to the invention may comprise at least one sequestering agent. Sequestering agents, also called chelating agents, limit the accumulation of metal ions in the fluid.

[0109] Examples of sequestering agents include those derived from phosphonic acids and phosphonates, such as diethylenetriaminepentamethylphosphonic acid (DTPMPA), amino tri(methylene phosphonic) acid (ATMP), hydroxyethane diphosphonic acid (HEDP), 1,1-hydroxyethylidene diphosphonate, 2-hydroxyethylamine di(methylene phosphonic) acid (HEAMBP), diethylene triamino penta(methylene phosphonic) acid (DTMP), multifunctional organic acids and hydroxylated acids, such as ethylenediaminetetraacetic acid (EDTA), pteroyl-L-glutamic acid (PGLU), organic polyacids, such as maleic acid and polyaspartic acid, and carbohydrates, such as inulin, carboxymethylinulin, and carboxymethylchitosan.

[0110] In particular, the lubricating fluid according to the invention may comprise from 0.001 to 2% by weight of sequestering agent(s), preferably from 0.01 to 1.0% by weight of sequestering agent(s), relative to the total weight of said fluid.

[0111] - Anti-wear additives and Zou extreme pressure:

[0112] The lubricating fluid according to the invention may include at least one anti-wear and / or extreme pressure additive.

[0113] Organosulfur, organophosphorus and / or organophosphosulfur compounds can be used as anti-wear and / or extreme pressure agents, in addition to sulfur compounds of general formula (I) according to the invention.

[0114] Sulfur compounds can be selected from polysulfides, including sulfur olefins, alkyl or alkylphenol polysulfides, water-soluble salts of sulfonic acids, sulfur fatty acids, and esters of sulfur fatty acids, preferably in their neutralized, water-emulsifiable form. If necessary, their neutralized form can be obtained with an alkalizing agent, such as sodium hydroxide, potassium hydroxide, or an alkanolamine, such as monoethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, and triisopropanolamine.

[0115] Sulfur fatty acids can comprise from 8 to 22 carbon atoms, preferably from 12 to 18 carbon atoms.

[0116] Phosphorus compounds may be selected from among amine phosphates; phosphates such as phosphate esters, in particular alkyl, alkenyl or aryl phosphates; polyphosphates; phosphonates such as phosphonate esters, in particular alkyl, alkenyl or aryl phosphonates; polyphosphonates; carbamyl phosphates; phosphinates; and mixtures thereof.

[0117] Phosphorus compounds are preferably present in a lubricating fluid according to the invention in the form of salts, particularly as phosphate, phosphonate, or phosphinate ions, neutralized by a suitable counter-ion. The salts may be salts of alkali or alkaline earth metals, ammonium, alkanolamines, particularly C2-C8, or alkane amines, particularly C2-C8, phosphate, phosphonate, or phosphinate.

[0118] Examples of compounds particularly suitable for the invention are phosphate esters, phosphonate esters and their salts, preferably phosphate esters and their salts.

[0119] Phosphate esters, also known as phosphoric acid esters, can be obtained by reacting at least one alcohol, linear or branched, ethoxylated or not, preferably at least one alcohol having between 1 and 12 carbon atoms, more preferably between 2 and 8 carbon atoms, with phosphorus pentoxide of formula P₂O₅ or with phosphoric acid. Preferably, a phosphorus compound used in a fluid according to the invention is selected from among the salts of phosphate esters, in particular the alkali metal salts of phosphate esters, especially C1-C8 alkyl or dialkyl phosphate esters.

[0120] Examples of phosphosulfur compounds include thiophosphates, particularly metal alkylthiophosphates, thiophosphites, including thiophosphoric and thiophosphorous acids, esters of these acids, their salts, dithiophosphites, and dithiophosphates. Examples of such compounds include amine dithiophosphates, monobutylthiophosphate, monooctylthiophosphate, monolaurylthiophosphate, dibutylthiophosphate, dilaurylthiophosphate, tributylthiophosphate, trioctylthiophosphate, triphenylthiophosphate, trilaurylthiophosphate, monobutylthiophosphite, monooctylthiophosphite, monolaurylthiophosphite, dibutylthiophosphite, dilaurylthiophosphite, tributylthiophosphite, trioctylthiophosphite, triphenylthiophosphite, trilaurylthiophosphite, and their salts.

[0121] Examples of salts of thiophosphoric acid and thiophosphorous acid esters are those obtained by reaction with a nitrogenous compound such as ammonia or an amine or zinc oxide or zinc chloride.

[0122] Their total quantity can be between 0.1 and 10%, preferably between 0.1 and 5%, by weight relative to the total weight of the fluid.

[0123] According to one embodiment, said fluid comprises only one or more sulfur compounds of general formula (I) according to the invention as an anti-wear and / or extreme pressure additive. - Biocides

[0124] The fluid may also contain one or more biocides with fungicidal and / or bactericidal action. Such biocides may be chosen from parabens, aldehydes, reactive acetylacetone compounds, isothiazolinones, triazines, phenolic compounds, acid salts, halogenated compounds, quaternary ammonium compounds, alcohols, and mixtures thereof.

[0125] For example, the biocide(s) may be chosen from among tetrakis Hydroxymethyl Phosphonium Sulfate (THPS), possibly substituted benzisothiazolinones (BIT), such as N-butyl-1,2-benzisothiazolin-3-one, methylisothiazolinones (MIT), mixtures of methylisothiazolinone and chloromethylisothiazolinone (MIT / CMIT), orthophenyl-phenol (OPP) or its sodium salt, 3-iodo-2-propynylbutylcarbamate (IPBC), chloro-cresol and N,N-methylene-bis-morpholine (MBM), sorbic acid and mixtures thereof. More specifically, the biocide(s) may be chosen from among triazines, orthophenylphenol (OPP) or its sodium salt, 3-iodo-2-propynylbutylcarbamate, chlorocresol, benzisothiazolinones and N,N-methyleneisomorpholine, preferably from among the triazines.

[0126] For example, the total quantity of biocide(s) may be between 0.1 and 10%, preferably between 0.1 and 5% by weight, relative to the total weight of the fluid.

[0127] - Antifreeze additives

[0128] Antifreeze additives can be chosen from glycols, glycerol, diglycerol, triglycerol, and mixtures thereof.

[0129] Glycols are diols in which the two hydroxyl groups are borne by different carbon atoms, preferably by vicinal carbon atoms.

[0130] Preferably, glycols are alkylene glycols, particularly those with 2 to 10 carbon atoms, and especially those with 2 to 6 carbon atoms. Monoethylene glycol and propylene glycol are examples.

[0131] In particular, the lubricating fluid according to the invention does not comprise chlorinated compounds or, more generally, halogenated compounds as extreme-pressure and / or anti-wear additives. In one embodiment, said fluid does not comprise phosphates, borates, phosphonates, phosphites, or hypophosphites. In another embodiment, said fluid does not comprise any extreme-pressure and / or anti-wear additive other than the sulfur compound(s) of general formula (I).

[0132] In particular, the fluid according to the invention does not comprise a long-chain aliphatic amine and / or does not comprise a tertiary cycloalkylamine. Specifically, said fluid does not comprise a primary C8-C24 amine and / or does not comprise ethoxylated tertiary cycloalkylamine.

[0133] A preferred fluid according to the invention comprises (or is made up of):

[0134] - between 1 and 15% by weight of a sulfur compound of general formula (I) as defined above, relative to the total weight of said fluid;

[0135] - between 1 and 10% by weight of a polyalkylene glycol as defined above, relative to the total weight of said fluid;

[0136] - between 1 and 15% by weight of functional additive(s), relative to the total weight of said fluid; and the remainder in water.

[0137] Another preferred fluid according to the invention comprises (or is made up of):

[0138] - between 1 and 15% by weight of a sulfur compound of general formula (I) as defined above, relative to the total weight of said fluid;

[0139] - between 15 and 40% by weight of a polyalkylene glycol as defined above, relative to the total weight of said fluid;

[0140] - between 1 and 15% by weight of functional additive(s), relative to the total weight of said fluid; and the remainder in water.

[0141] The fluids according to the invention can be prepared by methods conventional to a person skilled in the art, in particular by mixing the different components.

[0142] Description of the figures

[0143] Figure 1 represents the experimental setup used for the Reichert test.

[0144] Figure 2 represents the noise index obtained for four different fluid formulations.

[0145] The following examples are given for illustrative purposes only and are not limiting to the present invention.

[0146] EXAMPLES

[0147] Materials used for the examples

[0148] Polyethylene glycol having a molar mass of 2000 g / mol (Mn), hereinafter referred to as PEG 2000, marketed by Sigma-Aldrich.

[0149] GENAPOL PS marketed by Clariant (statistical copolymer EO / PO, having a molar mass of 15,000 g / mol).

[0150] GENAPOL RPF1740 marketed by Clariant (EO / PO block copolymer, having a molar mass of 2300 g / mol).

[0151] L-Homocysteine.

[0152] Sebacic acid, marketed by Sigma-Aldrich.

[0153] Monoethanolamine, marketed by Sigma-Aldrich.

[0154] Example 1: Improved anti-wear effect of a lubricating fluid according to the invention

[0155] We tested the lubricating properties of different water-based fluids.

[0156] A) Preparation of lubricating fluids

[0157] The following four lubricant fluid formulations were prepared:

[0158] 1- A "reference" formulation without extreme pressure / anti-wear and lubrication additives, which corresponds to:

[0159] - 5% by weight of sebacic acid (corrosion inhibitor);

[0160] - 5% by weight of monoethanolamine (alkaline reserve); and

[0161] - the remaining demineralized water.

[0162] 2- A "5% PEG 2000" formulation, which contains:

[0163] - 5% by weight of PEG 2000,

[0164] - 5% by weight of sebacic acid (corrosion inhibitor),

[0165] - 5% by weight of monoethanolamine (alkaline reserve), and

[0166] - the remaining demineralized water.

[0167] 3- A "5% HCY" formulation, which contains:

[0168] - 5% by weight of homocysteine ​​disodium (HCY),

[0169] - 5% by weight of sebacic acid (corrosion inhibitor),

[0170] - 5% by weight of monoethanolamine (alkaline reserve), and

[0171] - the remaining demineralized water. 4-A formulation "5% PEG 2000 + 5% HOY", which contains:

[0172] - 5% by weight of PEG 2000,

[0173] - 5% by weight of homocysteine ​​disodium,

[0174] - 5% by weight of sebacic acid (corrosion inhibitor),

[0175] - 5% by weight of monoethanolamine (alkaline reserve), and

[0176] - the remaining demineralized water.

[0177] For formulations containing homocysteine, the preferred order of addition of components is as follows: homocysteine, then

[0178] - demineralized water, then

[0179] - sodium hydroxide to obtain the disodium salt of homocysteine, then

[0180] - sebacic acid, then

[0181] - possibly PAG, and monoethanolamine.

[0182] The percentages are expressed as a percentage of the total weight of each formulation.

[0183] Finally, the four formulations were each diluted to 10% by weight in water to obtain the final aqueous fluids.

[0184] B) Reichert Test

[0185] The Reichert test is a test used to determine the anti-wear properties of the lubricating fluids prepared above.

[0186] B. 1) Principle of the test

[0187] A tightly clamped test roller is pressed, by a double lever system, against a rough rotating collector ring ("grinding ring"), the lower third of which is immersed in the fluid whose load capacity is being tested.

[0188] The rotational speed of the drive motor and that of the slip ring which rotates directly with the motor are determined so that a sufficient quantity of fluid can always enter the point of contact (friction wear point) between the test roller and the test ring.

[0189] As the slip ring rotates, abrasion zones (elliptical wear marks) are produced on the test roller – their size depends on the test roller's load capacity. The smaller this wear mark is after 100m of travel, the greater the load capacity (specific pressure).

[0190] Furthermore, a grinding noise is audible during the test operation. This noise level is also recorded (the distance at which the noise is no longer audible is determined).

[0191] This test is explained in Encyclopedia of Lubricants and Lubrication, 2014, p.2142-2144 and Figure 1 shows the experimental setup used.

[0192] B.2) Test conditions

[0193] The measuring ring and roller are made of hardened steel of different hardnesses (ring: 60 HRC hardness and roller: 62 HRC hardness).

[0194] A mass equivalent to 1500 gf (gram-force) is placed on the lever located on the left side of the device. The circular speed of the ring is 1.70 ms -1 , thus the test distance of 100m is reached after about 1 minute.

[0195] Fluid quantity: 30 g

[0196] Test mechanism: ring and roller, axes intersecting perpendicularly

[0197] Sliding speed: 1.7 m / sec

[0198] Test duration: 100m run (approximately 1 min.)

[0199] Ring and roller material: 100 CR6 steel

[0200] Normal load: 294.3 N

[0201] Type of friction: Sliding friction

[0202] B.3) Results:

[0203] The results are shown in Figure 2.

[0204] Noise index

[0205] The results show that the "5% PEG 2000" fluid has no effect because the same noise index as the reference formulation is obtained, i.e. 100 m (i.e. 0 m reduction).

[0206] The noise index of the fluid "5% HCY" is 43 m.

[0207] We can calculate the portion of the reduction in the noise index due to homocysteine: 100 - 43 = 57 m.

[0208] The noise index of the fluid "5% PEG 2000 + 5% HCY" is 26.

[0209] We can calculate the proportion of the reduction in the noise index due to the combination of homocysteine ​​and PEG 2000: 100 - 26 = 74 m.

[0210] However, the reduction in the theoretical noise index of a fluid comprising 5% HCY and 5% PEG 2000 would be 0+57=57 m.

[0211] Experimentally, the reduction in the noise index is 74 m, significantly greater. The combination of a polyalkylene glycol and a sulfur compound as described in the invention provides an improved anti-wear effect, or even a synergistic effect on the noise index.

[0212] Signs of wear

[0213] [Table 1]

[0214] The combination of a polyalkylene glycol and a sulfur compound as per the invention makes it possible to obtain an improved anti-wear effect, with a reduced wear footprint.

[0215] Example 2: Reduction in the amount of PAG while maintaining lubricating properties

[0216] ESSA1 1:

[0217] The following three formulations were prepared:

[0218] Formulation A: 5% by weight of GENAPOL RPF1740 and the rest water.

[0219] Formulation B: 4% by weight of GENAPOL RPF 1740, 1% by weight of homocysteine ​​disodium and the rest water.

[0220] Formulation C: 5% by weight of homocysteine ​​disodium and the rest water.

[0221] The percentages are expressed as a percentage of the total weight of each formulation.

[0222] Then, the formulations were each diluted to 10% by weight in water to obtain the final fluids.

[0223] We performed a Reichert test under the same conditions as in example 1.

[0224] Noise levels and wear areas are shown in the table below:

[0225] [Table 21 11

[0226] The combination of a sulfur compound and a polyalkylene glycol according to the invention makes it possible to reduce the amount of polyalkylene glycol, while maintaining the lubricating properties of the fluid, in particular the anti-wear properties.

[0227] TEST 2:

[0228] A second series of tests was carried out with the aim of further reducing the concentration of polyalkylene glycol.

[0229] We have prepared the following two formulations:

[0230] Formulation A': 5% by weight of GENAPOL PS, 5% by weight of sebacic acid and 5% by weight of monoethanolamine and the rest water.

[0231] Formulation B': 2% by weight of GENAPOL PS, 5% by weight of homocysteine ​​disodium, 5% by weight of sebacic acid, 5% by weight of monoethanolamine and the rest water.

[0232] The percentages are expressed by weight, relative to the total weight of each formulation.

[0233] A Reichert test is performed under the same conditions as in example 1.

[0234] The noise levels and wear areas are shown in the table below:

[0235] [Table 3]

[0236] The combination according to the invention makes it possible to greatly reduce the quantity of PAGs, while obtaining similar lubricating properties, in particular anti-wear properties.

[0237] Extreme pressure effect of a fluid according to the invention

[0238] A. Preparation of formulations

[0239] Three aqueous-based hydraulic fluid formulations were prepared:

[0240] Formulation A:

[0241] - 30% by weight of GENAPOL PS,

[0242] - 5% by weight of alkaline reserve (monoethanolamine), and

[0243] - the remaining water. Formulation B:

[0244] - 30% by weight of GENAPOL PS,

[0245] - 5% by weight of alkaline reserve (monoethanolamine),

[0246] - 5% disodium homocysteine, and the rest water.

[0247] Formulation C:

[0248] - 25% by weight of GENAPOL PS,

[0249] - 5% alkaline reserve (monoethanolamine),

[0250] - 5% disodium homocysteine, and the rest water.

[0251] The percentages are expressed as a percentage of the total weight of each formulation.

[0252] The formulations are used without dilution to obtain the final hydraulic fluid.

[0253] B. Test Conditions

[0254] The 4-ball test is performed according to the standardized method ASTM D2783-21.

[0255] In this test, the extreme pressure measurement is performed by rotating a stainless steel ball on three stationary stainless steel balls, all four of which are fully coated with a lubricating fluid. A load is applied to the balls and gradually increased (every minute according to the parameters below) until the balls weld together. The balls are changed before each increase in load.

[0256] The extreme pressure effect corresponds to the load value at which the four balls weld together, preventing the top ball from rotating on the other three. The greater the load, the greater the extreme pressure effect.

[0257] The conditions are as follows:

[0258] Speed: 1450 + / - 50 rpm

[0259] 100 Cr6 steel balls

[0260] C. Results

[0261] The results obtained are given in the table below. [Table 4]

[0262] The results show a good extreme-pressure effect of the fluid according to the invention. This effect is maintained when the amount of PAG is decreased.

[0263] Example 4: Anti-corrosion effect of a fluid according to the invention

[0264] Two formulations were prepared:

[0265] A formulation A (according to the invention) which contains:

[0266] - 5% by weight of GENAPOL PS,

[0267] - 5% by weight of homocysteine ​​disodium,

[0268] - 5% by weight of monoethanolamine (alkaline reserve), and

[0269] - the remaining demineralized water.

[0270] Formulation B (comparative) which contains:

[0271] - 5% by weight of GENAPOL PS,

[0272] - 5% by weight of sebacic acid,

[0273] - 5% by weight of monoethanolamine (alkaline reserve), and

[0274] - the remaining demineralized water.

[0275] The anti-corrosion properties of formulations A and B, at different dilution rates, were evaluated via the standardized test DIN 51360-2, the experimental conditions of which are as follows:

[0276] Gray cast iron shavings are deposited on filter paper soaked in the formulation diluted in hard water. The assembly is then placed in a chamber at room temperature. After a specified time, the shavings are examined for traces of corrosion, yielding the following results: [Table 51]

[0277] [Table 6]

[0278] The results show that the fluids according to the invention possess anti-corrosion properties. These properties are even better than those obtained with a fluid containing a conventional corrosion inhibitor (sebacic acid).

Claims

Claims 1. A lubricating fluid comprising: a) a sulfur compound of the following general formula (I), or one of its salts: R3-X-CH(NRiR2)-(CH2)n-(S)p-Z (I) in which: - Z is a hydrogen atom or a group -(CH2) n -CH(NRiR2)-X-R3; Ri and R2, which may be identical or different, are selected from: a hydrogen atom or a saturated or unsaturated, linear, branched or cyclic, aromatic or non-aromatic hydrocarbon chain having from 1 to 20 carbon atoms and which may comprise one or more heteroatom(s); - X is selected from -C(=O)-, -CH2- or -CN; - R3 is: (i) either zero when X represents -CN, (ii) or a hydrogen atom, (iii) or - OR a , R abeing a hydrogen atom or a saturated or unsaturated, linear, branched or cyclic, aromatic or non-aromatic hydrocarbon chain of 1 to 20 carbon atoms and which may comprise one or more heteroatom(s); (iv) either -NRbRc, Rb and R c , which are identical or different, being chosen from: a hydrogen atom or a saturated or unsaturated, linear, branched or cyclic, aromatic or non-aromatic hydrocarbon chain of 1 to 20 carbon atoms and which may comprise one or more heteroatom(s); n is an integer equal to 1 or 2; and p is an integer between 1 and 8; provided that if Z is a hydrogen atom, then p is equal to 1; b) a polyalkylene glycol; c) water; and d) optionally one or more functional additive(s).

2. Lubricating fluid according to claim 1, in which said sulfur-containing compound is of the following general formula (la), or one of its salts: R3-X-CH(NRiR2)-(CH2) n-SH (la) in which Ri, R2, R3, X and n are as defined in claim 1. Tl 3. The lubricating fluid according to claim 1, wherein said sulfur compound of general formula (I) is chosen from the group consisting of: homocysteine, homocystine, dihomocysteine trisulfide, dihomocysteine tetrasulfide, dihomocysteine pentasulfide and one of their salts, preferably homocysteine.

4. The lubricating fluid according to any one of the preceding claims, wherein the quantity of sulfur compound of general formula (I) is between 0.1 and 30% by weight, preferably between 0.1 and 20% by weight, more preferably still between 1 and 7% by weight, relative to the total weight of the fluid.

5. A lubricating fluid according to any one of the preceding claims, wherein said polyalkylene glycol is selected from: polyethylene glycol (PEG), polypropylene glycol (PPG), polybutylene glycol, copolymers comprising ethylene oxide and / or propylene oxide and / or butylene oxide units, and mixtures thereof.

6. A lubricating fluid according to any one of the preceding claims, wherein said polyalkylene glycol is a copolymer comprising ethylene oxide and propylene oxide units (EO / PO).

7. A lubricating fluid according to any one of the preceding claims, wherein the amount of said polyalkylene glycol is between 0.1 and 40% by weight, preferably between 1 and 35% by weight, more preferably still between 1 and 30% by weight relative to the total weight of the fluid.

8. A lubricating fluid according to any one of the preceding claims, wherein the amount of water is at least 20% by weight, preferably at least 30% by weight, based on the total weight of said fluid.

9. A lubricating fluid according to any one of the preceding claims, wherein the molar ratio [sulfur compound of general formula (I) / polyalkylene glycol] is between 0.025 and 20,000, more preferably between 0.1 and 1000.

10. Use of a lubricating fluid as defined in any one of claims 1 to 9, for the lubrication and / or cleaning of mechanical devices, for the working of metals, as a hydraulic fluid, or also as a fluid for mining operations.

11. Use of a sulfur compound of general formula (I) or one of its salts, as defined in any one of claims 1 to 3 in combination with a polyalkylene glycol, to improve the tribological properties of a lubricating fluid.

12. Method of lubricating a metal or a composite comprising a step of bringing said metal or said composite into contact with a lubricating fluid as defined in any one of claims 1 to 9.

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