USE OF A POLYALKOXYSILOXANE-BASED OIL AS A LUBRICANT

Description

TECHNICAL FIELD

[0001] The present invention relates to the technical field of lubricants, in particular base oils for lubricating applications. TECHNOLOGICAL BACKGROUND

[0002] In this field, a lubricating composition is generally composed of a base oil to which a number of additives are added to adjust properties such as foaming, corrosion resistance, oxidation resistance (via antioxidants) and wear resistance (via anti-wear or extreme pressure additives).

[0003] Silicone oils, also known as polydimethylsiloxanes (PDMS), are used as base oils in certain industrial applications, notably as lubricants or dielectric fluids, in cosmetics, and in instrumentation. Their main advantage lies in their excellent fluidity, particularly at low temperatures, which is characterized by a high viscosity index. Silicone chemistry is also recognized for its impressive thermal resistance and fire resistance, offering users a very high level of safety during operation.

[0004] Moreover, this type of chemistry is also distinguished by a significantly improved toxicological aspect compared to that of other chemistries such as PolyAlphaOlefins (some grades of which can be fatal if ingested and enter the respiratory tract) or Phosphate esters which, in addition, are category 2 carcinogens because of n-tributylphosphate.

[0005] However, silicone oils, particularly PDMS, exhibit very low lubricating properties, as described in the publication by E.D. Brown, "Methyl Alkyl Silicones, A New Class of Lubricants," Asle Trans. 1966, 9, 31-35. Furthermore, silicone oils, especially PDMS, have poor compatibility with other components, particularly with commonly used additives. Therefore, it is difficult to substantially modify the properties of silicone oils by adding additives, as is generally the case in the field of lubricant compositions.

[0006] US patent 10,011,801 describes a process for copolymerizing polyalkylphenylsiloxanes and alkylfluoroalkylsiloxanes, the resulting copolymer having lubricating properties. The copolymer formed comprises perfluorinated R1 groups.

[0007] Document WO 2020 / 020476 concerns the supply of a new hybrid grease with low coefficients of friction and high wear protection, which can be used over a wide temperature range. The new hybrid grease is based on a combination of silicone oil-based grease and grease based on synthetic hydrocarbon oils, mineral oils, or polyglycols.

[0008] Document EP 0 475 440 describes a process for preparing an organopolysiloxane compound substituted with a higher alkoxy group (comprising at least 4 carbon atoms bonded to the silicon atom) suitable for use as an ingredient in a wide variety of toiletries and cosmetics.

[0009] In this context, the Applicant has developed a new family of polyalkoxysiloxane derivatives with significantly better lubrication properties than polydimethylsiloxanes, while retaining other desirable physicochemical properties in ranges close to those of polydimethylsiloxanes, such as a high viscosity index, high thermal resistance, and non-flammability. These polyalkoxysiloxane derivatives also exhibit compatibility with other components, particularly additives and other families of synthetic and mineral base oils, unlike conventional polydimethylsiloxanes. This allows for a significant expansion of their use by offering the possibility of modifying, adjusting, and / or optimizing the properties of lubricating compositions whose base oil includes such a polyalkoxysiloxane. SUMMARY OF THE INVENTION

[0010] The present invention is described in and by the attached claims.

[0011] The present invention relates to the use of a polyalkoxysiloxane base oil of formula (I): in which Each R1 is independently an alkyl group at C1 to C4, each R2 is independently an alkyl or alkenyl group at C2 to C22 or a phenyl group, R3 is a methyl group, x is equal to 0, y is an integer between 1 and 500, or a mixture of polyalkoxysiloxane base oils of formula (I), as a lubricating agent.

[0012] Other non-limiting and advantageous characteristics of use, taken individually or in all technically possible combinations, are as follows: The polyalkoxysiloxane base oil comprises one or more polyalkoxysiloxanes of formula (I); x is 0 and each R1 is independently chosen from the methyl, ethyl and phenyl groups, in particular R1 is independently chosen from the methyl and ethyl groups, in general R1 is a methyl group; typically R1 is not a phenyl group; each R2 is independently an alkyl or alkenyl group in C4 to C18, preferably in C4 to C12, in particular R2 is independently an alkyl group in C4 to C18; typically R2 is not a phenyl group; according to one feature of the invention, each R2 is independently an alkyl or alkenyl group at C4 to C22, or a phenyl, generally when R2 is a phenyl group, it preferably represents (by number) 70% or less, preferably 60% or less and typically 50% or less of the motif repeated y times; each R3 is a methyl group;The polyalkoxysiloxane is chosen from the group consisting of polyalkoxysiloxanes of formula (I) in which: x=0, y is between 1 and 140, each R1 is a methyl group, each R3 is a methyl group, and each R2 is an n-butyl chain; x=0, y is between 1 and 140, each R1 is a methyl group, each R3 is a methyl group, and each R2 is a heptyl chain; x=0, y is between 1 and 140, each R1 is a methyl group, each R3 is a methyl group, and each R2 is a dodecyl chain; x=0, y is between 1 and 140, each R1 is a methyl group, each R3 is a methyl group, and each R2 is a tetradecyl chain; and x=0, y is between 1 and 140, each R1 is a methyl group, each R3 is a methyl group, and each R2 is chosen from the group consisting of octadecenyl and hexadecyl chains; the polyalkoxysiloxane base oil of formula (I) is used as a lubricating agent to form a dielectric fluid, a coolant fluid or a hydraulic fluid;The polyalkoxysiloxane base oil of formula (I) is used in mixture with at least one other base oil, as a co-base.

[0013] The invention also relates to a lubricating composition comprising a polyalkoxysiloxane base oil of formula (I) as described above and at least one additive.

[0014] The polyalkoxysiloxane base oil comprises one or more polyalkoxysiloxanes of formula (II).

[0015] Preferably, said at least one additive is chosen from the group consisting of extreme-pressure additives, anti-wear additives, detergent additives, viscosity-modifying additives, anti-foaming additives, antioxidant additives, flame retardants, corrosion inhibitors and a combination of at least two of said additives.

[0016] Advantageously, said composition includes at least one flame retardant additive, so as to obtain a non-flammable hydraulic fluid.

[0017] The invention also relates to polyalkoxysiloxanes of formula (II): in which each R1 is independently an alkyl group in C1 to C4, such as a methyl group, each R2 is independently an alkyl or alkenyl group in C4 to C16, preferably an alkyl or alkenyl group in C4 to C12 or a phenyl, x is 0, y is an integer between 1 and 500.

[0018] According to one feature of the invention, each R2 is independently an alkyl or alkenyl group at C4 to C16, or a phenyl; preferably when R2 is a phenyl group, it represents (by number) 70% or less, preferably 60% or less and typically 50% or less of the motif repeated y times.

[0019] According to the invention, "R2 is a phenyl group, this represents (by number) 70% or less of the motif repeated y times" includes the following values ​​(%) and any interval between these values: 70; 69; 68; 67; 66; 65; 64; 63; 62; 61; 60; 59; 58; 57; 56; 55; 54; 53; 52; 51; 50; 49; 48; 47; 46; 45; 44; 43; 42; 41; 40; 39; 38; 37; 36; 35; 34; 33; 32; 30; 25; 20; 15; 10; 5, etc.

[0020] The invention also relates to particular polyalkoxysiloxanes, selected from the group consisting of polyalkoxysiloxanes of formula (II) in which: x=0, y is between 1 and 140, preferably between 40 and 80, each R1 is a methyl group, and each R2 is an n-butyl chain (PAS4), x=0, y is between 1 and 140, preferably between 40 and 80, each R1 is a methyl group, and each R2 is a heptyl chain (PAS7), x=0, y is between 1 and 140, preferably between 40 and 80, each R1 is a methyl group, and each R2 is a dodecyl chain (PAS12), x=0, y is between 1 and 140, preferably between 40 and 80, each R1 is a methyl group, each R3 is a methyl group, and each R2 is a tetradecyl chain (PAS14), and x=0, y is between 1 and 140, preferably between 40 and 80, each R1 is a methyl, and each R2 is chosen from the group consisting of octadecenyl and hexadecyl chains (PAS16-18).

[0021] In the present invention, unless otherwise specified, the term "comprises" and its derivatives shall be understood as non-limiting and not excluding the presence of other components or steps. In certain particular embodiments, the term "comprises" may be understood as "consisting essentially of" or "being made up of".

[0022] Unless otherwise specified, the intervals mentioned in the present invention are understood to include the limits.

[0023] Of course, the different features, variants and embodiments of the invention can be combined with each other in various ways as long as they are not incompatible or mutually exclusive. BRIEF DESCRIPTION OF THE FIGURE

[0024] Furthermore, various other features of the invention become apparent from the attached description made with reference to the drawings which illustrate non-limiting embodiments of the invention and where: [ Fig. 1 ] presents the results of friction coefficient measurement for different polyalkoxysiloxanes according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention is described in and by the attached claims.

[0026] The present invention relates firstly to the use of a polyalkoxysiloxane base oil of formula (I) in which Each R1 is independently an alkyl group at C1 to C4, each R2 is independently an alkyl or alkenyl group at C2 to C22 or a phenyl group, R3 is a methyl group, x is equal to 0, y is an integer between 1 and 500, or a mixture of polyalkoxysiloxane base oils of formula (I), as a lubricating agent.

[0027] The term "C1-Cj alkyl group" refers to a saturated hydrocarbon group, linear, branched, or cyclic, comprising i to j carbon atoms, and optionally interrupted by one or more heteroatoms, such as oxygen or sulfur. Examples of C1-C18 alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, n-pentyl, n-hexyl, cyclohexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tetradecyl, n-hexadecyl, and n-octadecenyl. Generally, a C1-Cj alkyl group according to the invention is a linear saturated group or a branched saturated group, preferably a linear saturated group.

[0028] The term "C1 to C18 alkenyl group" refers to an unsaturated, linear, branched, or cyclic hydrocarbon group comprising i to j carbon atoms, and optionally interrupted by one or more heteroatoms, such as oxygen or sulfur. The alkenyl group may include one or more unsaturations, such as double bonds between two carbon atoms. Preferably, the alkenyl group comprises a single unsaturation. Preferably, the unsaturation(s) of the alkenyl group are double bonds between two carbon atoms. Examples of C2 to C18 alkenyl groups include ethenyl, propenyl, butenyl, pentenyl, hexenyl, cyclohexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tetradecenyl, hexadecenyl, and octadecenyl. Preferably, an alkenyl group according to the invention is linear or branched, in particular linear.

[0029] According to the invention, "phenyl" is commonly understood to mean a univalent group of benzene comprising six carbon atoms bonded to five hydrogen atoms.

[0030] In one embodiment, each R1 is independently chosen from the group consisting of a methyl group, an ethyl group. Preferably, all R1s are methyl groups, or all R1s are ethyl groups. In particular, all R1s are methyl groups.

[0031] In one embodiment, each R2 is independently an alkyl or alkenyl group at C2 to C22, preferably at C2 to C18, preferably at C4 to C18, preferably at C4 to C16, preferably at C4 to C12.

[0032] In one embodiment, each R2 is independently an alkyl group at C2 to C22, preferably at C2 to C18, preferably at C4 to C18, preferably at C4 to C16, preferably at C4 to C12.

[0033] According to the invention, by a group comprising a number of carbons ranging from "C2 to C22", the following values ​​are understood to mean, or any other interval between these values: 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22.

[0034] The R2 chains can all be identical within the same compound of formula (I), but they can also differ in pairs. In particular, using R2 chains of different lengths within the same compound of formula (I) can combine the advantages in terms of properties of compounds of formula (I) with R2 chains of each length.

[0035] According to one embodiment, in the same compound of formula (I), all the R2 chains are identical.

[0036] According to another embodiment, in the same compound of formula (I), the R2 chains are different. In this embodiment, each R2 is independently an alkyl or alkenyl group at C2 to C18, preferably at C2 to C16, typically at C2 to C14, such as at C7, and a phenyl group. Preferably, when present in the compound of formula (I), the phenyl group represents 70% or less, preferably 60% or less, and typically 50% or less of the repeated y-times motif. By way of example, R2 is independently an alkyl group at C2 to C18, preferably at C2 to C16, typically at C2 to C14, such as at C7, and a phenyl group, the latter representing 50% or less of the repeated y-times motif.

[0037] According to the invention, each R3 is a methyl group.

[0038] According to an embodiment not covered by the invention, x is an integer between 0 and 50, y is an integer between 1 and 500.

[0039] A range "between 0 and 50" means the following values ​​or any interval between them: 1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 45; 50. Similarly, a range "between 1 and 500" means the following values ​​or any interval between them: 1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 45; 50; 55; 60; 65; 70; 75; 80; 85; 90; 95; 105; 110; 115; 120; 125; 130; 135; 140; 145; 150; 155; 160; 165; 170; 175; 180; 185; 190; 195; 200; 205; 210; 215; 220; 225; 230; 235; 240; 245; 250; 260; 270; 280; 290; 300; 310; 320; 330; 340; 350; 360; 370; 380; 390; 400; 410; 420; 430; 440; 450; 460; 470; 480; 490; 500.

[0040] In general, y is between 1 and 150, in particular between 1 and 140. Typically, y is between 1 and 80. Preferably, the sum x+y is between 1 and 500.

[0041] The y / x ratio is strictly greater than 0.5. A range "strictly greater than 0.5" includes the following values ​​or any interval between these values: 0.5; 0.6; 0.7; 0.8; 0.9; 1.0; 1.1; 1.2; 1.3; 1.4; 1.5; 1.6; 1.7; 2.5; 3.5; 3.5; 4.0; 4.5; 5.0; 5.5; 6.0; 6.5; 10; 15; 20; 25; 30; 35; 40; 45; 50; 55; 60; 65; 70; 75; 80; 95; 100; 110; 120; 130; 140; 150; 160; 170; 180; 190; 200; 210; 220; 230; 240; 250;etc.

[0042] The molar mass of polyalkoxysiloxane of formula (I) is typically between 300 g / mol and 200,000 g / mol, preferably between 300 g / mol and 30,000 g / mol. The molar mass can notably be measured by gel permeation chromatography (GPC).

[0043] According to the invention, x is 0. This means that the polyalkoxysiloxane comprises only alkoxylated (or phenoxylated) units. In a preferred embodiment, x is zero and y is between 1 and 140, in particular x is zero and y is between 1 and 80.

[0044] According to the invention, the polyalkoxysiloxane oil may comprise a mixture of derivatives of formula (I) of different lengths, i.e., for which the y values ​​are not all the same. In this case, it is possible to define an average y value for a polyalkoxysiloxane oil according to the invention. Preferably, the average y value in a polyalkoxysiloxane oil according to the invention is between 50 and 60.

[0045] According to the invention, the polyalkoxysiloxane oil may comprise a mixture of polyalkoxysiloxane base oils of formula (I) having different substitutions among R1, R2 or R3. According to one feature of the invention, the mixture of polyalkoxysiloxane base oils of formula (I) may correspond to and / or is obtained from a physical mixture of different oils previously synthesized separately.

[0046] According to another feature of the invention, the polyalkoxysiloxane base oil mixture of formula (I) can be obtained during the same chemical synthesis, for example when a mixture of alcohols having varying chain lengths is used. Thus, during the same synthesis, a polyalkoxysiloxane base oil mixture of formula (I) is obtained in which the R2 group varies between the oils (a polyalkoxysiloxane base oil mixture of formula (I) can thus be obtained comprising a first polyalkoxysiloxane oil whose R2 group is a hexadecyl group (C16) and a polyalkoxysiloxane oil whose R2 group is an octadecenyl group (C18)).

[0047] By "lubricating agent" we mean an agent which, when used alone or in a composition, reduces friction between two surfaces moving relative to each other.

[0048] The Applicant unexpectedly discovered that the polyalkoxysiloxane base oils of formula (I) according to the invention have lower coefficients of friction (CoF) than polysiloxanes, such as PDMS and polyalphaolefins (PAOs), and even lower than those of other organophosphate-based fluids used in hydraulics, under specific conditions. This confirms their superior lubricity compared to these other fluids. In particular, the CoF measured for the polyalkoxysiloxanes of formula (I) is less than 0.1, indicating good performance. Under the same conditions, the CoF measured for the reference polydimethylsiloxanes, organophosphates (marketed, for example, under the brand name Skydrol®), and polyalphaolefins is greater than or equal to 0.1. The specific conditions are as follows: mixed lubrication system, speed 100 mm / s, pure sliding, load of 40 N (pressure 1.02 GPa), temperature 75°C.

[0049] The "coefficient of friction (CoF)" refers to a dimensionless characteristic intrinsic to a base oil, indicating the ability of a base oil film to reduce friction between two surfaces. The value of the coefficient of friction, also known as lubricating power, is highly dependent on the lubrication regime. The coefficient of friction can be measured using a tensile testing machine such as a Mini Traction Machine (MTM) equipped with an AISI 52100 stainless steel ball and disc. Specifically, tests can be performed at a speed of 100 mm / s, in pure sliding (Sliding Rolling Ratio of 200%), under a load of 40 N equivalent to a contact pressure of 1.02 GPa and at a temperature of 75°C, for one hour.

[0050] Friction is a force that opposes the relative motion of two surfaces in contact. It depends on several factors, such as the surface condition, temperature, normal power, sliding speed, etc. Significant friction between two surfaces (characterized by a high coefficient of friction) will lead to wear of the moving parts. The best way to reduce this risk is to add a lubricant between the surfaces, so that, in a hydrodynamic lubrication regime, they are not in direct contact. The coefficient of friction is a function of the system's characteristics, and its evolution is graphically illustrated by the Stribeck curve.

[0051] The choice of the hydro-dynamic regime makes it possible to characterize the lubricating power of a base oil, under relatively standard pressure, temperature and speed conditions for undemanding lubrication systems such as pumps and other accessories present in hydraulic circuits.

[0052] The polyalkoxysiloxane base oils of formula (I) have high viscosity indices, in particular viscosity indices greater than 200, preferably greater than 300, approaching those recorded for PDMS (>350) and significantly higher than those of polyalphaolefins, in particular. This means that the polyalkoxysiloxane base oils according to the invention have high fluidity over a wide temperature range, especially at low temperatures. Low temperature is preferably defined as a temperature below -40°C, in particular below -50°C. The viscosity index can be measured by any technique known in the art. It is preferably performed according to the standardized method known as ASTMD2270.

[0053] Polyalkoxysiloxane base oils of formula (I) may be used either alone, or as a mixture of at least two polyalkoxysiloxane base oils of formula (I), or in a mixture with at least one other base oil, as a co-base or major base, preferably as a co-base. Preferably, the polyalkoxysiloxane base oil of formula (I) is used in the absence of any other base oil.

[0054] Other base oils that can be blended with the polyalkoxysiloxane base oils according to the invention include polyalphaolefins, polydimethylsiloxanes, and synthetic esters, particularly diesters. These other base oils are known to those skilled in the art and will therefore not be described in detail below.

[0055] Indeed, the polyalkoxysiloxane base oils according to the invention, unlike polydimethylsiloxanes, are compatible with these various other base oils, particularly at high temperatures, but also with certain other base oils at low temperatures, notably other low-viscosity base oils. The polyalkoxysiloxane base oil according to the invention and the other base oil can be used in the mixture in any mass proportion from 100:0 to 0:100 (excluding limits), preferably from 10:90 to 90:10 (including limits), thus encompassing the following mass proportions (including limits): 10:90, 50:50, or 90:10.

[0056] The polyalkoxysiloxane base oils of formula (I) according to the invention can also be used as a dielectric fluid. Indeed, their physicochemical characteristics, such as their insulating properties, pour point, kinematic viscosity, flash point, fire point, and / or auto-ignition temperature, are perfectly suited for these base oils to constitute a good dielectric fluid. Furthermore, according to the present invention, the polyalkoxysiloxane base oil of formula (I) preferably performs both lubrication and dielectric functions simultaneously.This is particularly suitable when the polyalkoxysiloxane base oil of formula (I) according to the invention is used as a lubricant in a system requiring electrical insulation, cooling, arc extinguishing and / or limitation, and / or reduction of partial discharges, such as, for example, electrical transformers: traction transformers, power transformers, wind turbine transformers, offshore transformers, distribution transformers, etc.

[0057] In addition, the polyalkoxysiloxane base oil of formula (I) according to the invention is perfectly suited for lubricating hydraulic systems, whether for small, independent systems such as actuators (a few liters of fluid), or for large, centralized hydraulic systems transmitting commands (>100L of fluid). It can also be used with fluids found in landing gear, radar fluids, and other dielectric fluids used in military or space applications. Thanks to its low coefficient of friction (CoF), the polyalkoxysiloxane base oil of formula (I) according to the invention can, in particular, perfectly lubricate the pumps of a hydraulic system, providing a high level of protection against long-term wear and, consequently, ensuring excellent longevity of the equipment and mechanical components.

[0058] Due to their advantageous technical characteristics as previously mentioned, the polyalkoxysiloxane base oils of formula (I) according to the invention can also be used as hydraulic fluids. The performance level of these alternative formulations can easily be justified as being equivalent to or superior to that of reference fluids such as, for example, phosphate esters, which are widely used in commercial aviation (see Example 2 below).

[0059] The polyalkoxysiloxanes of formula (I) according to the invention can be synthesized by any technique known to those skilled in the art. In particular, the polyalkoxysiloxanes of formula (I) can be synthesized by processes such as those described in patent applications EP0475440 and WO2014099497. However, the Applicant has observed that replacing the reported catalytic systems (palladium or platinum catalysts, possibly with the addition of carboxylic acids) with bases well known to those skilled in the art (e.g., cesium fluoride, CsF) could advantageously improve the conversion rate and therefore the reaction yield as well as its selectivity.

[0060] As an example, the catalytic system may include at least one or more catalysts chosen from among: CsF, CsOH, Cs(TFA), KOH, Ba(OH)₂, Ca(OH)₂, Sr(OH)₂, Ba(OH)₂, CH₃COOLi, NaOH, metal hydroxides, metal carboxylates, organic bases such as tertiary amines, or other aromatic or non-basic nitrogen heterocycles (pyridine, morpholine, alkylated piperidine and piperazine, etc.). Generally, the catalytic system includes cesium fluoride (CsF) or cesium hydroxide (CsOH), and is advantageously cesium hydroxide (CsOH). The latter compound has the advantage of being less toxic than CsOH and is also simpler to use.

[0061] The invention also relates to a lubricating composition comprising a polyalkoxysiloxane base oil of formula (I) as defined above or a mixture thereof, and at least one additive.

[0062] By "additive" we mean in particular a chemical product which is added to the lubricating composition to improve certain properties for its application as a lubricating agent.

[0063] At least one additive included in the lubricating composition according to the invention may be chosen from among the additives well known to those skilled in the art. In particular, it may be chosen from the group consisting of additives suitable for extreme pressures, anti-wear additives, detergent additives, viscosity-modifying additives, anti-foaming additives, amine and phenolic antioxidant additives, corrosion inhibitors, flame-retardant additives, and a combination of at least two of said additives.

[0064] Examples of additives suitable for extreme pressures include zinc dithiophosphate and MoS2 or molybdenum disulfide.

[0065] Examples of anti-wear and / or extreme-pressure additives include sulfides, chlorides, organophosphates, aryl and / or alkyl phosphates and metallic phosphates, for example zinc dialkyldithiophosphates (ZDDP).

[0066] Examples of anti-foaming additives include silicones.

[0067] Examples of flame-retardant additives include organophosphates with a high auto-ignition point.

[0068] Examples of antioxidant additives include phenols, aromatic amines, and sulfur-containing additives such as zinc dithiophosphates. Additives marketed under the names Vanlube 81® and Naugalube® 438 (4,4'-dioctyldiphenylamine), and Irganox® LO6 (octylated N-phenyl-1-naphthylamine) are examples of aromatic organic amines. Additives marketed under the names Irganox® L109 (hexamethylene glycol bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)) and Ionol® 220 AH (4,4'-methylenebis(2,6-di-tert-butylphenol)) are examples of phenolic antioxidant additives.

[0069] Examples of oil-soluble corrosion inhibitors include zinc-based agents, sulfonates, sorbitan esters, phosphates, and amine phosphates.

[0070] Examples of detergent and / or viscosity-modifying additives are well known to those skilled in the art.

[0071] In one embodiment, the lubricating composition according to the invention comprises at least two, at least three, at least four, at least five, at least six, different additives.

[0072] In one embodiment, the composition comprises fewer than ten additives or ten different additives.

[0073] Each of the additives present in the lubricating composition according to the invention is advantageously present in an amount ranging from 0.001% to 35% by weight, and preferably between 0.1% and 15% by weight, relative to the total weight of said lubricating composition.

[0074] The lubricating composition according to the invention advantageously comprises an amount of polyalkoxysiloxane base oil according to the invention ranging from 50% to 99.9999% by weight relative to the total weight of the lubricating composition, preferably an amount of polyalkoxysiloxane base oil according to the invention ranging from 60% to 99%, in particular from 75% to 95%. The other components of said composition are preferably selected from other base oils and additives, in particular additives as described above or other types of additives known in Art.

[0075] Other base oils that can be mixed with a base oil according to the invention or that can be included in a lubricating composition according to the invention are the same as those mentioned above and can include, in particular, mineral lubricating oils, or synthetic lubricating oils such as synthetic esters, or polyalphaolefins.

[0076] The lubricating composition according to the invention can be used in various fields, including industry, energy, marine, automotive, and / or aerospace. Specifically, it can be used as a hydraulic fluid, engine oil, power transfer oil, and / or turbine lubricant. It can be advantageously used in all fields where silicone oils are employed, while generally offering more favorable characteristics.

[0077] The invention also relates to a polyalkoxysiloxane of formula (II) suitable for and / or configured to form a base oil of lubricating composition. in which: each R1 is independently an alkyl group in C1 to C4, each R2 is independently an alkyl or alkenyl group in C4 to C16, preferably in C4 to C14, especially in C4 to C12 or a phenyl, x is 0 and y is an integer between 1 and 500.

[0078] According to the invention, by C4 to C16 alkyl or alkenyl group, a group comprising from 4 to 16 carbon atoms or any range between: 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16.

[0079] Polyalkoxysiloxanes of formula (II) constitute a specific subgroup of polyalkoxysiloxanes of formula (I) as previously defined. Therefore, the characteristics specified above also apply to the general formula (II) unless otherwise specified below.

[0080] In one embodiment, each R1 in formula (II) is independently chosen from the group consisting of a methyl group, an ethyl group. Preferably, all R1s are methyl groups, all R1s are ethyl groups. In particular, all R1s are methyl groups.

[0081] In one embodiment, each R2 in formula (II) is independently an alkyl or alkenyl group, preferably alkyl, at C4 to C14, preferably at C4 to C12.

[0082] In one embodiment, each R2 in formula (II) is independently an alkyl group in C4 to C14, preferably in C4 to C12.

[0083] The R2 chains can all be identical in the same compound of formula (II), but they can also be different in pairs.

[0084] Thus, according to one embodiment, in the same compound of formula (II), all the R2 chains are identical.

[0085] According to another embodiment, in the same compound of formula (II), the R2 chains differ in the y-repeated motifs. In this embodiment, each R2 is independently an alkyl or alkenyl group at C2 to C16, preferably at C2 to C14, typically at C2 to C12, such as at C7, and a phenyl group. Generally, the phenyl group represents, by number, 70% or less, preferably 60% or less, in particular 50% or less of the y-repeated motif. By way of example, R2 is independently an alkyl group at C2 to C16, preferably at C2 to C14, typically at C2 to C12, such as at C7, and a phenyl group, the latter representing, by number, 50% or less of the y-repeated motif.

[0086] In formula (II), according to an embodiment not covered by the invention, x is an integer between 0 and 50, y is an integer between 1 and 500. Preferably, x is between 0 and 25. Preferably, y is between 1 and 150, preferably between 1 and 140. In particular, y is between 1 and 80. Preferably, the sum x+y is between 1 and 500.

[0087] A range "between 0 and 50" means the following values ​​or any interval between them: 1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 45; 50. Similarly, a range "between 1 and 500" means the following values ​​or any interval between them: 1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 45; 50; 55; 60; 65; 70; 75; 80; 85; 90; 95; 105; 110; 115; 120; 125; 130; 135; 140; 145; 150; 155; 160; 165; 170; 175; 180; 185; 190; 195; 200; 205; 210; 215; 220; 225; 230; 235; 240; 245; 250; 260; 270; 280; 290; 300; 310; 320; 330; 340; 350; 360; 370; 380; 390; 400; 410; 420; 430; 440; 450; 460; 470; 480; 490; 500.

[0088] The y / x ratio is strictly greater than 0.5. A range "strictly greater than 0.5" includes the following values ​​or any interval between these values: 0.5; 0.6; 0.7; 0.8; 0.9; 1.0; 1.1; 1.2; 1.3; 1.4; 1.5; 1.6; 1.7; 2.5; 3.5; 3.5; 4.0; 4.5; 5.0; 5.5; 6.0; 6.5; 10; 15; 20; 25; 30; 35; 40; 45; 50; 55; 60; 65; 70; 75; 80; 95; 100; 110; 120; 130; 140; 150; 160; 170; 180; 190; 200; 210; 220; 230; 240; 250;etc.

[0089] The molar mass of polyalkoxysiloxane of formula (II) is typically between 300 g / mol and 200,000 g / mol, preferably between 300 g / mol and 30,000 g / mol.

[0090] According to the invention, x is 0 in formula (II). This means that the polyalkoxysiloxane comprises only alkoxylated (or phenoxylated) units. In a preferred embodiment, x is zero and y is between 1 and 140 in formula (II), in particular x is zero and y is between 1 and 80.

[0091] The invention also relates to particular polyalkoxysiloxanes, selected from the group consisting of polyalkoxysiloxanes of formula (II) in which: x=0, y is between 1 and 140, each R1 is a methyl, and each R2 is an n-butyl chain (PAS4), x=0, y is between 1 and 140, each R1 is a methyl, and each R2 is a heptyl chain (PAS7), x=0, y is between 1 and 140, each R1 is a methyl, and each R2 is a dodecyl chain (PAS12), x=0, y is between 1 and 140, each R1 is a methyl, and each R2 is a tetradecyl chain (PAS14), and x=0, y is between 1 and 140, each R1 is a methyl, and each R2 is chosen from the group consisting of octadecenyl and hexadecyl chains (PAS16-18).

[0092] In one embodiment, independently for each of the particular polyalkoxysilanes according to the invention, y is between 1 and 80. In another embodiment, independently for each of the polyalkoxysiloxanes according to the invention, an average value of y is between 50 and 60.

[0093] The invention may also relate to a lubricating composition comprising, as a lubricating agent, the polyalkoxysiloxane base oil of formula (I) or (II) (namely, which may comprise one or more polyalkoxysiloxanes of formula (I) or (II)), and at least one additive, with optionally at least one other base oil (different from the polyalkoxysiloxanes of formula (I) or (II) according to the invention), as a co-base; these compounds being as defined above. EXAMPLES

[0094] The following examples are intended to illustrate the invention without limiting its scope. Example 1: Synthesis of polyalkoxysiloxanes according to the invention (the catalyst is Cesium Fluoride CsF)

[0095] The synthesis of polyalkoxysiloxanes according to the invention is carried out following the following protocol: - Terms

[0096] The synthesis of polyalkoxysiloxane compounds according to the invention must be carried out under anhydrous conditions requiring the washing and drying of all the elements of the assembly under nitrogen purging; - Device

[0097] In order to carry out the synthesis of polyalkoxysiloxanes according to the invention, a 6L grooved flanged neck cylindrical “Schott” type reactor marketed by the company Belleville SA was used, the latter being surmounted by a tetracol lid.

[0098] The reactor is equipped with a water condenser to allow the alcohol vapors to condense during the reaction and is maintained under a flow of nitrogen, as well as a mechanical stirring system. - Synthesis

[0099] a) an alcohol (as defined in Table 1 below) is introduced in excess (approximately 10% by mass relative to the stoichiometric amount corresponding to the number of moles of Si-H contained in a PolyMethylHydroSiloxane which will be used below) into the reactor and maintained under a nitrogen flow until a temperature of 75-80°C; b) when the reaction medium is anhydrous, the catalyst (Cesium Fluoride CsF) is introduced (ideally 0.0.005% by mass relative to the total mass of the reaction medium); c) Polymethylhydrosiloxane (PMHS), labeled Silres® BS 94 (marketed by Wacker), is then added dropwise, controlling the flow rate to avoid a runaway reaction; d) the precise progress of the reaction can be determined by infrared by checking the decrease, then the disappearance of the characteristic "Si-H" band (≈2170 cm⁻¹); e) once the reaction is complete, the catalyst is extracted from the reaction medium by filtration and the excess alcohol is then distilled under vacuum.

[0100] The polyalkoxysiloxanes according to the invention which were synthesized following the above protocol are presented in Table 1 below. [Table 1] NOT Alcohol introduced into the synthesis R1 R2 R3 x Polymeric fraction y PAS4 n -butanol methyl n -butyle methyl 0 1 à 140 PAS5 n -pentanol methyl n -pentyle methyl 0 1 à 140 PAS6 n -hexanol methyl n -hexyl methyl 0 1 à 140 PAS7 n -heptanol methyl n -heptyle methyl 0 1 à 140 PAS8 n -octanol methyl n -octyl methyl 0 1 à 140 PAS9 n -nonanol methyl n -nonyle methyl 0 1 à 140 PAS10 n -decanol methyl n -decyl methyl 0 1 à 140 PAS12 lauric alcohol methyl n -dodecyl methyl 0 1 à 140 PAS16-18 Cetyl and oleic alcohol mixture (NAFOL 1618) methyl n -hexadecyl methyl 0 1 à 140 n -octadecenyl

[0101] The average value of y for each of the polyalkoxysiloxane oils in Table 1 is between 50 and 60. Example 2: Synthesis of polyalkoxysiloxanes according to the invention (the catalyst is Cesium hydroxide CsOH)

[0102] The synthesis of polyalkoxysiloxanes according to example 2 is illustrated below:

[0103] The synthesis of polyalkoxysiloxanes according to the invention is carried out following the following protocol: - The conditions and the system used are identical to those in example 1 described above; - Synthesis

[0104] a) 972 g of heptanol is introduced in excess into the reactor and maintained under a nitrogen flow until a temperature of 20°C is reached for 10 minutes; b) 0.939 g of catalyst (Cesium Hydroxide CsOHn.H2O2) is introduced (ideally 0.07% by mass relative to the total mass of the reaction medium); c) 400 g of PolymethylHydroSiloxane (PMHS), labeled Silres®< BS 94 (marketed by Wacker), is then added dropwise using a dropping funnel, controlling the flow rate (on the order of 1.7 mL / min) to avoid a runaway reaction; d) the reaction is exothermic, the temperature of the reaction medium is gradually raised to 65°C before stabilizing; e) the precise progress of the reaction can be determined by infrared by checking the decrease, then the disappearance of the characteristic band “Si-H” (≈2170cm -1< );f) After the complete addition of Polymethyl Hydrosiloxane (PMHS), the reaction mixture is allowed to gradually return to room temperature (approximately 3 hours); g) once the reaction is complete, the catalyst is extracted from the reaction mixture by filtration through a bed of dicalite and silica, and the excess alcohol is then distilled under reduced pressure (2 mbar, 100°C for 2 hours); h) the distillation residue is filtered through filter paper before packaging.

[0105] The polyalkoxysiloxanes synthesized according to this example are as follows: [Table 2] NOT Alcohol introduced into the synthesis R1 R2 R3 x Polymeric fraction y PAS7 n -heptanol methyl n -heptyle methyl 0 1 à 140

[0106] The average value of y for each of the polyalkoxysiloxane oils in Table 1 is around 61. Example 3: Characterization of the polyalkoxysiloxanes from examples 1 and 2 Tested features * The coefficient of friction

[0107] The coefficient of friction (CoF) was measured for each polyalkoxylsiloxane synthesized in Example 1, under the following conditions: mixed lubrication regime, speed 100 mm / s, pure sliding, load of 40 N (pressure 1.02 GPa), temperature 75°C.

[0108] In particular, the coefficient of friction is measured using a Mini Traction Machine (MTM) equipped with AISI 52100 steel ball and disc, at a speed of 100 mm / s, in pure sliding (Sliding Rolling Ratio of 200%), under a load of 40 N equivalent to a contact pressure of 1.02 GPa and at a temperature of 75°C, for one hour. * Kinematic viscosity

[0109] The kinematic viscosity at different temperatures (100°C, 40°C, -40°C) was measured according to ASTM D445 / 2532 at 35 minutes, 3 hours, 72 hours and after 72 hours respectively. * Viscosity index

[0110] The viscosity index was measured according to the ASTM D2270 method standard. * Flashpoint

[0111] The flash point was measured according to ASTM D92. Fire point

[0112] The point of ignition was measured according to ASTM D92.

[0113] The auto-ignition point was measured according to ASTM E659. * The pour point*

[0114] The pour point was measured according to ASTM D97. * KRL mechanical shear stability after 100h

[0115] The mechanical shear stability KRL after 100h was measured according to the modified CEC L 45-A-99 method. Results

[0116] There figure 1presents the measurement results of the coefficient of friction of polyalkoxysiloxanes according to example 1, as well as the measurement results for the reference oils PDMS 20 and PAO 8 and for three reference hydraulic fluids (FH2 (PAO / ester mixture), FH42 (PAO / ester mixture), Skydrol ®< type V (organophosphate-based aeronautical hydraulic fluid)) respectively approved according to the military standards MIL-PRF-83282, MIL-PRF-87257 and civil standard BMS 3-11 Type V. PAO8 designates a polyalphaolefin with a kinematic viscosity of 8 cSt at 100°C.

[0117] Under these conditions, the coefficient of friction values ​​of the polyalkoxysiloxanes according to the invention (Example 1) are less than 0.1 and are therefore significantly more lubricating than the reference aeronautical hydraulic fluid used in commercial aviation, namely Skydrol V (BMS 3-11 type V). Among the best polyalkoxysiloxanes in the prepared series, some achieve coefficients of friction (CoF) close to or even below 0.05, representing a very substantial improvement compared to the reference military fluids MIL-PRF-83282 and MIL-PRF-87257. This test demonstrates that the polyalkoxysiloxane compounds of the invention offer the potential for improved protection of hydraulic systems (less and slower wear of moving mechanical parts in contact with each other).

[0118] Tables 3 and 3bis below gather the results of the measurements of the physico-chemical properties for the polyalkoxysiloxanes synthesized in examples 1 and 2, and the corresponding values ​​for reference lubricants: PDMS20 and PAO8. [Table 3] Features Reference oils Base oils according to the invention PDMS20 PAO8 PAS4 Ex.1 PAS5 Ex.1 PAS6 Ex.1 PAS7 Ex.1 Coefficient of Friction (CoF) >0,25 0,100 0,049 0,061 0,087 0,056 Kinematic viscosity at (cSt): 100°C 6,30 8 6,48 5,57 5,59 6,24 40°C 14,70 48 20,24 15,89 17,24 18,9 -40°C 19000 460 256 421 730 Viscosity index 467 139 324 345 312 325 COC flash point (°C) 248 260 152 160 194 220 Pour point (°C) <-72 -48 <-72 -93 -93 <-72 Base oils according to the invention PAS8 Ex.1 PAS9 Ex.1 PAS10 Ex.1 PAS12 Ex.1 PAS16-18 Ex.1 PAS7 Ex.2 Coefficient of Friction (CoF) 0,068 0,096 0,091 0,041 Kinematic viscosity at (cSt): 100°C 7,11 7,49 7,76 9,31 14,0 5,25 40°C 25,77 28,12 28,91 37,8 69,4 16 -40°C 1390 fixed fixed 643 Viscosity index 262 254 259 242 211 309 COC flash point (°C) 216 266 248 214 Pour point (°C) -60 -39 -6 9 -96 Fire point (ASTM D 92) 234 Water content (ASTM D 1533) in mg / kg 183

[0119] The grey boxes in table 3 correspond to measurements that were not carried out or could not be carried out given the behavior of the oil under the measurement conditions.

[0120] The coefficient of friction (CoF) values ​​demonstrate the superiority in terms of lubricating power of the polyalkoxylsiloxanes according to the invention compared to polydimethylsiloxane oils and polyalphaolefins.

[0121] The kinematic viscosity and viscosity index values ​​demonstrate that the polyalkoxysiloxane oils according to the invention possess a very interesting fluidity relatively close to that of polydimethylsiloxane oils over a wide thermal range.

[0122] The pour point values, in particular those below -45°C, kinematic viscosity values, in particular those below 28 cSt at 40°C and / or those below 15000 cSt at -40°C, and flash point values ​​demonstrate that the polyalkoxysiloxane oils according to the invention can advantageously combine their lubricating properties with dielectric fluid properties for use at very low temperatures compatible with very cold regions (Russia, Canada, etc.). [Table 4] Characteristic Unit PAS7 Base Oil Phosphate ester (Skydrol® product) type V) Flash of lightning °C 220 166 Fire point °C 246 192 KRL mechanical shear stability after 100h (CEC-L-45-A-99) * Variation of kinematic viscosity at 40°C % 3,0 -26,4 * Variation of kinematic viscosity at 100°C % -0,9 -32,7 * Variation in viscosity index -- -17 -136

[0123] The results in Table 4 show that the base oil according to the invention possesses superior safety and shear stability characteristics (simulating the severe mechanical stresses exerted on the fluid, particularly in pumps operating in a pressurized environment) compared to the reference hydraulic fluid used in commercial aviation (e.g., Skydrol®), making it a prime candidate for potential replacement. It should be noted that the compounds of the invention exhibit very high performance under shear stress compared to Skydrol® type V (the reference fluid in the aeronautical market), for which the rheology decreases significantly after only 100 hours of shear (the drop in viscosity index indicating drastically different behavior at high and low temperatures). Example 4: Study of compatibility with other base oils

[0124] Mixtures of a base oil according to the invention (Example 1) with various other base oils were prepared, and their compatibility was studied for different proportions of base oil according to the invention: other base oil (0:100, 10:90, 50:50, 90:10, and 100:0), and at different temperatures (100°C, ambient temperature, 5.9°C, and -30°C). The base oil according to the invention is an oil of formula (I) with R1=methyl, R2=heptyl, R3=methyl, x=0, and y=1 to 140.

[0125] An absence of cloudiness or separation demonstrating very good compatibility of the base oil according to the invention with the other base oil was observed for mixtures in all proportions and at all temperatures with polyalphaolefins with kinematic viscosity at 100°C of 4, 6 and 8 cSt respectively, designated PAO4, PAO6 and PAO8.

[0126] Similarly, very good compatibility of the base oil according to the invention has been demonstrated at high temperature (100°C) in all proportions with the polyalphaolefin PAO100 of kinematic viscosity at 100°C of 100 cSt.

[0127] Finally, very good compatibility of the base oil according to the invention has been demonstrated at 100°C, ambient temperature and 5.9°C in all proportions with the paraffinic mineral oils known under the names 150 Neutral Solvent ®< , 600 Neutral Solvent ®< and BSS ®< (Bright Stock Solvent). Example 5: Study of compatibility with anti-wear additives

[0128] In this example, the compatibility of the polyalkoxysiloxane base oil according to the invention (here, the compound) was measured. PAS7 described in example 1) with the following anti-wear additives: a) tri(methyl silyl) phosphate, b) tris(trimethylsiloxy)boron c) (diethylphosphatoethyl)triethoxysilane. by measuring in particular the mechanical performance according to the ASTM D4172 standard (4-ball wear, 1h, 40kg, 75°C 1200 revolutions per minute).

[0129] The results are presented in Table 5 below: [Table 5] 1 2 3 4 5 6 7 Composition PAS7 (oil only) PAS7 + a) 1% PAS7 + a) 3% PAS7 + b) 1% PAS7 + b) 3% PAS7 + c) 1% PAS7 + c) 3% Mechanical performance (wear diameter) 1,54 1,06 0,85 1,3 1,26 1,12 0,93

[0130] This example shows that adding anti-wear additives to the polyalkoxysiloxane base oil according to the invention makes it possible to form a lubricating composition with significantly improved mechanical performance. Example 6: Study of compatibility with flame-retardant additives

[0131] In this example, the compatibility of the polyalkoxysiloxane base oil according to the invention (here, the PAS7 compound described in Example 1) with flame-retardant additives as listed below was measured, as well as their effects on the auto-ignition temperature of the mixtures obtained.

[0132] The results are presented in Table 6 below: [Table 6] Base oil Flame retardant Mass content of flame retardant additive Auto-ignition temperature (°C) Organophosphates (HYJET V) N / A N / A 391 Organophosphates (SKYDROL ®< PE 5) N / A N / A 396 PAS7 none 0% 385 PAS7 Isodecyl diphenyl phosphate 5% 393 PAS7 isodecyl diphenyl phosphate 10% 403 PAS7 isodecyl diphenyl phosphate 15% 407 PAS7 Phenol, isobutylene phosphate (3:1) 5% 390 PAS7 Phenol, isobutylene phosphate (3:1) 15% 409

[0133] This example shows that the addition of flame-retardant additives is chemically possible and allows the polyalkoxysiloxane base oil according to the invention to form a lubricating composition having an auto-ignition point higher than that of the base oil (PAS7) as well as those of reference hydraulic fluids such as Skydrol ®< type V and Hyjet V. This is undeniably a very advantageous (fire) safety feature. Example 7: Another synthesis of polyalkoxysiloxanes according to the invention (from two alcohols)

[0134] For this comparative study, the experimental conditions are identical to those described for Example 1 above, i.e., the polymethylhydrosiloxane (PMHS) is Silres®< BS94 and the catalyst is cesium fluoride (CsF), except that the starting alcohol was replaced by a mixture of n-heptanol and phenol, varying their molar ratios: 50% phenol / 50% n-heptanol and 25% phenol / 75% n-heptanol. For information purposes, the characteristics of the polyalkoxysiloxanes obtained were compared to those of the PAS7 compound synthesized according to Example 1 of the invention.

[0135] The polyalkoxysiloxanes that were synthesized following the protocol above are presented in Table 7 below. [Table 7] NOT Alcohol introduced into the synthesis R1 R2 R3 x Polymeric fraction y PAS7 (example 1) n -heptanol methyl n -heptyle methyl 0 1 à 140 PAS7(a) (example 7) 75% n-heptanol / 25% phenol / methyl n -heptyl or phenyl methyl 0 1 à 140 PAS7(b) (example 7) 50% n-heptanol / 50% phenol / methyl n -heptyl or phenyl methyl 0 1 à 140

[0136] The average value of y for each of the polyalkoxysiloxane oils in Table 1 is between 50 and 60.

[0137] The characterization of the polyalkoxysiloxanes obtained is illustrated in Table 8 below. The characteristics tested are those defined in Example 3. [Table 8] Features PAS7 PAS7(a) PAS7(b) Coefficient of Friction (CoF) 0,056 0,116 0,087 Kinematic viscosity at (cSt): 100°C 6,24 6,89 6,01 40°C 18,9 23,1 21,0 -40°C 730 1495 2662 Viscosity index 325 291 263 Water content (ppm) (ASTM D 1533) 64 6 13 Pour point (°C) <-72 -84 -75

[0138] This test shows on the one hand that the polyalkoxylsiloxanes according to the invention PAS7(a) and PAS7(b) in which the R2 group varies within the same compound and can be either a phenyl group or a heptyl group, have a very low coefficient of friction (well below 0.5), equivalent to the polyalkoxysiloxane PAS7 of example 1. These compounds PAS7(a) and PAS7(b) comprising a phenyl R2 group therefore also have excellent lubricating power.

[0139] Of course, various other modifications can be made to the invention within the scope of the attached claims.

Claims

1. Use of a polyalkoxysiloxane base oil of formula (I) wherein each R1 is independently a C1 to C4 alkyl group, each R2 is independently a C2 to C22 alkyl or alkenyl group or a phenyl, R3 is a methyl group, x is equal to 0, y is an integer between 1 and 500, or a mixture of polyalkoxysiloxane base oils of formula (I), as lubricating agent.

2. Use according to claim 1, wherein each R1 is independently selected from the methyl and ethyl groups.

3. Use according to claim 2, wherein each R1 is a methyl group.

4. Use according to any one of claims 1 to 3, wherein each R2 is independently a C4 to C18 alkyl or alkenyl group, preferably a C4 to C12.

5. Use according to any one of claims 1 to 3, wherein each R2 is independently a C2 to C14 alkyl or alkenyl group and a phenyl group, preferably the phenyl group represents, in number, 70% or less, preferably 60% or less, and typically 50% or less of the unit repeated y times.

6. Use according to any one of claims 1 to 5, wherein y is between 1 and 140.

7. Use according to any one of claims 1 to 6, wherein the polyalkoxysiloxane is selected from the group consisting of the polyalkoxysiloxanes of formula (I) wherein: x = 0, y is between 1 and 140, each R1 is a methyl, each R3 is a methyl and each R2 is an n-butyl chain, x = 0, y is between 1 and 140, each R1 is a methyl, each R3 is a methyl and each R2 is a heptyl chain, x = 0, y is between 1 and 140, each R1 is a methyl, each R3 is a methyl and each R2 is a dodecyl chain, x = 0, y is between 1 and 140, each R1 is a methyl, each R3 is a methyl and each R2 is a tetradecyl chain, and x = 0, y is between 1 and 140, each R1 is a methyl, each R3 is a methyl and each R2 is selected from the group consisting of octadecenyl and hexadecyl chains; x= 0, y is between 1 and 140, each R1 is a methyl, each R3 is a methyl and each R2 is independently an n-heptyl or phenyl, phenyl representing, in number, 50% or less of the unit repeated y times.

8. Use of a polyalkoxysiloxane base oil of formula (I) such as defined in any one of claims 1 to 7, in mixture with at least one other base oil as a co-base.

9. Lubricating composition, characterised in that it comprises a polyalkoxysiloxane base oil of formula (I) such as defined in any one of the preceding claims and at least one additive.

10. Lubricating composition according to claim 9, wherein said at least one additive is selected from the group consisting of extreme-pressure additives, antiwear additives, detergent additives, viscosity-modifying additives, antifoaming additives, antioxidant additives, flame retardants, corrosion inhibitors and a combination of at least two of said additives.

11. Lubricating composition according to claim 9 or 10, wherein said composition comprises at least one flame-retarding additive, so as to obtain a non-flammable hydraulic fluid.

12. Polyalkoxysiloxane of formula (II): wherein each R1 is independently a C1 to C4 alkyl group, each R2 is independently a C4 to C16 alkyl or alkenyl group, or a phenyl, x is 0, y is an integer between 1 and 500.

13. Polyalkoxysiloxane of formula (II) according to claim 12, wherein each R1 is selected independently from a methyl and ethyl groups ; each R2 is independently a C4 to C14 alkyl or alkenyl group, preferably C4 to C12, or a phenyl group, provided that, when R2 is a phenyl group, the latter represents at least 50% of the unit repeated y times. each R3 is a methyl group; x = 0; and y is between 1 and 140.

14. Polyalkoxysiloxane of formula (I) according to claim 12 or 13, selected from the group consisting of the polyalkoxysiloxanes of formula (II) in which: x = 0, y is between 1 and 140, each R1 is a methyl, and each R2 is an n-butyl chain, x = 0, y is between 1 and 140, each R1 is a methyl, and each R2 is a heptyl chain, x = 0, y is between 1 and 140, each R1 is a methyl, and each R2 is a dodecyl chain, x = 0, y is between 1 and 140, each R1 is a methyl, and each R2 is a tetradecyl chain, and x = 0, y is between 1 and 140, each R1 is a methyl, and each R2 is selected from the group consisting of octadecenyl and hexadecyl chains, x= 0, y is between 1 and 140, each R1 is a methyl, each R3 is a methyl, and each R2 is independently an n-heptyl or phenyl, the phenyl representing, in number, 50% or less of the unit repeated y times.