Lubricant Compositions Containing Traction Coefficient Additives - Patent application

Abstract

The present invention relates to a lubricant composition suitable for use in electric vehicles. The lubricant composition includes a traction coefficient additive, the traction coefficient additive being an ester, the ester being the reaction product of at least one saturated branched aliphatic monohydric alcohol having from 12 to 32 carbon atoms and at least one aliphatic carboxylic acid having from 6 to 32 carbon atoms. The lubricant composition described herein provides an electric vehicle gear oil and imparts desirable traction coefficient properties in use.

Description

[Technical field]

[0001] The present invention relates to lubricant compositions suitable for use in electric vehicles that contain a traction coefficient additive. The lubricant compositions described herein find utility, among other things, in electric vehicle gear oils, and especially in electric vehicle transmission fluids, and provide improved traction properties in use. [Background technology]

[0002] An electric vehicle is a vehicle that is propelled using one or more electric motors. Electric vehicles can be fully electric (also known as pure electric or all-electric vehicles) or hybrid in nature (in hybrid electric vehicles, propulsion can sometimes be achieved from alternative means such as hydrocarbon-derived fuels). Electric vehicles also include range-extended electric vehicles in which the vehicle is powered by an electric motor and a plug-in battery, although vehicles also include auxiliary combustion engines that are used only to supplement the battery charge and not as the primary propulsion source. The present invention is suitable for use in all of these types of electric vehicles.

[0003] Gear oils are a subclass of lubricants that typically contain a lubricating base stock (or base oil) as its major component. The choice of lubricating base stock utilized in a lubricant can have a significant impact on properties such as oxidation and thermal stability, volatility, low temperature fluidity, ability to dissolve additives, contaminants, and decomposition products, and traction. More specifically, it has been traditionally taught by the industry that the traction coefficient is an inherent property of the base stock fluid (i.e., based on the chemical composition of the base oil) and that the traction coefficient is not affected by additives. It is widely believed in the industry that the viscosity of the base stock fluid determines the lubricant's coefficient of traction in service.

[0004] The American Petroleum Institute (API) defines five groups of lubricant base stocks (API Publication 1509).

[0005] Groups I, II, and III are mineral oils classified by the amount of saturates and sulfur they contain and their viscosity index. Table 1 below illustrates these API classifications for Groups I, II, and III.

[0006] [Table 1]

[0007] Group I base stocks are solvent refined mineral oils and are the least expensive base stocks to produce. They provide satisfactory oxidation stability, volatility, low temperature performance, and traction properties, and have especially good solvency for additives and contaminants.

[0008] Group II base stocks are primarily hydrotreated mineral oils that typically offer improved volatility and oxidation stability compared to Group I base stocks. Use of Group II stocks has grown to about 30% of the U.S. market.

[0009] Group III (including Group III+) base stocks can be highly hydrotreated mineral oils or produced by isomerization of wax or paraffins. They have better oxidation stability and volatility than Group I and II base stocks, but are known to have a limited range of commercially available viscosities. Group III+ base stocks include those derived from GTL (gas to liquid) fuel streams.

[0010] Group IV base stocks differ from Groups I-III in that they are synthetic base stocks, e.g., polyalphaolefins (PAOs). PAOs have good oxidative stability, volatility, and low pour points. Drawbacks include moderate solubility of polar additives, e.g., anti-wear additives.

[0011] Group V base stocks are all base stocks not included in Groups I through IV. Examples include alkyl naphthalenes, alkyl aromatics, vegetable oils, esters, polycarbonates, silicone oils, and polyalkylene glycols.

[0012] The rapid move towards electrification of passenger vehicles has exceeded the understanding and specifications of OEMs and regulators of current gear oil specifications. Current generation hybrid and electric vehicles still use standard automatic transmission fluid (ATF) formulations that were not specifically designed for this application. Current gear oils do not meet OEMs' dynamic requirements due to rapid advancements in electric vehicle technology, ATF base fluids, and add packs. Furthermore, because electric motors in EVs are highly efficient, gear lubricant losses in EV powertrain systems can be very large. Reducing energy losses provides improved battery life in EVs during use, which means that EVs will require less frequent charging as battery range increases.

[0013] Furthermore, thermal management of components in electric vehicles is becoming increasingly important. In vehicle batteries, thermal management is crucial to ensure safe running and use. Currently, much research is being conducted to explore immersion-cooled battery systems, where the battery is in direct contact with a dielectric cooling fluid. Fluids with high thermal properties, such as heat capacity and thermal conductivity, are therefore required for this application. Cooling of electronic power systems, such as electric motors and transmissions, is also required to keep them functioning effectively without overheating. Removing excess heat from electronic systems also helps to reduce electrical resistance and therefore improve engine efficiency. Thus, lubricant compositions suitable for use in electric vehicles are significantly different from those developed for use in automotive combustion engines.

[0014] Thus, despite the continued development of lubricant technology for transmissions and gearboxes in internal combustion engines, hybrids and electric vehicles, there remains a need for lubricant formulations with improved energy efficiency over the life of the lubricant. More specifically, there is a need for lubricant technology optimized and tailored to meet the requirements of electric vehicle gearboxes, which differ from those of traditional combustion engines. Thus, there remains an active need for new base oils that provide high performance in electric engines (especially low traction and high thermal conductivity) but are commercially viable for the electric vehicle passenger car market.

[0015] It is an object of the present invention to provide a lubricant composition suitable for use in the gearbox of an electric vehicle, thereby providing improved traction and thus minimizing energy loss. As well as providing traction, the lubricant composition should have sufficient oxidation stability, good low temperature properties and compatibility with materials such as elastomers and copper. Summary of the Invention

[0016] Accordingly, the present invention provides a lubricant composition suitable for use in electric vehicles comprising a traction coefficient additive, the traction coefficient additive being an ester, the ester being i) at least one saturated branched chain aliphatic monohydric alcohol having 12 to 24 carbon atoms; ii) It is the reaction product of at least one aliphatic carboxylic acid having 6 to 24 carbon atoms.

[0017] The present invention also provides a method of reducing the traction coefficient in a gearbox comprising using a lubricant according to the first aspect of the invention.

[0018] The traction coefficient additives described herein can advantageously improve the performance of the gearbox in which the lubricant composition is applied by providing a reduced traction coefficient.

[0019] The traction coefficient additives described herein can be used as traction coefficient reducing additives in lubricant compositions, more particularly in gear oils for gearboxes, especially in gear oils for electric vehicle gearboxes. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] It is understood that any upper or lower quantity or range limits used herein can be independently combined.

[0021] When describing the number of carbon atoms in a substituent (e.g., "C1-C6"), it will be understood that the number refers to the total number of carbon atoms present in the substituent, including any present in any branching groups. Additionally, when describing the number of carbon atoms in, for example, a fatty acid, this refers to the total number of carbon atoms, including the carbon atoms in the carboxylic acid and any carbon atoms present in any branching groups.

[0022] As used herein with respect to the described and claimed inventions, depending on the context, the term "wt. %" refers to the weight percentage of the component being stated as a percentage of the total weight of the lubricant composition. When the context refers to a particular component, such as NOACK evaporation loss, the term "wt. %" refers to the weight percentage of the total weight of the component.

[0023] In accordance with the present invention, there is provided a lubricant composition suitable for use in electric vehicles comprising a traction coefficient additive, the traction coefficient additive being an ester, the ester being iii) at least one saturated branched chain aliphatic monohydric alcohol having 12 to 32 carbon atoms; iv) The reaction product of at least one aliphatic carboxylic acid having 6 to 32 carbon atoms.

[0024] Suitably, the aliphatic carboxylic acid may be saturated or unsaturated, straight chain or branched chain. Preferably, the aliphatic carboxylic acid is saturated, thereby providing improved oxidative stability.

[0025] Desirably, the carboxylic acid may be derived from vegetable fats and / or oils. Thus, preferably, the aliphatic carboxylic acid may be a fatty acid. The fatty acid may be saturated or unsaturated. The fatty acid may be straight-chained or branched. Preferably, the fatty acid is saturated. Naturally, fatty acids having an even number of carbons in the fatty chain are more abundant in nature and therefore more readily and cheaply available, and therefore these forms of fatty acids may be preferred, particularly those having C6, C8, C10, C12, C14, C16, C18, and C20 chain lengths. The fatty acid may be understood to contain a medium chain fatty acid chain, providing a fatty chain containing preferably 6 to 18 carbons.

[0026] Preferably, the carboxylic acid is a mono- or dicarboxylic acid and the ester is a mono- or diester, most preferably the carboxylic acid is a mono-carboxylic acid and the ester is a mono-ester. Suitably, the carboxylic acid may be selected from one or more of the following: Hexenoic acid, heptanoic acid, octanoic acid, decanoic acid, dodecanoic acid, myristic acid, stearic acid, behenic acid, hexenoic acid, linoleic acid, linoleic acid, palmitoleic acid, oleic acid, elaidic acid, erucic acid, nervonic acid, isohexanoic acid, 4-methylpentanoic acid, isooctanoic acid, 2-ethylhexanoic acid, isostearic acid, isobehenic acid, 2-ethyl-1-butanoic acid, 2-butyl-octanoic acid, 2-hexyl-decanoic acid, 2-octyldodecanoic acid, 2-decyltetradecanoic acid, adipic acid, suberic acid, azelaic acid, sebacic acid, ferrogenic acid, cyclohexanedicarboxylic acid, fatty acids, tall oil, reaction products with acrylic acid, and alkenylsuccinic acid.

[0027] Suitably, the at least one saturated branched aliphatic monohydric alcohol having from 12 to 32 carbon atoms can be obtained from any suitable source and can typically be selected from Guerbet alcohols, oxo alcohols, aldol condensation derived alcohols, and mixtures thereof.

[0028] Preferably, the at least one saturated branched aliphatic monohydric alcohol has 12 to 32 carbon atoms, more preferably 12 to 24 carbon atoms, most preferably 12 to 20 carbon atoms. The preferred shorter chain length has a positive effect on the ester viscosity for the intended use. More specifically, the at least one saturated branched aliphatic monohydric alcohol having 12 to 32 carbon atoms is an alcohol branched at the β-position on the main carbon chain. Preferably, such alcohols may be selected from 2-butyloctanol, isomyristoyl alcohol, 2-hexyldecanol, isostearyl alcohol, 2-octyldecanol-1,2-heptylundecanol-1,2-octadodecanol-1,2-nonyltridecanol-1 and 2-decyltetradecanol-1, and mixtures of two or more such alcohols. Such alcohols are advantageously Guerbet alcohols, and preferably the alcohols may be selected from C12 Guerbet alcohols, C14 Guerbet alcohols, C16 Guerbet alcohols, C18 Guerbet alcohols, C20 Guerbet alcohols, and C24 Guerbet alcohols, or mixtures thereof. The use of the preferred Guerbet alcohols is believed to provide esters with unusually high thermal conductivity for their given viscosity, i.e., the thermal conductivity of the traction coefficient additives is surprising, making the lubricant compositions of the present invention suitable for use in electric vehicles. The monoesters according to the present invention have also been found to have low viscosity, low polarity and low volatility, while providing exceptionally low traction, particularly compared to other esters.

[0029] Suitably, the traction coefficient is greater than 0.131 W / mK at 40° C., preferably greater than 0.135 W / mK at 40° C., more preferably greater than 0.141 W / mK at 40° C., and most preferably greater than 0.151 W / mK at 40° C. Thermal conductivity is measured according to the method described below with respect to the Examples.

[0030] Preferably, the traction coefficient additive has a kinematic viscosity at 100° C. of 8.0 or less, preferably 6.0 cSt or less, and most preferably 4.0 cSt or less, and in some particularly preferred embodiments, the traction coefficient additive has a kinematic viscosity at 100° C. of 3.3 cSt or less. Additionally or alternatively, the traction coefficient additive has a kinematic viscosity at 40° C. of 30 cSt or less, preferably 20 cSt or less, and most preferably 10 cSt or less.

[0031] Preferably, the traction coefficient additive has a viscosity index of at least 130, preferably at least 140.

[0032] Preferably, the traction coefficient additive has a pour point of -30°C or less, more specifically -35°C or less, especially -40°C or less.

[0033] Preferably, the traction coefficient additive has a NOACK evaporation loss at 250° C. of 22 wt% or less, preferably 20 wt% or less, more preferably 18 wt% or less. Additionally or alternatively, preferably, the traction coefficient additive has a NOACK evaporation loss at 200° C. of 9 wt% or less, preferably 7 wt% or less, more preferably 5 wt% or less.

[0034] Preferably, the traction coefficient additive has a flash point of at least 200°C, more preferably at least 210°C, and more specifically at least 220°C.

[0035] Preferably the traction coefficient additive has a non-polarity index (NPI) of at least 80, preferably at least 90, as described in EP-B-0792334.

[0036] Preferably, the traction coefficient additive is stable when kept for one week at −20° C. This low temperature stability can be tested by storing about 30 ml of a sample in a glass vial and placing the vial in a −20° C. freezer unit for one week, checking the sample at regular intervals and noting any signs of crystal formation or gelling.

[0037] A further advantage of the traction coefficient additive of the present invention is that it has been found to have minimal swelling effect on FKM and / or HNBR elastomers and therefore can be used at higher loading levels relative to conventional lubricant additives. Thus, in some less preferred embodiments, the lubricant composition may consist solely of the traction coefficient additive. Alternatively, the lubricant composition may comprise a majority of the traction coefficient additive such that the lubricant composition comprises more than 50 wt% of the traction coefficient additive. However, preferably, the lubricant composition comprises up to 50 wt% of the traction coefficient additive. Preferably, the lubricant composition comprises at least 3 wt%, more preferably at least 5 wt% of the traction coefficient additive. The lubricant composition may comprise up to 45 wt%, more preferably up to 35 wt%, especially up to 25 wt% of the traction coefficient additive. In one embodiment of the present invention, the lubricant composition comprises 3 wt% to 25 wt%, preferably 5 wt% to 20 wt% of the traction coefficient additive.

[0038] Preferably, the lubricant composition comprises at least one base oil, suitably the at least one base oil is selected from Group I to Group IV base oils, or a mixture of two or more thereof. Preferably, the lubricant composition comprises at least one base oil as a major component of the lubricant composition. One advantage of the present invention is that the traction coefficient additive has good compatibility with Group I to Group IV base oils, allowing flexibility in the selection of base oils in the lubricant composition. However, for use in electric vehicles, lubricant compositions comprising at least one of Group III or Group IV base oils may be preferred. Examples of suitable Group III base oils include mineral oils, as well as Group III+GTL base oils made by converting natural gas (i.e., methane and higher alkanes) to synthesis gas (carbon monoxide and hydrogen) and then via oligomerization (e.g., via the Fischer-Tropsch process) to high molecular weight molecules that are hydrocracked to produce isoparaffins in the required lubricant boiling point / viscosity range. Examples of suitable Group IV base oils include C8 to C 12 Poly-alpha-olefins (PAOs) are derived from alpha-olefins and have kinematic viscosities ranging from 2 cSt to 8 cSt at 100° C. An example of a PAO is SpectraSyn MaX available from Exxon. Examples of Group V base oils include polyalkylene glycols (PAGs), alkylbenzenes and esters (where it is understood that Group V ester base oils are esters other than the traction coefficient additives described above). Examples of esters include Priolube 3970™, TMPnC8 / nC 10 Polyol esters are included.

[0039] In one embodiment of the present invention, the lubricant composition consists essentially of the traction coefficient additive and at least two base oils, the at least two base oils including at least one Group V base oil, particularly an ester.

[0040] In one embodiment, the gear oil formulation is non-aqueous. However, it will be understood that the components of the lubricant composition may contain small amounts of residual water (moisture) and thus may be present in the lubricant composition. The lubricant composition may contain less than 5 wt.% water based on the total weight of the composition. More preferably, the lubricant composition is substantially free of water, i.e., contains less than 2 wt.%, less than 1 wt.%, or preferably less than 0.5 wt.% water based on the total weight of the composition. Preferably, the lubricant composition is substantially anhydrous.

[0041] To render the lubricant composition more suitable for its intended use, in particular for use as a gear oil, the composition may contain one or more of the following additive types: 1. Dispersants: for example, alkenyl succinimides, alkenyl succinic acid esters, alkenyl succinimides modified with other organic compounds, alkenyl succinimides modified by post-treatment with ethylene carbonate or boric acid, pentaerythritol, phenate-salicylates and their post-treated analogues, alkali metal or mixed alkali metal, alkaline earth metal borates, dispersions of hydrated alkali metal borates, dispersions of alkaline earth metal borates, polyamide ashless dispersants, etc., or mixtures of such dispersants. 2. Antioxidants: Antioxidants reduce the tendency of mineral oils to deteriorate in service, the deterioration being evidenced by oxidation products such as sludge and varnish-like deposits on metal surfaces and by an increase in viscosity. Examples of the antioxidant include 4,4'-methylene-bis(2,6-di-tert-butylphenol), 4,4'-bis(2,6-di-tert-butylphenol), 4,4'-bis(2-methyl-6-tert-butylphenol), 2,2'-methylene-bis(4-methyl-6-tert-butylphenol), 4,4'-butylidene-bis(3-methyl-6-tert-butylphenol), 4,4'-isopropylidene-bis(2,6-di-tert-butylphenol), 2,2'-methylene-bis(4-methyl-6-nonylphenol), 2,2'-isobutylidene-bis(4,6-dimethylphenol), 2,2'-methylenebis(4-methyl-6-cyclohexylphenol), 2, phenolic antioxidants such as 6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, 2,6-di-tert-butylphenol, 2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-l-dimethylamino-p-cresol, 2,6-di-tert-4-(N,N'-dimethylaminomethylphenol), 4,4'-thiobis(2-methyl-6-tert-butylphenol), 2,2'-thiobis(4-methyl-6-tert-butylphenol), bis(3-methyl-4-hydroxy-5-tert-butylbenzyl)-sulfide, and bis(3,5-di-tert-butyl-4-hydroxybenzyl). Other types of antioxidants include alkylated diphenylamines (e.g., Irganox L-57, e.g., BASF), metal dithiocarbamates (e.g., zinc dithiocarbamate), and methylene bis(dibutyldithiocarbamate). 3. Anti-Wear Agents: As the name suggests, these agents reduce wear on moving metal parts. Examples of such agents include phosphates, phosphites, carbamates, esters, sulfur-containing compounds, and molybdenum complexes. 4. Emulsifiers: For example, linear alcohol ethoxylates. 5. Demulsifiers: For example, addition products of alkylphenols and ethylene oxide, polyoxyethylene alkyl ethers, and polyoxyethylene sorbitan esters. 6. Extreme pressure agents (EP agents): for example, zinc dialkyl dithiophosphates (primary alkyl, secondary alkyl, and aryl types), sulfurized oils, diphenyl sulfide, methyl trichlorostearate, chlorinated naphthalenes, fluoroalkyl polysiloxanes, and lead naphthenate. A preferred EP agent is zinc dialkyl dithiophosphate (ZnDTP), for example, as one of the co-additive components for antiwear hydraulic oil compositions. 7. Multifunctional additives: for example, sulfurized oxymolybdenum dithiocarbamate, sulfurized oxymolybdenum organic phosphorodithioate, oxymolybdenum monoglycehde, oxymolybdenum diethylate amide, amine-molybdenum complex compounds, and sulfur-containing molybdenum complex compounds. 8. Viscosity index improvers: for example, polymethacrylate polymers, ethylene-propylene copolymers, styrene-isoprene copolymers, hydrogenated styrene-isoprene copolymers, polyisobutylene, and dispersant type viscosity index improvers. 9. Pour point depressants: for example, polymethacrylate polymers. It is an advantage of the present invention that the pour points of the compounds of formula (I) are suitable for use as gearbox oils, however, embodiments utilizing relatively long chain linear molecules may benefit from the addition of a pour point depressant. Additionally, the presence of some alternative additives may adversely affect the pour point of the formulation, making the addition of a pour point depressant attractive. 10. Foam suppressors: for example, alkyl methacrylate polymers and dimethyl silicone polymers. 11. Friction Modifiers: include amides, amines, and partial aliphatic esters of polyhydric alcohols, and may include, for example, glycerol monooleate, oleylamide, and alternative friction modifiers available from Croda under the tradename "Perfad" or available from Nouryon under the tradename "Ethomeen."

[0042] Suitably, the lubricant composition may comprise at least 0.5 wt%, preferably at least 1 wt%, more preferably at least 5 wt%, of one or more additive types, based on the total weight of the formulation. The lubricant composition may comprise up to 30 wt%, preferably up to 20 wt%, more preferably up to 10 wt%, of one or more additive types, based on the total weight of the formulation.

[0043] Other additives may also be present in the lubricant composition of known functional groups at levels of 0.01 to 30 wt%, more preferably 0.01 to 20 wt%, especially 0.01 to 10 wt%, based on the total weight of the lubricant composition. These may include detergents, corrosion inhibitors, rust inhibitors, and mixtures thereof. Corrosion inhibitors include sarcosine derivatives, such as Crodasinic O available from Croda Europe Ltd. Ashless detergents include carboxylic acid dispersants, amine dispersants, Mannich dispersants, and polymeric dispersants. Ash-containing dispersants include neutral and basic alkaline earth metal salts of acidic organic compounds. Additives may have two or more functional groups in a single material.

[0044] The additive(s) may be available in the form of a commercially available additive pack. Such additive packs vary in composition depending on the required use of the additive pack. Those skilled in the art may select a suitable commercially available additive pack for gear oil. An example of an additive pack that is particularly suitable for gear oil is Evogen 5201, e.g., Lubrizol, USA, which is specifically designed for use in electric vehicles.

[0045] The lubricant composition preferably comprises at least 0.05 wt.-%, more preferably at least 0.5 wt.-%, in particular at least 1 wt.-%, especially at least 1.5 wt.-% of further additives (additive pack), based on the total weight of the lubricant composition. The lubricant composition preferably comprises up to 15 wt.-%, more preferably up to 10 wt.-%, in particular up to 4 wt.-%, especially up to 2.5 wt.-% of further additives (additive pack), based on the total weight of the lubricant composition.

[0046] Notwithstanding the above examples, to make a lubricant composition suitable for use in an electric vehicle, the selection of any additive should take into consideration copper compatibility (due to the requirements of electric motors), as well as providing or exhibiting low (but not necessarily zero) electrical conductivity; however, not all additives commonly utilized in combustion engine automotive engines are suitable for use in electric ah vehicle powertrain fluids.

[0047] The lubricant composition may have a kinematic viscosity according to an ISO grade, which is defined as the midpoint kinematic viscosity of a sample at 40° C. in cSt (mm 2 The viscosity is specified in units of viscosity per second ( / sec). For example, ISO100 has a viscosity of 100±10 cSt, and ISO1000 has a viscosity of 1000±100 cSt. The lubricant composition preferably has a viscosity in the range of ISO10 to ISO1500, more preferably ISO68 to ISO680.

[0048] The present invention also provides a gear oil comprising the above lubricant composition. Gear oils can be considered lubricating fluids and may have utility in other areas as lubricants, even when heat transfer and traction are not important. The gear oil formulation may be suitable for use as industrial, automotive and / or marine gear oils for use in any type of transmission system. However, the gear oil is suitable for providing a gearbox oil, more specifically an integrated gearbox oil suitable for use in electric vehicles, because the above lubricant composition provides advantageous heat transfer properties and desirable traction properties when in use. Additionally, providing good thermal properties in the gear oil may enhance engine life.

[0049] It is also envisioned that the base oil may find utility as a heat transfer fluid. Such a heat transfer fluid may provide a means of removing heat from a system. Such a system that requires or benefits from the use of a heat transfer fluid may be a mechanical system or an electrical system. The base oil of the present invention may be well suited for use as a heat transfer fluid in electrical systems, and more particularly, as a heat transfer fluid in electrical vehicles.

[0050] According to another embodiment of the present invention, there is provided a method for improving energy efficiency in an electric vehicle, comprising using a lubricant composition according to the first aspect of the present invention in the powertrain of the electric vehicle. The lubricant composition may be used in various systems in the powertrain, such as axles, differentials, transmissions, battery packs, and power electronics. The lubricant composition has suitable properties for use in an electric vehicle powertrain, including traction, thermal, conductivity, and viscosity properties optimized for use in an electric vehicle. In a further alternative, there is provided a method for improving heat removal from an electric vehicle powertrain, comprising using a base oil according to the first aspect of the present invention in the electric vehicle powertrain.

[0051] Additionally or alternatively, a method for improving energy efficiency in an electric vehicle is provided, comprising using a lubricant composition according to an aspect of the present invention in a gearbox of the electric vehicle. More specifically, the method for improving energy efficiency in an electric vehicle comprises providing a lubricant composition in an integrated gearbox. Thus, there is provided a use of the lubricant composition as described herein in a vehicle powertrain, more specifically in an electric vehicle integrated gearbox. More specifically, the lubricant composition or gear oil may be used in systems in the powertrain, such as axles, differentials, transmissions, battery packs, and power electronics.

[0052] The lubricant compositions according to the present invention include those suitable for use in electric vehicle powertrains. More specifically, the lubricant compositions are gear oils suitable for use in both gear systems integrated with and not integrated with electric motors. Such systems include axles, differentials, and transmissions. It should be noted that an electric vehicle may be provided with two or more electric motors. [Brief description of the drawings]

[0053] The invention will now be described with reference to the following examples and the accompanying drawings. [Figure 1] 1 shows MTM traction coefficient data for experimental and commercial samples at 40° C. [Diagram 2] 1 shows MTM traction coefficient data for experimental and commercial samples at 60° C. [Diagram 3] 1 shows MTM traction coefficient data for experimental and commercial samples at 75° C. [Figure 4] 1 shows MTM traction coefficient data for experimental and commercial samples at 100° C. [Diagram 5] 1 shows MTM traction coefficient data for experimental and commercial samples at 120° C.

[0054] material The following materials are utilized in this example: Group III base oil Yubase 4. ex.SK lubricants Group IV base oil SpectraSyn PAO 4 ex. ExxonMobil. Commercially available low viscosity conventional automotive friction modifying additive - Priolube 3959 ex. Croda.

[0055] The invention will now be further described with reference to the following examples.

[0056] 1. Example The samples according to the present invention are detailed below in Table 1. The method of producing the sample materials is by conventional esterification methods known to those skilled in the art. The alcohol in each of the ester example samples is provided by Guerbet alcohols available under the trade name ISOFOL from Sasol.

[0057] [Table 2]

[0058] 2. Testing The following tests were used to evaluate the properties of the example base oils. 2.1 Oxidative stability was measured using an Anton Paar RapidOxy machine. 4 grams of sample is placed in a pressure vessel and charged with oxygen at 700 kPa before being heated to 140°C. The time taken for the pressure to drop by 10% is measured as the Oxidative Induction Time (OIT). This provides a relative measure of the resistance of the tested sample to oxidative degradation; the longer the OIT, the more oxidatively stable the sample. 2.2 Kinematic Viscosity (KV) was measured at 100°C and 40°C using an Anton Paar SVM Viscometer. The Viscosity Index (VI) of the tested materials is also provided. The higher the VI value, the more stable the material is over a temperature range. 2.3 Thermal conductivity was measured using a Thermtest THW-L2 based on the hot wire transient method. To create a reliable average, ten data points were collected at temperatures of 40°C and 80°C, with the fluid allowed to settle for 5 minutes between each data point. The test power was set to give a measured output power of 70-90mW and a temperature rise of approximately 3°C, and the test time was set to 1 second. 2.4 Pour point testing was performed on an ISL Mini Pour Point 5Gs to determine the minimum temperature at which the material will still flow which correlates to ASTM D97 and D2500. 2.5 The coefficient of friction was measured using a Mini Traction Machine (MTM) and testing was performed on a PCS MTM 1. All specimens required to set up the MTM, as well as standard specimens supplied by PCS (as shown in Table 2 below), were sonicated three times for 15 minutes in heptane using a Camsonix C940 ultrasonic bath, draining the heptane and then refreshing after each sonication. All specimens were dried using nitrogen before assembly in the MTM. The test profile shows a 0-100% Sliding to Rolling Ratio (SRR) at 16N and 41 data points are taken at a given temperature to create a traction curve. This is repeated at 40°C, 60°C, 75°C, 100°C, and 120°C to show performance over a wide range of temperatures. The test parameters are detailed in Table 3. 2.6 NOACK volatility at 250°C is measured according to standard test method ASTM D5800. Because electric vehicles do not operate at such high temperatures, a modified test based on ASTM D5800 but at a temperature of 200°C is also performed to provide the NOACK volatility at 200°C measurement. 2.7 Hydrolytic stability measured over 15 days (RR1006). 250 g of oil and 25 g of water are mixed in a conical flask and fitted with a water trap (airlock). This is placed in a 90° C. oven for 15 days and the acid number is tested once every few days. 2.8 Seal Swelling Test: Seals of different materials are immersed in the sample to be tested at 100°C for 2 weeks.

[0059] [Table 3]

[0060] [Table 4]

[0061] 3. Test Data The sample materials detailed in Table 1, as well as commercial base oils from Group III and Group IV detailed above, were subjected to the tests outlined in Section 2 above.

[0062] [Table 5]

[0063] The example samples of the present invention have physical properties that make them useful as additives for use in lubricant compositions for use in electric vehicles. In particular, the balance between kinematic viscosity (KV) and thermal conductivity is particularly desirable for use in the powertrain of electric vehicles. More specifically, the sample designated DE10766 provides a material that is particularly suitable for use in electric vehicles, as it has a low kinematic viscosity material of only 2.9 cSt at 100°C, an excellent viscosity index (VI), and a very low NOACK of its viscosity (NOACK data is provided in Table 5 below). The pour point of DE10766 is also acceptable for use in electric vehicle lubricant applications.

[0064] [Table 6]

[0065] Sample DE10766 was further tested to determine its electrical breakdown voltage, an important consideration for a fluid suitable for use in electric vehicles. The electrical breakdown voltage values ​​were lower than expected.

[0066] [Table 7]

[0067] The hydrolytic stability of sample DE10766 is excellent, varying by only 0.38 AV units over 15 days. This is surprising for a monoester, and it is believed that the use of Guerbet alcohols provides some resistance to hydrolysis, although the reasons for this are not yet understood.

[0068] Compatibility with engine seals is another important feature of any additive used in electric vehicle powertrains. Sample DE10766 was also tested to access seal swelling over a two week period. Generally, low viscosity materials are highly polar and will swell the elastomer significantly. As a comparison additive, Priolube 3959, a commercial lubricant additive diester with a KV of 2.5 cSt at 100°C, was also tested. Since a small amount of seal swelling is desirable, the DE10766 results are desirable where as Priolube 3959 would swell the elastomer to a greater, undesirable extent. The 1% swelling results in FKM mean that DE10766 can be used at high treat rates, despite its low viscosity.

[0069] [Table 8]

[0070] At 40°C, all of the samples tested had significantly lower traction than both Group III and Group IV base oils, and this trend continued at 60°C and 75°C, but at 100°C, the sample designated as 731-90 began to show high traction coefficients at low slide-to-roll ratios. It is believed that the very low viscosity of the 731-90 sample (KV at 100°C of 2.2 cSt) is the reason for this, and the sample is unable to maintain a lubricant film under low slide-to-roll ratio conditions. At 120°C, the sample designated as 731-84 also shows an increase in traction coefficient. The reason for this is unclear as sample 731-84 has a higher viscosity than DE10766, which allows it to maintain low traction levels even at high temperatures, and it is expected that a higher viscosity material would perform better. However, in any case, it is believed that all example samples prepared herein have utility for use in electric vehicle powertrains, since the operating temperatures in electric vehicles are typically below 100°C. In particular, DE10766 appears to be an excellent choice for use in reducing traction in electric vehicles. It is low viscosity, low polarity, and has excellent thermal properties, low traction properties, and elastomer compatibility. Furthermore, other example samples tested herein show that samples containing Isophor 12 and Isophor 16 provide desirably very low traction.

Claims

1. A method for lubricating a gearbox of an electric vehicle, comprising the step of lubricating the gearbox with a gear oil, wherein the gear oil comprises a base oil and an amount of a traction coefficient additive effective to reduce the traction coefficient characteristics of the gear oil, the traction coefficient additive comprises an ester, and the ester is a reaction product of at least one saturated aliphatic carboxylic acid having 6 to 32 carbon atoms and a saturated branched-chain aliphatic monohydric alcohol having 12 to 24 carbon atoms.

2. The method according to claim 1, wherein the base oil comprises a mixture of two or more base oils selected from at least two different groups of Group I, Group II, Group III, and Group IV.

3. The method according to claim 1, wherein the saturated branched-chain aliphatic monohydric alcohol is selected from the group consisting of C12 gerb alcohol, C14 gerb alcohol, C16 gerb alcohol, C18 gerb alcohol, C20 gerb alcohol, and C24 gerb alcohol.

4. The method according to claim 1, wherein the saturated branched-chain aliphatic monohydric alcohol is 2-octyldodecanoic acid.

5. The method according to claim 1, wherein the saturated branched-chain aliphatic monohydric alcohol is 2-butyl-octanoic acid.

6. The method according to claim 1, wherein the saturated aliphatic carboxylic acid is straight-chain.

7. The method according to claim 1, wherein the saturated aliphatic carboxylic acid is branched-chain.

8. The method according to claim 1, wherein the ester is a heptanoate ester.

9. The method according to claim 1, wherein the ester is a stearate ester.

10. The method according to claim 1, wherein the ester is isostearate ester.

11. The method according to claim 1, wherein the ester is 2-octyldodecyl isooctadecanoate.

12. The method according to claim 1, wherein the ester is 2-hexyldecyl heptanoate.

13. The method according to claim 1, wherein the ester is 2-butyloctyl octadecenoate.

14. The method according to claim 1, wherein the ester is bis(2-butyloctyl) decanedioate.

15. The method according to claim 1, wherein the ester is 2-octyldodecyl isooctadecanoate.

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