Lubricating oil composition

Abstract

A composition that is used as a lubricating oil includes: a lubricant base oil including a mineral base oil or no mineral base oil, a poly-α-olefin base oil or no poly-α-olefin base oil, and at least one ester base oil, wherein the ester base oil is an ester of at least one saturated or unsaturated fatty acid having no branched chain structure, and at least one alcohol, and the content of the ester base oil is no less than 15 mass % based on total mass of the lubricant base oil, the ester base oil including no fatty acid having a branched chain structure, and the kinematic viscosity of the composition at 40° C. is 2.0 to 23.0 mm2 / s.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to lubricating oil compositions.BACKGROUND ART

[0002] In recent years, electric vehicles using electric motors as power sources for running, and hybrid vehicles using electric motors and internal combustion engines together as power sources for running have been attracting interest in view of energy efficiency and environmental compatibility. Electric motors generate heat following operation, whereas including heat-sensitive components such as coils and magnets. Thus, these vehicles, which use electric motors as power sources for running, are provided with means of cooling the electric motors. As the means of cooling electric motors, air cooling, water cooling, and oil cooling are known. Among them, the oil cooling system is to circulate oil around the insides of electric motors, thereby bringing heat generating components (such as coils, cores and magnets) in the electric motors into direct contact with coolant (oil), so that a cooling effect can be produced. Electric motors using the oil cooling system are lubricated and cooled at the same time by circulating oil (lubricating oil) around the insides thereof.

[0003] Patent literatures 1 to 9 disclose technologies relating to various lubricating oils.CITATION LISTPatent Literature[Patent Literature 1] JP 2020-172642 A

[0005] [Patent Literature 2] JP 2010-090210 A

[0006] [Patent Literature 3] JP 2015-098592 A

[0007] [Patent Literature 4] JP 2009-249496 A

[0008] [Patent Literature 5] JP 2017-155079 A

[0009] [Patent Literature 6] JP 2016-194002 A

[0010] [Patent Literature 7] WO 2020 / 095968 A1

[0011] [Patent Literature 8] JP 2019-151804 A

[0012] [Patent Literature 9] JP 2020-105347 ASUMMARY OF INVENTIONTechnical Problem

[0013] As described above, lubricating compositions to be used in electric vehicles and hybrid vehicles are required to have both cooling characteristics and fuel efficiency (lubricity) for electric motors, inverters, etc., but conventionally, do not always have both enough.

[0014] An object of the present disclosure is to provide a lubricating oil composition capable of having both cooling characteristics and fuel efficiency.Solution to Problem

[0015] The present disclosure encompasses the following modes [1] to [5].

[0016] [1] A composition that is used as a lubricating oil, the composition comprising: a lubricant base oil comprising a mineral base oil or no mineral base oil, a poly-α-olefin base oil or no poly-α-olefin base oil, and at least one ester base oil, wherein the ester base oil is an ester of at least one saturated or unsaturated fatty acid having no branched chain structure, and at least one alcohol, and a content of the ester base oil is no less than 15 mass % based on total mass of the lubricant base oil, the ester base oil comprising no fatty acid having a branched chain structure, and kinematic viscosity of the composition at 40° C. is 2.0 to 23.0 mm2 / s.

[0017] [2] The lubricating oil composition according to [1], comprising: at least one poly(meth)acrylate or no poly(meth)acrylate, the poly(meth)acrylate having a weight average molecular weight of no more than 30,000 in an amount of no more than 10 mass % based on total mass of the composition.

[0018] [3] The lubricating oil composition according to [1] or [2], wherein the composition is used to lubricate electric motors.

[0019] [4] The lubricating oil composition according to [1] or [2], wherein the composition is used to lubricate gears.

[0020] [5] The lubricating oil composition according to [1] or [2], wherein the composition is used to lubricate electric motors or to lubricate the electric motors and transmissions, in automobiles comprising the electric motors.Advantageous Effects of Invention

[0021] A lubricating oil composition according to the present disclosure is capable of having both cooling characteristics and fuel efficiency.DESCRIPTION OF EMBODIMENTS

[0022] The present disclosure will be hereinafter described. In the present description, the expression “A to B” concerning the numerical values A and B shall be equivalent to “no less than A and no more than B” unless otherwise specified. In such expression, if a unit is added to the numerical value B only, the same unit shall be applied to the numerical value A. In the present description, the word “or” shall mean a logical sum unless otherwise specified.

[0023] In the present description, the “weight average molecular weight” means the weight average molecular weight measured by gel permeation chromatography (GPC) in terms of standard polystyrene. The measurement conditions for GPC are as follows.[GPC Measurement Conditions]device: ACQUITY (registered trademark) APC UV RI System, manufactured by Waters Corporation

[0025] column: two columns of ACQUITY (registered trademark) APC XT900A manufactured by Waters Corporation (gel particle size: 2.5 μm, column size (inner diameter×length): 4.6 mm×150 mm), and one column of ACQUITY (registered trademark) APC XT200A manufactured by Waters Corporation (gel particle size: 2.5 μm, column size (inner diameter x length): 4.6 mm×150 mm) are connected in series in this order from the upstream side,

[0026] column temperature: 40° C.

[0027] sample solution: tetrahydrofuran solution having a sample concentration of 1.0 mass %

[0028] eluent: tetrahydrofuran

[0029] solution injection volume: 20.0 μL

[0030] detector: differential refractometer

[0031] standard material: standard polystyrene (Agilent EasiCal (registered trademark) PS-1 manufactured by Agilent Technologies, Inc.), eight points (molecular weight: 2698000, 597500, 290300, 133500, 70500, 30230, 9590 and 2970)

[0032] If the weight average molecular weight measured under the foregoing conditions is less than 10000, the columns and the standard material are changed according to the following conditions, and the weight average molecular weight is measured again.

[0033] column: one column of ACQUITY (registered trademark) APC XT125A manufactured by Waters Corporation (gel particle size: 2.5 μm, column size (inner diameter×length): 4.6 mm×150 mm), and two columns of ACQUITY (registered trademark) APC XT45A manufactured by Waters Corporation (gel particle size: 1.7 μm, column size (inner diameter x length): 4.6 mm×150 mm) are connected in series in this order from the upstream side standard material: standard polystyrene (Agilent EasiCal (registered trademark) PS-1 manufactured by Agilent Technologies, Inc.), 10 points (molecular weight: 30230, 9590, 2970, 890, 786, 682, 578, 474, 370 and 266)

[0034] A lubricating oil composition according to the present disclosure comprises a lubricant base oil comprising a mineral base oil or no mineral base oil, an α-olefin base oil or no α-olefin base oil, and at least one ester base oil.1. Composition of Lubricant Base Oil1.1. Ester Base Oil

[0035] The ester base oil contained in the lubricant base oil of the present disclosure is an ester of at least one alcohol, and at least one saturated or unsaturated fatty acid having no branched chain structure. The ester base oil comprises no fatty acid having a branched chain structure.

[0036] The ester base oil falls under the group V base oil of API base stock categories.

[0037] The alcohol constituting the ester may be a monohydric or a polyhydric alcohol.

[0038] As a monohydric alcohol, usually a C1-24, preferably a C1-12, and more preferably a C1-8 monohydric alcohol is used. As such an alcohol, a straight or a branched chain alcohol may be used, and a saturated or an unsaturated alcohol may be used. Specific examples of a C1-24 alcohol as used herein include methanol, ethanol, straight or branched chain propanol, straight or branched chain butanols, straight or branched chain pentanols, straight or branched chain hexanols, straight or branched chain heptanols, straight or branched chain octanols, straight or branched chain nonanol, straight or branched chain decanol, straight or branched chain undecanol, straight or branched chain dodecanol, straight or branched chain tridecanol, straight or branched chain tetradecanol, straight or branched chain pentadecanol, straight or branched chain hexadecanol, straight or branched chain heptadecanol, straight or branched chain octadecanol, straight or branched chain nonadecanol, straight or branched chain icosanol, straight or branched chain henicosanol, straight or branched chain tricosanol, and straight or branched chain tetracosanol, and mixtures thereof.

[0039] As a polyhydric alcohol, a polyhydric alcohol usually having 2 to 10, preferably having 2 to 6 hydroxy groups is used. Specific examples of a polyhydric alcohol having 2 to 10 hydroxy groups as used herein include: dihydric alcohols such as ethylene glycol, diethylene glycol, polyethylene glycol (trimer to pentadecamer of ethylene glycol), propylene glycol, dipropylene glycol, polypropylene glycol (trimer to pentadecamer of propylene glycol), 1,3-propanediol, 1,2-propanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl-1,2-propanediol, 2-methyl-1,3-propanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, and neopentyl glycol; polyhydric alcohols such as glycerol, polyglycerol (dimer to octamer of glycerol, such as diglycerol, triglycerol, and tetraglycerol), trimethylol alkanes (including trimethylolethane, trimethylolpropane, and trimethylolbutane) and dimers to octamers thereof, pentaerythritol and dimer to tetramer thereof, 1,2,4-butanetriol, 1,3,5-pentanetriol, 1,2,6-hexanetriol, 1,2,3,4-butanetetrol, sorbitol, sorbitan, sorbitol glycerol condensates, adonitol, arabitol, xylitol, and mannitol; and saccharides such as xylose, arabinose, ribose, rhamnose, glucose, fructose, galactose, mannose, sorbose, cellobiose, maltose, isomaltose, trehalose, and sucrose; and mixtures thereof.

[0040] Among these polyhydric alcohols, polyhydric alcohols having 2 to 6 hydroxy groups such as ethylene glycol, diethylene glycol, polyethylene glycol (trimer to decamer of ethylene glycol), propylene glycol, dipropylene glycol, polypropylene glycol (trimer to decamer of propylene glycol), 13-propanediol, 2-methyl-1,2-propanediol, 2-methyl-13-propanediol, neopentyl glycol, glycerol, diglycerol, triglycerol, trimethylol alkanes (including trimethylolethane, trimethylolpropane, and trimethylolbutane) and dimers to tetramers thereof, pentaerythritol, dipentaerythritol, 1,2,4-butanetriol, 1,3,5-pentanetriol, 1,2,6-hexanetriol, 1,2,3,4-butanetetrol, sorbitol, sorbitan, sorbitol glycerol condensates, adonitol, arabitol, xylitol, and mannitol, and mixtures thereof are preferable. Further, ethylene glycol, propylene glycol, neopentyl glycol, glycerol, trimethylolethane, trimethylolpropane, pentaerythritol, and sorbitan, and mixtures thereof are more preferable. Among them, neopentyl glycol, trimethylolethane, trimethylolpropane, and pentaerythritol, and mixtures thereof are most preferable since greater thermo-oxidative stability is obtained.

[0041] As a monoprotoic acid that may constitute the ester, a C2-24 fatty acid is used. This fatty acid has no branched chain structure, that is, is of a straight chain, and may be saturated or unsaturated. The ester base oil comprises no fatty acid having a branched chain structure.

[0042] Examples include: saturated fatty acids such as acetic acid, propionic acid, straight chain butanoic acid, straight chain pentanoic acid, straight chain hexanoic acid, straight chain heptanoic acid, straight chain octanoic acid, straight chain nonanoic acid, straight chain decanoic acid, straight chain undecanoic acid, straight chain dodecanoic acid, straight chain tridecanoic acid, straight chain tetradecanoic acid, straight chain pentadecanoic acid, straight chain hexadecanoic acid, straight chain heptadecanoic acid, straight chain octadecanoic acid, straight chain nonadecanoic acid, straight chain icosanoic acid, straight chain heneicosanoic acid, straight chain docosanoic acid, straight chain tricosanoic acid, and straight chain tetracosanoic acid; and unsaturated fatty acids such as acrylic acid, straight chain butenoic acid, straight chain pentenoic acid, straight chain hexenoic acid, straight chain heptenoic acid, straight chain octenoic acid, straight chain nonenoic acid, straight chain decenoic acid, straight chain undecenoic acid, straight chain dodecenoic acid, straight chain tridecenoic acid, straight chain tetradecenoic acid, straight chain pentadecenoic acid, straight chain hexadecenoic acid, straight chain heptadecenoic acid, straight chain octadecenoic acid, straight chain nonadecenoic acid, straight chain icosenoic acid, straight chain henicosenoic acid, straight chain docosenoic acid, straight chain tricosenoic acid, and straight chain tetracosenoic acid; and mixtures thereof.

[0043] Among them, in view of a further improvement in lubricity and handleability, C3-20 saturated fatty acids, and C3-22 unsaturated fatty acids, and mixtures thereof are particularly preferable, and C4-18 saturated and unsaturated fatty acids, and mixtures thereof are more preferable, and in view of oxidation stability, C4-18 saturated fatty acids are most preferable.

[0044] Examples of a polyprotic acid as used herein include C2-16 diprotic acids, and trimellitic acid. A C2-16 diprotic acid as used herein has no branched chain structure, that is, is of a straight chain, and may be saturated or unsaturated. Specific examples include ethanedioic acid, propanedioic acid, straight chain butanedioic acid, straight chain pentanedioic acid, straight chain hexanedioic acid, straight chain heptanedioic acid, straight chain octanedioic acid, straight chain nonanedioic acid, straight chain decanedioic acid, straight chain undecanedioic acid, straight chain dodecanedioic acid, straight chain tridecanedioic acid, straight chain tetradecanedioic acid, straight chain heptadecanedioic acid, straight chain hexadecanedioic acid, straight chain hexenedioic acid, straight chain heptenedioic acid, straight chain octenedioic acid, straight chain nonenedioic acid, straight chain decenedioic acid, straight chain undecenedioic acid, straight chain dodecenedioic acid, straight chain tridecenedioic acid, straight chain tetradecenedioic acid, straight chain heptadecenedioic acid, and straight chain hexadecenedioic acid, and mixtures thereof.

[0045] Any of the foregoing alcohols and any of the foregoing acids are combined to constitute the ester without particularly limitations. The ester obtained when any of the foregoing polyhydric alcohols is used as the alcohol component may be a complete ester in which all the hydroxy groups in the polyhydric alcohol are esterified, or a partial ester in which a part of the hydroxy groups is not esterified but remains as the hydroxyl groups themselves.

[0046] The ester base oil may comprise only one of the foregoing ester compounds, or a mixture of at least two of the foregoing ester compounds.

[0047] The kinematic viscosity of the ester base oil at 40° C. is not particularly limited, but is preferably 3 to 25 mm2 / s. This kinematic viscosity of more than 25 mm2 / s leads to a tendency to too poor viscosity-temperature characteristics and too poor low-temperature viscosity characteristics. This kinematic viscosity of less than 3 mm2 / s may lead to insufficient oil film formation at lubricating points, and as such, a poorer metal fatigue preventing property and a poorer load-bearing property, and leads to a tendency to a greater evaporation loss of the lubricant base oil.

[0048] The kinematic viscosity of the ester base oil at 100° C. is not particularly limited, either, but is preferably 1.0 to 10 mm2 / s. This kinematic viscosity of more than 10 mm2 / s leads to a tendency to too poor viscosity-temperature characteristics and too poor low-temperature viscosity characteristics. On the contrary, this kinematic viscosity of less than 1.0 mm2 / s may lead to insufficient oil film formation at lubricating points, and as such, a poorer metal fatigue preventing property and a poorer load-bearing property, and leads to a tendency to a greater evaporation loss of the lubricant base oil.

[0049] The kinematic viscosity of the ester base oil at 0° C. is not particularly limited, either, but is preferably 10 to 130 mm2 / s. This kinematic viscosity of more than 130 mm2 / s leads to a tendency to too poor viscosity-temperature characteristics and too poor low-temperature viscosity characteristics. On the contrary, the kinematic viscosity of less than 10 mm2 / s may lead to insufficient oil film formation at lubricating points, and as such, a poorer metal fatigue preventing property and a poorer load-bearing property, and leads to a tendency to a greater evaporation loss of the lubricant base oil.

[0050] The viscosity index of the ester base oil is not particularly limited, but is preferably 100 to 300. The lubricating oil composition excellent in viscosity-temperature characteristics and in low-temperature viscosity characteristics can be obtained by setting this viscosity index to no less than 100. Setting this viscosity index to no more than 300 can lead to improved mix stability with components, and improved storage stability.

[0051] The density of the ester base oil is not particularly limited, but is preferably 0.80 g / cm3 to 1.0 g / cm3. Setting this density to no less than 0.80 g / cm3 can lead to achievement in all of viscosity-temperature characteristics and low-temperature performance, and anti-wear performance and a fatigue preventing property at a high level. This density of less than 0.80 g / cm3 may lead to insufficient oil film formation at lubricating points, and as such, leads to a tendency to deterioration in metal fatigue preventing property and in load-bearing property. Setting this density to no more than 1.0 g / cm3 can lead to improved solubility for other components.

[0052] The thermal conductivity of the ester base oil is not particularly limited, but is preferably 0.120 to 0.180 W / mK. The heat capacity required of the lubricating oil composition is easily obtained by setting this thermal conductivity within this range.

[0053] The content of the ester base oil in the lubricant base oil of the present invention is not particularly limited as long as the ester base oil is contained, and the lubricant base oil described later is capable of satisfying certain heat capacity, thermal conductivity and kinematic viscosity, but is preferably no less than 15 mass %, and more preferably 30 mass % on the basis of the lubricant base oil. All the lubricant base oil may be the ester base oil. That is, the upper limit of this content is 100 mass %.1.2 Mineral Base Oil

[0054] The lubricant base oil comprises a mineral base oil or no mineral base oil. This mineral base oil can be considered as follows when is contained. The mineral base oil falls under any of groups I to III base oils of API base stock categories. The Group I base oil encompasses mineral base oils each containing more than 0.03 mass % sulfur and / or less than 90 mass % saturates, and each having a viscosity index of no less than 80 and less than 120. The Group II base oil encompasses mineral base oils each containing no more than 0.03 mass % sulfur and no less than 90 mass % saturates, and each having a viscosity index of no less than 80 and less than 120. The Group III base oil encompasses mineral base oils each containing no more than 0.03 mass % sulfur and no less than 90 mass % saturates, and each having a viscosity index of no less than 120.

[0055] Examples of the mineral base oil include: paraffinic base oils, normal-paraffinic base oils, and isoparaffinic base oils that are refined with lubricating oil fractions obtained by atmospheric distillation and / or vacuum distillation of crude oils through one, or two or more selected from refining processes such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid washing, and white clay treatment in combination; and mixtures thereof. The API Group II base oil and the API Group III base oil are usually produced via hydrocracking.

[0056] The % Cp of the mineral base oil is preferably no less than 60, and more preferably no less than 65 in view of further improving the viscosity-temperature characteristics and the fuel efficiency of the composition; is preferably no more than 99, more preferably no more than 95, and further preferably no more than 94 in view of improving the solubility for additives; and in one embodiment, can be 60 to 99, or 60 to 95, or 65 to 95, or 65 to 94.

[0057] The % CA of the mineral base oil is preferably no more than 2, more preferably no more than 1, further preferably no more than 0.8, and especially preferably no more than 0.5 in view of further improving the viscosity-temperature characteristics and the fuel efficiency of the composition.

[0058] The % CN of the mineral base oil is preferably no less than 1, and more preferably no less than 4 in view of improving the solubility for additives; is preferably no more than 40, and more preferably no more than 35 in view of further improving the viscosity-temperature characteristics and the fuel efficiency of the composition; and can be 1 to 40, or 4 to 35.

[0059] In the present description, the % CP, the % CN and the % CA mean the percentage of the paraffinic carbon number to the total carbon number, the percentage of the naphthenic carbon number to the total carbon number, and the percentage of the aromatic carbon number to the total carbon number, respectively, which are obtained by the method conforming to ASTM D 3238-85 (ring analysis by the n-d-M method). That is, the foregoing preferred ranges of the % Cp, the % CN and the % CA are based on the values obtained according to this method. For example, the value of the % CN obtained according to this method can be more than 0 even if the lubricant base oil has no naphthene content.

[0060] The saturates of the mineral base oil is preferably no less than 90 mass %, more preferably no less than 95 mass %, and further preferably no less than 99 mass % on the basis of the total mass of the lubricant base oil in view of improving the viscosity-temperature characteristics of the composition. In the present description, the saturates means the value measured conforming to ASTM D 2007-93.

[0061] The aromatic content in the mineral base oil on the basis of the total mass of the lubricant base oil is preferably 0 to 10 mass %, more preferably 0 to 5 mass %, and especially preferably 0 to 1 mass %; and in one mode, can be no less than 0.1 mass %. The aromatic content of no more than this upper limit can lead to improvement in low-temperature viscosity characteristics and in viscosity-temperature characteristics of the fresh oil, and in addition, further improvement in fuel efficiency, and in reduction in evaporation loss of the lubricating oil to reduce the consumption of the lubricating oil; and also allows additives to exert the effects thereof effectively when the additives are incorporated into the lubricant base oil. The lubricant base oil may have no aromatic content, whereas the aromatic content of no less than the aforementioned lower limit can lead to improvement in solubility of additives.

[0062] In the present description, the aromatic content means the value measured conforming to ASTM D 2007-93. Generally, the aromatic content includes alkylbenzenes and alkylnaphthalenes; anthracenes, phenanthrenes and alkylated products thereof; further, compounds each having four or more fused benzene rings; and aromatic compounds each having a heteroatom, such as pyridine, quinoline, phenol, and naphthol.

[0063] The kinematic viscosity of the mineral base oil at 40° C. is not particularly limited, but is preferably 5 to 20 mm2 / s. This kinematic viscosity of more than 20 mm2 / s leads to a tendency to too poor viscosity-temperature characteristics and too poor low-temperature viscosity characteristics. This kinematic viscosity of less than 5 mm2 / s may lead to insufficient oil film formation at lubricating points, and as such, a poorer metal fatigue preventing property and a poorer load-bearing property, and leads to a tendency to a greater evaporation loss of the lubricant base oil.

[0064] The kinematic viscosity of the mineral base oil at 100° C. is not particularly limited, either, but is preferably 1.0 to 5 mm2 / s. This kinematic viscosity of more than 5 mm2 / s leads to a tendency to too poor viscosity-temperature characteristics and too poor low-temperature viscosity characteristics. This kinematic viscosity of less than 1.0 mm2 / s may lead to insufficient oil film formation at lubricating points, and as such, a poorer metal fatigue preventing property and a poorer load-bearing property, and leads to a tendency to a greater evaporation loss of the lubricant base oil.

[0065] The pour point of the mineral base oil is not particularly limited, but is preferably no more than −15° C., more preferably no more than −17.5° C., further preferably no more than −20° C., particularly preferably no more than −22.5° C., and most preferably no more than −25° C. The lower limit of this pour point is not particularly limited, but this pour point is preferably no less than −45° C., more preferably no less than −40° C., further preferably no less than −35° C., and particularly preferably no less than −30° C. in view of low-temperature viscosity characteristics. The lubricating oil composition excellent in low-temperature viscosity characteristics can be obtained by setting the pour point of the mineral base oil to no more than −15° C.

[0066] Here, the pour point is the pour point (° C.) measured conforming to JIS K 2269:1987.

[0067] The viscosity index of the mineral base oil is not particularly limited, but is preferably 100 to 135. The lubricating oil composition excellent in viscosity-temperature characteristics and in low-temperature viscosity characteristics can be obtained by setting this viscosity index to no less than 100. In contrast, setting this viscosity index to no more than 135 causes the lubricating oil composition to be excellent in solubility for additives and sludge.

[0068] The density of the mineral base oil is not particularly limited, but is preferably 0.80 g / cm3 to 1.0 g / cm3. This density of no less than 0.80 g / cm3 can lead to achievement in all of viscosity-temperature characteristics and low-temperature performance, and anti-wear performance and a fatigue preventing property at a high level. This density of less than 0.80 g / cm3 may lead to insufficient oil film formation at lubricating points, and as such, leads to a tendency to deterioration in metal fatigue preventing property and load-bearing property. Setting this density to no more than 1.0 g / cm3 can lead to improved solubility for other components.

[0069] The thermal conductivity of the mineral base oil is not particularly limited, but is preferably 0.120 to 0.180 W / mK. The heat capacity required of the lubricating oil composition is easily obtained by setting the thermal conductivity within this range.

[0070] The mineral base oil in the lubricant base oil contained in the lubricating oil composition according to the present disclosure is optional as described above. Therefore, the content of the mineral base oil when the mineral base oil is contained is not particularly limited. In the present disclosure, it is essential to comprise the ester base oil, and the lubricant base oil described later can be configured to satisfy certain heat capacity, thermal conductivity, and kinematic viscosity.

[0071] When contained in the lubricant base oil, the mineral base oil may be one mineral base oil, or may be constituted by at least two mineral base oils.1.3. Poly-a-Olefin Base Oil

[0072] The lubricant base oil comprises a poly-α-olefin base oil or no poly-α-olefin base oil. A poly-α-olefin base oil as used herein falls under the group IV base oil of API base stock categories. Examples of Group IV base oils include: oligomers and co-oligomers of C2-32, preferably C6-16 α-olefins, such as ethylene-propylene copolymers, polybutene, 1-octene oligomers, and 1-decene oligomers, and hydrogenated products thereof; and hydrogenated products thereof.

[0073] Preferred ranges of the kinematic viscosity, the viscosity index, and the thermal conductivity, etc. when such a poly-α-olefin base oil is contained in the lubricant base oil can be considered in the same manner as those of the mineral base oil.1.4. Additives

[0074] The lubricating oil composition according to the present disclosure may comprise an additive if necessary. Examples of such an additive include the following.1.4.1. Viscosity Index Improver

[0075] The lubricating oil composition may further comprise at least one polymer having a viscosity index improving effect (viscosity index improver). Examples of a viscosity index improver as used herein include non-dispersant or dispersant poly(meth)acrylates, (meth)acrylate-olefin copolymers, non-dispersant or dispersant ethylene-α-olefin copolymers or hydrogenated products thereof, polyisobutylene or hydrogenated products thereof, hydrogenated styrene-diene copolymers, styrene-maleic anhydride / ester copolymers, and polyalkylstyrene. In this description, “(meth)acrylate” means “acrylate and / or methacrylate”. As such a viscosity index improver, one polymer may be used alone, and at least two polymers may be used in combination.

[0076] In one embodiment, as such a viscosity index improver, a dispersant or a non-dispersant poly(meth)acrylate, or combination thereof may be preferably used. A dispersant poly(meth)acrylate may be preferably used. In this description, a dispersant poly(meth)acrylate compound has a functional group comprising a nitrogen atom, whereas a non-dispersant poly(meth)acrylate compound has no functional group comprising a nitrogen atom.

[0077] The weight-average molecular weight of a viscosity index improver as used herein can be determined appropriately according to the use of the lubricating oil composition, and is preferably no more than 30,000 for balancing improvement in viscosity index against improvement in solubility in the lubricant base oil, storage stability, and shear stability.

[0078] When the lubricating oil composition comprises such a viscosity index improver, the content of the viscosity index improver can be determined appropriately as a content that allows desired kinematic viscosity and desired viscosity-temperature characteristics to be obtained as the entire lubricating oil composition, and is preferably suppressed to be no more than 10 mass %.

[0079] In the present disclosure, the content of the viscosity index improver can be suppressed since it is not necessary to always use the viscosity index improver.1.4.2 Metallic Detergent

[0080] The lubricating oil composition may further comprise at least one metallic detergent. Examples of a metallic detergent as used herein include salicylate detergents, sulfonate detergents, and phenate detergents. A metallic detergent as used herein may comprise one metallic detergent only, and may comprise at least two metallic detergents. Generally, in the lubricating oil field, organic acid metal bases capable of forming micelles in lubricant base oils (such as alkali or alkaline earth metal alkylsalicylates, alkali or alkaline earth metal alkylbenzene sulfonates, and alkali or alkaline earth metal alkylphenates), or mixtures of such organic acid metal bases and basic metal salts (including hydroxides, carbonates and borates of alkali or alkaline earth metals constituting such organic acid metal bases) are used as a metallic detergent. Such an organic acid usually has, in a molecule thereof, at least one polar group capable of forming a salt along with a metal base (typically a metal oxide and / or metal hydroxide) and having Broensted acidity (such as a carboxy group, a sulfo group, and a phenolic hydroxy group), and at least one lipophilic group such as a straight or branched chain alkyl group (for example, C6 or more straight or branched chain alkyl).

[0081] A metallic detergent as used herein may be carbonate salt-overbased (examples of a carbonate salt here include alkali metal carbonate salts such as sodium carbonate and potassium carbonate, and alkaline earth metal carbonate salts such as calcium carbonate and magnesium carbonate), and may be borate salt-overbased (examples of a borate salt here include alkali metal borate salts such as sodium borate and potassium borate, and alkaline earth metal borate salts such as calcium borate and magnesium borate).

[0082] A metallic detergent as used herein comprises at least one overbased calcium or magnesium sulfonate detergent, at least one overbased calcium or magnesium salicylate detergent, and / or at least one overbased calcium or magnesium phenate detergent, and can preferably comprise at least one overbased calcium sulfonate detergent, and / or at least one overbased calcium salicylate detergent. A calcium sulfonate detergent, a calcium salicylate detergent, and a calcium phenate detergent here are each preferably calcium carbonate-overbased; and a magnesium sulfonate detergent, a magnesium salicylate detergent, and a magnesium phenate detergent here are each preferably magnesium carbonate-overbased.

[0083] When the lubricating oil composition comprises such a metallic detergent, the content of the metallic detergent can be determined appropriately according to the use of the lubricating oil composition. For example, when the lubricating oil composition is used for lubrication of gears of transmissions (such as manual transmissions, automatic transmissions, and continuously variable transmissions), the content of the metallic detergent in the lubricating oil composition on the basis of the total mass of the lubricating oil composition in terms of metal is preferably no less than 50 mass ppm, or no less than 100 mass ppm in view of improving anti-wear performance, anti-seizure performance, fatigue resistance, and the torque transmitting capacity of wet clutches; is preferably no more than 300 mass ppm, or no more than 250 mass ppm in view of improving fuel efficiency and fatigue resistance, and in view of further suppressing deterioration in electrical insulation; and can be 50 to 300 mass ppm, or 100 to 250 mass ppm.

[0084] Here, the metal content (in terms of metal) is the metal content (mass ppm) measured conforming to JIS K 9116.1.4.3. Others

[0085] The lubricating oil composition according to the present disclosure may comprise any other various additives if necessary as long as the performance thereof is satisfied. Such additives are not particularly limited, and any additives that are conventionally used in the lubricating oil field may be incorporated. Specific examples of such lubricating oil additives include ashless dispersants, antioxidants, extreme-pressure agents, anti-wear agents, friction modifiers, pour point depressants, corrosion inhibitors, anti-rust agents, demulsifiers, metal deactivators, and defoaming agents. One of these additives may be used alone, and two or more of these additives may be used in combination.

[0086] As an ashless dispersant, any ashless dispersant that is used for lubricating oils may be used. Examples of such an ashless dispersant include mono or bis-succinimide having at least one C40-400 straight or branched chain alkyl or alkenyl in one molecule thereof, benzylamine having at least one C40-400 alkyl or alkenyl in one molecule thereof, and polyamine having at least one C40-400 alkyl or alkenyl in one molecule thereof, and modified products thereof by boron compounds, carboxylic acids, phosphoric acid, etc. Any one or at least any two selected from the foregoing may be incorporated to be used.

[0087] Examples of an antioxidant as used herein include phenolic or amine ashless antioxidants, and copper or molybdenum metallic antioxidants.

[0088] Examples of a friction modifier as used herein include: ashless friction modifiers of fatty acid esters, fatty amines, and fatty acid amides; and metallic friction modifiers of molybdenum dithiocarbamate, molybdenum dithiophosphate, etc.

[0089] As an extreme-pressure agent and an anti-wear agent as used herein, any known extreme-pressure agent and anti-wear agent that are used for lubricating oils may be used. For example, a sulfur-based, a phosphorus-based, or a sulfur-phosphorus-based extreme pressure agent or the like may be used. Specific examples include phosphite esters, thiophosphite esters, dithiophosphite esters, trithiophosphite esters, phosphate esters, thiophosphate esters, dithiophosphate esters, and trithiophosphate esters, amine salts thereof, metal salts thereof, and derivatives thereof, dithiocarbamates, zinc dithiocarbamate, molybdenum dithiocarbamate, disulfides, polysulfides, sulfurized olefins, and sulfurized fats.

[0090] As a pour point depressant as used herein, for example, a polymethacrylate polymer compatible with the lubricant base oil to be used may be used.

[0091] Examples of a corrosion inhibitor as used herein include benzotriazole, tolyltriazole, thiadiazole, and imidazole compounds.

[0092] Examples of an anti-rust agent as used herein include petroleum sulfonate, alkylbenzenesulfonate, dinonylnaphthalenesulfonate, alkenylsuccinate esters, and polyol esters.

[0093] Examples of a demulsifier as used herein include polyalkylene glycol-based nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, and polyoxyethylene alkylnaphthyl ethers.

[0094] Examples of a metal deactivator as used herein include imidazolines, pyrimidine derivatives, alkylthiadiazole, mercaptobenzothiazole, benzotriazole or derivatives thereof, 1,3,4-thiadiazole polysulfide, 1,3,4-thiadiazolyl-2,5-bis(dialkyl dithiocarbamate), 2-(alkyldithio)benzimidazole, and β-(o-carboxybenzylthio) propionitrile.

[0095] Examples of a defoaming agent as used herein include silicone oils having a kinematic viscosity at 25° C. of no less than 0.1 mm2 / s and less than 100 mm2 / s, alkenylsuccinimide derivatives, esters of a polyhydroxy aliphatic alcohol and a long-chain fatty acid, methyl salicylate, and o-hydroxybenzylalcohol.2. Properties of Lubricating Oil Composition

[0096] The above-described lubricating oil composition is configured to satisfy the following properties, thereby having both cooling characteristics and fuel efficiency.2.1. Kinematic Viscosity

[0097] In the present disclosure, the kinematic viscosity of the lubricating oil composition at 40° C. is 2 to 23 mm2 / s. This leads to good fuel efficiency in view of fluidity of the lubricating oil composition. This kinematic viscosity is preferably no less than 7 mm2 / s, and preferably no more than 20 mm2 / s. This kinematic viscosity at 40° C. of less than 2 mm2 / s may lead to problems with oil film retention at lubrication positions, and evaporation. This kinematic viscosity at 40° C. of more than 23 mm2 / s may lead to poorer fuel efficiency.2.2. Density, Specific Heat, and Heat Capacity

[0098] In the present disclosure, the heat capacity of the lubricating oil composition per unit area at 15° C. is no less than 1.65 J / K·cm3. This causes the lubricating oil composition to have good cooling characteristics. This heat capacity is more preferably no less than 1.70 J / K·cm3. The upper limit of this heat capacity is not particularly limited, and this heat capacity is no more than 2.00 J / K·cm3, and more preferably no more than 1.90 J / K·cm3.

[0099] Here, the heat capacity of the lubricating oil composition per unit area at 15° C. can be obtained from the product of the density (g / cm3) and the specific heat (J / K·g) of the lubricating oil composition at 15° C. This density at 15° C. is not particularly limited, but is preferably no less than 0.83 g / cm3, and more preferably no less than 0.85 g / cm3. This specific heat at 15° C. is not particularly limited, but is preferably 1.90 J / K·g.2.3. Thermal Conductivity

[0100] In the present disclosure, the thermal conductivity of the lubricating oil composition at 15° C. is no less than 0.138 W / m·K. This causes the lubricating oil composition to have good cooling characteristics. This thermal conductivity is more preferably no less than 0.140 W / m·K. The upper limit of this thermal conductivity of the lubricating oil composition is not particularly limited, but this thermal conductivity is no more than 0.20 W / m·K, and is more preferably 0.18 W / m·K.2.4. Traction Coefficient

[0101] In the present disclosure, the traction coefficient of the lubricating oil composition is no more than 0.004 at 25° C. This leads to good fuel efficiency in the lubricating oil composition in view of fluidity. This traction coefficient is more preferably no more than 0.003. Here, the traction coefficient at 25° C. is the value measured using an EHL (elastohydrodynamic lubrication) testing machine that is described in Journal of Japanese Society of Tribologists. 1992, vol. 37, no. 2, pp. 126-133, with a disc (material: SUJ-2, diameter: 130 mm, thickness: 12 mm, roughness Ra=4.5 nm) and a roller (material: SUJ-2, diameter: 25.4 mm, roughness Ra=6 nm) in combination under the following conditions: oil temperature: 25° C., surface pressure: 0.44 GPa, slip rate: 3%, and speed: 0.75 m / s.3. Effects Etc

[0102] The lubricating oil composition according to the present disclosure having the aforementioned composition, and satisfying the aforementioned kinematic viscosity, heat capacity, thermal conductivity, and traction coefficient can be configured to have both cooling characteristics and fuel efficiency.

[0103] Since the lubricating oil composition according to the present disclosure can be configured to have both cooling characteristics and fuel efficiency, this lubricating oil composition can be preferably used as an electric motor oil, a transmission oil, and an electric motor-transmission (gear mechanism) common lubricating oil for electric vehicles and hybrid vehicles, and a lubricating oil / cooling oil for electric drive modules comprising electric motors and transmissions (gear mechanisms). The lubricating oil composition according to the present disclosure can be also preferably used as a cooling oil for cooling the components of electric power systems including batteries and power electronics. In one embodiment, the lubricating oil composition according to the present invention can be preferably used as a common lubricating / cooling oil for: transmissions (gear mechanisms) or electric motors, or combination thereof; and a batteries or at least one power electronics, or combination thereof.EXAMPLES4. Examples

[0104] Hereinafter the present disclosure will be more specifically described using examples and comparative examples. The present invention is not limited to these examples at all.

[0105] As shown in tables 3 and 4, lubricating oil compositions according to the present disclosure (examples 1 to 9), and lubricating oil compositions of comparative examples (comparative examples 1 to 10) were each prepared. With respect to each of the obtained compositions, the properties of kinematic viscosity, density, specific heat, heat capacity per unit capacity, thermal conductivity, and traction coefficient were obtained, and tables 3 and 4 were compiled from these results together.4.1. Mineral Base Oil

[0106] Mineral base oils A-1 to A-6 were as shown in table 1.4.2. Poly-α-Olefin Base Oil

[0107] Poly-α-olefin base oils B-1 and B-2 were as shown in table 1.4.3. Ester Base Oil

[0108] Ester base oils C-1 to C-6 were as shown in table 1. Alcohols and acids constituting the ester base oils were as follows:

[0109] C-1:2-ethylhexanol for alcohol, oleic acid (straight chain structure) for acid

[0110] C-2:2-ethylhexanol for alcohol, azelaic acid (straight chain structure) for acid

[0111] C-3: neopentyl glycol for alcohol, isooctanoic acid (branched chain structure) for acid

[0112] C-4: triglycerol for alcohol, decanoic acid (straight chain structure) for acid

[0113] C-5: isooctanol for alcohol, isooctanoic acid (branched chain structure) for acid

[0114] C-6:2-ethylhexanol for alcohol, lauric acid (straight chain structure) for acidTABLE 1group ofkinematickinematicAPI basetractiondensitythermalviscosityviscositystockcoefficient(15° C.)conductivity(40° C.)(100° C.)viscositybase oilcategories(25° C.)g / cm3W / mKmm2 / smm2 / sindexremarksmineralA-1II0.01290.8330.1318.9032.471100oilA-2II0.01030.83140.1318.112.341103A-3II0.00470.80760.149.1862.628124A-4II0.00990.8350.1419.124.188124A-5II0.00640.82610.14318.254.166135A-60.02480.82890.1162.3931.05—PAOB-1IV0.00590.79790.13551.677—B-2IV0.00520.82030.14618.414.099125esterC-1V0.00230.87120.1458.3122.69185straight chain structureC-2V0.00340.9220.14610.83146straight chain structureC-3V0.01360.9190.1287.5082.05853branched chain structureC-4V0.00280.94940.15114.883.641132straight chain structureC-5V0.01560.8630.1282.7161.077—branched chain structureC-6V0.00470.86310.1415.11.8—straight chain structure4.4. Additives

[0115] The presence or not of additives are shown in tables 3 and 4. Specific components of the additives were as shown in table 2.TABLE 2typeadditive amountviscosity indexPMA (Mw = 17,700)7 mass % includingimproverdiluent oilashless dispersantsuccinimide (Mw =6 mass % as packageapproximately 5,000)metallic detergentCa salicylateP-based extreme-di(3-thiaundecyl)phosphitepressure agentacid phosphate (C18)4.5. Properties

[0116] The properties described in the description including the examples and the comparative examples are as follows.

[0117] The kinematic viscosity at each temperature is the kinematic viscosity at each temperature measured conforming to JIS K 2283-2000 by the use of an automated viscometer (trade name: “CAV-2100” manufactured by Cannon Instrument Company) as a measuring device.

[0118] The viscosity index is the viscosity index measured conforming to JIS K 2283-2000 by the use of an automated viscometer (trade name: “CAV-2100” manufactured by Cannon Instrument Company) as a measuring device.

[0119] The density is the density (g / cm3) measured conforming to JIS K 2249-1:2011.

[0120] The specific heat is the specific heat (J / K·g) measured using a differential scanning calorimeter conforming to JIS K 7123.

[0121] The heat capacity per unit area is the heat capacity (J / K·cm3) obtained from the product of the obtained density and specific heat.

[0122] The thermal conductivity is the thermal conductivity (W / m·K) measured conforming to ASTM D 5334.

[0123] The traction coefficient is the traction coefficient measured using an EHL testing machine under the following conditions as described above: oil temperature: 25° C., surface pressure: 0.44 GPa, slip rate: 3%, and speed: 0.75 m / s.4.6. Results etc.

[0124] With respect to the compositions, the components and property values are shown in tables 3 and 4.TABLE 3examplesitemunit123456789mineralA-1mass %base oilA-2mass %A-3mass %251675A-4mass %111111A-5mass %5A-6mass %PAOB-1mass %494949375B-2mass %esterC-1mass %70891320C-2mass %10C-3mass %C-4mass %4040401184C-5mass %C-6mass %5090total ester content in base oilmass %40404070100846310020content of ester having nomass %40404070100846310020branched chain structure in baseoiladditiveviscosity index improver—◯—◯◯◯◯◯◯◯ashless dispersant, metallic—◯——◯◯◯◯◯◯detergent, and P-based extreme-pressure agentcompositionkinematic viscosity (40° C.)mm2 / s9.87.910.512.513.119.88.58.411.6density (15° C.)g / cm30.840.830.840.860.880.920.850.880.82specific heat (15° C.)J / K · g1.992.052.021.981.951.961.961.942.06heat capacity per unit area (15° C.)J / K · cm31.671.701.701.701.721.801.671.701.69thermal conductivity (15° C.)W / mK0.1420.1400.1410.1440.1460.1490.1390.1420.141traction coefficient (25° C.)—0.0030.0030.0030.0030.0020.0030.0020.0020.004TABLE 4comparative examplesitemunit12345678910mineral oilA-1mass %8816A-2mass %128371A-3mass %7313A-4mass %7141010A-5mass %10A-6mass %29PAOB-1mass %17879080B-2mass %esterC-1mass %C-2mass %1010C-3mass %70207710C-4mass %23C-5mass %70C-6mass %total ester content in base oilmass %0107090100001010content of ester having nomass %0010002300100branched chain structure in baseadditiveviscosity index improver—◯◯◯◯◯◯◯◯◯◯ashless dispersant, metallic—◯◯◯◯◯◯◯◯◯◯detergent, P-based extreme-pressure agentcompo-kinematic viscosity (40° C.)mm2 / s12.012.111.912.56.114.08.08.18.18.0sitiondensity (15° C.)g / cm30.840.810.840.890.880.930.840.810.820.82specific heat (15° C.)J / K · g1.962.041.881.911.941.891.842.032.032.00heat capacity per unitJ / K ·1.651.651.581.701.711.751.551.641.661.64area (15° C.)cm3thermal conductivity (15° C.)W / mK0.1320.1390.1330.1330.1300.1330.1270.1360.1360.135traction coefficient (25° C.)—0.0130.0050.0100.0130.0090.0110.0090.0030.0030.004As shown in tables 3 and 4, the lubricating oil compositions of the examples satisfied all the properties of kinematic viscosity, traction coefficient, heat capacity per unit capacity, and thermal conductivity, and could be configured to have both cooling characteristics and fuel efficiency. In contrast, the lubricating oil compositions of the comparative examples were not capable of satisfying at least one of these properties, and could not be considered to have both cooling characteristics and fuel efficiency.

Claims

1. A composition that is used as a lubricating oil, the composition comprising:a lubricant base oil comprising a mineral base oil or no mineral base oil, a poly-α-olefin base oil or no poly-α-olefin base oil, and at least one ester base oil, whereinthe ester base oil is an ester of at least one saturated or unsaturated fatty acid having no branched chain structure, and at least one alcohol, and a content of the ester base oil is no less than 15 mass % based on total mass of the lubricant base oil, the ester base oil comprising no fatty acid having a branched chain structure, andkinematic viscosity of the composition at 40° C. is 2.0 to 23.0 mm2 / s.

2. The lubricating oil composition according to claim 1, comprising:at least one poly(meth)acrylate or no poly(meth)acrylate, the poly(meth)acrylate having a weight average molecular weight of no more than 30,000 in an amount of no more than 10 mass % based on total mass of the composition.

3. The lubricating oil composition according to claim 1, whereinthe composition is used to lubricate electric motors.

4. The lubricating oil composition according to claim 1, whereinthe composition is used to lubricate gears.

5. The lubricating oil composition according to claim 1, wherein the composition is used to lubricate electric motors or to lubricate the electric motors and transmissions, in automobiles comprising the electric motors.