Lubricant composition for automatic transmission

Abstract

 The invention provides a lubricant composition for automatic transmissions excellent in sustainability of anti-shudder property, low-temperature viscosity characteristics, and oxidation stability, and excellent and well-balanced in durability of friction characteristics, energy-conserving performance, and anti-fatigue performance on gears. The composition contains a base oil having a kinematic viscosity at 100 °C of 3.7-4.1 mm²/s, and composed of base oil having the viscosityof 2.5-4.5 mm²/s, and base oil having the viscosity of 10-40 mm²/s; 1-20 mass% poly(meth)acrylate viscosity index improver having Mw of 15000-30000; 2-4 mass% imide friction modifier having a C8-C30 hydrocarbon group; 0.01-0.04 mass% phosphorus extreme pressure agent in terms of phosphorus; and 0.01-0.04 mass% ashless dispersant having at least one alkyl or alkenyl group of a number average molecular weight of ≥2000 in terms of nitrogen, all with respect to the total amount of the composition, and has the viscosity of 5.6-5.8 mm²/s.

Description

[0001] The present invention relates to a lubricant composition for automatic transmissions, in particular, to a lubricant composition for automatic transmissions, particularly for vehicles, which prolongs fatigue life of gears irrespective of its low viscosity, and which is excellent and well-balanced in sustainability of anti-shudder property, low-temperature viscosity characteristics, oxidation stability, and durability of friction characteristics.

[0002] There is recently a pressing need for saving energy consumption of vehicles, or energy conservation, in order to address environmental issues, for example, by reduction of CO₂ emission. Engines and automatic transmissions are strongly demanded to contribute to energy-saving, and lubricants therefor are desired to have lower agitation resistance and friction resistance.

[0003] One means for making automatic transmissions energy-conserving is to lower the viscosity of a lubricant used therein. By lowering the viscosity of lubricants used in a vehicle automatic transmission for example, which has a torque converter, a wet clutch, gear-bearing mechanisms, an oil pump, and a hydraulic control mechanism, the agitation resistance and friction resistance in these parts are reduced, which results in improvement in power transmission efficiency and thus in vehicle fuel efficiency.

[0004] However, the lowered viscosity of lubricants used in these parts may remarkably shorten their fatigue life, causing seizure or the like to result in troubles in the transmission. In particular, when a phosphorus extreme pressure agent is used for improving extreme pressure property of a low-viscosity lubricant, fatigue life of gears is remarkably impaired, so that it is usually hard to lower the viscosityof such lubricant. On the other hand, a sulfur extreme pressure agent may improve fatigue life of gears, but impairs oxidation stability, so that a large amount of antioxidants is required.

[0005] As a conventional automatic transmission fluid for vehicles capable of maintaining various performances, such as transmission characteristics, for a prolonged period of time, there are reported optimized blends of synthetic and/or mineral lubricant base oils, anti-wear agent, extreme pressure agent, metal detergent, ashless dispersant, friction modifier, viscosity index improver, and the like (JP-3-39399-A, JP-7-268375-A, JP-2000-63869-A, and JP-2001-262176-A).

[0006] These compositions, however, are not intended to improve fuel efficiency, and thus have a high kinematic viscosity. No discussion is made in these publications regarding the effect of reduction in lubricant viscosity on fatigue life of gears. Sufficient discussion has not been made on a composition that may solve such a problem. There are recently proposed automatic transmission fluids having a lowered viscosity, for example, in JP-2004-169025-A, JP-2004-155924-A, andJP-2004-155873-A. These lubricants have excellent sustainability of anti-shudder property, low-temperature viscosity characteristics, and oxidation stability, but are yet to be improved in excellence and balance of durability of friction characteristics, contribution to energy conservation, and anti-fatigue performance on gears.

[0007] It is an object of the present invention to provide a lubricant composition for automatic transmissions which has excellent sustainability of anti-shudder property, low-temperature viscosity characteristics, and oxidation stability,andisexcellentand well-balancedin durability of friction characteristics, energy-conserving performance, and anti-fatigue performance on gears, in particular a lubricant composition for automatic transmissions, particularly suitable for vehicle automatic transmissions, having both energy-conserving performance and anti-fatigue performance on gears.

[0008] The present inventors have made intensive studies for solving the above problems to find out that a low-viscosity lubricant composition for automatic transmissions wherein particular base oils and particular additives are combined, may solve the above problems, thereby completing the present invention.

[0009] According to the present invention, there is provided a lubricant composition for automatic transmissions comprising:

(A) a lubricant base oil having a kinematic viscosity at 100 °C of 3.7 to 4.1 mm²/s, and consisting of lubricant base oil (A1) having a kinematic viscosity at 100 °C of 2.5 to 4.5 mm²/s and lubricant base oil (A2) having a kinematic viscosity at 100 °C of 10 to 40 mm²/s;

(B) a poly(meth)acrylate viscosity index improver having a weight average molecular weight of 15000 to 30000 (sometimes referred to as component (B) hereinbelow) at 1 to 20 mass% of the total amount of the composition;

(C) an imide friction modifier having a hydrocarbon group with 8 to 30 carbon atoms (sometimes referred to as component (C) hereinbelow) at 2 to 4 mass% of the total amount of the composition;

(D) a phosphorus extreme pressure agent (sometimes referred to as component (D) hereinbelow) at 0.01 to 0.04 mass% of the total amount of the composition in terms of phosphorus;

(E) an ashless dispersant having at least one alkyl or alkenyl group of a number average molecular weight of not lower than 2000 (sometimes referred to as component (E) hereinbelow) at 0.01 to 0.04 mass% of the total amount of the composition in terms of nitrogen;
wherein said composition has a kinematic viscosity at 100 °C of 5.6 to 5.8 mm²/s (sometimes referred to as the present composition hereinbelow).

[0010] The present composition is capable of prolonging fatigue life of gears irrespective of its low viscosity, has excellent sustainability of anti-shudder property, low-temperature viscosity characteristics, and oxidation stability, and is excellent and well-balanced in durability of friction characteristics, energy-conserving performance, and anti-fatigue performance on gears. The present composition is particularly suitable for vehicle automatic transmissions, has both energy-conserving performance and anti-fatigue performance on gears, and is capable of achieving energy conservation of vehicles.

[0011] The present invention will now be explained in detail.

[0012] The present composition is characterized in that the particular lubricant base oil (A) and particular components (B) to (E) are contained at a good balance, and the kinematic viscosity at 100 °C is in the range of 5.6 to 5.8 mm²/s.

[0013] Having the particular composition and the kinematic viscosity in the particular range mentioned above, the present composition is capable of prolonging fatigue life of gears irrespective of its low viscosity, and is excellent and well-balanced in sustainability of anti-shudder property, low-temperature viscosity characteristics, oxidation stability, and durability of friction characteristics.

[0014] If the kinematic viscosity at 100 °C of the present composition is higher than 5.8 mm²/s, the energy-conserving performance given by reduction of agitation resistance and excellent low-temperature viscosity characteristics cannot be achieved sufficiently. If the kinematic viscosity is lower than 5.6 mm²/s, fatigue life of gears cannot be prolonged sufficiently.

[0015] According to the present invention, the lubricant base oil (A) includes lubricant base oil (A1) having a kinematic viscosity at 100 °C of 2.5 to 4.5 mm²/s (sometimes referred to as component (A1) hereinbelow), i.e. one or more lubricant base oils (A1) selected from the group consisting of mineral and synthetic base oils having the particular kinematic viscosity, and lubricant base oil (A2) having a kinematic viscosity at 100 °C of 10 to 40 mm²/s (sometimes referred to as component (A2) hereinbelow).

[0016] In component (A1), the mineral base oil may be, for example, paraffin or naphthene mineral base oils refined by atmospheric-distilling crude oil followed by vacuum-distillation of the atmospheric residue, and refining the resulting lubricant fraction by one or a suitable combination of solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, contact dewaxing, hydrorefining, washing with sulfuric acid, and clay treatment; or normal paraffin or isoparaffin. One or a combination of two or more of these base oils at any ratio may be used.

[0017] Examples of preferred mineral base oils may be as follows:

(1) distillate oil obtained by atmospheric distillation of paraffinic and/or mixed-base crude oils;

(2) distillate oil obtained by vacuum-distilling the atmospheric residue of paraffinic and/or mixed-base crude oils (WVGO);

(3) wax obtained by lubricant dewaxing and/or Fischer-Tropsch wax produced by GTL process and the like;

(4) mild-hydrocracked (MHC) oil of one or a mixture of two or more of above (1) to (3);

(5) mixed oil of two or more of above (1) to (4);

(6) deasphalted oil (DAO) of above (1), (2), (3), (4), or (5) ;

(7) mild-hydrocracked (MHC) oil of above (6); and

(8) lubricant obtained by refining a mixed oil of two or more of above (1) to (7) as a charge stock and/or a lubricant fraction recovered from this charge stock, by ordinary refining processes, and recovering the lubricant fraction.

[0018] The ordinary refining processes as used herein are not particularly limited, and any refining processes used in production of lubricant base oils may be employed. Examples of the ordinary refining processes may include (a) hydrorefining, such as hydrocracking or hydrofinishing; (b) solvent refining, such as furfural solvent extraction; (c) dewaxing, such as solvent dewaxing or contact dewaxing; (d) clay refining using acid clay or activated clay; and (e) chemical (acid or alkali) refining, such as washing with sulfuric acid or caustic soda. One or any combination in any order of these refining processes may be employed in the present invention.

[0019] Particularly preferred mineral base oils may be those obtained by subjecting a base oil selected from above (1) to (8) to the following treatment.

[0020] That is, hydrocracked mineral oils and/or wax-isomerized isoparaffin base oils may preferably be used, which are obtained by subjecting the lubricant fraction of a base oil selected from above (1) to (8) to hydrocracking or wax-isomerization, then subjecting the resulting product or the lubricant fraction thereof to dewaxing, such as solvent or contact dewaxing, followed by solvent refining, or to solvent ref ining followed by dewaxing, such as solvent or contact dewaxing. The hydrocracked mineral oil and/or wax-isomerized isoparaffin base oil is preferably used in an amount of not less than 30 mass%, more preferably not less than 50 mass%, most preferably not less than 70 mass% of the total amount of the mineral base oil.

[0021] In component (A1), the synthetic base oil may be, for example, poly-α-olefin or hydrides thereof, isobutene oligomer or hydrides thereof, isoparaffin, alkylbenzene, alkylnaphthalene; diesters, such as ditridecyl glutarate, di-2-ethylhexyl adipate, isodecyl adipate, ditridecyl adipate, or di-2-ethylhexyl sebacate; polyol esters, such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol-2-ethylhexanoate, or pentaerythritol pelargonate; polyoxyalkylene glycols, dialkyldiphenyl ethers, or polyphenyl ethers.

[0022] Among the above synthetic base oils, for example, poly-α-olefin or hydrides thereof may preferably be used. Poly-α-olefin may be an oligomer or a co-oligomer of α-olefin having usually 2 to 32, preferably 6 to 16 carbon atoms. More specifically, 1-octene oligomer, 1-decene oligomer, ethylene-propylene co-oligomer, or hydrides thereof, may be used.

[0023] The poly-α-olefin may be prepared by any process without specific limitation, for example, by polymerizing α-olefin in the presence of a polymerization catalyst, such as a Friedel-Crafts catalyst, including aluminum trichloride, boron trifluoride, or a complex of boron trifluoride with water, alcohol, such as ethanol, propanol, or butanol, carboxylic acid, or an ester, such as ethyl acetate or ethyl propionate.

[0024] It is sufficient that component (A1) contains at least one mineral or synthetic base oil. For example, two or more mineral base oils, two or more synthetic base oils, a mixture thereof, or a mixture of at least one mineral base oil and at least one synthetic base oil may be used. When component (A1) is a mixture, the mixing ratio of the base oils therein may be selected arbitrarily, as long as the kinematic viscosity at 100 °C of each base oil is 2.5 to 4.5 mm²/s.

[0025] Component (A1) may preferably be one or more base oils selected from the following base oils (A1a) to (A1c):

(A1a) mineral base oils having a kinematic viscosity at 100 °C of 2.5 to 3.5 mm²/s, preferably 2.5 to 3.2 mm²/s;

(A1b) mineral base oils having a kinematic viscosity at 100 °C of 3.5 to 4.5 mm²/s, preferably 3.8 to 4.3 mm²/s; and

(A1c) poly-α-olefin base oils having a kinematic viscosity at 100 °C of 2.5 to 4.5 mm²/s, preferably 3.8 to 4.3 mm²/s.

[0026] The %C_A of component (A1), such as base oils (A1a) to (A1c), is not particularly limited, and may preferably be not higher than 3, more preferably not higher than 2, and particularly preferably not higher than 1. With the %C_A of component (A1), or even lubricant base oil (A), of not higher than 3, a composition having still more excellent oxidation stability may be obtained.

[0027] As used herein, the %C_A refers to a percent of the number of aromatic carbons with respect to the total carbon number determined in accordance with ASTM D 3238-85.

[0028] The viscosity index of component (A1), such as base oils (A1a) to (A1c), is not particularly limited, and may preferably be not lower than 80, more preferably not lower tan 90, particularly preferably not lower than 110, and usually not higher than 200, more preferably not higher than 160. With the viscosity index of not lower than 80, a composition exhibiting excellent viscosity characteristics from lower temperatures to higher temperatures may be obtained, but with too high a viscosity index, the effect of the composition on fatigue life of gears may be lowered.

[0029] The sulfur content of component (A1), such as base oils (A1a) to (A1c), is not particularly limited, and may preferably be not higher than 0.05 mass%, more preferably not higher than 0.02 mass%, particularly preferably not higher than 0.005 mass% of the total amount of component (A1). With a reduced sulfur content of component (A1), a composition having still more excellent oxidation stability may be obtained.

[0030] Each of base oils (A1a) to (A1c) may be used alone or mixed in any combination or ratio. It is particularly preferred to use a combination of base oil (A1a) and base oil (A1b) and/or (A1c). When a combination of base oil (A1a) and/or (A1b) and base oil (A1c) is used, the content of base oil (A1c) is preferably 1 to 50 mass%, more preferably 3 to 20 mass%, particularly preferably 5 to 15 mass% of the total amount of lubricant base oil (A). With the content of base oil (A1c) being about 5 to 15 mass%, a composition having excellent anti-fatigue performance, low-temperature characteristics, and oxidation stability may be obtained effectively at low cost.

[0031] In lubricant base oil (A), component (A2) acts for further improving fatigue life of gears, and may be one or more base oils selected from the following base oils (A2a) to (A2c) :

(A2a) mineral and/or synthetic base oils, preferably mineral base oils, having a kinematic viscosity at 100 °C of 10 to 15 mm²/s, preferably 10 to 12 mm²/s;

(A2b) mineral and/or synthetic base oils, preferably mineral base oils, having a kinematic viscosity at 100 °C of 15 to 25 mm²/s, preferably 17 to 23 mm²/s; and

(A2c) mineral and/or synthetic base oils, preferably mineral base oils, having a kinematic viscosity at 100 °C of 25 to 40 mm²/s, preferably 28 to 35 mm²/s.

[0032] The %C_A of component (A2), such as base oils (A2a) to (A2c), is usually 0 to 40 but not particularly limited, and may preferably be not lower than 2, more preferably not lower than 4, particularly preferably not lower than 6, and preferably not higher than 15, more preferably not higher than 10, particularly preferably not higher than 8, for balancing good oxidation stability and anti-fatigue performance.

[0033] The viscosity index of component (A2), such as base oils (A2a) to (A2c), is not particularly limited, and may preferably be not lower than 80, more preferably not lower than 90, particularly preferably not lower than 95, and usually not higher than 200, preferably not higher than 120, more preferably not higher than 110, particularly preferably not higher than 100. With the viscosity index of not lower than 80, a composition exhibiting excellent viscosity characteristics from lower temperatures to higher temperatures may be obtained, but with too high a viscosity index, the effect of the composition on fatigue life of gears may be lowered.

[0034] The sulfur content of component (A2), such as base oils (A2a) to (A2c), is not particularly limited, and may usually be 0 to 2 mass%, preferably 0.05 to 1.5 mass%, more preferably 0.3 to 1.2 mass%, still more preferably 0.5 to 1 mass%, particularly preferably 0.7 to 1. 0 mass% of the total amount of component (A2). With component (A2) having a relatively high sulfur content, anti-fatigue performance may be improved, whereas with component (A2) preferably having a sulfur content of not higher than 1.0 mass%, a composition having still more excellent oxidation stability may be obtained.

[0035] According to the present invention, it is preferred to use, as component (A2), base oil (A2b) or (A2c) for improving anti-fatigue performance, and base oil (A2b) for balancing the oxidation stability and anti-fatigue performance. With base oil (A1c) used as component (A1), a composition having excellent oxidation stability and low-temperature viscosity characteristics and anti-fatigue performance, may be obtained.

[0036] In lubricant base oil (A), the contents of components (A1) and (A2) are not particularly limited, and the content of component (A1) may preferably be 70 to 97 mass%, more preferably 85 to 95 mass%, and the content of component (A2) may preferably be 3 to 30 mass%, more preferably 5 to 15 mass%, of the total amount of lubricant base oil (A) .

[0037] Lubricant base oil (A), which is composed of components (A1) and (A2), has a kinematic viscosity at 100 °C of 3.7 to 4.1 mm²/s, preferably 3.9 to 4.1 mm²/s . At a kinematic viscosity at 100 °C of not higher than 4.1 mm²/s, fluid resistance is lowered, so that a lubricant composition exhibiting still lower friction resistance at lubricating sites may be obtained. A composition having excellent low-temperature viscosity, for example, a Brookfield viscosity at -40 °C of not higher than 15000 mPa·s, may be obtained. On the other hand, at a kinematic viscosity at 100 °C of not lower than 3.7 mm²/s, a composition may be obtained which is capable of forming a sufficient oil film, has still more excellent lubricity and anti-fatigue performance, and exhibits still lower base oil evaporation loss under high-temperature conditions.

[0038] The %C_A of lubricant base oil (A) is not particularly limited, and may preferably be not higher than 3, more preferably not higher than 2, particularly preferably not higher than 1, and preferably not lower than 0.1, more preferably not lower than 0.5. With the %C_A of lubricant base oil (A) of not higher than 3, a composition having still more excellent oxidation stability may be obtained.

[0039] The viscosity index of lubricant base oil (A) is not particularly limited, and may preferably be not lower than 80, more preferably not lower than 90, particularly preferably not lower than 110. With the viscosity index of not lower than 80, a composition exhibiting excellent viscosity characteristics from lower temperatures to higher temperatures may be obtained.

[0040] The sulfur content of lubricant base oil (A) is not particularly limited, and may preferably be 0 to 0.3 mass%, more preferably 0.03 to 0.2 mass%, particularly preferably 0.06 to 0.1 mass%. With the sulfur content of lubricant base oil (A) in the above range, in particular 0.03 to 0.2 mass%, anti-fatigue performance and oxidation stability may be balanced.

[0041] In the present composition, component (B) is a poly(meth)acrylate viscosity index improver having a weight average molecular weight of 15000 to 30000, which is obtained by diluting a poly (meth) acrylate compound with a diluent. The weight average molecular weight of the poly(meth)acrylate compound may preferably be 17000 to 25000, more preferably 18000 to 24000, for further improvement in anti-fatigue performance.

[0042] As used herein, the weight average molecular weight means a weight average molecular weight measured with 150-C ALC/GPC system manufactured by WATERS CORPORATION equipped with two GMHHR-M (7.8 mmID x 30 cm) columns manufactured by TOSOH CORPORATION arranged in series, using tetrahydrofuran as a solvent, at 23 °C, at a flow rate of 1 mL/min, a sample concentration of 1 mass%, sample injection volume of 75 µL, and determined with a differential refractive index detector (RI) against a calibration curve obtained from polystyrene standard.

[0043] The poly(meth)acrylate in the poly(meth)acrylate compound constituting component (B) maypreferably be those having a structural unit represented by the formula (1):

[0044] In the formula (1), R¹ stands for a hydrogen atom or a methyl group, preferably a methyl group, and R² stands for a hydrocarbon group having 1 to 30 carbon atoms or a group represented by the formula - (R) a-E, wherein R stands for an alkylene group having 1 to 30 carbon atoms, E stands for an amine or heterocyclic residue having 1 to 2 nitrogen atoms and 0 to 2 oxygen atoms, and a is 0 or 1.

[0045] Examples of the alkyl group having 1 to 30 carbon atoms represented by R² may include methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, icosyl, docosyl, tetracosyl, hexacosyl, and octacosyl groups. These alkyl groups may be either straight or branched.

[0046] Examples of the alkylene group having 1 to 30 carbon atoms represented by R may include methylene, ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, tridecylene, tetradecylene, pentadecylene, hexadecylene, heptadecylene, and octadecylene groups. These alkylene groups may be either straight or branched.

[0047] Examples of the amine residue represented by E may include dimethylamino, diethylamino, dipropylamino, dibutylamino, anilino, toluidino, xylidino, acetylamino, and benzoylamino groups. Examples of the heterocyclic residue represented by E may include morpholino, pyrrolyl, pyrrolino, pyridyl, methylpyridyl, pyrrolidinyl, piperidinyl, quinonyl, pyrrolidonyl, pyrrolidono, imidazolino, and pyrazino groups.

[0048] Examples of the poly (meth) acrylate having a structural unit represented by the formula (1) may include poly(meth)acrylates prepared by polymerizing or copolymerizing one or more monomers represented by the formula (1a) :

         CH₂=CH(R¹)-C(=O)-OR²     (1a)

wherein R¹ and R² are the same as those in the formula (1).

[0049] Examples of the monomers represented by the formula (1a) may include the following monomers (Ba) to (Be).

[0050] Monomer (Ba) is a (meth) acrylate having an alkyl group with 1 to 4 carbon atoms, and may specifically be methyl(meth)acrylate, ethyl(meth)acrylate, n- or i-propyl(meth)acrylate, n-, i-, or sec-butyl (meth) acrylate, with methyl(meth)acrylate being preferred.

[0051] Monomer (Bb) is a (meth)acrylate having an alkyl or alkenyl group with 5 to 15 carbon atoms, and may specifically be octyl(meth)acrylate, nonyl(meth)acrylate, decyl(meth)acrylate, undecyl(meth)acrylate, dodecyl(meth)acrylate, tridecyl(meth)acrylate, tetradecyl(meth)acrylate, pentadecyl(meth)acrylate, octenyl(meth)acrylate, nonenyl(meth)acrylate, decenyl(meth)acrylate, undecenyl(meth)acrylate, dodecenyl(meth)acrylate, tridecenyl(meth)acrylate, tetradecenyl(meth)acrylate, or pentadecenyl(meth)acrylate. These may be either straight or branched. (Meth) acrylates mainly containing straight alkyl groups with 12 to 15 carbon atoms are preferred.

[0052] Monomer (Bc) is a (meth) acrylate having a straight alkyl or alkenyl group with 16 to 30 carbon atoms, preferably a straight alkyl group with 16 to 20 carbon atoms, more preferably a straight alkyl group with 16 or 18 carbon atoms. Specific examples of monomer (Bc) may include n-hexadecyl(meth)acrylate, n-octadecyl(meth)acrylate, n-icosyl(meth)acrylate, n-docosyl(meth)acrylate, n-tetracosyl(meth)acrylate, n-hexacosyl(meth)acrylate, and n-octacosyl(meth)acrylate, with n-hexadecyl(meth)acrylate and n-octadecyl(meth)acrylate being preferred.

[0053] Monomer (Bd) is a (meth) acrylate having a branched alkyl or alkenyl group with 16 to 30 carbon atoms, preferably a branched alkyl group with 20 to 28 carbon atoms, more preferably a branched alkyl group with 22 to 26 carbon atoms. Specific examples of monomer (Bd) may include branched hexadecyl(meth)acrylate, branched octadecyl(meth)acrylate, branched icosyl(meth)acrylate, branched docosyl(meth)acrylate, branched tetracosyl(meth)acrylate, branched hexacosyl(meth)acrylate, and branched octacosyl(meth)acrylate. (Meth)acrylates represented by the formula -C-C(R³)R⁴, having a branched alkyl group with 16 to 30, preferably 20 to 28, more preferably 22 to 26 carbon atoms are preferred. In the formula, R³ and R⁴ are not particularly limited as long as the carbon number of -C-C (R³) R⁴ is 16 to 30, and R³ may preferably be a straight alkyl group having 6 to 12, more preferably 10 to 12 carbon atoms, and R⁴ may preferably be a straight alkyl group having 10 to 16, more preferably 14 to 16 carbon atoms.

[0054] Specific examples of monomer (Bd) may include (meth)acrylates having a branched alkyl group with 20 to 30 carbon atoms, such as 2-decyl-tetradecyl (meth) acrylate, 2-dodecyl-hexadecyl(meth)acrylate, and 2-decyl-tetradecyloxyethyl(meth)acrylate.

[0055] Monomer (Be) is a monomer having a polar group. Examples of monomer (Be) may include vinyl monomers having an amido group, monomers having a nitro group, vinyl monomers having a primary to tertiary amino group, or vinyl monomers having a nitrogen-containing heterocyclicgroup;chlorides, nitrides, or phosphates thereof; lower alkyl monocarboxylates, such as those having 1 to 8 carbon atoms, vinyl monomers having a quaternary ammonium salt group, amphoteric vinyl monomers containing oxygen and nitrogen, monomers having a nitrile group, vinyl aliphatic hydrocarbon monomers, vinyl alicyclic hydrocarbon monomers, vinyl aromatic hydrocarbon monomers, vinyl esters, vinyl ethers, vinyl ketones, vinyl monomers having an epoxy group, vinyl monomers having a halogen, unsaturated carboxylates, vinyl monomers having a hydroxyl group, vinyl monomers having a polyoxyalkylene chain, vinyl monomers having an ionic group, such as anionic, phosphate, sulfonate, or sulfate group; monovalent metal salts, divalent metal salts, amine salts, or ammonium salts thereof.

[0056] As monomer (Be), monomers containing nitrogen are preferred among these, which may be, for example, 4-diphenylamine (meth)acrylamide, 2-diphenylamine (meth)acrylamide, dimethylaminoethyl (meth)acrylamide, diethylaminoethyl (meth)acrylamide, dimethylaminopropyl (meth)acrylamide, dimethylaminomethyl methacrylate, diethylaminomethyl methacrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, morpholinomethyl methacrylate, morpholinoethyl methacrylate, 2-vinyl-5-methylpyridine, or N-vinylpyrrolidone.

[0057] Component (B) in the present invention may preferably be a viscosity index improver containing a poly(meth)acrylate compound obtained by polymerizing or copolymerizing one or more monomers selected from above monomers (Ba) to (Be).

[0058] More preferred examples of such poly(meth)acrylate compound may include:

1) non-dispersant type poly(meth)acrylate which is a copolymer of monomers (Ba) and (Bb), or hydrides thereof;

2) non-dispersant type poly(meth)acrylate which is a copolymer of monomers (Ba), (Bb), and (Bc), or hydrides thereof;

3) non-dispersant type poly(meth)acrylate which is a copolymer of monomers (Ba), (Bb), (Bc), and (Bd) or hydrides thereof;

4) dispersant type poly (meth) acrylate which is a copolymer of monomers (Ba), (Bb), and (Be), or hydrides thereof;

5) dispersant type poly (meth) acrylate which is a copolymer of monomers (Ba), (Bb), (Bc), and (Be), or hydrides thereof; and

6) dispersant type poly (meth) acrylate which is a copolymer of monomers (Ba), (Bb), (Bc), (Bd), and (Be), or hydrides thereof.

[0059] Among these, non-dispersant type poly(meth)acrylate compounds 1) to 3) above are more preferred, and non-dispersant type poly(meth)acrylate compounds 2) and 3) are still more preferred, and non-dispersant type poly (meth) acrylate compound 3) is particularly preferred.

[0060] In the present composition, component (B) is usually provided in a state wherein the poly (meth) acrylate compound has been diluted to about 10 to 80 mass% with a diluent in light of easy handling and solubility in lubricant base oil (A) . Thus the content of component (B) including the diluent is 1 to 20 mass%, preferably 2 to 12 mass%, more preferably 3 to 8 mass% of the total amount of the composition.

[0061] If the content of component (B) is over the above range, improvement in anti-fatigue performance in proportion to the content may not be expected, shear stability is poor, the initial extreme pressure property is hard to be maintained for a prolonged period of time, and the effect on fuel efficiency resulting from the reduction of viscosity may be lowered.

[0062] The content of component (B) may suitably be selected from the above range depending on the kind of the poly (meth) acrylate compound or the ratio of the diluent, so that the kinematic viscosity at 100 °C of the present composition is 5.6 to 5.8 mm²/s.

[0063] In the present composition, component (C) is an imide friction modifier having a hydrocarbon group with 8 to 30 carbon atoms.

[0064] Component (C) is not particularly limited as long as it is a compound having an imide structure and a hydrocarbon group with 8 to 30 carbon atoms, and may preferably be, for example, a succinimide represented by the formula (2) or (3) and/or a derivative thereof:

[0065] In the formula (2), R⁵ stands for a straight or branched hydrocarbon group having 8 to 30 carbon atoms, R⁶ stands for a hydrogen atom or a straight or branched hydrocarbon group having 1 to 30 carbon atoms, R⁷ stands for a hydrocarbon group having 1 to 4 carbon atoms, and m is an integer of 1 to 7.

[0066] In the formula (3), R⁸ and R⁹ independently stand for a straight or branched hydrocarbon group having 8 to 30 carbon atoms, R¹⁰ and R¹¹ independently stand for a hydrocarbon group having 1 to 4 carbon atoms, and n is an integer of 1 to 7.

[0067] R⁵ in the formula (2) and R⁸ and R⁹ in the formula (3) independently stand for a straight or branched hydrocarbon group having 8 to 30, preferably 12 to 25 carbon atoms. Examples of such a hydrocarbon group may include alkyl and alkenyl groups, with an alkyl group being preferred. Examples of the alkyl and alkenyl groups may include octyl, octenyl, nonyl, nonenyl, decyl, decenyl, dodecyl, dodecenyl, octadecyl, and octadecenyl groups, as well as straight or branched alkyl group having up to 30 carbon atoms. If the carbon number of the hydrocarbon group is less than 8 or more than 30, sufficient anti-shudder property is hardly obtained. Thus it is particularly preferable that the hydrocarbon group is a branched alkyl group having 8 to 30, more preferably 10 to 25 carbon atoms. With a branched alkyl group having 8 to 30 carbon atoms, deterioration of torque capacity of various wet clutches may be decreased compared to the case with a straight alkyl group, and a composition excellent in both capability of maintaining torque capacity and sustainability of anti-shudder property, may be obtained.

[0068] R⁷ in the formula (2) and R¹⁰ and R¹¹ in the formula (3) independently stand for a hydrocarbon group having 1 to 4 carbon atoms. The hydrocarbon group may be an alkylene group having 1 to 4 carbon atoms, preferably an alkylene group having 2 or 3 carbon atoms, such as an ethylene or propylene group.

[0069] R⁶ in the formula (2) may be, for example, a straight or branched alkyl or alkenyl group having 1 to 30 carbon atoms, preferably a branched alkyl or alkenyl group having 1 to 30, more preferably 8 to 30, and particularly preferably 10 to 25 carbon atoms, with the branched alkyl group being particularly preferred.

[0070] In the formulae (2) and (3), n and m each denote an integer of 1 to 7. For obtaining a composition with still higher sustainability of anti-shudder property, n and m each preferably denote 1, 2, or 3, particularly preferably 1.

[0071] The succinimide compound represented by the formula (2) or (3) may be prepared by a known method, for example, by reacting an alkyl or alkenyl succinic anhydride and polyamine. Specifically, monosuccinimide represented by the formula (2) wherein R⁶ is a hydrogen atom may be prepared, for example, by gradually adding dropwise 1 mole of succinic anhydride having a straight or branched alkyl or alkenyl group with 8 to 30 carbon atoms to 1 mole or more of polyamine, such as diethylenetriamine, triethylenetetramine, or tetraethylenepentamine, in a nitrogen atmosphere at 130 to 180 °C, preferably 140 to 175 °C, allowing to react for 1 to 10 hours, preferably 2 to 6 hours, and distilling off the unreacted polyamine. Monosuccinimide represented by the formula (2) wherein R⁶ is a hydrocarbon group having 1 to 30 carbon atoms maybe prepared, for example, by reacting N-octadecyl-1,3-propanediamine and the succinic anhydride mentioned above in the same way as outlined above. Bissuccinimide represented by the formula (3) may be prepared by adding dropwise 0.5 mole of the polyamine mentioned above to 1 mole of the succinic anhydride mentioned above under the same conditions as outlined above, allowing to react in the same way, and evaporating the generated moisture.

[0072] Examples of the derivatives of the succinimide represented by the formula (2) or (3) may include compounds resulting from modification of the succinimide with boric acid, phosphoric acid, carboxylic acid, or derivatives thereof, sulfuric compounds, or triazoles. Specific examples of and methods for producing the derivatives may be those specifically disclosed in JP-2002-105478-A.

[0073] In the present invention, as component (C), use of bis-type succinimide represented by the formula (3) is particularly preferred compared to use of mono-type succinimide represented by the formula (2), for its capability of giving higher sustainability of anti-shudder property to the composition.

[0074] In the present composition, the content of component (C) is 2 to 4 mass%, preferably 2.5 to 3.5 mass% of the total amount of the composition. At a content of less than 2 mass%, the sustainability of anti-shudder property may not be made to achieve the higher goal of the present invention, for example, 1000 hour or longer life of anti-shudder property, whereas at a content of more than 4 mass%, anti-fatigue performance may be impaired.

[0075] Component (D) in the present composition is a phosphorus extreme pressure agent. Specific examples of component (D) may include monophosphates, diphosphates, and triphosphates, monophosphites, diphosphites, triphosphites, having an alkyl or aryl group with 3 to 30, preferably 4 to 18 carbon atoms, and amine or alkanolamine salts thereof. Among these, phosphates and phosphites having an alkyl group with 3 to 30 carbon atoms are preferred, and phosphites having an alkyl group with 3 to 30 carbon atoms are particularly preferred.

[0076] In the present composition, the content of component (D) is 0.01 to 0.04 mass%, preferably 0.02 to 0.04 mass% of the total amount of the composition in terms of phosphorus. If the content of component (D) in terms of phosphorus is less than the above range, sustainability of anti-shudder property tends to be lowered, whereas if the content is over the above range, anti-fatigue performance tends to be impaired.

[0077] In the present composition, component (E) is an ashless dispersant having at least one alkyl or alkenyl group of a number average molecular weight of not lower than 2000.

[0078] Typical examples of component (E) may include succinimide, benzylamine, or polyamine ashless dispersants having at least one alkyl or alkenyl group of a number average molecular weight of not lower than 2000. Among these, the succinimide ashless dispersants having at least one alkyl or alkenyl group are preferred, and bis-type succinimide ashless dispersants having at least two alkyl or alkenyl groups are particularly preferred.

[0079] The number average molecular weight of the alkyl or alkenyl group may preferably be 2000 to 5000, more preferably 2100 to 3500, still more preferably 2200 to 3000. The alkyl or alkenyl group may be either straight or branched. A branched alkyl or alkenyl group derived from oligomer of olefins, such as propylene, 1-butene, or isobutene, or co-oligomer of ethylene and propylene, is preferred, and a polybutenyl group derived from poly(iso)butene is particularly preferred.

[0080] With the number average molecular weight of the alkyl or alkenyl group being not lower than 2000, anti-shudder property may be improved, and durability of friction characteristics and anti-fatigue performance on gears may be improved. With the number average molecular weight of not higher than 5000, a compound having still more excellent low-temperature characteristics may easily be obtained.

[0081] Specific examples of the succinimide having at least one alkyl or alkenyl group of a number average molecular weight of not lower than 2000 may include compounds represented by the formula (4-a) or (4-b):

[0082] In the formulae, R¹², R¹³, and R¹⁴ independently stand for an alkyl or alkenyl group having a number average molecular weight of not lower than 2000, preferably 2000 to 5000, more preferably a poly(iso)butenyl group; r is an integer of 1 to 5, preferably 2 to 4; and s is an integer of 0 to 4, preferably 1 to 3.

[0083] The above-mentioned succinimide includes so-called monosuccinimides represented by the formula (4-a) wherein succinic anhydride is added to one end of polyamine, and so-called bissuccinimides represented by the formula (4-b) wherein succinic anhydride is added to both ends of polyamine. In the present composition, either one of or a mixture of these succinimides may be contained, and bis-type succinimides composed mainly of the bissuccinimides are particularly preferred.

[0084] The succinimide may be prepared by any process without particular limitation, for example, by reacting polybutene or polyisobutylene having a number average molecular weight of not lower than 2000 with maleic anhydride at 100 to 200 °C, and reacting the resulting poly(iso)butenyl succinate with polyamine. The polyamine may be, for example, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, or pentaethylenehexamine.

[0085] Specific examples of benzylamine having at least one alkyl or alkenyl group of a number average molecular weight of not lower than 2000 may include compounds represented by the formula (4-c):

[0086] In the formula, R¹⁵ stands for an alkyl or alkenyl group having a number average molecular weight of not lower than 2000, preferably 2000 to 5000, more preferably a poly(iso)butenyl group, and t is an integer of 1 to 5, preferably 2 to 4.

[0087] The benzylamine represented by the formula (4-c) may be prepared by any process without particular limitation, for example, by reacting polyolefin, such as propylene oligomer, polybutene, or ethylene-α-olefin copolymer, with phenol to give alkylphenol, which is then reacted with formaldehyde and polyamine, such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, or pentaethylenehexamine, by the Mannich reaction or the like.

[0088] Specific examples of the polyamine having at least one alkyl or alkenyl group with a number average molecular weight of not lower than 2000 may include compounds represented by the formula (4-d):

         R¹⁶-NH-(CH₂CH₂NH)_q-H     (4 - d)

[0089] In the formula, R¹⁶ stands for an alkyl or alkenyl group having a number average molecular weight of not lower than 2000, preferably 2000 to 5000, more preferably a poly(iso)butenyl group, and q is an integer of 1 to 5, preferably 2 to 4.

[0090] The polyamine represented by the formula (4-d) may be prepared by any process without particular limitation, for example, by chlorinating polyolefin, such as propylene oligomer, polybutene, or ethylene-α-olefin copolymer, and reacting with ammonia or polyamine, such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, or pentaethylenehexamine.

[0091] Component (E) also includes derivatives of the above nitrogen-containing compounds, such as succinimide, benzylamine, or polyamine. Examples of such derivatives may include so-called acid-modified compounds obtained by reacting, to the nitrogen-containing compounds, monocarboxylic acid having 2 to 30 carbon atoms, such as aliphatic acid, polycarboxylic acid having 2 to 30 carbon atoms, such as oxalic, phthalic, trimellitic, or pyromellitic acid, or anhydrides or esters thereof, alkylene oxide having 2 to 6 carbon atoms, or hydroxy(poly)oxyalkylenecarbonate or the like, to neutralize or amidify all or part of the residual amino and/or imino groups; so-called boron-modified compounds obtained by reacting, to the nitrogen-containing compounds, boroncompounds, suchasboricacid, boratesalts, orboronic esters, to neutralize or amidify all or part of the residual amino and/or imino groups; sulfur-modified compounds obtained by reacting a sulfur compound to the nitrogen-containing compounds; and modified compounds obtained by modifying the nitrogen-containing compounds by a combination of two or more modifications selected from the acid-, boron-, and sulfur-modifications.

[0092] Among the derivatives discussed above, boron-modified compounds of alkyl- or alkenylsuccinimide having a number average molecular weight of not lower than 2000 give finest anti-fatigue performance on gears, so that it is particularly preferred that component (E) contains such boron-modified compounds as an essential component.

[0093] The mass ratio of boron to nitrogen (B/N ratio) of the boron-modified compounds of the nitrogen-containing compounds is not particularly limited, and may preferably be not lower than 0.1, more preferably not lower than 0.2, and preferably not higher than 0.6, more preferably not higher than 0.3. With a boron-modified compound having a B/N ratio within the above range, a compound having excellent anti-fatigue performance on gears may be obtained.

[0094] The nitrogen content of component (E) is arbitrary, and may usually be 0.01 to 10 mass%, preferably 0.1 to 3 mass%, particularly preferably 0.2 to 1 mass%, in light of abrasion resistance, oxidation stability, and friction characteristics.

[0095] In the present composition, the minimum content of component (E) is not lower than 0.01 mass%, preferably 0.02 mass% of the total amount of the composition in terms of nitrogen, while the maximum content is not higher than 0.04 mass%, preferably 0.035 mass% of the total amount of the composition in terms of nitrogen.

[0096] If the content of component (E) is less than 0.01 mass%, durability of friction characteristics and torque capacity are hard to be maintained, and oxidation stability tends to be deteriorated. Even if the content is more than 0. 04 mass%, sufficient effect in proportion to the content is not achieved, and low-temperature fluidity of the composition or anti-fatigue performance on gears may be deteriorated, thus not being preferred.

[0097] When a boron-modified compound of the nitrogen-containing compound is essentially contained as component (E), the minimum content thereof is not lower than 0.003 mass%, preferably not lower than 0.004 mass% of the total amount of the composition in terms of boron, while the maximum content thereof is not higher than 0.01 mass%, preferably not higher than 0.008 mass% of the total amount of the composition in terms of boron.

[0098] With the content of the boron-modified compound as component (E) within the above-mentioned preferred range in terms of boron, durability of friction characteristics, torque capacity, low-temperature fluidity, and anti-fatigue performance on gears may be maintained at high levels in good balance.

[0099] The viscosity index of the present composition is preferably 95 to 200, more preferably 150 to 180 for excellent viscosity-temperature characteristics.
Further, a suitable kinematic viscosity at 40 °C of the composition is 25 to 30 mm²/s.

[0100] The present composition, in order to further improve its performance, or to impart performances necessary as a lubricant oil for automatic transmissions, may optionally contain various additives, such as viscosity index improvers, friction modifiers other than component (C), extreme pressure agents other than component (D), dispersants other than component (E), metal detergents, antioxidants, corrosion inhibitors, rust inhibitors, demulsifiers, metal deactivators, pour point depressants, seal swelling agents, foam inhibitors, and coloring agents, alone or in combination, as necessary.

[0101] Examples of the viscosity index improver may include known non-dispersant or dispersant type polymethacrylates (other than component (B)), non-dispersant or dispersant type ethylene-α-olefin copolymers or hydrides thereof, polyisobutylene or hydrides thereof, styrene-diene hydrogenated copolymers, styrene-maleic anhydride ester copolymers, and polyalkylstyrenes.

[0102] The content of the viscosity index improver other than component (B), if contained, in the present composition is not particularly limited as long as the conditions of the kinematic viscosity at 100 °C of the composition is fulfilled, and may usually be 0.1 to 15 mass%, preferably 0.5 to 5 mass% of the total amount of the composition.

[0103] The friction modifier other than component (C) may be any compound usually used as a friction modifier for a lubricant, and may preferably be, for example, amine compounds, fatty acids, fatty acid esters, fatty acid amides, or fatty acid metal salts, having in their molecule at least one alkyl or alkenyl group having 6 to 30 carbon atoms, preferably at least one straight alkyl or alkenyl group having 6 to 30 carbon atoms.

[0104] In the present invention, any one or more compounds selected from the friction modifiers mentioned above may be contained at any content, which may usually be 0.01 to 5.0 mass%, preferably 0.03 to 3.0 mass% of the total amount of the composition.

[0105] As an extreme pressure agent other than component (D), it is preferred to add an extreme pressure agent composed of at least one sulfur extreme pressure agent selected from the group consisting of sulfurized oils and fats, sulfurized olefins, dihydrocarbyl polysulfides, dithiocarbamates, thiadiazoles, and benzothiazoles, and/or at least one phosphorus-sulfur extreme pressure agent selected from the group consisting of thiophosphorous acids, thiophosphorous monoesters, thiophosphorous diesters, thiophosphorous triesters, dithiophosphorous acids, dithiophosphorous monoesters, dithiophosphorous diesters, dithiophosphorous triesters, trithiophosphorous acids, trithiophosphorous monoesters, trithiophosphorous diesters, trithiophosphoroustriesters, and salts thereof.

[0106] The content of the extreme pressure agent other than component (D), if contained, in the present composition may suitably be selected depending on its kind.

[0107] The dispersant other than component (E) maybe an ashless dispersant, such as succinimide, benzylamine, polyamine, and/or boron compound derivatives thereof, having a hydrocarbon group with 40 to 400 carbon atoms, other than component (E).

[0108] In the present invention, any one or more compounds selected from the dispersants mentioned above may be contained at any content, which may usually be 0.01 to 15 mass%, preferably 0.1 to 8 mass% of the total amount of the composition.

[0109] Examples of the metal detergents may include alkaline earth metal sulfonates, alkaline earth metal phenates, and alkaline earth metal salicylates.

[0110] In the present invention, any one or more compounds selected from the metal detergents mentioned above may be contained at any content, which may usually be 0.01 to 10 mass%, preferably 0.1 to 5 mass% of the total amount of the composition.

[0111] The antioxidants may be those commonly used for lubricants, such as phenol or amine compounds.

[0112] Specific examples of the antioxidants may include alkylphenols, such as 2,6-di-tert-butyl-4-methylphenol, bisphenols, such as methylene-4,4-bisphenol(2,6-di-tert-butyl-4-methylphenol), naphthylamines, such as phenyl-α-naphthylamine, dialkyldiphenylamines, zinc dialkyl dithiophosphates, such as zinc di-2-ethylhexyl dithiophosphate, and esters of (3,5-di-tert-butyl-4-hydroxyphenyl) fatty acid (such as propionic acid) or (3-methyl-5-tert-butyl-4-hydroxyphenyl) fatty acid (such as propionic acid) and monohydric or polyhydric alcohols, such as methanol, octanol, octadecanol, 1,6-hexadiol, neopentyl glycol, thiodiethylene glycol, triethylene glycol, or pentaerythritol.

[0113] Any one or more of the compounds selected from the above may be contained at any content, which may usually be 0.01 to 5 mass%, preferably 0.1 to 3 mass% of the total amount of the composition.

[0114] Examples of the corrosion inhibitor may include benzotriazol, tolyltriazole, thiadiazole, and imidazole compounds.

[0115] Examples of the rust inhibitor may include petroleum sulfonates, alkylbenzene sulfonates, dinonylnaphthalene sulfonates, alkenyl succinates, and esters of polyhydric alcohols.

[0116] Examples of the demulsifier may include polyalkylene glycol nonionic surfactants, such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, and polyoxyethylene alkylnaphthyl ethers.

[0117] Examples of the metal deactivator may include imidazoline, pyrimidine derivatives, alkylthiadiazoles, mercaptobenzothiazoles, benzotriazoles or derivatives thereof, 1,3,4-thiadiazolepolysulfides, 1,3,4-thiadiazolyl-2,5-bisdialkyldithiocarbamates, 2-(alkyldithio)benzoimidazoles, and β-(o-carboxybenzylthio)propionitrile.

[0118] The pour point depressant may be selected from known pour point depressants depending on the lubricant base oil, and may preferably be polymethacrylates having a weight average molecular weight of 20000 to 500000, more preferably 50000 to 300000, particularly preferably 80000 to 200000.

[0119] The foam inhibitor may be any compound usually used as a foam inhibitor for lubricants, for example, silicones, such as dimethyl silicon or fluorosilicon.

[0120] The seal swelling agent may be any compound usually used as a seal swelling agent for lubricants, for example, ester, sulfur, or aromatic seal swelling agents.

[0121] The coloring agent may be any compound usually used, and may be contained at any content, which may usually be 0.001 to 1.0 mass% of the total amount of the composition.

[0122] The contents of the above additives, if contained, in the present composition are: 0.005 to 5 mass% for the corrosion inhibitors, rust inhibitors, or demulsifiers; 0.005 to 2 mass% for the pour point depressants or metal deactivators; 0.01 to 5 mass% for the seal swelling agents; and 0.0005 to 1 mass% for the foam inhibitors.

[0123] The present invention will now be explained in detail with reference to Examples and Comparative Examples, which are illustrative only and do not intend to limit the present invention.

Examples 1-3 and Comparative Examples 1-9

[0124] Lubricant compositions for automatic transmissions of the present invention (Examples 1 to 3) were prepared with the compositions shown in Table 1. The following performance evaluation tests were conducted on these compositions. The results are also shown in Table 1.

[0125] Lubricant compositions for automatic transmissions for comparison (Comparative Examples 1 to 9) were also prepared with the compositions shown in Table 1. The same performance evaluation tests were conducted on these compositions. The results are shown in Table 1.

[Sustainability of Anti-shudder Property]

[0126] In accordance with "Test Procedure of Anti-shudder Property of Automatic Transmission Fluids " provided in JASO M349-98, the low velocity friction test was conducted at an oil temperature of 120 °C during the durability test to evaluate the life of the anti-shudder property of the compositions of Examples and Comparative Examples. Incidentally, the life of the reference oil provided in this test procedure is 72 hours, so that the life of the anti-shudder property equivalent to or longer than this is determined to be excellent. However, the present invention aimed at 1000 hour or longer life, and after 1500 hours, the test was discontinued.

[Low-Temperature Viscosity Measurement]

[0127] In accordance with "Test Procedure of Low-Temperature Viscosity of Gear Oils" provided in JPI-5S-26-85, the low-temperature viscosities at -40 °C of the lubricant compositions for automatic transmissions were measured using a low-temperature oil bath. The present invention aimed at a viscosity of not higher than 15000 mPa·s, but a viscosity of not lower than 10000 mPa·s was found to be desirable for good anti-fatigue performance.

[Oxidation Stability]

[0128] The test oils were subjected to forced degradation by ISOT test (150 °C, 96 hours) in accordance with JIS K 2514, and the increase in acid number (mgKOH/g) was measured.

[SAE No. 2 Test]

[0129] Using SAE No. 2 test machine, durability of transmission characteristics of a wet clutch was evaluated in accordance with JASO M348-95 "Test Procedure of Friction Characteristics of Automatic Transmission Fluids" except for the following severe test conditions.

<Test Conditions>

Oil Temperature: 120 °C; Revolution: 3000 rpm; Inertial Mass: 0.5 kg·m²; Contact Pressure: 1.9 MPa

[0130] Only the kinematic friction test was conducted. The clutch was idled at 3000 rpm with an inertial mass of 0.5 kg·m², and then pressed with applied pressure to stop the rotation. The friction coefficient was calculated from the torque generated at a relative revolution of 1200 rpm of the clutch, and recorded.

<Criteria of Evaluation>

[0131] The cycle number at which the kinematic friction coefficient was decreased for 0.02 or more from the average kinematic friction coefficient of 1 to 100 cycles was taken as a durable life cycle of each composition. The durability was evaluated as extremely excellent at 8000 cycles or more, and the test was discontinued at 12000 cycles.

[Gear Fatigue Life Test on Actual Device]

[0132] Using a commercial 3-speed automatic transmission (AY2 unit) manufactured by JATCO LTD., fatigue life (pitching life) of the reduction gear under loading by a motoring device was evaluated.

<Test Conditions>

[0133] Oil Temperature: 120 °C; Revolution: 1000 rpm; Load Torque: 422 N (output shaft torque); Gear: set at 1st position

<Criteria of Evaluation>

[0134] The transmission was overhauled every 1000000 cycles, and the cycle number at which pitching occurred was taken as the life.

[0135] From Table 1, it is understood that the lubricant compositions for automatic transmissions according to the present invention (Examples 1 to 3) containing the particular amounts of components (A) to (E) of the present invention provided long fatigue life irrespective of their low viscosity, exhibited excellent sustainability of anti-shudder property, low-temperature viscosity characteristics, and oxidation stability, and were excellent and balanced in durability of friction characteristics, energy-conserving performance, and anti-fatigue performance on gears. On the other hand, it is understood that when the components defined in the present invention were not contained in good balance, any one or more of the above performances were not satisfactory (Comparative Examples 1 to 9).

Table 1

	Example			Comparative Example
	1	2	3	1	2	3	4	5	6	7	8	9
Base Oil (A) (based on total amount of base oil)
Base oil (A1a)1) (mass%)	30	30	40	50	20	30	5	30	30	30	30	30
Base oil (A1b)2) (mass%)	60	50	50	40	70	60	95	60	60	60	60	60
Base oil (A1c)3) (mass%)	-	10
Base oil (A2a)4) (mass%)	10	10	10	10	10	10		10	10	10	10	10
Base Oil Property Kinematic Viscosity at 100 °C (mm2/s)	4.0	4.0	3.8	3.6	4.2	4.0	4.0	4.0	4.0	4.0	4.0	4.0

Additives (based on total amount of composition)
(B) VM-A105) (mass%)	5	5	6	7	3.8	-	5	5.2	5.1	5	5.8	3
VM-B116) (mass%)	-	-	-	-	-	5	-	-	-	-	-	-
(C) Imide FM7) (mass%)	3	3	3	3	3	3	3	-	3	3	3	3
(D) Phosphorus ((P) mass% Extreme Pressure Agent8)	0.03	0.03	0.03	0.03	0.03	0.03	0.03	0.03	0.03	-	0.03	0.03
Phosphoirus-Sulfur Extreme Pressure Agent9) ((P) mass%)	-	-	-	-	-	-	-	-	-	0.03	-	-
(E) Ashless Dispersant10) (mass%)	3	3	3	3	3	3	3	3	-	3	1	6
Ashless Dispersant (low-molecular weight mono-type)11) (mass%)	-	-	-	-	-	-	-	-	3	-	-	-
N content (mass%)	0.027	0.027	0.027	0.027	0.027	0.027	0.027	0.027	0.063	0.027	0.009	0.054
Additive Package12) (mass%)	5	5	5	5	5	5	5	5	5	5	5	5

Composition Properties and Test Results
Kinematic viscosity at 100 °C (mm2/s)	5.7	5.7	5.7	5.7	5.7	5.7	5.7	5.7	5.7	5.7	5.7	5.7
Viscosity index	162	164	166	168	157	165	161	163	161	162	164	155
Anti-shudder life (h)	1500	1500	1500	1500	1500	1500	1500	24	1200	120	1500	1200
Low-temperature viscosity (BF method: -40 °C) (mPa·s)	14800	12300	13500	12400	18800	13500	15300	13700	16400	15000	13800	22300
Increase in acid number (ISOT 150 °C, after 96 hrs) (mgKOH/g)	-0.11	-0.13	-0.11	-0.12	0.11	-0.12	-0.12	-0.14	-0.03	0.15	0.02	-0.02
Durable cycle number of friction characteristics in SAE No. 2 test	12000	12000	12000	12000	12000	12000	12000	4000	6000	4000	4000	12000
Gear fatigue test on actual device (thousand cycles)	3000	3000	3000	2000	3000	2000	2000	5000	2000	2000	3000	2000

Note for Table 1: 1) Hydrocracked mineral oil (kinematic viscosity at 100 °C: 2.6 mm2/s; %CA: 0; sulfur content: <0.001 mass%; viscosity index: 105) 2) Hydrocracked mineral oil (kinematic viscosity at 100 °C: 4.2 mm2/s; %CA: 0; sulfur content: <0.001 mass%; viscosity index: 125) 3) Poly-α-olefin base oil (kinematic viscosity at 100 °C: 4.0 mm2/s; %CA: 0; sulfur content: 0; viscosity index: 124) 4) Solvent-refined mineral oil (kinematic viscosity at 100 °C: 21.9 mm2/s; %CA: 7; sulfur content: 0.91 mass%; viscosity index: 95) 5) Non-dispersant type polymethacrylate additive (Mw: 20000) 6) Non-dispersant type polymethacrylate additive (Mw: 50000) 7) Imide friction modifier having C18 hydrocarbon group 8) Alkylphosphite 9) ZnDTP 10) Polybutenyl succinimide ashless dispersant (molecular weight of polybutenyl group Mw: 2500; bis-type; N content: 0.9 mass%, B content: 0.2 mass%) 11) Polybutenyl succinimide ashless dispersant (molecular weight of polybutenyl group Mw: 1300; mono-type; N content: 2.1 mass%) 12) Metal detergent, anti-oxidant, viscosity index improver, dispersant, demulsifier, foam inhibitor, etc.

Claims

1. A lubricant composition for automatic transmissions comprising:

(A) a lubricant base oil having a kinematic viscosity at 100 °C of 3.7 to 4.1 mm²/s, and consisting of:

lubricant base oil (A1) having a kinematic viscosity at 100 °C of 2.5 to 4.5 mm²/s, and

lubricant base oil (A2) having a kinematic viscosity at 100 °C of 10 to 40 mm²/s;

(B) a poly(meth)acrylate viscosity index improver having a weight average molecular weight of 15000 to 30000 at 1 to 20 mass% of the total amount of the composition;

(C) an imide friction modifier having a hydrocarbon group with 8 to 30 carbon atoms at 2 to 4 mass% of the total amount of the composition;

(D) a phosphorus extreme pressure agent at 0.01 to 0. 04 mass% of the total amount of the composition in terms of phosphorus; and

(E) an ashless dispersant having at least one alkyl or alkenyl group of a number average molecular weight of not lower than 2000 at 0.01 to 0.04 mass% of the total amount of the composition in terms of nitrogen;

wherein said composition has a kinematic viscosity at 100 °C of 5.6 to 5.8 mm²/s.

2. Use of a composition for the manufacture of a lubricant for automatic transmissions, said composition comprising:

(A) a lubricant base oil having a kinematic viscosity at 100 °C of 3.7 to 4.1 mm²/s, and consisting of:

lubricant base oil (A1) having a kinematic viscosity at 100 °C of 2.5 to 4.5 mm²/s, and

lubricant base oil (A2) having a kinematic viscosity at 100 °C of 10 to 40 mm²/s;

(B) a poly(meth)acrylate viscosity index improver having a weight average molecular weight of 15000 to 30000 at 1 to 20 mass% of the total amount of the composition;

(C) an imide friction modifier having a hydrocarbon group with 8 to 30 carbon atoms at 2 to 4 mass% of the total amount of the composition;

(D) a phosphorus extreme pressure agent at 0.01 to 0.04 mass% of the total amount of the composition in terms of phosphorus; and

(E) an ashless dispersant having at least one alkyl or alkenyl group of a number average molecular weight of not lower than 2000 at 0.01 to 0.04 mass% of the total amount of the composition in terms of nitrogen;

wherein said composition has a kinematic viscosity at 100 °C of 5.6 to 5.8 mm²/s.