Lubricating oil composition for automatic transmission

Abstract

A lubricating oil composition for automatic transmission comprises a base oil having a pour point of −25° C. or lower and a kinematic viscosity of 2 to 7 mm2/s at 100° C., (A) a over-based calcium sulfonate having a total base number of 300 to 500 mg KOH/g, (B) a succinimide having boron atom and/or a succinimide having no boron atom, which are substituted with a hydrocarbon group (an alkyl groups or alkenyl groups having an average molecular weight of 1,000 to 3,500), and, and (C) a phosphorous acid ester-based compound, each in a specific amount. The composition has a great statical friction coefficient (&mgr;s), an excellent transmission torque capacity and a friction characteristic having a &mgr; ratio of 1 or smaller, exhibiting excellent resistance to transmission shock on clutching.

Description

TECHNICAL FIELD

[0001] The present invention relates to a lubricating oil composition for automatic transmission. More particularly, the present invention relates to a lubricating oil composition for automatic transmission which has a great statical friction coefficient (&mgr;s), an excellent transmission torque capacity and a friction characteristic having a &mgr; ratio of 1 or smaller, exhibiting excellent resistance to transmission shock on clutching, and achieves the improvement in the energy saving property and the decrease in the size and the weight of the automatic transmission.

BACKGROUND ART

[0002] An automatic transmission fluid (referred to as ATF, hereinafter) is a lubricating oil composition used for automatic transmissions of automobiles having a torque converter, a gear mechanism, a hydraulic mechanism and a wet type clutch at the inside.

[0003] In order that the mechanism of the automatic transmission works smoothly, ATF is required to have many functions such as the function as a medium for transmitting power in the torque converter, the hydraulic system and the control system, lubrication of bearings of gears and the wet type clutch, the function as a medium for adjustment of the temperature, lubrication of friction materials and retention of the suitable friction property. As the automatic transmission is more widely used in the automobile field in recent years, the requirement for ATF having a more excellent friction property is increasing.

[0004] To improve the efficiency of transmission of the automatic transmission and also to improve the torque of transmission in the wet type clutch portion for decreasing the size and the weight of the transmission, it is required that the friction coefficient of the clutch be increased. (The torque capacity of a clutch is evaluated, in general, by the SAE No. 2 tester using an actually used clutch.)

[0005] In general, the transmission torque capacity is evaluated by the statical friction coefficient &mgr;s. The transmission torque capacity can be increased by increasing the value of &mgr;s. When the transmission torque capacity is increased, i.e., the statical friction coefficient &mgr;s is increased, in general, the friction coefficient &mgr;0 immediately before stopping by clutching is also increased. As the result, in general, the ratio of &mgr;0 as the index of the transmission shock to the dynamical friction coefficient &mgr;d (the &mgr; ratio) deteriorates and it has been considered difficult that the &mgr; ratio is kept at 1 or greater while &mgr;s is increased.

[0006] However, it is apparent that the &mgr; ratio and the value of &mgr;s can be improved simultaneously only when the dynamical friction coefficient is increased. The development of the technology for increasing &mgr;d has been conducted actively but it is the actual situation that no satisfactory results have been obtained.

[0007] In the transmission using conventional ATF, the number and the diameter of the disk plate are decreased to improve the transmission efficiency and to decrease the size and the weight of the transmission. This causes a problem in that the transmission torque capacity becomes insufficient due to a small friction coefficient.

[0008] Although various proposals have been made on ATF, these proposals have some problems and the satisfactory results cannot be always obtained. For example, in the technology disclosed in Japanese Patent No. 3184113, the transmission shock on clutching is great although the torque capacity is good. In the technology disclosed in Japanese Patent Application Laid-Open No. 2000-160183, the friction coefficient is small and the transmission torque capacity is insufficient although the transmission shock on clutching is absent. When an ashless dispersant containing boron is used and boron is contained in an amount of 0.035% by mass or more (350 ppm by mass or more) as disclosed in Japanese Patent Application Laid-Open No. 2001-279286, a problem arises in that the transmission shock is great and precipitates are formed since water tends to be absorbed due to the great amount of boron.

DISCLOSURE OF THE INVENTION

[0009] Under the above circumstances, the present invention has an object of providing a lubricating oil composition for automatic transmission which has a great statical friction coefficient (&mgr;s) to achieve the improvement in the energy saving property, a friction characteristic having a &mgr; ratio of 1 or smaller, exhibiting excellent resistance to transmission shock on clutching, to achieve the improvement torque capacity for the decrease in the size and the weight of the automatic transmission.

[0010] As the result of the intensive studies by the present inventors to develop the lubricating oil composition for automatic transmission having the above excellent properties, it was found that the above object could be achieved with a composition comprising a combination of a base oil having a specific fluidity, a over-based calcium sulfonate having a total base number in a specific range, a succinimide substituted with a hydrocarbon group and having boron atom or no boron atom such as polybutenylsuccinimide having no boron atom and polybutenylsuccinimide having boron atoms, and a phosphorous acid ester-based compound, each in a specific amount based on the entire amount of the composition. The present invention has been completed based on this knowledge.

[0011] The present invention provides a lubricating oil composition for automatic transmission which comprises a base oil having a pour point of −25° C. or lower and a kinematic viscosity of 2 to 7 mm2/s at a temperature of 100° C., (A) a over-based calcium sulfonate having a total base number of 300 to 500 mg KOH/g in an amount in a range of 2,000 to 3,500 ppm by mass as calcium, (B) at least one of a succinimide substituted with a hydrocarbon group and having no boron atom and a succinimide substituted with a hydrocarbon group and having boron atom, the succinimides having an alkyl group or an alkenyl group having an average molecular weight of 1,000 to 3,500, in an amount such that an amount of nitrogen is in a range of 100 to 500 ppm by mass and an amount of boron is in a range of 0 to 300 ppm by mass, and (C) a phosphorous acid ester-based compound in an amount in a range of 100 to 300 ppm by mass as phosphorus, each amount being based on an entire amount of the composition.

PREFERRED EMBODIMENT TO CARRY OUT THE INVENTION

[0012] In the lubricating oil composition for automatic transmission of the resent invention, a base oil having a pour point of −25° C. or lower and a kinematic viscosity of 2 to 7 mm2/s at the temperature of 100° C. is used. When the pour point of the base oil exceeds −25° C., the fluidity at low temperatures is insufficient. When the kinematic viscosity is smaller than 2 mm2/s at the temperature of 100° C., the vapor pressure is excessively great and the flash point decreases. Moreover, friction at the sliding portions such as bearings of gears and the clutch in the automatic transmission increases. A kinematic viscosity exceeding than 7 mm2/s is not necessary for the design of the automatic transmission and there is the possibility that delay in clutching takes places in transmission when the kinematic viscosity exceeds 7 mm2/s.

[0013] The type of the base oil is not particularly limited and any of mineral oils and synthetic oils can be used. As the mineral oil, various conventional mineral oils can be used. Examples of the mineral oil include paraffinic mineral oils, intermediate mineral oils and naphthenic mineral oils. Specific examples of the mineral oil include light neutral oil, intermediate neutral oil, heavy neutral oil and bright stock purified with a solvent or hydrogen.

[0014] As the synthetic oil, various conventional synthetic oils can be used. Examples of the synthetic oil include poly-&agr;-olefins (including copolymers of &agr;-olefins), polybutene, polyol esters, esters of dibasic acids, esters of phosphoric acid, polyphenyl ether, alkylbenzenes, alkylnaphthalenes, polyoxyalkylene glycols, neopentyl glycol, silicone oils, trimethylolpropane, pentaerythritol and hindered esters. The base oil may be used singly or in combination of two or more. The mineral oil and the synthetic oil may be used in combination. In the present invention, paraffinic highly purified base oils having a value of % CA of 0.1% by mass or smaller are preferable from the standpoint of the properties of the lubricating oil composition. The % CA is the fraction of aromatic components obtained in accordance with the n-d-M method of the ring analysis.

[0015] The lubricating oil composition of the present invention comprises an over-based calcium sulfonate as component (A). The over-based calcium sulfonate is a salt selected from calcium salts of various sulfonic acids and, in general, obtained by carbonation of a calcium salt of a sulfonic acid. Examples of the sulfonic acid include aromatic petroleum sulfonic acids, alkylsulfonic acids, aryl sulfonic acids and alkylarylsulfonic acids. Specific examples of the sulfonic acid include dodecylbenzenesulfonic acid, dilaurylcetylbenzenesulfonic acid, benzenesulfonic acid substituted with paraffin wax, benzenesulfonic acid substituted with polyolefins, benzenesulfonic acid substituted with polyisobutylene and naphthalene-sulfonic acid.

[0016] In the present invention, a over-based calcium sulfonate having a total base number in the range of 300 to 500 mg KOH/g is used. When the total base number is smaller than 300 mg KOH/g, the friction oeficient is not sufficiently great. It is difficult that a over-based calcium sulfonate having a total base number exceeding 500 mg KOH/g is produced.

[0017] The over-based calcium sulfonate may be used singly or in combination of two or more. The amount is selected in the range of 2,000 to 3,500 ppm by mass as the amount of calcium based on the entire amount of the composition. When the amount is less than 2,000 ppm by mass, the friction coefficient is not sufficiently great and the effect of cleaning is insufficient. When the amount exceeds 3,500 ppm by mass, it is difficult that the value of &mgr; is adjusted at 1 or smaller although the friction coefficient is sufficiently great.

[0018] The lubricating oil composition of the present invention comprises, as component (B), at least one of a succinimide substituted with a hydrocarbon group and having no boron atom and a succinimide substituted with a hydrocarbon group and having boron atom. The succinimides have an alkyl group or an alkenyl group having an average molecular weight of 1,000 to 3,500 and preferably a polybutenyl group.

[0019] When the average molecular weight of the alkyl group or the alkenyl group in the succinimide substituted with a hydrocarbon group and having boron atom or no boron atom is outside the range of 1,000 to 3,500, the object of the present invention is not achieved. It is preferable that the hydrocarbon group is a polybutenyl group and, more preferably a polybutenyl group having an average molecular weight in the range of 1,000 to 2,500.

[0020] Examples of the succinimide substituted with a hydrocarbon group and having no boron atom include monopolybutenylsuccinimides represented by general formula (I): 1

[0021] wherein R1 represents a polybutenyl group having an average molecular weight in the range of 1,000 to 3,500, R2 represents an alkylene group aving 2 to 4 carbon atoms, and m represents an integer of 1 to 10; and ispolybutenylsuccinimides represented by general formula (II): 2

[0022] wherein R3 and R4 each represent a polybutenyl group having an average molecular weight in the range of 1,000 to 3,500, R3 and R4 may represent the same group or different groups; R5 and R6 each represent an alkylene group having 2 to 4 carbon atoms, R5 and R6 may represent the same group or different groups, and n represents 0 or an integer of 1 to 10.

[0023] These polybutenylsuccinimides can be produced, in general, by reacting a polyalkylenepolyamine with polybutenylsuccinic anhydride which is obtained by the reaction of polybutene and maleic anhydride. The monobutenylsuccinimide, the bisbutenylsuccinimide or a mixture of these compounds can be obtained when the relative amounts of the nolybutenylsuccinic anhydride and the polyalkylenepolyamine are changed in the reaction.

[0024] Examples of the polyalkylenepolyamine used above include polyethylenepolyamine, polypropylenepolyamine and polybutylene-polyamine. Among these compounds, polyethylenepolyamine is preferable.

[0025] Examples of the polybutenylsuccinimide having boron atom include compounds obtained by reacting the polybutenylsuccinimide having no boron atom (the mono-compound and/or the bis-compound) with a boron compound. Examples of the boron compound include boric acid, boric acid anhydride, boron halides, esters of boric acid, amides of boric acid and boron oxide.

[0026] In the present invention, at least one compound selected from the monopolybutenylsuccinimides, bispolybutenylsuccinimides and polybutenylsuccinimides having boron atom described above is preferable as component (B). The amount is selected in the range such that the amount of nitrogen is in the range of 100 to 500 ppm by mass and preferably in the range of 150 to 350 ppm by mass and the amount of boron is in the range of 0 to 300 ppm by mass. When the amount of nitrogen is less than 100 ppm by mass, the torque transmission capacity decreases and dispersion of the degradation products deteriorates. When the amount of nitrogen exceeds 500 ppm by mass or the amount of boron exceeds 300 ppm by mass, transmission shock and shudder tend to take place.

[0027] The lubricating composition of the present invention comprises a phosphorous acid ester-based compound as component (C). Examples of the phosphorous acid ester-based compound include compounds represented by general formula (III): 3

[0028] wherein R7 to R9 each represent hydrogen atom or a hydrocarbon group having 4 to 30 carbon atoms, the atoms and the groups represented by R7 to R9 may be the same with or different from each other, and at least one of R7 to R9 represents a hydrocarbon group having 4 to 30 carbon atoms.

[0029] Examples of the hydrocarbon group having 4 to 30 carbon atoms represented by R7 to R9 in general formula (III) include linear, branched and cyclic alkyl groups and alkenyl groups having 4 to 30 carbon atoms, aryl groups having 6 to 30 carbon atoms, alkylaryl groups having 7 to 30 carbon atoms and arylalkyl groups having 7 to 30 carbon atoms. Specific examples of the above hydrocarbon group include butyl group, pentyl group, hexyl group, cyclohexyl group, octyl group, decyl group, lauryl group, myristyl group, palmityl group, stearyl group, oleyl group, eicosyl group, phenyl group, xylyl group, benzyl group and phenethyl group.

[0030] The phosphorous acid ester-based compound represented by general formula (III) include phosphorous acid esters represented by general formula (III) in which R7 to R9 each represent a hydrocarbon group, acidic phosphorous acid esters represented by general formula (III) in which one of R7 to R9 represents hydrogen atom and the rest of R7 to R9 each represent a hydrocarbon group (monohydrogenphosphites) and acidic phosphorous acid esters represented by general formula (III) in which two of R7 to R9 each represent hydrogen atom and the rest of R7 to R9 represents a hydrocarbon group (dihydrogenphosphites). In the present invention, acidic phosphorous acid esters are preferable and monohydrogenphosphites are more preferable.

[0031] Examples of the monohydrogenphosphite include dibutyl hydrogenphosphite, dilauryl hydrogenphosphite, dioleyl hydrogen-phosphite, distearyl hydrogenphosphite, diphenyl hydrogenphosphite and dibenzyl hydrogenphosphite. Among these compounds, dilauryl hydrogenphosphite and dioleyl hydrogenphosphite are preferable.

[0032] In the present invention, the phosphorous acid ester-based compound described above may be used singly or in combination of two or more. The amount is in the range of 100 to 300 ppm by mass as the amount of phosphorus based on the entire amount of the composition. When the amount is less than 100 ppm by mass, it is difficult that the value of &mgr; is adjusted at 1.0 or smaller and the object of the present invention is not achieved. When the amount exceeds 300 ppm by mass, the friction coefficient decreases and the transmission torque is insufficient.

[0033] Where desired, the lubricating oil composition of the present invention may further comprise other additives such as antioxidants, viscosity index improvers, extreme pressure agents, friction modifiers, rust preventives, corrosion inhibitors and defoaming agents as long as the object of the present invention is not adversely affected.

[0034] Examples of the antioxidant include amine-based antioxidants such as alkylated diphenylamine, phenyl-&agr;-naphthylamine and alkylated &agr;-naphthylamine; phenol-based antioxidants such as 2,6-di-t-butyl-4-methylphenol and 4,4′-methylenebis(2,6-di-t-butylphenol); and sulfur-based antioxidants such as dilauryl thiodipropionate and distearyl thiodipropionate.

[0035] Examples of the viscosity index improver include polymethacrylate-based viscosity index improvers, polyisobutylene-based viscosity index improvers, ethylene-propylene copolymer-based viscosity index improvers and hydrogenated styrene-butadiene copolymer-based viscosity index improvers.

[0036] Examples of the extreme pressure agent and the friction modifier include esters of phosphoric acid, carboxylic acids, esters of carboxylic acids, oils and fats, amides of carboxylic acids (reaction products of amines such as polyalkylenepolyamines, alkanolamines and alkylamines with carboxylic acids), alkylamines, N-alkylalkanolamines and partial esters of carboxylic acids and polyhydric alcohols.

[0037] Examples of the rust preventive include alkenylsuccinic acids and partial esters of alkenylsuccinic acids. Examples of the corrosion inhibitor include benzotriazole and benzimidazole. Examples of the defoaming agent include dimethylpolysiloxane and polyacrylates.

[0038] The lubricating oil composition of the present invention having the above composition has, in general, a value of &mgr;s of 1.2 or greater, exhibiting the excellent transmission torque capacity, and, at the same time, a &mgr; ratio of 1 or smaller, exhibiting the excellent resistance to transmission shock. Therefore, the improvement in the energy saving property and the decreases in the size and the weight of the automatic transmission can be achieved.

[0039] The lubricating oil composition of the present invention can be applied to transmissions equipped with an automatic transmission and a friction materal mechanism of the wet type. joints having a friction material mechanism of the wet type for braking and automatic/non-stage transmissions having a lock up clutch equipped with a continuous slipping mechanism.

EXAMPLE

[0040] The present invention will be described more specifically with reference to examples in the following. However, the present invention is not limited to the examples.

Examples 1 to 3 and Comparative Examples 1 to 8

[0041] Lubricating oil compositions were prepared in accordance with the formulations shown in Table 1 and the dynamical friction coefficient (&mgr;d) and the stillness friction coefficient (&mgr;0) were evaluated under the dynamic condition and the statical friction coefficient (&mgr;s) was evaluated under the static condition using the SAE No. 2 friction tester. The conditions of the evaluations are shown in the following.

[0042] A cellulose-based clutch material actually used for transmissions was used under the following conditions: the face pressure: 0.2 to 0.3 N/mm2; the oil temperature: 100° C.; the dynamic speed of rotation: 3,000 rpm; and the static speed of rotation: 0.7 rpm.

[0043] Under the above test conditions, &mgr;d and &mgr;s at 3,000 rpm were measured and the &mgr; ratio (&mgr;0/&mgr;d) was obtained. When the &mgr; ratio is 1 or smaller, the resistance to transmission shock is excellent.

[0044] The results are shown in Table 1.

[0045] The components used for the compositions are shown in the following.

[0046] (1) Base Oil

[0047] A paraffinic oil having a pour point of −30° C., a kinematic viscosity of 3.5 mm2/s at 100° C. and a % CA of 0.1% by mass or smaller.

[0048] (2) Over-Based Calcium Sulfonates

[0049] 250TBN: calcium sulfonate having a total base number of 230 mg/KOH.

[0050] 300TBN: calcium sulfonate having a total base number of 330 mg/KOH.

[0051] 400TBN: calcium sulfonate having a total base number of 400 mg/KOH.

[0052] (3) Polybutenylsuccinimides

[0053] High molecular weight Mw2000: a polybutenylsuccinimide having a nitrogen content of 1.5% by mass and no boron atom, in which the average molecular weight of the polybutenyl group was 2,000.

[0054] B-based Mw1000-1: a polybutenylsuccinimide having a nitrogen content of 1.6% by mass and a boron content of 0.4% by mass, in which the average molecular weight of the polybutenyl group was 1,000.

[0055] B-based Mw1000-2: a polybutenylsuccinimide having a nitrogen content of 2.0% by mass and a boron content of 2.0% by mass, in which the average molecular weight of the polybutenyl group was 1,000.

[0056] Low molecular weight Mw1000>: a polybutenylsuccinimide having a nitrogen content of 3.6% by mass and no boron atom, in which the average molecular weight of the polybutenyl group was 500.

[0057] (4) Acidic Phosphorous Acid Ester

[0058] Dioleyl hydrogenphosphite

[0059] (5) Antioxidant

[0060] A combination of a phenol-based antioxidant and an amine-based antioxidant

[0061] (6) Viscosity Index Improver

[0062] A PMA-based viscosity index improver 1 TABLE 1-1 Comparative Example Example 1 2 3 1 2 3 Composition (% by mass) paraffinic base oil rest rest rest rest rest rest calcium sulfonate 250 TBN — — — — — — 300 TBN — —  3.0 — — — 400 TBN  2.0  2.0 —  1.0  2.0  2.0 polybutenylsuccinimide high molecular weight  2.0 —  2.0  2.0 — — Mw2000 B-based Mw1000-1 —  2.0 — — — — B-based Mw1000-2 — — — —  2.0 — low molecular weight — — — — —  4.5 Mw1000> acidic phosphorous acid ester  0.4  0.3  0.4  0.4  0.4  0.4 antioxidant  1.0  1.0  1.0  1.0  1.0  1.0 viscosity index improver etc 11*1 11*1 11*1 11*1 11*1 11*1 Elements in additives added to the composition (ppm by mass) Ca 3000 3000 3000 1500 3000 3000 N  300  200  300  300  300  300 P  250  200  250  250  200  250 B  0  100  0  0  350  0 Friction properties by SAE No. 2 test &mgr;d  0.16  0.16  0.16  0.13  0.14  0.14 &mgr;s  0.12  0.13  0.13  0.08  0.09  0.09 &mgr; ratio  0.92  0.90  0.94  0.94  0.92  0.90

[0063] 2 TABLE 1-2 Comparative Example 4 5 6 7 8*2 Composition (% by mass) paraffinic base oil rest rest rest rest rest calcium sulfonate 250 TBN 5.0 — — — — 300 TBN — — — — — 400 TBN — 2.0 4.0 2.0 2.0 polybutenylsuccinimide high molecular weight 2.0 2.0 2.0 4.0 — Mw2000 B-based Mw1000-1 — — — — — B-based Mw1000-2 — — — — — low molecular weight — — — — — Mw1000> acidic phosphorous acid 0.4 — 0.8 0.4 0.4 ester antioxidant 1.0 1.0 1.0 1.0 1.0 viscosity index improver 11*1 11*1 11*1 11*1 11*1 etc Elements in additives added to the composition (ppm by mass) Ca 3000 3000 6000 3000 3000 N 300 300 300 600 0 P 250 0 500 250 250 B 0 0 0 0 0 Friction properties by SAE No. 2 test &mgr;d 0.13 0.17 0.17 0.13 0.14 &mgr;s 0.08 0.16 0.12 0.09 0.12 &mgr; ratio 0.93 1.05 1.01 0.90 0.95 Notes *1Viscosity index improver: 10% and others: 1% *2Precipitates were formed.

[0064] As shown in Table 1, the lubricating oil compositions of the present invention all showed &mgr;s of 0.12 or greater, exhibiting great transmission torque capacities, and &mgr; ratios of 1.0 or smaller, exhibiting excellent resistance to transmission shock.

INDUSTRIAL APPLICABILITY

[0065] In accordance with the present invention, the lubricating oil composition for automatic transmission which has a great statical friction coefficient (&mgr;s), an excellent transmission torque capacity and a friction characteristic having a &mgr; ratio of 1 or smaller, exhibiting excellent resistance to transmission shock on clutching, and achieves the improvement the energy saving property and the decrease in the size and the weight of the automatic transmission, can be provided.

Claims

1. A lubricating oil composition for automatic transmission which comprises a base oil having a pour point of −25° C. or lower and a kinematic viscosity of 2 to 7 mm2/s at a temperature of 100° C., (A) a over-based calcium sulfonate having a total base number of 300 to 500 mg KOH/g in an amount in a range of 2,000 to 3,500 ppm by mass as calcium, (B) at least one of a succinimide substituted with a hydrocarbon group and having no boron atom and a succinimide substituted with a hydrocarbon group and having boron atom, the succinimides having an alkyl group or an alkenyl group having an average molecular weight of 1,000 to 3,500, in an amount such that an amount of nitrogen is in a range of 100 to 500 ppm by mass and an amount of boron is in a range of 0 to 300 ppm by mass, and (C) a phosphorous acid ester-based compound in an amount in a range of 100 to 300 ppm by mass as phosphorus, each amount being based on an entire amount of the composition.

2. A lubricating oil composition for automatic transmission according to claim 1, wherein the base oil is a paraffinic highly purified based oil having a value of % CA of 0.1% by mass or smaller.

3. A lubricating oil composition for automatic transmission according to any one claims 1 and 2, wherein the phosphorous acid ester-based compound of component (C) is an acidic phosphorous acid ester.