Lubricating oil composition

Abstract

 A lubricating oil composition for an automatic or continuously variable transmission, which comprises a lubricating base stock and 0.1 to 10 wt.%, based on the whole weight of the composition, of a compound composed of at least one of alkyl- or alkenyl-substituted cyclic dicarboxylic anhydrides. The composition can achieve a sufficient friction coefficient in a high sliding speed range in the automatic or continuously variable transmission.

Description

[0001] This invention relates to a lubricating oil composition, and more specifically to a lubricating oil composition having high torque capacity and excellent judder vibration preventing performance and suited for use in automatic transmissions and continuously variable transmissions (of the belt, traction and like types) for automotive vehicles, each of which transmissions has a torque converter equipped with a lock-up clutch.

[0002] An automatic or continuously variable transmission fluid is a lubricating oil for use in automatic or continuously variable transmissions for automotive vehicles and the like, which are equipped with torque converters, gear mechanisms, hydraulic systems, wet clutches and the like, respectively. This automatic or continuously variable transmission fluid is required to have many functions, for example, as a power transmitting medium for the torque converters, the hydraulic systems, control systems and the like, as a lubricating medium and temperature-controlling heating medium for gears, bearings, the wet clutches and the like, and also as a lubricating medium and friction characteristic medium for friction materials.

[0003] In recent years, lock-up clutches effective for improved fuel economy have been increasingly adopted in automatic transmissions and the like for automotive vehicles. In this mechanism, the transmission is built in a torque converter. It is the function of the lock-up clutch to transmit directly drive force of an engine to the transmission in accordance with running conditions. Switching of driving through the torque converter and direct driving at adequate timings makes it possible to improve the efficiency of the torque converter.

[0004] In the conventional automatic transmission or the like, the lock-up clutch is actuated only in a high vehicle speed range and is not used in a low vehicle speed range where engine torque varies considerably.

[0005] Recently, slip control which permits actuation of the lock-up clutch even in the low vehicle speed range of the automatic transmission is performed. Actuation of the lock-up clutch in the low vehicle speed range, however, develops a problem in that abnormal body vibrations called "judder" frequently occur at a friction surface of the lock-up clutch. Especially in a slip-controlled lock-up clutch, judder tends to occur where the friction coefficient decreases as the relative sliding speed increases. For the prevention of occurrence of this judder, there is accordingly a demand for an automatic or continuously variable transmission fluid having good µ-V (friction coefficient - sliding speed) characteristic, that is, a friction characteristic of a positive gradient such that the friction coefficient becomes higher with the sliding speed.

[0006] It has already been proposed to use a phosphate, a fatty acid ester, a fatty acid amide or the like as a friction modifier in an automatic transmission fluid as disclosed, for example, in JP kokai 63-254196. However, the addition of such a friction modifier involves problems in that the friction coefficient of the lock-up clutch is lowered in a low sliding speed range and the torque capacity upon engagement of the clutch is insufficient.

[0007] Accordingly, with a view to increasing torque capacity, it has also been proposed to use a metal detergent, an ashless dispersant and/or the like as disclosed, for example, in JP kokai 5ÿ2D105892, JP kokai 8-127789, JP kokai 8-319494 and so on. Despite these proposals, not many automatic or continuously variable transmission fluids have heretofore been available with sufficient friction characteristics.

[0008] µV characteristic and torque capacity are in a trade-off relation as described above. It is necessary to heighten the torque capacity while maintaining the µ-V characteristic in a positive gradient. For this purpose, it has been strongly desired to develop a technique that makes it possible to increase friction coefficient in a high sliding speed range.

[0009] With the foregoing situation of developments in view there is a need for a lubricating oil composition for an automatic or continuously variable transmission having a torque converter with a lock-up clutch, which achieves a sufficient friction coefficient in a high sliding speed range of the automatic or continuously variable transmission.

[0010] The present inventors have found that the addition of a compound, which is composed of at least one cyclic dicarboxylic acid anhydride substituted by one or more specific alkyl or alkenyl groups, in an effective proportion to a lubricating base stock can provide a lubricating oil composition for an automatic or continuously variable transmission, which achieves a sufficient friction coefficient in a high sliding speed range while retaining lubricating oil properties required as a lubricating oil for the automatic or continuously variable transmission.

[0011] According to the present invention, there is thus provided a lubricating oil composition which comprises a lubricating base stock and a compound composed of at least one cyclic dicarboxylic acid anhydride substituted by one or more alkyl or alkenyl groups.

[0012] The present invention, as described above, relates to the lubricating oil composition comprising the lubricating base oil and the specific compound added thereto. As preferred embodiments, the present invention includes the following lubricating oil compositions.

(i) A lubricating oil composition for an automatic or continuously variable transmission, which comprises a lubricating base stock and 0.1 wt.% to 7 wt.%, based on the whole composition, of at least one compound selected from cyclic dicarboxylic acid anhydrides each of which has been substituted by one or more alkyl or alkenyl groups.

(ii) A lubricating oil composition for an automatic or continuously variable transmission, which comprises a lubricating base stock and 0.1 wt.% to 7 wt.%, based on the whole composition, of at least one compound selected from cyclic dicarboxylic acid anhydrides represented by the formulas (1) to (4), respectively.

(iii) A lubricating oil composition for an automatic or continuously variable transmission, as described above under one of (i) and (ii), which comprises at least one of cyclic dicarboxylic acid anhydrides of the formula (1) to (4) in which R is a C_6-300 alkyl or alkenyl group having a molecular weight of from 80 to 4,200.

(iv) A lubricating oil composition for an automatic or continuously variable transmission, which comprises a lubricating base stock and the above-described compound and further at least one additive component selected from viscosity index improvers, pour-point depressants, ashless dispersants, metal detergents, oxidation inhibitors, antiwear agents, extreme pressure additives, metal deactivators, corrosion inhibitors, foam inhibitors and other additive components required for lubricating oil compositions for automatic or continuously variable transmissions.

[0013] The present invention will hereinafter be described in detail.

(1) Lubricating base stock

[0014] No particular limitation is imposed on the base stock employed in the lubricating oil composition of the present invention for the automatic or continuously variable transmission, and any base oil can be used insofar as it is generally used as a lubricating base stock. Lubricating base stocks which meet this requirement can include mineral oils, synthetic oils and blends thereof.

[0015] Illustrative of the mineral oils are solvent-refined raffinates available by treating feed oils for lubricating oils, said feed oils having been derived from atmospheric distillation or vacuum distillation of crude oils, with an aromatic extracting solvent such as phenol, furfural or N-methylpyrrolidone; hydrotreated oils available by contact of feed oils for lubricating oils with hydrogen in the presence of a hydrotreating catalyst under hydrotreatment conditions; isomerized oils available by contact of waxes with hydrogen in the presence of an isomerizing catalyst under isomerizing conditions; and lubricating oil fractions available by combining solvent refining steps with hydrotreating steps, isomerizing steps or the like. In all the above production processes, one or more steps such as a dewaxing step, a hydrofinishing step and/or a clay treatment step can be adopted as desired in a usual manner. Specific examples of the mineral oils can include light neutral oil, medium neutral oil, heavy neutral oil, bright stock and the like. A base stock can be prepared by suitably blending two or more of them to meet required properties.

[0016] Illustrative of the synthetic oils are poly(α-olefins), α-olefin oligomers, polybutene, alkylbenzenes, polyol esters, dibasic acid esters, polyoxyalkylene glycols, polyoxyalkylene glycol ethers, and silicone oils.

[0017] These base stocks can be used either singly or in combination. It is possible to use one or more mineral oils and one or more synthetic oils in combination. The base stock for use in the present invention generally has a kinematic viscosity of from 2 to 20 mm²/s at 100°C with a range of from 3 to 15 mm²/s being preferred. An excessively high kinematic viscosity leads to an increase in low-temperature viscosity and hence to a deterioration in low-temperature smooth operability, while an unduly low kinematic viscosity leads to a problem in that wearing increases at sliding parts such as gear bearings, a clutch and the like in an automatic transmission.

(2) Additive components

[0018] Illustrative of cyclic dicarboxylic anhydride substituted by one or more alkyl or alkenyl groups and having property to increase the friction coefficient in a high sliding speed range according to the present invention, are succinic anhydride, maleic anhydride, glutaric anhydride, glutaconic anhydride, adipic anhydride, itaconic anhydride, citraconic anhydride, phthalic anhydride, cyclohexyl-1,2-dicarboxylic acid anhydride, cis-4-cyclohexene-1,2-dicarboxylic anhydride, endo-bicyclo-(2,2,1)-5-heptene-2,3-dicarboxylic anhydride, alkylcyclohexyl-1,2-dicarboxylic anhydride, 3,6-methylenecyclohexyl- 1,2-dicarboxylic anhydride, 2-alkyl-3,6-dimethylenecylohexyl-1,2-dicarboxylic anhydride, each of which has been substituted by one or more alkyl or alkenyl groups. Among these, particularly useful are cyclic dicarboxylic acid anhydrides substituted by one or more alkyl or alkenyl groups, which are represented by the following formulas, respectively:

wherein R represents an alkyl or alkenyl group, X represents a C_nH_2n-1 or
C_nH_2n-3, and n stands for an integer of from 2 to 11.

[0019] The alkyl or alkenyl group represented by the formula (1) has 6 to 300 carbon atoms, with 8 to 120 carbon atoms being preferred. A carbon number smaller than 6 leads to a reduction in the solubility in lubricating oil and, on the other hand, a carbon number greater than 300 also results in a reduction in the solubility in lubricating oil. Further, its molecular weight ranges from 80 to 4,200, preferably from 110 to 1,700. A molecular weight smaller than 80 leads to a reduction in the solubility in lubricating oil, and a molecular weight greater than 4,200 also results in a reduction in the solubility in the lubricating oil.

wherein R represents an alkyl or alkenyl group, and n stands for an integer of from 1 to 4.

[0020] The formula (2) represents alkyl- or alkenyl-substituted cyclohexyl-1,2-dicarboxylic acid anhydrides, in which the alkyl or alkenyl group(s) represented by R has 6 to 300 carbon atoms, with 8 to 120 carbon atoms being preferred. A carbon number smaller than 6 leads to a reduction in the solubility in lubricating oil and, on the other hand, a carbon number greater than 300 also results in a reduction in the solubility in lubricating oil. Further, its molecular weight ranges from 80 to 4,200, preferably from 110 to 1,700. A molecular weight smaller than 80 leads to a reduction in the solubility in lubricating oil, and a molecular weight greater than 4,200 also results in a reduction in the solubility in lubricating oil. n is preferably an integer of from 1 to 2. The preferred position(s) of the substituent group(s) is the 4- or 5-position.

wherein R represents an alkyl or alkenyl, m stands for an integer of from 1 to 4, and n stands for an integer of from 1 to 2.

[0021] The formula (3) represents alkyl- or alkenyl-substituted 3,6-methylenecyclohexyl-1,2-dicarboxylic acid anhydrides, in which the alkyl or alkenyl group(s) represented by R has 6 to 300 carbon atoms, with 8 to 120 carbon atoms being preferred. A carbon number smaller than 6 leads to a reduction in the solubility in lubricating oil and, on the other hand, a carbon number greater than 300 also results in a reduction in the solubility in lubricating oil. Further, its molecular weight ranges from 80 to 4,200, preferably from 110 to 1,700. A molecular weight smaller than 80 leads to a reduction in the solubility in lubricating oil, and a molecular weight greater than 4,200 also results in a reduction in the solubility in lubricating oil. The preferred position(s) of the substituent group(s) is the 4- or 5-position. On the other hand, m which indicates the number of methylene group(s) is preferably an integer from 1 to 2.

wherein R represents an alkyl or alkenyl group, and n stands for an integer of from 1 to 4.

[0022] The formula (4) represents alkyl- or alkenyl-substituted cyclic phthalic anhydrides, in which the alkyl or alkenyl group(s) represented by R has 6 to 300 carbon atoms, with 8 to 120 carbon atoms being preferred. A carbon number smaller than 6 leads to a reduction in the solubility in lubricating oil and, on the other hand, a carbon number greater than 300 also results in a reduction in the solubility in lubricating oil. Further, its molecular weight ranges from 80 to 4,200, preferably from 110 to 1,700. A molecular weight smaller than 80 leads to a reduction in the solubility in lubricating oil, and a molecular weight greater than 4,200 also results in a reduction in the solubility in lubricating oil. n is preferably an integer from 1 to 2. The preferred position(s) of the substituent group(s) is the 4- or 5-position.

[0023] The compound which is composed of at least one of alkyl- or alkenyl-substituted cyclic dicarboxylic acid anhydrides such as those described above is added to the lubricating base stock. Its content may range from 0.1 wt.% to 10 wt.%, preferably from 0.1 wt.% to 7.0 wt.% based on the whole weight of the composition. A content lower than 0.1wt.% is unlikely to show sufficient judder vibration preventing performance, whereas a content higher than 10 wt.% tends to lead to a reduction in oxidation stability and moreover, is unlikely to bring about extra friction characteristic improving effects.

[0024] Owing to the inclusion of at least one of these compounds as an essential component, the lubricating oil composition according to the present invention, when used as an automatic or continuously variable transmission fluid, can exhibit a marked effect so that a friction coefficient is increased in a high sliding speed range.

[0025] Insofar as the object of the present invention that a friction coefficient is increased in a high sliding speed range is not impaired, one or more friction-modifying compounds other than the alkyl- or alkenyl-substituted cyclic dicarboxylic acid anhydrides can also be added.

[0026] Illustrative of such friction-modifying compounds other than the alkyl- or alkenyl-substituted cyclic dicarboxylic acid anhydrides, said other friction-modifying compounds being an optional component for the present invention, are molybdenum dithiophosphate, molybdenum dithiocarbamate, phosphates, phosphites, acid phosphate amine salts, fatty acids, higher alcohols, fatty acid esters, oil and fats, polyhydric alcohol esters, sorbitan esters, amine compounds, amide compounds, and imide compounds. These friction-modifiers can generally lower a friction of coefficient in the entire sliding speed range. Whenever these friction modifiers are combined with the alkyl- or alkenyl-substituted cyclic dicarboxylic anhydride as an essential component for the present invention, they can be formed into such a composition as modifying friction to facilitate an increase in the friction coefficient in a high sliding speed range and also the provision of a positive gradient for the µ-V characteristic without substantially lowering the friction coefficient in a low speed range. Among these, particularly preferred can be amide compounds and imide compounds, which can bring about significant friction characteristics improving effects when combined with the alkyl- or alkenyl-substituted cyclic dicarboxylic anhydride.

(3) Other additive components

[0027] In the lubricating oil composition according to the present invention, the above-described compound or compounds are added as an essential component to the lubricating base stock. It is also possible to add various additives - i.e., viscosity index improvers, pour-point depressants, metal detergents, oxidation inhibitors, antiwear agents, extreme pressure additives, metal deactivators, corrosion inhibitors, foam inhibitors, coloring matters and the like - as needed to extents not impairing the object of the present invention.

[0028] Illustrative of the viscosity index improvers are generally polymethacrylate, olefin copolymers (polyisobutylene and ethylene-propylene copolymers), polyalkylstyrene, hydrogenated styrene-butadiene copolymers, and styrene-maleic anhydride ester copolymers. For example, polymethacrylate can be used preferably. They can be used generally in a proportion of from 3 to 35 wt.%.

[0029] Illustrative of the pour-point depressants are generally ethylenevinyl acetate copolymers, condensates between chlorinated paraffins and naphthalene, condensates between chlorinated paraffins and phenol, polymethacrylates, and polyalkylstyrenes. For example, polymethacrylates can be used preferably. They can be used generally in a proportion of from 0.01 to 5 wt.%.

[0030] Illustrative of the ashless dispersants are polyalkenylsuccinimides, polyalkenylsuccinamides, benzylamines, succinates, succinate-amides, and boron-containing ashless dispersants. Among these, polyalkenylsuccinimides (polybutenylsuccinimide) can be used preferably. They can be used generally in a proportion of from 0.1 to 10 wt.%.

[0031] Illustrative of the metal detergents are the sulfonates, phenates, salicylates and phosphonates of Ca, Mg, Ba and the like. They can be used generally in a proportion of from 0.05 to 5 wt.%.

[0032] Illustrative of the oxidation inhibitors are generally amine-type oxidation inhibitors such as alkylated diphenylamines, phenyl-α-naphthylamine and alkylated phenyl-α-naphthylamines; phenol-type oxidation inhibitors such as 2,6-di(tertiary butyl)~phenol and 4,4'-methylene bis-[2,6-(ditertiary butyl) phenol]; sulfur-containing oxidation inhibitors such as dilauryl-3,3'- thiodipropionate; phosphorus-containing oxidation inhibitors such as phosphites; and zinc dithiophosphate. For example, amine-type oxidation inhibitors and phenol-type oxidation inhibitors can be used preferably. They can be used generally in a proportion of from 0.05 to 5 wt.%.

[0033] Illustrative of the antiwear agents are generally metal (Zn, Pb, Sb, Mo and the like) dithiophosphates, metal (Zn, Mo and the like) dithiocarbamates, metal (Pb and the like) naphthenoates, fatty acid metal (Pb and the like) salts, vulcanized oils and fats, sulfur compounds, boron compounds, phosphates, phosphites, and acid phosphate amine salts. For example, phosphates and metal dithiophosphates can be used preferably. They can be used generally in a proportion of from 0.05 to 5 wt.%.

[0034] Illustrative of the extreme pressure additives are vulcanized oils and fats, dibenzyl disulfide, dibutyl disulfide, zinc dithiophosphate, phosphates, phosphites, and acid phosphate amine salts. They can be used generally in a proportion of from 0.05 to 3 wt.%.

[0035] Illustrative of the metal deactivators are benzo~triazole, triazole derivatives, benzotriazole derivatives, and thiadiazole derivatives. They can be used generally in a proportion of from 0.001 to 3 wt.%.

[0036] In addition, other additives such as corrosion inhibitors, foam inhibitors, coloring matters and the like can also be used as needed in the lubricating oil composition according to the present invention.

[0037] Preferred contents of the above-described various additives can be indicated as shown below in terms of proportions based on the whole weight of the composition.

	Preferred Content (wt.%)		Content (wt. %)
Viscosity index improver	4	- 30	3	- 35
Pour-point depressant	0.5	- 3	0.01	- 5
Ashless dispersant	0.1	- 5	0.1	- 10
Metal detergent	0.1	- 3	0.05	- 5
Oxidation inhibitor	0.1	- 3	0.05	- 5
Antiwear agent	0.1	- 2	0.05	- 5
Extreme pressure additive	0.1	- 2	0.05	- 5
Metal deactivator	0.01	- 2	0.001	- 3
Corrosion inhibitor	0.01	- 5	0.01	- 10
Foam inhibitor	0.0001	- 1	0.0001	- 2

Examples

[0038] The present invention will hereinafter be described in further detail by Examples and Comparative Examples. It should however be borne in mind that the present invention is not particularly limited to these Examples. The friction coefficient and lubricating oil solubility in each of the Examples and Comparative Examples were measured and ranked by the following methods.

(1) Friction coefficient measuring method

[0039] Each friction coefficient was measured by the judder preventing performance testing method for automatic transmission fluid as specified in JASO M349-95. Using a low-speed sliding friction testing machine [a modified LVFA (Low Velocity Friction Apparatus)] as a testing machine, the measurement and ranking were conducted under the following test conditions and by the following ranking method, respectively.

(i) Test conditions

[0040] 

• Specimen A friction plate (friction material: SD-1777) and a steel plate as specified in JASO M349-95.
• Running-in conditions:After a test friction material is immersed for 30 minutes in a test oil, the test friction material is kept sliding for 30 minutes at an oil temperature of 80°C a contact pressure of 1 MPa and a sliding speed of 0.6 m/s.
• Test procedures: The friction material, which has been subjected to the conditioning test, is immersed for 30 minutes in a test oil and is then subjected to a formal test under the following conditions:
• Oil quantity: 100 cc
• Oil temperature: 60°C
• Contact pressure: 1.0 MPa
• Rotational speed (high sliding speed): 200 rpm
• µ200: Friction coefficient at 200 rpm
• Friction coefficient measuring method: Following the method specified in JASO M349-95, a friction coefficient upon elapsed time of 2 seconds after initiation of a measurement under the respective preset conditions is recorded as a friction coefficient under the preset conditions.

(ii) Ranking method

[0041] The steel plate was fixed, and the friction plate was rotated under a preset load. The resulting torque (friction coefficient) was measured. A high friction coefficient is interpreted to provide a high torque capacity.

(2) Lubricating oil solubility ranking method

[0042] A lubricating oil composition is prepared by adding a desired additive in a predetermined proportion to a lubricating base stock. This lubricating oil composition is left over in a dark place for 14 days, and the existence or non-existence of turbidity and deposit is visually determined. The additive is determined to have good solubility if neither turbidity nor deposit is seen.

(3) Examples and Comparative Examples

Examples 1-3

[0043] Respective lubricating oil compositions were prepared by using as a lubricating base stock a solvent-refined paraffin-base mineral oil (kinematic viscosity at 100°C: 4 mm²/s) and adding to the lubricating base stock an alkenyl-substituted succinic anhydride of 1,000 in molecular weight in a proportion of 1.0 wt.% in Example 1, in a proportion of 2.5 wt.% in Example 2 and in a proportion of 5.0 wt.% in Example 3, all based on the corresponding whole compositions. Measurement of the coefficients of friction achieved by these lubricating oil compositions and ranking of the lubricating oil solubility of alkenyl-substituted succinic anhydride were conducted. The results of these measurement and ranking are shown in Table 1. The friction coefficient was 0.138 in Example 1. Similarly, the coefficients of friction were 0.140 and 0.142 in Examples 2 and 3, respectively. Further, the lubricating oil solubility was ranked "good".

Examples 4-11

[0044] Lubricating oil compositions were prepared by mixing the lubricating base stock component and the corresponding components useful in the present invention, which are all shown in Table 1, in the corresponding proportions also presented in the same table. With respect to each composition, the measurement of a friction coefficient and the ranking of lubricating oil solubility were conducted. The results of these measurement and ranking are shown in Table 1.

Comparative Examples 1-7

[0045] Lubricating oil compositions were prepared by mixing the lubricating base stock component and the various corresponding additive components, which are all shown in Table 2, in the corresponding proportions also presented in the same table. With respect to each composition, the measurement of a friction coefficient and the ranking of lubricating oil solubility were conducted. The results of these measurement and ranking are shown in Table 2.

[0046] Concerning a lubricating oil composition for automatic or continuously variable transmissions with a built-in torque converters equipped with lock-up clutches, it is a general approach to provide the µ-V characteristic with a positive gradient by lowering a friction coefficient in a low sliding speed range with a friction-modifying compound. However, a reduction in the friction coefficient in the low sliding speed range also leads to a reduction in the torque capacity. This approach therefore contradicts with the object of the development that high torque capacity be obtained. There has accordingly be a desire for a compound which, without substantially lowering the friction coefficient in the low speed range, can increase the friction coefficient in the high sliding speed range and can hence provide the µ-V characteristic with a positive gradient. In the Examples of the present invention, a target was therefore set up for the provision of lubricating oil compositions capable of achieving a friction coefficient µ200 higher than 0.120 at a high sliding speed.

[0047] From the above Examples and Comparative Examples, it became clear that each of the Examples furnished a high-quality lubricating oil composition, which can achieve the high friction coefficient at a high sliding speed and can meet the target as an automatic or continuously variable transmission fluid, owing to the addition of the alkyl- or alkenyl-substituted cyclic dicarboxylic anhydride, which is an essential component in the present invention, in the specific proportion. Described specifically, taking the results of Example 1 by way of example, the friction coefficient at the high sliding speed was 0.138. As this value is greater than 0.120, it is evident that the lubricating oil composition is extremely good in power transmitting performance. Similarly, Examples 2-11 also furnished lubricating oil compositions which are of high quality as automatic or continuously variable transmission fluids.

[0048] In Comparative Example 1, on the other hand, the ranking was conducted using only the lubricating base stock which did not contain the essential ingredient for the present invention. The friction coefficient in the high sliding speed range was 0.104, thereby failing to reach the target value. It is also appreciated that, although polyisobutylenes were added in the specific proportion in Comparative Examples 2 and 3 as compounds equivalent to the substituent groups of an alkyl- or alkenyl-substituted succinic anhydride as an essential component for the present invention, the lubricating oil compositions involve a problem because the coefficients of friction in the high sliding speed range were as low as 0.096 and 0.104, respectively. It is also understood that, although alkenyl-substituted succinimides commonly incorporated in lubricating oil compositions were added in the specific proportion in Comparative Examples 4 and 5 as compounds similar to alkyl- or alkenyl-substituted succinic anhydrides as essential components for the present invention, the lubricating oil compositions involve a problem because the coefficients of friction in the high sliding speed range were still as low as 0.110 and 0.113, respectively, as in Comparative Examples 1-3. It is also envisaged that, although an alkenyl-substituted succinic anhydride similar to one useful as an essential component in the present invention is added in the specific proportion in Comparative Example 6, the substituent group is an alkenyl group having a molecular weight of 4,500 and the alkenyl-substituted succinic anhydride has a problem in the solubility in the lubricating oil (base stock) despite the high and good friction coefficient at the high sliding speed. It is also understood that, although an alkenyl-substituted succinic anhydride as an essential component for the present invention is added in Comparative Example 7, its proportion is as small as 0.05 wt.% and the lubricating oil composition involves a problem in the friction coefficient in the high sliding speed range.

[0049] It is thus clear from the foregoing that, unless an alkyl- or alkenyl-substituted cyclic dicarboxylic acid anhydride as an essential component for the present invention is added in a particular proportion, the friction coefficient in the high sliding speed range cannot be increased, thereby failing to obtain a lubricating oil composition having high quality as an automatic or continuously variable transmission fluid. In other words, it has become evident that the addition of an alkyl- or alkenyl-substituted cyclic dicarboxylic anhydride led by an alkyl- or alkenyl-substituted succinic anhydride in a lubricating base stock can increase the friction coefficient in the high sliding speed range without a substantial reduction in the friction coefficient in the low speed range, thereby making it possible to obtain a lubricating oil composition of high quality.

[0050] Owing to the addition of the specific compound in the lubricating base stock, the lubricating oil composition according to the present invention for an automatic or continuously variable transmission with a built-in torque converter equipped with a lock-up clutch has excellent property that it achieves a sufficient friction coefficient in a high sliding speed range in the automatic or continuously variable transmission.

Claims

1. A lubricating oil composition comprising a lubricating base stock and a compound composed of at least one cyclic dicarboxylic acid anhydride substituted by one or more alkyl or alkenyl groups.

2. A lubricating oil composition according to claim 1, wherein said at least one cyclic dicarboxylic acid anhydride substituted by said one or more alkyl or alkenyl groups is any one of compounds represented by the following formulas (1) to (4), respectively:

wherein R represents a C_6-300 alkyl or alkenyl group having a molecular weight of from 80 to 4,200, X represents a C_nH_2n-1 or C_nH2n-3, and n stands for an integer of from 2 to 11.

wherein R represents a C_6-300 alkyl or alkenyl group having a molecular weight of from 80 to 4,200, and n stands for an integer of from 1 to 4.

wherein R represents a C_6-300 alkyl or alkenyl group having a molecular weight of from 80 to 4,200, m stands for an integer of from 1 to 4, and n stands for an integer of from 1 to 2.

wherein R represents a C_6-300 alkyl or alkenyl group having a molecular weight of from 80 to 4,200, and n stands for an integer of from 1 to 4.

3. A lubricating oil composition according to claim 1 or 2, comprising 0.1 to 10 wt.%, based on the whole weight of said composition, of at least one of said compounds of the formulas (1) to (4).