Lubricant compositions

Abstract

Lubricant compositions comprises a lubricant base oil blended with a boron-containing ashless dispersant (A) in an amount of 0.035 or more percent of boron by mass, and an alkaline earth metal sulfonate having a total base number of 50 to 500 mgKOH/g (B) in an amount of 0.01 or more percent of alkaline earth metal by mass, all the percentages being based on the total mass of the composition and if necessary a phosphorus additive. The lubricant compositions are superior in effect to enhance the dynamic- and static-friction coefficients in a wet clutch.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to lubricant compositions, and more particularly to lubricant compositions for automatic transmissions or continuously variable transmissions, which are effective to enhance friction properties in wet clutches.

[0003] 2. Description of the Prior Art

[0004] An automatic transmission is equipped with a torque converter, wet clutches, gears and bearings mechanism and a hydraulic control mechanism to control these parts, the transmission gear ratio is determined by the number of gear tooth of the intermeshing gears. An automatic transmission has several kinds of gear mechanisms, and a desired transmission gear ratio is obtained by selecting suitable gears determining the ratio depending on ground speed and load. The automatic transmission has a number of wet clutches, and some of them are coupled and the others are idle so as to select the gears determining a transmission gear ratio. Upon changing a transmission gear ratio, the clutches having been coupled are released and other clutches are brought into coupling. Therefore, in order to accomplish quick gear change, the wet clutches are required to be high in dynamic friction coefficient. Furthermore, in order to transmit engine torque, the coupled wet clutches must not slip until being released and need to be high in static friction coefficient. These friction coefficients are strongly affected by the materials of clutches as well as the properties of the lubricant to be used.

[0005] In the design of automatic transmissions, the size, number, and pressing hydraulic pressure of wet clutches are determined by the output of the engine to be combined therewith. In the case where the friction coefficient in wet clutches is low, their size, number, and pressing hydraulic pressure are required to be increased. The increase of size and number of wet clutches would invite the large sized automatic transmission, while the increase of pressing hydraulic pressure would impose excessive load on the hydraulic pump incorporated in the automatic transmission.

[0006] On the other hand, unlike in the automatic transmissions, in the continuous variable transmissions, wet clutches are not used for gear change but are used as switching means for going forward and backward.

[0007] With regard to the enhancement of friction coefficient in a wet clutch, Japanese Patent Laid-Open Publication No. 6-240275 proposes that a lubricant composition comprising an overbased calcium phenate, a succinimide-based ashless dispersant, a borated succinimide is effective to enhance static friction coefficient. In Japanese Patent Laid-Open Publication No. 8-127789 proposes that a lubricant composition comprising bis- and mono-types alkenyl succinimides, and low and high molecular weight products of polydimethylsiloxane is effective to enhance dynamic and static friction coefficient. Many lubricant compositions produced by combining these techniques have been put in use but are not satisfactory in terms of the improvement of fuel efficiency. Therefore, further improvement has been demanded.

[0008] In view of the issues concerning the global environment, for the purpose of mainly decreasing the exhausted carbon dioxide, it is now urgent issue that the fuel efficiency of automobiles are improved. A great demand has been made on automatic transmissions which can contribute to improve fuel efficiency. In this connection, a strong demand had been made on the development of a lubricant which is effective to enhance the friction coefficient in a wet clutch so as to downsize a transmission and reduce pump loss. For the continuously variable transmissions, the enhanced friction coefficient in a wet clutch is also effective to downsize a transmission and reduce pump loss. As well as the automatic transmission fluid, there arise a strong demand on the development of a lubricant for continuously variable transmissions which is effective to enhance the friction coefficient in the wet clutch.

[0009] The present invention meets such demands and are intended to provide a lubricant composition which exhibits excellent effects to enhance the dynamic and static friction coefficients particularly in wet clutches.

BREIF SUMMARY OF THE INVENTION

[0010] According to the present invention, there is provided a lubricant composition which comprises a lubricant base oil blended with a boron-containing ashless dispersant hereinafter referred to as Component (A) in an amount of 0.035 or more percent of boron by mass, and an alkaline earth metal sulfonate having a total base number of 50 to 500 mgKOH/g hereinafter referred to as Component (B) in an amount of 0.01 or more percent of alkaline earth metal by mass, all the percentages being based on the total mass of the composition.

[0011] The lubricant composition is preferably blended with a phosphorus additive hereinafter referred to as Component (C).

DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in more details below.

[0013] Eligible lubricant base oils in the lubricant composition of the present invention are any mineral oils and/or synthetic oils which are used as base oil of conventional lubricants.

[0014] Specific examples of mineral oils which may be used include paraffinic- and naphthenic-mineral oils which are produced by subjecting lubricant fractions resulting from the atmospheric distillation and the vacuum distillation of crude oil to refining processes such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid washing, and clay treatment in suitable combination; and n-paraffinic mineral oils.

[0015] Although not restricted, examples of synthetic oils are poly-&agr;-olefins such as 1-octene oligomer, 1-decene oligomer, and ethylene-propylene oligomer, and hydrides thereof, isobutene oligomer and hydrides thereof, isoparaffin, alkylbenzene, alkylnaphthalene, diesters such as ditridecyl glutarate, di-2-ethylhexyl adipate, diisodecyl adipate, ditridecyl adipate, and di-2-ethylhexyl sebacate), polyol esters such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol-2-ethyl hexanoate, and pentaerythritol pelargonate, polyoxyalkylene glycol, dialkyldiphenyl ether, and polyphenylether.

[0016] Although not restricted, the kinematic viscosity at 100° C. of these lubricant base oils is usually within the range of 1 to 10 mm2/s, preferably 2 to 8 mm2/S.

[0017] Component (A) in the lubricant composition of the present invention is a boron-containing ashless dispersant.

[0018] It is important that Component (A) contains boron. An ashless dispersant which is free of boron is not preferred for Component (A) of the present invention because it is poor in effect to enhance the friction coefficient in a wet clutch. No particular limitation is imposed on the boron content in Component (A). However, since excellent effects in the enhancement of friction coefficient in a wet clutch, and anti-corrosion and oxidation stability properties can be obtained, the lower limit of boron content is preferably 0.2 percent by mass, and more preferably 0.4 percent by mass, while the upper limit is preferably 4 percent by mass, and more preferably 2.5 percent by mass.

[0019] Examples of Component (A) are products resulting from the modification of nitrogen-containing compounds having at least one alkyl or alkenyl group in their molecules or a derivative thereof with a boric compound. The alkyl or alkenyl group may be straight-chain or branched. Preferred are branched alkyl and alkenyl groups derived from oligomers of olefins such as propylene, 1-butene, and isobutylene, or cooligomers of ethylene and propylene.

[0020] The carbon number of the alkyl or alkenyl group is preferably 40 to 400, and more preferably 60 to 350. Alkyl or alkenyl groups having less than 40 carbon atoms are not preferred because the compound would be deteriorated in solubility with a lubricant base oil, while those having more than 400 carbon atoms are not preferred because the resulting lubricant composition would be deteriorated in low-temperature fluidity.

[0021] Specific examples of the nitrogen-containing compound and derivatives thereof are one or more compounds selected from the following compounds:

[0022] (A-1) succinimides having in the molecules at least one alkyl or alkenyl group having 40 to 400 carbon atoms, or derivatives thereof;

[0023] (A-2) benzylamines having in the molecules at least one alkyl or alkenyl group having 40 to 400 carbon atoms, or derivatives thereof; and

[0024] (A-3) polyamines having at least one alkyl or alkenyl group having 40 to 400 carbon atoms and derivatives thereof.

[0025] Specific examples of the succinimides (A-1) are compounds represented by the formula 1

[0026] wherein R1 is an alkyl or alkenyl group having 40 to 400, preferably 60 to 350 carbon atoms, and a is an integer of 1 to 5, preferably 2 to 4; and compounds represented by the formula 2

[0027] wherein R2 and R3 are each independently an alkyl or alkenyl group having 40 to 400, preferably 60 to 350 carbon atoms, and b is an integer of 0 to 4, preferably 1 to 3.

[0028] The succinimides can be classified as mono-type succinimides, as represented by formula (1), in which succinic anhydride is added to one end of a polyamine and bis-type succinimides, as represented by formula (2), in which succinic anhydride is added to both ends of a polyamine. Both types of succinimides and mixtures thereof are eligible as Component (A-1).

[0029] Specific examples of the benzylamines (A-2) are compounds represented by the formula 3

[0030] wherein R4 is an alkyl or alkenyl group having 40 to 400, preferably 60 to 350 carbon atoms, c is an integer of 1 to 5, preferably 2 to 4.

[0031] No particular limitation is imposed on a method for producing the benzylamines. For example, benzylamines may be produced by reacting phenol with a polyolefin having 40 to 400 carbon atoms, such as propylene oligomer, polybutene, and ethylene-&agr;-copolymer to obtain an alkylphenol and then subjecting it to the Mannich reaction with formaldhyde and a polyamine such as diethylenetriamine, triethylenetetraamine, tetraethylenepentamine, and pentaethylenehexamine.

[0032] Specific examples of the polyamines (A-3) are compounds represented by the formula 4

[0033] wherein R5 is an alkyl or alkenyl group having 40 to 400, preferably 60 to 350 carbon atoms, and d is an integer of 1 to 5, preferably 2 to 4.

[0034] No particular limitation is imposed on a method for producing such polyamines. For Example, polyamines may be produced by chloridizing a polyolefin having 40 to 400 carbon atoms, such as propylene oligomer, polybutene, and an ethylene-&agr;-copolymer and then reacting the resulting product with ammonia or a polyamine such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine.

[0035] Specific examples of derivatives of the nitrogen-containing compounds are a carboxylic acid-modified compound obtained by allowing the above-described nitrogen-containing compound to react with monocarboxylic acid having 2 to 30 carbon atoms, such as fatty acid or polycarboxylic acid having 2 to 30 carbon atoms, such as oxalic acid, phthalic acid, trimellitic acid, and pyromellitic acid to neutralize the whole or part of the remaining amino and/or imino groups or to convert the same into amides; a sulfur-modified compound obtained by allowing the above-described nitrogen-containing compound to react with a sulfuric compound; and mixtures thereof.

[0036] Component (A) of the present invention is a product obtained by modifying the above-mentioned nitrogen-containing compound or derivative thereof with a boric compound.

[0037] No particular limitation is imposed on a method for modifying the nitrogen-containing compound or derivative thereof with a boric compound and any suitable method may be employed. For instance, the modification may be conducted by allowing the above-mentioned nitrogen-containing compound or derivative thereof to react with a boric compound such as boric acid, a boric acid salt, and a borate and then neutralizing the whole or part of the amino and/or imino groups remaining in the nitrogen-containing compound or derivative thereof or converting the same into amide.

[0038] Specific examples of boric acid are orthoboric acid, metaboric acid, and tetraboric acid.

[0039] Specific examples of boric acid salt are alkali metal salts, alkaline earth metal salts or ammonium salts of boric acid. More specific examples are lithium borates such as lithium metaborate, lithium tetraborate, lithium pentaborate, lithium perborate; sodium borates such as sodium metaborate, sodium diborate, sodium tetraborate, sodium pentaborate, sodium hexaborate, and sodium octaborate; potassium borates such as potassium metaborate, potassium tetraborate, potassium pentaborate, potassium hexaborate, and potassium octaborate; calcium borates such as calcium metaborate, calcium diborate, tricalcium tetraborate, pentacalcium tetraborate, and calcium hexaborate; magnesium borates such as magnesium metaborate, magnesium diborate, trimagnesium tetraborate, pentamagnesium tetraborate, and magnesium hexaborate; and ammonium borates such as ammonium metaborate, ammonium tetraborate, ammonium pentaborate, and-ammonium octaborate.

[0040] Borates may be esters of boric acid with an alkyl alcohol having 1 to 6 carbon atoms. Specific examples are monomethylborate, dimethylborate, trimethylborate, monoethylborate, diethylborate, triethylborate, monopropylborate, dipropylborate, tripropylborate, monobutylborate, dibutylborate, and tributylborate.

[0041] Component (A) may be one or more products obtained by modifying the nitrogen-containing compounds (A-1), (A-2) or (A-3), or a derivative thereof with a boric compound. However, preferred are those obtained by modifying the succinimides (A-1) or derivatives thereof with a boric compound because they are excellent in effect to enhance the friction coefficient in a wet clutch.

[0042] The lower limit content of Component (A) in the inventive lubricant composition is 0.035 percent of boron by mass, preferably 0.04 percent of boron by mass, based on the total mass of the composition. A content less than 0.035 percent of boron by mass is not preferred because Component (A) in such an amount is poor in enhancing the friction coefficient in a wet clutch. Although not restricted, in order to maintain the storage stability and oxidation stability of the resulting lubricant composition, the upper limit content of Component (A) is 0.5 percent of boron by mass, preferably 0.3 percent of boron by mass, particularly preferably 0.2 percent of boron by mass, based on the total mass of the composition.

[0043] Component (B) of the present invention is an alkaline earth metal sulfonate having a total base number of 50 to 500 mgKOH/g. The lower limit total base number of Component (B) 50 mgKOH/g, preferably 100 mgKOH/g, and more preferably 150 mgKOH/g, while the upper limit is 500 mgKOH/g, and preferably 450 mgKOH/g. A total base number less than 50 mgKOH/g is not preferred because Component (B) having such a total base number is poor in effect to enhance the friction coefficient, while a total base number in excess of 500 mgKOH/g because Component (B) having such a total base number would deteriorate the storage stability of the resulting lubricant composition. The term “total base number” used herein denotes a total base number measured by the perchloric acid potentiometric titration method in accordance with section 7 of JIS K2501 “Petroleum products and lubricants-Determination of neutralization number”.

[0044] Eligible alkaline earth metals are magnesium and calcium.

[0045] Specific examples of the alkaline earth metal sulfonates are alkaline earth metal salts preferably magnesium salt or calcium salt of an alkyl aromatic sulfonic acid obtained by sulfonating an alkyl aromatic compound having a molecular weight of 100 to 1,500, preferably 200 to 700. Specific examples of the alkyl aromatic sulfonic acid are petroleum sulfonic acids and synthetic sulfonic acids.

[0046] The petroleum sulfonic acid may be mahogany acid obtained by sulfonating the alkyl aromatic compound contained in the lubricant fraction of mineral oil or by-produced upon production of white oil. The synthetic sulfonic acid may be those obtained by sulfonating an alkyl benzene having a straight-chain or branched alkyl group, which may be by-produced from a plant for producing alkyl benzene used as materials of detergents, or sulfonating dinonylnaphthalene. Although not restricted, there may be used fuming sulfuric acid and sulfuric acid as a sulfonating agent. Alkaline earth metal sulfonates are sold and available with diluted a light lubricant base oil or-the like. Preferred for the present invention are those having the metal in an amount of 1.0 to 20 percent by mass, and preferably 2.0 to 16 percent by mass.

[0047] The lower limit content of Component (B) in the inventive lubricant composition is 0.01 percent of alkaline earth metal by mass, preferably 0.02 percent of alkaline earth metal by mass, based on the total composition. Component (B) which is less than 0.01 percent by mass is poor in effect to enhance the friction coefficient in a wet clutch. Although not restricted, in order to maintain the storage stability and oxidation stability of the resulting lubricant composition, the upper limit content of Component (B) is preferably 0.5 percent by mass, more preferably 0.4 percent by mass, and particularly preferably 0.3 percent by mass.

[0048] The lubricant composition of the present invention further contains a phosphorus additive hereinafter referred to as Component (C). The addition of Component (C) makes it possible to further enhance the friction coefficient in a wet clutch.

[0049] Examples of component (C) are zinc alkyldithiophosphate, phosphoric acid, phosphorous acid, monophosphates, diphosphates, triphosphates, monophosphites, diphosphites, triphosphites, salts of phosphates and phosphites, and mixtures thereof.

[0050] These exemplified compounds, besides phosphoric acid and phosphorus acid, are compounds having a hydrocarbon group having 2 to 30, preferably 3 to 20 carbon atoms.

[0051] Specific examples of the hydrocarbon group having 2 to 30 are alkyl groups, cycloalkyl groups, alkyl-substituted cycloalkyl groups, alkenyl groups, aryl groups, alkyl-substituted aryl groups, and aryl-substituted alkyl groups.

[0052] Examples of the alkyl group are ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, and octadecyl groups, all of which may be straight-chain or branched.

[0053] Examples of the cycloalkyl groups are those having 5 to 7 carbon atoms, such as cyclopentyl, cyclohexyl, and cycloheptyl groups. Examples of the alkylcycloalkyl groups are those having 6 to 11 carbon atoms, of which the cycloalkyl group may possess an alkyl substituent at any position, such as methylcyclopentyl, dimethylcyclopentyl, methylethylcyclopentyl, diethylcyclopentyl, methylcyclohexyl, dimethylcyclohexyl, methylethylcyclohexyl, diethylcyclohexyl, methylcycloheptyl, dimethylcycloheptyl, methylethylcycloheptyl and diethylcycloheptyl groups.

[0054] Examples of the alkenyl groups are butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl, hexadecenyl, heptadecenyl and octadecenyl groups, all of which may be straight-chain or branched and the position of which the double bond may vary.

[0055] Examples of the aryl groups are phenyl and naphthyl. Examples of the alkylaryl groups are those having 7 to 18 carbon atoms, such as tolyl, xylyl, ethylphenyl, propylphenyl, butylphenyl, pentylphenyl, hexylphenyl, heptyl phenyl, octylphenyl, nonylphenyl, decylphenyl, undecylphenyl, and dodecylphenyl groups, all of which the alkyl group may be straight-chain or branched and may bond to any position of the aryl group. Examples of the arylalkyl groups are those having 7 to 12 carbon atoms, such as benzyl, phenylethyl, phenylpropyl, phenylbutyl, phenylpentyl and phenylhexyl groups, all of which may be straight-chain or branched.

[0056] Specific examples of the salts of phosphites and phosphates are those obtained by allowing a monophosphate, a diphosphate, a monophosphite, or a diphosphite to react with a nitrogen-containing compound such as ammonia or an amine compound having in its molecules only hydrocarbon or hydroxyl-containing groups having 1 to 8 carbon atoms so as to neutralize the whole or part of the remaining acid hydrogen.

[0057] Specific examples of the nitrogen-containing compound are ammonia; alkylamines, of which the alkyl group may be straight-chain or branched, such as monomethylamine, monoethylamine, monopropylamine, monobutylamine, monopentylamine, monohexylamine, monoheptylamine, monooctylamine, dimethylamine, methylethylamine, diethylamine, methylpropylamine, ethylpropylamine, dipropylamine, methylbutylamine, ethylbutylamine, propylbutylamine, dibutylamine, dipentylamine, dihexylamine, diheptylamine and dioctylamine; alkanolamines, of which the alkanol group may be straight-chain or branched, such as monomethanolamine, monoethanolamine, monopropanolamine, monobutanolamine, monopentanolamine, monohexanolamine, monoheptanolamine, monooctanolamine, monononanolamine, dimethanolamine, methanolethanolamine, diethanolamine, methanolpropanolamine, ethanolpropanolamine, dipropanolamine, methanolbutanolamine, ethanolbutanolamine, propanolbutanolamine, dibutanolamine, dipentanolamine, dihexanolamine, diheptanolamine and dioctanolamine; and mixtures thereof. One or more Compounds (C) may be added to the lubricant composition of the present invention.

[0058] Preferred compounds as Component (C) are phosphoric acid; phosphorus acid; zinc alkyldithiophosphates, of which the alkyl group may be straight-chain or branched, such as zinc dipropyldithiophosphate, zinc dibutyldithiophosphate, zinc dipentyldithiophospahte, zinc dihexyldithiophospahte, zinc diheptyldithiophospahte, and zinc dioctyldithiophospahte; monoalkyl phosphates, of which the alkyl group may be straight-chain or branched, such as monopropyl phosphate, monobutyl phosphate, monopentyl phosphate, monohexyl phosphate, monoheptyl phospahte and monooctyl phosphate; mono(alkyl)aryl phosphates such as monophenyl phospahte and monocresyl phosphate; dialkyl phosphates, of which the alkyl group may be straight-chain or branched, such as dipropyl phosphate, dibutyl phosphate, dipentyl phqspahte, dihexyl phosphate, diheptyl phosphate and dioctyl phospahte; di(alkyl)aryl phosphates such as diphenyl phosphate and dicresyl phospahte; trialkyl phosphates, of which the alkyl group may be straight-chain or branched, such as tripropyl phosphate, tributyl phosphate, tripentyl phosphate, trihexyl phosphate, triheptyl phosphate and trioctyl phosphate; tri(alkyl)aryl phosphates such as triphenyl phosphate and tricresyl phosphate; monoalkyl phosphites, of which the alkyl group may be straight-chain or branched, such as monopropyl phosphite, monobutyl phosphite, monopentyl phosphite, monohexyl phosphite, monoheptyl phosphite and monooctyl phosphite; mono(alkyl)aryl phosphites such as monophenyl phosphite and monocresyl phosphite; dialkyl phosphites, of which the alkyl group may be straight-chain or branched, such as dipropyl phosphite, dibutyl phosphite, dipentyl phosphite, dihexyl phosphite, diheptyl phosphite and dioctyl phosphite; di(alkyl)aryl phosphites such as diphenyl phosphite and dicresyl phosphite; trialkyl phosphites, of which the alkyl group may be straight-chain or branched, such as tripropyl phosphite, tributyl phosphite, tripentyl phosphite, trihexyl phosphite, triheptyl phosphite and trioctyl phosphite; tri(alkyl)aryl phosphites, of which the alkyl group may be straight-chain or branched, such as triphenyl phosphite and tricresyl phosphite; salts of the above-mentioned phosphates and phosphites; and mixtures thereof.

[0059] No particular limitation is imposed on the content of Component (C) in the lubricant composition of the invention. In order to attain the enhanced friction coefficient in a wet clutch and the excellent oxidation stability of the resulting lubricant composition, Component (C) is added in an amount of preferably 0.005 to 0.2 percent of phosphorus by mass, and more preferably 0.01 to 0.15 percent of phosphorus by mass, based on the total mass of the composition.

[0060] In the present invention, a lubricant composition having an enhanced friction coefficient in a wet clutch can be obtained by blending a lubricant base oil with Components (A) and (B), and preferably Component (C). For the purpose of further enhancing the properties of the inventive lubricant composition, there may be blended an ashless dispersant other than Component (A), a metallic detergent other than Component (B), a friction modifier, a viscosity index improver, an extreme pressure additive, an oxidation inhibitor, a corrosion inhibitor, an anti-foaming agent, and dyes. These additives may be used singularly or in combination.

[0061] These additives are described below.

[0062] Eligible ashless dispersants other than Component (A) are succinimide-containing ashless dispersants which are free of boron. The succinimide-containing ashless dispersants may be Components (A-1) mentioned above or derivatives thereof. The addition of such succinimide-containing ashless dispersants can further enhance the detergency of the resulting lubricant.

[0063] In the present invention, one or more of these ashless dispersants may be added in suitable amounts. The amount of the ashless dispersants is 0.05 to 10 percent by mass, and preferably 1 to 7 percent by mass, based on the total mass of the lubricant composition.

[0064] Eligible metallic detergents which may be used in the present invention are any compounds other than Component (B), which are generally used as metallic detergents for a lubricant. Examples of these compounds are phenates, salicylates and naphthenate of an alkali metal or alkaline earth metal. One or more of these compounds may be used. Preferred alkali metals are sodium and potassium, while preferred alkaline earth metals are calcium and magnesium. Specific examples of preferred metallic detergents are phenates and salicylates of calcium or magnesium. The total base number and content of these metallic detergents may be selected depending on the required properties of a lubricant.

[0065] Friction modifiers which can be used in combination with the lubricant composition of the present invention are any compounds which are generally used as friction modifiers for a lubricant. Specific examples of the friction modifiers are amine compounds, fatty acid esters, fatty acid amides and fatty acid metallic salts which have in the molecules at least one alkyl or alkenyl group having 6 to 30 carbon atoms, particularly straight-chain alkyl or alkenyl group having 6 to 30 carbon atoms.

[0066] Examples of the amine compounds are aliphatic monoamines, straight-chain or branched, preferably straight-chain aliphatic polyamines, and alkyleneoxide adducts thereof. Examples of the fatty acid esters are esters of straight-chain or branched, but preferably straight-chain fatty acid having 7 to 31 carbon atoms with aliphatic monohydric alcohols or aliphatic polyhydric alcohols. Examples of the fatty acid amides are amides of straight-chain or branched, but preferably straight-chain fatty acids having 7 to 31 carbon atoms with aliphatic monoamines or aliphatic polyamines. Examples of the fatty acid metallic salts are alkaline earth metal salts, such as magnesium salts and calcium salts, and zinc salts, of straight-chain or branched, but preferably straight-chain fatty acids having 7 to 31 carbon atoms.

[0067] One or more of these friction modifiers may be added to the lubricant composition of the present invention. The friction modifiers are added in an amount of 0.01 to 5.0 percent by mass, preferably 0.03to 3.0 percent by mass, based on the total lubricant composition.

[0068] Specific examples of the viscosity index improvers which can be used in combination with the lubricant of the present invention are non-dispersion type viscosity index improvers such as polymers or copolymers of one or monomers selected from various methacrylates, and hydrides thereof and dispersion type viscosity index improvers obtained by copolymerizing various methacrylates containing a nitrogen-containing compound. Specific examples of other viscosity index improvers are non-dispersion or dispersion type ethylene-&agr;-olefin copolymers of which &agr;-olefin may be propylene, 1-butene, and 1-pentene, and hydrides thereof; polyisobutylene and hydrides thereof, hydrogenated products of styrene-diene copolymers; styrene-maleic anhydride ester copolymers; and polyalkylstyrenes.

[0069] The molecular weight of these viscosity index improvers is preferably selected in view of shear stability. Specifically, it is desired that the dispersion type and non-dispersion type-polymethacrylates have a number-average molecular weight of 5,000 to 150,000, preferably 5,000 to 35,000. It is also desired that the polyisobutylenes and hydrides thereof have a number-average molecular weight of 800 to 5,000, preferably 1,000 to 4,000. Ethylene-&agr;-olefin copolymers and hydrides thereof have a number-average molecular weight of 800 to 150,000, preferably 3,000 to 12,000.

[0070] Among these viscosity index improvers, the use of ethylene-&agr;-olefin copolymers or hydrides thereof result in a lubricant composition having an excellent shear stability.

[0071] In the present invention, one or more compounds selected from these viscosity index improvers may be blended in suitable amounts. The viscosity index improvers are added in an amount of preferably 0.1 to 40.0 percent by mass, based on the total mass of the lubricant of the present invention.

[0072] Extreme pressure agents which may be used in combination with the lubricant composition of the present invention are any compounds which are generally used as extreme pressure agents for a lubricant. Specific examples are sulfuric compounds such as disulfides, olefin sulfides, and sulfurized fats and oils. One or more of these compounds is preferably added in an amount of 0.01 to 5.0 percent by mass based on the total mass of the lubricant composition.

[0073] Oxidation inhibitors which may be used in combination with the lubricant composition of the present invention are any compounds which are generally used as oxidation inhibitors for a lubricant. Eligible oxidation inhibitors are phenol- or amine-based compounds. These may be used individually or in combination. Specific examples of the oxidation inhibitor are alkylphenols such as 2-6-di-tert-butyl-4-methylphenol, bisphenols such as methylene-4,4-bisphenol(2,6-di-tert-butyl-4-methylphenol), naphtylamines such as phenyl-&agr;-naphtylamine, dialkyldiphenylamines, zinc dialkyldithiophosphates such as zinc di-2-ethylhexyldithiophosphate, esters of 3,5-di-tert-butyl-4-hydroxyphenyl fatty acid (propionic acid) with a mono- or poly-hydric alcohol such as methanol, octadecanol, 1,6 hexanediol, neopentyl glycol, thiodiethylene glycol, triethylene glycol or pentaerythritol.

[0074] One or more compounds selected from these oxidation inhibitors may be added in suitable amounts. In the present invention, the use of combination of the amine-based ones and the phenol-based ones are preferred.

[0075] The oxidation inhibitor is blended in an amount of 0.01 to 5.0 percent by mass based on the total mass of the lubricant composition.

[0076] Corrosion inhibitors which may be used in combination with the lubricant composition of the present invention are any compounds which are generally used as corrosion inhibitors for lubricants. Eligible corrosion inhibitors are benzotriazole-, tolyltriazole-, thiodiazole-and imidazole-based compounds. These may be used individually or in combination. The corrosion inhibitor is usually blended in an amount of 0.01 to 3.0 percent by mass, based on the total mass of the lubricant composition.

[0077] Anti-foaming agents which may be used in combination with the lubricant composition of the present invention are any compounds which are generally used as anti-foaming agents for lubricants. Eligible anti-foaming agents are silicones such as dimethylsilicon and fluorosilicon. These may be used individually or in combination. The anti-foaming agents is usually blended in an amount of 0.001 to 0.05 percent by mass, based on the total mass of the lubricant composition.

[0078] Dyes which may be used in combination with the lubricant composition of the present invention are any suitable ones which are added in suitable amounts. The dye is usually blended in an amount of 0.001 to 1.0 percent by mass, based on the total mass of the lubricant composition.

[0079] The invention will be further described by way of the following examples and comparative examples which are provided for illustrative purposes only.

EXAMPLES

[0080] [Preparation of Lubricant Compositions]

[0081] Lubricant compositions according to the present invention (Examples 1 to 4) and those for comparison (Comparative Examples 1 to 4) were prepared in accordance with the formulations given in Table 1.

[0082] [Properties Evaluation by Friction Properties Test]

[0083] Friction properties test for a wet clutch was conducted on each of the lubricant compositions. The test was conducted in accordance with JASO M348-95 “Test method for friction property of automatic transmission fluids” using an SAE #2. friction tester. This test is consisted of a dynamic friction test and a static friction test. In the dynamic friction test, after a clutch was rotated at 3,600 rpm and 0.343 kg·m2 under no load, the rotation of the clutch was stopped by being pressed by applying thereto pressure. Friction coefficient was calculated from the torque occurring at a relative rotation of 1,800 and the resulting value was defined as dynamic friction coefficient. In the static friction test, a clutch was rotated at a relative rotation of 0.72 rpm while being pressed by applying thereto pressure and friction coefficient was calculated from the torque occurring thereupon. The friction coefficient at the maximum torque at which the clutch starts to slide at the low speed of rotation was defined as static friction coefficient. In this test, each of the dynamic- and static-friction tests was conducted repeatedly and the properties to enhance the friction coefficient of each of the composition were evaluated with the friction coefficient after 1,000 cycles. 1 TABLE 1 Examples Comparative Examples Lubricant Composition (mass %) 1 2 3 4 1 2 3 4 Refined mineral oil1) 89.598 89.198 89.218 86.398 89.598 91.398 90.758 84.798 (A) Ashless dispersant Boron-containing ashless dispersant A2) 3.3 3.3 3.3 — — 1.5 3.3 3.3 Boron-containing ashless dispersant B3) — — — 6.5 — — — — Ashless dispersant4) — — — — 3.3 — — — Boron content mass % 0.066 0.066 0.066 0.058 0 0.03 0.066 0.066 (B) Metal sulfonate Ca sulfonate A5) 1.2 — 1.2 1.2 1.2 1.2 0.04 — Mg sulfonate6) — 1.6 — — — — — — Ca sulfonate B7) — — — — — — — 6 Alkaline earth metal content mass % 0.15 0.15 0.15 0.15 0.15 0.15 0.005 0.15 (C) Phosphorus additive8) — — 0.38 — — — — — Phosphorus content mass % — — 0.05 — — — — — Viscosity index improver9) 5 5 5 5 5 5 5 5 Oxidation inhibitor A10) 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Oxidation inhibitor B11) 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Friction modifier A12) 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 Friction modifier B13) 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Corrosion inhibitor14) 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 Anti-foaming agent15) 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 Results of properties evaluation Dynamic friction coefficient 0.153 0.152 0.156 0.151 0.135 0.140 0.142 0.142 Static friction coefficient 0.140 0.139 0.140 0.144 0.131 0.119 0.123 0.127 1)hydrogenated refined mineral oil (kinematic viscosity at 100° C.: 4 mm2/s, viscosity index: 120) 2)boron-modified polybutenylsuccinimide (mono type average-weight molecular weight of polybutenyl group: 1,000, boron content: 2 mass %) 3)boron-modified polybutenylsuccinimide (bis type, average-weight molecular weight of polybutenyl group: 1,000, boron content: 0.9 mass %) 4)polybutenylsuccinimide (mono type, average-weight molecular weight of polybutenyl group: 1,000, containing no boron) 5)petroleum-based Ca sulfonate (total base number: 300 mgKOH/g, Ca content: 12.2 mass %) 6)petroleum-based Mg sulfonate (total base number: 400 mgKOH/g, Mg content: 9.4 mass %) 7)synthetic Ca sulfonate (total base number: 13 mgKOH/g, Ca content: 2.5 mass %) 8)diphenylhydrogenphosphite (phosphorus content: 13.2 mass %) 9)dispersion type polymethacrylate 10)dialkyldiphenylamine-based 11)bisphenol-based 12)amine-based 13)amide-based 14)tolyltriazole 15)polydimethylsiloxane

[0084] As apparent from the results given in Table 1, the lubricant compositions of the present invention (Examples 1 to 4) are high in dynamic and static friction coefficients and thus exhibit an excellent effect to enhance the friction coefficient in a wet clutch.

[0085] Whereas, Comparative Example 1 which contains a boron-free ashless dispersant, Comparative Example 2 which contains boron in an amount less that that defined by the present invention, Comparative Examples 3 which contains an alkaline earth metal sulfonate in an amount less than that defined by the present invention, and Comparative Example 4 which contains an alkaline earth metal sulfonate and whose total base number is less than that defined by the present invention, are all low in friction coefficient.

Claims

1. A lubricant composition which comprises a lubricant base oil blended with a boron-containing ashless dispersant (A) in an amount of 0.035 or more percent of boron by mass, and an alkaline earth metal sulfonate having a total base number of 50 to 500 mgKOH/g (B) in an amount of 0.01 or more percent of alkaline earth metal by mass, all the percentages being based on the total mass of the composition.

2. The lubricant composition according to

claim 1 wherein said ashless dispersant is a compound obtained by modifying a nitrogen-containing compound having in its molecule at least one alkyl or alkenyl group, or a derivative thereof, with a boric compound.

3. The lubricant composition according to

claim 2 wherein said boric compound is selected from the group consisting of boric acid, boric acid salts, and borates.

4. The lubricant composition according to

claim 1 which further contains a phosphorus additive (C).

5. The lubricant composition according to

claim 4 wherein said phosphorus additive is blended in an amount of 0.005 to 0.2 percent by mass of phosphorus, based on the total mass of the composition.

6. The lubricant composition according to any one of

claims 1 to

5 which further contains one or more additives selected from the group consisting of metallic detergents, friction modifiers, viscosity index improvers, extreme pressure agents, oxidation inhibitors, corrosion inhibitors, anti-foaming agents, and dyes.