Lubricating Oil Composition

Abstract

A lubricating oil composition which is excellent in wear resistance, despite its low phosphorus content, low sulfur content and low sulfuric acid ash content, and which exhibits excellent friction reducing effect even when used for a DLC-treated sliding part, is provided by compounding a specific sulfur-containing compound and a specific polar group-containing compound in a base oil.

Description

TECHNICAL FIELD

[0001] The present invention relates to a lubricating oil composition and, more specifically, to a lubricating oil composition which is excellent in wear resistance, despite its low phosphorus content, low sulfur content and low sulfuric acid ash content, and which exhibits excellent friction reducing effect even when used for a diamond-like carbon (DLC)-treated sliding part.

BACKGROUND ART

[0002] Current automobile engines use an oxidation catalyst, a three way catalyst, an NOx occlusion reduction catalyst, a diesel particulate filter (DPF), etc. for purification of exhaust gases. These exhaust gas purification devices are known to be adversely affected by metal components, phosphorus components and sulfur components contained in the engine oil. Thus, it is known to be necessary to reduce these components in order to prevent the deterioration of these devices.

[0003] Various technical developments of automobiles, etc. have been made in recent years for the purpose of reducing fuel consumption. For example, there may be mentioned a surface treatment technique in sliding parts such as engines.

[0004] A zinc dithiophosphate (Zn-DTP) has been conventionally used over the years as a wear resisting and antioxidation agent for a lubricating oil for use in an internal combustion engine such as a gasoline engine, a diesel engine or a gas engine and is now still accepted as an important essential additive for such a lubricating oil for internal combustion engines.

[0005] The zinc dithiophosphate, which generates sulfuric acid and phosphoric acid upon being decomposed, however, may consume basic compounds contained in the engine oil and accelerate the deterioration of the lubricant oil with the result that oil change intervals are extremely short. Additionally, the zinc dithiophosphate tends to form a sludge when subjected to high temperature conditions and to cause deterioration of the detergency inside an engine. Moreover, the zinc dithiophosphate which contains, in the molecule thereof, a large amount of phosphorus and sulfur components in addition to a metal (zinc) component is considered to cause an adverse influence on an exhaust gas purifying device. In this circumstance, it is desired to develop a lubricating oil composition which excels in a wear resistance without use of the zinc dithiophosphate.

[0006] In the circumstance in which development of techniques for surface treatment of sliding parts is being made as described above, there is a demand for further improvement of lubricating oil compositions. For example, when a conventional lubricating oil is used for a sliding part which has been subjected to a DLC treatment, there is often a case in which an expected friction reducing effect is not achievable. Thus, there is a demand for a lubricating oil composition which exhibits excellent friction reducing effect, even when used for DLC-treated sliding parts, to achieve further lower-fuel consumption.

[0007] With a view toward solving these problems, various lubricating oil additives and lubricating oil compositions have been hitherto proposed. For example, Patent Documents 1 to 3 disclose lubricating oil additives and lubricating oil compositions which contain as a principle component a disulfide compound having a specific structure. Patent Document 4 discloses an engine oil which is alleged to be able to reduce sulfur and phosphorus that serve as a poisoning substance of reduction catalysts and to excel in friction reducing performance. Patent Document 5 discloses a low friction sliding mechanism which has a sliding surface using diamond and which is provided with a lubricating oil composition containing a specific additive.

PRIOR ART DOCUMENT

Patent Document

[0008] Patent Document 1: JP-A-2004-262964 [0009] Patent Document 2: JP-A-2004-262965 [0010] Patent Document 3: JP-A-2008-056876 [0011] Patent Document 4: JP-A-2007-131792 [0012] Patent Document 5: JP-A-2008-56735

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

[0013] Although development of various lubricating oil additives and lubricating oil compositions has been thus far been made as described above, the lubricating oil compositions disclosed in the above documents are not fully satisfactory when taking into consideration that lubricating oils are generally required to satisfy various performances, such as performance against catalytic poisoning, wear resistance and friction reducing effect, at the same time. In particular, it has been difficult to provide a lubricating oil composition, which exhibits performances comparable to or better than those of the conventional ones, without using zinc dithiophosphate which is a very effective additive for improving wear resistance and oxidation resistance.

[0014] The present invention has been made in view of the foregoing circumstances and is aimed at the provision of a lubricating oil composition which is excellent in wear resistance, despite its low phosphorus content, low sulfur content and low sulfuric acid ash content, and which exhibits excellent friction reducing effect even when used for a DLC-treated sliding part.

Means for Solving the Problem

[0015] The present inventors have made an earnest study and found that the above-described object can be achieved by using a specific sulfur-containing compound in combination with a specific polar group-containing compound. The present invention has been completed based on such finding.

[0016] Namely, the present invention provides:

<1> A lubricating oil composition comprising

[0017] a base oil, (A) at least one selected from sulfur-containing compounds represented by the general formulas (I) and (II) shown below, and (B) a polar group-containing compound which has at least one polar group selected from amino groups, amide groups and a hydroxyl group and which has a C.sub.3 to C.sub.24 alkyl group;

##STR00001##

[0018] (in the above formulas, R.sup.1 to R.sup.12 each independently represent a hydrogen atom; a hydrocarbon group selected from alkyl groups, cycloalkyl groups, alkenyl groups, cycloalkenyl groups and aryl groups; or a hetero atom-containing group having an atom which is selected from an oxygen atom, a nitrogen atom and a sulfur atom and which is contained in the above hydrocarbon group; Ys each independently represent a divalent group selected from --O--, --S--, --SO--, --SO.sub.2--, --(C.dbd.O)O--, --(C.dbd.O)NH--, --O(C.dbd.O)NH--, --C(.dbd.O)--, --N(H)--, --NHCONH--, --N.dbd.N--, --NH--C(.dbd.NH)--NH--, --S--C(.dbd.O)--, --NH--C(.dbd.S)-- and --NH--C(.dbd.S)--NH--; x represents an integer of 1 to 3; and ns each independently represent an integer of 1 to 5);

<2> The lubricating oil composition according to above <1>, wherein (B) the polar group-containing compound, which has at least one polar groups selected from amino groups, amide groups and a hydroxyl group and which has a C.sub.3 to C.sub.24 alkyl group, is at least one compound selected from the group consisting of glycerol partial esters of fatty acids, glycerol monoether compounds, amine compounds and amide compounds; <3> The lubricating oil composition according to above <1>, wherein (B) the polar group-containing compound, which has at least one polar groups selected from amino groups, amide groups and a hydroxyl group and which has a C.sub.3 to C.sub.24 alkyl group, is a glycerol monoester of a fatty acid represented by the general formula (III) or (IV) shown below or a glycerol monoether compound represented by the general formula (V) or (VI) shown below:

##STR00002##

wherein R.sup.13 and R.sup.14 each independently represent a C.sub.3 to C.sub.24 alkyl group; <4> The lubricating oil composition according to about <1>, wherein (B) the polar group-containing compound, which has at least one polar groups selected from amino groups, amide groups and a hydroxyl group and which has a C.sub.3 to C.sub.24 alkyl group, is an amine compound represented by the general formula (VII) shown below or an amide compound represented by the general formula (VIII) shown below:

##STR00003##

wherein R.sup.15 and R.sup.17 each independently represent a C.sub.3 to C.sub.24 alkyl group, and R.sup.16 and R.sup.18 each independently represent a hydrogen atom or a group having a hydroxyl group substituted for a terminal hydrogen atom of a straight chained C.sub.2 to C.sub.4 alkyl group; <5> The lubricating oil composition according to any one of above <1> to <4>, wherein the lubricating oil composition has a phosphorus content of 0.5% by mass or less and a sulfuric acid ash content of 0.6% by mass or less, each based on the total mass of the lubricating oil composition; <6> The lubricating oil composition according to any one of above <1> to <5>, wherein the lubricating oil composition has a phosphorus content of 0% by mass and a sulfuric acid ash content of 0.1% by mass or less, each based on the total mass of the lubricating oil composition; and <7> The lubricating oil composition according to any one of above <1> to <6>, wherein the lubricating oil composition is used for a sliding part which has been treated with diamond-like carbon (DLC).

Effect of the Invention

[0019] According to the present invention, there is provided a lubricating oil composition which is excellent in wear resistance, despite its low phosphorus content, low sulfur content and a low sulfuric acid ash content, and which exhibits excellent friction reducing effect even when used for a DLC-treated sliding part.

EMBODIMENTS OF THE INVENTION

[0020] The lubricating oil composition of the present invention is characterized in that a specific sulfur-containing compound and a specific polar group-containing compound are compounded in a base oil.

Base Oil:

[0021] The base oil used in the present invention is not specifically limited and may be appropriately selected from any mineral oils and synthetic oils that are conventionally used as a base oil for lubricant oils.

[0022] Examples of the mineral oils include those which are obtained by subjecting a lube-oil distillate (which is obtained by vacuum distillation of an atmospheric residue produced by atmospheric distillation of a crude oil) to one or more refining treatments such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing and hydrorefining, and those which are produced by isomerizing waxes or GTL waxes.

[0023] Examples of the synthetic oils include polybutene, polyolefins (.alpha.-olefin homopolymers and copolymers (such as ethylene-.alpha.-olefin copolymers)), various esters (such as polyol esters, dibasic acid esters and phosphoric acid esters), various ethers (such as polyphenyl ethers), polyglycols, alkyl benzenes and alkyl naphthalenes. Among these synthetic oils, particularly preferred are polyolefins and polyol esters.

[0024] In the present invention, the above mineral oils may be used alone or in combination of two or more thereof as the base oil. Also, the above synthetic oils may be used alone or in combination of two or more thereof. Further, one or more mineral oils may be used in combination with one or more synthetic oils.

[0025] The viscosity of the base oil is not specifically limited. However, it is preferred that the base oil have a kinematic viscosity at 100.degree. C. of 2 to 30 mm.sup.2/s, more preferably 3 to 15 mm.sup.2/s, still more preferably 4 to 10 mm.sup.2/s.

[0026] When the kinematic viscosity at 100.degree. C. is 2 mm.sup.2/s or more, an evaporation loss is small. When the kinematic viscosity is 30 mm.sup.2/s or less, a power loss by viscosity resistance can be suppressed so that a fuel consumption improving effect is obtainable.

[0027] It is also preferred that the base oil have a % C.sub.A value of 3.0 or less as measured by ring analysis and a sulfur content of 50 ppm by mass or less. As used herein, the term "% C.sub.A value as measured by ring analysis" means a proportion (percentage) of an aromatic component which is calculated by the n-d-M ring analysis method. The sulfur content as used herein means the value as measured according to JIS K 2541.

[0028] The base oil having a % C.sub.A value of 3.0 or less and a sulfur content of 50 ppm by mass or less exhibits good oxidation stability and can give a lubricant oil composition that can suppress an increase of the acid value and formation of a sludge. The % C.sub.A value is more preferably 1.0 or less, still more preferably 0.5 or less. The sulfur content is more preferably 30 ppm by mass or less.

[0029] It is further preferred that the base oil have a viscosity index of 70 or more, more preferably 100 or more, still more preferably 120 or more. When the viscosity index of the base oil is 70 or more, a change in viscosity of the base oil by a change in temperature is small.

Sulfur-Containing Compound:

[0030] The lubricating oil composition of the present invention contains a sulfur-containing compound represented by the following general formula (I) or (II):

##STR00004##

[0031] In the general formulas (I) and (II), R.sup.1 to R.sup.12 each independently represent a hydrogen atom; a hydrocarbon group selected from alkyl groups, cycloalkyl groups, alkenyl groups, cycloalkenyl groups and aryl groups; or a hetero atom-containing group having an atom which is selected from an oxygen atom, a nitrogen atom and a sulfur atom and which is contained in the above hydrocarbon group.

[0032] The alkyl group represented by R.sup.1 to R.sup.12 is preferably a C.sub.1 to C.sub.30 alkyl group, preferably a C.sub.1 to C.sub.24 alkyl group. Specific examples of the alkyl group include n-butyl groups, isobutyl groups, sec-butyl groups, tert-butyl groups, various hexyl groups, various octyl groups, various decyl groups, various dodecyl groups, various tetradecyl groups, various hexadecyl groups, various octadecyl groups and various icosyl groups. The alkyl group may be substituted with an aromatic group, examples of which include a benzyl group and a phenethyl group.

[0033] The cycloalkyl group represented by R.sup.1 to R.sup.12 is preferably a C.sub.3 to C.sub.30 cycloalkyl group, more preferably a C.sub.3 to C.sub.24 cycloalkyl group. Specific examples of the cycloalkyl group include a cyclopropyl group, a cyclopentyl group, a cyclohexyl group, a methylcyclopentyl group, a dimethylcyclopentyl group, a methylethylcyclopentyl group, a diethylcyclopentyl group, a methylcyclohexyl group, a dimethylcyclohexyl group, a methylethylcyclohexyl group and a diethylcyclohexyl group. The cycloalkyl group may be substituted with an aromatic group, examples of which include a phenylcyclopentyl group and a phenylcyclohexyl group.

[0034] The alkenyl group represented by R.sup.1 to R.sup.12 is preferably a C.sub.2 to C.sub.30 alkenyl group, more preferably a C.sub.2 to C.sub.24 alkenyl group. Specific examples of the alkenyl group include a vinyl group, an allyl group, a 1-butenyl group, a 2-butenyl group, a 3-butenyl group, 1-methylvinyl group, a 1-methylallyl group, a 1,1-dimethylallyl group, a 2-methylallyl group, a nonenyl group, a decenyl group and an octadecenyl group. The alkenyl group may be substituted with an aromatic group.

[0035] The cycloalkenyl group represented by R.sup.1 to R.sup.12 is preferably a C.sub.3 to C.sub.30 cycloalkenyl group, more preferably a C.sub.3 to C.sub.24 cycloalkenyl group. Specific examples of the cycloalkenyl group include a cyclobutenyl group and a methylcyclobutenyl group. The cycloalkenyl group may be substituted with an aromatic group.

[0036] The aryl group represented by R.sup.1 to R.sup.12 is preferably a C.sub.6 to C.sub.30 aryl group, more preferably a C.sub.6 to C.sub.24 aryl group. Specific examples of the aryl group include a phenyl group, a tolyl group, a xylyl group, a naphthyl group, a butylphenyl group, an octylphenyl group and a nonylphenyl group.

[0037] In the general formulas (I) and (II), Ys each independently represent a divalent group selected from --O--, --S--, --SO--, --SO.sub.2--, --(C.dbd.O)O--, --(C.dbd.O)NH--, --O(C.dbd.O)NH--, --C(.dbd.O)--, --N(H)--, --NHCONH--, --N.dbd.N--, --NH--C(.dbd.NH)--NH--, --S--C(.dbd.O)--, --NH--C(.dbd.S)-- and --NH--C(.dbd.S)--NH--.

[0038] In the general formulas (I) and (II), x is an integer of 1 to 3, preferably 2, and ns each independently represent an integer of 1 to 5, preferably 1 or 2.

[0039] As the sulfur-containing compound represented by the general formula (I), there may be mentioned, for example, compounds of the formulas shown below:

##STR00005## ##STR00006## ##STR00007## ##STR00008## ##STR00009## ##STR00010## ##STR00011##

[0040] The following compounds are also examples of the compound represented by the general formula (I), i.e. such examples include: bis(methoxycarbonylmethyl)disulfide, bis(ethoxycarbonylmethyl)disulfide, bis(n-propoxycarbonylmethyl)disulfide, bis(isopropoxycarbonylmethyl)disulfide, bis(n-butoxycarbonylmethyl)disulfide, bis(n-octoxycarbonylmethyl)disulfide, bis(n-dodecyloxycarbonylmethyl)disulfide, bis(cyclopropoxycarbonylmethyl)disulfide, 1,1-bis(2-methoxycarbonylethyl)disulfide, 1,1-bis(3-methoxycarbonyl-n-propyl)disulfide, 1,1-bis(4-methoxycarbonyl-n-butyl)disulfide, 1,1-bis(2-ethoxycarbonylethyl)disulfide, 1,1-bis(2-n-propoxycarbonylethyl)disulfide, 1,1-bis(2-isopropoxycarbonylethyl)disulfide and 1,1-bis(2-cyclopropoxycarbonylethyl)disulfide.

[0041] Specific examples of the compound represented by the general formula (II) include tetramethyl dithiomalate, tetraethyl dithiomalate, tetra-1-propyl dithiomalate, tetra-2-propyl dithiomalate, tetra-1-butyl dithiomalate, tetra-2-butyl dithiomalate, tetraisobutyl dithiomalate, tetra-1-hexyl dithiomalate, tetra-1-octyl dithiomalate, tetra-1-(2-ethyl)hexyl dithiomalate, tetra-1-(3,5,5-trimethyl)hexyl dithiomalate, tetra-1-decyl dithiomalate, tetra-1-dodecyl dithiomalate, tetra-1-hexadecyl dithiomalate, tetra-1-octadecyl dithiomalate, tetrabenzyl dithiomalate, tetra-.alpha.-(methyl)benzyl dithiomalate, tetra-.alpha.,.alpha.-dimethylbenzyl dithiomalate, tetra-1-(2-methoxy)ethyl dithiomalate, tetra-1-(2-ethoxy)ethyl dithiomalate, tetra-1-(2-butoxy)ethyl dithiomalate, tetra-1-(2-ethoxy)ethyl dithiomalate, tetra-1-(2-butoxybutoxy)ethyl dithiomalate and tetra-1-(2-phenoxy)ethyl dithiomalate.

[0042] In the present invention the sulfur-containing compounds represented by the general formulas (I) or (II) may be used singly or as a mixture of two or more thereof. The compounding amount of the sulfur-containing compound is preferably 0.01% to 5.0% by mass, more preferably 0.1% to 2.0% by mass, based on the total mass of the composition. When the compounding amount is 0.01% by mass or more, a sufficient wear resistance is obtainable. When the compounding amount exceeds 5.0% by mass, there is a possibility that the effect proportional to the amount added is not obtainable.

Polar Group-Containing Compound:

[0043] The lubricating oil composition of the present invention contains a polar group-containing compound which has at least one polar group selected from amino groups, amide groups and a hydroxyl group and which has an alkyl group having a specific number of carbon atoms

[0044] The polar group-containing compound used in the present invention is a compound having a C.sub.3 to C.sub.24, preferably C.sub.8 to C.sub.20 alkyl group. When the number of carbon atoms is less than 3, the solubility of the compound is low. Although better friction reducing effect is generally obtainable as the number of carbon atoms increases, an effect proportional to the increased number of carbon atoms is hardly obtainable, when the number of carbon atoms exceeds 24.

[0045] As the polar group-containing compound, there may be mentioned, for example, those which are selected from glycerol partial esters of fatty acids, glycerol monoether compounds, amine compounds and amide compounds and which have the above-described alkyl group.

[0046] The glycerol partial ester of a fatty acid may be, for example, a compound obtained by reaction of glycerol with a fatty acid. Examples of the fatty acid include acetic acid, propionic acid, butanoic acid (butyric acid), pentanoic acid (valeric acid), isopentanoic acid (isovaleric acid), hexanoic acid (caproic acid), heptanoic acid, isoheptanoic acid, octanoic acid (caprylic acid), 2-ethylhexanoic acid, isooctanoic acid, nonanoic acid (pelargonic acid), isononanoic acid, decanoic acid (capric acid), isodecanoic acid, undecanoic acid, isoundecanoic acid, dodecanoic acid (lauric acid), isododecanoic acid, tridecanoic acid, isotridecanoic acid, tetradecanoic acid (myristic acid), hexadecanoic acid (palmitic acid), octadecanoic acid (stearic acid), isostearic acid, eicosanoic acid (arachidic acid), docosanoic acid (behenic acid), tetracosanoic acid (lignoceric acid), hexacosanoic acid (cerotic acid), octacosanoic acid (montanic acid), 10-undecenoic acid, zomaric acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, gadoleic acid, erucic acid and selacholeic acid. A mixed fatty acid obtainable from natural fats and oils may also be used. Among these fatty acids, C.sub.10 to C.sub.18 fatty acids are preferred, C.sub.10 to C.sub.12 saturated fatty acids are more preferred, oleic acid and elaidic acid are further preferred and oleic acid is most preferred from the standpoint of the friction reducing effect of the lubricant oil.

[0047] As the glycerol partial ester of a fatty acid, there may be mentioned, for example, glycerol monoesters of fatty acids represented by the general formulas (III) and (IV) shown below. These compounds may be obtained by, for example, direct esterification between a fatty acid and glycerol or interesterification of a fat and oil with glycerol.

##STR00012##

[0048] In the general formulas (III) and (IV), R.sup.13s each independently represent a C.sub.3 to C.sub.24 alkyl group. In the present invention, the glycerol monoesters of fatty acids represented by the general formulas (III) and (IV) may be used singly or as a mixture thereof. In use of these glycerol monoesters of fatty acids, glycerol diesters and/or triesters of fatty acids may be contained therein.

[0049] The glycerol monoether compounds may be, for example, compounds obtained by reaction of glycerol with an aliphatic alcohol. Examples of the aliphatic alcohol include propanol, butanol, oleyl alcohol and stearyl alcohol. Above all, from the standpoint of friction reducing effect of the lubricant oil, oleyl alcohol and stearyl alcohol are preferred, and oleyl alcohol is particularly preferred.

[0050] As the glycerol monoether compound, there may be mentioned, for example, glycerol monoether compound represented by the general formulas (V) and (VI) shown below:

##STR00013##

[0051] In the general formulas (V) and (VI), R.sup.14s each independently represent a C.sub.3 to C.sub.24 alkyl group. In the present invention, the glycerol monoether compounds represented by the general formula (V) and (VI) may be used singly or as a mixture thereof. In use of these glycerol monoether compounds, glycerol diethers and/or glycerol triethers may be contained therein.

[0052] Examples of the amine compound include alkylamine compounds and alkanolamine compounds. The alkyl group of the alkylamine compound may be the alkyl group which is contained in the above-described fatty acids. As the amine compound, there may be mentioned, for example, those represented by the following general formula (VII):

##STR00014##

[0053] In the general formula (VII), R.sup.15 represents a C.sub.3 to C.sub.24 alkyl group, and R.sup.16s each independently represent a hydrogen atom or a group having a hydroxyl group substituted for a terminal hydrogen atom of a straight chained C.sub.2 to C.sub.4 alkyl group (such as --(CH.sub.2).sub.2--OH). In the present invention, the amine compounds represented by the general formula (VII) may be used singly or as a mixture thereof.

[0054] Specific examples of the amine compounds include monoethanolamine, diethanolamine, triethanolamine, N-methylethanolamine, N,N-dimethylethanolamine, N-ethylethanolamine, N,N-diethylethanolamine, N-isopropylethanolamine, N,N-diisopropylethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, N-methylisopropanolamine, N,N-dimethylisopropanolamine, N-ethylisopropanolamine, N,N-diethylisopropanolamine, N-isopropylisopropanolamine, N,N-diisopropylisopropanolamine, mono-n-propanolamine, di-n-propanolamine, tri-n-propanolamine, N-methyl-n-propanolamine, N,N-dimethyl-n-propanolamine, N-ethyl-n-propanolamine, N,N-diethyl-n-propanolamine, N-isopropyl-n-propanolamine, N,N-diisopropyl-n-propanolamine, monobutanolamine, dibutanolamine, tributanolamine, N-methylbutanolamine, N,N-dimethylbutanolamine, N-ethylbutanolamine, N,N-diethylbutanolamine, N-isopropylbutanolamine and N,N-diisopropylbutanolamine.

[0055] As the amide compound, there may be mentioned compounds obtainable by reaction of mono- to tetravalent carboxylic acid with an alkylamine or an alkanolamine.

[0056] The monovalent carboxylic acid may have an alkyl group such a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, an icosyl group, a pentaicosyl group, a docosyl group, a tricosyl group, a tetracosyl group, a pentacosyl group, a hexacosyl group, a heptacosyl group, an octacosyl group, a nonacosyl group and a triacontyl group.

[0057] Examples of the monovalent carboxylic acid include caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid and lignoceric acid. Examples of the di- to tetravalent carboxylic acids include polycarboxylic acids such as oxalic acid, phthalic acid, trimellitic acid and pyromellitic acid.

[0058] As the amide compound, there may be mentioned a compound represented by the following general formula (VIII).

##STR00015##

[0059] In the general formula (VIII), R.sup.17 represents a C.sub.3 to C.sub.24 alkyl group, and R.sup.18s each independently represent a hydrogen atom or a group having a hydroxyl group substituted for a terminal hydrogen atom of a straight chained C.sub.2 to C.sub.4 alkyl group (such as --(CH.sub.2).sub.2--OH). In the present invention, the amide compounds represented by the general formula (VIII) may be used singly or as a mixture thereof.

[0060] Specific examples of the amide compound include oleic acid monoethanolamide, oleic acid diethanolamide, oleic acid monopropanolamide and oleic acid dipropanolamide.

[0061] The polar group-containing compound used in the present invention may also be a compound obtainable by reaction of the above-described polar group-containing compound with a molybdenum compound. Examples of the molybdenum compound include molybdenum oxide, molybdenum halide and molybdic acid. In this reaction, the molybdenum compound is preferably used in a molar ratio of 0.01 to 10 moles, more preferably 0.05 to 5 moles, per mole of the polar group-containing compound.

[0062] The reaction may be carried out using a solvent, for example an organic solvent such as a hydrocarbon oil, hexane, heptane, octane, toluene and xylene.

[0063] The reaction temperature for the above reaction is not specifically limited but is preferably 50 to 250.degree. C., more preferably 100 to 200.degree. C.

[0064] The polar group-containing compound used in the present invention may also be a compound obtainable by reaction of the above-described polar group-containing compound with a boron compound. Examples of the boron compound include boron oxide, a boron halide, boric acid, boric anhydride and an ester of boric acid. In this reaction, the boron compound is preferably used in a molar ratio of 0.01 to 10 moles, more preferably 0.05 to 5 moles, per mole of the polar group-containing compound.

[0065] The reaction may be carried out using a solvent, for example an organic solvent such as a hydrocarbon oil, hexane, heptane, octane, toluene and xylene.

[0066] The reaction temperature for the above reaction is not specifically limited but is preferably 50 to 250.degree. C., more preferably 100 to 200.degree. C.

[0067] In the present invention, the polar group-containing compounds may be used singly or as a mixture thereof. The compounding amount of the polar group-containing compound is preferably 0.01% to 5.0% by mass, more preferably 0.1% to 2.0% by mass, based on the total mass of the composition. When the compounding amount is 0.01% by mass or more, a sufficient friction reducing effect is obtainable. When the compounding amount is 5.0% by mass or more, there is a possibility that undissolved residues may be present.

[0068] In the lubricating oil composition of the present invention, a customarily employed additive may be compounded as long as the effect thereof is not adversely affected. Examples of the additive include an antioxidant, an ashless dispersant, a metallic detergent, a viscosity index improver, a pour point depressant, a metal deactivator, a rust inhibitor and a defoaming agent.

[0069] The above-mentioned antioxidant is preferably a phosphorus-free antioxidant. Examples of the phosphorus-free antioxidant include a phenol-based antioxidant, an amine-based antioxidant, a molybdenum/amine complex-based antioxidant and a sulfur-based antioxidant.

[0070] Specific examples of the phenol-based antioxidant include 4,4'-methylenebis(2,6-di-t-butylphenol), 4,4'-bis(2,6-di-t-butylphenol), 4,4'-bis(2-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-t-butylphenol), 2,2'-methylenebis(4-methyl-6-t-butylphenol), 4,4'-butylidenebis(3-methyl-6-t-butylphenol), 4,4'-isopropylidenebis(2,6-di-t-butylphenol), 2,2'-methylenebis(4-methyl-6-nonylphenol), 2,2'-isobutylidenebis(4,6-dimethylphenol), 2,2'-methylenebis(4-methyl-6-cyclohexylphenol), 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, 2,4-dimethyl-6-t-butylphenol, 2,6-di-t-amyl-p-cresol, 2,6-di-t-butyl-4-(N,N'-dimethylaminomethylphenol), 4,4'-thiobis(2-methyl-6-t-butylphenol), 4,4'-thiobis(3-methyl-6-t-butylphenol), 2,2'-thiobis(4-methyl-6-t-butylphenol), bis(3-methyl-4-hydroxy-5-t-butylbenzyl)sulfide, bis(3,5-di-t-butyl-4-hydroxybenzyl)sulfide, n-octyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate, n-octadecyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate, and 2,2'-thio[diethyl-bis-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate].

[0071] Above all, especially preferred are bisphenol-based antioxidants and ester group-containing phenol-based antioxidants.

[0072] Specific examples of the amine-based antioxidant include monoalkyldiphenylamines such as monooctyldiphenylamine and monononyldiphenylamine; dialkyldiphenylamines such as 4,4'-dibutyldiphenylamine, 4,4'-dipentyldiphenylamine, 4,4'-dihexyldiphenylamine, 4,4'-diheptyldiphenylamine, 4,4'-dioctyldiphenylamine and 4,4'-dinonyldiphenylamine; polyalkyldiphenylamines such as tetrabutyldiphenylamine, tetrahexyldiphenylamine, tetraoctyldiphenylamine and tetranonyldiphenylamine; .alpha.-naphthylamine; phenyl-.alpha.-naphthylamine; and alkyl-substituted phenyl-.alpha.-naphthylamines such as butylphenyl-.alpha.-naphthylamine, pentylphenyl-.alpha.-naphthylamine, hexylphenyl-.alpha.-naphthylamine, heptylphenyl-.alpha.-naphthylamine, octylphenyl-.alpha.-naphthylamine and nonylphenyl-.alpha.-naphthylamine.

[0073] Above all, the dialkyldiphenylamine-based and naphthylamines-based antioxidants are preferred.

[0074] As the molybdenum/amine complex-based antioxidants, there may be mentioned, for example, hexavalent molybdenum compounds. Specific examples of such compounds include those which are obtained by reacting molybdenum trioxide and/or molybdic acid with an amine compound and those which are obtained by the production method described in JP-A-2003-252887.

[0075] The amine compound to be reacted with the hexavalent molybdenum compound is not particularly limited, and there may be mentioned monoamines, diamines, polyamines and alkanol amines. Specific examples of the amine compound include alkyl amines having an C.sub.1 to C.sub.30 alkyl group or groups (the alkyl group may be either linear or branched) such as methylamine, ethylamine, dimethylamine, diethylamine, methylethylamine and methylpropylamine; alkenyl amines containing a C.sub.2 to C.sub.30 alkenyl group or groups (the alkenyl group may be linear or branched) such as ethenyl amine, propenyl amine, butenyl amine, octenyl amine and oleyl amine; alkanol amines containing a C.sub.1 to C.sub.30 alkanol group or groups (the alkanol group may be linear or branched) such as methanol amine, ethanol amine, methanol ethanol amine and methanol propanol amine; alkylene diamines containing a C.sub.1 to C.sub.30 alkylene group or groups such as methylenediamine, ethylenediamine, propylenediamine and butylenediamine; polyamines such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine and pentaethylenehexamine; compounds, such as undecyldiethylamine, undecyldiethanol amine, dodecyldipropanol amine, oleyldiethanol amine, oleylpropylenediamine and stearyltertraethylenepentamine, which are obtained by further introducing a C.sub.8 to C.sub.20 alkyl or alkenyl group into the above monoamines, diamines or polyamines; heterocyclic compounds such as imidazoline; alkyleneoxide adducts of these compounds; and mixtures of these compounds.

[0076] In addition, as the molybdenum/amine complex-based antioxidants, there may be mentioned, for example, sulfur-containing molybdenum complexes of succinic imide as described in JP-B-3-22438 and JP-A-2004-2866.

[0077] As the sulfur-based antioxidant, there may be mentioned, for example, phenothiazine, pentaerythritol-tetrakis-(3-lauryl thiopropionate), didodecyl sulfide, dioctadecyl sulfide, didodecyl thiodipropionate, dioctadecyl thiodipropionate, dimyristyl thiodipropionate, dodecyloctadecyl thiodipropionate and 2-mercaptobenzoimidazole.

[0078] Among these antioxidants, from the standpoint of reducing a metal content and a sulfur content, phenol-based antioxidants and amine-based antioxidants are preferred. The above antioxidants may be used singly or as a mixture of two or more thereof. From the standpoint of improved oxidation stability, a mixture of one or more kinds of phenol-based antioxidants and one or more kinds of amine-based oxidants is preferably used.

[0079] The compounding amount of the antioxidant is generally 0.1% to 5% by mass, more preferably from 0.1% to 3% by mass, based on the total mass of the composition.

[0080] As the above-mentioned ashless dispersant, there may be used any ashless dispersant which is generally used for lubricant oils. Examples of the ashless dispersant include a mono-type succinimide compound represented by the following general formula (IX) or a bis-type succinimide compound represented by the following general formula (X):

##STR00016##

[0081] In the above general formulas (IX) and (X), R.sup.19, R.sup.21 and R.sup.24 each represent an alkenyl or alkyl group having a number-average molecular weight of 500 to 4,000. The groups R.sup.21 and R.sup.24 may be the same or different. The number-average molecular weight of R.sup.19, R.sup.21 and R.sup.24 is preferably from 1,000 to 4,000.

[0082] When the number-average molecular weight of R.sup.19, R.sup.21 and R.sup.24 is 500 or more, the solubility of the compound in the base oil is good. When the number-average molecular weight is 4,000 or less, there is no fear of deterioration of the dispersancy.

[0083] In the formulas, R.sup.20, R.sup.22 and R.sup.23 each represent a C.sub.2 to C.sub.5 alkylene group. The groups R.sup.22 and R.sup.23 may be the same or different. The symbol r is an integer of 1 to 10, s is 0 or an integer of 1 to 10. The symbol r is preferably 2 to 5, more preferably 3 or 4. When r is 1 or more, good dispersancy may be obtained. When r is 10 or less, the compound exhibits good solubility in the base oil.

[0084] Further, in the general formula (X), s is preferably 1 to 4, more preferably 2 or 3. The symbol s that lies within the above-specified range is preferred for reasons of the dispersancy and solubility in the base oil.

[0085] Examples of the alkenyl group include a polybutenyl group, a polyisobutenyl group and an ethylene-propylene copolymer group. Examples of the alkyl group include those which are obtained by hydrogenating these alkenyl groups. Typical examples of the suitable alkenyl group include a polybutenyl group and a polyisobutenyl group. The polybutenyl group may be obtained by polymerizing a mixture of 1-butene and isobutene, or high-purity isobutene.

[0086] Typical examples of the suitable alkyl group include those which are obtained by hydrogenating a polybutenyl group and a polyisobutenyl group.

[0087] The above alkenylsuccinimide compound or alkylsuccinimide compound may be generally produced by reacting a polyamine with an alkenylsuccinic anhydride obtained by reaction of a polyolefin with maleic anhydride, or with an alkylsuccinic anhydride, obtained by hydrogenating the alkenylsuccinic anhydride. Also, the above mono-type succinimide compound or bis-type succinimide compound may be produced by varying a reaction ratio between the alkenylsuccinic anhydride or alkylsuccinic anhydride and the polyamine.

[0088] As an olefin monomer from which the above polyolefin is formed, there may be used a C.sub.2 to C.sub.8 .alpha.-olefin or a mixture of two or more thereof. Among them, a mixture of isobutene and butene-1 may be suitably used.

[0089] Examples of the polyamine include primary diamines such as ethylenediamine, propylenediamine, butylenediamine and pentylenediamine; polyalkylene polyamines such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, di(methylethylene)triamine, dibutylenetriamine, tributylenetetramine and pentapentylenehexamine; and piperazine derivatives such as aminoethylpiperazine.

[0090] In addition to the above alkenyl or alkylsuccinimide compound, there may also be used boron derivatives thereof and/or organic acid-modified products thereof as the ashless dispersant.

[0091] The boron derivatives of the alkenyl- or alkylsuccinimide compound may be produced by an ordinary method. For example, the boron derivatives may be produced by first reacting the above polyolefin with maleic anhydride to obtain an alkenylsuccinic anhydride, and then reacting the resulting alkenylsuccinic anhydride with an intermediate product obtained by reacting the above polyamine with a boron compound, such as boron oxide, a boron halide, boric acid, boric anhydride, a boric acid ester and an ammonium salt of orthoboric acid, to imidize the alkenylsuccinic anhydride.

[0092] The content of boron in the boron derivatives is not particularly limited, and is preferably in the range of 0.05% to 5% by mass, more preferably 0.1% to 3% by mass, in terms of boron element.

[0093] The compounding amount of the ashless dispersant is preferably 0.5% to 15% by mass, more preferably 1% to 10% by mass, still more preferably 3 to 7% by mass, based on a total amount of the lubricating oil composition.

[0094] When the compounding amount is less than 0.5% by mass, the effect on base number retaining property at high temperatures is small. When the compounding amount exceeds 15% by mass, the fluidity at low temperatures of the lubricating oil composition is considerably deteriorated. Thus, either case is not preferable.

[0095] As the above-mentioned metallic detergent, there may be used any alkaline earth metal-based detergents which are employed for ordinary lubricating oils. Examples of the alkaline earth metal-based detergent include alkaline earth metal sulfonates, alkaline earth metal phenates, alkaline earth metal salicylates and mixtures of two or more thereof.

[0096] As the alkaline earth metal sulfonates, there may be mentioned alkaline earth metal salts of an alkyl aromatic sulfonic acid obtained by sulfonating an alkyl aromatic compound having a molecular weight of 300 to 1,500, preferably 400 to 700. Among them, magnesium salts and/or calcium salts, especially calcium salts are preferred.

[0097] As the alkaline earth metal phenates, there may be mentioned alkaline earth metal salts of alkylphenols, alkylphenol sulfides and Mannich reaction products of alkylphenols. Among them, magnesium salts and/or calcium salts, especially calcium salts are preferred.

[0098] As the alkaline earth metal salicylates, there may be mentioned alkaline earth metal salts of alkyl salicylic acids. Among them magnesium salts and/or calcium salts, especially calcium salts are preferred.

[0099] The alkyl group contained in the compounds constituting the above alkaline earth metal-based detergents is preferably a C.sub.4 to C.sub.30 alkyl group, more preferably a C.sub.6 to C.sub.18 linear or branched alkyl group. These alkyl groups may be straight chained or branched.

[0100] These alkyl groups may be primary alkyl groups, secondary alkyl groups or tertiary alkyl groups.

[0101] As the alkaline earth metal sulfonates, alkaline earth metal phenates and alkaline earth metal salicylates, there may be used neutral alkaline earth metal sulfonates, neutral alkaline earth metal phenates and neutral alkaline earth metal salicylates which may be produced by directly reacting the above alkyl aromatic sulfonic acids, alkylphenols, alkylphenol sulfides, Mannich reaction products of alkylphenols, alkyl salicylic acids or the like with an alkaline earth metal base such as an oxide or a hydroxide of an alkaline earth metal such as magnesium and/or calcium or which may be produced by once forming an alkali metal salt thereof and then converting the alkali metal salt into an alkaline earth metal salt. Further, there may also be used basic alkaline earth metal sulfonates, basic alkaline earth metal phenates and basic alkaline earth metal salicylates which may be produced by heating neutral alkaline earth metal sulfonates, neutral alkaline earth metal phenates and neutral alkaline earth metal salicylates together with an excess amount of an alkaline earth metal salt or an alkaline earth metal base in the presence of water. Furthermore, there may also be used perbasic alkaline earth metal sulfonates, perbasic alkaline earth metal phenates and perbasic alkaline earth metal salicylates which may be produced by reacting neutral alkaline earth metal sulfonates, neutral alkaline earth metal phenates and neutral alkaline earth metal salicylates with an alkaline earth metal carbonate or an alkaline earth metal borate in the presence of carbon dioxide.

[0102] The metallic detergent used in the present invention is preferably an alkaline earth metal salicylate or alkaline earth phenate, especially a perbasic salicylate or perbasic phenate, for reasons of reducing a sulfur content of the composition.

[0103] The total base number of the metallic detergent used in the present invention is preferably 10 to 500 mg KOH/g, more preferably 15 to 450 mg KOH/g. The metallic detergent may be selected from these detergents and used singly or in combination of two or more thereof.

[0104] The term "total base number" as used herein means the value as measured by a potentiometric titration method (base number/perchlorate method) according to the Item 7 of JIS K 2501 "Petroleum Products and Lubricants-Neutralization Number Testing Method."

[0105] The metal ratio of the metallic detergent used in the present invention is not specifically limited. The metallic detergent having a metal ratio of 20 or less may be generally used singly or as a mixture of two or more thereof. The metallic detergent having a metal ratio of preferably 3 or less, more preferably 1.5 or less, still more preferably 1.2 or less, is particularly suitably used for reasons of further improved oxidation stability, base number retaining property, high-temperature detergency, etc.

[0106] Meanwhile, the term "metal ratio" as used herein means a ratio represented by the formula: (valence of a metal element).times.(content (mol %) of the metal element)/(content (mol %) of a soap group) wherein the metal element is calcium, magnesium, etc., and the soap group is a sulfonic group, a phenol group, a salicylic group, etc.

[0107] The compounding amount of the metallic detergent is preferably 0.01% to 20% by mass, more preferably 0.1% to 10% by mass, still more preferably 0.5% to 5% by mass, based on the total amount of the lubricating oil composition.

[0108] A compounding amount of the metallic detergent less than 0.01% by mass is not preferable because performances such as high temperature detergency, oxidation stability and base number retaining property are not easily obtainable. When the amount of the metallic detergent compounded is 20% by mass or less, an effect proportional to the compounding amount of the metallic detergent may be generally obtained. In spite of the above specified range, however, it is important that the upper limit of the compounding amount of the metallic detergent should be as low as possible. By so doing, the metal content, namely sulfuric acid ash content, of the lubricating oil composition is reduced, with the result that the exhaust gas purification device of automobiles is prevented from being deteriorated.

[0109] The metallic detergent may be used singly or in combination of two or more thereof as long as the content thereof lies within the above-specified range.

[0110] Specifically, among the above-mentioned metallic detergents, perbasic calcium salicylate and perbasic calcium phenate are particularly preferred. Among the above-mentioned ashless dispersants, the above-mentioned bis-polybutenylsuccinimide is particularly preferred. Meanwhile, it is preferred that perbasic calcium salicylate and perbasic calcium phenate each have a total base number of 100 to 500 mgKOH/g, more preferably 200 to 500 mgKOH/g.

[0111] As the above-mentioned viscosity index improver, there may be mentioned, for example, polymethacrylates, dispersion type polymethacrylates, olefin-based copolymers (such as ethylene-propylene copolymers), dispersion type olefin-based copolymers and styrene-based copolymers (such as styrene-diene copolymers and styrene-isoprene copolymers).

[0112] The compounding amount of the viscosity index improver is preferably 0.5% to 15% by mass, more preferably 1% to 10% by mass, based on the total amount of the lubricating oil composition from the standpoint of effects attained by addition thereof.

[0113] As the above-mentioned pour point depressant, there may be mentioned, for example, polymethacrylates having a weight-average molecular weight of about 5,000 to about 50,000.

[0114] The compounding amount of the pour point depressant is generally 0.1% to 2% by mass, more preferably 0.1% to 1% by mass, based on the total amount of the lubricating oil composition from the standpoint of effects attained by addition thereof.

[0115] As the metal deactivator, there may be mentioned, for example, benzotriazole-based compounds, tolyl triazole-based compounds, thiadiazole-based compounds and imidazole-based compounds.

[0116] The compounding amount of the metal deactivator is preferably 0.01% to 3% by mass, more preferably 0.01% to 1% by mass, based on the total amount of the lubricating oil composition.

[0117] As the rust inhibitor, there may be mentioned, for example, petroleum sulfonates, alkylbenzene sulfonates, dinonylnaphthalene sulfonates, alkenylsuccinic acid esters and polyhydric alcohol esters.

[0118] The compounding amount of the rust inhibitor is preferably 0.01% to 1% by mass, more preferably 0.05% to 0.5% by mass, based on the total amount of the lubricating oil composition from the standpoint of effects attained by addition thereof.

[0119] As the above-mentioned defoaming agent, there may be mentioned, for example, silicone oils, fluorosilicone oils and fluoroalkyl ethers. The compounding amount of the defoaming agent is preferably 0.005% to 0.5% by mass, more preferably 0.01% to 0.2% by mass, based on the total amount of the lubricating oil composition from the standpoint of a balance between the defoaming effect and economy.

[0120] The lubricating oil composition of the present invention may further contain a friction modifier, an anti-wear agent and an extreme pressure agent, if necessary. The friction modifier herein is a compound other than the polar group-containing compounds which are an essential ingredient of the present invention. The compounding amount of the friction modifier agent is preferably 0.01% to 2% by mass, more preferably 0.01% to 1% by mass or less, based on the total amount of the lubricating oil composition.

[0121] As the anti-wear agent or the extreme-pressure agent, there may be mentioned sulfur containing compounds such as zinc dithiophosphate, zinc phosphate, zinc dithiocarbamate, molybdenum dithiocarbamate, molybdenum dithiophosphate, disulfides (other than the sulfur-containing compounds of the general formula (I) or (II) used in the present invention; dibenzyldisulfide is an example thereof), sulfurized olefins, sulfurized oils and fats, sulfurized esters, thiocarbonates, thiocarbamates and polysulfides; phosphorus containing compounds such as phosphorous acid esters, phosphoric acid esters, phosphonic acid esters and amine salts or metal salts of these esters; and sulfur- and phosphorus-containing anti-wear agents such as thiophosphorous acid esters, thiophosphoric acid esters, thiophosphonic acid esters and amine salts or metal salts of these esters.

[0122] The compounding amount of the anti-wear agent or the extreme-pressure agent to be compounded should be such that the phosphorus content, sulfur content and metal content of the lubricating oil are not excessively large by addition thereof.

[0123] The lubricating oil composition of the present invention may be formulated as described in the foregoing and preferably has the following properties:

[0124] (1) the sulfuric acid ash content (JIS K 2272) is 0.6% by mass or less, more preferably 0.1% by mass or less; and

[0125] (2) the phosphorus content (JPI-5S-38-92) is 0.5% by mass or less, more preferably 0% by mass.

[0126] Additionally, it is more preferred that the following properties are met:

[0127] (3) the sulfur content (JIS K 2541) is 0.4% by mass or less, more preferably 0.2% by mass or less; and

[0128] (4) the boron content is 0.4% by mass or less, more preferably 0.2% by mass or less.

[0129] The lubricating oil composition of the present which satisfies the above properties can suppress deterioration of an oxidation catalyst, a three way catalyst, an NOx occlusion reduction catalyst, a diesel particulate filter (DPF), etc. which are used in automobile engines.

[0130] The lubricating oil composition of the present invention uses a combination of the above-described sulfur-containing compound and the polar group-containing compound. As a result of such combined use, there are achieved wear resisting and friction reducing effects which are far superior to those attained by separate use thereof. Accordingly, even when zinc dithiophosphate which has been hitherto often used as a lubricant oil additive is not used, the lubricating oil composition shows sufficiently excellent lubricating performance and makes it possible to achieve properties of low sulfuric acid ash, etc. Furthermore, the lubricating oil composition of the present invention exhibits excellent friction reducing effect even when used for a DLC-treated sliding part as described hereinafter.

[0131] The lubricating oil composition of the present invention can be suitably used as a lubricant oil for use in an internal combustion engine, such as a gasoline engine, a diesel engine or a gas engine, for two-wheeled vehicles, four-wheeled vehicles, power generators, ships or the like, and is particularly suited for internal combustion engines equipped with an exhaust gas purification device because of its low phosphorus content, low sulfur content and low sulfuric acid ash content.

[0132] The lubricating oil composition of the present invention is also suitably used for applications other than those described above. Especially, since the lubricating oil composition of the present invention shows excellent wear resistance and friction reducing effect, it can be used for lubrication of internal combustion engines, automatic transmissions, continuously variable transmissions, manual transmissions, power steerings, shock absorbers, compressors, cooling medium compressors, refrigerators, hydraulic pumps and clutch pulleys. Namely, the lubricating oil composition of the present invention may be used as internal combustion engine oils, automatic transmission oils, continuously variable transmission oils, manual transmission oils, power steering oils, shock absorber oils, compressor oils, refrigerator oils, hydraulic pump oils and clutch pulley lubricating oils and greases.

[0133] The lubricating oil composition of the present invention exhibits friction reducing effect and excellent wear resistance not only for a sliding surface of a metal such as a steel but also for a sliding surface having a DLC film on at least a portion thereof.

[0134] It is preferred that the hydrogen content of such a DLC be 40% by atom or less, more preferably 30% by atom or less, particularly preferably 20% by atom or less.

[0135] A counter member with which the sliding surface of such a DLC film-bearing member is to be brought into contact is not specifically limited and may be, for example, an iron or iron alloy member, aluminum alloy member or an organic material such as a resin or rubber material.

EXAMPLES

[0136] The present invention will be next described in more detail by way of Examples and Comparative Examples. The scope of the present invention, however, is not limited to these examples in any way.

Methods for Measuring Properties and Performances:

[0137] The properties and performances of the lubricating oil compositions obtained in the following Examples and Comparative Examples are measured by the methods shown below.

(1) Kinematic Viscosity:

[0138] Measured according to JIS K 2283.

(2) Phosphorus Content:

[0139] Measured according to JPI-5S-38-92.

(3) Sulfur Content:

[0140] Measured according to JIS K 2541.

(4) Boron Content:

[0141] Measured according to JPI-5S-38-92.

(5) Sulfuric Acid Ash Content:

[0142] Measured according to JIS K 2272.

(6) Reciprocating Friction Test

[0143] The test was carried out with a reciprocating friction tester to determine a friction coefficient using a test plate and a test ball shown below under the conditions shown below.

Test plate: SUJ-2 plate Test ball: SUJ-2 ball (1/2 in) treated with DLC (hydrogen content: 20%)

--Test Conditions--

[0144] Testing temperature: 100.degree. C.

[0145] Load: 200 g

[0146] Amplitude: 10 mm

[0147] Sliding speed: 1.0 mm/sec

(7) Frictional Wear Test

[0148] The test was carried out with a reciprocating friction tester to determine wear track length using a test plate and a test ball shown below under the conditions shown below.

Test plate: SUJ-2 plate Test ball: SUJ-2 ball (10 mm diameter)

--Test Conditions--

[0149] Testing temperature: 100.degree. C.

[0150] Load: 200 N

[0151] Amplitude: 10 mm

[0152] Frequency: 10 Hz

[0153] Testing time: 30 min

Examples 1 to 7 and Comparative Examples 1 to 9

[0154] The base oil and additives shown in Table 1 were blended in the proportion shown in Table 1 to prepare lubricating oil compositions. The properties, formulations and performances of the compositions are also shown in Table 1.

TABLE-US-00001 TABLE 1 Example 1 2 3 4 5 6 7 Formulation Base oil 88.60 88.60 88.60 88.60 88.60 88.60 87.80 Composition Sulfur-containing compound A 0.40 0.40 0.40 0.40 0.40 0.40 -- (% by mass) Sulfur-containing compound B -- -- -- -- -- -- 1.20 Polar group-containing compound A 0.50 -- -- -- -- -- 0.50 Polar group-containing compound B -- 0.50 -- -- -- -- -- Polar group-containing compound C -- -- 0.50 -- -- -- -- Polar group-containing compound D -- -- -- 0.50 -- -- -- Polar group-containing compound E -- -- -- -- 0.50 -- -- Polar group-containing compound F -- -- -- -- -- 0.50 -- Viscosity index improver 5.00 5.00 5.00 5.00 5.00 5.00 5.00 Pour point depressant 0.30 0.30 0.30 0.30 0.30 0.30 0.30 Polybutenylsuccinimide 4.00 4.00 4.00 4.00 4.00 4.00 4.00 Phenol-based antioxidant 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Amine-based antioxidant 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Zinc dialkyldithiophosphate -- -- -- -- -- -- -- Etc. 0.20 0.20 0.20 0.20 0.20 0.20 0.20 Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Properties Phosphorus content (% by mass) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Sulfur content (% by mass) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Boron content (% by mass) 0.08 0.08 0.08 0.08 0.08 0.08 0.08 Sulfuric acid ash content (% by mass) 0.05 0.05 0.05 0.05 0.05 0.05 0.05 Reciprocating friction test (friction coefficient) 0.115 0.117 0.113 0.110 0.116 0.117 0.114 Frictional wear test (wear track diameter; mm) 0.46 0.45 0.45 0.44 0.42 0.43 0.45

TABLE-US-00002 TABLE 2 Comparative Example 1 2 3 4 5 6 7 8 9 Formulation Base oil 89.10 89.00 89.00 89.00 89.00 89.00 89.00 88.40 87.90 Composition Sulfur-containing 0.40 -- -- -- -- -- -- -- -- (% by mass) compound A Polar group-containing -- 0.50 -- -- -- -- -- -- -- compound A Polar group-containing -- -- 0.50 -- -- -- -- -- -- compound B Polar group-containing -- -- -- 0.50 -- -- -- -- -- compound C Polar group-containing -- -- -- -- 0.50 -- -- -- -- compound D Polar group-containing -- -- -- -- -- 0.50 -- -- -- compound E Polar group-containing -- -- -- -- -- -- 0.50 0.50 0.50 compound F Viscosity index improver 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 Pour point depressant 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 Polybutenylsuccinimide 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 Phenol-based antioxidant 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Amine-based antioxidant 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Zinc -- -- -- -- -- -- -- 0.60 0.60 dialkyldithiophosphate Etc. 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Properties Phosphorus content 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.05 (% by mass) Sulfur content (% by mass) 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 Boron content (% by mass) 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 Sulfuric acid ash content 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.17 0.17 (% by mass) Reciprocating friction test 0.140 0.125 0.128 0.124 0.125 0.126 0.124 0.155 0.143 (friction coefficient) Frictional wear test (wear track diameter; 0.52 0.64 0.65 0.63 0.63 0.60 0.61 0.48 0.50 mm) Note: Base oil: Hydrogenated refined base oil (kinematic viscosity at 40.degree. C.: 21 mm.sup.2/s; kinematic viscosity at 100.degree. C.: 4.5 mm.sup.2/s; viscosity index: 127; % C.sub.A: 0.0; sulfur content: less than 20 ppm by mass; NOACK test evaporation amount: 13.3% by mass) Sulfur-containing compound A: 1,1-bis(octoxycarbonylmethyl) disulfide Sulfur-containing compound B: Tetra-1-hexyl dithiomalate Polar group-containing compound A: Glycerol monoolate Polar group-containing compound B: Oleic acid diethanolamide Polar group-containing compound C: Glycerol monooleyl ether Polar group-containing compound D: N,N-Dipolyoxyethylene-N-oleylamine Polar group-containing compound E: Reaction product between glycerol monoolate and boric acid Polar group-containing compound F: Reaction product between oleic acid diethanolamide and boric acid Viscosity index improver: Polymethacrylate (weight average molecular weight: 420,000; resin content: 39% by mass) Pour point depressant: Polyalkyl methacrylate (weight average molecular weight: 6,000) Polybutenylsuccinimide: Number-average molecular weight of polybutenyl group: 1,000; Nitrogen content: 1.76% by mass; Boron content: 1.9% by mass Phenol-based antioxidant: Octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate Amine-based antioxidant: Dialkyldiphenylamine (nitrogen content: 4.62% by mass) Zinc dialkyldithiophosphate (Zn content: 9.0% by mass, phosphorus content: 8.2% by mass, sulfur content: 17.1% by mass, alkyl group: mixture of secondary butyl group and secondary hexyl group) Etc.: Defoaming agent and metal deactivator

[0155] As shown in Tables 1 and 2, the lubricating oil compositions of Examples 1 to 7 gives low friction coefficients and small wear track diameters due to the synergetic effect of the combined use of the sulfur-containing compound and the polar group-containing compound.

[0156] Namely, in Comparative Example 1 which does not use a polar group-containing compound, the friction coefficient is 0.140. In Comparative Examples 2 to 7 which do not use a sulfur-containing compound, the friction coefficient is 0.124 to 0.128. However, when these compounds are used in combination, the friction coefficient can be reduced to 0.110 to 0.117 (Examples 1 to 7). Similarly, although Comparative Example 1 which does not use a polar group-containing compound gives a wear track diameter of 0.52 and Comparative Examples 2 to 7 which do not use a sulfur-containing compound give a wear track diameter of 0.60 to 0.65, the wear track diameter can be reduced to 0.42 to 0.46 by using these compounds in combination (Examples 1 to 7).

[0157] Also, it will be appreciated by comparison between Examples 1 to 7 and Comparative Example 8 that the effect achieved by the combined use of the additives according to the present invention is superior to the effect attained by using zinc dialkyldithiophosphate alone. Further, as will be appreciated from the results in Comparative Example 9, when the zinc dialkyldithiophosphate is substituted for the sulfur-containing compound of the present invention and is used in combination with the polar group-containing compound, it is impossible to obtain such a low friction coefficient and a small wear track diameter as attained in Examples 1 to 7.

[0158] As described in the foregoing, the combined use of the specific sulfur-containing compound and the specific polar group-containing compound can make it possible to obtain higher wear resistance than that attained by the zinc dialkyldithiophosphate. Consequently, it is possible to provide a lubricating oil composition which is excellent in wear resistance, despite its low phosphorus content, low sulfur content and low sulfuric acid ash content, and which exhibits excellent friction reducing effect.

INDUSTRIAL APPLICABILITY

[0159] According to the present invention there is provided a lubricating oil composition which is excellent in wear resistance, despite its low phosphorus content, low sulfur content and low sulfuric acid ash content, and which exhibits excellent friction reducing effect even when used for a DLC-treated sliding part. The lubricating oil composition according to the present invention, therefore, can be particularly suitably used as a lubricating oil composition for internal combustion engines such as gasoline engines, diesel engines and gas engines.

Claims

1. A lubricating oil composition, comprising: (A) a base oil; (B) at least one selected from the group consisting of a sulfur-comprising compound of formula (I) and formula (II): ##STR00017## wherein: R.sup.1 to R.sup.12 are each independently is a hydrogen atom; a hydrocarbon group selected from the group consisting of an alkyl group, a cycloalkyl group, an alkenyl group, a cycloalkenyl group, and an aryl group; or a hetero atom-comprising group comprising an atom selected from an oxygen atom, a nitrogen atom, and a sulfur atom, which is contained a hydrocarbon group selected from the group consisting of an alkyl group, a cycloalkyl group, an alkenyl group, a cycloalkenyl group, and an aryl group; each Y is independently a divalent group selected from the group consisting of --O--, --S--, --SO--, --SO.sub.2--, --(C.dbd.O)O--, --(C.dbd.O)NH--, --O(C.dbd.O)NH--, --C(.dbd.O)--, --N(H)--, --NHCONH--, --N.dbd.N--, --NH--C(.dbd.NH)--NH--, --S--C(.dbd.O)--, --NH--C(.dbd.S)--, and --NH--C(.dbd.S)--NH--; x is an integer from 1 to 3; and each n is independently an integer from 1 to 5; and (C) a polar group-comprising compound comprising at least one polar group selected from the group consisting of an amino group, an amide group, and a hydroxyl group, and which comprises a C.sub.3 to C.sub.24 alkyl group.

2. The composition of claim 1, wherein compound (C) is at least one selected from the group consisting of a glycerol partial ester of a fatty acid, a glycerol monoether compound, an amine compound, and an amide compound.

3. The composition of claim 1, wherein compound (C) is a glycerol monoester of a fatty acid represented by the general formula (III) or (IV): ##STR00018## wherein each R.sup.13 is independently a C.sub.3 to C.sub.24 alkyl group, or a glycerol monoether compound of formula (V) or (VI): ##STR00019## wherein each R.sup.14 is independently a C.sub.3 to C.sub.24 alkyl group.

4. The composition of claim 1, wherein the compound (C) is an amine compound of formula (VII): ##STR00020## wherein R.sup.15 is a C.sub.3 to C.sub.24 alkyl group and each R.sup.16 is independently a hydrogen atom or a group comprising a hydroxyl group substituted for a terminal hydrogen atom of a straight chained C.sub.2 to C.sub.4 alkyl group, or an amide compound of formula (VIII): ##STR00021## wherein is a C.sub.3 to C.sub.24 alkyl group, and each R.sup.18 is independently a hydrogen atom or a group comprising a hydroxyl group substituted for a terminal hydrogen atom of a straight chained C.sub.2 to C.sub.4 alkyl group.

5. The composition of claim 1, comprising a phosphorus content of 0.5% by mass or less and a sulfuric acid ash content of 0.6% by mass or less, each based on a total mass of the composition.

6. The composition of claim 1, comprising a phosphorus content of 0% by mass and a sulfuric acid ash content of 0.1% by mass or less, each based on a total mass of the composition.

7. The composition of claim 1, being adapted for a sliding part which is treated with diamond-like carbon (DLC).

8. The composition of claim 1, wherein the base oil (A) has a kinematic viscosity at 100.degree. C. of 2 to 30 mm.sup.2/s.

9. The composition of claim 1, wherein the base oil (A) has a viscosity index of 70 or more.

10. The composition of claim 1, comprising 0.01% to 5.0% by mass of the compound (B), based on a total mass of the composition.

11. The composition of claim 1, comprising 0.1% to 2.0% by mass of the compound (B), based on a total mass of the composition.

12. The composition of claim 1, comprising 0.01% to 5.0% by mass of the compound (C), based on a total mass of the composition.

13. The composition of claim 1, comprising 0.1% to 2.0% by mass of the compound (C), based on a total mass of the composition.

14. The composition of claim 1, wherein the compound (B) of formula (I) is at least one selected from the group consisting bis(methoxycarbonylmethyl)disulfide, bis(ethoxycarbonylmethyl)disulfide, bis(n-propoxycarbonylmethyl)disulfide, bis(isopropoxycarbonylmethyl)disulfide, bis(n-butoxycarbonylmethyl)disulfide, bis(n-octoxycarbonylmethyl)disulfide, bis(n-dodecyloxycarbonylmethyl)disulfide, bis(cyclopropoxycarbonylmethyl)disulfide, 1,1-bis(2-methoxycarbonylethyl)disulfide, 1,1-bis(3-methoxycarbonyl-n-propyl)disulfide, 1,1-bis(4-methoxycarbonyl-n-butyl)disulfide, 1,1-bis(2-ethoxycarbonylethyl)disulfide, 1,1-bis(2-n-propoxycarbonylethyl)disulfide, 1,1-bis(2-isopropoxycarbonylethyl)disulfide, and 1,1-bis(2-cyclopropoxycarbonylethyl)disulfide.

15. The composition of claim 1, wherein the compound (B) of formula (II) is at least one selected from the group consisting of tetramethyl dithiomalate, tetraethyl dithiomalate, tetra-1-propyl dithiomalate, tetra-2-propyl dithiomalate, tetra-1-butyl dithiomalate, tetra-2-butyl dithiomalate, tetraisobutyl dithiomalate, tetra-1-hexyl dithiomalate, tetra-1-octyl dithiomalate, tetra-1-(2-ethyl)hexyl dithiomalate, tetra-1-(3,5,5-trimethyl)hexyl dithiomalate, tetra-1-decyl dithiomalate, tetra-1-dodecyl dithiomalate, tetra-1-hexadecyl dithiomalate, tetra-1-octadecyl dithiomalate, tetrabenzyl dithiomalate, tetra-.alpha.-(methyl)benzyl dithiomalate, tetra-.alpha.,.alpha.-dimethylbenzyl dithiomalate, tetra-1-(2-methoxy)ethyl dithiomalate, tetra-1-(2-ethoxy)ethyl dithiomalate, tetra-1-(2-butoxy)ethyl dithiomalate, tetra-1-(2-ethoxy)ethyl dithiomalate, tetra-1-(2-butoxybutoxy)ethyl dithiomalate and tetra-1-(2-phenoxy)ethyl dithiomalate.