Background of the Invention

Field of the Invention

The present invention relates to a lubricating oil composition, in particular, to a lubricating composition having excellent wear resistance and extreme-pressure lubricity and a low coefficient of friction. The lubricating oil composition of the present invention can be minimized in phosphorus content or made phosphorus-free, and usable in a wide variety of fields such as, for example, a lubricating oil for an internal combustion engineer or an engine oil.

Description of the Related Art

The engine oil is usually effective in lubricating various sliding parts such as a piston ring and a cylinder liner; bearings of a crankshaft and connecting rod; valve mechanisms including a cam and valve lifter; drivers such as an oil pump and distributor, and also in cooling the inside of an engine, deterging and dispersing combustion products and preventing the rusting and corrosion thereof.

Thus, various properties are required of the engine oil and, in addition, the required properties are continuously changing according to improvements in performance and output of the engine and driving conditions which grow more and more severe. Therefore, various additives such as an antiwear agent, metallic detergent, ashless dispersant and oxidation inhibitor are incorporated into the engine oil for the purpose of satisfying the various properties required thereof.

An important basic function of the engineer oil is to insure smoothly ninning of engine while preventing it from wear and seizure under any conditions. Although a fluid lubrication state is usually realized in an engine lubricating part, a boundary lubrication state is apt to be realized in a valve mechanism system or at top and bottom dead centers of a piston. The

- 2 -

antiwear properties of the engine oil in the boundary lubrication state are usually realized by the addition of a zinc dialkyldithiophosphate (ZnDTP).

However, ZnDTP has a problem that, since phosphorus (P) contained therein poisons catalyst and oxygen sensors, exhaust gas-control systems are deteriorated. Further, the wear resistance is inclined to be lost by an interaction between ZnDTP and other additives such as succinimide and alkylamine. Under these circumstances, a lubricating oil composition having a minimum phosphorus content or free from phosphorus is required.

On the other hand, a friction modifier is added to a lubricating oil to be used for a gasoline engine in order to reduce the friction loss and fuel consumption, since the energy loss is high in friction parts in which the lubricatng oil is concerned in the gasoline engine. As the friction modifier, and extreme-pressure agent such as a molybdenum sulfϊde dialkyldithiocarbamate or an oiliness agent such as octadecylamine is used, the molybdenum sulfide dialkyldithiocarbamate which is a dithiocarbamate-type organomolybdenum compound has a particularly excellent effect of reducing the friction. However, the wear resistance and friction characteristics of the molybdenum sulfide dialkyldithiocarbamate used hitherto are yet insufficient, and further improve- ent in the properties of engine oils is demanded, since more reduction in the fuel consumption is eagerly demanded now.

Present Invention

The object of the present invention is to provide a lubricating oil composition excellent in both wear resistance and friction characteristics.

The object of the present invention is to provide a lubricating oil composition containing an extreme-pressure agent which is a dithiocarbamate- type organometallic compound having friction characteristics and wear resistance superior to those of an conventional extreme-pressure agent.

Another object of the present invention is to provide a lubricating oil composition which contains also a dithiophosphate-type organometallic compound to further improve the wear resistance and friction characteristics.

It was noted that an ordinary dithiocarbamate-type organometal compound such as a molybdenum sulfide dialkyldithiocarbamate has such a structure that the alkyl group is bonded to the nitrogen atom through its primary carbon atoms (namely, -CH2-N<). After intensive investigations on the alkyl group, it was found that when a metal dithiocarbamate which is a dithio- arbamate-type organometallic compound having such a structure that the alkyl group is bonded to the nitrogen atom through its secondary carbon atom (namely, >CH-N<), the metal atom being molybdenum or tungsten, is incorporated into a lube base oil, wear resistance and friction characteristics more excellent than those of a corresponding conventional lubricating oil can be obtained surprisingly.

The friction characteristics and wear resistance of the lubricating oil composition of the present invention are further improved by adding ZnDTP and/or zinc dithiocarbamate (ZnDTC), in addition to the above-described metal dithiocarbamate, to the lube base oil. When the metal dithiocarbamate of the present invention is used in combination with ZnDTP, ZnDTP content can be reduced to enable reduction of the phosphorus content, since the metal dithio¬ carbamate is excellent in the wear resistance and friction characteristics. Further, when the metal dithiocarbamate of the present invention is used solely or in combination with ZnDTC without using ZnDTP, the phosphorus-free lubricating oil composition can be obtained.

The coefficient of friction of the lubricating oil composition of the present invention can be further reduced by adding a fatty acid ester and/or an organic amide compound. As a matter of course, various other additives for lubricating oils can be added, if desired, to the lubricating oil composition of the present invention.

The lubricating oil composition of the present invention is usable in wide fields in which excellent wear resistance and friction characteristics are necessitated, such as in the field of engine oils.

The present invention has been completed on the basis of these findings.

Thus, the present invention provides

1. a lubricating oil composition comprising a lube base oil and 0.01 to 10% by weight, based on the total weight of the composition, of a metal dithiocarbamate (A) represented by the general formula (1):

wherein M represents a molybdenum atom or tungsten atom, Rj, R , R3 and R4 may be the same or different from one another and each represent a hydrogen atom, saturated or unsaturated C1-C20 al yl group, C6-C26 cycloalkyl or alkyl-substituted cycloalkyl group, C6-C26 aryl, alkylaryl or arylalkyl group, or such a hydrocarbon group as above which further contains an ester bond, ether bond, hydroxyl group or carboxyl group, with the proviso that at least one of the four symbols Rj, R2, R3 and R4 must be a C5-C20 alkyl group having a secondary carbon atom, through which the group is bonded to a nitrogen atom (N) in the formula, X\ and X2 each represent an oxygen or sulfur atom, and Yj and Y2 each represent an oxygen or sulfur atom.

The present invention provides the following lubricating oil compositions 2 to 6 as preferred embodiments:

2. a lubricating oil composition which contains a lube base oil and, in addition to the above-mentioned metal dithiocarbamate (A), 0.01 to 7% by weight, based on the total weight of the composition, at least one organozinc compound selected from the group consisting of zinc dithiophosphates (B) of the general formula (2):

wherein R5, R , R7 and R$ may be the same or different from one another and each represent a hydrogen atom, saturated or unsaturated C1-C20 alkyl group, C6-C26 cycloalkyl or alkyl substituted cycloalkyl group, C6-C26 aryl, alkylaryl or arylalkyl group, or such a hydrocarbon group as above which further contains an ester bond, ether bond, hydroxyl group or carboxyl group, and zinc dithiocarbamates (C) of the general formula (3):

wherein R^, R ¹ ^ R11 and W - may be the same or different from one another and each represent a hydrogen atom, saturated or unsaturated C1-C20 alkyl group, C6-C25 cycloalkyl or alkyl-substituted cycloalkyl group, C6-C26 aryl, alkylaryl or arylalkyl group, or such a hydrocarbon group as above which further contains as ester bond, ether bond, hydroxyl group or carboxyl group,

3. a lubricating oil composition according to above item 1 or 2, wherein at least two of R ¹ , R2, R3 and R"4 in the general formula (1) for the metal dithiocarbamate (A) each represent an alkyl group having a secondary carbon atom, through which the group is bonded to the nitrogen atom (N) in the formula,

4. a lubricating oil composition according to above item 1 or 2, wherein at least three of \\\, R^, R3 and R^ in the general formula (1) for the metal dithiocarbamate (A) each represent an alkyl group having a secondary carbon atom, through which the group is bonded to the nitrogen atom (N) in the formula,

5. a lubricating oil composition according to any of above items 1 to 4, wherein R ¹ , R^, R3 _? and R4 in the general formula (1) for the metal dithiocarbamate (A) each represent a C5-C20 alkyl group, and

6. a lubricating oil composition according to any of above items 1 to 5, which comprises a lube base oil containing a fatty acid ester, a

organic amide or a mixture of fatty acid ester and organic anide in addition to the metal dithiocarbamate (A) or in addition to the combination of the metal dithiocarbamate (A) with zinc dithiophosphate (B) and or zinc dithiocarbamate (C).

The detailed description will be made on the present invention.

Base Oil:

The base oils usable in the present invention are not particularly limited, and include known mineral oils and synthetic oils. The mineral oils include raffinates obtained by solvent refining of a starting material for the lubricating oil with an aromatic solvent such as phenol or furfural; hydrotreated oils obtained by the hydrotreatment of the starting material with a hydrogena- tion catalyst such as cobalt or molybdenum supported on a silica/alumina carrier; and mineral oils such as a lubricating oil fraction obtained by isomerization of a wax. They include, for example, 60 neutral oil, 100 neutral oil, 150 neutral oil, 300 neutral oil, 500 neutral oil and bright stock. Examples of the synthetic oils include poly-a-olefins, polybutenes, alkylbenzenes, poly- alkylene glycol esters, polyol esters and dibasic acid esters. These base oils can be used singly or in the form of a mixture of two or more of them. A mixture of such a mineral oil and synthetic oil is also usable. The kinematic viscosity of the lube base oil is usually preferably in the range of 3 to 20 cSt at 100°C when the oil is used as the engine oil.

Metal dithiocarbamate (A

The metal dithiocarbamates used in the present invention are organometallic compounds represented by the above-described general formula

(1).

In the general formula (1), M represents a molybdenum atom (Mo) or tungsten atom (W). The molybdenum atom is particularly preferred to the tungsten atom.

X ¹ , X^, γl and Y^ represent an oxygen atom or sulfur atom independently from one another. These four atoms contain 0 to 3 oxygen atoms and 1 to 4 sulfur atoms in total.

R ¹ , R2, R3 and R"4 may be the same of different from one another and each represent a hydrogen atom, saturated or unsaturated C1-C20 alkyl group, C6-C26 cycloalkyl or alkyl-substituted cycloalkyl group, C6-C26 aryl, alkylaryl or arylalkyl group, or such a hydrocarbon group as above which further contains an ester bond, ether bond, hydroxyl group or carboxyl group. Each of R to R^ is preferably a C5-C20 alkyl group, more preferably C_-C\% alkyl group.

At least one of R*, R^, R3 and R^ must be a C5-C20 alkyl group having a secondary carbon atom, through which the group is bonded to a nitrogen atom (N) in the above general formula. Preferably at least two, more preferably at least three of R ¹ , R^, R3, or R each represent the C5-C20 alkyl group bonded to the nitrogen atom through the secondary carbon atom.

The alkyl groups having the secondary carbon atom include, for example, 1-methylpentyl, 1-ethylbutyl, 1-methylheptyl, 1-ethylhexyl, 1- methyloctyl, 1-ethylheptyl, 1-methylnonyl, 1-ethyloctyl, 1-methylundecyl, 1- ethyldecyl, 1-methylheptadecyl and 1-ethylhexadecyl groups.

These alkyl groups having the secondary carbon atom must be bonded to the nitrogen atom (N) in the general formula (1) through the secondary carbon atom. For example, 1-methylnonyl group is bonded to the nitrogen atom through the secondary carbon atom thereof as shown by the following formula:

Thus, for example, although 2-ethylhexyl group

[(CH ₃ (CH2)3CH(C ₂ H5)CH2-]

contains a secondary carbon atom, it is not included in the above-defined alkyl group having the specified secondary carbon atom, since it is bonded to the nitrogen atom through the primary carbon atom.

The groups other than the alkyl groups having the specified secondary carbon atoms are preferably C5-C20 hydrocarbon groups such as saturated or unsaturated alkyl groups and alkylaryl groups. Examples of such hydrocarbon groups include saturated alkyl groups and unsaturated alkyl groups (alkenyl groups) such as n-hexyl, 2-ethylhexyl, n-octyl, nonyl, decyl, lauryl, tridecyl, oleyl and linoleyl groups, and alkylaryl groups such as nonylphenyl groups. Among them, particularly preferred are C -Ci 8 alkyl groups.

These metal dithiocarbamates (A) can be used either singularly or in combination of two or more of them. The relative amount of the metal dithiocarbamate (A) is about 0.01 to 10% by weight, preferably 0.05 to 7a% by weight and more preferably 0.1 to 5% by weight, based on the total weight of the whole lubricating oil composition. When the relative amount of the metal dithiocarbamate (A) is insufficient, it is difficult to obtain sufficient friction characteristics and wear resistance and, on the contrary, an excess amount thereof is economically disadvantageous and causes reduction in the wear resistance.

Zinc dithiophosphate (B and zinc dithiocarbamate (C

In addition to the above-described metal dithiocarbamate (A), various lubricating oil additives can be incorporated into the lubricating oil composition of the present invention. Among them, a zinc dithiophosphate (B) of the above general formula (2) and/or zinc dithiocarbamate (C) of the above general formula (3) is particularly effective in further improving the wear resistance and reducing the coefficient of friction as intended in the present invention.

In the above-described general formula (2), each of R^ to R^ represents preferably a C2-C13 hydrocarbon group, more preferably C2-C13 alkyl group.

In the above-described general formula (3), each of R9 to R12 represents preferably a C2-C1 hydrocarbon group, more preferably C2-C13 alkyl group.

The relative amount of the zinc dithiophosphate (B) or zinc dithiocarbamate (C) or mixture thereof is about 0.01 to 7% by weight, preferably 0.03 to 5% by weight and more preferably 0.05 to 2% by weight, based on the total weight of the whole lubricating oil composition. When the relative amount of (B) or (C) or mixture thereof is insufficient, the effect obtained by the combination thereof with the metal dithiocarbamate (A) is insufficient for obtaining the sufficient effect of improving the wear resistance and reducing the coefficient of friction and, on the contrary, an excess amount thereof is economically disadvantageous and might impair the friction characteristics.

Other additives

The lubricating oil composition of the present invention may contain various additives, if desired, depending on the field in which it is used, such as engine oils. The additives include, for example, a friction modifier, metallic detergent, ashless detergent-dispersant, oxidation inhibitor, viscosity- index improver, rust inhibitor, antifoamer, corrosion inhibitor, pour-point depressant and wear resistant additive other than those described above.

Preferred examples of the friction modifiers include fatty acid esters such as fatty acid glycerol monoesters and diesters, and fatty acid/ sorbitan monoesters and diesters; and fatty acid amides such as oleamide and lauramide. When the fatty acid ester is used in an amount of usually 0.01 to 10% by weight, preferably 0.05 to 5% by weight, based on the total weight of the whole composition, the wear resistance and friction characteristics are further improved. When the organic amide compound is used in an amount of usually 0.01 to 5% by weight, preferably 0.05 to 2% by weight, based on the total weight of the whole composition, the copper-corroding properties are controlled and the coefficient of friction can be reduced from the initial stage.

The fatty acid glycerides are monoesters and diesters of the following general formulae (4) and (5):

CH ₂ 0C0R ¹³

(4) CHOH

I

CH ₂ OH

CH ₂ OCOR ¹⁴

CHOCOR ¹⁵

(5) I

CH ₂ OH

wherein R ^χ 3, R*4 and R ¹ ^ each represent a saturated or unsaturated C8-C22 alkyl group.

The fatty acid esters are usable either singularly or in combina¬ tion of two or more of them. When the metal dithiocarbamate (A) is used in combination with the fatty acid ester, wear resistance and friction characteristics more improved than those obtained when they are used separately can be obtained. Particularly when the fatty acid ester is added to the combination of the metal dithiocarbamate (A) and zinc dithiophosphate (B) and/or zinc dithiocarbamate (C), a lubricating oil composition having improved wear resistance and friction properties and a remarkably reduced coefficient of friction can be obtained.

The organic amide compounds are those represented by the following general formula (6):

wherein R*6 and R ¹ ^ may be the same or different from each other an each represent a hydrogen atom, C1-C20 alkyl group, C6-C26 cycloalkyl group, C6-C26 aryl, alkylaryl or arylalkyl group, or C2-C20 alkylene oxide group, and R ¹⁸ represents a hydrogen atom, C1-C20 alkyl group, C6-C26 cycloalkyl group, C6-C26 aryl, alkylaryl or arylalkyl group, or such a hydrocarbon group as above further containing an ester bond, ether bond, hydroxyl group or carboxyl group.

The alkylene oxide group in the general formula (6) are those represented by the following general formula (7) or (8):

wherein R ¹ ^ and R^O each represent a hydrogen atom or methyl group, and n represents an integer of 1 to 10 inclusive.

In the general formula (6), R ¹ ^ and R*? each preferably represent a hydrogen atom, C2-C6 alkyl group, Cg-C ^ cycloalkyl group, C8-C14 alkylaryl group or alkylene oxide group wherein n represents 1 to 5, and R ^l8 preferably represents a saturated or unsaturated C^-Cis alkyl group, C12-C24 cycloalkyl group or C12-C24 alkylaryl group.

The metallic detergents include, for example, phenates, sulfonates and phosphonates of barium (Ba), calcium (Ca) and magnesium (Mg). Among them, overbased Ca sulfonate, Mg sulfonate and Ca phenate and neutral Ca sulfonate are preferred. The metallic detergent is used usually in an amount of 0.1 to 10% by weight (based on the whole composition).

The ashless detergent-dispersants include, for example, benzyl- amine, boron derivatives of benzylamine, alkenylsuccinimides and boron derivatives of alkenylsuccinimides. The ashless detergent dispersant is used usually in an amount of 0.5 to 15% by weight (based on the whole composi¬ tion).

The oxidation inhibitors include, for example, amine oxidation inhibitors such as alkylated diphenylamines, pheyl-α-naphthylamines and alkylated α-naphthylamines; and phenolic oxidation inhibitors such as 2,6-di-t- butylphenol). The oxidation inhibitor is used usually in an amount of 0.05 to 2.0% by weight (based on the whole composition).

The viscosity-index improvers include, for example, polymethyl methacrylates, polyisobutylenes, ethylene/propylene copolymers and hydro- genated styrene-butadiene copolymers. The viscosity-index improver is used usually in an amount of 0.5 to 35% by weight (based on the whole composi¬ tion).

The rust inhibitors include, for example, alkenylsuccinic acids and partial esters of them, which can be suitably added to the composition.

The antifoamers include, for example, dimethyl polysiloxanes and polyacrylates, which can be suitably added to the composition.

The following Examples and Comparative Examples will further illustrate the present invention, which by no means limit the invention.

The properties were evaluated by a method described below.

Friction and Wear Characteristics

The wear track diameter (mm) and coefficient of friction were determined by the Shell four-ball test 30 min. after the initiation of the test. The test conditions were as follows:

load: 40 kg oil temperature: 90°C rotation rate: 1800 rpm, and test time: 30 minutes

Example 1:

A test oil was prepared by mixing a lube base oil and components given in Table 1. The components used were as follows:

(1) base oil:

mineral oil: 150 neutral oil having a kinematic viscosity at 100°C of 5.1 cSt (mm^/s), and synthetic oil: a mixed oil comprising 80% by weight of a poly-α-olefin and 20% by weight of diisodecyl adipate.

(2) Molybdenum dithiocarbamate: molybdenum sulfide dithiocarbamates of the general formula (1) where X* and X^ each represent an oxygen atom (O) and γl and Y^ each represent a sulfur atom (S) was used, having the following different hydrocarbon groups as indicated in Table 1 :

1. hydrocarbon group = p-C8-C 13 alkyl: commercially available primary alkyl groups (mixed C8-C 3 alkyl groups) - containing molybdenum sulfide dithiocarbamate,

2. hydrocarbon group = p-C 13 alkyl: commercially available primary alkyl groups (C13 alkyl groups) - containing molybdenum sulfide dithiocarbamate,

3. hydrocarbon group = 1-methylheptyl: molybdenum sulfide dithio¬ carbamate wherein R ¹ to R4 each represent a 1-methylheptyl group,

4. hydrocarbon group = 1-methylnonyl: molybdenum sulfide dithio¬ carbamate wherein R to R ⁴ each represent a 1-methylnonyl group,

5. hydrocarbon group = 50% of a p-C 13 alkyl and 50% of 1-methyl- nonyl: molybdenum sulfide dithiocarbamate where R to R4 each represent a mixed alkyl group comprising a primary alkyl group (C13 alkyl group) and 1-methylnonyl group in a ratio of 50:50,

(3) Zinc dithiophosphate: commercially available i-C3/s-C6 alkyl type, and

(4) Zinc dithiocarbamate: commercially available n-C5/i-C5 alkyl type.

In Table 1, the relative amount of each of the additives is given in terms of percent by weight based on the whole composition, and die balance is the base oil (total: 100% by weight). The amount of added molybdenum sulfide dithiocarbamate is given in terms of the molybdenum atom content. The results are given in Table 1.

TABLE 1

Zinc dithio¬ Zinc dithio¬ Wear track

Experiment Molybdenum Dithiocarbamate phosphate carbamate Diameter Coefficient Number Base Oil Hydrocarbon Group Mo Content ppm % % mm of Friction

1 * mineral oil — ~ 0.10 — 0.48 0.1 15

2* mineral oil P-CR-C Π alkyl 700 0.10 — 0.34 0.066

3* mineral oil p-C 1 -t alkyl 700 0.10 — 0.59 0.060

4 mineral oil 1-methylheptyl 300 0.10 — 0.45 0.048

5 mineral oil 1 -methylnonyl 700 0.10 — 0.48 0.043

6 mineral oil p-C _| 3 alkyl 50% 700 0.10 ~ 0.42 0.045 l-menthylnonyl 50%

7* mineral oil P-CR-C Π alkyl 700 — 0.10 0.55 0.075

8 mineral oil 1 -methylnonyl 700 — 0.10 0.43 0.057 g* synthetic oil P-CR-C Π alkyl 700 0.10 — 0.49 0.070

10 synthetic oil 1 -methylnonyl 700 0.10 — 0.42 0.050

Experiment Examples with the symbol "*" are comparative examples, and others are examples of the present invention.

It is apparent from Table 1 that the lubricating oil compositions of the present invention (Experiment Nos. 4 to 6, 8 and 10) containing the speci¬ fied secondary alkyl group have a wear resistance and a reduced coefficient of friction which were far superior to those of the lubricating oil compositions (Experiment Nos. 2, 3, 7 and 8) containing ordinary molybdenum sulfide di _¬ thiocarbamate bonded to the nitrogen atom through the primary carbon atom.

Example 2

A test oil was prepared by mixing a lube base oil with components given in Table 2. The components used were as follows:

(1) the base oil, molybdenum sulfide dithiocarbamate and zinc dithio¬ phosphate were the same as those identified in Example 1.

(2) Fatty acid glyceride:

CH ₂ OCOR CH ₂ OCOR

I I

CHOH 50 wt% CHOCOR 50 wt%

I I

CH ₂ OH CH ₂ OH

wherein R represents an oleyl group.

(3) Organic amide compound: oleamide

The results are given in Table 2

TABLE 2

Zinc dithio¬ Fatty Acid Fatty Acid Coefficient

Experiment Molybdenum Dithiocarbamate phosphate Glyceride Amide of Number Base Oil Hydrocarbon Group Mo Content ppm % % % Friction

1 1 mineral oil 1 -methylnonyl 700 0.10 0.10 — 0.039

12 mineral oil p-C 13 alkyl 50% 700 0.10 0.10 ~ 0.043 1 -methylnonyl 50%

I

13* mineral oil p-C _π alkyl 700 0.10 0. 10 0.059

14 mineral oil 1 -methylnonyl 700 0.10 — 0.10 0.040

15 synthetic oil 1 -methylnonyl 700 0.10 0.10 — 0.048

Experiment Examples with the symbol "*" are comparative examples, and others are examples of the present invention.

It is apparent from the results given in Table 2 that the coefficient of friction is further reduced when the fatty acid ester or fatty acid amide is added.

According to the present invention, the lubricating oil composi¬ tion having wear resistance and friction characteristics far superior to those of a conventional metal sulfide dithiocarbamate containing composition can be provided. The lubricating oil composition of the present invention can be minimized in the phosphorus content or made phosphorus-free. Thus, the lubricating oil composition of the present invention can be used in various fields such as a field of lubricating oils for internal combustion engines, etc.