ENGINES

Field of the Invention

The present invention relates to a lubricating oil composition for internal combustion engines, which contains prescribed components.

Background of the Invention

Many lubricating oil compositions have been proposed in the past. For example, WO2005014763 proposes a lubricating oil that contains a lubricant base oil and a sulfur-containing molybdenum complex. Meanwhile, "Wear Analysis of DLC Coating in Oil Containing Mo-DTC", Takatoshi Shinyoshi, Yoshio Fuwa, Yoshinori Ozaki, JSAE 20077103 SAE 2007-01-1969 proposes a case in which wear of a film is facilitated when MoDTC is combined with a hydrogen-containing DLC film (a-C:H) .

In the case of the lubricating oil composition according to WO2005014763, however, evaluations are conducted using a real engine. Friction reduction performance evaluations are conducted on systems in which both DLC contacting faces and non-DLC contacting faces not having a DLC film, such as ordinary steel materials for engines, are simultaneously lubricated, but if a steel material lubrication effect is greater than a DLC contacting face lubrication effect, it is not clear whether this is suitable for DLC films. In addition, a means for solving the problem in "Wear Analysis of DLC Coating in Oil Containing Mo-DTC", Takatoshi Shinyoshi, Yoshio Fuwa, Yoshinori Ozaki, JSAE 20077103 SAE 2007-01-1969 is not given .

With these circumstances in mind, the purpose of the present invention is to provide a lubricating oil composition for internal combustion engines, which can realize low friction in both internal combustion engines equipped with DLC and internal combustion engines not equipped with DLC.

Summary of the Invention

As a result of diligent research carried out in order to achieve this purpose, the inventors of the present invention found that by blending a polyalkylene glycol (PAG) and a specific organic molybdenum compound as additional components in a lubricating oil composition for internal combustion engines, specifying the molecular weight of the PAG and specifying the blending quantity of the PAG, it was possible to realize low friction in internal combustion engines regardless of the presence or absence of DLC, and thereby completed the present invention. That is, the present invention is a lubricating oil composition for internal combustion engines, which is characterized in that the lubricating oil composition for internal combustion engines is a composition obtained by blending a molybdenum dithiocarbamate (MoDTC) and a polyalkylene glycol (PAG) in a base oil, the weight average molecular weight of the polyalkylene glycol is 2750-4500, and the content of the polyalkylene glycol is not less than 0.005 mass% and less than 10 mass% relative to the total mass of the composition.

Furthermore, the lubricating oil composition for internal combustion engines of the present invention may be a lubricating oil composition for internal combustion engines that can be used in both internal combustion engines equipped with DLC and internal combustion engines not equipped with DLC (conventional internal combustion engines) .

Detailed Description of the Invention

According to the present invention, it is possible to provide a lubricating oil composition for internal combustion engines, which can be used in both internal combustion engines equipped with DLC and internal combustion engines not equipped with DLC. Explanations will now be given of the constitution, properties and intended uses of lubricating oil compositions for internal combustion engines according to the present aspect .

In the lubricating oil compositions for internal combustion engines according to the present aspect, a molybdenum dithiocarbamate (MoDTC) and a polyalkylene glycol (PAG) are blended in a base oil, and other additives may, if necessary, also be blended in the base oil. A detailed explanation will now be given of the lubricating oil compositions for internal combustion engines according to the present aspect, but the present invention is in no way limited to these compositions.

The base oil used in the present aspect is not particularly limited, and mineral oils, synthetic oils, vegetable and animal oils and mixtures thereof used in conventional lubricating oils and grease compositions can be used as appropriate. Specific examples thereof include base oils of groups 1 to 5 in the API (American Petroleum Institute) base oil categories. Here, API base oil categories are broad classifications of base oil materials defined by the American Petroleum Institute in order to prepare guidelines for lubricant base oils. In order to achieve excellent oxidation stability, a base oil belonging to group 3 is preferred.

The kinematic viscosity of the base oil is not particularly limited, but from practical perspectives such as preventing wear and achieving fuel efficiency, the kinematic viscosity at 100°C is preferably 2-32 mm ² /s, and more preferably 3-8 mm ² /s.

The viscosity index of the base oil is not particularly limited, but from practical perspectives such as preventing wear and achieving fuel efficiency, the viscosity index is preferably 10-200, and more preferably 100-200.

The molybdenum dithiocarbamate (MoDTC) used in the present aspect can be, for example, a molybdenum dialkyldithiocarbamate represented by formula (1) below.

In formula 1, R ¹ to R ⁴ each denote an alkyl group, and X ¹ to X ⁴ each denote an oxygen atom or sulfur atom.

The alkyl groups R ¹ , R ² , R ³ and R ⁴ contained in the molybdenum dialkyldithiocarbamate represented by formula (1) are each independently a lipophilic group having 2-30 carbon atoms, and it is preferable for at least one of these four lipophilic groups to be a secondary lipophilic group.

Here, the molybdenum dithiocarbamate (MoDTC) used in the present aspect is preferably a molybdenum

dithiocarbamate represented by formula (2) below.

In formula (2), R ¹ to R ⁴ each denote an alkyl group.

The content of the molybdenum dithiocarbamate used in the present aspect is not particularly limited, but is preferably 50-1500 ppm in terms of molybdenum content relative to the total mass of lubricating oil composition.

The polyalkylene glycol (PAG) is a compound in which a plurality of alkylene glycols are polymerized, and is represented by the general formula HO- ( C _n H _m O) _S -H or the general formula HO- (C _n H _m O) _s -OH, but is not particularly limited. In order to use a material having low oil solubility, it is preferable to use one or more compounds selected from among polyethylene glycol, polypropylene glycol and polybutylene glycol. From the perspective of compatibility with the base oil, polypropylene glycol and polybutylene glycol are more preferred. In addition, the weight average molecular weight of the polyalkylene glycol according to the present aspect is 2750-4500, and preferably 3000-4000. By setting the weight average molecular weight of the polyalkylene glycol to fall within such a range, compatibility with the base oil is improved and the coefficient of friction can be lowered.

Furthermore, the polyalkylene glycol according to the present aspect is contained at a quantity of not less than 0.05 mass% and less than 10 mass%, preferably 0.5-8.0 mass%, and more preferably 1.0-5.0 mass%, relative to the total mass of the lubricating oil composition. By setting the content of the polyalkylene glycol to fall within such a range, compatibility with the base oil is improved and the lubricating properties of the polyalkylene glycol can be exhibited.

One or more of a variety of additives, such as ash-free dispersing agents, wear prevention agents, extreme pressure additives, metal-based cleaning agents, antioxidants, viscosity index-improving agents, friction modifiers, rust inhibitors, non-ionic surfactants, demulsifiers, metal deactivating agents and anti-foaming agents, can be blended as optional components in the lubricating oil compositions according to the present aspect . The HTHS viscosity of the lubricating oil composition of the present invention at 150°C and 10 ⁶ s ^_1 is preferably 3.5 mPa-s or less, more preferably 3.0 mPa-s or less, further preferably 2.8 mPa-s or less, and particularly preferably 2.7 mPa-s or less. In addition, this HTHS viscosity is preferably 1.4 mPa-s or more, more preferably 2.0 mPa-s or more, further preferably 2.3 mPa-s or more, particularly preferably 2.5 mPa-s or more, and most preferably 2.6 mPa-s or more. Moreover, this high temperature high shear viscosity is a numerical value determined using the test method disclosed in ASTM D5481.

The scope of use of the lubricating oil compositions according to the present aspect is not particularly limited as long as the compositions are used in internal combustion engines .

In particular, according to the lubricating oil compositions according to the present aspect, by setting the weight average molecular weight and content of the polyalkylene glycol to fall within specified ranges in the composition, which is obtained by blending a molybdenum dithiocarbamate and the polyalkylene glycol in a base oil, it is possible to reduce friction on surfaces not equipped with DLC as well as significantly reducing friction on surfaces equipped with DLC, and it is therefore possible to realize low friction regardless of the presence or absence of DLC . As a result, the lubricating oil composition according to the present aspect can be used in both internal combustion engines equipped with DLC and internal combustion engines not equipped with DLC (that is, can be used as a lubricating oil composition for internal combustion engines equipped with DLC and internal combustion engines not equipped with DLC) .

Moreover, an internal combustion engine equipped with DLC is an internal combustion engine in which all, or at least some, surfaces that are friction surfaces are coated with DLC. In addition, DLC (diamond-like carbon) generally means an amorphous substance constituted mainly from elemental carbon, and in which the bonding between carbon atoms comprises both a diamond structure (SP3 bonds) and graphite bonds (SP2 bonds) . Specifically, these include a-C (amorphous carbon) comprising only elemental carbon, hydrogen-containing a-C : H (hydrogenated amorphous carbon) and MeC, which partially contains a metal element such as titanium (Ti) or molybdenum (Mo) . Furthermore, the coefficient of friction tends to increase as the content of hydrogen in DLC increases, but it is possible to select an arbitrary hydrogen content, such as 10 atom% or less, 5 atom% or less, or 0.5 atom% or less. Examples

The present invention will now be explained in greater detail through the use of working examples and comparative examples, but is not limited to these examples. Preparation of lubricating oil composition

The raw materials used in the working examples are as follows .

Base oil

A GTL (gas-to-liquid) base oil synthesized by the Fischer-Tropsch process, which belongs to group 3, has a kinematic viscosity at 100°C of 7.58 mm ² /s, and has a viscosity index of 141.

Additives

PAG

PEG#400 (manufactured by NOF Corporation, PEG, weight average molecular weight 100)

D-250 (manufactured by NOF Corporation, PPG, OH groups at both terminals, weight average molecular weight 250) D-1000 (manufactured by NOF Corporation, PPG, OH groups at both terminals, weight average molecular weight 1000) D-2000 (manufactured by NOF Corporation, PPG, OH groups at both terminals, weight average molecular weight 2000) D-4000 (manufactured by NOF Corporation, PPG, OH groups at both terminals, weight average molecular weight 4000) MB7 (manufactured by NOF Corporation, PPG, OH group at one terminal, weight average molecular weight 700)

MB700 (manufactured by NOF Corporation, PPG, OH group at one terminal, weight average molecular weight 3000)

50MB-2 (manufactured by NOF Corporation, PPG-PEG, weight average molecular weight 200)

MoDTC

Sakuralube-165 (manufactured by ADEKA, molybdenum content 4.5 mass%, sulfur content 5.0 massl)

1. Viscosity index improving agent

Polymethacrylate-based viscosity index improving agent Packaged additives

GF-5DI (compositional details: metal cleaning agent, ash-free dispersing agent, zinc dithiophosphate, rust inhibitor, corrosion prevention agent, antioxidant, ash-free friction modifier, and the like) .

Lubricating oil compositions were obtained by blending and formulating the raw materials mentioned above at the proportions (mass percentages) shown in the Tables. Tests

Friction test

The lubricating oil compositions of the working examples and comparative examples were applied to sliding surfaces consisting of a DLC-coated sliding member, which was obtained by coating with a diamond-like carbon having a hydrogen content of 0.5 atom%, and a sliding member consisting of a SUJ2 material, and a friction test was carried out . Using a cylinder-on-disk SRV friction tester (ASTM D 6425 ) , the coefficient of friction was measured. The conditions are as follows: Temperature: 80°C, frequency: 50 Hz, load: 100 N. A similar friction test was carried out by applying the lubricating oil compositions of the working examples and comparative examples to sliding surfaces consisting of a SUJ2 sliding member not coated with a DLC and a sliding member consisting of a SUJ2 material. The results are shown in the tables.

As shown in the tables, the present invention achieves a friction-reducing effect on both metal contacting faces if a DLC film is present. Because a low coefficient of friction reduces the amount of frictional heat generated, it is possible to suppress softening of a surface caused by carbonization (an increase in the amount of SP2 bonding) caused by friction in the case of a DLC surface and improve the wear resistance of the DLC, and a reduction in frictional resistance leads to a reduction in stress within a DLC film and between a DLC film and the member therebelow, and lessens the problem of a DLC coating film detaching . In this way, the present invention contributes to fuel efficiency and maintenance of a favorable state in a DLC lubrication system.

According to the present invention, in the case of metal surfaces also, the organic molybdenum compound exhibits functionality, a reduction in friction occurs and fuel efficiency is realized even in a non-DLC lubricating system.

Table 1 - comparative Examples 1-8 and working Examples 1-2

Table 2 - comparative Examples 9-16 and working Example 3