LUBRICATING OIL COMPOSITION

Abstract

 A lubricating oil composition contains (A) a base oil for a lubricating oil in a range of 5 mass% to 70 mass% based on a total amount of the composition, the base oil containing a sulfur content of 0.03 mass% or less and having a kinematic viscosity at 100 degrees C in a range of 0.5 mm²/s to 1.5 mm²/s, and (B) a poly-alpha-olefm in a range of 15 mass% to 30 mass% based on the total amount of the composition, the poly-alpha-olefin having a kinematic viscosity at 100 degrees C of 150 mm²/s or more.

Description

TECHNICAL FIELD

[0001] The present invention relates to a lubricating oil composition.

BACKGROUND ART

[0002] In recent years, due to a global issue of carbon dioxide emission and worldwide increase of energy demand, saving automobile fuel has been demanded more and more. Under such circumstances, it has been demanded that a transmission (i.e., a component of an automobile) also contributes to fuel-saving more than ever.
For instance, one of fuel-saving methods of the transmission is lowering a viscosity of a lubricating oil. However, the low-viscosity lubricating oil exhibits a low oil-film retention at high temperatures, thereby occasionally generating seizure, shortening a fatigue life and the like.

[0003] Patent Literature 1 reports a lubricating oil composition capable of maintaining a gear shifting performance for a long period of time, in which various additives are contained for optimization. However, since the invention disclosed in Patent Literature 1 is not directed to fuel-saving, a kinematic viscosity of the lubricating oil composition is high and a fatigue life thereof when the viscosity is lowered has not been studied.
Currently, a lubricating oil composition having a viscosity index of about 150 to 250 is commercially available. However, in order to further promote fuel-saving, decreasing a viscosity at low temperatures while maintaining a viscosity at high temperatures, in short, viscosity-index improvement has been demanded from the viewpoint of low-temperature startability.
For instance, a viscosity index of a lubricating oil composition disclosed in Patent Literature 2 is improved with polymethacrylate (PMA) as a viscosity index improver. Moreover, in lubricating oil compositions disclosed in Patent Literatures 3 and 4, a viscosity property is improved with a high-viscosity synthetic oil (poly-alpha-olefin: PAO) and an olefin copolymer (OCP) effective for improving a fatigue life is further contained.

CITATION LIST

PATENT LITERATURE(S)

[0004] 

Patent Literature 1: JP-A-2001-262176

Patent Literature 2: JP-A-2006-117852

Patent Literature 3: JP-A-2008-208220

Patent Literature 4: JP-A-2008-208221

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] Although it is effective that the lubricating oil composition contains the viscosity index improver in order to increase the viscosity at high temperatures, it is feared that abrasion and fatigue are caused by a large decrease in the viscosity at the time of shearing (i.e., a decrease in shear stability) and a consequent decrease in oil-film formability. Moreover, since fatigue-life improvement and viscosity-index improvement are in inverse proportion to each other, the lubricating oil composition of Patent Literature 2 has a poor oil-film retention whereas having an improved viscosity index. In Patent Literatures 3 and 4, although OCP is blended in the lubricating oil composition, the lubricating oil composition does not exhibit the viscosity index that is equal to or exceeds the viscosity index of PAO. Thus, the viscosity-index improvement has not been studied. In other words, technique to improve the viscosity index while having a sufficient fatigue life has been neither realized nor studied.

[0006] Accordingly, an object of the invention is to provide a lubricating oil composition that is adapted to maintain a required shear stability, exhibit an excellent low-temperature viscosity property and a high fuel-saving performance, and maintain a fatigue life.

MEANS FOR SOLVING THE PROBLEMS

[0007] As result of dedicated studies for solving the problem, the inventor has found that the following combination of a specific base oil and a specific additive can solve the problem, and has achieved the invention.

[0008] Specifically, the invention provides a lubricating oil composition as follows.
(1) A lubricating oil composition according to an aspect of the invention contains (A) a base oil for a lubricating oil in a range of 5 mass% to 70 mass% based on a total amount of the composition, the base oil containing a sulfur content of 0.03 mass% or less and having a kinematic viscosity at 100 degrees C in a range of 0.5 mm²/s to 1.5 mm²/s, and (B) a poly-alpha-olefin in a range of 15 mass% to 30 mass% based on the total amount of the composition, the poly-alpha-olefin having a kinematic viscosity at 100 degrees C of 150 mm²/s or more.

[0009] (2) The lubricating oil composition according to the above aspect of the invention further contains silicone oil having a kinematic viscosity at 100 degrees C in a range of 0.5 mm²ls to 4 mm²/s_.

[0010] (3) The lubricating oil composition according to the above aspect of the invention further contains a polymethacrylate having a mass average molecular weight (Mw) of 100000 or more.

[0011] (4) The lubricating oil composition according to the above aspect of the invention further contains a poly-alpha-olefin having a kinematic viscosity at 100 degrees C in a range of 1.5 mm²/s to 2.5 mm²/s.

[0012] (5) In the lubricating oil composition according to the above aspect of the invention, the kinematic viscosity at 100 degrees C of the component (A) is in a range of 0.7 mm²/s to 1.3 mm²/s, and the kinematic viscosity at 100 degrees C of the component (B) is in a range of 150 mm²/s to 400 mm²/s, the lubricating oil composition further containing: a polymethacrylate having a mass average molecular weight of 300000 or more in a range of 0.1 mass% to 5.0 mass% based on the total amount of the composition; and a poly-alpha-olefin having the kinematic viscosity at 100 degrees C in a range of 1.7 mm²/s to 2.1 mm²/s in a range of 0.1 mass% to 60 mass% based on the total amount of the composition.

[0013] (6) In the lubricating oil composition according to the above aspect of the invention, the lubricating oil composition is used for a transmission.

ADVANTAGES OF THE INVENTION

[0014] According to the invention, a lubricating oil composition maintaining shear stability on the conventional level and exhibiting an excellent low-temperature viscosity property can be provided. Moreover, when the lubricating oil composition is used as a lubricating oil for a transmission and the like, fuel-saving property is high and a fatigue life is maintainable.

DESCRIPTION OF EMBODIMENT(S)

[0015] A lubricating oil composition of the invention contains the aforementioned components. The invention will be described below in detail.

Component (A)

[0016] The lubricating oil composition according to the invention contains a base oil for a lubricating oil in a range of 5 mass% to 70 mass% based on the total amount of the composition as the component (A), in which the base oil contains a sulfur content of 0.03 mass% or less and has a kinematic viscosity at 100 degrees C in a range of 0.5 mm²/s to 1.5 mm²/s, preferably of 0.7 mm²/s to 1.3 mm²/s.
When the sulfur content in the component (A) exceeds 0.03 mass%, oxidation stability is deteriorated.
When the kinematic viscosity at 100 degrees C is less than 0.5 mm²/s, vaporizability may be significantly increased to raise consumption of the lubricating oil. When the kinematic viscosity at 100 degrees C exceeds 1.5 mm²/s, the viscosity index is decreased.
The viscosity index of the lubricating oil composition is decreased when the content of the base oil is less than 5 mass%. The viscosity thereof is decreased when the content of the base oil exceeds 70 mass%. Accordingly, such a lubricating oil composition is not preferable for use in a transmission since abrasion of mechanical components may be increased.

[0017] Although both a mineral oil and a synthetic oil are usable as the base oil as the component (A), the mineral oil is preferably usable because of its lower viscosity.
The mineral oil is subject to no limitation as long as being generally usable as the base oil for the lubricating oil for the transmission. Preferable examples of the mineral oil include paraffinic and naphthenic base oils which can be obtained by subjecting a lubricating oil fraction produced by atmospheric- and vacuum-distillation of a crude oil, to any suitable combination of refining processes including solvent-deasphalting, solvent-extracting, hydrocracking, solvent-dewaxing, catalytic-dewaxing, hydrorefining, sulfuric acid treatment and clay treatment.

[0018] As the component (A), the mineral oil and synthetic oil having the kinematic viscosity at 100 degrees C in the above range may be used alone or in a mixture of two or more selected from the mineral oil and synthetic oil at any rate.

Component (B)

[0019] The lubricating oil composition according to the invention contains a poly-alpha-olefin in a range of 15 mass% to 30 mass% based on the total amount of the composition as the component (B), in which the poly-alpha-olefin has a kinematic viscosity at 100 degrees C of 150 mm²/s or more, preferably in a range of 150 mm²/s to 400 mm²/s.
When the kinematic viscosity at 100 degrees C is less than 150 mm²/s, the viscosity index is decreased. Accordingly, the lubricating oil composition is not preferable for use in a transmission since abrasion of mechanical components may be increased.
The viscosity index of the finished oil is decreased when the content of the poly-alpha-olefin is less than 15 mass%. Accordingly, the finished oil is not preferable for use in a transmission since abrasion of mechanical components may be increased. When the content of the poly-alpha-olefin exceeds 30 mass%, the viscosity may be increased to unfavorably increase friction loss.

[0020] The lubricating oil composition according to the invention may contain the components (A) and (B), thereby maintaining shear stability on the conventional level and exhibiting an excellent low-temperature viscosity property.

Other Components

[0021] In addition to the above components (A) and (B), the lubricating oil composition according to the invention further contains the following components, thereby enhancing the low-temperature viscosity property.
When silicone oil is blended, it is preferable to use silicone oil having a kinematic viscosity at 100 degrees C in a range of 0.5 mm²/s to 4 mm²/s. A flash point of the lubricating oil composition may be unfavorably significantly decreased when the kinematic viscosity at 100 degrees C is less than 0.5 mm²/s. The viscosity index may be unfavorably decreased when the kinematic viscosity at 100 degrees C exceeds 4 mm²/s.
The content of the silicone oil is preferably in a range of 5 mass% to 50 mass% based on the total amount of the composition. Conventionally, the silicone oil exhibits a poor solubility although exhibiting a high viscosity index. When the lubricating oil composition according to the invention contains the components at the respective contents as described above, the viscosity index of the lubricating oil composition is improvable without deteriorating the solubility thereof. However, when the silicone oil exceeds 50 mass%, the solubility of the lubricating oil composition is decreased, so that the silicone oil may be separated or precipitated.

[0022] It is preferable that the lubricating oil composition according to the invention further contains a polymethacrylate having a mass average molecular weight of 100000 or more. Although the polymethacrylate is blended so as to improve the viscosity index, the polymethacrylate may not provide a sufficient viscosity index improvability with the mass average molecular weight of less than 100000. The content of the polymethacrylate is preferably in a range of 0.1 mass% to 5.0 mass% based on the total amount of the composition. The polymethacrylate may not provide a sufficient viscosity index improvability with the content of less than 0.1 mass%. The polymethacrylate may lower the shear stability of the lubricating oil composition with the content of more than 5.0 mass%.

[0023] It is preferable that the lubricating oil composition according to the invention further contains a poly-alpha-olefin preferably having a kinematic viscosity at 100 degrees C in a range of 1.5 mm²/s to 2.5 mm²/s, more preferably of 1.7 mm²/s to 2.1 mm²/s.
The vaporizability may be unfavorably increased when the kinematic viscosity at 100 degrees C is less than 1.5 mm²/s. The viscosity of the lubricating oil composition may be increased to raise the friction loss when the kinematic viscosity at 100 degrees C exceeds 2.5 mm²/s.
The content of PAO is preferably in a range of 0.1 mass% to 60 mass% based on the total amount of the composition. The viscosity index of the lubricating oil composition may be lowered when the content of PAO exceeds 60 mass%.

Other Additives

[0024] The lubricating oil composition according to the invention may be added as necessary with other additives such as a detergent, an ashless dispersant, an antiwear agent, a friction modifier, a rust inhibitor, a metal deactivator, an antifoaming agent, an antioxidant and a coloring agent, as long as advantages of the invention are not hampered.

[0025] The detergent is exemplified by a metal detergent such as a neutral metal sulfonate, a neutral metal phenate, a neutral metal salicylate, a neutral metal phosphonate, a basic sulfonate, a basic phenate, a basic salicylate, an overbased sulfonate, an overbased salicylate and an overbased phosphonate. The content of the detergent is preferably approximately in a range of 0.01 mass% to 10 mass% based on the total amount of the composition.
Examples of the ashless dispersant include: succinimides; boron-containing succinimides; benzil amines; boron-containing benzil amines; succinates; and monovalent or divalent carboxylic amides represented by fatty acid or succinic acid. The content of the ashless dispersant is preferably approximately in a range of 0.1 mass% to 20 mass% based on the total amount of the composition.

[0026] Examples of the antiwear agent include: a sulfur antiwear agent such as a salt of thiophosphoric acid and a metal (e.g., Zn, Pb, Sb) and a salt of thiocarbamic acid and a metal (e.g., Zn); and a phosphorus antiwear agent such as a phosphate ester (tricresyl phosphate). The content of the antiwear agent is preferably approximately in a range of 0.05 mass% to 5 mass% based on the total amount of the composition.
The friction modifier is exemplified by a partial ester of polyhydric alcohol such as neopentyl glycol monolaurate, trimethylolpropane monolaurate, and glycerin monooleate (monoglyceride oleate). The content of the friction modifier is preferably approximately in a range of 0.05 mass% to 4 mass% based on the total amount of the composition.

[0027] Examples of the rust inhibitor are a fatty acid, an alkenyl succinic acid half ester, a fatty acid soap, an alkyl sulfonate, a fatty acid ester of polyhydric alcohol, a fatty acid amide, an oxidized paraffin, and an alkyl polyoxyethylene ether. The content of the rust inhibitor is preferably approximately in a range of 0.01 mass% to 3 mass% based on the total amount of the composition.
Examples of the metal deactivator include benzotriazole, a benzotriazole derivative, triazole, a triazole derivative, imidazole, an imidazole derivative and thiadiazole, which are used alone or in combination of two or more thereof. The content of the metal deactivator is preferably approximately in a range of 0.01 mass% to 5 mass% based on the total amount of the composition.

[0028] Examples of the antifoaming agent include a silicone compound and an ester compound, which may be used alone or in a combination of two or more. The content of the antifoaming agent is preferably approximately in a range of 0.05 mass% to 5 mass% based on the total amount of the composition.
Preferable examples of the antioxidant include: a hindered phenolic antioxidant, an amine antioxidant, and zinc alkyldithiophosphate (ZnDTP). A bisphenol antioxidant and an ester group-containing phenol antioxidant are particularly preferable as the phenolic antioxidant. A dialkyldiphenylamine antioxidant and a naphthylamine antioxidant are preferable as the amine antioxidant. The content of the antioxidant is preferably approximately in a range of 0.05 mass% to 7 mass%.

[0029] The lubricating oil composition containing the above components according to the invention satisfies the requirements of the components, the content and the viscosity described above, whereby the low-temperature viscosity can be significantly decreased while the high-temperature viscosity and the viscosity after shearing can be maintained on the conventional level. Accordingly, when used as a lubricating oil composition for a transmission, the lubricating oil composition according to the invention provides advantages of a high fuel-saving performance and maintainability of a fatigue life, and does not impair machine durability and reliability.

Examples

[0030]  Next, examples of the invention will be described below in detail. However, it should be noted that the scope of the invention is by no means limited by the examples.

Examples 1 to 5 and Comparatives 1 to 5

[0031] Lubricating oil compositions were prepared according to the blend composition set forth in Table 1. The prepared compositions were measured in terms of a kinematic viscosity at 100 degrees C, a kinematic viscosity at -40 degrees C and a viscosity after shearing according to the following method.
The components described in Table 1 are as follows.
Mineral oil-1: Mineral oil containing a sulfur content of 0.03 mass% or less and having a kinematic viscosity at 100 degrees C of 1.0 mm²/s and a kinematic viscosity at 40 degrees C of 2.6 mm²/s
Mineral oil-2: Mineral oil containing a sulfur content of 0.03 mass% or less and having a kinematic viscosity at 100 degrees C of 6.5 mm²/s and a kinematic viscosity at 40 degrees C of 37 mm²/s
PAO-1: PAO having a kinematic viscosity at 100 degrees C of 300 mm²/s
PAO-2: PAO having a kinematic viscosity at 100 degrees C of 1.8 mm²/s
PAO-3: PAO having a kinematic viscosity at 100 degrees C of 100 mm²/s
Ester: Butoxyethyl oleate ester having a kinematic viscosity at 100 degrees C of 100 mm²/s
Silicone 1: Polydimethyl siloxane having a kinematic viscosity at 100 degrees C of 0.9 mm²/s
Silicone 2: Polydimethyl siloxane having a kinematic viscosity at 100 degrees C of 3.4 mm²/s
PMA-1: Polymethacrylate having a mass average molecular weight of 500000
PMA-2: Polymethacrylate having a mass average molecular weight of 20000
Other Additives: Detergent (e.g., Ca sulfonates), Dispersant (e.g., succinimides), Extreme pressure additive and Antiwear agent (e.g., sulfides, phosphate compounds, sulfurated phosphate compounds), Antifoaming agent, Copper deactivator, etc.

Viscosity Index

[0032] Measurement was conducted according to JIS K2283.

Kinematic Viscosity at 100 degrees C and -40 degrees C

[0033] Measurement was conducted according to JIS K2283.

Kinematic Viscosity Before and After Shearing

[0034] Kinematic viscosities at 140 degrees C before and after the 30-hour test were measured according to JASO M-347.

[0035] 

Evaluation Results

[0036] Table 1 shows that the kinematic viscosity at -40 degrees C was significantly decreased while the kinematic viscosity at 100 degrees C and the kinematic viscosity after shearing were sufficiently maintained in Examples of the invention.
It is understood from Examples 2 and 3 that blending silicone significantly decreases the kinematic viscosity at -40 degrees C.
With respect to selection from the mineral oils and PAO of different viscosities, the content of each of the mineral oils and PAO and the content of PMA, it is understood from Examples 4 and 5 and Comparatives 2 to 5 that the advantages of the invention (i.e., to maintain the high-temperature kinematic viscosity and the kinematic viscosity after shearing and to significantly decrease the low-temperature kinematic viscosity) are attained when the components and the respective contents of the invention are satisfied. In Comparatives 3 and 4, blending the polymethacrylate improves the viscosity index, but decreases the viscosity after shearing. Accordingly, it is feared that abrasion resistance and anti-fatigue life may be decreased.

INDUSTRIAL APPLICABILITY

[0037] The invention is usable as a lubricating oil composition, particularly suitably usable as a lubricating oil for a transmission.

Claims

1. A lubricating oil composition comprising:

(A) a base oil for a lubricating oil in a range of 5 mass% to 70 mass% based on a total amount of the composition, the base oil comprising a sulfur content of 0.03 mass% or less and having a kinematic viscosity at 100 degrees C in a range of 0.5 mm²/s to 1.5 mm²/s; and

(B) a poly-alpha-olefin in a range of 15 mass% to 30 mass% based on the total amount of the composition, the poly-alpha-olefin having a kinematic viscosity at 100 degrees C of 150 mm²/s or more.

2. The lubricating oil composition according to claim 1, further comprising:

silicone oil having a kinematic viscosity at 100 degrees C in a range of 0.5 mm²/s to 4 mm²/s_.

3. The lubricating oil composition according to claim 1 or 2, further comprising:

a polymethacrylate having a mass average molecular weight of 100000 or more.

4. The lubricating oil composition according to any one of claims 1 to 3, further comprising:

a poly-alpha-olefin having a kinematic viscosity at 100 degrees C in a range of 1.5 mm²/s to 2.5 mm²/s.

5. The lubricating oil composition according to claim 1, wherein
the kinematic viscosity at 100 degrees C of the component (A) is in a range of 0.7 mm²/s to 1.3 mm²/s, and
the kinematic viscosity at 100 degrees C of the component (B) is in a range of 150 mm²/s to 400 mm²/s,
the lubricating oil composition further comprising:

a polymethacrylate having a mass average molecular weight of 300000 or more in a range of 0.1 mass% to 5.0 mass% based on the total amount of the composition; and

a poly-alpha-olefin having the kinematic viscosity at 100 degrees C in a range of 1.7 mm²/s to 2.1mm²/s in a range of 0.1 mass% to 60 mass% based on the total amount of the composition.

6. The lubricating oil composition according to any one of claims 1 to 5, wherein the lubricating oil composition is used for a transmission.