WO2024195513A1

WO2024195513A1 - Gear oil composition

Info

Publication number: WO2024195513A1
Application number: PCT/JP2024/008237
Authority: WO
Inventors: 泰徳菅野; 亜喜良多田
Original assignee: Ｅｎｅｏｓ株式会社
Priority date: 2023-03-20
Filing date: 2024-03-05
Publication date: 2024-09-26

Abstract

A gear oil composition comprising (A) a base oil composition, (B) a specific thickening agent, and (C) a specific dispersant, wherein the component (A) contains a specific base oil (A-1) at a specific proportion, the amount of a mineral oil-based base oil contained in the component (A) is at a specific proportion or more, the component (B) is a specific poly(meth)acrylate, the component (C) is at least one selected from the group consisting of bis-type unmodified succinimides and bis-type boric acid-modified succinimides, the contained amounts of the component (B) and the component (C) are each in a specific range, and the kinematic viscosity of the gear oil composition at 40°C and the kinematic viscosity of the gear oil composition at 100°C are each not more than a specific value.

Description

Gear Oil Composition

The present invention relates to a gear oil composition.

Lubricating oil compositions are used for a variety of purposes, and various types of compositions have been researched to exhibit characteristics suited to each application. In this context, the use of lower viscosity lubricating oil compositions is being considered, particularly for lubricating oil compositions (gear oil compositions) used in gear devices, from the standpoints of environmental compatibility and energy efficiency (fuel economy, etc.).

For example, International Publication No. 2017/073748 (Patent Document 1) discloses that a lubricating oil composition containing a lubricating base oil; a dispersant-type poly(meth)acrylate compound; a thiadiazole compound; a polysulfide compound; and a friction modifier and having a kinetic viscosity at 100°C of 7.0 ^mm2 /s or more and 35.0 ^mm2 /s or less is suitable as a lubricating oil composition for gear oil.

In addition, JP 2021-080429 A (Patent Document 2) discloses that a lubricating oil composition containing (A) a lubricating base oil, (B) a viscosity index improver, (C) a sulfur-based extreme pressure agent, (D) a phosphorus-based extreme pressure agent, and (E) an ashless dispersant, and having a kinetic viscosity of 2 to 10 mm ² /s at 100 ° C., is suitable as a lubricating oil composition for gear oil. In addition, Patent Document 2 also discloses that at least one selected from non-dispersant polymethacrylate and dispersant polymethacrylate is preferable as the viscosity index improver.

Furthermore, JP 2007-284564 A (Patent Document 3) describes a lubricating oil composition comprising (A1) a lubricating base oil having a kinetic viscosity at 100° C. of 2.5 to 4.5 mm ² /s and (A2) a lubricating base oil having a kinetic viscosity at 100° C. of 10 to 40 mm ² /s and a lubricating base oil having a kinetic viscosity at 100° C. of 3.7 to 4.1 mm ² The present invention discloses a lubricating oil composition for automatic transmissions having a kinetic viscosity at 100° C. of 5.6 to 5.8 mm 2 /s, which is prepared by blending lubricating base oil (A) having a kinetic viscosity of 100° C./s with 1 to 20 mass % of a poly(meth)acrylate viscosity index improver having a weight average molecular weight of 15,000 to 30,000, based on the total amount of the composition, (B) 2 to 4 mass % of an imide friction modifier having a hydrocarbon group having 8 to 30 carbon atoms, based on the total amount of the composition, (C) 0.01 to 0.04 mass % of a phosphorus-based extreme pressure agent in terms of phosphorus, based on the total amount of the composition, and ( ^E ) 0.01 to 0.04 mass % of an ashless dispersant having at least one alkyl or alkenyl group having a number average molecular weight of 2,000 or more, based on the total amount of the composition in terms of nitrogen content.

International Publication No. 2017/073748 JP 2021-080429 A JP 2007-284564 A

However, the lubricating oil compositions described in Patent Documents 1 to 3 leave room for improvement in terms of improving load-bearing performance (particularly anti-seizure performance) while still achieving low viscosity.

The present invention was made in consideration of the problems with the above-mentioned conventional technology, and aims to provide a gear oil composition that can have a low kinetic viscosity based on the kinetic viscosity at 40°C and 100°C, while also having excellent anti-seizure performance and excellent oxidation stability.

In general, the lower the viscosity of a lubricating oil composition, the lower the oil film forming ability and the more likely metal contact sites are to occur, which in turn reduces the anti-seizure performance. Thus, in a lubricating oil composition, lowering the viscosity and improving the anti-seizure performance are in a contradictory relationship (trade-off relationship) in which improving one characteristic reduces the other. For this reason, conventional gear oil compositions were unable to achieve both characteristics, and even if it was possible to reduce the kinetic viscosity of the composition, it was not possible to achieve excellent load-bearing performance (especially anti-seizure performance) when the viscosity was reduced.

Under these circumstances, the present inventors have conducted extensive research to achieve the above object, and first considered adding an ester-based base oil to the base oil composition used in the lubricating oil composition to reduce viscosity and improve anti-seizure performance, but it was found that such a formulation, even if it is possible to reduce viscosity and improve anti-seizure performance, makes it difficult to obtain excellent oxidation stability, and is not necessarily sufficient in terms of practical use (see Comparative Example 9 of the present application). Then, in light of such findings, the present inventors conducted further extensive research and found that a gear oil composition containing (A) a base oil composition, (B) a poly(meth)acrylate-based thickener, and (C) a succinimide dispersant; the (A) component is a poly(meth)acrylate-based thickener having a kinetic viscosity at 100°C of 3.5 to 4.5 ^mm2 a base oil composition containing 10 mass% or more of a mineral base oil (A-1) having a viscosity index of 140 or more based on the total amount of the base oil composition; the content of the mineral base oil in the (A) component is 93 mass% or more; the (B) component is a poly(meth)acrylate having a weight average molecular weight of 15,000 or less; the (C) component is at least one selected from the group consisting of a bis-type unmodified succinimide and a bis-type boric acid modified succinimide; the content of the (B) component is 1 to 4 mass% based on the total amount of the gear oil composition; the content of the (C) component is 1 to 3 mass% based on the total amount of the gear oil composition; the kinetic viscosity of the gear oil composition at 40°C is 36.0 mm ² /s or less; and the kinetic viscosity of the gear oil composition at 100°C is 8.0 mm ² /s or less, it has surprisingly been found that it is possible to achieve both low viscosity and improved anti-seizure performance, which are in a contradictory relationship, while also achieving excellent oxidation stability, and that it is possible to achieve a low kinetic viscosity based on the kinetic viscosities at 40°C and 100°C, while also achieving excellent anti-seizure performance and excellent oxidation stability, and thus the present invention has been completed.

In other words, the present invention provides the following aspects:

[1] A gear oil composition comprising: (A) a base oil composition; (B) a poly(meth)acrylate thickener; and (C) a succinimide dispersant;
The component (A) is a base oil composition containing 10 mass % or more of a mineral base oil (A-1) having a kinematic viscosity at 100°C of 3.5 to 4.5 ^mm2 /s and a viscosity index of 140 or more, based on the total amount of the base oil composition;
The content of the mineral oil base oil in the component (A) is 93% by mass or more,
The component (B) is a poly(meth)acrylate having a weight average molecular weight of 15,000 or less,
The component (C) is at least one selected from the group consisting of bis-type unmodified succinimides and bis-type boric acid modified succinimides,
The content of the component (B) based on the total amount of the gear oil composition is 1 to 4 mass %,
The content of the component (C) based on the total amount of the gear oil composition is 1 to 3 mass %,
The gear oil composition has a kinematic viscosity at 40°C of 36.0 ^mm2 /s or less, and
The gear oil composition has a kinematic viscosity at 100° C. of 8.0 mm ² /s or less.

[2] The gear oil composition according to [1], wherein the component (C) is a boric acid modified compound of a bis-type succinimide compound.

[3] The gear oil composition according to [1] or [ ² ], wherein the component (A) contains 40 mass% or more of a mineral base oil (A-2) having a kinematic viscosity at 100°C of 6.0 to 7.0 mm2/s and a viscosity index of 140 or more, based on the total amount of the base oil composition.

[4] A gear oil composition according to any one of [1] to [3], which is a gear oil composition for an electric vehicle transmission.

The present invention makes it possible to provide a gear oil composition that has a low kinetic viscosity based on the kinetic viscosity at 40°C and 100°C, while still having excellent anti-seizure performance.

The present invention will be described in detail below with reference to preferred embodiments. In this specification, unless otherwise specified, the expression "X to Y" for numerical values X and Y means "X or more and Y or less." In such expressions, when a unit is assigned only to numerical value Y, the unit is also applied to numerical value X.

The gear oil composition of the present invention comprises
A gear oil composition comprising: (A) a base oil composition; (B) a poly(meth)acrylate thickener; and (C) a succinimide dispersant;
The component (A) is a base oil composition containing 10 mass % or more of a mineral base oil (A-1) having a kinematic viscosity at 100°C of 3.5 to 4.5 ^mm2 /s and a viscosity index of 140 or more, based on the total amount of the base oil composition;
The content of the mineral oil base oil in the component (A) is 93% by mass or more,
The component (B) is a poly(meth)acrylate having a weight average molecular weight of 15,000 or less,
The component (C) is at least one selected from the group consisting of bis-type unmodified succinimides and bis-type boric acid modified succinimides,
The content of the component (B) based on the total amount of the gear oil composition is 1 to 4 mass %,
The content of the component (C) based on the total amount of the gear oil composition is 1 to 3 mass %,
The gear oil composition has a kinematic viscosity at 40°C of 36.0 ^mm2 /s or less, and
The gear oil composition has a kinematic viscosity at 100° C. of 8.0 mm ² /s or less.

[Component (A): Base oil composition]
The gear oil composition of the present invention contains a base oil composition as component (A). The base oil composition used as component (A) contains the mineral oil-based base oil (A-1). The base oil composition contains 10 mass % or more of the base oil (A-1) based on the total amount of the base oil composition. It is a composition containing a base oil containing at least 5% by mass of the base oil.

The base oil (A-1) contained in such a base oil composition is a mineral base oil that satisfies the conditions that the kinematic viscosity at 100°C is 3.5 to 4.5 mm ² /s and the viscosity index is 140 or more. Thus, the base oil (A-1) is a mineral base oil, and is a high-performance base oil having a low kinematic viscosity at 100°C of 3.5 to 4.5 mm ² /s and an extremely high viscosity index of 140 or more. In this specification, the kinematic viscosity of the base oil or composition at 40°C or 100°C means the kinematic viscosity at each temperature (40°C or 100°C) specified in JIS K 2283-2000. In addition, in this specification, the "viscosity index" of the base oil or composition means the viscosity index measured in accordance with JIS K 2283-2000.

As described above, the base oil (A-1) must be a mineral oil-based base oil having a kinetic viscosity at 100°C of 3.5 to 4.5 ^mm2 /s. By making the kinetic viscosity at 100°C of such base oil (A-1) 3.5 ^mm2 /s or more, the oil film forming performance at the lubricated parts is improved, and it is possible to improve the seizure resistance. On the other hand, by making the kinetic viscosity at 100°C of the base oil (A-1) 4.5 ^mm2 /s or less, the finally obtained gear oil composition can be made lower in viscosity, and it is possible to reduce the stirring loss during use. The kinetic viscosity at 100°C of such base oil (A-1) is more preferably 3.7 to 4.3 ^mm2 /s, since a higher effect can be obtained in terms of improving the seizure resistance due to the oil film forming performance and improving the fuel saving performance due to the low viscosity.

The base oil (A-1) must have a viscosity index of 140 or more. By making the viscosity index of the base oil (A-1) 140 or more, it is possible to obtain a gear oil composition with smaller viscosity changes due to temperature, and it is possible to maintain a low viscosity under the conditions of use and further improve fuel economy performance. It is more preferable that the viscosity index of the base oil (A-1) is 141 or more, since this provides a greater effect in terms of fuel economy performance.

Moreover, the base oil (A-1) is more preferably a base oil having a kinetic viscosity at 40°C of 15.8 to 16.5 ^mm2 /s (more preferably 16.0 to 16.3 ^mm2 /s). When the kinetic viscosity at 40°C of such base oil (A-1) is not less than the lower limit, it is possible to improve the oil film forming ability at the lubricated parts and provide more excellent lubricity compared to when the kinetic viscosity is below the lower limit, and it is possible to reduce the evaporation loss of the gear oil composition and reduce the consumption of the gear oil composition. On the other hand, when the kinetic viscosity at 40°C of the base oil (A-1) is not more than the upper limit, it is possible to obtain a higher effect in terms of low temperature viscosity characteristics and fuel saving performance of the gear oil composition compared to when the kinetic viscosity exceeds the upper limit.

Furthermore, the base oil (A-1) is preferably a base oil having a sulfur content of 10 ppm by mass or less (more preferably 8 ppm by mass or less, and even more preferably 5 ppm by mass or less). When the sulfur content is below the upper limit, it is possible to obtain a gear oil composition having higher oxidation stability compared to when the sulfur content exceeds the upper limit. Furthermore, the "sulfur content" in the base oil can be measured in accordance with ASTM D4951.

The base oil (A-1) is preferably a base oil having a pour point of -20°C or less. When the pour point is equal to or less than the upper limit, the low-temperature fluidity of the final gear oil composition can be improved compared to when the pour point exceeds the upper limit. Although there is no particular restriction on the lower limit of the pour point, it is more preferable that the pour point is -40°C or more from the viewpoint of enabling a higher viscosity index. In this specification, "pour point" means the pour point measured in accordance with JIS K 2269-1987.

Furthermore, it is preferable that the base oil (A-1) is a base oil having a flash point of 200°C or higher. Furthermore, by setting the flash point at or above the lower limit, safety during use at high temperatures tends to be improved compared to when the flash point is below the lower limit. In this specification, "flash point" refers to the flash point measured in accordance with JIS K 2265-4-2007 (Cleveland Opening Method).

The base oil (A-1) is preferably a base oil having a % _CP of 90 to 95. By making the % _CP of the base oil (A-1) equal to or greater than the lower limit, it becomes possible to further improve the viscosity-temperature characteristics, fuel saving performance, and effects obtained by adding components other than the base oil (including other additives, etc.). On the other hand, by making the % _CP of the base oil equal to or less than the upper limit, it becomes possible to increase the solubility of other components such as extreme pressure agents.

The base oil (A-1) is preferably a base oil having a % _CA of 0. By making the % _CA of the base oil (A-1) 0, it is possible to further improve the viscosity-temperature characteristics and also to further improve the fuel economy performance.

The base oil (A-1) is preferably a base oil having a % C _N of 5 to 10. By making the % C _N of the base oil (A-1) equal to or less than the upper limit, it is possible to improve the viscosity-temperature characteristics and further improve fuel economy. By making the % C _N equal to or more than the lower limit, it is possible to increase the solubility of additives.

In this specification, %C _P , %C _N and %C _A respectively mean the percentage of paraffin carbon number to the total carbon number, the percentage of naphthene carbon number to the total carbon number and the percentage of aromatic carbon number to the total carbon number, which are determined by a method (nd-M ring analysis) in accordance with ASTM D 3238-85. In other words, the preferred ranges of %C _P , %C _N and %C _A described above are based on the values determined by the above method, and for example, even in the case of a lubricating base oil that does not contain naphthene content, the %C _N determined by the above method may show a value exceeding 0.

Furthermore, the base oil (A-1) may be any mineral oil-based base oil, and is not particularly limited, but is preferably a mineral oil-based hydrocarbon oil. Examples of such mineral oil-based hydrocarbon oils include paraffinic base oils, naphthenic base oils, aromatic base oils, wax isomerized base oils, solvent refined base oils, hydrorefined base oils, hydrocracked base oils, normal paraffins, and isoparaffins, which are obtained by using at least one selected from the group consisting of kerosene fractions obtained by distillation of paraffinic, naphthenic, or aromatic crude oils; normal paraffins obtained by extraction operations from kerosene fractions; and lubricating oil fractions obtained by distillation of paraffinic, naphthenic, or aromatic crude oils; or wax derived from crude oils such as slack wax obtained by a lubricating oil dewaxing process and/or synthetic waxes such as Fischer-Tropsch wax and GTL wax obtained by a gas-to-liquid (GTL) process or the like; and refining the oils by one or a combination of two or more suitable refining treatments such as solvent deasphalting, solvent extraction, hydrocracking, hydroisomerization, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid washing, and clay treatment. As such base oil (A-1), wax isomerized base oil is particularly preferred. These mineral oil-based hydrocarbon oils may be used alone or in combination of two or more in any ratio.

The base oil (A-1) is preferably a mineral oil-based base oil having a saturated content of 90% by mass or more (more preferably 95% by mass or more, and even more preferably 99% by mass or more). By having the saturated content be equal to or greater than the lower limit, the viscosity-temperature characteristics can be improved. Here, in this specification, the saturated content means a value measured in accordance with ASTM D 2007-93.

Furthermore, the base oil (A-1) is preferably a mineral oil-based base oil having an aromatic content of 0 to 10 mass% (more preferably 0 to 5 mass%, particularly preferably 0 to 1 mass% or less). In one embodiment of such a mineral oil-based base oil, the aromatic content may be 0.1 mass% or more. By setting the aromatic content to the above upper limit or less, it is possible to improve the viscosity-temperature characteristics and low-temperature viscosity characteristics. In this specification, the aromatic content means a value measured in accordance with ASTM D 2007-93. Aromatics usually include alkylbenzenes, alkylnaphthalenes, anthracene, phenanthrene and their alkylated products, compounds with 4 or more condensed benzene rings, pyridines, quinolines, phenols, naphthols and other aromatic compounds having heteroatoms.

Furthermore, such base oil (A-1) is preferably a Group III base oil in the base oil classification by API (American Petroleum Institute) (hereinafter, the API base oil classification group may be simply referred to as "API group"). API Group III base oils are mineral oil-based base oils with a sulfur content of 0.03 mass% or less, a saturate content of 90 mass% or more, and a viscosity index of 120 or more.

Furthermore, as described above, the base oil composition (component (A)) according to the present invention contains 10 mass% or more of the base oil (A-1) based on the total amount of the base oil composition. By making the content of such base oil (A-1) 10 mass% or more based on the total amount of the base oil composition, it becomes possible to improve the viscosity-temperature characteristics and further improve fuel economy. From the viewpoint of further improving fuel economy, the content of such base oil (A-1) is more preferably 50 to 70 mass%, and even more preferably 50 to 60 mass%.

Furthermore, the base oil composition (component (A)) according to the present invention has a mineral base oil content of 93 mass% or more (more preferably 95 to 100 mass%, and even more preferably 98 to 100 mass%) in the base oil composition (base oil (A)). That is, in the base oil composition (component (A)) according to the present invention, the mineral base oil content is 93 mass% or more based on the total amount of the base oil composition. By setting the content of such mineral base oil to the above lower limit or more, it is possible to further improve oxidation stability.

Furthermore, the base oil composition (component (A)) according to the present invention can contain other base oils selected appropriately in addition to the base oil (A-1) so that the base oil composition satisfies the above conditions. The base oil other than the base oil (A-1) to be contained in such a base oil composition may be any base oil that can cause the resulting composition to satisfy the above conditions of the base oil composition, and can be appropriately selected from known base oils depending on the type of base oil (A-1). For example, other mineral oil-based base oils, synthetic oils made of polyalphaolefins (PAO), etc. may be appropriately used.

Among these other base oils, it is particularly preferable to use a mineral oil-based base oil (A-2) having a kinematic viscosity at 100° C. of 6.0 to 7.0 mm ² /s and a viscosity index of 140 or more as the base oil to be used in combination with the base oil (A-1), since it is possible to improve the viscosity-temperature characteristics. By using the base oil (A-2) together with the base oil (A-1), it is possible to improve the viscosity-temperature characteristics more efficiently. Hereinafter, the base oil (A-2) that can be suitably used in combination with the base oil (A-1) in the base oil composition (component (A)) will be described.

Such a base oil (A-2) is a mineral oil-based base oil having a kinetic viscosity at 100 ° C. of 6.0 to 7.0 mm ² /s. When the kinetic viscosity at 100 ° C. of such a base oil (A-2) is equal to or higher than the lower limit, it is possible to improve the oil film forming property at the lubricated portion and to make the lubrication property more excellent compared to the case where it is below the lower limit, and it is possible to reduce the evaporation loss of the gear oil composition and to reduce the consumption amount of the gear oil composition. On the other hand, when it is equal to or lower than the upper limit, it is possible to obtain a higher effect in terms of improving the low-temperature viscosity characteristics and fuel saving performance of the gear oil composition compared to the case where it exceeds the upper limit. The kinetic viscosity at 100 ° C. of such a base oil (A-2) is more preferably 6.0 to 6.5 mm ² / s (more preferably 6.1 to 6.5 mm ² / s) because a higher effect is obtained in terms of the low viscosity, fuel saving, and oil film forming property of the gear oil composition.

The base oil (A-2) is a mineral oil-based base oil with a viscosity index of 140 or more. By making the viscosity index of such base oil (A-2) 140 or more, it is possible to obtain a gear oil composition with smaller viscosity changes due to temperature, and it is possible to maintain a low viscosity state under usage conditions and further improve fuel efficiency performance. The viscosity index of such base oil (A-2) is more preferably 140 to 145, as this provides a greater effect in terms of fuel efficiency performance.

Moreover, the base oil (A-2) is more preferably a base oil having a kinetic viscosity at 40° C. of 33.5 to 34.5 mm ² /s (more preferably 34.0 to 34.3 mm ² /s). When the kinetic viscosity at 40° C. of such base oil (A-2) is not less than the above-mentioned lower limit, a higher effect in terms of oil film formation tends to be obtained compared to when the kinetic viscosity is below the above-mentioned lower limit, while when the kinetic viscosity at 40° C. of base oil (A-2) is not more than the above-mentioned upper limit, a higher effect in terms of lowering the viscosity of the gear oil composition and fuel saving tends to be obtained compared to when the kinetic viscosity exceeds the above-mentioned upper limit.

Furthermore, it is preferable that the base oil (A-2) is a base oil having a sulfur content of 1 mass ppm or less. When the sulfur content is equal to or less than the upper limit, it is possible to improve oxidation stability compared to when the sulfur content exceeds the upper limit.

The base oil (A-2) is preferably a base oil having a pour point of -10°C or lower. When the pour point is equal to or lower than the upper limit, it is possible to improve the low-temperature fluidity of the entire gear oil composition compared to when the pour point exceeds the upper limit. The lower limit of the pour point is not particularly limited, but from the viewpoint of enabling a higher viscosity index, it is more preferable that the pour point is -40°C or higher. Furthermore, the base oil (A-2) is preferably a base oil having a flash point of 200°C or higher (more preferably 230°C or higher). By setting the flash point at or above the lower limit, safety during high-temperature use tends to be improved compared to when the flash point is below the lower limit.

The base oil (A-2) is preferably a base oil having a % _CP of 70 to 90. By making the % _CP of the base oil (A-2) equal to or greater than the lower limit, it becomes possible to further improve the viscosity-temperature characteristics, fuel saving performance, and effects obtained by adding components other than the base oil (including other additives, etc.). On the other hand, by making the % _CP of the base oil equal to or less than the upper limit, it becomes possible to increase the solubility of other components such as extreme pressure agents.

The base oil (A-2) is preferably a base oil having a % _CA of 0 to 30. By setting the % _CA of the base oil (A-2) to the above upper limit or less, it is possible to further improve the viscosity-temperature characteristics and also to further improve the fuel economy performance.

The base oil (A-2) is preferably a base oil having a % _CN of 0 to 30. By setting the % _CN of the base oil (A-2) to the above upper limit or less, it is possible to improve the viscosity-temperature characteristics and further improve fuel economy.

Furthermore, the base oil (A-2) is preferably a mineral oil-based base oil having a saturated content of 90% by mass or more (more preferably 95% by mass or more, and even more preferably 99% by mass or more). By having the saturated content be equal to or greater than the lower limit, it is possible to improve the viscosity-temperature characteristics. Here, in this specification, the saturated content means a value measured in accordance with ASTM D 2007-93.

Furthermore, the base oil (A-2) is preferably a mineral oil-based base oil having an aromatic content of 0 to 10 mass% (more preferably 0 to 5 mass%, particularly preferably 0 to 1 mass% or less). In one embodiment of such a mineral oil-based base oil, the aromatic content may be 0.1 mass% or more. By setting the aromatic content to the above upper limit or less, it is possible to improve the viscosity-temperature characteristics and low-temperature viscosity characteristics. In this specification, the aromatic content means a value measured in accordance with ASTM D 2007-93. Aromatics usually include alkylbenzenes, alkylnaphthalenes, anthracene, phenanthrene and their alkylated products, compounds with 4 or more condensed benzene rings, pyridines, quinolines, phenols, naphthols, and other aromatic compounds having heteroatoms.

The base oil (A-2) may be a mineral oil, and is not particularly limited, but is preferably a mineral hydrocarbon oil (e.g., hydrotreated base oil, etc.). Furthermore, such a base oil (A-2) is preferably an API Group III base oil.

In addition, in the base oil composition (component (A)) according to the present invention, when the base oil (A-2) is used together with the base oil (A-1), the content of the base oil (A-2) is preferably 30 mass% or more (more preferably 30 to 50 mass%, and even more preferably 40 to 50 mass%) based on the total amount of the base oil composition. By making the content of the base oil (A-2) equal to or more than the lower limit, it is possible to improve the oil film forming ability at the lubricated parts and provide better lubricity compared to when it is less than the lower limit, and it is possible to reduce the evaporation loss of the gear oil composition and reduce the consumption of the gear oil composition. On the other hand, by making the content of the base oil (A-2) equal to or less than the upper limit, it is possible to obtain a higher effect in terms of low-temperature viscosity characteristics and fuel saving performance of the gear oil composition compared to when it is less than the upper limit.

Furthermore, when such a base oil composition (component (A)) contains the base oil (A-2), it is possible to further improve the oil film forming ability at the lubricated parts, thereby making the lubrication more excellent, and it is also possible to further improve the fuel saving performance. From this viewpoint, it is more preferable that the content (total amount) of the base oil (A-1) and the base oil (A-2) relative to the total amount of the base oil composition is 93 mass% or more (more preferably 95 to 100 mass%, and particularly preferably 98 to 100 mass%). Note that, as a preferred embodiment of such a base oil composition (component (A)), for example, one consisting only of the base oil (A-1) and the base oil (A-2) may be used.

Moreover, the base oil composition (component (A)) according to the present invention is more preferably a mixture of mineral oil-based hydrocarbon base oils. A preferred embodiment of such a base oil composition that is a mixture of hydrocarbon base oils is, for example, a mixture of all base oil components contained in the base oil composition (when the base oil composition consists only of the base oil (A-1) and the base oil (A-2), all of the base oil (A-1) and the base oil (A-2)) as mineral hydrocarbon oils.

The base oil composition (component (A)) according to the present invention preferably has a kinetic viscosity at 40° C. of 34.0 to 36.0 mm ² /s (more preferably 34.5 to 36.0 mm ² /s). When the kinetic viscosity at 40° C. of such a base oil composition is not less than the above-mentioned lower limit, it is possible to further improve the oil film formability at lubricated parts and provide more excellent lubricity, and it is possible to further reduce the evaporation loss of the gear oil composition and thus reduce the consumption of the gear oil composition. On the other hand, when the kinetic viscosity is not more than the above-mentioned upper limit, it is possible to further improve the low temperature viscosity characteristics and fuel saving performance of the gear oil composition compared to when the kinetic viscosity exceeds the above-mentioned upper limit.

The base oil composition (component (A)) according to the present invention preferably has a kinetic viscosity at 100° C. of 6.0 to 8.0 mm ² /s (more preferably 7.0 to 8.0 mm ² /s). When the kinetic viscosity at 100° C. of such a base oil composition is not less than the above-mentioned lower limit, the oil film forming performance at lubricated points is higher and seizure resistance can be further improved compared to when the kinetic viscosity is below the above-mentioned lower limit, and on the other hand, when the kinetic viscosity is not more than the above-mentioned upper limit, the gear oil composition has a lower viscosity and it is possible to further reduce stirring loss during use compared to when the kinetic viscosity exceeds the above-mentioned upper limit.

Furthermore, the base oil composition (component (A)) according to the present invention is preferably a base oil having a viscosity index of 140 or more. By making the viscosity index of such a base oil composition 140 or more, it is possible to obtain a gear oil composition with smaller viscosity changes due to temperature, and it is possible to maintain a low viscosity state under usage conditions and further improve fuel economy performance.

Furthermore, the base oil composition (component (A)) according to the present invention preferably has a sulfur content of less than 10 ppm by mass (more preferably 0 to 5 ppm by mass). When the sulfur content in such a base oil composition is equal to or less than the upper limit, there is a tendency for a higher effect in terms of oxidation stability to be obtained compared to when the sulfur content exceeds the upper limit.

[Component (B): Poly(meth)acrylate-based thickener]
The gear oil composition of the present invention contains a poly(meth)acrylate thickener as component (B). In the present invention, the poly(meth)acrylate thickener (component (B)) is a poly(meth)acrylate having a weight average molecular weight of 15,000 or less. When the weight average molecular weight of the poly(meth)acrylate used as component (B) is 15,000 or less, it is possible to further improve the seizure resistance under poor lubrication conditions compared to when the weight average molecular weight exceeds 15,000. The "weight average molecular weight" of such a poly(meth)acrylate means the value determined by gel permeation chromatography (GPC) (the molecular weight obtained by standard polystyrene conversion).

Moreover, it is more preferable that the poly(meth)acrylate used as component (B) has a weight-average molecular weight of 11,000 to 13,000. By setting the weight-average molecular weight of the poly(meth)acrylate within the above range, there is a tendency for even greater effects to be obtained in terms of improving seizure resistance under poor lubrication conditions.

The poly(meth)acrylate used as component (B) may have a weight average molecular weight of 15,000 or less, and other conditions (e.g., structure, etc.) are not particularly limited. Any poly(meth)acrylate compound known to be usable as a thickener (which may be a so-called viscosity index improver) in the field of lubricating oils (e.g., poly(meth)acrylate compounds described in paragraphs [0079] to [0096] of JP 2010-195894 A) having a weight average molecular weight of 15,000 or less may be used. In this specification, "(meth)acrylate" means acrylate and/or methacrylate. Such poly(meth)acrylate may be either non-dispersed or dispersed. Such component (B) may be used alone or in combination of two or more.

[Component (C): Succinimide Dispersant]
The gear oil composition of the present invention contains a succinimide dispersant as component (C). In the present invention, the succinimide dispersant (component (C)) may be at least one of bis-type unmodified succinimide and bis-type boric acid modified succinimide, and is not particularly limited. However, from the viewpoint of further improving wear resistance, it is particularly preferable that the succinimide is a bis-type boric acid modified succinimide. In addition, such bis-type unmodified succinimide and bis-type boric acid modified succinimide are not particularly limited, and known ashless dispersants (for example, unmodified succinimide described in JP 2022-041877 A, bis-type among boric acid modified succinimides, etc.) can be appropriately used.

As such a bis-type unmodified succinimide, a bis-type succinimide having at least one alkyl or alkenyl group in the molecule is preferred, and is represented by the following formula (I):

(In the formula, R ¹ and R ² each independently represent an alkyl group having 40 to 400 carbon atoms (more preferably 60 to 350) or an alkenyl group having 40 to 400 carbon atoms (more preferably 60 to 350), and n represents an integer of 0 to 4 (more preferably 1 to 4, even more preferably 1 to 3).)
More preferred are compounds represented by the formula (I). As the alkyl or alkenyl group that can be selected as R ¹ and R ² in the formula (I), a branched alkyl or alkenyl group derived from an oligomer of isobutene called polyisobutylene, or a polybutenyl group is more preferred. In addition, the alkyl or alkenyl group that can be selected as R ¹ and R ² in the formula (I) preferably has a number average molecular weight of 800 to 3500. There is no particular restriction on the method for producing such a bis-type succinimide, and a known method can be appropriately adopted. For example, a condensation reaction product can be obtained by reacting an alkyl or alkenyl succinic acid having an alkyl or alkenyl group having 40 to 400 carbon atoms or an anhydride thereof with a polyamine to imidize both ends of the polyamine chain.

In addition, in the present invention, a "bis-type boric acid modified succinimide" is a compound modified by allowing boric acid to act on the bis-type unmodified succinimide, thereby neutralizing or amidating some or all of the amino groups and/or imino groups in the bis-type unmodified succinimide.

From the viewpoint of further improving oxidation stability, such a succinimide dispersant (component (C)) preferably has a weight average molecular weight (Mw) of 2500 to 3500 (more preferably 2800 to 3200). The "weight average molecular weight" of such a succinimide dispersant and the "number average molecular weight" of the alkyl or alkenyl groups ( ^R1 and ^R2 ) in the compound represented by formula (I) above each mean a value determined by gel permeation chromatography (GPC) (a molecular weight obtained in terms of standard polystyrene).

Furthermore, from the viewpoint of oxidation stability, such a succinimide dispersant (component (C)) is more preferably one having a nitrogen content of 1.8 to 2.4 mass% (more preferably 2.0 to 2.3 mass%). Furthermore, from the viewpoint of abrasion resistance, such a succinimide dispersant (component (C)) is preferably a bis-type boric acid modified succinimide, and among these, a boric acid modified compound of a bis-type alkenyl succinimide (a compound modified by neutralizing or amidating a bis-type alkenyl succinimide with boric acid) can be particularly preferably used.

[Gear Oil Composition]
The gear oil composition of the present invention is a gear oil composition (a lubricating oil composition for gear oils) containing the above-mentioned component (A), component (B), and component (C).

In the gear oil composition of the present invention, the content of the component (A) is not particularly limited, but is preferably 75 to 95 mass % (more preferably 80 to 90 mass %) based on the total amount of the gear oil composition. When the content of the component (A) is equal to or greater than the lower limit, a higher effect in terms of oxidation stability can be obtained compared to when the content is below the lower limit, and on the other hand, when the content is equal to or less than the upper limit, a higher effect can be obtained in terms of improving the additive effect at the lubricated points and improving lubricity compared to when the content exceeds the upper limit.

In such a gear oil composition of the present invention, the content of the (B) component must be 1 to 4 mass % (more preferably 2 to 3 mass %) based on the total amount of the gear oil composition. When the content of the (B) component is 1 mass % or more, it is possible to improve the oil film forming ability compared to when it is less than 1 mass %, and it is possible to achieve excellent seizure resistance. On the other hand, when the content of the (B) component is 4 mass % or less, it is possible to suppress the inhibition of film formation of the additive compared to when it exceeds 4 mass %, and it is also possible to achieve excellent seizure resistance.

In addition, in the gear oil composition of the present invention, the content of the (C) component (total amount of the (C) components) is preferably 1 to 3 mass% based on the total amount of the gear oil composition. When the content of the (C) component is equal to or greater than the lower limit, it is possible to obtain a higher effect in terms of sludge dispersibility and corrosion prevention compared to when the content is below the lower limit, and on the other hand, when the content is equal to or less than the upper limit, it is possible to obtain a higher effect in terms of electrical insulation compared to when the content exceeds the upper limit.

In the gear oil composition of the present invention, the sulfur content based on the total amount of the gear oil composition is preferably 2.0 to 2.3 mass%. In the gear oil composition of the present invention, the phosphorus content based on the total amount of the gear oil composition is more preferably 1400 to 1800 mass ppm. In the gear oil composition of the present invention, the boron content based on the total amount of the gear oil composition is more preferably 50 to 70 mass ppm. The sulfur, phosphorus and boron contents in the gear oil composition can be measured in accordance with ASTM D4951.

The gear oil composition of the present invention may further contain additives in addition to the components (A) to (C). Such additives may be any known additive used in the field of lubricating oil compositions (more preferably gear oil compositions), and are not particularly limited. Preferred examples include, but are not limited to, (D) an extreme pressure agent; and (E) a viscosity index improver consisting of at least one of an ethylene and α-olefin copolymer and its hydrogenated product.

As the extreme pressure agent (component (D)), any known extreme pressure agent (such as a known sulfur-based extreme pressure agent or phosphorus-based extreme pressure agent) can be used as appropriate, and is not particularly limited. However, from the viewpoint of extreme pressure properties and wear resistance, it is particularly preferable to use a sulfur-based extreme pressure agent.

As such sulfur-based extreme pressure agents, sulfur-based extreme pressure agents known in the field of lubricating oil compositions (e.g., polysulfide compounds, sulfurized esters, sulfurized oils and fats, thiadiazole compounds, etc.) can be appropriately used and are not particularly limited, but from the viewpoint of further improving seizure resistance, sulfur-based extreme pressure agents made of polysulfide compounds are preferred. In this specification, polysulfide compounds refer to all compounds crosslinked by two or more sulfurs.

Furthermore, as such polysulfide compounds, polysulfide compounds having a sulfur content of 30 to 65 mass % (more preferably 30 to 50 mass %) are preferred. When the sulfur content in such polysulfide compounds is equal to or greater than the lower limit, it is possible to obtain a higher effect in terms of extreme pressure properties compared to when the content is below the lower limit, and on the other hand, when the content is equal to or less than the upper limit, it is possible to obtain a higher effect in terms of oxidation stability compared to when the content exceeds the upper limit.

Among the polysulfide compounds used as the sulfur-based extreme pressure agents, dihydrocarbyl polysulfide compounds are more preferred. Examples of such dihydrocarbyl polysulfide compounds include dibutyl polysulfide, dihexyl polysulfide, dioctyl polysulfide, dinonyl polysulfide, didecyl polysulfide, didodecyl polysulfide, ditetradecyl polysulfide, dihexadecyl polysulfide, dioctadecyl polysulfide, dieicosyl polysulfide, diphenyl polysulfide, dibenzyl polysulfide, diphenethyl polysulfide, polypropenyl polysulfide, polybutenyl polysulfide, and mixtures thereof. These sulfur-based extreme pressure agents can be used alone or in combination of two or more.

The copolymer and its hydrogenated product used in the viscosity index improver (component (E)) may be of the so-called non-dispersion type or of the dispersion type. The copolymer and its hydrogenated product used in the viscosity index improver (E) preferably have a number average molecular weight of 2,000 to 10,000. If the number average molecular weight is equal to or greater than the lower limit, a higher effect can be obtained in terms of improving the viscosity-temperature characteristics compared to when the number average molecular weight is less than the lower limit, and if the number average molecular weight is equal to or less than the upper limit, a higher effect can be obtained in terms of shear stability compared to when the number average molecular weight exceeds the upper limit. The "number average molecular weight" of the copolymer and the like used as component (E) means the value determined by gel permeation chromatography (GPC) (molecular weight obtained in standard polystyrene conversion). When component (E) is used in the gear oil composition of the present invention, it is more preferable that the content of component (E) based on the total amount of the gear oil composition is 3.0 to 4.5 mass%.

Although the above-mentioned (D) and (E) components have been described as suitable additives that can be suitably used in the gear oil composition of the present invention, the additives that can be used in the gear oil composition of the present invention are not limited to the above-mentioned (D) and (E) components, and may include, within the scope that does not impair the effects of the present invention, for example, pour point depressants, metal deactivators, friction modifiers, dispersants other than the dispersants described above, antioxidants, rubber swelling agents, defoamers, Other known additive components such as diluent oil, rust inhibitor, demulsifier, colorant, corrosion inhibitor, antiwear agent, extreme pressure agent, metal detergent, acid scavenger, etc. (for example, those described in JP 2016-3258 A, WO 2015/056783 A, WO 2016-160312 A, WO 2003-155492 A, WO 2017/073748 A, WO 2020-76004 A, etc.) can also be used as appropriate.

In addition, when additive components other than the (D) and (E) components are used, the total amount (total quantity) of the other additive components is more preferably 1.0 to 2.0 mass% based on the total amount of the gear oil composition. When the total amount of such other additive components is equal to or greater than the lower limit, it is possible to obtain a higher effect in terms of sludge dispersibility and corrosion prevention compared to when it is less than the lower limit, and on the other hand, when it is equal to or less than the upper limit, it is possible to obtain a higher effect in terms of oxidation stability compared to when it exceeds the upper limit.

The various additives that can be used in such gear oil compositions may be prepared separately for each component and added, or may be prepared and added as a mixture with other components. As such a mixture of other components, a commercially available package product (e.g., an additive package containing a dispersant, friction modifier, corrosion inhibitor, etc.) may be appropriately used. In this regard, for example, as one embodiment, the (D) and (E) components may be prepared individually, while a separate additive package containing a dispersant, friction modifier, corrosion inhibitor, etc. may be prepared and used in combination.

[Gear oil composition characteristics, manufacturing method, uses, etc.]
The gear oil composition of the present invention must have a kinetic viscosity of 36.0 mm ² /s or less at 40° C. When the gear oil composition has a kinetic viscosity of 36.0 mm ² /s or less at 40° C., it is possible to reduce the stirring resistance in a relatively low temperature range near 40° C. (preferably about 20 to 60° C.) compared with a case where the kinetic viscosity exceeds 36.0 mm ² /s, and it is possible to further improve the power transmission efficiency even in a low temperature state such as immediately after the start of use, and it is possible to further improve the fuel saving performance. In addition, although the lower limit of the kinetic viscosity of the gear oil composition at 40° C. is not particularly limited, it is preferable that the kinetic viscosity of the gear oil composition at 40° C. is 34.0 mm ² /s or more. When the kinetic viscosity of the gear oil composition at 40°C is equal to or higher than the lower limit, the oil film forming and oil film retaining properties of the gear oil composition at lubricated points are improved in a relatively low temperature range around 40°C (preferably about 20 to 60°C), making it possible to maintain a better lubricated state and further improve fuel economy performance. In addition, the kinetic viscosity of the gear oil composition at 40°C is more preferably 34.5 to 35.0 ^mm2 /s, since this provides a greater effect in improving anti-seizure performance and fuel economy performance.

The gear oil composition of the present invention must have a kinetic viscosity of 8.0 mm ² /s or less at 100°C. When the gear oil composition has a kinetic viscosity of 8.0 mm ² /s or less at 100°C, it can have a lower viscosity in a relatively high temperature range near 100°C compared to when the kinetic viscosity exceeds 8.0 mm ² /s, and the stirring resistance can be reduced, so that the power transmission efficiency can be improved and the fuel saving performance can be improved. In addition, the lower limit of the kinetic viscosity of such a gear oil composition at 100°C is not particularly limited, but the kinetic viscosity of the gear oil composition at 100°C is preferably 6.5 mm ² /s or more. When the kinetic viscosity of the gear oil composition at 100°C is the lower limit or more, the oil film forming ability and oil film retention ability of the gear oil composition at the lubricated parts are further improved in a relatively high temperature range near 100°C (preferably about 80 to 120°C), and the oil film can be more uniformly maintained, so that the seizure resistance during use can be made more advanced. The kinetic viscosity of the gear oil composition at 100° C. is more preferably 7.0 to 7.5 mm ² /s, since this provides a greater effect in terms of improving anti-seizure performance and fuel economy performance.

The gear oil composition of the present invention preferably has a viscosity index greater than 170, and more preferably equal to or greater than 171. When the viscosity index is greater than 170, it is possible to further improve the viscosity-temperature characteristics and anti-wear properties of the lubricating oil composition, as well as to further improve fuel economy performance, compared to when the viscosity index is 170 or less.

The gear oil composition of the present invention preferably has a Brookfield viscosity (BF viscosity) of 15 Pa·s or less at -40°C. By setting the BF viscosity to the above upper limit or less, it is possible to further improve low-temperature fluidity. Such a Brookfield viscosity (BF viscosity) can be measured in accordance with ASTM D2983.

The method for producing the gear oil composition of the present invention is not particularly limited, and the gear oil composition may be prepared by appropriately selecting and mixing each of the components to be contained so as to obtain the gear oil composition of the present invention (satisfying the above conditions).

The use of the gear oil composition of the present invention is not particularly limited, and it can be suitably used, for example, as a lubricant for gear mechanisms such as transmissions (e.g., manual transmissions in various automobiles), differential gears, etc. The gear oil composition of the present invention can also be used, for example, as a common lubricant (dual-purpose oil) for both the transmissions and differential gears in various automobiles. Among such uses, the gear oil composition of the present invention is particularly preferably used as a gear oil composition for the transmissions of electric automobiles, from the viewpoint of excellent seizure resistance under poor lubrication.

The present invention will be explained in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

(Ingredients used in each example)
First, the base oils, thickeners, extreme pressure agents and other additives used in the examples are shown below.

[Base oil]
<Base oil (A1)>
Wax isomerized base oil (mineral hydrocarbon oil, API Group III, kinematic viscosity at 40° C. (KV40): 16.19 mm ² /s, kinematic viscosity at 100° C. (KV100): 3.912 mm ² /s, viscosity index (VI): 141, sulfur content in base oil (sulfur content in base oil): less than 10 ppm by mass).
<Base oil (A2)>
Hydrotreated base oil [mineral hydrocarbon oil, API Group III, kinematic viscosity at 40° C. (KV40): 34.1 mm ² /s, kinematic viscosity at 100° C. (KV100): 6.437 mm ² /s, viscosity index (VI): 144, sulfur content in base oil (sulfur content in base oil): less than 1 ppm by mass]
<Base oil (A3)>
Hydrotreated base oil [mineral hydrocarbon oil, API Group III, kinematic viscosity at 40°C (KV40): 19.4 ^mm2 /s, kinematic viscosity at 100°C (KV100): 4.23 ^mm2 /s, viscosity index (VI): 124, sulfur content in base oil (sulfur content in base oil): less than 1 ppm by mass].

<Base oil (A4)>
Ester oil (manufactured by NOF Corporation, trade name "Unistar H381R", kinematic viscosity at 40° C. (KV40): 48.7 mm ² /s, kinematic viscosity at 100° C. (KV100): 9.8 mm ² /s, viscosity index (VI): 192).

[Poly(meth)acrylate-based thickener]
<Thickener (B1)>
Polymethacrylate with a weight average molecular weight (Mw) of 12,200 [non-dispersed type]
<Thickener (B2): Comparative component>
Polymethacrylate with a weight average molecular weight (Mw) of 17,600 [non-dispersed type]
<Thickener (B3): Comparative component>
Polymethacrylate [non-dispersed type] with a weight average molecular weight (Mw) of 9610.

[Dispersant]
<Dispersant (C1)>
Bis-type boric acid modified succinimide [weight average molecular weight: 3000, nitrogen content: 2.2 mass%, boron content: 0.4 mass%]
<Dispersant (C2)>
Monotype boric acid modified succinimide [weight average molecular weight: 5000, nitrogen content: 2.0 mass%, boron content: 1.9 mass%]
[Other added ingredients (additives)]
Extreme Pressure Agent (D1)
Dibutyl polysulfide [Sulfur content in compound: 29.09% by mass]
<Extreme pressure agent (D2)>
Polysulfide [Sulfur content in compound: 30.91% by mass]
Viscosity Index Improver (E1)
Copolymer of ethylene and α-olefin [number average molecular weight (Mn): 2600]

(Examples 1 to 2 and Comparative Examples 1 to 9)
The gear oil compositions of Examples 1 and 2 and Comparative Examples 1 to 9 were each prepared using the above-mentioned components so as to have the composition shown in Table 1. In Table 1, in the "Composition of gear oil composition" section, "-" indicates that the component was not used. In Table 1, in the "Composition of gear oil composition" section, "mass%" represents the content (mass%) of the base oil based on mass relative to the total amount of the base oil composition (mixed base oil), and "inmass%" represents the content (mass%) based on mass relative to the total amount of the gear oil composition.

[Method for evaluating the characteristics of the gear oil compositions obtained in each example, etc.]
<Measurement of kinematic viscosity and viscosity index of gear oil composition>
The gear oil compositions obtained in each Example were measured for kinematic viscosity at 40°C, kinematic viscosity at 100°C, and viscosity index in accordance with JIS K2283-2000. The measurement results are shown in Table 1. In this specification, a gear oil composition that satisfies the conditions (low viscosity conditions) of a kinematic viscosity at 40°C of 36.0 ^mm2 /s or less and a kinematic viscosity at 100°C of 8.0 ^mm2 /s or less is evaluated as a low-viscosity gear oil composition. Table 1 includes an item for whether or not the low-viscosity condition is met, and those that meet the low-viscosity condition (low-viscosity ones) are indicated as "met", and those that do not meet the condition are indicated as "not met". Since a gear oil composition is required to have a low viscosity, particularly from the viewpoint of fuel saving, when the low-viscosity condition is met (met), it can be evaluated that a higher effect is obtained in terms of fuel saving.

<Measurement of Brookfield Viscosity (BF Viscosity) of Gear Oil Composition>
The BF viscosity at -40°C of the gear oil compositions obtained in each Example was measured in accordance with ASTM D2983 using a Brookfield viscosity thermostat/Brookfield viscometer as the measuring device at a temperature of -40°C. The results are shown in Table 1. Note that when the BF viscosity value is 15 Pa s or less, it can be evaluated that the fluidity at low temperatures is at a high level.

<Evaluation test of seizure resistance of gear oil composition>
Using a block-on-ring tester (LFW-1) conforming to ASTM D 2174, a block-shaped test piece (Falex H-60 Test Block, SAE01 Steel: hereinafter simply referred to as "block") and a ring-shaped test piece (Falex S-10 Test Ring, SAE4620 First, the gear oil composition was supplied to an oil bath so that the ring was in line contact (block-on-ring) with the gear oil composition in the oil bath (about half of the ring was immersed in the oil bath), and a break-in was carried out for 1 minute under the following conditions (break-in conditions): oil temperature: room temperature (about 25° C.), sliding speed: 1 m/s, surface pressure (load) on the contact surface between the ring and the block: 0 GPa (no load). After spreading over the contact surface of the block, the drain cock was opened to extract all of the gear oil composition from the oil bath, creating a poor lubrication state (a state in which the gear oil composition was not replenished except for the gear oil composition spread during the break-in), and then the oil temperature of the gear oil composition was left to the course of the test, and the block-on-ring tester was operated (tested) under the conditions of a sliding speed of 2.5 m/s and a surface pressure (load) of the contact surface between the ring and the block of 0.25 GPa, and the time until seizure occurred (seizure time) was determined. Note that, since the friction coefficient μ and vibration that can be measured by the block-on-ring tester increase when seizure occurs, in this test, the elapsed time from the start of the above operation (test) after the break-in operation described above until the increase in the friction coefficient μ occurs was determined as the "seizure time". The results obtained are shown in Table 1. Note that a seizure time of 900 seconds or more can be evaluated as excellent seizure resistance.

<Wear resistance test>
The gear oil compositions obtained in each Example were subjected to a Shell four-ball test (ASTM D4172) under the conditions of load: 392 N, rotation speed: 100 rpm, temperature: 40° C., and test time: 1 hour, and the wear scar diameter (mm) was measured. The results are shown in Table 1. A wear scar diameter of 0.4 mm or less can be evaluated as having excellent wear resistance.

<Test to confirm the presence or absence of strong acid value after ISOT test>
An ISOT test (temperature: 135°C, time: 96 hours) was conducted in accordance with JIS K 2514-1, and the gear oil composition after the ISOT test was confirmed for strong acid number in accordance with JIS K2501:2003. The results of such confirmation tests are shown in Table 1. In Table 1, those in which a strong acid number was confirmed are indicated as "Yes", and those in which a strong acid number was not confirmed are indicated as "No".

As is clear from the results shown in Table 1, first, in Examples 1 and 2, the base oil composition used in the gear oil composition contains 10 mass% or more of a mineral base oil (A1) that satisfies the conditions that the kinematic viscosity at 100°C is 3.5 to 4.5 ^mm2 /s and the viscosity index is 140 or more, and the content of the mineral base oil (base oil (A1) and base oil (A2)) in the base oil composition is 100 mass%. Furthermore, the gear oil compositions obtained in Examples 1 and 2 contain 1 to 4 mass% of a thickener made of a poly(meth)acrylate having a weight average molecular weight of 15,000 or less based on the total amount of the gear oil composition. Furthermore, the gear oil compositions obtained in Examples 1 and 2 contain 1.4 mass% of a bis-type succinimide dispersant (bis-type boric acid modified succinimide). Furthermore, the gear oil compositions obtained in Examples 1 and 2 satisfied the conditions that the kinematic viscosity at 40°C was 36.0 mm ² /s or less and the kinematic viscosity at 100°C was 8.0 mm ² /s or less (furthermore, the viscosity index also showed a value greater than 170). The gear oil compositions obtained in Examples 1 and 2 having such characteristics satisfied the low viscosity condition (conformed to the low viscosity condition) and had high seizure resistance. In addition, the gear oil compositions obtained in Examples 1 and 2 were not confirmed to have a strong acid value after the ISOT test, and it was also confirmed that they had excellent oxidation stability. Thus, it was confirmed that the gear oil compositions obtained in Examples 1 and 2 were low-viscosity and excellent seizure resistance gear oil compositions, as well as excellent oxidation stability. It was also confirmed that the gear oil compositions obtained in Examples 1 and 2 had excellent wear resistance, with a wear scar diameter of 0.4 mm or less after a Shell four-ball test, and had a BF viscosity of 15 Pa s or less, indicating a high level of fluidity at low temperatures.

On the other hand, the gear oil composition obtained in Comparative Example 1, in which polymethacrylate with a weight-average molecular weight of 12,200 was used as a thickener but the content of said polymethacrylate was 5 mass% based on the total amount of the gear oil composition, satisfied the above-mentioned condition of low viscosity, but did not have sufficient seizure resistance, and was unable to achieve both low viscosity and excellent seizure resistance.

In addition, the gear oil compositions obtained in Comparative Examples 2 and 3, in which polymethacrylate with a weight-average molecular weight of 17,600 was used as a thickener instead of polymethacrylate with a weight-average molecular weight of 12,200, met the low viscosity condition, but did not have sufficient seizure resistance, and were unable to achieve both low viscosity and excellent seizure resistance.

In addition, the gear oil compositions obtained in Comparative Examples 4 to 6, which did not use a thickener, met the low viscosity requirement, but did not have sufficient seizure resistance, and were unable to achieve both low viscosity and excellent seizure resistance.

Furthermore, the gear oil composition obtained in Comparative Example 7, which did not use base oil (A1) but instead used a combination of base oil (A2) and base oil (A3) with a viscosity index of 124, achieved a sufficient level of seizure resistance, but did not satisfy the low viscosity condition, and was unable to achieve both low viscosity and excellent seizure resistance.

In addition, the gear oil composition obtained in Comparative Example 8, in which the base oil composition consisted only of base oil (A2), had a sufficient level of anti-seizure properties, but did not satisfy the low viscosity condition, and was unable to achieve both low viscosity and excellent anti-seizure properties.

Furthermore, the gear oil composition obtained in Comparative Example 9, which used base oil (A4) without using a thickener, achieved both low viscosity and excellent seizure resistance, but was confirmed to have a strong acid value after the ISOT test, and was not sufficient in terms of oxidation stability. Furthermore, when Comparative Example 4 and Comparative Example 9 are compared, they have similar compositions except for the composition of the base oil composition, so it was found that when the content of mineral oil-based base oil is less than 93 mass% (Comparative Example 9), the oxidation stability decreases.

The characteristics of the gear oil compositions obtained in Examples 1-2 and Comparative Examples 1-9 shown in Table 1 demonstrate that the present invention can provide a gear oil composition that has low viscosity and excellent fuel economy performance while also having excellent anti-seizure performance and excellent oxidation stability.

As explained above, according to the present invention, it is possible to provide a gear oil composition that can have a low kinetic viscosity based on the kinetic viscosity at 40°C and 100°C, while having excellent anti-seizure performance and excellent oxidation stability. Due to its properties, the gear oil composition of the present invention is particularly useful as a gear oil composition for the transmission of an electric vehicle.

Claims

A gear oil composition comprising: (A) a base oil composition; (B) a poly(meth)acrylate thickener; and (C) a succinimide dispersant;
The component (A) is a base oil composition containing 10 mass % or more of a mineral base oil (A-1) having a kinematic viscosity at 100°C of 3.5 to 4.5 mm2 /s and a viscosity index of 140 or more, based on the total amount of the base oil composition;
The content of the mineral oil base oil in the component (A) is 93% by mass or more,
The component (B) is a poly(meth)acrylate having a weight average molecular weight of 15,000 or less,
The component (C) is at least one selected from the group consisting of bis-type unmodified succinimides and bis-type boric acid modified succinimides,
The content of the component (B) based on the total amount of the gear oil composition is 1 to 4 mass %,
The content of the component (C) based on the total amount of the gear oil composition is 1 to 3 mass %,
The gear oil composition has a kinematic viscosity at 40°C of 36.0 mm2 /s or less, and
The gear oil composition has a kinematic viscosity at 100° C. of 8.0 mm 2 /s or less.
The gear oil composition according to claim 1, wherein component (C) is a bis-type boric acid modified succinimide.
2. The gear oil composition according to claim 1, wherein the component (A) contains 40 mass% or more of a mineral base oil (A-2) having a kinematic viscosity at 100°C of 6.0 to 7.0 mm2 /s and a viscosity index of 140 or more, based on the total amount of the base oil composition.
The gear oil composition according to claim 1, which is a gear oil composition for an electric vehicle transmission.