Lubricating oil composition

Abstract

To provide a lubricant composition for a heavy-duty diesel engine that satisfies various standard values for a multigrade engine oil and has even better low-temperature fluidity.SOLUTION: A lubricant composition for a heavy-duty diesel engine comprises: a base oil (A) with a %CP of 90 or more as specified by ASTM D3238 (Standard Test Method for Carbon Distribution and Structural Group Analysis of Petroleum by n-d-M Method); and a viscosity index improver (B) containing a hydrogenated styrene-diene polymer (b) having a weight average molecular weight (Mw) of less than 580,000, wherein HTHS viscosity at 150°C is 3.5 mPa s or more, CCS viscosity at -35°C is 6,200 mPa s or less, and MRV viscosity at -40°C is 60,000 mPa s or less. A lubrication method comprises using the lubricant composition for the heavy-duty diesel engine.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a lubricating oil composition. [Background technology]

[0002] Lubricating oil compositions used in internal combustion engines such as gasoline and diesel engines require lubrication performance over a wide temperature range, including lubrication performance at low temperatures from engine startup in cold regions to very early stages of driving (immediately after engine startup), and lubrication performance at high temperatures and high loads during engine operation. For this reason, lubricating oil compositions are required to have minimal viscosity changes at both low and high temperatures. To meet these requirements, multigrade engine oils exist, and their viscosity is specified in SAE J 300-201501. Multigrade engine oils are required to conform to the aforementioned specifications for high-temperature high-shear viscosity at 150°C (HTHS viscosity at 150°C), low-temperature cranking viscosity (CCS viscosity), and low-temperature pumping viscosity (MRV viscosity). For example, Patent Document 1 discloses improving low-temperature fluidity, evaporability, and oxidation stability by using a specific base oil. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2007-137952 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, Patent Document 1 uses a specific synthetic oil as the base oil, thus limiting the materials that can be used. Furthermore, there is a demand for further improvement in low-temperature fluidity in extremely cold regions. The present invention has been made in view of the above problems, and aims to provide a lubricating oil composition for heavy-duty diesel engines that satisfies various standard values ​​for multi-grade engine oils while having even better low-temperature fluidity. [Means for solving the problem]

[0005] As a result of diligent research by the inventors, we have found that the above problems can be solved by a heavy-duty diesel engine lubricant composition containing a specific viscosity index improver in combination with a specific base oil, and have completed the present invention.

[0006] In other words, the present invention provides the following [1] and [2]. [1] %C as defined in ASTM D3238 (Standard Test Method for Analysis of Carbon Distribution and Structural Groups in Petroleum by the ndM Method) P However, it contains a base oil (A) with a weight-average molecular weight (Mw) of 90 or more, and a viscosity index improver (B) containing a hydrogenated styrene-diene polymer (b) with a weight-average molecular weight (Mw) of less than 580,000. The high-temperature high-shear viscosity (HTHS viscosity at 150°C) is 3.5 mPa·s or higher. The low-temperature cranking viscosity (CCS viscosity at -35°C) is 6,200 mPa·s or less. The low-temperature pumping viscosity (MRV viscosity at -40°C) is 60,000 mPa·s or less. Lubricant composition for heavy-duty diesel engines. [2] A method for lubricating a heavy-duty diesel engine using the lubricating oil composition described in [1]. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a lubricating oil composition for heavy-duty diesel engines that satisfies various specifications for multi-grade engine oils while also having superior low-temperature fluidity. [Modes for carrying out the invention]

[0008] The embodiments of the present invention (hereinafter sometimes referred to as "these embodiments") will be described below. The upper and lower limit values of the numerical ranges described in this specification can be arbitrarily combined. For example, when the numerical ranges "A to B" and "C to D" are described, the numerical ranges "A to D" and "C to B" are also included in the scope of the present invention. Also, the numerical range "lower limit value to upper limit value" described in this specification means, unless otherwise specified, that it is not less than the lower limit value and not more than the upper limit value. In addition, in this specification, the numerical values in the examples are numerical values that can be used as the upper limit value or the lower limit value. In this specification, for example, "(meth)acrylate" is used as a term indicating both "acrylate" and "methacrylate", and the same applies to other similar terms and the same notations.

[0009] [Lubricating Oil Composition for Heavy Duty Diesel Engines] The lubricating oil composition for a heavy-duty diesel engine of the present embodiment contains a base oil (A) with a %C P of 90 or more defined by ASTM D3238 (Standard Test Method for Carbon Distribution and Structural Group Analysis of Petroleum by the n-d-M Method), and a viscosity index improver (B) containing a hydrogenated styrene-diene polymer (b) with a weight average molecular weight (Mw) of less than 580,000. The high-temperature high-shear viscosity at 150°C (HTHS viscosity at 150°C) is 3.5 mPa·s or more, the low-temperature cranking viscosity at -35°C (CCS viscosity at -35°C) is 6,200 mPa·s or less, and the low-temperature pumping viscosity at -40°C (MRV viscosity at -40°C) is 60,000 mPa·s or less.

[0010] As described above, for multi-grade engine oils, it is required that the high-temperature high-shear viscosity at 150°C (HTHS viscosity at 150°C), low-temperature cranking viscosity (CCS viscosity), low-temperature pumping viscosity (MRV viscosity), etc. comply with the standard values. The lubricant composition of the present embodiment contains a mineral oil (A) and a viscosity index improver (B) described below, and has a high-temperature high-shear viscosity at 150°C (HTHS viscosity at 150°C), a low-temperature cranking viscosity (CCS viscosity), and a low-temperature pumping viscosity (MRV viscosity) that satisfy the standard values of a multigrade engine oil, while reducing the viscosity at -40°C measured by the standard test method for the low-temperature, low-shear rate, viscosity / temperature dependence of lubricating oils using the temperature scanning method described in ASTM D5133 (hereinafter referred to as "viscosity at -40°C measured by ASTM D51). It was found that it has excellent low-temperature fluidity. And it was found that various properties of the lubricant composition of the present embodiment are particularly suitable for heavy-duty diesel engines.

[0011] <HTHS viscosity at 150°C> The HTHS viscosity at 150°C of the lubricant composition for heavy-duty diesel engines of the present embodiment is 3.5 mPa·s or more. If it is less than 3.5 mPa·s, the formation of the oil film becomes insufficient and it cannot function as a lubricant composition. As the upper limit value, in order to improve the low-temperature fluidity, it is preferably 4.0 mPa·s or less. The HTHS viscosity at 150°C can be measured, for example, by the method described in the examples.

[0012] <CCS viscosity at -35°C> The CCS viscosity at - / 35°C of the lubricant composition for heavy-duty diesel engines of the present embodiment is 6,200 mPa·s or less. If it exceeds 6,200 mPa·s, the low-temperature fluidity is not improved. In order to improve the low-temperature fluidity, it is preferably 5,500 mPa·s or less, more preferably 5,000 mPa·s or less, and even more preferably 4,900 mPa·s or less. The lower limit value is not particularly limited. The CCS viscosity at -35°C can be measured, for example, by the method described in the examples.

[0013] <MRV viscosity at -40°C> The MRV viscosity of the lubricating oil composition for a heavy-duty diesel engine of the present embodiment at -40°C is 60,000 mPa·s or less. If it exceeds 6,000 mPa·s, the low-temperature fluidity cannot be improved. In order to improve the low-temperature fluidity, it is preferably 40,000 mPa·s or less, and more preferably 30,000 mPa·s or less. The lower limit is not particularly limited. The MRV viscosity at -40°C can be measured, for example, by the method described in the examples.

[0014] Hereinafter, each component contained in the lubricating oil composition for a heavy-duty diesel engine of the present embodiment will be described.

[0015] <Base oil (A)> The base oil (A) used in the present embodiment has %C P of 90 or more as defined by ASTM D3238 (Standard Test Method for Carbon Distribution and Structure Group Analysis of Petroleum by the n-d-M Method). P If it is less than 90, the low-temperature fluidity cannot be sufficiently improved. %C P Since it can improve the low-temperature fluidity, it is preferably 91 or more.

[0016] %C A Since it can improve the low-temperature fluidity, it is preferably 1 or less. %C N Since it can improve the low-temperature fluidity, it is preferably less than 10, and more preferably less than nine. The lower limit is not particularly limited, but from the viewpoint of easy availability, it is preferably 3 or more.

[0017] In this specification, %C P %, %C N and %C A mean the ratios (percentages) of the paraffin content, naphthene content, and aromatic content defined by ASTM D3238 (Standard Test Method for Carbon Distribution and Structure Group Analysis of Petroleum by the n-d-M Method).

[0018] The content of the base oil (A) in the total amount (100% by mass) of the heavy-duty diesel engine lubricant composition is preferably 50.0% by mass or more and 80.0% by mass or less, more preferably 55.0% by mass or more and 79.0% by mass or less, and even more preferably 60.0% by mass or more and 78.0% by mass or less, in order to improve low-temperature fluidity.

[0019] The base oil (A) used in this embodiment is preferably a base oil classified as Group II or III of the base oil categories of the American Petroleum Institute (API), and more preferably a base oil classified as Group III.

[0020] The kinematic viscosity and viscosity index of the base oil (A) are preferably within the following ranges, with the upper limit being from the viewpoint of achieving good fuel efficiency, and the lower limit being from the viewpoint of reducing the loss of the lubricating oil composition due to evaporation and ensuring oil film retention.

[0021] The kinematic viscosity of the base oil (A) at 40°C (kinematic viscosity (40°C)) is 13.0 mm 2 Preferably 15.0 mm / s or more. 2 / s or more is more preferable, and 18.0 mm 2 A value of / s or higher is even more preferable. The upper limit is 25.0 mm. 2 Preferably less than / s, and 23.0 mm 2 A value of / s or less is more preferable, and 22.0 or less is even more preferable.

[0022] The kinematic viscosity (kinematic viscosity (100°C)) of the base oil (A) at 100°C is 3.5 mm 2 Preferably 3.7 mm or more 2 / s or more is more preferable, and 3.8mm 2 A value of / s or higher is even more preferable, with an upper limit of 5.0 mm. 2 A value of / s or less is preferable.

[0023] The viscosity index of the base oil (A) is preferably 120 or higher. The upper limit is not particularly limited. The kinematic viscosity at 40°C, the kinematic viscosity at 100°C, and the viscosity index can be measured or calculated, for example, by the method described in the examples. Furthermore, if the base oil (A) is a mixed base oil containing two or more base oils, it is preferable that the kinematic viscosity and viscosity index of the mixed base oil are within the above range.

[0024] <Viscosity index improver (B)> The viscosity index improver (B) used in this embodiment contains a styrene-hydrogenated polymer (b) with a weight-average molecular weight (Mw) of less than 580,000. If a styrene-hydrogenated polymer (b) with a weight-average molecular weight of 580,000 or more is included, the low-temperature fluidity deteriorates.

[0025] The viscosity index improver (B) used in this embodiment is obtained by diluting the solid component with a diluent oil, and contains the hydrogenated styrene-diene polymer (b) as the solid component. The diluent oil can be any type commonly used as a base oil in lubricating oil compositions, but a base oil classified as Group I or III of the base oil category is preferred. A base oil classified as Group III is particularly preferred.

[0026] (Hydrogenated styrene-diene polymer (b)) The hydrogenated styrene-diene polymer (b) used in this embodiment has a weight-average molecular weight (Mw) of less than 580,000. If the Mw is 580,000 or more, excellent low-temperature fluidity cannot be obtained.

[0027] The weight-average molecular weight (Mw) of the hydrogenated styrene-diene polymer (b) used in this embodiment is preferably less than 560,000, and more preferably less than 550,000, in order to improve low-temperature fluidity. The lower limit is preferably 100,000 or more, more preferably 120,000 or more, and even more preferably 140,000 or more.

[0028] In this embodiment, the molecular weight distribution (Mw / Mn) of the hydrogenated styrene-diene polymer (b) is preferably 1.0 or more and less than 2.0 in order to improve low-temperature fluidity. In this specification, the weight-average molecular weight (Mw) and number-average molecular weight (Mn) of each component are values ​​on a standard polystyrene basis measured by gel permeation chromatography (GPC), which can be measured, for example, by the method described in the Examples.

[0029] The hydrogenated styrene-diene polymer (b) used in this embodiment may be any copolymer having constituent units derived from styrene and diene, respectively.

[0030] The diene may be a non-conjugated diene or a conjugated diene, but a conjugated diene is preferred, and isoprene or butadiene is more preferred.

[0031] The content of the hydride styrene-diene polymer (b) in the total amount (100% by mass) of the heavy-duty diesel engine lubricating oil composition is preferably 1.5% by mass or more, more preferably 1.6% by mass or more, even more preferably 1.7% by mass or more, with an upper limit of preferably 5.0% by mass or less, more preferably 4.0% by mass or less, even more preferably 3.0% by mass or less, even more preferably 2.5% by mass or less, and particularly preferably 2.2% by mass or less.

[0032] The solid component of the viscosity index improver contained in the lubricating oil composition for heavy-duty diesel engines may be only styrene-hydrogenated diene polymer (b), or a combination of two or more may be used. When two or more are used in combination, poly(meth)acrylate is preferred.

[0033] <Pour point depressant (C)> The heavy-duty diesel engine lubricant composition of this embodiment may further contain a pour point depressant (C). Including a pour point depressant (C) can further improve low-temperature fluidity. Examples of the pour point depressant include polymethacrylate, alkylated aromatic compounds, copolymers of fumarate and vinyl acetate, and copolymers of ethylene and vinyl acetate, with polymethacrylate having a weight-average molecular weight (Mw) of 40,000 to 200,000 being preferred. Furthermore, the weight-average molecular weight (Mw) of these polymers used as the pour point depressant is preferably 50,000 to 150,000, and more preferably 50,000 to 100,000. The molecular weight distribution (Mw / Mn) is preferably 1.3 or more and less than 2.0, and more preferably 1.6 or more and 1.8 or less, in order to improve low-temperature fluidity. These may be used individually or in combination of two or more types.

[0034] The amount of the pour point depressant (C) in the total lubricating oil composition (100% by mass) is not particularly limited, but is used in a proportion of 0.1% by mass or more and 1.0% by mass or less.

[0035] <Other ingredients> The heavy-duty diesel engine lubricant composition of this embodiment may contain other components besides those mentioned above, as necessary, as long as they do not impair the effects of the present invention. Examples of additives used as other components, particularly in heavy-duty applications, include metal-based detergents, antioxidants, wear inhibitors, defoamers, dispersants, and metal deactivators. These may be used individually or in combination of two or more types.

[0036] (Metal-based cleaning agent) Examples of the aforementioned metal-based detergents include organic acid metal salt compounds containing metal atoms selected from alkali metals and alkaline earth metals. Specifically, examples include metal salicylates, metal phenates, and metal sulfonates containing metal atoms selected from alkali metals and alkaline earth metals.

[0037] From the viewpoint of improving high-temperature cleaning and dispersibility, and from the viewpoint of solubility in base oil, it is preferable that the material be one or more selected from calcium salicylate, calcium phenate, calcium sulfonate, magnesium salicylate, magnesium phenate, and magnesium sulfonate.

[0038] These metal-based detergents may be neutral salts, basic salts, overbasic salts, or mixtures thereof. The base number is not particularly limited.

[0039] (Antioxidant) Examples of the aforementioned antioxidants include amine-based antioxidants and phenol-based antioxidants. Examples of amine-based antioxidants include diphenylamine and alkylated diphenylamines having an alkyl group with 3 to 20 carbon atoms.

[0040] Examples of phenolic antioxidants include 2,6-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, isooctyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 4,4'-methylenebis(2,6-di-tert-butylphenol), and 2,2'-methylenebis(4-ethyl-6-tert-butylphenol). These may be used individually or in combination of two or more types.

[0041] (Abrasion-resistant agent) Zinc dialkyldithiophosphate (ZnDTP) is preferred as the wear-resistant agent.

[0042] (Antifoaming agent) Examples of the antifoaming agent include silicone oils such as dimethylpolysiloxane, fluorosilicone oils, and fluoroalkyl ethers. These may be used individually or in combination of two or more types.

[0043] (Dispersant) As the dispersant, one or more selected from non-boron-modified succinimides and boron-modified succinimides can be used, and it is preferable to combine one or more selected from non-boron-modified succinimides with one or more selected from boron-modified succinimides.

[0044] (Metal deactivator) Examples of the metal deactivators include benzotriazole compounds, tolyltriazole compounds, and thiadiazole compounds. These may be used individually or in combination of two or more types.

[0045] The content of the other components mentioned above can be adjusted as appropriate within a range that does not impair the effects of the present invention. The heavy-duty diesel engine lubricant composition of this embodiment may contain only a base oil (A), a viscosity index improver (B), and a pour point depressant (C) added as needed. However, if other components are included, the total content of the other components is preferably 0.1% by mass or more and 30.0% by mass or less, more preferably 5.0% by mass or more and 20.0% by mass or less, and even more preferably 8.0% by mass or more and 15.0% by mass or less, based on the total amount (100% by mass) of the heavy-duty diesel engine lubricant composition.

[0046] [Physical properties of lubricating oil compositions] (Kinematic viscosity at 40°C) The kinematic viscosity (kinematic viscosity (40°C)) of the heavy-duty diesel engine lubricant composition of this embodiment at 40°C is 65.0 mm². 2 / s or more 85.0mm 2 It is preferable that the kinematic viscosity is less than or equal to / s. The kinematic viscosity at 40°C can be measured, for example, by the method described in the examples.

[0047] (Kinematic viscosity at 100°C) The kinematic viscosity (kinematic viscosity (100°C)) of the heavy-duty diesel engine lubricant composition of this embodiment at 100°C is 9.3 mm in order to maintain oil film formation and achieve high wear resistance. 2 / s or more 16.3mm 2 It is preferable that the value be less than or equal to / s, and 11.5 mm 2 / s or more 16.0mm 2 It is more preferable that the kinematic viscosity is less than or equal to / s. The kinematic viscosity at 100°C can be measured, for example, by the method described in the examples.

[0048] (viscosity index) The viscosity index of the heavy-duty diesel engine lubricant composition of this embodiment is preferably 150 or higher, more preferably 170 or higher, even more preferably 180 or higher, and even more preferably 188 or higher, in order to improve low-temperature fluidity, maintain oil film formation over a wide temperature range, and achieve high wear resistance. The upper limit is not particularly limited, but is preferably 250 or lower. The viscosity index can be measured, for example, by the method described in the examples.

[0049] (Pour point) The pour point of the heavy-duty diesel engine lubricant composition of this embodiment is preferably -70.0°C to -35.0°C, more preferably -66.0°C to -40.0°C, and even more preferably -63.0°C to -42.0°C, in order to improve low-temperature fluidity. The pour point can be measured, for example, by the method described in the examples.

[0050] <Uses of lubricating oil compositions> The lubricating oil composition for heavy-duty diesel engines according to this embodiment exhibits excellent low-temperature fluidity. The heavy-duty diesel engine lubricant composition of this embodiment is used for heavy-duty diesel engines. Heavy-duty diesel engines are diesel engines installed in various vehicles and machines that operate under harsh operating conditions, such as large trucks, large buses, construction machinery, and agricultural machinery, and require a lubricant composition with excellent low-temperature fluidity.

[0051] [Engine lubrication methods] The lubrication method for a heavy-duty diesel engine in this embodiment is a method for lubricating a heavy-duty diesel engine using the heavy-duty diesel engine lubricating oil composition described above. The lubricating oil composition for heavy-duty diesel engines of this embodiment improves low-temperature fluidity and exhibits excellent lubrication performance over a wide temperature range. Therefore, the lubrication method for heavy-duty diesel engines of this embodiment can provide the heavy-duty diesel engine with excellent lubrication performance in a wide range of operating environments.

[0052] [One aspect of the present invention provided] According to one aspect of the present invention, the following [1] to [4] are provided. [1] %C as defined in ASTM D3238 P A heavy-duty diesel engine lubricant composition comprising a base oil (A) with a molecular weight of 90 or higher, and a viscosity index improver (B) containing a hydrogenated styrene-diene polymer (b) with a weight-average molecular weight (Mw) of less than 580,000, wherein the high-temperature high-shear viscosity (HTHS viscosity at 150°C) is 3.5 mPa·s or higher, the low-temperature cranking viscosity (CCS viscosity at -35°C) is 6,200 mPa·s or lower, and the low-temperature pumping viscosity (MRV viscosity at -40°C) is 60,000 mPa·s or lower. [2] The heavy-duty diesel engine lubricant composition according to [1], wherein the content of the hydrogenated styrene-diene polymer (b) is 1.5% by mass or more based on the total amount (100% by mass) of the lubricant composition. [3] The heavy-duty diesel engine lubricant composition according to [1] or [2], wherein the molecular weight distribution (Mw / Mn) of the hydrogenated styrene-diene polymer (b) is 1.0 or more and less than 2.0. [4] A method for lubricating a heavy-duty diesel engine using a lubricating oil composition described in any of [1] to [3]. [Examples]

[0053] The present invention will be specifically described by the following examples, but the present invention is not limited to these examples. The various properties of each component used in the examples and comparative examples and the resulting lubricating oil compositions were measured by the following methods.

[0054] (1)Kinematic viscosity In accordance with ASTM D455, the kinematic viscosity at 40°C and the kinematic viscosity at 100°C were measured. (2) Viscosity index Measurements were taken in accordance with ASTM D2270. (3) Low-temperature cranking viscosity at -35°C (CCS viscosity at -35°C) The cranking viscosity at -35°C (CCS viscosity (-35°C)) was measured in accordance with ASTM D 5293.

[0055] (4) High-temperature high-shear viscosity at 150°C (HTHS viscosity at 150°C) The HTHS viscosity at 150°C (HTHS viscosity (150°C)) was measured or calculated in accordance with ASTM D 4683. (5) Low-temperature pumping viscosity at -40°C (MRV viscosity at -40°C) The low-temperature pumping viscosity (MRV viscosity (-40°C)) at -40°C was measured in accordance with ASTM D 4684. (6) Pour point The pour point was measured in accordance with the method described in JIS K2269-1987.3.

[0056] (7) Viscosity at -40°C as measured by the standard test method for low temperature, low shear rate, viscosity / temperature dependence of lubricating oils using the temperature scanning method described in ASTM D5133. The viscosity at -40°C was measured in accordance with the standard test method for low-temperature, low-shear-rate viscosity / temperature dependence of lubricating oils using the temperature scanning method described in ASTM D5133. A value of 30,000 mPa·s or less was considered acceptable.

[0057] (8) Weight average molecular weight (Mw), number average molecular weight (Mn), molecular weight distribution (Mw / Mn) Mw, Mn, and Mw / Mn were determined in accordance with JIS K 7252-4:2016 and JIS K 7252-2:2016. The measurements were performed by gel permeation chromatography (GPC). For the GPC method, a Tosoh HLC-8321GPC / HT was used, and two Tosoh GMHHR-H(S)HT columns were connected in series for the measurements. Trichlorobenzene was used as the measurement solvent, with a column temperature of 145°C and a flow rate of 1.0 ml / min.

[0058] (9) Ring analysis (%C A , %C N and %C P ) The percentage of aromatic content calculated by the ring analysis ndM method is %C. A The percentage of naphthenes (percentage) is %C N The percentage of paraffin content is %C P This is indicated as such and is specified in ASTM D3238 (Standard Test Method for Analysis of Carbon Distribution and Structural Groups in Petroleum by the ndM Method).

[0059] (Examples 1-4 and Comparative Examples 1-5) The following were used as base oil (A), viscosity index improver (B), pour point depressant (C), and other additives.

[0060] <Base oil (A)> Base oil (A): Base oils 1 to 4 having the physical properties listed in Table 1 were used.

[0061] [Table 1]

[0062] In the table, "< 1" means %C A This means that the value is less than 1.

[0063] <Viscosity index improver (B)> Viscosity index improvers (B): As styrene-hydrogenated diene polymers (b) or comparative styrene-hydrogenated diene copolymers, styrene-hydrogenated diene copolymers with Mw and Mw / Mn values ​​as shown below were used, in the diluent oil at a total viscosity index improver basis (100% by mass) content as shown below. The content of viscosity index improvers 1 to 6 in Tables 2 and 3 refers to the content of viscosity index improvers 1 to 6 in the total lubricant composition (100% by mass). Furthermore, the diluent oils shown below refer to the base oil categories of the American Petroleum Institute (API).

[0064] Viscosity index improver 1: Hydrogenated styrene-diene copolymer (Mw=450,000, Mw / Mn=1.6), content = 15.0% by mass, diluent = Group III Viscosity index improver 2: Hydrogenated styrene-diene copolymer (Mw=550,000, Mw / Mn=1.5), content = 13.8% by mass, diluent = Group I Viscosity index improver 3: Hydrogenated styrene-diene copolymer (Mw=150,000, Mw / Mn=1.1), content = 7.6% by mass, diluent = Group III Viscosity index improver 4: Hydrogenated styrene-diene copolymer (Mw=600,000, Mw / Mn=1.9), content = 10.7% by mass, diluent = Group I Viscosity index improver 5: Hydrogenated styrene-diene copolymer (Mw=600,000, Mw / Mn=1.9), content = 10.7% by mass, diluent = Group III Viscosity index improver 6: Hydrogenated styrene-diene copolymer (Mw=690,000, Mw / Mn=1.8), content = 9.0% by mass, diluent = Group I

[0065] <Pour point depressant (C)> Pour point depressant (C): Polymethacrylate (VISCOPLEX 1-500 (manufactured by EVONIK)) was used as the pour point depressant.

[0066] <Other additives> Metal-based cleaning agents, antioxidants, wear-resistant agents, defoaming agents, dispersants, etc.

[0067] To each base oil listed in Tables 2 and 3, each component listed in Tables 2 and 3 was added in the amounts listed in Tables 2 and 3, and thoroughly mixed to obtain a lubricating oil composition. Tables 2 and 3 show the content of styrene-hydrogenated polymer (100% by mass) in each lubricating oil composition based on the total amount. A blank space in the table indicates that the composition does not contain the corresponding ingredient. Tables 2 and 3 show the various properties of the lubricating oil compositions of Examples 1-4 and Comparative Examples 1-5.

[0068] [Table 2]

[0069] [Table 3]

[0070] As can be seen from Table 2, the lubricating oil compositions of Examples 1 to 4, which satisfy all the components of the present invention, were found to have low viscosity at -40°C as measured by ASTM D5133, and while the CCS viscosity at -35°C and MRV viscosity at -40°C met the standard values, they also exhibited excellent low-temperature fluidity. On the other hand, the lubricating oil compositions of Comparative Examples 1 to 3, which do not contain viscosity index improver (B) as shown in Table 3, showed high viscosity at -40°C as measured by ASTM D5133, indicating poor low-temperature fluidity. The lubricating oil compositions of Comparative Examples 4 and 5, which do not contain base oil (A), also showed high viscosity at -40°C as measured by ASTM D5133, indicating poor low-temperature fluidity compared to the lubricating oil compositions of Examples 1 to 4.

Claims

1. %C as defined in ASTM D3238 Ｐ However, it contains a base oil (A) with a weight-average molecular weight (Mw) of 90 or more, and a viscosity index improver (B) containing a hydrogenated styrene-diene polymer (b) with a weight-average molecular weight (Mw) of less than 580,000. The high-temperature high-shear viscosity (HTHS viscosity at 150°C) at 150°C is 3.5 mPa·s or higher. The low-temperature cranking viscosity (CCS viscosity at -35°C) is 6,200 mPa·s or less. The low-temperature pumping viscosity (MRV viscosity at -40°C) is 60,000 mPa·s or less. Lubricant composition for heavy-duty diesel engines.

2. The heavy-duty diesel engine lubricant composition according to claim 1, wherein the content of the hydrogenated styrene-diene polymer (b) on a basis of the total amount (100% by mass) of the lubricant composition is 1.5% by mass or more.

3. The heavy-duty diesel engine lubricant composition according to claim 1 or 2, wherein the molecular weight distribution (Mw / Mn) of the hydrogenated styrene-diene polymer (b) is 1.0 or more and less than 2.

0.

4. A method for lubricating a heavy-duty diesel engine using the lubricating oil composition described in claim 1 or 2.

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