EP4644513A1

EP4644513A1 - Lubricating oil composition

Info

Publication number: EP4644513A1
Application number: EP23911960.5A
Authority: EP
Inventors: Kenji Kobayashi; Toshiki Kameyama
Original assignee: Idemitsu Kosan Co Ltd
Current assignee: Idemitsu Kosan Co Ltd
Priority date: 2022-12-27
Filing date: 2023-12-22
Publication date: 2025-11-05
Also published as: WO2024143180A1; JP2024093384A; CN120344642A

Abstract

There is a need for a shock absorber lubricating oil composition having excellent hydraulic response and component protection. Provided is a lubricating oil composition used for lubricating shock absorbers, the lubricating oil composition containing: a base oil (A) having a kinematic viscosity of 5.8 mm<sup>2</sup>/s or less at 100°C; a polyalkyl (meth)acrylate (B) having a weight average molecular weight of at least 100,000; a phosphorus-containing compound (C) having an acid value of at least 1.0 mgKOH/g; and a fatty acid ester (D).

Description

Technical Field

The present invention relates to a lubricating oil composition, and a method for using the lubricating oil composition.

Background Art

A shock absorber is a mechanism used by being filled with a lubricating oil composition for a shock absorber, and installed on an automobile body for the purpose of producing a damping force that damps the vibration of an automobile body, and at the same time, is required to optimize the frictional characteristics of a sliding part to control ride comfort of an automobile body, to suppress frictional wear of a sliding part to ensure durability, etc.
Various lubricating oil compositions for a shock absorber that can be suitably used in such a shock absorber have been developed.
For example, Patent Literature 1 discloses a lubricating oil composition containing a base oil, a predetermined zinc dithiophosphate, calcium sulfonate, and a seal sweller.

Citation List

Patent Literature

Patent Literature 1: Japanese Patent Laid-Open No. 2022-022721

Summary of Invention

Technical Problem

By the way, good hydraulic response and high component protection are required for a lubricating oil composition for a shock absorber. For example, air bubbles formed in the oil (air bubbles in oil) reduce the bulk modulus of the lubricating oil composition, which may decrease the damping force response in a hydraulic mechanism. In addition, when the coefficient of friction (rubber friction coefficient) between sliding parts (e.g., rubber and metal) is high, a high resistance will be generated in the sliding parts, which may reduce the response. Furthermore, from the viewpoint of improving the durability of the shock absorber, it is important to provide appropriate lubrication to reduce the wear width of the sliding parts and to improve component protection. Thus, in a lubricating oil composition for a shock absorber, the characteristic of air bubbles in the oil and the rubber friction coefficient are related to hydraulic response, and the wear width is related to component protection.
Under these circumstances, there is a need for a lubricating oil composition for a shock absorber having excellent hydraulic response and component protection.

Solution to Problem

The present invention provides a lubricating oil composition used for lubrication of a shock absorber, containing a base oil having a predetermined kinematic viscosity, a polyalkyl (meth)acrylate (B) having a predetermined weight average molecular weight, a phosphorus-containing compound (C) having an acid number of 1.0 mg KOH/g or more, and a fatty acid ester (D).
Specifically, the present invention provides the following embodiments [1] to [10].

[1] A lubricating oil composition used for lubrication of a shock absorber, comprising: a base oil (A) having a kinematic viscosity at 100°C of 5.8 mm²/s or less, a polyalkyl (meth)acrylate (B) having a weight average molecular weight of 100,000 or more, a phosphorus-containing compound (C) having an acid number of 1.0 mg KOH/g or more, and a fatty acid ester (D).
[2] The lubricating oil composition according to [1], wherein a content of the polyalkyl (meth)acrylate (B) is 0.1 mass% or more based on the total amount of the lubricating oil composition.
[3] The lubricating oil composition according to [1] or [2], wherein a content of an olefin copolymer having a weight average molecular weight of less than 100,000 is less than 0.05 mass% based on the total amount of the lubricating oil composition.
[4] The lubricating oil composition according to any one of [1] to [3], wherein the phosphorus-containing compound (C) comprises one or more selected from an acid phosphoric acid ester (C1) and an acid phosphorous acid ester (C2).
[5] The lubricating oil composition according to any one of [1] to [4], wherein the fatty acid ester (D) comprises an unsaturated fatty acid ester (D1).
[6] The lubricating oil composition according to any one of [1] to [5], wherein a content ratio by mass of the phosphorus-containing compound (C) to the fatty acid ester (D), [(C)/(D)], is 0.3 to 10.
[7] The lubricating oil composition according to any one of [1] to [6], wherein a content of an anti-foaming agent is less than 0.05 mass% based on the total amount of the lubricating oil composition.
[8] The lubricating oil composition according to any one of [1] to [7], wherein a kinematic viscosity at 100°C is 6.0 mm²/s or less.
[9] Use of the lubricating oil composition according to any one of [1] to [8], wherein the lubricating oil composition is applied to lubrication of a shock absorber.

Advantageous Effects of Invention

The lubricating oil composition of a preferred embodiment of the present invention has excellent hydraulic response and component protection. Therefore, the lubricating oil composition of one embodiment of the present invention can be suitably used for lubrication of a shock absorber.

Description of Embodiments

In the numerical range described in the present specification, upper limit and lower limit values can be freely combined. For example, with the description "preferably 30 to 100, more preferably 40 to 80" as a numerical range, the range of "30 to 80" and the range of "40 to 100" are also included in the numerical range described in the present specification. In addition, for example, with the description "preferably 30 or more, more preferably 40 or more, and preferably 100 or less, more preferably 80 or less" as a numerical range, the range of "30 to 80" and the range of "40 to 100" are also included in the numerical range described in the present specification.
In addition, for example, the description of "60 to 100" as the numerical range described in the present specification means a range of "60 or more (60 or more than 60) and 100 or less (100 or less than 100)".
Furthermore, in defining the upper limit values and lower limit values described in the present specification, the numerical range from the lower limit value to the upper limit value can be defined by appropriately selecting from each option and freely combining them.
In addition, the various requirements described in the present specification as preferred embodiments can be combined in multiple combinations.

[Constitution of lubricating oil composition]

One embodiment of the present invention is directed to a lubricating oil composition used for lubrication of a shock absorber, which contains a base oil (A) having a kinematic viscosity at 100°C of 5.8 mm²/s or less (hereinafter also referred to as "component (A)"), a polyalkyl (meth)acrylate (B) having a weight average molecular weight of 100,000 or more (hereinafter also referred to as "component (B)"), a phosphorus-containing compound (C) having an acid number of 1.0 mg KOH/g or more (hereinafter also referred to as "component (C)"), and a fatty acid ester (D) (hereinafter also referred to as "component (D)").
As mentioned above, good hydraulic response and component protection are required for a lubricating oil composition used for a shock absorber. To obtain good hydraulic response, the viscosity of the lubricating oil composition and the additives must be set appropriately. The present inventors have found that by using a base oil (A) having a specific viscosity and combining it with the components (B) to (D) as additives, it is possible to provide a lubricating oil composition with good hydraulic response. In addition, the present inventors have also found that by combining these components (A) to (D), it is possible to provide a lubricating oil composition with good hydraulic response, as well as high component protection.
Since the lubricating oil composition of one embodiment of the present invention has such properties, it can be suitably used for lubrication of a shock absorber.
The lubricating oil composition of one embodiment of the present invention may further contain a lubricating oil additive other than the components (B) to (D) when needed as long as the effects of the present invention are not impaired.
In the lubricating oil composition of one embodiment of the present invention, the total content of the components (A) and (B) is preferably 60 mass% or more, more preferably 70 mass% or more, still more preferably 80 mass% or more, still much more preferably 90 mass% or more, and particularly preferably 95 mass% or more, based on the total amount (100 mass%) of the lubricating oil composition.
In the lubricating oil composition of one embodiment of the present invention, the total content of the components (B) to (D) is preferably 0.5 mass% or more, more preferably 1.0 mass% or more, still more preferably 1.5 mass% or more, and particularly preferably 1.8 mass% or more, based on the total amount (100 mass%) of the lubricating oil composition.
In the lubricating oil composition of one embodiment of the present invention, from the viewpoint of obtaining a lubricating oil composition with good frictional characteristics and wear resistance, the content ratio by mass of the component (C) to the component (D), [(C)/(D)], is preferably 0.1 or more, more preferably 0.2 or more, and still more preferably 0.5 or more.
On the other hand, from the viewpoint of improving the blending balance of each component and retaining the properties of the lubricating oil composition, the mass ratio is preferably 10 or less, more preferably 7 or less, and still more preferably 5 or less.
Hereinafter, details of each component contained in the lubricating oil composition of one embodiment of the present invention will be described.

The base oil contained in the lubricating oil composition of one embodiment of the present invention may be a mineral oil or a synthetic oil, and a mixture of mineral oil and synthetic oil may be used.
Examples of the mineral oils include atmospheric residues obtained by subjecting crude oils, such as paraffinic crude oil, intermediate base crude oil and naphthenic crude oil, to atmospheric distillation; distillates obtained by subjecting these atmospheric residues to vacuum distillation; and refined oils obtained by subjecting the distillates to one or more of refining treatments, such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, and hydrorefining.
Examples of the synthetic oils include poly-α-olefins, such as an α-olefin, a homopolymer thereof, and an α-olefin copolymer (for example, an α-olefin copolymer having 8 to 14 carbon atoms such as an ethylene-α-olefin copolymer); isoparaffin; polyalkylene glycol; ester oils, such as polyol ester, dibasic acid ester, and phosphoric acid ester; ether oils, such as polyphenyl ether; alkylbenzene; alkylnaphthalene; synthetic oil (GTL) obtained by isomerizing wax produced from natural gas through Fischer-Tropsch process or the like (GTL wax (Gas To Liquids WAX)); synthetic oil (CTL) obtained by isomerizing wax produced from coal through Fischer-Tropsch process or the like (CTL wax (Coal To Liquids WAX); and synthetic oil (BTL) obtained by isomerizing wax produced from biomass through Fischer-Tropsch process or the like (BTL wax (Biomass To Liquids WAX).
Among these, the base oil used in one embodiment of the present invention preferably contains at least one selected from mineral oils classified in Group II and Group III of API (American Petroleum Institute) base oil categories, and synthetic oils. In one embodiment of the present invention, these base oils may be used singly, or may be used in combination of two or more.
From the viewpoint of suppressing the generation of air bubbles due to vibration and obtaining a lubricating oil composition with improved hydraulic response, the kinematic viscosity of the base oil (A) used in one embodiment of the present invention at 100°C is 5.8 mm²/s or less, but preferably 5.5 mm²/s or less, more preferably 5.0 mm²/s or less, still more preferably 4.5 mm²/s or less, still much more preferably 4.0 mm²/s or less, still much more preferably 3.5 mm²/s or less, and particularly preferably 3.0 mm²/s or less.
On the other hand, from the viewpoint of obtaining a lubricating oil composition with improved component protection by having good oil film-retaining properties and enhanced lubricating performance, the kinematic viscosity of the base oil (A) at 100°C is preferably 1.0 mm²/s or more, more preferably 1.2 mm²/s or more, and still more preferably 1.4 mm²/s or more.
The viscosity index of the base oil (A) used in one embodiment of the present invention is appropriately set depending on the applications of the lubricating oil composition, and is preferably 70 or more, more preferably 80 or more, still more preferably 90 or more, and particularly preferably 100 or more.
When a mixed oil that is a combination of two or more base oils is used as the component (A) in one embodiment of the present invention, the kinematic viscosity and the viscosity index of the mixed oil are preferably in the above ranges.
In the present specification, the kinematic viscosity and the viscosity index mean values measured or calculated in accordance with JIS K2283:2000.
The content of the base oil (A) in the lubricating oil composition of one embodiment of the present invention is usually 55 mass% or more, preferably 60 mass% or more, more preferably 70 mass% or more, still more preferably 80 mass% or more, and particularly preferably 90 mass% or more, and it is preferably 99.9 mass% or less, more preferably 99.0 mass% or less, and still more preferably 98.5 mass% or less, based on the total amount (100 mass%) of the lubricating oil composition.

The lubricating oil composition of the present invention contains a polyalkyl (meth)acrylate having a weight average molecular weight (Mw) of 100,000 or more, as the component (B). When the Mw is less than 100,000, a relatively high number of air bubbles is generated in the oil. The air bubbles formed in the oil may lead to a reduction in the bulk modulus of the lubricating oil composition, resulting in a decrease in the damping force response. Therefore, it is preferable to use a polyalkyl (meth)acrylate having a Mw of 100,000 or more as the component (B).
From the viewpoint of suppressing the generation of air bubbles due to vibration and obtaining a lubricating oil composition with improved hydraulic response, the weight average molecular weight (Mw) of the polyalkyl (meth)acrylate used in one embodiment of the present invention is 100,000 or more, but preferably 120,000 or more, more preferably 140,000 or more, still more preferably 160,000 or more, still much more preferably 180,000 or more, and particularly preferably 190,000 or more, and may also be 200,000 or more, 250,000 or more, 300,000 or more, 350,000 or more, 400,000 or more, 450,000 or more, 500,000 or more, 540,000 or more, or 600,000 or more.
On the other hand, from the viewpoint of improving solubility in the base oil and obtaining a lubricating oil composition with good storage stability, the weight average molecular weight (Mw) of the polyalkyl (meth)acrylate is preferably 900,000 or less, 800,000 or less, or 700,000 or less.
In the present specification, the weight average molecular weight (Mw) is a value measured in terms of standard polystyrene by a gel permeation chromatography (GPC) method, and specifically means a value measured by the method described in the Examples.
The content of the component (B) in the lubricating oil composition of one embodiment of the present invention is, from the viewpoint of suppressing the generation of air bubbles due to vibration and obtaining a lubricating oil composition with improved hydraulic response, preferably 0.1 mass% or more, more preferably 0.5 mass% or more, still more preferably 0.8 mass% or more, still much more preferably 1.2 mass% or more, and particularly preferably 1.5 mass% or more, and from the viewpoint of improving solubility in the base oil, and obtaining a lubricating oil composition with good storage stability, it is preferably 10.0 mass% or less, more preferably 8.0 mass% or less, still more preferably 5.0 mass% or less, still much more preferably 3.0 mass% or less, and particularly preferably 2.0 mass% or less, based on the total amount (100 mass%) of the lubricating oil composition.
The component (B) used in one embodiment of the present invention may be a polymer having a structural unit derived from an alkyl acrylate or an alkyl methacrylate (hereinafter, collectively referred to as "alkyl (meth)acrylate"), or may be a copolymer having a structural unit derived from a monomer other than an alkyl (meth)acrylate.
The number of carbon atoms of the alkyl group in the alkyl (meth)acrylate may be 1 or more, 3 or more, 5 or more, or 10 or more, and may be 60 or less, 40 or less, 30 or less, or 20 or less.
In addition, in the component (B) used in one embodiment of the present invention, the content of the structural unit derived from the alkyl (meth)acrylate may be 10 mol% or more, 30 mol% or more, 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more, based on the total amount (100 mol%) of the structural unit of the component (B).
The lubricating oil composition of one embodiment of the present invention may or may not contain an olefin copolymer having a weight average molecular weight (Mw) of less than 100,000, but from the viewpoint of suppressing the generation of air bubbles due to vibration and obtaining a lubricating oil composition with improved hydraulic response, it is preferably substantially free of olefin copolymer having a Mw of less than 100,000. This is because in the present invention, a relatively high number of air bubbles is generated in the oil when such olefin copolymer is contained, which can reduce the hydraulic response.
Here, "substantially free of olefin copolymer having a Mw of less than 100,000" means to exclude the lubricating oil compositions in which this olefin copolymer is intentionally blended, and while it does not mean to exclude the embodiments in which this olefin copolymer is unintentionally blended, the content of such olefin copolymer is preferably as small as possible.
The content of the olefin copolymer having a Mw of less than 100,000 contained in the lubricating oil composition of one embodiment of the present invention is preferably less than 0.05 mass%, more preferably less than 0.03 mass%, still more preferably less than 0.01 mass%, and particularly preferably less than 0.001 mass%, based on the total amount (100 mass%) of the lubricating oil composition.
The olefin copolymer having a Mw of less than 100,000 is specifically a copolymer having a structural unit derived from a monomer having an alkenyl group, and examples thereof include a copolymer of an α-olefin having 2 to 20 carbon atoms, and more specific examples thereof include an ethylene-α-olefin copolymer.
The lubricating oil composition of one embodiment of the present invention may contain an olefin copolymer other than a polyalkyl (meth)acrylate having a Mw of 100,000 or more as long as it has a Mw of 100,000 or more.

The lubricating oil composition of the present invention contains a phosphorus-containing compound (C) having an acid number of 1.0 mg KOH/g or more, as the component (C). When the lubricating oil composition of the present invention contains the component (C), the wear resistance is improved, which allows to obtain a lubricating oil composition with a small wear width of the sliding parts and high component protection. The phosphorus-containing compound (C) is not particularly limited as long as it is a compound that has an acid value of 1.0 mg KOH/g or more and contains a phosphorus atom. The lubricating oil composition of one embodiment of the present invention preferably contains one or more selected from an acid phosphoric acid ester (C1) and an acid phosphorous acid ester (C2), as the component (C).
The acid phosphoric acid ester (C1) may be an acid phosphoric acid monoester or an acid phosphoric acid diester. Examples of acid phosphoric acid monoesters include ethyl acid phosphate, propyl acid phosphate, butyl acid phosphate, and ethylhexyl acid phosphate.
Examples of acid phosphoric acid diesters include diethyl acid phosphate, dipropyl acid phosphate, dibutyl acid phosphate, and diethylhexyl acid phosphate.
These acid phosphoric acid esters (C1) may be used alone, or two or more of them may be used in combination.
The acid phosphorous acid ester (C2) may be an acid phosphorous acid monoester or an acid phosphorous acid diester. Examples of acid phosphorous acid monoesters include ethyl hydrogen phosphite, propyl hydrogen phosphite, butyl hydrogen phosphite, lauryl hydrogen phosphite, oleyl hydrogen phosphite, and ethylhexyl hydrogen phosphite.
Examples of acid phosphorous acid diesters include dihexyl hydrogen phosphite, diheptyl hydrogen phosphite, dioctyl hydrogen phosphite, dilauryl hydrogen phosphite, dioleyl hydrogen phosphite, and diethylhexyl hydrogen phosphite.
These acid phosphorous acid esters (C2) may be used alone, or two or more of them may be used in combination.
The acid phosphoric acid ester (C1) and the acid phosphorous acid ester (C2) may be each in the form of an amine salt. Examples of the amines that form amine salts with these phosphoric acid esters include monosubstituted amines such as butylamine, pentylamine, hexylamine, cyclohexylamine, octylamine, laurylamine, stearylamine, oleylamine, and benzylamine; di-substituted amines such as dibutylamine, dipentylamine, dihexylamine, dicyclohexylamine, dioctylamine, dilaurylamine, distearylamine, dioleylamine, dibenzylamine, stearyl monoethanolamine, decyl monoethanolamine, hexyl monopropanolamine, benzyl monoethanolamine, phenyl monoethanolamine, and tolyl monopropanolamine; and trisubstituted amines such as tributylamine, tripentylamine, trihexylamine, tricyclohexylamine, trioctylamine, trilaurylamine, tristearylamine, trioleylamine, tribenzylamine, dioleyl monoethanolamine, dilauryl monopropanolamine, dioctyl monoethanolamine, dihexyl monopropanolamine, dibutyl monopropanolamine, oleyl diethanolamine, stearyl dipropanolamine, lauryl diethanolamine, octyl dipropanolamine, butyl diethanolamine, benzyl diethanolamine, phenyl diethanolamine, tolyl dipropanolamine, xylyl diethanolamine, triethanolamine, and tripropanolamine.
In addition, the lubricating oil composition of one embodiment of the present invention also preferably contains a phosphorus-containing compound (C) other than the acid phosphoric acid ester (C1) and acid phosphorous acid ester (C2), as the component (C). Specific examples thereof include zinc dialkyldithiophosphate (ZnDTP), zinc phosphate, zinc dithiocarbamate, molybdenum dithiocarbamate, molybdenum dithiophosphate, and compounds containing sulfur atoms such as disulfides, sulfurized olefins, sulfurized fats and oils, sulfurized esters, thiocarbonates, thiocarbamates, and polysulfides.
In one embodiment of the present invention, the acid number of the phosphorus-containing compound (C) is, from the viewpoint of improving wear resistance and obtaining a lubricating oil composition with a small wear width of the sliding parts and high component protection, 1.0 mg KOH/g or more, but the acid number is preferably 2.0 mg KOH/g or more, 3.0 mg KOH/g or more, 4.0 mg KOH/g or more, 4.8 mg KOH/g or more, 5.0 mg KOH/g or more, 10.0 mg KOH/g or more, 20.0 mg KOH/g or more, 30.0 mg KOH/g or more, 40.0 mg KOH/g or more, 50.0 mg KOH/g or more, 58.0 mg KOH/g or more, 60.0 mg KOH/g or more, 70.0 mg KOH/g or more, 80.0 mg KOH/g or more, 90.0 mg KOH/g or more, 100 mg KOH/g or more, 110 mg KOH/g or more, 120 mg KOH/g or more, 128 mg KOH/g or more, or 130 mg KOH/g or more.
The upper limit value of the acid number is not particularly limited, but examples thereof include 300 mg KOH/g or less, 250 mg KOH/g or less, and 200 mg KOH/g or less.
In the present specification, the acid number means a value measured in accordance with JIS K2501:2003 (indicator method).
In addition, the lubricating oil composition of one embodiment of the present invention may contain a phosphorus-containing compound other than the phosphorus-containing compound (C) having an acid number of 1.0 mg KOH/g or more. Examples of such phosphorus-containing compounds include neutral phosphorus compounds having an acid number of less than 1.0 mg KOH/g, and specifically include tricresyl phosphate, dicresyl phenyl phosphate, cresyl diphenyl phosphate, tris(ethylphenyl) phosphate, di(ethylphenyl) phenyl phosphate, ethylphenyl diphenyl phosphate, tris(n-propylphenyl) phosphate, di(n-propylphenyl) phenyl phosphate, n-propylphenyl diphenyl phosphate, tris(isopropylphenyl) phosphate, di(isopropylphenyl) phenyl phosphate, and isopropylphenyl diphenyl phosphate.
The content of the component (C) in the lubricating oil composition of one embodiment of the present invention is, from the viewpoint of improving wear resistance and obtaining a lubricating oil composition with a small wear width of the sliding parts and high component protection, preferably more than 0.2 mass%, more preferably 0.25 mass% or more, still more preferably 0.3 mass% or more, still much more preferably 0.4 mass% or more, and particularly preferably 0.5 mass% or more, and from the viewpoint of improving thermal stability, it is preferably 5.0 mass% or less, more preferably 4.0 mass% or less, and still more preferably 3.0 mass% or less, based on the total amount (100 mass%) of the lubricating oil composition.
In addition, in the lubricating oil composition of one embodiment of the present invention, the content of the component (C) in terms of phosphorus atoms is, from the viewpoint of improving wear resistance and obtaining a lubricating oil composition with a small wear width of the sliding parts and high component protection, preferably 100 ppm by mass or more, more preferably 120 ppm by mass or more, still more preferably 140 ppm by mass or more, still much more preferably 160 ppm by mass or more, and particularly preferably 180 ppm by mass or more, and may be 200 ppm by mass or more, 250 ppm by mass or more, 300 ppm by mass or more, 350 ppm by mass or more, or 400 ppm by mass or more, based on the total amount (100 mass%) of the lubricating oil composition.
On the other hand, from the viewpoint of improving thermal stability, the content of the component (C) in terms of phosphorus atoms is preferably 1200 ppm by mass or less, more preferably 1000 ppm by mass or less, still more preferably 800 ppm by mass or less, and still much more preferably 600 ppm by mass or less.

The lubricating oil composition of the present invention contains a fatty acid ester (D), as the component (D). When the lubricating oil composition of the present invention contains the component (D), the coefficient of friction is reduced, which allows to suppress the generation of a high resistance in the sliding parts, resulting in a lubricating oil composition with a high hydraulic response.
The fatty acid ester (D) is a condensation product of an aliphatic carboxylic acid with an alcohol. Examples of aliphatic carboxylic acids include saturated or unsaturated aliphatic monocarboxylic acids, aliphatic dicarboxylic acids, aliphatic tricarboxylic acids, and aliphatic tetracarboxylic acids. The lubricating oil composition of one embodiment of the present invention contains at least an unsaturated fatty acid ester (D1), as the component (D). The aliphatic carboxylic acid may be either a linear or cyclic aliphatic carboxylic acid. Furthermore, the number of carbon atoms of the aliphatic carboxylic acid is preferably 6 to 40, more preferably 8 to 32, and still more preferably 12 to 24.
Examples of saturated aliphatic carboxylic acids include saturated aliphatic monocarboxylic acids such as capric acid, neodecanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, and lignoceric acid; and saturated aliphatic dicarboxylic acids such as adipic acid, azelaic acid, and sebacic acids.
Examples of unsaturated aliphatic carboxylic acids include undecylenic acid, oleic acid, elaidic acid, erucic acid, nervonic acid, linoleic acid, ricinoleic acid, γ-linolenic acid, arachidonic acid, α-linolenic acid, stearidonic acid, eicosapentaenoic acid and docosahexaenoic acid.
Examples of alcohols include aliphatic alcohols. The aliphatic alcohol may be a monohydric or polyhydric alcohol, and may be either saturated or unsaturated. Furthermore, the aliphatic alcohol may be linear or branched. The number of carbon atoms of the alcohol is preferably 1 to 30, and more preferably 2 to 24.
Specific examples of the alcohol include methanol, ethanol, allyl alcohol, propanol, butanol, pentanol, hexanol, octanol, decanol, dodecanol, tetradecanol, hexadecanol, octadecanol, butenol, pentenol, hexenol, octenol, decenol, dodecenol, tetradecenol, hexadecenol, octadecenol, ethylene glycol, propylene glycol, neopentyl glycol, glycerol, trimethylolethane, trimethylolpropane, pentaerythritol, and sorbitan.
In one embodiment of the present invention, specific examples of the component (D) include glycerol fatty acid monoesters (monoglycerides) and glycerol fatty acid diesters (diglycerides) when the alcohol is glycerol.
When the alcohol is sorbitan, examples include sorbitan fatty acid esters such as sorbitan monostearate, sorbitan tristearate, sorbitan monooleate and sorbitan trioleate.
When the alcohol is pentaerythritol, examples include pentaerythritol fatty acid esters such as pentaerythritol monooleate, pentaerythritol dioleate, and pentaerythritol tetraoleate.
The content of the component (D) in the lubricating oil composition of one embodiment of the present invention is, from the viewpoint of reducing the coefficient of friction and obtaining a lubricating oil composition with a high hydraulic response, preferably 0.1 mass% or more, more preferably 0.2 mass% or more, still more preferably 0.3 mass% or more, and from the viewpoint of maintaining good compatibility with elastic members such as rubber, it is preferably 5.0 mass% or less, more preferably 4.0 mass% or less, and still more preferably 3.0 mass% or less, based on the total amount (100 mass%) of the lubricating oil composition.

The lubricating oil composition of one embodiment of the present invention may be blended with general purpose additives (hereinafter also referred to as "general purpose additives") that do not correspond to the components (B) to (D) described above and are blended in general lubricating oil compositions, as long as the effects of the present invention are not impaired.
Examples of such general purpose additives include an antioxidant, an ashless dispersant, a metal-based detergent, a viscosity index improver, a fluidity promoter, an extreme pressure agent, a corrosion inhibitor, a friction modifier, and an anti-wear agent. These general purpose additives can be used alone, or a plurality of them can be used in combination.
In addition, an additive package consisting of a plurality of these general purpose additives may be used in the lubricating oil composition of one embodiment of the present invention.
The contents of these general purpose additives are each preferably 0.001 to 10 mass%, and more preferably 0.01 to 5 mass%, based on the total amount (100 mass%) of the lubricating oil composition.
In addition, the total content of the general purpose additives is preferably 0.01 to 40 mass%, and more preferably 0.1 to 35 mass%, based on the total amount (100 mass%) of the lubricating oil composition.
The lubricating oil composition of one embodiment of the present invention may or may not contain an anti-foaming agent, but from the viewpoint of suppressing air bubbles forming in the oil, it is preferably substantially free of anti-foaming agent. "Substantially free of anti-foaming agent" means to exclude the lubricating oil compositions in which an anti-foaming agent is intentionally blended, and while it does not mean to exclude the embodiments in which they are unintentionally blended, the content of such anti-foaming agent is preferably as small as possible.
The content of anti-foaming agent contained in the lubricating oil composition of one embodiment of the present invention is preferably less than 0.05 mass%, more preferably less than 0.03 mass%, still more preferably less than 0.01 mass%, and particularly preferably less than 0.001 mass%, based on the total amount (100 mass%) of the lubricating oil composition. Generally, anti-foaming agents are added for the purpose of suppressing foaming of the lubricating oil composition, break down bubbles, and the like, but in the present invention, it was found that adding an anti-foaming agent actually increased the amount of air bubbles in the oil. Therefore, in order to suppress air bubbles forming in the oil, the content of anti-foaming agent is preferably controlled within the above range.
Examples of the anti-foaming agent include silicone anti-foaming agents such as alkyl silicone anti-foaming agents and fluorosilicone anti-foaming agents.

The method for producing the lubricating oil composition of one embodiment of the present invention is not particularly limited, but from the viewpoint of productivity, preferable is a method having a step of adding the components (B) to (D), and if necessary, various additives to the component (A).
The order of blending of each component can be set as appropriate, but the resin components such as the component (B) are preferably in a form of a solution dissolved in a diluent oil and the solution is preferably blended with the component (A), from the viewpoint of compatibility with the component (A).

[Properties of lubricating oil composition]

The kinematic viscosity of the lubricating oil composition of one embodiment of the present invention at 100°C is preferably 1.0 mm²/s or more, more preferably 1.2 mm²/s or more, still more preferably 1.4 mm²/s or more, and from the viewpoint of suppressing unintended increase of resistance in the shock absorber and maintaining proper fluid resistance, it is preferably 6.0 mm²/s or less, more preferably 5.5 mm²/s or less, still more preferably 5.0 mm²/s or less, still much more preferably 4.5 mm²/s or less, and particularly preferably 4.0 mm²/s or less.
The viscosity index of the lubricating oil composition of one embodiment of the present invention is preferably 70 or more, more preferably 80 or more, still more preferably 90 or more, and particularly preferably 100 or more.

[Characteristics and use application of lubricating oil composition]

The lubricating oil composition of the present invention has excellent hydraulic response and component protection. Examples of specific indicators for evaluating hydraulic response include the number of air bubbles in the oil, as measured by the method described in the Examples below, and the rubber friction coefficient when a reciprocating sliding test is conducted as described in the Examples below.
Examples of specific indicators for evaluating component protection include the wear width when a reciprocating dynamic friction test is conducted as described in the Examples below.
The number of air bubbles in the oil, as measured by the method described in the Examples below, using the lubricating oil composition of one embodiment of the present invention, is preferably 80 or less, more preferably 75 or less, still more preferably 70 or less, still much more preferably 65 or less, and particularly preferably 60 or less. The presence of air bubbles in the oil reduces the bulk modulus of the lubricating oil composition, which may decrease the response in a hydraulic mechanism such as a shock absorber. Therefore, it can be said that the smaller the number of air bubbles in the oil is in this evaluation, the better the hydraulic response of the lubricating oil composition becomes.
The rubber friction coefficient when a reciprocating sliding test is conducted as described in the Examples below, using the lubricating oil composition of one embodiment of the present invention, is preferably 0.70 or less, more preferably 0.65 or less, still more preferably 0.60 or less, and particularly preferably 0.50 or less. A high rubber friction coefficient causes a high resistance in mechanisms where sliding between an oil seal (rubber) and a metal material occurs, such as shock absorbers, which may decrease the response. Thus, it can be said that the smaller the rubber friction coefficient is, the better the hydraulic response of the lubricating oil composition becomes.
The wear width when a reciprocating dynamic friction test is conducted as described in the Examples below, using the lubricating oil composition of one embodiment of the present invention, is preferably 0.57 mm or less, more preferably 0.55 mm or less, still more preferably 0.50 mm or less, and particularly preferably 0.45 mm or less. It can be said that as the wear width is smaller, the wear resistance is better, and in a mechanism with reciprocating sliding motion, such as a shock absorber, the component protection of the lubricating oil composition is higher.
Since the lubricating oil composition of one embodiment of the present invention has the above characteristics, it can be suitably applied to the lubrication of various equipment, and may be suitably applied to, for example, lubricating oil for a shock absorber, hydraulic oil, hydraulic oil for construction machinery, power steering fluid, turbine oil, compressor oil, machine tool lubricating oil, cutting oil, gear oil, fluid bearing oil, and rolling bearing oil. Among these, the lubricating oil composition of one embodiment of the present invention can be suitably applied to a shock absorber. More specifically, the lubricating oil composition of one embodiment of the present invention can be used for any of a double cylinder type shock absorber and a single cylinder type shock absorber, and can be preferably used for any of shock absorbers for motorcycles and for four-wheeled vehicles.
Therefore, the present invention also provides a shock absorber of the following [I], and a use of a lubricating oil composition of the following [II].

[1] A shock absorber filled with the aforementioned lubricating oil composition of one embodiment of the present invention.
[II] Use of a lubricating oil composition, wherein the aforementioned lubricating oil composition of one embodiment of the present invention is applied to lubrication of a shock absorber.

Examples

Next, the present invention will be described in much more detail with reference to the Examples, but the present invention is in no way limited to these Examples. The various physical property values of each component used in the Examples and Comparative Examples and the resulting lubricating oil compositions were measured in accordance with the following methods.

(1) Kinematic viscosity, viscosity index

The kinematic viscosity and viscosity index were measured and calculated in accordance with JIS K2283:2000.

(2) Weight average molecular weight (Mw)

Measurements were made using a gel permeation chromatograph (HPLC Model 1260 manufactured by Agilent) under the following conditions and the values measured in terms of standard polystyrene were used.

(Measurement conditions)

Column: Two serially connected "Shodex LF404."
Column temperature: 35°C
Developing solvent: Chloroform
Flow rate: 0.3 mL/min

(3) Acid number

The acid number was measured in accordance with JIS K2501:2003 (indicator method).

(4) Hydroxyl value

The hydroxyl value was measured in accordance with JIS K 0070:1992.

(5) Characteristic of air bubbles in oil

A rectangular glass container with a base area of 5 cm long and 5 cm wide was filled with 50 mL of the lubricating oil composition to be measured, heated to 100°C, and shaken for 60 seconds at an amplitude of ±10 mm and frequency of 10 Hz. The state of the lubricating oil composition (oil) 40 seconds after the end of shaking was photographed from the side of the glass container using a speed camera to measure the number of air bubbles in the oil. The shooting range of the speed camera was a square area with sides of 0.5 cm and centered at a position 2.5 cm above the bottom of the container, and was observed from the side of the glass container. A number of air bubbles in the oil, as measured by the above method, of 80 or less was considered as pass, and the measurements in (3) and (4) below were not performed on the samples with a number of air bubbles in the oil exceeding 80. It can be said that the smaller the number of air bubbles in the oil is in this evaluation, the better the hydraulic response of the lubricating oil composition becomes.

(6) Rubber friction coefficient

Using a reciprocating dynamic friction tester, a Cr-plated steel plate and nitrile rubber (A437, manufactured by NOK) covering a 1/2 steel ball were subjected to reciprocating sliding with a 0.1 mL drop of the lubricating oil composition to be measured, and the maximum friction coefficient detected at the 100th cycle was recorded. The load was 4 N, the amplitude was ±3 mm, and the frequency was 1 Hz. The Cr-plated steel plate was heated to 40°C. A rubber friction coefficient, as measured by the above method, of 0.70 or less was considered as pass, and the measurement in (2) above was not performed on the samples with a rubber friction coefficient exceeding 0.70. It can be said that the smaller the rubber friction coefficient is, the better the hydraulic response of the lubricating oil composition becomes.

(7) Wear resistance

Using a Bowden type reciprocating dynamic friction tester, an SPCC-SB steel plate heated to 100°C and a 1/2-inch SUJ2 steel ball were subjected to reciprocating sliding with a 0.1 mL drop of the lubricating oil composition to be measured, and the width of the wear mark at the sliding center of the SPCC-SB steel plate after 400 cycles was recorded. The load was 20 N, the amplitude was ±5 mm, and the speed was 50 mm/s, with the reciprocating motion performed at a constant speed. A wear width, as measured by the above method, of 0.57 mm or less was considered as pass, and the measurement in (2) above was not performed on the samples with a wear width exceeding 0.57 mm. It can be said that as the wear width is smaller, the wear resistance is better, and in a mechanism with reciprocating sliding motion, such as a shock absorber, the component protection of the lubricating oil composition is higher.

Examples 1 to 11 and Comparative Examples 1 to 5

The components (A) to (D) shown in Table 1 were added and mixed in amounts shown in Table 1, thereby preparing each lubricating oil composition. The prepared lubricating oil compositions are substantially free of olefin copolymer with a Mw of less than 100,000 and anti-foaming agent (each of which having a content of less than 0.05 mass%). The blending amount of the component (B) in Table 1 describes the blending amount in terms of resin excluding the diluent solvent.
Details of each component used in the preparation of the lubricating oil composition are as follows.

Mineral oil (a1): Mineral oil classified in Group III of the API base oil categories, kinematic viscosity at 100°C = 2.2 mm²/s, viscosity index = 109, density (15°C): 0.82 g/cm³.
Mineral oil (a2): Mineral oil classified in Group III of the API base oil categories, kinematic viscosity at 100°C = 2.7 mm²/s, viscosity index = 111, density (15°C): 0.81 g/cm³.
Mineral oil (a3): Mineral oil classified in Group III of the API base oil categories, kinematic viscosity at 100°C = 6.0 mm²/s, viscosity index = 132, density (15°C): 0.84 g/cm³.

PMA (b1): Polyalkyl methacrylate, Mw = 190,000.
PMA (b2): Polyalkyl methacrylate, Mw = 540,000.
PMA (b3): Polyalkyl methacrylate, Mw = 36,000.

<Phosphorus-containing compound (C)>

Phosphorus-containing compound (c1): Dioleyl hydrogen phosphite, acid number (indicator method) = 4.8 mg KOH/g
Phosphorus-containing compound (c2): Mixture of dilauryl hydrogen phosphite and monolauryl hydrogen phosphite, acid number (indicator method) = 58 mg KOH/g
Phosphorus-containing compound (c3): Zinc dialkyl dithiophosphate, acid number (indicator method) = 128 mg KOH/g
Phosphorus-containing compound (c4): Tricresyl phosphate, acid number (indicator method) = 0.01 mg KOH/g
Phosphorus-containing compound (c5): Isopropylated triaryl phosphate compound, acid number (indicator method) = 0.05 mg KOH/g

Fatty acid ester (d1): Oleic acid monoglyceride, hydroxyl value = 156 mg KOH/g
Fatty acid ester (d2): Sorbitan monooleate, hydroxyl value = 235 mg KOH/g
Fatty acid ester (d3): Pentaerythritol dioleate, hydroxyl value = 156 mg KOH/g

For the lubricating oil compositions prepared in the Examples and Comparative Examples, various physical property values were measured and calculated in accordance with the aforementioned measurement methods. The results of them are set forth in Table 1.

[Table. 1]

Table 1

				Example 1	Example 2	Example 3	Example 4	Example 5	Example 6	Example 7	Example 8	Example 9	Example 10	Example 11	Comparative Example 1	Comparative Example 2	Comparative Example 3	Comparative Example 4	Comparative Example 5
Composition of lubricating oil composition	Component (A)	Mineral oil (a1)	mass%	97.0	97.0	96.5	97.0	97.0	97.8		96.2	96.5	96.2	98.2		91.8	97.0	97.0	97.2
		Mineral oil (a2)	mass%							97.0
		Mineral oil (a3)	mass%												97.0
	Component (B)	PMA(b1)	mass%	2.0	2.0	2.0	2.0	2.0		2.0	2.0	2.0	2.0	0.8	2.0		2.0	2.0	2.0
		PMA(b2)	mass%						1.2
		PMA(b3)	mass%													7.2
	Component (C)	Phosphorus-containing compound (c1)	mass%	0.5	0.5	0.5			0.5	0.5		1.0	1.5	0.5	0.5	0.5
		Phosphorus-containing compound (c2)	mass%				0.5	0.5
		Phosphorus-containing compound (c3)	mass%								0.8								0.8
		Phosphorus-containing compound (c4)	mass%														0.5
		Phosphorus-containing compound (c5)	mass%															0.5
	Component (D)	Fatty acid ester (d1)	mass%	0.5			0.5		0.5	0.5		0.5	0.3	0.5	0.5	0.5	0.5	0.5
		Fatty acid ester (d2)	mass%		0.5			0.5
		Fatty acid ester (d3)	mass%			1.0					1.0
	Content ratio (by mass) of Component (C)/Component (D)		-	1	1	0.5	1	1	1	1	0.8	2	5	1	1	1	1	1	-
	Total			100	100	100	100	100	100	100	100	100	100	100	100	100	100	100	100
Properties of lubricating oil composition	Kinematic viscosity at 100°C		mm²/s	3.7	3.7	3.7	3.7	3.7	3.7	3.9	3.8	3.7	3.7	2.9	8.1	3.7	3.7	3.7	3.7
Test results	Air bubbles in oil		bubbles	57	35	31	15	18	23	18	49	11	33	15	260	145	Not measured	Not measured	Not measured
	Rubber friction coefficient		-	0.10	0.09	0.09	0.15	0.15	0.18	0.13	0.43	0.07	0.10	0.10	measured	measured	0.73	0.90	1.01
	Wear width		mm	0.36	0.39	0.37	0.38	0.38	0.34	0.37	0.42	0.44	0.38	0.39	Not measured	measured	0.49	0.58	0.41

According to Table 1, the lubricating oil compositions of Examples 1 to 11, which contain a base oil (A) having a kinematic viscosity at 100°C of 5.8 mm²/s or less, a polyalkyl (meth)acrylate (B) having a weight average molecular weight of 100,000 or more, a phosphorus-containing compound (C) having an acid number of 1.0 mg KOH/g or more, and a fatty acid ester (D), had excellent hydraulic response and component protection compared to Comparative Examples 1 to 5. Specifically, the lubricating oil compositions of Examples 1 to 11 had fewer air bubbles in the oil and better hydraulic response than the lubricating oil compositions of Comparative Examples 1 and 2. In addition, the lubricating oil compositions of Examples 1 to 11 had a smaller rubber friction coefficient and wear width than the lubricating oil compositions of Comparative Examples 3 to 5, and had both good hydraulic response and component protection.

Claims

A lubricating oil composition used for lubrication of a shock absorber, comprising: a base oil (A) having a kinematic viscosity at 100°C of 5.8 mm²/s or less, a polyalkyl (meth)acrylate (B) having a weight average molecular weight of 100,000 or more, a phosphorus-containing compound (C) having an acid number of 1.0 mg KOH/g or more, and a fatty acid ester (D).
The lubricating oil composition according to claim 1, wherein a content of the polyalkyl (meth)acrylate (B) is 0.1 mass% or more based on the total amount of the lubricating oil composition.
The lubricating oil composition according to claim 1 or 2, wherein a content of an olefin copolymer having a weight average molecular weight of less than 100,000 is less than 0.05 mass% based on the total amount of the lubricating oil composition.
The lubricating oil composition according to any one of claims 1 to 3, wherein the phosphorus-containing compound (C) comprises one or more selected from an acid phosphoric acid ester (C1) and an acid phosphorous acid ester (C2).
The lubricating oil composition according to any one of claims 1 to 4, wherein the fatty acid ester (D) comprises an unsaturated fatty acid ester (D1).
The lubricating oil composition according to any one of claims 1 to 5, wherein a content ratio by mass of the phosphorus-containing compound (C) to the fatty acid ester (D), [(C)/(D)], is 0.3 to 10.
The lubricating oil composition according to any one of claims 1 to 6, wherein a content of an anti-foaming agent is less than 0.05 mass% based on the total amount of the lubricating oil composition.
The lubricating oil composition according to any one of claims 1 to 7, wherein a kinematic viscosity at 100°C is 6.0 mm²/s or less.
Use of the lubricating oil composition according to any one of claims 1 to 8, wherein the lubricating oil composition is applied to lubrication of a shock absorber.