Lubricating oil composition and method for using lubricating oil composition

Abstract

The present invention addresses the problem of providing a lubricating oil composition that has excellent permeability through water-bearing strata, as well as excellent evaporation-suppressing properties and coking resistance. The problem was solved by a lubricating oil composition that contains a polyalkylene glycol compound (A), a naphthylamine-based antioxidant (B), and a diphenylamine-based antioxidant (C). The molar ratio [(EO) / (RO)] of oxyethylene units (EO) to oxyalkylene units (RO) other than oxyethylene units (EO) in the polyalkylene glycol compound (A) is 1.0 or more, and the content of the polyalkylene glycol compound (A) is more than 50 mass% based on the total amount of the lubricating oil composition.

Description

Lubricating oil composition and method of using the lubricating oil composition

[0001] The present invention relates to a lubricating oil composition and a method of using the lubricating oil composition.

[0002] Lubricating oil compositions are used in various fields for the purpose of preventing wear of contact members and imparting lubricity, etc. For example, as a lubricating oil composition for an air compressor, a base oil containing a polyalkylene glycol and a rust inhibitor are contained, and a lubricating oil composition in which the content of the polyalkylene glycol is 65.0% by mass or more based on the total amount of the composition is known (see Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2020-063371

[0004] By the way, in recent years, as one of the effective countermeasures against global warming, carbon dioxide capture and storage technology (CCS: Carbon Dioxide Capture and Storage, hereinafter also referred to as "CCS technology") has attracted attention. In CCS technology, a compressor is used when injecting CO ２ deep into the ground. In the process of advancing the study on CCS technology, the present inventors have recognized that the lubricating oil composition used for lubricating the compressor may mix into the CO ２ pressure transmission line and reach the formation for storing CO ２ (hereinafter also referred to as "CO ２ storage layer"). When the lubricating oil composition reaches the CO ２ storage layer, a part of the CO ２ storage layer may be blocked, and the injectability of CO ２ may be reduced. Therefore, in order to suppress the reduction of the injectability of CO ２ , it is required to create a lubricating oil composition having excellent permeability through a "formation containing water" such as a CO ２ storage layer. However, the lubricating oil composition described in Patent Document 1 has insufficient permeability through a formation containing water. Further, in CCS technology, when CO ２Lubricating oil compositions used in compressors (especially reciprocating compressors) for pressurizing materials require both evaporation suppression and coking resistance. However, achieving both evaporation suppression and coking resistance simultaneously in lubricating oil compositions is inherently difficult, and achieving both while also ensuring permeability through water-containing geological formations is extremely challenging.

[0005] The present invention aims to provide a lubricating oil composition that exhibits excellent permeability through water-containing geological formations, as well as excellent evaporation suppression and coking resistance.

[0006] The present invention provides the following [1] and [2]: [1] A lubricating oil composition comprising a polyalkylene glycol compound (A), a naphthylamine-based antioxidant (B), and a diphenylamine-based antioxidant (C), wherein the molar ratio [(EO) / (RO)] of oxyethylene units (EO) of the polyalkylene glycol compound (A) to oxyalkylene units (RO) other than the oxyethylene units (EO) is 1.0 or more, and the content of the polyalkylene glycol compound (A) is more than 50% by mass on a basis of the total amount of the lubricating oil composition. [2] A method for using the lubricating oil composition described in [1] above for lubrication of a compressor.

[0007] According to the present invention, it is possible to provide a lubricating oil composition that has excellent permeability to water-containing geological formations, as well as excellent evaporation suppression and coking resistance.

[0008] The upper and lower limits of the numerical ranges described herein can be combined in any way. For example, if 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 within the scope of the present invention. Furthermore, unless otherwise specified, the numerical ranges "lower limit to upper limit" described herein mean greater than or equal to the lower limit and less than or equal to the upper limit. In addition, in this specification, the numerical values ​​in the examples are numerical values ​​that can be used as upper or lower limits.

[0009] [Description of Lubricating Oil Composition] The lubricating oil composition of this embodiment contains a polyalkylene glycol compound (A), a naphthylamine-based antioxidant (B), and a diphenylamine-based antioxidant (C). The molar ratio [(EO) / (RO)] of oxyethylene units (EO) to oxyalkylene units other than oxyethylene units (RO) of the polyalkylene glycol compound (A) is 1.0 or more. The content of the polyalkylene glycol compound (A) is more than 50% by mass on a total basis of the lubricating oil composition.

[0010] The inventors diligently conducted studies to solve the above problems. As a result, they found that by using polyalkylene glycol compound (A) as a base oil and adjusting the molar ratio [(EO) / (RO)] of oxyalkylene units other than oxyethylene units (EO) in polyalkylene glycol compound (A) to 1.0 or higher, the hydrophilicity of polyalkylene glycol compound (A) can be improved, thereby providing a lubricating oil composition with excellent permeability to water-containing geological formations. Furthermore, they found that by using polyalkylene glycol compound (A) as a base oil and blending naphthylamine-based antioxidant (B) and diphenylamine-based antioxidant (C) into the lubricating oil composition, it is possible to prepare a lubricating oil composition that exhibits excellent permeability to water-containing geological formations while also having excellent evaporation suppression and coking resistance. Based on these findings, the inventors conducted further studies and completed the present invention.

[0011] In the lubricating oil composition of this embodiment, polyalkylene glycol compound (A) is the main component of the lubricating oil composition, and its content is more than 50% by mass on a basis of the total amount of the lubricating oil composition. In the lubricating oil composition of this embodiment, the content of polyalkylene glycol compound (A) is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and still more preferably 90% by mass or more, on a basis of the total amount of the lubricating oil composition, from the viewpoint of improving the permeability of water-containing geological formations. Furthermore, the content of polyalkylene glycol compound (A) is preferably less than 100% by mass, more preferably less than 99.8% by mass, even more preferably less than 99.5% by mass, and still more preferably less than 99.0% by mass, on a basis of the total amount of the lubricating oil composition.

[0012] The lubricating oil composition of this embodiment may consist only of a polyalkylene glycol compound (A), a naphthylamine-based antioxidant (B), and a diphenylamine-based antioxidant (C), or it may contain components other than the polyalkylene glycol compound (A), naphthylamine-based antioxidant (B), and diphenylamine-based antioxidant (C) (hereinafter also referred to as "other components"). The total content of the polyalkylene glycol compound (A), naphthylamine-based antioxidant (B), and diphenylamine-based antioxidant (C) is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, based on the total amount of the lubricating oil composition. Furthermore, the total content of the polyalkylene glycol compound (A), naphthylamine-based antioxidant (B), and diphenylamine-based antioxidant (C) may be 100% by mass on a basis of the total amount of the lubricating oil composition. However, from the viewpoint of ensuring room for other components, it is preferably less than 100% by mass, more preferably 99.6% by mass or less, and even more preferably 99.4% by mass or less, on a basis of the total amount of the lubricating oil composition.

[0013] The components contained in the lubricating oil composition of this embodiment, as well as components that may be contained in the lubricating oil composition of this embodiment, will be described in detail below. In the following description, "polyalkylene glycol compound (A)", "naphthylamine-based antioxidant (B)", and "diphenylamine-based antioxidant (C)" will also be referred to as "component (A)", "component (B)", and "component (C)", respectively. Furthermore, in the following description, "polyalkylene glycol compound (A)" will also be referred to as "PAG compound (A)".

[0014] <PAG Compound (A)> The lubricating oil composition of this embodiment contains PAG compound (A). The molar ratio [(EO) / (RO)] of oxyethylene units (EO) and oxyalkylene units (RO) other than oxyethylene units (EO) in PAG compound (A) is 1.0 or higher. A molar ratio [(EO) / (RO)] of 1.0 or higher enhances the hydrophilicity of PAG compound (A), resulting in excellent permeability through water-containing strata. Here, from the viewpoint of further enhancing the hydrophilicity of PAG compound (A) and improving permeability through water-containing strata, the molar ratio [(EO) / (RO)] is preferably 1.1 or higher, more preferably 1.2 or higher, even more preferably 1.3 or higher, even more preferably 1.4 or higher, even more preferably 1.5 or higher, and even more preferably 1.6 or higher. Furthermore, there is no particular upper limit to the molar ratio [(EO) / (RO)], but it is usually 99.0 or less, may be 50.0 or less, 10.0 or less, 5.0 or less, or 3.0 or less. In addition, the alkylene group R of the oxyalkylene unit (RO) other than the oxyethylene unit (EO) is preferably an alkylene group having 3 to 5 carbon atoms, more preferably an alkylene group having 3 to 4 carbon atoms, and even more preferably an alkylene group having 3 carbon atoms. In addition, the PAG compound (A) may be used alone or in combination of two or more types.

[0015] Here, the PAG compound (A) preferably includes the PAG compound (A1) represented by the following general formula (a1).

[0016] In the above general formula (a1), R １１ R is a hydrogen atom, a monovalent hydrocarbon group having 1 to 10 carbon atoms, an acyl group having 2 to 10 carbon atoms, a divalent to hexavalent hydrocarbon group having 1 to 10 carbon atoms, or a heterocyclic group having 3 to 10 ring-forming atoms. １２ X is a hydrogen atom, a monovalent hydrocarbon group having 1 to 10 carbon atoms, an acyl group having 2 to 10 carbon atoms, or a heterocyclic group having 3 to 10 ring-forming atoms. X is -(EO) ｍ１ (RO) ｍ２ -. E is an ethylene group, and R is an alkylene group with 3 to 5 carbon atoms. m1 is a number from 8 to 56. m2 is a number from 4 to 36. n １１ = m1 + m2. １２ n is an integer between 1 and 6. １２ If the number is 2 or more, there are multiple R １２ They may be the same as each other, or they may be different from each other. Also, n １２ If there are two or more X's, the multiple X's may be the same as each other or may be different from each other.

[0017] R in general formula (a1) １１ and R １２ From the viewpoint of improving oxidation stability and storage stability, it is preferable that at least one of them is a monovalent hydrocarbon group having 1 to 10 carbon atoms, an acyl group having 2 to 10 carbon atoms, a divalent to 6 carbon atoms having 1 to 10 carbon atoms, or a heterocyclic group having 3 to 10 ring-forming atoms, and it is more preferable that at least one of them is a monovalent hydrocarbon group having 1 to 10 carbon atoms. That is, PAG compound (A) is preferably a polyalkylene glycol monohydrocarbyl ether with one end sealed with a monovalent hydrocarbon group having 1 to 10 carbon atoms, or a polyalkylene glycol dihydrocarbyl ether with both ends sealed with monovalent hydrocarbon groups having 1 to 10 carbon atoms, and it is more preferable that it is a polyalkylene glycol monohydrocarbyl ether with one end sealed with a monovalent hydrocarbon group having 1 to 10 carbon atoms. Furthermore, from the viewpoint of improving oxidation stability and storage stability, R is particularly preferable. １１ and R １２ It is preferable that both are monovalent hydrocarbon groups having 1 to 10 carbon atoms. Here, R １１ and R１２ This includes linear and branched structures.

[0018] R １１ and R １２ Examples of monovalent hydrocarbon groups having 1 to 10 carbon atoms include alkyl groups such as methyl, ethyl, propyl (e.g., n-propyl, isopropyl), butyl (e.g., linear n-butyl, as well as branched isobutyl, s-butyl, and t-butyl groups; the same applies to the groups exemplified below), alkyl groups such as pentyl, hexyl, heptyl, octyl, nonyl, and decyl; cycloalkyl groups such as cyclopentyl, cyclohexyl, methylcyclohexyl, ethylcyclohexyl, propylcyclohexyl, and dimethylcyclohexyl; aryl groups such as phenyl, methylphenyl, ethylphenyl, dimethylphenyl, propylphenyl, trimethylphenyl, butylphenyl, and naphthyl; and arylalkyl groups such as benzyl, phenylethyl, methylbenzyl, phenylpropyl, and phenylbutyl. Furthermore, examples include alkenyl groups, cycloalkenyl groups, and arylalkenyl groups, respectively, obtained by removing two hydrogen atoms from the alkyl groups, cycloalkyl groups, and arylalkyl groups mentioned above. In addition, the number of carbon atoms in the monovalent hydrocarbon group is preferably 1 or more, preferably 10 or less as the upper limit, more preferably 6 or less, and even more preferably 4 or less, from the viewpoint of improving oxidation stability and storage stability.

[0019] R １１ and R １２ Regarding the acyl group having 2 to 10 carbon atoms, the hydrocarbon group portion of the acyl group is as described above (R １１ and R １２ Among the monovalent hydrocarbon groups exemplified above, those having 1 to 9 carbon atoms are examples, and they may be linear, branched, or cyclic. Furthermore, the number of carbon atoms in the acyl group is preferably 2 or more, preferably 10 or less as the upper limit, and more preferably 6 or less, from the viewpoint of improving oxidation stability and storage stability.

[0020] R １１ As for the divalent to hexavalent hydrocarbon group, the above R １１Examples include residues obtained by removing 1 to 5 hydrogen atoms from a monovalent hydrocarbon group, and residues obtained by removing a hydroxyl group from polyhydric alcohols such as trimethylolpropane, glycerin, pentaerythritol, sorbitol, 1,2,3-trihydroxycyclohexane, and 1,3,5-trihydroxycyclohexane. From the viewpoint of improving oxidation stability and storage stability, the number of carbon atoms in the divalent to hexavalent hydrocarbon group is preferably 1 or more, preferably 10 or less as the upper limit, more preferably 6 or less, and even more preferably 4 or less.

[0021] R １１ and R １２ Examples of heterocyclic groups having 3 to 10 ring-forming atoms include oxygen-containing heterocyclic groups or sulfur-containing heterocyclic groups. The heterocyclic group may be a saturated ring or an unsaturated ring. Examples of oxygen-containing heterocyclic groups include oxygen-containing saturated heterocyclic rings such as 1,3-propylene oxide, tetrahydrofuran, tetrahydropyran, and hexamethylene oxide, and residues obtained by removing 1 to 6 hydrogen atoms from oxygen-containing unsaturated heterocyclic rings such as acetylene oxide, furan, pyran, oxycycloheptatriene, isobenzofuran, and isochromene. Furthermore, examples of sulfur atom-containing heterocyclic groups include sulfur atom-containing saturated heterocyclic groups such as ethylene sulfide, trimethylene sulfide, tetrahydrothiophene, tetrahydrothiopyran, and hexamethylene sulfide, and sulfur atom-containing unsaturated heterocyclic groups such as acetylene sulfide, thiophene, thiapyran, and thiotripyridene, from which 1 to 6 hydrogen atoms have been removed. From the viewpoint of improving oxidation stability and storage stability, the number of ring-forming atoms of the heterocyclic group is preferably 3 or more, more preferably 5 or more, with an upper limit of preferably 10 or less, and more preferably 6 or less.

[0022] n １２ R is an integer from 1 to 6, and R in the general formula (1) above. １１ It is determined according to the number of bonding sites with R. For example, R １１ If it is a monovalent hydrocarbon group such as an alkyl group or cycloalkyl group, or an acyl group, then n １２ This becomes 1. In other words, R １１If the group is a hydrocarbon group or a heterocyclic group, and the valency of the group is 1, 2, 3, 4, 5, and 6, then n １２ These are 1, 2, 3, 4, 5, and 6, respectively. １２ From the viewpoint of improving oxidation stability and storage stability, it is preferably 1 or more, preferably 4 or less as the upper limit, more preferably 3 or less, and particularly preferably 1.

[0023] X is -(EO) ｍ１ (RO) ｍ２ - is the case. E is an ethylene group, and R is an alkylene group having 3 to 5 carbon atoms. R is preferably an alkylene group having 3 to 4 carbon atoms, and more preferably an alkylene group having 3 carbon atoms. As an alkylene group having 3 to 5 carbon atoms for R, for example, a trimethylene group (-CH ２ CH ２ CH ２ -), 1-methylethylene group (propylene group) (-CH(CH ３ )CH ２ C3 alkylene groups such as (-) and tetramethylene groups (-CH ２ CH ２ CH ２ CH ２ -), 1-methyltrimethylene group (-CH(CH ３ )CH ２ CH ２ -), 2-methyltrimethylene group (-CH ２ CH (CH ３ )CH ２ -), butylene group (-C(CH ３ ) ２ CH ２ -), 1-ethylethylene group (-CH(CH ２ CH ３ )CH ２ -), 1,2-dimethylethylene group (-CH(CH ３ )-CH(CH ３ )-) C4 alkylene groups, pentamethylene groups (-CH ２ CH ２ CH ２ CH ２ CH ２ -), tert-pentylene group (-C(CH ３ ) ２ CH２ CH ２ Examples include C5 alkylene groups such as (-CH(CH). Among these, from the viewpoint of improving oxidation stability and storage stability, 1-methylethylene group (propylene group) (-CH(CH)) are used. ３ )CH ２ -) is preferable.

[0024] - (EO) ｍ１ (RO) ｍ２ - can be any combination, such as random or block, but from the viewpoint of availability, the combination is preferably random.

[0025] m1 is a number from 8 to 56, preferably 10 to 48, more preferably 12 to 40, and even more preferably 13 to 32. m2 is a number from 4 to 36, preferably 5 to 30, more preferably 6 to 25, and even more preferably 7 to 20. In this embodiment, the molar ratio [(EO) / (RO)] must be 1.0 or higher. Therefore, m1 / m2 must also be 1.0 or higher. This increases the hydrophilicity of the PAG compound (A) and improves the permeability of water-containing strata. From the viewpoint of further increasing the hydrophilicity of the PAG compound (A) and improving the permeability of water-containing strata, m1 / m2 is preferably 1.1 or higher, more preferably 1.2 or higher, even more preferably 1.3 or higher, even more preferably 1.4 or higher, even more preferably 1.5 or higher, and even more preferably 1.6 or higher. Furthermore, there is no particular upper limit to m1 / m2, but it is usually 99.0 or less, and may also be 50.0 or less, 10.0 or less, 5.0 or less, or 3.0 or less.

[0026] n １１ n is the sum of m1 and m2, and is a value that is appropriately set according to the molecular weight of the compound represented by the general formula (a1) above. １１ n is a number between 12 and 92, preferably 15 to 78, more preferably 18 to 65, and even more preferably 20 to 52. When using two or more different compounds represented by the general formula (a1), n １１The value is the average (weighted average), and it is sufficient if that average is within the above range.

[0027] The molecular weight of the PAG compound (A) is preferably 200 or more, more preferably 500 or more, and even more preferably 800 or more, from the viewpoint of improving oxidation stability and storage stability, and improving the viscosity index of the lubricating oil composition, with an upper limit of preferably 10,000 or less, more preferably 5,000 or less, and even more preferably 3,000 or less. The upper and lower limits of these numerical ranges can be arbitrarily combined. Specifically, it is preferably 200 to 10,000, more preferably 500 to 5,000, and even more preferably 800 to 3,000. Hereinafter, the molecular weight refers to the value measured by the method described in the examples later.

[0028] Furthermore, PAG compound (A1) may be used alone or in combination of two or more types.

[0029] <Method for producing PAG compound (A) and PAG compound (A1)> PAG compound (A) and PAG compound (A1) can be produced by known methods, such as appropriately selecting an initiator and polymerizing a desired alkylene oxide in the presence of a catalyst such as potassium hydroxide, and the production method is not particularly limited. Commercially available PAG compound (A) and PAG compound (A1) can also be used.

[0030] <Naphthylamine-based antioxidant (B)> The lubricating oil composition of this embodiment contains a naphthylamine-based antioxidant (B). By including a naphthylamine-based antioxidant (B) together with the PAG compound (A) and the diphenylamine-based antioxidant (C) in the lubricating oil composition of this embodiment, the effects of the present invention are achieved, and in particular, the evaporation suppression properties of the lubricating oil composition can be greatly improved. The naphthylamine-based antioxidant (B) may be used alone or in combination of two or more types.

[0031] As the naphthylamine-based antioxidant (B), it is preferable to include one or more selected from the group consisting of phenylnaphthylamine (B1) and alkylphenylnaphthylamine (B2) from the viewpoint of improving the evaporation suppression of the lubricating oil composition. The content of one or more selected from the group consisting of phenylnaphthylamine (B1) and alkylphenylnaphthylamine (B2) in the naphthylamine-based antioxidant (B) is preferably 50% to 100% by mass, more preferably 60% to 100% by mass, even more preferably 70% to 100% by mass, even more preferably 80% to 100% by mass, and still more preferably 90% to 100% by mass, based on the total amount of the naphthylamine-based antioxidant (B).

[0032] Phenylnaphthylamine (B1) is an amine having an unsubstituted phenyl group and an unsubstituted naphthyl group, represented by the following general formula (b1), and is a compound that differs from alkylphenylnaphthylamine (B2) in that the phenyl group is unsubstituted.

[0033]

[0034] Examples of phenylnaphthylamine (B1) include phenyl-α-naphthylamine and phenyl-β-naphthylamine.

[0035] Alkylphenylnaphthylamine (B2) is an amine in which a phenyl group is substituted with an alkyl group, represented by the following general formula (b2).

[0036] Phenylnaphthylamine (B1) may be used alone or in combination of two or more types.

[0037]

[0038] In the above general formula (b2), R ｂ１ R represents an alkyl group. p1 is an integer from 1 to 5, preferably an integer from 1 to 3, more preferably 1 or 2, and even more preferably 1. ｂ１ If there are multiple instances (when p1 is 2 or more), multiple R ｂ１They may be identical to each other, or they may be different to each other.

[0039] R ｂ１ The number of carbon atoms in the alkyl group that can be selected is preferably 1 to 30, from the viewpoint of improving the oxidation stability of the lubricating oil composition and the storage stability of alkylphenylnaphthylamine (B2). In addition to these viewpoints, from the viewpoint of solubility in polyalkylene glycol compound (A), ｂ１ The number of carbon atoms in the alkyl group that can be selected is more preferably 1 to 20, even more preferably 4 to 16, and even more preferably 6 to 14.

[0040] Examples of alkylphenylnaphthylamine (B2) include alkylphenyl-α-naphthylamine in which the phenyl group is substituted with the alkyl group mentioned above, and alkylphenyl-β-naphthylamine substituted with the alkyl group mentioned above. Among these, alkylphenyl-α-naphthylamine in which the phenyl group is substituted with the alkyl group mentioned above is preferred, and N-(octylphenyl)naphthalene-1-amine is more preferred.

[0041] Alkylphenylnaphthylamine (B2) may be used alone or in combination of two or more types.

[0042] Here, from the viewpoint of further improving the evaporation suppression properties of the lubricating oil composition, it is preferable that the naphthylamine-based antioxidant (B) contains alkylphenylnaphthylamine (B2). From the viewpoint of improving the evaporation suppression properties of the lubricating oil composition, the content of alkylphenylnaphthylamine (B2) in the naphthylamine-based antioxidant (B) is preferably 50% to 100% by mass, more preferably 60% to 100% by mass, even more preferably 70% to 100% by mass, even more preferably 80% to 100% by mass, and still more preferably 90% to 100% by mass, based on the total amount of the naphthylamine-based antioxidant (B).

[0043] (Content of naphthylamine antioxidant (B)) The content of naphthylamine antioxidant (B) is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, even more preferably 0.2% by mass or more, even more preferably 0.4% by mass or more, even more preferably 0.5% by mass or more, even more preferably 0.6% by mass or more, and even more preferably 0.7% by mass or more, based on the total amount of the lubricating oil composition, from the viewpoint of improving the oxidation stability of the lubricating oil composition. Furthermore, the content of naphthylamine antioxidant (B) is preferably 3.0% by mass or less, more preferably 2.0% by mass or less, even more preferably 1.5% by mass or less, even more preferably 1.2% by mass or less, and even more preferably 1.0% by mass or less, based on the total amount of the lubricating oil composition, from the viewpoint of solubility in polyalkylene glycol compound (A), etc. Specifically, the amount is preferably 0.01% to 3.0% by mass, more preferably 0.1% to 3.0% by mass, even more preferably 0.2% to 2.0% by mass, even more preferably 0.4% to 2.0% by mass, still more preferably 0.5% to 1.5% by mass, and even more preferably 0.6% to 1.2% by mass.

[0044] <Diphenylamine-based antioxidant (C)> The lubricating oil composition of this embodiment contains a diphenylamine-based antioxidant (C). By including a diphenylamine-based antioxidant (C) together with the PAG compound (A) and the naphthylamine-based antioxidant (B) in the lubricating oil composition of this embodiment, the effects of the present invention are achieved, and in particular, the coking resistance of the lubricating oil composition can be greatly improved. The diphenylamine-based antioxidant (C) may be used alone or in combination of two or more types.

[0045] As the diphenylamine-based antioxidant (C), unsubstituted diphenylamine may be used, but from the viewpoint of improving the evaporation suppression property of the lubricating oil composition, it is preferable to contain an alkyldiphenylamine (C1). The content of the alkyldiphenylamine (C1) in the diphenylamine-based antioxidant (C) is preferably 50% by mass to 100% by mass, more preferably 60% by mass to 100% by mass, still more preferably 70% by mass to 100% by mass, even more preferably 80% by mass to 100% by mass, and still even more preferably 90% by mass to 100% by mass, based on the total amount of the diphenylamine-based antioxidant (C), from the viewpoint of improving the evaporation suppression property of the lubricating oil composition.

[0046] The alkyldiphenylamine (C1) is an amine represented by the following general formula (c1).

[0047]

[0048] In the above general formula (c1), R ｃ１ and R ｃ２ each independently represent an alkyl group. In the above general formula (c1), p2 and p3 are each independently an integer of 0 to 5, preferably an integer of 0 to 3, more preferably an integer of 0 to 2, and still more preferably 0 or 1. However, p2 + p3 ≧ 1. In each of the above general formulas, when there are a plurality of R ｃ１ , the plurality of R ｃ１ may be the same as each other or different from each other. Similarly, when there are a plurality of R ｃ２ , the plurality of R ｃ２ may be the same as each other or different from each other.

[0049] R ｃ１ and R ｃ２ The number of carbon atoms of the alkyl group that can be selected as is usually 1 to 30, preferably 1 to 20, more preferably 4 to 16, still more preferably 4 to 14, and even more preferably 4 to 12, each independently, from the viewpoint of improving the solubility in the base oil.

[0050] Examples of alkyldiphenylamines (C1) include monoalkyldiphenylamines such as monooctyldiphenylamine and monononyldiphenylamine; dialkyldiphenylamines such as 4,4'-dibutyldiphenylamine, 4,4'-dipentyldiphenylamine, 4,4'-dihexyldiphenylamine, 4,4'-diheptyldiphenylamine, 4,4'-dioctyldiphenylamine, and 4,4'-dinonyldiphenylamine; and polyalkyldiphenylamines such as tetrabutyldiphenylamine, tetrahexyldiphenylamine, tetraoctyldiphenylamine, and tetranonyldiphenylamine. Among these, monoalkyldiphenylamines and dialkyldiphenylamines are preferred, and dialkyldiphenylamines are more preferred.

[0051] Alkyldiphenylamine (C1) may be used alone or in combination of two or more types.

[0052] (Content of diphenylamine antioxidant (C)) The content of diphenylamine antioxidant (C) is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, even more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and still more preferably 0.4% by mass or more, based on the total amount of the lubricating oil composition, from the viewpoint of improving the oxidation stability of the lubricating oil composition. Furthermore, the content of diphenylamine antioxidant (C) is preferably 3% by mass or less, more preferably 2% by mass or less, even more preferably 1.5% by mass or less, even more preferably 1.2% by mass or less, even more preferably 1.0% by mass or less, even more preferably 0.8% by mass or less, and even more preferably 0.6% by mass or less, based on the total amount of the lubricating oil composition, from the viewpoint of solubility in polyalkylene glycol compound (A), etc. Specifically, the amount is preferably 0.01% to 3% by mass, more preferably 0.1% to 2% by mass, even more preferably 0.2% to 1.5% by mass, even more preferably 0.2% to 1.2% by mass, still more preferably 0.3% to 1.0% by mass, even more preferably 0.3% to 0.8% by mass, and even more preferably 0.4% to 0.6% by mass.

[0053] <Total Content of Components (B) and (C)> In the lubricating oil composition of this embodiment, the total content of components (B) and (C) is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, even more preferably 0.7% by mass or more, even more preferably 0.8% by mass or more, even more preferably 0.9% by mass or more, even more preferably 1.0% by mass or more, even more preferably 1.1% by mass or more, even more preferably 1.2% by mass or more, even more preferably 1.3% by mass or more, even more preferably 1.4% by mass or more, and particularly preferably 1.5% by mass or more, based on the total amount of the lubricating oil composition. From a similar viewpoint, preferably it is 6.0% by mass or less, more preferably 5.0% by mass or less, even 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, even more preferably 2.2% by mass or less, even more preferably 2.0% by mass or less, even more preferably 1.9% by mass or less, even more preferably 1.8% by mass or less, even more preferably 1.7% by mass or less, and particularly preferably 1.6% or less. The upper and lower limits of these numerical ranges can be arbitrarily combined. Specifically, the preferred amount is 0.5% to 6.0% by mass, more preferably 0.6% to 5.0% by mass, even more preferably 0.7% to 4.0% by mass, even more preferably 0.8% to 3.0% by mass, still more preferably 0.9% to 2.5% by mass, even more preferably 1.0% to 2.2% by mass, even more preferably 1.1% to 2.0% by mass, even more preferably 1.2% to 1.9% by mass, and even more preferably 1.3% to 1.8% by mass.

[0054] <Various ratios of component (B) and component (C)> In the lubricating oil composition of this embodiment, the content ratio of component (C) to component (B) [(C) / (B)] is preferably 0.2 or more, more preferably 0.3 or more, even more preferably 0.4 or more, and even more preferably 0.5 or more, in terms of mass ratio, from the viewpoint of achieving both evaporation suppression and good coking resistance. Also, preferably 6.0 or less, more preferably 4.0 or less, even more preferably 3.0 or less, even more preferably 2.5 or less, even more preferably 2.3 or less, even more preferably 2.0 or less, even more preferably 1.5 or less, even more preferably 1.2 or less, and even more preferably 1.0 or less. The upper and lower limits of these numerical ranges can be arbitrarily combined. Specifically, the values ​​are preferably 0.2 to 6.0, more preferably 0.2 to 4.0, even more preferably 0.3 to 3.0, even more preferably 0.3 to 2.5, still more preferably 0.4 to 2.3, even more preferably 0.5 to 2.0, even more preferably 0.5 to 1.5, even more preferably 0.5 to 1.2, and even more preferably 0.5 to 1.0.

[0055] In the lubricating oil composition of this embodiment, the ratio of the content of component (B) to the total content of component (C) [(B) / {(B)+(C)}] is preferably 0.3 or more, more preferably 0.4 or more, even more preferably 0.5 or more, and even more preferably 0.6 or more, in terms of mass ratio, from the viewpoint of achieving both evaporation suppression and good coking resistance.

[0056] <Other Components> The lubricating oil composition of the present embodiment may contain other components in addition to components (A) to (C). Examples of the other components include one or more selected from mineral oils and synthetic oils other than component (A), one or more selected from antioxidants other than component (B) and component (C), extreme pressure agents, detergent dispersants, pour point depressants, viscosity index improvers, rust inhibitors, metal deactivators, antifoaming agents, and friction modifiers, and one or more selected from lubricant additives such as these. Among these, the lubricating oil composition of the present embodiment preferably contains a benzotriazole compound which is a rust inhibitor and an antifoaming agent. Further, in the lubricating oil composition of the present embodiment, from the viewpoint of the permeability of the formation containing water, the content of the polyol ester compound is preferably small. Specifically, the content of the polyol ester compound is preferably less than 20% by mass, more preferably less than 10% by mass, still more preferably less than 5% by mass, even more preferably less than 3% by mass, still even more preferably less than 1% by mass, and most preferably does not contain a polyol ester compound, based on the total amount of the lubricating oil composition.

[0057] <Benzotriazole Compound> As the benzotriazole compound, any compound having benzotriazole can be used without particular limitation, and examples include 1,2,3-benzotriazole represented by the following general formula (d), alkylbenzotriazole represented by the following general formula (d-1), and aminoalkylbenzotriazole represented by the following general formula (d-2). Among these, aminoalkylbenzotriazole represented by the following general formula (d-2) is preferred.

[0058]

[0059] In the above general formulas (d-1) and (d-2), R Ｄ１ is, independently of each other, an alkyl group having 1 to 4 carbon atoms, and the alkyl group may be a linear alkyl group or a branched alkyl group. Further, the alkyl group may have a hydroxyl group. When there are a plurality of R Ｄ１ s, the plurality of R Ｄ１These may be the same as or different from each other. a is an integer between 1 and 4, preferably 1 or 2. b is an integer between 0 and 4, preferably 0 to 2, more preferably 0 or 1, and even more preferably 0. R Ｄ２ This is a methylene group or an ethylene group. Ｄ３ and R Ｄ４ Each of these is independently a hydrogen atom or an alkyl group having 1 to 18 carbon atoms, and the alkyl group may be a linear alkyl group or a branched alkyl group. Furthermore, the alkyl group may have a hydroxyl group.

[0060] Examples of benzotriazole compounds include alkylbenzotriazoles such as 1,2,3-benzotriazole, methylbenzotriazole, dimethylbenzotriazole, and ethylbenzotriazole; and aminoalkylbenzotriazoles such as (dihydroxyethylaminomethyl)methylbenzotriazole, (dioctylaminomethyl)methylbenzotriazole, [N-(ethylhexyl)aminomethyl]methylbenzotriazole, and 1-[N,N-bis(2-ethylhexyl)aminomethyl]-1H-benzotriazole. Among these, 1-[N,N-bis(2-ethylhexyl)aminomethyl]-1H-benzotriazole is preferred. When the lubricating oil composition of this embodiment contains a benzotriazole compound, the content of the benzotriazole compound is preferably 0.01% to 0.3% by mass, more preferably 0.01% to 0.2% by mass, and even more preferably 0.01% to 0.1% by mass, based on the total amount of the lubricating oil composition. The benzotriazole compound may be used alone or in combination of two or more types.

[0061] <Antifoaming agent> Examples of antifoaming agents include silicone-based antifoaming agents, fluorine-based antifoaming agents, and polyacrylate-based antifoaming agents, with silicone-based antifoaming agents being preferred among these. As an example of a silicone-based antifoaming agent, a silicone-based antifoaming agent containing polydimethylsiloxane as an active ingredient is preferred. When the lubricating oil composition of this embodiment contains an antifoaming agent, the antifoaming agent content (amount of active ingredient) is preferably 0.005% to 0.015% by mass, and more preferably 0.0005% to 0.0015% by mass, based on the total amount of the lubricating oil composition. An antifoaming agent may be used alone, or two or more types may be used in combination.

[0062] <Physical Properties of the Lubricating Oil Composition> The lubricating oil composition of this embodiment preferably satisfies the following physical properties.

[0063] (Kinematic viscosity at 40°C) The lubricating oil composition of this embodiment preferably has a kinematic viscosity at 40°C of 19.8 mm. ２ / s ~ 352mm ２ / s, more preferably 28.8 mm ２ / s ~ 242mm ２ / s, more preferably 28.8 mm ２ / s ~ 165mm ２ It is / s.

[0064] (Diatomaceous earth permeability) The lubricating oil composition of this embodiment has a diatomaceous earth permeability measured by the method described in the examples below, preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more.

[0065] (Evaporation Inhibition) The lubricating oil composition of this embodiment has an evaporation rate, as measured by the method described in the examples below, preferably 40% or less, more preferably 35% or less, even more preferably 30% or less, even more preferably 20% or less, still more preferably 10% or less, even more preferably 5% or less, and even more preferably 2% or less.

[0066] (Caulking Resistance) The lubricating oil composition of this embodiment has a carbon content of preferably 40 mg or less, more preferably 30 mg or less, and even more preferably 20 mg or less after the panel caulking test described in the examples below.

[0067] [Method for Manufacturing the Lubricating Oil Composition] The method for manufacturing the lubricating oil composition of this embodiment is not particularly limited. For example, the method for manufacturing the lubricating oil composition of this embodiment includes a step of mixing a polyalkylene glycol compound (A), a naphthylamine-based antioxidant (B), and a diphenylamine-based antioxidant (C), wherein the polyalkylene glycol compound (A) is blended in an amount of more than 50% by mass on a basis of the total amount of the lubricating oil composition. Furthermore, the molar ratio [(EO) / (RO)] of oxyethylene units (EO) to oxyalkylene units other than oxyethylene units (EO) of the polyalkylene glycol compound (A) is 1.0 or more. In addition, the method may further include a step of blending the other components mentioned above as needed. There are no particular limitations on the method of mixing each component, but for example, one method is to blend each component with the polyalkylene glycol compound (A). Alternatively, each component may be blended after being made into a solution (dispersion) by adding a diluent oil or the like. After blending each component, it is preferable to stir and disperse it uniformly by a known method. The preferred embodiments of component (A), component (B), component (C), and other components are as described above. Furthermore, the amounts and ratios of component (A), component (B), component (C), and other components are preferably those corresponding to the preferred content and content ratio of component (A), component (B), component (C), and other components as described above.

[0068] [Uses of the lubricating oil composition] The lubricating oil composition according to this embodiment has excellent permeability to water-containing geological formations. Therefore, the lubricating oil composition of this embodiment is CO ２ Because it has excellent permeability to "water-containing layers" such as reservoirs, even when it permeates into water-containing layers, it suppresses blockage of pores in the layers, etc., CO ２It can penetrate well into the geological formation. Therefore, it can be suitably used as a compressor oil for CCS, for example, and is particularly suitable as a reciprocating compressor oil for CCS. Thus, the lubricating oil composition according to this embodiment can provide the following methods of use (1) and (2). (1) A method of using the lubricating oil composition according to this embodiment for lubrication of a compressor used when injecting carbon dioxide into the ground in carbon dioxide capture and storage technology. (2) A method of using the lubricating oil composition according to this embodiment for lubrication of a reciprocating compressor used when injecting carbon dioxide into the ground in carbon dioxide capture and storage technology.

[0069] Herein, the lubricating oil composition according to this embodiment is not limited to the above-mentioned uses, but can be widely used, for example, as a lubricating oil composition for machinery and equipment. Specifically, it is preferably used as an industrial equipment oil. Examples of such industrial equipment oils include hydraulic oil, turbine oil, compressor oil, machine tool oil, or gear oil, and among these, it is preferably used as a compressor oil. Examples of compressors include centrifugal and axial flow turbo compressors, reciprocating compressors using pistons and diaphragms, screw type, movable vane type, scroll type, and tooth type rotary compressors. Accordingly, the lubricating oil composition according to this embodiment provides the following methods of use (3) to (7). (3) A method of using the lubricating oil composition according to this embodiment for lubrication of industrial equipment. (4) A method of using the lubricating oil composition according to this embodiment for lubrication of hydraulic actuators, turbines, compressors, machine tools, or gears. (5) A method of using the lubricating oil composition according to this embodiment for lubrication of compressors. (6) A method of using the lubricating oil composition according to this embodiment for lubrication of a rotary air compressor or a reciprocating air compressor. (7) A method of using the lubricating oil composition according to this embodiment for lubrication of a reciprocating air compressor.

[0070] Furthermore, the following lubrication methods are also provided in this embodiment: (1') A lubrication method for lubricating a compressor used when injecting carbon dioxide into the ground in carbon dioxide capture and storage technology, using the lubricating oil composition according to this embodiment. (2') A lubrication method for lubricating a reciprocating compressor used when injecting carbon dioxide into the ground in carbon dioxide capture and storage technology, using the lubricating oil composition according to this embodiment. (3') A lubrication method for lubricating industrial equipment, using the lubricating oil composition according to this embodiment. (4') A lubrication method for lubricating a hydraulic actuator, turbine, compressor, machine tool, or gear, using the lubricating oil composition according to this embodiment. (5') A lubrication method for lubricating a compressor with the lubricating oil composition according to this embodiment. (6') A lubrication method for lubricating a rotary air compressor or a reciprocating air compressor, using the lubricating oil composition according to this embodiment. (7') A lubrication method for lubricating a reciprocating air compressor, using the lubricating oil composition according to this embodiment.

[0071] [An Embodiment of the Invention Provided] According to one embodiment of the present invention, the following [1] to

[11] are provided. [1] A lubricating oil composition comprising a polyalkylene glycol compound (A), a naphthylamine-based antioxidant (B), and a diphenylamine-based antioxidant (C), wherein the molar ratio [(EO) / (RO)] of oxyethylene units (EO) of the polyalkylene glycol compound (A) to oxyalkylene units (RO) other than the oxyethylene units (EO) is 1.0 or more, and the content of the polyalkylene glycol compound (A) is more than 50% by mass on a basis of the total amount of the lubricating oil composition. [2] The lubricating oil composition according to [1], wherein the molar ratio [(EO) / (RO)] is 1.2 or more. [3] The lubricating oil composition according to [1] or [2], wherein the alkylene group R of the oxyalkylene unit (RO) is an alkylene group having 3 to 5 carbon atoms. [4] The lubricating oil composition according to any one of [1] to [3] above, wherein the total content of the naphthylamine antioxidant (B) and the diphenylamine antioxidant (C) is 0.5% to 6.0% by mass on a basis of the total amount of the lubricating oil composition. [5] The lubricating oil composition according to any one of [1] to [4] above, wherein the content ratio of the diphenylamine antioxidant (C) to the naphthylamine antioxidant (B) [(C) / (B)] is 0.2 to 6.0 by mass. [6] The lubricating oil composition according to any one of [1] to [5] above, wherein the content ratio of the diphenylamine antioxidant (C) to the naphthylamine antioxidant (B) [(C) / (B)] is 0.3 to 3.0 by mass. [7] The lubricating oil composition according to any one of [1] to [6] above, wherein the content ratio of the diphenylamine antioxidant (C) to the naphthylamine antioxidant (B) [(C) / (B)] is 0.5 to 2.0 by mass. [8] The lubricating oil composition according to any one of [1] to [7] above, wherein the total content of the naphthylamine antioxidant (B) and the diphenylamine antioxidant (C) is 1.1% to 2.0% by mass on a basis of the total amount of the lubricating oil composition.[9] The lubricating oil composition according to any one of [1] to [8] above, wherein the ratio of the content of the naphthylamine antioxidant (B) to the total content of the naphthylamine antioxidant (B) and the diphenylamine antioxidant (C) [(B) / {(B)+(C)}] is 0.5 or more by mass.

[10] A method of using the lubricating oil composition according to any one of [1] to [9] above for lubrication of a compressor.

[11] The method of using the lubricating oil composition according to

[10] above, wherein the compressor is a reciprocating compressor used when injecting carbon dioxide into the ground in carbon dioxide capture and storage technology.

[0072] The present invention will be specifically described by the following examples, but the present invention is not limited to the following examples.

[0073] [Methods for measuring various physical properties] The various physical properties of the raw materials used in this example were measured by the following methods: (1) Kinematic viscosity at 40°C: Measured in accordance with JIS K2283:2000. (2) Molecular weight: The molecular weight of the PAG compound was calculated by the following method. １ The PAG compound to be measured was measured using 1H-NMR, and the molar ratio [(EO) / (RO)] was calculated from the integral ratio of the peaks derived from EO and RO. Next, １３ The PAG compound to be measured was analyzed using 13C NMR. The integral ratio of the peaks originating from the terminal alkyl group was set to 1, and the integral ratio of the peaks originating from RO to this was defined as the molar average degree of polymerization of RO. Then, the molar average degree of polymerization of EO was calculated from the previously calculated molar ratio [(EO) / (RO)]. Based on these calculation results, the composition (average composition) and molecular weight (average molecular weight) of the PAG compound were calculated. １ H-NMR > Instrument name: JNM-ECZ400R Measurement conditions: Frequency 400 MHz Measurement solvent: CDCl ３ < １３ C-NMR > Instrument name: JNM-ECZ400R Measurement conditions: Frequency 100MHz, Measurement mode BCM, DEPT, NON Measurement solvent: CDCl ３

[0074] [Examples 1-3 and Comparative Examples 1-5] The following components were mixed to prepare lubricating oil compositions with the compositions shown in Table 1, and evaluations 1-3 below were performed. The numerical units of the compound compositions in Table 1 are in "mass%".

[0075] <Polyalkylene glycol compound (A)> ・"PAG1" EO / PO = 63 / 37 (molar ratio) Molecular weight: 1,200 Kinematic viscosity at 40°C: 53.63 mm ２ The structural formula of PAG1 is shown below. Note that nBu in the formula represents an n-butyl group. nBuO-(PO) ８ - (EO) １４ -H

[0076] • "PAG2" EO / PO = 61 / 39 (molar ratio) Molecular weight: 2,400 The structural formula of PAG2 is shown below. Note that nBu in the formula represents an n-butyl group. nBuO-(PO) １７ - (EO) ２７ -H

[0077] <Polyalkylene glycol compound (A)'> ・"PAG3" EO / BO = 10 / 90 (molar ratio), BO: butylene oxide Molecular weight: 1,000 (estimated value) Kinematic viscosity at 40°C: 68 mm ２ The structural formula for PAG3 is shown below: nBuO-(BO) ９ - (EO) １ -H ・"PAG4" EO / PO = 0 / 100 (molar ratio) Kinematic viscosity at 40°C: 100 mm ２ PAG4 is a PAG compound that consists only of repeating PO units, with both ends sealed by methyl groups. Its molecular weight has not been measured.

[0078] <Naphthylamine-based antioxidant (B)> N-(octylphenyl)naphthalene-1-amine N-(octylphenyl)naphthalene-1-amine is a compound corresponding to alkylphenylnaphthylamine (B2), and in the above general formula (b2), R ｂ１ This is an octyl group, and p1 = 1.

[0079] <Diphenylamine-based antioxidant (C)> Unsubstituted diphenylamine, and in the above general formula (c1), R ｃ１ and Rｃ２ However, each is independently an octyl group or a tert-butyl group, and the mixture is an alkyldiphenylamine (C2) mixture in which p2 = 0 or 1, p3 = 0 or 1, and p2 + p3 = 1 or more.

[0080] <Benzotriazole Compounds> 1-[N,N-bis(2-ethylhexyl)aminomethyl]-1H-benzotriazole is a compound corresponding to the aminoalkylbenzotriazole represented by the above general formula (d-2), where b=0, R Ｄ１ is a methylene group, R Ｄ３ and R Ｄ４ It is a 2-ethylhexyl group.

[0081] <Antifoaming agent> Silicone-based antifoaming agent containing polydimethylsiloxane as the active ingredient (100x dilution)

[0082] <Evaluation 1: Evaluation of diatomaceous earth permeability> The lubricating oil compositions of Examples 1 to 3 and Comparative Examples 1 to 4 were evaluated for their permeability to diatomaceous earth. The procedure for evaluating diatomaceous earth permeability is as follows: (1) Preparation of a filter using diatomaceous earth as a filter medium (1-1) A vacuum filtration filter holder (manufactured by ADVANTEC, model number: KGS-47 17311500) was placed in a 1,000 mL filtration bottle. A stainless steel mesh and Millipore paper (made of polycarbonate, aqueous 0.8 μm) were placed inside the vacuum filtration filter holder with the Millipore paper facing upwards. (1-2) 20 g of diatomaceous earth and 100 mL of deionized water were placed in a 200 mL beaker and mixed with a spatula to suspend. Observation of the diatomaceous earth used with an electron microscope revealed that its size (diameter) was 30 to 100 μm and it had pores of about 1 to 5 μm. (1-3) Diatomaceous earth suspended in water was added from the top of the filter funnel of the vacuum filtration filter holder, and the pressure was reduced using an evaporator. At this time, in order to form a uniform diatomaceous earth layer, the diatomaceous earth suspended in water was added by letting it run down the wall with a spatula or similar tool. If any diatomaceous earth remained in the beaker, deionized water was added to add all of the remaining diatomaceous earth in the beaker. Through the above operations, a filter using diatomaceous earth as the filter medium was prepared in the vacuum filtration filter holder.

[0083] (2) Method for evaluating diatomaceous earth permeability (2-1) 6 g of the sample (lubricating oil composition) and 300 g of deionized water were placed in a 500 mL beaker and stirred at 180 rpm for 5 minutes at room temperature using a magnetic stirrer to obtain an aqueous sample solution. (2-2) 102 g of the aqueous sample solution was placed at room temperature from the top of the filter funnel of the vacuum filtration filter holder (with diatomaceous earth filter material) prepared in (1) above. Then, the pressure was reduced for 3 minutes after the liquid level reached 0. This operation was repeated twice. (2-3) The vacuum device was turned off, and the diatomaceous earth in the vacuum filtration filter holder was removed and placed in a 200 mL beaker. (2-4) Acetone was added to the beaker containing the diatomaceous earth in (2-3) up to the 100 mL line and stirred with a magnetic stirrer for 5 minutes. (2-5) A glass filter was placed above a filter bottle of any size and the pressure was reduced, and the sample from (2-4) above was poured in from above the glass filter. (2-6) Approximately 100 mL of acetone was poured in to wash the diatomaceous earth. (2-7) The solution accumulated in the filter bottle was placed in a beaker whose weight was measured and dried in a fume hood. The solution accumulated in the filter bottle contained the sample that had been captured by the diatomaceous earth in the vacuum filtration filter holder. (2-8) When the weight of the beaker stopped changing, the amount of sample in the beaker x [g] was calculated. Then, the diatomaceous earth permeability [%] was calculated from the amount of sample in the beaker x [g] relative to the 102 g of sample added. If the amount of sample in the beaker was 0 g, the diatomaceous earth permeability was 100%. In other words, the less weight (residual amount) of oil sample remaining in the diatomaceous earth, the better the diatomaceous earth permeability and the better the permeability of the water-containing layer. In this example, samples with a diatomaceous earth permeability of 80% or more were considered acceptable.

[0084] <Evaluation 2: Evaporation Test> For the lubricating oil compositions of Examples 1 to 3 and Comparative Examples 1 and 4, which showed good diatomaceous earth permeability, as well as the lubricating oil composition of Comparative Example 5, the evaporation rate of the lubricating oil composition after 24 hours was measured using a test machine specified in JIS 2540:2000 under the following conditions: (Conditions) ・Temperature: 150℃ ・Oil volume: 2g ・Air volume: 10L / min A lower evaporation rate indicates better evaporation suppression. In this example, a composition with an evaporation rate of 40% or less was considered acceptable.

[0085] <Evaluation 3: Panel Caulking Test> The lubricating oil compositions of Examples 1-3 and Comparative Example 4, which showed good diatomaceous earth permeability and evaporation suppression, as well as the lubricating oil composition of Comparative Example 5, were tested for 3 hours in a cycle of 15 seconds of splash time and 45 seconds of stop time, under conditions of panel temperature of 300°C and oil temperature of 80°C, in accordance with Fed. Test Method Std. 791-3462. After the test, the amount of carbon adhering to the panel (carbon adhesion amount (mg)) was evaluated. A smaller carbon adhesion amount indicates better caulking resistance. In this example, a carbon adhesion amount of 40 mg or less was considered acceptable.

[0086] The results are shown in Table 1. Note that the panel caulking test results for Comparative Example 1 are predicted values.

[0087]

[0088] From the results shown in Table 1, the following can be seen: The lubricating oil compositions of Examples 1 to 3 exhibit excellent permeability through water-containing geological formations, as well as excellent evaporation suppression and coking resistance. In contrast, the lubricating oil composition of Comparative Example 1, which does not contain naphthylamine-based antioxidant (B) and diphenylamine-based antioxidant (C), exhibits excellent permeability through water-containing geological formations and coking resistance, but poor evaporation suppression. Furthermore, as with the lubricating oil compositions of Comparative Examples 2 and 3, when the PAG compound [(EO) / (PO)] (in other words, [(EO) / (RO)]) is less than 1.0, the permeability through water-containing geological formations is poor. Moreover, the lubricating oil composition of Comparative Example 4, which does not contain diphenylamine-based antioxidant (C), exhibits excellent permeability through water-containing geological formations and evaporation suppression, but poor coking resistance. Furthermore, the lubricating oil composition of Comparative Example 5, which does not contain naphthylamine-based antioxidant (B), exhibits poor evaporation suppression.

Claims

1. A lubricating oil composition comprising a polyalkylene glycol compound (A), a naphthylamine-based antioxidant (B), and a diphenylamine-based antioxidant (C), wherein the molar ratio [(EO) / (RO)] of oxyethylene units (EO) of the polyalkylene glycol compound (A) to oxyalkylene units (RO) other than the oxyethylene units (EO) is 1.0 or more, and the content of the polyalkylene glycol compound (A) is more than 50% by mass on a total basis of the lubricating oil composition.

2. The lubricating oil composition according to claim 1, wherein the molar ratio [(EO) / (RO)] is 1.2 or greater.

3. The lubricating oil composition according to claim 1, wherein the alkylene group R of the oxyalkylene unit (RO) is an alkylene group having 3 to 5 carbon atoms.

4. The lubricating oil composition according to claim 1, wherein the total content of the naphthylamine-based antioxidant (B) and the diphenylamine-based antioxidant (C) is 0.5% by mass to 6.0% by mass on a basis of the total amount of the lubricating oil composition.

5. The lubricating oil composition according to claim 1, wherein the content ratio of the diphenylamine antioxidant (C) to the naphthylamine antioxidant (B) [(C) / (B)] is 0.2 to 6.0 by mass.

6. The lubricating oil composition according to claim 1, wherein the content ratio of the diphenylamine antioxidant (C) to the naphthylamine antioxidant (B) [(C) / (B)] is 0.3 to 3.0 by mass.

7. The lubricating oil composition according to claim 1, wherein the content ratio of the diphenylamine antioxidant (C) to the naphthylamine antioxidant (B) [(C) / (B)] is 0.5 to 2.0 by mass.

8. The lubricating oil composition according to claim 1, wherein the total content of the naphthylamine-based antioxidant (B) and the diphenylamine-based antioxidant (C) is 1.1% by mass to 2.0% by mass on a basis of the total amount of the lubricating oil composition.

9. The lubricating oil composition according to claim 1, wherein the ratio of the content of the naphthylamine antioxidant (B) to the total content of the naphthylamine antioxidant (B) and the diphenylamine antioxidant (C) [(B) / {(B)+(C)}] is 0.5 or more by mass.

10. A method for using the lubricating oil composition described in claim 1 for the lubrication of a compressor.

11. The method of using the lubricating oil composition according to claim 10, wherein the compressor is a reciprocating compressor used when injecting carbon dioxide into the ground in carbon dioxide capture and storage technology.

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