Lubricating Oil Composition And Method Of Using The Lubricating Oil Composition

Description

FIELD OF THE INVENTION

The present invention relates to a lubricating oil composition and a method of using a lubricating oil composition.

BACKGROUND OF THE INVENTION

A lubricating oil composition has been used in various fields for the purpose of, for example, preventing the abrasion of a contact member to impart lubricity thereto.

For example, the following lubricating oil composition has been known as a lubricating oil composition for an air compressor (see Japanese Patent Application Laid-open No. 2020-063371): the lubricating oil composition contains a base oil containing a polyalkylene glycol and a rust inhibitor, and the content of the polyalkylene glycol is 65.0 mass % or more with respect to the total amount of the composition.

SUMMARY OF THE INVENTION

Incidentally, in recent years, a carbon dioxide capture and storage (CCS) technology (hereinafter also referred to as “CCS technology”) has been attracting attention as one effective measure against global warming. In the CCS technology, a compressor is used to inject CO₂ deep into the ground. During an investigation on the CCS technology, the inventors of the present invention have recognized that a lubricating oil composition to be used in the lubrication of the compressor may be mixed in a pumping line for CO₂ to reach a stratum for storing CO₂ (hereinafter also referred to as “CO₂-storing layer”). When the lubricating oil composition reaches the CO₂-storing layer, part of the CO₂-storing layer may be clogged reducing the injectability of CO₂.

To suppress the reduction in injectability of CO₂, the creation of a lubricating oil composition excellent in passability through a “water-containing stratum” such as a CO₂-storing layer has been required. However, the lubricating oil composition described in Japanese Patent Application Laid-open No. 2020-063371 is insufficient in passability through a water-containing stratum.

In addition, in the CCS technology, the lubricating oil composition to be used in the compressor (in particular, a reciprocating compressor) to be used when CO₂ is injected into the ground is required to have an evaporation-suppressing property and coking resistance.

However, in the lubricating oil composition, the achievement itself of both the evaporation-suppressing property and the coking resistance is difficult, and it is extremely difficult to cause the composition to exhibit passability through a water-containing stratum as well as to achieve both of these properties.

An object of the present invention is to provide a lubricating oil composition, which is excellent in passability through a water-containing stratum, and is also excellent in evaporation-suppressing property and coking resistance.

According to the present invention, there are provided the following items [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 a molar ratio [(EO)/(RO)] of an oxyethylene unit (EO) to an oxyalkylene unit (RO) except the oxyethylene unit (EO) in the polyalkylene glycol compound (A) is 1.0 or more, and wherein a content of the polyalkylene glycol compound (A) is more than 50 mass % with respect to a total amount of the lubricating oil composition.
  • [2] A method of using a lubricating oil composition, comprising using the lubricating oil composition of the above-mentioned item [1] in lubrication of a compressor.

According to the present invention, the lubricating oil composition, which is excellent in passability through a water-containing stratum, and is also excellent in evaporation-suppressing property and coking resistance can be provided.

DETAILED DESCRIPTION OF THE INVENTION

The upper limit values and lower limit values of numerical ranges described herein may be freely combined. For example, when the range of “from A to B” and the range of “from C to D” are described as numerical ranges, the numerical range of “from A to D” and the numerical range of “from C to B” are also included in the scope of the present invention.

In addition, the numerical range of “from a lower limit value to an upper limit value” described herein means that the value is the lower limit value or more and the upper limit value or less unless otherwise stated.

In addition, in this description, the numerical values of Examples are numerical values that may each be used as an upper limit value or a lower limit value.

[Aspect of Lubricating Oil Composition]

A lubricating oil composition of at least one embodiment of the present invention includes: a polyalkylene glycol compound (A); a naphthylamine-based antioxidant (B); and a diphenylamine-based antioxidant (C).

The molar ratio [(EO)/(RO)] of an oxyethylene unit (EO) to an oxyalkylene unit (RO) except the oxyethylene unit (EO) in the polyalkylene glycol compound (A) is 1.0 or more.

In addition, the content of the polyalkylene glycol compound (A) is more than 50 mass % with respect to the total amount of the lubricating oil composition.

The inventors of the present invention have made extensive investigations with a view to solving the above-mentioned problems. As a result, the inventors have found that when the polyalkylene glycol compound (A) is used as a base oil, and the molar ratio [(EO)/(RO)] of the oxyethylene unit (EO) to the oxyalkylene unit (RO) except the oxyethylene unit (EO) in the polyalkylene glycol compound (A) is adjusted to 1.0 or more, the hydrophilicity of the polyalkylene glycol compound (A) is improved, and hence a lubricating oil composition excellent in passability through a water-containing stratum can be provided.

In addition, the inventors have found that when the polyalkylene glycol compound (A) is used as the base oil, and the naphthylamine-based antioxidant (B) and the diphenylamine-based antioxidant (C) are blended into the lubricating oil composition, there can be prepared a lubricating oil composition, which is excellent in evaporation-suppressing property and coking resistance while exhibiting passability through a water-containing stratum.

The inventors of the present invention have further made various investigations on the basis of those investigation results and have completed the present invention.

In the lubricating oil composition of the at least one embodiment, the polyalkylene glycol compound (A) is the main component of the lubricating oil composition, and its content is more than 50 mass % with respect to the total amount of the lubricating oil composition.

In the lubricating oil composition of the at least one embodiment, from the viewpoint of improving its passability through a water-containing stratum, the content of the polyalkylene glycol compound (A) is preferably 60 mass % or more, more preferably 70 mass % or more, still more preferably 80 mass % or more, still further more preferably 90 mass % or more with respect to the total amount of the lubricating oil composition. In addition, the content of the polyalkylene glycol compound (A) is preferably less than 100 mass %, more preferably less than 99.8 mass %, still more preferably less than 99.5 mass %, still further more preferably less than 99.0 mass % with respect to the total amount of the lubricating oil composition.

The lubricating oil composition of at least one embodiment may be formed only of the polyalkylene glycol compound (A), the naphthylamine-based antioxidant (B), and the diphenylamine-based antioxidant (C), or may include a component except the polyalkylene glycol compound (A), the naphthylamine-based antioxidant (B), and the diphenylamine-based antioxidant (C) (hereinafter also referred to as “other component”).

The total content of the polyalkylene glycol compound (A), the naphthylamine-based antioxidant (B), and the diphenylamine-based antioxidant (C) is preferably 80 mass % or more, more preferably 90 mass % or more, still more preferably 95 mass % or more with respect to the total amount of the lubricating oil composition.

In addition, the total content of the polyalkylene glycol compound (A), the naphthylamine-based antioxidant (B), and the diphenylamine-based antioxidant (C) may be 100 mass % with respect to the total amount of the lubricating oil composition. However, from the viewpoint of securing room for the incorporation of the other component, the total content is preferably less than 100 mass %, more preferably 99.6 mass % or less, still more preferably 99.4 mass % or less with respect to the total amount of the lubricating oil composition.

The components to be incorporated into the lubricating oil composition of at least one embodiment, a component that may be incorporated into the lubricating oil composition of at least one embodiment, and the like are described in detail below.

In the following description, the “polyalkylene glycol compound (A),” the “naphthylamine-based antioxidant (B),” and the “diphenylamine-based antioxidant (C)” are also referred to as “component (A),” “component (B),” and “component (C),” respectively.

In the following description, the “polyalkylene glycol compound (A)” is also referred to as “PAG compound (A).”

<PAG Compound (A)>

The lubricating oil composition of the at least one embodiment includes the PAG compound (A).

The molar ratio [(EO)/(RO)] of the oxyethylene unit (EO) to the oxyalkylene unit (RO) except the oxyethylene unit (EO) in the PAG compound (A) is 1.0 or more.

When the molar ratio [(EO)/(RO)] is 1.0 or more, the hydrophilicity of the PAG compound (A) is improved, and hence the passability of the composition through a water-containing stratum can be made excellent.

Herein, from the viewpoint of further improving the hydrophilicity of the PAG compound (A) to make the passability through a water-containing stratum more excellent, the molar ratio [(EO)/(RO)] is preferably 1.1 or more, more preferably 1.2 or more, still more preferably 1.3 or more, still further more preferably 1.4 or more, yet still further more preferably 1.5 or more, even more preferably 1.6 or more.

Although the upper limit value of the molar ratio [(EO)/(RO)] is not particularly limited, the upper limit value is typically 99.0 or less, and may be 50.0 or less, may be 10.0 or less, may be 5.0 or less, or may be 3.0 or less.

In addition, the alkylene group R of the oxyalkylene unit (RO) except the oxyethylene unit (EO) is preferably an alkylene group having 3 to 5 carbon atoms, more preferably an alkylene group having 3 or 4 carbon atoms, still more preferably an alkylene group having 3 carbon atoms.

In addition, the PAG compounds (A) may be used alone or in combination thereof.

Herein, a PAG compound (A1) represented by the following general formula (a1) is preferably contained as the PAG compound (A).

In the general formula (a1), R¹¹ represents 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.

R¹² represents 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 represents -(EO)_m1(RO)_m2—. E represents an ethylene group and R represents an alkylene group having 3 to 5 carbon atoms.

m1 represents a number from 8 to 56.

m2 represents a number from 4 to 36.

n₁₁=m1+m2.

n₁₂ represents an integer of from 1 to 6.

When n₁₂ represents 2 or more, R¹²s present in plurality may be identical to or different from each other. In addition, when n₁₂ represents 2 or more, Xs present in plurality may be identical to or different from each other.

From the viewpoint of improving the oxidation stability and storage stability of the lubricating oil composition, at least one of R¹¹ or R¹² in the general formula (a1) preferably represents 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, that is, preferably does not represent a hydrogen atom, and more preferably represents a monovalent hydrocarbon group having 1 to 10 carbon atoms. That is, the PAG compound (A) is preferably a polyalkylene glycol monohydrocarbyl ether one terminal of which is blocked with a monovalent hydrocarbon group having 1 to 10 carbon atoms, or a polyalkylene glycol dihydrocarbyl ether both terminals of which are each blocked with a monovalent hydrocarbon group having 1 to 10 carbon atoms, and is more preferably a polyalkylene glycol monohydrocarbyl ether one terminal of which is blocked with a monovalent hydrocarbon group having 1 to 10 carbon atoms. In addition, from the viewpoint of improving the oxidation stability and the storage stability, it is particularly preferred that both of R¹¹ and R¹² each represent a monovalent hydrocarbon group having 1 to 10 carbon atoms. Herein, R¹¹ and R¹² may each represent a linear or branched group.

Examples of the monovalent hydrocarbon group having 1 to 10 carbon atoms that is represented by any one of R¹¹ and R¹² include: alkyl groups, such as a methyl group, an ethyl group, a propyl group (e.g., a n-propyl group or an isopropyl group), a butyl group (e.g., a branched butyl group, such as an isobutyl group, a s-butyl group, or a t-butyl group, is included in addition to a linear butyl group such as a n-butyl group; the same holds true for groups given as examples below), a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, and a decyl group; cycloalkyl groups, such as a cyclopentyl group, a cyclohexyl group, a methylcyclohexyl group, an ethylcyclohexyl group, a propylcyclohexyl group, and a dimethylcyclohexyl group; aryl groups, such as a phenyl group, a methylphenyl group, an ethylphenyl group, a dimethylphenyl group, a propylphenyl group, a trimethylphenyl group, a butylphenyl group, and a naphthyl group; and arylalkyl groups, such as a benzyl group, a phenylethyl group, a methylbenzyl group, a phenylpropyl group, and a phenylbutyl group. The examples also include alkenyl groups, cycloalkenyl groups, and arylalkenyl groups obtained by removing two hydrogen atoms from the above-mentioned alkyl groups, cycloalkyl groups, and arylalkyl groups, respectively.

In addition, from the viewpoint of improving the oxidation stability and the storage stability, the number of the carbon atoms of the monovalent hydrocarbon group is preferably 1 or more, and its upper limit is preferably 10 or less, more preferably 6 or less, still more preferably 4 or less.

With regard to the acyl group having 2 to 10 carbon atoms that is represented by any one of R¹¹ and R¹², the hydrocarbon group moiety of the acyl group is, for example, a group having 1 to 9 carbon atoms out of the above-mentioned monovalent hydrocarbon groups given as examples of R¹¹ and R¹², and may be any one of a linear group, a branched group, and a cyclic group.

In addition, from the viewpoint of improving the oxidation stability and the storage stability, the number of the carbon atoms of the acyl group is preferably 2 or more, and its upper limit is preferably 10 or less, more preferably 6 or less.

The divalent to hexavalent hydrocarbon group represented by R¹¹ is, for example, a residue obtained by further removing one to five hydrogen atoms from the above-mentioned monovalent hydrocarbon group represented by R¹¹, or a residue obtained by removing a hydroxy group from a polyhydric alcohol, such as trimethylolpropane, glycerin, pentaerythritol, sorbitol, 1,2,3-trihydroxycyclohexane, or 1,3,5-trihydroxycyclohexane. From the viewpoint of improving the oxidation stability and the storage stability, the number of the carbon atoms of the divalent to hexavalent hydrocarbon group is preferably 1 or more, and its upper limit is preferably 10 or less, more preferably 6 or less, still more preferably 4 or less.

The heterocyclic group having 3 to 10 ring-forming atoms that is represented by any one of R¹¹ and R¹² is, for example, an oxygen atom-containing heterocyclic group or a sulfur atom-containing heterocyclic group. The heterocyclic group may be a saturated ring or an unsaturated ring.

The oxygen atom-containing heterocyclic group is, for example, a residue obtained by removing one to six hydrogen atoms from an oxygen atom-containing saturated heterocycle, such as 1,3-propylene oxide, tetrahydrofuran, tetrahydropyran, or hexamethylene oxide, or an oxygen atom-containing unsaturated heterocycle, such as acetylene oxide, furan, pyran, oxycycloheptatriene, isobenzofuran, or isochromene.

In addition, the sulfur atom-containing heterocyclic group is, for example, a residue obtained by removing one to six hydrogen atoms from a sulfur atom-containing saturated heterocycle, such as ethylene sulfide, trimethylene sulfide, tetrahydrothiophene, tetrahydrothiopyran, or hexamethylene sulfide, or a sulfur atom-containing unsaturated heterocycle, such as acetylene sulfide, thiophene, thiapyran, or thiotripyridene.

From the viewpoint of improving the oxidation stability and the storage stability, the number of the ring-forming atoms of the heterocyclic group is preferably 3 or more, more preferably 5 or more, and its upper limit is preferably 10 or less, more preferably 6 or less.

n₁₂ represents an integer of from 1 to 6, and is determined in accordance with the number of bonding moieties with R¹¹ in the general formula (1). For example, when R¹¹ represents a monovalent hydrocarbon group, such as an alkyl group or a cycloalkyl group, or an acyl group, n₁₂ represents 1. In other words, when R¹¹ represents a hydrocarbon group or a heterocyclic group, and the valence of the group is 1, 2, 3, 4, 5, or 6, n₁₂ represents 1, 2, 3, 4, 5, or 6, respectively.

From the viewpoint of improving the oxidation stability and the storage stability, n₁₂ preferably represents 1 or more, and its upper limit is preferably 4 or less, more preferably 3 or less, particularly preferably 1.

X represents -(EO)_m1(RO)_m2—. E represents an ethylene group and R represents an alkylene group having 3 to 5 carbon atoms. R represents preferably an alkylene group having 3 or 4 carbon atoms, more preferably an alkylene group having 3 carbon atoms.

Examples of the alkylene group having 3 to 5 carbon atoms that is represented by R include: alkylene groups each having 3 carbon atoms, such as a trimethylene group (—CH₂CH₂CH₂—) and a 1-methylethylene group (propylene group) (—CH(CH₃)CH₂—); alkylene groups each having 4 carbon atoms, such as a tetramethylene group (—CH₂CH₂CH₂CH₂—), a 1-methyltrimethylene group (—CH(CH₃)CH₂CH₂—), a 2-methyltrimethylene group (—CH₂CH(CH₃)CH₂—), a butylene group (—C(CH₃)₂CH₂—), a 1-ethylethylene group (—CH(CH₂CH₃)CH₂—), and a 1,2-dimethylethylene group (—CH(CH₃)—CH(CH₃)—); and alkylene groups each having 5 carbon atoms, such as a pentamethylene group (—CH₂CH₂CH₂CH₂CH₂—) and a tert-pentylene group (—C(CH₃)₂CH₂CH₂—).

Among them, a 1-methylethylene group (propylene group) (—CH(CH₃)CH₂—) is preferred from the viewpoint of improving the oxidation stability and the storage stability.

Although the bonding mode of -(EO)_m1(RO)_m2— may be any one of, for example, a random mode and a block mode, the bonding mode is preferably a random mode from the viewpoint of, for example, ease of availability.

m1 represents a number from 8 to 56, preferably from 10 to 48, more preferably from 12 to 40, still more preferably from 13 to 32.

m2 represents a number from 4 to 36, preferably from 5 to 30, more preferably from 6 to 25, still more preferably from 7 to 20.

Herein, in the at least one embodiment, the molar ratio [(EO)/(RO)]needs to be 1.0 or more. Accordingly, the ratio “m1/m2” also needs to be 1.0 or more. Thus, the hydrophilicity of the PAG compound (A) is improved, and hence the passability of the lubricating oil composition through a water-containing stratum can be made excellent.

Herein, from the viewpoint of further improving the hydrophilicity of the PAG compound (A) to make the passability through a water-containing stratum more excellent, the ratio “m1/m2” is preferably 1.1 or more, more preferably 1.2 or more, still more preferably 1.3 or more, still further more preferably 1.4 or more, yet still further more preferably 1.5 or more, even more preferably 1.6 or more.

Although the upper limit value of the ratio “m1/m2” is not particularly limited, the upper limit value is typically 99.0 or less, and may be 50.0 or less, may be 10.0 or less, may be 5.0 or less, or may be 3.0 or less.

n₁₁ is the total of m1 and m2, and is a value appropriately set in accordance with the value of the molecular weight of the compound represented by the general formula (a1).

n₁₁ represents a number from 12 to 92, preferably from 15 to 78, more preferably from 18 to 65, still more preferably from 20 to 52.

When two or more different kinds of compounds each represented by the general formula (a1) are used, the value of n₁₁s is an average value (weighted average value), and the average value only needs to fall within the above-mentioned ranges.

From the viewpoints of improving the oxidation stability and the storage stability, and improving the viscosity index of the lubricating oil composition, the molecular weight of the PAG compound (A) is preferably 200 or more, more preferably 500 or more, still more preferably 800 or more, and its upper limit is preferably 10,000 or less, more preferably 5,000 or less, still more preferably 3,000 or less.

The upper limit values and lower limit values of those numerical ranges may be freely combined. Specifically, the molecular weight is preferably from 200 to 10,000, more preferably from 500 to 5,000, still more preferably from 800 to 3,000.

Herein, in this specification, the molecular weight means a value measured by a method described in Examples to be described later.

In addition, the PAG compounds (A1) may be used alone or in combination thereof.

<Methods of producing PAG Compound (A) and PAG Compound (A1)>

The PAG compound (A) and the PAG compound (A1) may each be appropriately produced by, for example, a known method including appropriately selecting an initiator and polymerizing a desired alkylene oxide in the presence of a catalyst such as a potassium hydroxide catalyst, and production methods therefor are not particularly limited.

Commercial products may be used as the PAG compound (A) and the PAG compound (A1).

<Naphthylamine-Based Antioxidant (B)>

The lubricating oil composition of the at least one embodiment includes the naphthylamine-based antioxidant (B).

When the lubricating oil composition of the at least one embodiment includes the naphthylamine-based antioxidant (B) together with the PAG compound (A) and the diphenylamine-based antioxidant (C), the effects of the present invention are exhibited, and in particular, the evaporation-suppressing property of the lubricating oil composition among the effects can be largely improved.

The naphthylamine-based antioxidants (B) may be used alone or in combination thereof.

From the viewpoint of improving the evaporation-suppressing property of the lubricating oil composition, one or more kinds selected from the group consisting of: a phenylnaphthylamine (B1); and an alkylphenylnaphthylamine (B2) are preferably contained as the naphthylamine-based antioxidant (B).

From the viewpoint of improving the evaporation-suppressing property of the lubricating oil composition, the content of the one or more kinds selected from the group consisting of: the phenylnaphthylamine (B1); and the alkylphenylnaphthylamine (B2) in the naphthylamine-based antioxidant (B) is preferably from 50 mass % to 100 mass %, more preferably from 60 mass % to 100 mass %, still more preferably from 70 mass % to 100 mass %, still further more preferably from 80 mass % to 100 mass %, yet still further more preferably from 90 mass % to 100 mass % with respect to the total amount of the naphthylamine-based antioxidant (B).

The phenylnaphthylamine (B1) is an amine represented by the following general formula (b1), the amine having an unsubstituted phenyl group and an unsubstituted naphthyl group, and is a compound different from the alkylphenylnaphthylamine (B2) in that its phenyl group is free of any substituent.

Examples of the phenylnaphthylamine (B1) include phenyl-α-naphthylamine and phenyl-β-naphthylamine.

The alkylphenylnaphthylamine (B2) is an amine represented by the following general formula (b2) in which a phenyl group is substituted with an alkyl group.

The phenylnaphthylamines (B1) may be used alone or in combination thereof.

In the general formula (b2), R^b1 represents an alkyl group.

p1 represents an integer of from 1 to 5, preferably an integer of from 1 to 3, more preferably 1 or 2, still more preferably 1.

When a plurality of R^b1s are present (when p1 represents 2 or more), the plurality of R^b1s may be identical to or different from each other.

From the viewpoint of improving the oxidation stability of the lubricating oil composition, and the viewpoint of the storage stability of the alkylphenylnaphthylamine (B2), the number of the carbon atoms of the alkyl group that may be selected as R^b1 is preferably from 1 to 30. In addition, from the viewpoint of the solubility of the alkylphenylnaphthylamine in the polyalkylene glycol compound (A) in addition to those viewpoints, the number of the carbon atoms of the alkyl group that may be selected as R^b1 is more preferably from 1 to 20, still more preferably from 4 to 16, still further more preferably from 6 to 14.

Examples of the alkylphenylnaphthylamine (B2) include an alkylphenyl-α-naphthylamine whose phenyl group is substituted with the above-mentioned alkyl group, and an alkylphenyl-β-naphthylamine whose phenyl group is substituted with the above-mentioned alkyl group.

Among them, an alkylphenyl-α-naphthylamine whose phenyl group is substituted with the above-mentioned alkyl group is preferred, and N-(octylphenyl)naphthalene-1-amine is more preferred.

The alkylphenylnaphthylamines (B2) may be used alone or in combination thereof.

Herein, from the viewpoint of further improving the evaporation-suppressing property of the lubricating oil composition, the naphthylamine-based antioxidant (B) preferably contains the alkylphenylnaphthylamine (B2).

From the viewpoint of improving the evaporation-suppressing property of the lubricating oil composition, the content of the alkylphenylnaphthylamine (B2) in the naphthylamine-based antioxidant (B) is preferably from 50 mass % to 100 mass %, more preferably from 60 mass % to 100 mass %, still more preferably from 70 mass % to 100 mass %, still further more preferably from 80 mass % to 100 mass %, yet still further more preferably from 90 mass % to 100 mass % with respect to the total amount of the naphthylamine-based antioxidant (B).

(Content of Naphthylamine-Based Antioxidant (B))

From, for example, the viewpoint of improving the oxidation stability of the lubricating oil composition, the content of the naphthylamine-based antioxidant (B) is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, still more preferably 0.2 mass % or more, still further more preferably 0.4 mass % or more, yet still further more preferably 0.5 mass % or more, even more preferably 0.6 mass % or more, still even more preferably 0.7 mass % or more with respect to the total amount of the lubricating oil composition.

In addition, from, for example, the viewpoint of the solubility in the polyalkylene glycol compound (A), the content of the naphthylamine-based antioxidant (B) is preferably 3.0 mass % or less, more preferably 2.0 mass % or less, still more preferably 1.5 mass % or less, still further more preferably 1.2 mass % or less, yet still further more preferably 1.0 mass % or less with respect to the total amount of the lubricating oil composition.

The upper limit values and lower limit values of those numerical ranges may be freely combined. Specifically, the content is preferably from 0.01 mass % to 3.0 mass %, more preferably from 0.1 mass % to 3.0 mass %, still more preferably from 0.2 mass % to 2.0 mass %, still further more preferably from 0.4 mass % to 2.0 mass %, yet still further more preferably from 0.5 mass % to 1.5 mass %, even more preferably from 0.6 mass % to 1.2 mass %.

<Diphenylamine-based Antioxidant (C)>

The lubricating oil composition of the at least one embodiment includes the diphenylamine-based antioxidant (C).

When the lubricating oil composition of the at least one embodiment includes the diphenylamine-based antioxidant (C) together with the PAG compound (A) and the naphthylamine-based antioxidant (B), the effects of the present invention are exhibited, and in particular, the coking resistance of the lubricating oil composition among the effects can be largely improved.

The diphenylamine-based antioxidants (C) may be used alone or in combination thereof.

Although the diphenylamine-based antioxidant (C) may be unsubstituted diphenylamine, the antioxidant preferably contains an alkyldiphenylamine (C1) from the viewpoint of improving the evaporation-suppressing property of the lubricating oil composition.

From the viewpoint of improving the evaporation-suppressing property of the lubricating oil composition, the content of the alkyldiphenylamine (C1) in the diphenylamine-based antioxidant (C) is preferably from 50 mass % to 100 mass %, more preferably from 60 mass % to 100 mass %, still more preferably from 70 mass % to 100 mass %, still further more preferably from 80 mass % to 100 mass %, yet still further more preferably from 90 mass % to 100 mass % with respect to the total amount of the diphenylamine-based antioxidant (C).

The alkyldiphenylamine (C1) is an amine represented by the following general formula (b1).

In the general formula (c1), R^c1 and R^c2 each independently represent an alkyl group.

In the general formula (c1), p2 and p3 each independently represent an integer of from 0 to 5, preferably an integer of from 0 to 3, more preferably an integer of from 0 to 2, still more preferably 0 or 1.

However, p2+p3>1.

When a plurality of R^c1s are present in the general formula, the plurality of R^c1s may be identical to or different from each other. Similarly, when a plurality of R^c2s are present, the plurality of R^c2s may be identical to or different from each other.

From the viewpoint of improving the solubility of the alkyldiphenylamine in the base oil, the numbers of the carbon atoms of the alkyl groups that may be selected as R^c1 and R^c2 are each independently typically from 1 to 30, preferably from 1 to 20, more preferably from 4 to 16, still more preferably from 4 to 14, still further more preferably from 4 to 12.

Examples of the alkyldiphenylamine (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 them, monoalkyldiphenylamines and dialkyldiphenylamines are preferred, and dialkyldiphenylamines are more preferred.

The alkyldiphenylamines (C1) may be used alone or in combination thereof.

(Content of Diphenylamine-Based Antioxidant (C))

From, for example, the viewpoint of improving the oxidation stability of the lubricating oil composition, the content of the diphenylamine-based antioxidant (C) is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, still more preferably 0.2 mass % or more, still further more preferably 0.3 mass % or more, yet still further more preferably 0.4 mass % or more with respect to the total amount of the lubricating oil composition.

In addition, from, for example, the viewpoint of the solubility in the polyalkylene glycol compound (A), the content of the diphenylamine-based antioxidant (C) is preferably 3 mass % or less, more preferably 2 mass % or less, still more preferably 1.5 mass % or less, still further more preferably 1.2 mass % or less, yet still further more preferably 1.0 mass % or less, even more preferably 0.8 mass % or less, still even more preferably 0.6 mass % or less with respect to the total amount of the lubricating oil composition.

The upper limit values and lower limit values of those numerical ranges may be freely combined. Specifically, the content is preferably from 0.01 mass % to 3 mass %, more preferably from 0.1 mass % to 2 mass %, still more preferably from 0.2 mass % to 1.5 mass %, still further more preferably from 0.2 mass % to 1.2 mass %, yet still further more preferably from 0.3 mass % to 1.0 mass %, even more preferably from 0.3 mass % to 0.8 mass %, still even more preferably from 0.4 mass % to 0.6 mass %.

<Total Content of Component (B) and Component (C)>

In the lubricating oil composition of the at least one embodiment, from the viewpoint of achieving both of an evaporation-suppressing property and satisfactory coking resistance, the total content of the component (B) and the component (C) is preferably 0.5 mass % or more, more preferably 0.6 mass % or more, still more preferably 0.7 mass % or more, still further more preferably 0.8 mass % or more, yet still further more preferably 0.9 mass % or more, even more preferably 1.0 mass % or more, still even more preferably 1.1 mass % or more, yet still even more preferably 1.2 mass % or more, further yet still even more preferably 1.3 mass % or more, even further yet still even more preferably 1.4 mass % or more, particularly preferably 1.5 mass % or more with respect to the total amount of the lubricating oil composition.

In addition, from the same viewpoint, the total content is preferably 6.0 mass % or less, more preferably 5.0 mass % or less, still more preferably 4.0 mass % or less, still further more preferably 3.0 mass % or less, yet still further more preferably 2.5 mass % or less, even more preferably 2.2 mass % or less, still even more preferably 2.0 mass % or less, yet still even more preferably 1.9 mass % or less, further yet still even more preferably 1.8 mass % or less, even further yet still even more preferably 1.7 mass % or less, particularly preferably 1.6 mass % or less.

The upper limit values and lower limit values of those numerical ranges may be freely combined. Specifically, the total content is preferably from 0.5 mass % to 6.0 mass %, more preferably from 0.6 mass % to 5.0 mass %, still more preferably from 0.7 mass % to 4.0 mass %, still further more preferably from 0.8 mass % to 3.0 mass %, yet still further more preferably from 0.9 mass % to 2.5 mass %, even more preferably from 1.0 mass % to 2.2 mass %, still even more preferably from 1.1 mass % to 2.0 mass %, yet still even more preferably from 1.2 mass % to 1.9 mass %, further yet still even more preferably from 1.3 mass % to 1.8 mass %.

<Various Ratios Concerning Component (B) and Component (C)>

In the lubricating oil composition of the at least one embodiment, from the viewpoint of achieving both of an evaporation-suppressing property and satisfactory coking resistance, the content ratio [(C)/(B)] of the component (C) to the component (B) is preferably 0.2 or more, more preferably 0.3 or more, still more preferably 0.4 or more, still further more preferably 0.5 or more in terms of mass ratio.

In addition, the content ratio [(C)/(B)] is preferably 6.0 or less, more preferably 4.0 or less, still more preferably 3.0 or less, still further more preferably 2.5 or less, yet still further more preferably 2.3 or less, even more preferably 2.0 or less, still even more preferably 1.5 or less, yet still even more preferably 1.2 or less, further yet still even more preferably 1.0 or less.

The upper limit values and lower limit values of those numerical ranges may be freely combined. Specifically, the content ratio [(C)/(B)] is preferably from 0.2 to 6.0, more preferably from 0.2 to 4.0, still more preferably from 0.3 to 3.0, still further more preferably from 0.3 to 2.5, yet still further more preferably from 0.4 to 2.3, even more preferably from 0.5 to 2.0, still even more preferably from 0.5 to 1.5, yet still even more preferably from 0.5 to 1.2, further yet still even more preferably from 0.5 to 1.0.

In the lubricating oil composition of the at least one embodiment, from the viewpoint of achieving both of an evaporation-suppressing property and satisfactory coking resistance, the content ratio [(B)/{(B)+(C)}] of the component (B) to the total content of the component (B) and the component (C) is preferably 0.3 or more, more preferably 0.4 or more, still more preferably 0.5 or more, still further more preferably 0.6 or more in terms of mass ratio.

<Other Component>

The lubricating oil composition of the at least one embodiment may include any other component except the components (A) to (C).

Examples of the other component include: one or more kinds selected from a mineral oil and a synthetic oil except the component (A); one or more kinds selected from antioxidants except the component (B) and the component (C); and one or more kinds selected from additives for lubricating oils, such as an extreme pressure agent, a detergent dispersant, a pour point depressant, a viscosity index improver, a rust inhibitor, a metal deactivator, an antifoaming agent, and a friction modifier.

The lubricating oil composition of the at least one embodiment preferably includes a benzotriazole compound that is a rust inhibitor and an antifoaming agent among those components.

<Benzotriazole Compound>

Although a compound having benzotriazole may be used as the benzotriazole compound without any particular limitation, examples thereof include 1,2,3-benzotriazole represented by the following general formula (d), an alkylbenzotriazole represented by the following general formula (d-1), and an aminoalkylbenzotriazole represented by the following general formula (d-2). Among them, an aminoalkylbenzotriazole represented by the following general formula (d-2) is preferred.

In the general formulae (d-1) and (d-2), R^D1s each independently represent an alkyl group having 1 to 4 carbon atoms, and the alkyl group may be a linear alkyl group or a branched alkyl group. In addition, the alkyl group may have a hydroxy group. When a plurality of R^D1s are present, the plurality of R^D1s may be identical to or different from each other.

“a” represents an integer of from 1 to 4, preferably 1 or 2.

“b” represents an integer of from 0 to 4, preferably an integer of from 0 to 2, more preferably 0 or 1, still more preferably 0.

R^D2 represents a methylene group or an ethylene group.

R^D3 and R^D4 each independently represent 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. In addition, the alkyl group may have a hydroxy group.

Examples of the benzotriazole compound 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 them, 1-[N,N-bis(2-ethylhexyl)aminomethyl]-1H-benzotriazole is preferred.

When the lubricating oil composition of the at least one embodiment includes the benzotriazole compound, the content of the benzotriazole compound is preferably from 0.01 mass % to 0.3 mass %, more preferably from 0.01 mass % to 0.2 mass %, still more preferably from 0.01 mass % to 0.1 mass % with respect to the total amount of the lubricating oil composition.

The benzotriazole compounds may be used alone or in combination thereof.

<Antifoaming Agent>

Examples of the antifoaming agent include a silicone-based antifoaming agent, a fluorine-based antifoaming agent, and a polyacrylate-based antifoaming agent. Among them, a silicone-based antifoaming agent is preferred.

The silicone-based antifoaming agent is preferably, for example, a silicone-based antifoaming agent containing a polydimethylsiloxane as an active ingredient.

When the lubricating oil composition of the at least one embodiment includes the antifoaming agent, the content (active ingredient amount) of the antifoaming agent is preferably from 0.005 mass % to 0.015 mass %, more preferably from 0.0005 mass % to 0.0015 mass % with respect to the total amount of the lubricating oil composition.

The antifoaming agents may be used alone or in combination thereof.

<Physical Properties of Lubricating Oil Composition>

The lubricating oil composition of the at least one embodiment preferably satisfies the following physical properties.

(40° C. Kinematic Viscosity)

The 40° C. kinematic viscosity of the lubricating oil composition of the at least one embodiment is preferably from 19.8 mm²/s to 352 mm²/s, more preferably from 28.8 mm²/s to 242 mm²/s, still more preferably from 28.8 mm²/s to 165 mm²/s.

(Diatomaceous Earth Passability)

The diatomaceous earth passability of the lubricating oil composition of the at least one embodiment measured by a method described in Examples to be described later is preferably 80% or more, more preferably 85% or more, still more preferably 90% or more.

(Evaporation-Suppressing Property)

The evaporation ratio of the lubricating oil composition of the at least one embodiment measured by a method described in Examples to be described later is preferably 40% or less, more preferably 35% or less, still more preferably 30% or less, still further more preferably 20% or less, yet still further more preferably 10% or less, even more preferably 5% or less, still even more preferably 2% or less.

(Coking Resistance)

The carbon amount of the lubricating oil composition of the at least one embodiment after a panel coking test described in Examples to be described later is preferably 40 mg or less, more preferably 30 mg or less, still more preferably 20 mg or less.

[Method of Producing Lubricating Oil Composition]

A method of producing a lubricating oil composition of at least one embodiment of the present invention is not particularly limited.

For example, the method of producing a lubricating oil composition of the at least one embodiment includes a step of mixing the polyalkylene glycol compound (A), the naphthylamine-based antioxidant (B), and the diphenylamine-based antioxidant (C), and in the step, the polyalkylene glycol compound (A) is blended at more than 50 mass % with respect to the total amount of the lubricating oil composition. In addition, the molar ratio [(EO)/(RO)] of the oxyethylene unit (EO) to the oxyalkylene unit (RO) except the oxyethylene unit (EO) in the polyalkylene glycol compound (A) is 1.0 or more.

In addition, the method may further include a step of blending the above-mentioned other component as required.

Although a method of mixing the respective components is not particularly limited, for example, a method including blending the respective components into the polyalkylene glycol compound (A) is available. In addition, each of the components may be blended after a diluting oil or the like has been added to bring the component into a solution (dispersion) form. After having been blended, the respective components are preferably dispersed in a uniform manner through stirring by a known method.

Preferred aspects of the component (A), the component (B), the component (C), and the other component are as described above.

In addition, the blending amounts and blending ratios of the component (A), the component (B), the component (C), and the other component are preferably set to blending amounts and blending ratios corresponding to preferred contents and preferred content ratios of the component (A), the component (B), the component (C), and the other component described above.

[Applications of Lubricating Oil Composition]

The lubricating oil composition according to the at least one embodiment is excellent in passability through a water-containing stratum. Accordingly, the lubricating oil composition of the at least one embodiment is excellent in passability through a “water-containing stratum” such as a CO₂-storing layer, and hence even when the composition permeates into the water-containing stratum, the clogging of, for example, a gap of the stratum is suppressed. Thus, CO₂ can be satisfactorily caused to permeate into the stratum. Accordingly, the composition may be suitably used as, for example, a compressor oil for CCS, and may be particularly suitably used as a reciprocating compressor oil for CCS.

According to the lubricating oil composition of the at least one embodiment, there can be provided the following usage methods (1) and (2):

• • (1) a method of using a lubricating oil composition, including using the lubricating oil composition according to the at least one embodiment in lubrication of a compressor to be used when carbon dioxide is pressed into a ground in a carbon dioxide capture and storage technology; and
  • (2) a method of using a lubricating oil composition, including using the lubricating oil composition according to the at least one embodiment in lubrication of a reciprocating compressor to be used when carbon dioxide is pressed into a ground in a carbon dioxide capture and storage technology.

Herein, the applications of the lubricating oil composition according to the at least one embodiment are not limited to those described above, and the composition may be widely used as, for example, a lubricating oil composition for machinery and equipment. Specifically, the composition is preferably used as an industrial equipment oil. The industrial equipment oil is, for example, a working fluid, a turbine oil, a compressor oil, a machine tool oil, or a gear oil. The composition is preferably used as a compressor oil among them.

Examples of the compressor include: centrifugal and axial-flow turbo compressors; a reciprocating compressor using a piston or a diaphragm; and screw-type, movable vane-type, scroll-type, and tooth-type rotary compressors.

Accordingly, the lubricating oil composition according to the at least one embodiment provides the following usage methods (3) to (7):

• • (3) a method of using a lubricating oil composition, including using the lubricating oil composition according to the at least one embodiment in lubrication of industrial equipment;
  • (4) a method of using a lubricating oil composition, including using the lubricating oil composition according to the at least one embodiment in lubrication of a hydraulic actuation device, a turbine, a compressor, a machine tool, or a gear;
  • (5) a method of using a lubricating oil composition, including using the lubricating oil composition according to the at least one embodiment in lubrication of a compressor;
  • (6) a method of using a lubricating oil composition, including using the lubricating oil composition according to the at least one embodiment in lubrication of a rotary air compressor or a reciprocating air compressor; and
  • (7) a method of using a lubricating oil composition, including using the lubricating oil composition according to the at least one embodiment in lubrication of a reciprocating air compressor.

[One Aspect of the Present Invention to be Provided]

According to one aspect of the present invention, there are provided the following items [1] to [11].

• • [1] A lubricating oil composition, including: a polyalkylene glycol compound (A); a naphthylamine-based antioxidant (B); and a diphenylamine-based antioxidant (C), wherein a molar ratio [(EO)/(RO)] of an oxyethylene unit (EO) to an oxyalkylene unit (RO) except the oxyethylene unit (EO) in the polyalkylene glycol compound (A) is 1.0 or more, and wherein a content of the polyalkylene glycol compound (A) is more than 50 mass % with respect to a total amount of the lubricating oil composition.
  • [2] The lubricating oil composition according to the above-mentioned item [1], wherein the molar ratio [(EO)/(RO)] is 1.2 or more.
  • [3] The lubricating oil composition according to the above-mentioned item [1] or [2], wherein an 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 the above-mentioned items [1] to [3], wherein a total content of the naphthylamine-based antioxidant (B) and the diphenylamine-based antioxidant (C) is from 0.5 mass % to 6.0 mass % with respect to the total amount of the lubricating oil composition.
  • [5] The lubricating oil composition according to any one of the above-mentioned items [1] to [4], wherein a content ratio [(C)/(B)] of the diphenylamine-based antioxidant (C) to the naphthylamine-based antioxidant (B) is from 0.2 to 6.0 in terms of mass ratio.
  • [6] The lubricating oil composition according to any one of the above-mentioned items [1] to [5], wherein a content ratio [(C)/(B)] of the diphenylamine-based antioxidant (C) to the naphthylamine-based antioxidant (B) is from 0.3 to 3.0 in terms of mass ratio.
  • [7] The lubricating oil composition according to any one of the above-mentioned items [1] to [6], wherein a content ratio [(C)/(B)] of the diphenylamine-based antioxidant (C) to the naphthylamine-based antioxidant (B) is from 0.5 to 2.0 in terms of mass ratio.
  • [8] The lubricating oil composition according to any one of the above-mentioned items [1] to [7], wherein a total content of the naphthylamine-based antioxidant (B) and the diphenylamine-based antioxidant (C) is from 1.1 mass % to 2.0 mass % with respect to the total amount of the lubricating oil composition.
  • [9] The lubricating oil composition according to any one of the above-mentioned items [1] to [8], wherein a content ratio [(B)/{(B)+(C)}] of the naphthylamine-based antioxidant (B) to a total content of the naphthylamine-based antioxidant (B) and the diphenylamine-based antioxidant (C) is 0.5 or more in terms of mass ratio.
  • [10] A method of using a lubricating oil composition, including using the lubricating oil composition of any one of the above-mentioned items [1] to [9] in lubrication of a compressor.
  • [11] The method of using a lubricating oil composition according to the above-mentioned item [10], wherein the compressor is a reciprocating compressor to be used when carbon dioxide is pressed into a ground in a carbon dioxide capture and storage technology.

EXAMPLES

The present invention is specifically described by way of Examples below. However, the present invention is not limited to Examples below.

[Methods of Measuring Various Physical Property Values]

The various physical property values of a raw material and the like used in this Example were measured by the following methods.

(1) 40° C. Kinematic Viscosity

A 40° C. kinematic viscosity was measured in conformity with JIS K2283:2000.

(2) Molecular Weight

The molecular weight of a PAG compound was calculated by the following method.

The PAG compound serving as a measurement object was subjected to measurement by using ¹H-NMR, and a molar ratio [(EO)/(RO)] was calculated from the integration ratios of peaks derived from its EO and RO.

Next, the PAG compound serving as a measurement object was subjected to measurement by using ¹³C-NMR, and the integration ratio of a peak derived from a terminal alkyl group thereof was defined as 1. The ratio of the integration ratio of the peak derived from the RO to the integration ratio was defined as the molar average polymerization degree of the RO. Then, the molar average polymerization degree of the EO was calculated from the molar ratio [(EO)/(RO)]calculated in advance. The composition (average composition) of the PAG compound and the molecular weight (average molecular weight) of the PAG compound were calculated on the basis of those calculation results.

<¹H-NMR>

• • Apparatus name: JNM-ECZ400R
  • Measurement condition: frequency: 400 MHz
  • Measurement solvent: CDCl₃

<¹³C-NMR>

• • Apparatus name: JNM-ECZ400R
  • Measurement conditions: frequency: 100 MHz, measurement modes: BCM, DEPT, and NON
  • Measurement solvent: CDCl₃

Examples 1 to 3 and Comparative Examples 1 to 5

The following respective components were mixed to prepare lubricating oil compositions each having composition shown in Table 1, and the compositions were subjected to the following evaluations 1 to 3.

The numerical values of blending composition in Table 1 are each represented in the unit of “mass %”.

<Polyalkylene Glycol Compound (A)>

“PAG 1”

EO / PO = 63 / 37 ⁢ ( molar ⁢ ratio )

• • Molecular weight: 1,200
  • 40° C. kinematic viscosity: 53.63 mm²/s

The structural formula of PAG 1 is shown below. nBu in the formula represents a n-butyl group.

“PAG 2”

EO / PO = 61 / 39 ⁢ ( molar ⁢ ratio )

• • Molecular weight: 2,400

The structural formula of PAG 2 is shown below. nBu in the formula represents a n-butyl group.

<Polyalkylene Glycol Compound (A)′>

“PAG 3”

• • EO/BO=10/90 (molar ratio), BO: butylene oxide
  • Molecular weight: 1,000 (estimated value)
  • 40° C. kinematic viscosity: 68 mm²/s

The structural formula of PAG 3 is shown below.

“PAG 4”

EO / PO = 0 / 100 ⁢ ( molar ⁢ ratio )

• • 40° C. kinematic viscosity: 100 mm²/s

PAG 4 is a PAG compound, which has only a PO repeating unit and both terminals of which are blocked with methyl groups. Its molecular weight is not measured.

<Naphthylamine-Based Antioxidant (B)>

N-(Octylphenyl)naphthalene-1-amine

N-(Octylphenyl)naphthalene-1-amine is a compound corresponding to the alkylphenylnaphtylamine (B2), and in the general formula (b2), R^b1 represents an octyl group and p1=1.

<Diphenylamine-Based Antioxidant (C)>

The antioxidant is a mixture of: unsubstituted diphenylamine; and the alkyldiphenylamine (c2) represented by the general formula (c1) in which R^c1 and R^c2 each independently represent an octyl group or a tert-butyl group, p2=0 or 1, and p3=0 or 1, provided that p2+p3 is 1 or more.

<Benzotriazole Compound>

1-[N,N-Bis(2-ethylhexyl)aminomethyl]-1H-benzotriazole

1-[N,N-Bis(2-ethylhexyl)aminomethyl]-1H-benzotriazole is a compound corresponding to the aminoalkylbenzotriazole represented by the general formula (d-2) in which b=0, R^D1 represents a methylene group, and R^D3 and R^D4 each represent a 2-ethylhexyl group.

<Antifoaming Agent>

A silicone-based antifoaming agent containing a polydimethylsiloxane as an active ingredient (100-fold diluted product)

<Evaluation 1: Evaluation of Diatomaceous Earth Passability>

The lubricating oil compositions of Examples 1 to 3 and Comparative Examples 1 to 4 were each evaluated for diatomaceous earth passability.

A procedure for the diatomaceous earth passability evaluation is described below.

(1) Preparation of Filter Containing Diatomaceous Earth as Filter Material

• • (1-1) A filter holder for vacuum filtration (manufactured by Advantec, model number: KGS-47 17311500) was arranged in a 1,000-milliliter filtration bottle. A stainless-steel net and millipore paper (made of a polycarbonate, aqueous 0.8 m) were arranged in the filter holder for vacuum filtration so that the millipore paper was placed above the net.
  • (1-2) 20 Grams of diatomaceous earth and 100 mL of ion-exchanged water were loaded into a 200-milliliter beaker, and were mixed with a dispensing spoon so that the diatomaceous earth was suspended in the water. As a result of the observation of the used diatomaceous earth with an electron microscope, the diatomaceous earth had a size (diameter) of from 30 m to 100 m, and had pores each having a diameter of from about 1 m to about 5 m.
  • (1-3) The diatomaceous earth suspended in the water was loaded from above the filter funnel of the filter holder for vacuum filtration, and a pressure in the holder was reduced with an evaporator. At this time, the diatomaceous earth suspended in the water was loaded while being moved along the wall surface of the holder with a dispensing spoon or the like so that a diatomaceous earth layer was able to be uniformly formed. When the diatomaceous earth remained in the beaker, ion-exchanged water was added, and all of the diatomaceous earth remaining in the beaker was loaded into the holder. Through the foregoing operation, a filter containing the diatomaceous earth as a filter material was prepared in the filter holder for vacuum filtration.
    (2) Method of evaluating Diatomaceous Earth Passability
  • (2-1) 6 Grams of a sample (lubricating oil composition) and 300 g of ion-exchanged water were loaded into a 500-milliliter beaker, and the mixture was stirred at room temperature with a magnetic stirrer at 180 rpm for 5 minutes to provide a sample aqueous solution.
  • (2-2) 102 Grams of the sample aqueous solution was loaded from above the filter funnel of the filter holder for vacuum filtration (containing the diatomaceous earth filter material) prepared in the above-mentioned section (1) at room temperature. Then, after the liquid level of the solution had reached 0, the pressure in the holder was continuously reduced for 3 minutes. The foregoing operation was repeated twice.
  • (2-3) A decompression device was turned off, and the diatomaceous earth in the filter holder for vacuum filtration was removed and stored in a 200-milliliter beaker.
  • (2-4) Acetone was loaded into the beaker storing the diatomaceous earth in the above-mentioned section (2-3) up to its 100-milliliter line, and the mixture was stirred with a magnetic stirrer for 5 minutes.
  • (2-5) A glass filter was arranged above a filtration bottle of any size, and a pressure in the filter was reduced, followed by the pouring of the sample obtained in the above-mentioned section (2-4) from above the glass filter.
  • (2-6) About 100 mL of acetone was further poured to wash the diatomaceous earth.
  • (2-7) The solution accumulating in the filtration bottle was loaded into a beaker whose weight had been measured, and was dried in a draft chamber. The solution accumulating in the filtration bottle is in a state in which the sample captured in the diatomaceous earth in the filter holder for vacuum filtration has migrated thereto.
  • (2-8) When the weight of the beaker no longer changed, the amount “x” [g] of the sample in the beaker was calculated. Then, diatomaceous earth passability [%] was determined from the ratio of the amount “x” [g] of the sample in the beaker to 102 g of the loaded sample. When the amount of the sample in the beaker is 0 g, the diatomaceous earth passability is 100%. In other words, it can be said that as the weight (remaining amount) of the oil sample remaining in the diatomaceous earth reduces, the sample becomes more excellent in diatomaceous earth passability, and hence becomes more excellent in passability through a water-containing stratum.

In this Example, a sample having a diatomaceous earth passability of 80% or more was judged to be acceptable.

<Evaluation 2: Evaporability Test>

With regard to each of the lubricating oil compositions of Examples 1 to 3, and Comparative Examples 1 and 4 each having satisfactory diatomaceous earth passability, and the lubricating oil composition of Comparative Example 5, the evaporation ratio of the lubricating oil composition 24 hours after the start of measurement under the following conditions was measured with a tester specified in JIS 2540:2000.

(Conditions)

• • Temperature: 150° C.
  • Oil amount: 2 g
  • Air amount: 10 L/min

It can be said that as the evaporation ratio reduces, the composition becomes more excellent in evaporation-suppressing property.

In this Example, a composition having an evaporation ratio of 40% or less was judged to be acceptable.

<Evaluation 3: Panel Coking Test>

The lubricating oil compositions of Examples 1 to 3 and Comparative Example 4 each having satisfactory diatomaceous earth passability and a satisfactory evaporation-suppressing property, and the lubricating oil composition of Comparative Example 5 were each tested in conformity with Fed. Test Method Std. 791-3462 under the conditions of a panel temperature of 300° C. and an oil temperature of 80° C. in cycles each formed of a splash time of 15 seconds and a stop time of 45 seconds for 3 hours. After the completion of the test, the amount of carbon adhering to a panel (carbon adhesion amount (mg)) was evaluated.

It can be said that as the carbon adhesion amount reduces, the composition becomes more excellent in coking resistance.

In this Example, a composition having a carbon adhesion amount of 40 mg or less was judged to be acceptable.

The results are shown in Table 1. The result of the panel coking test of Comparative Example 1 is a predicted value.

TABLE 1
						Comparative	Comparative
			Example 1	Example 2	Example 3	Example 1	Example 2
Lubricating oil	PAG	PAG 1	38.5	38.5	38.0	40
composition	compound	PAG 2	60	60	60.37	60
(mass %)	(A)
	PAG	PAG 3
	compound	PAG 4					100
	(A)′

	Naphthylamine-based antioxidant (B)	1.0	0.5	1.0
	Diphenylamine-based antioxidant (C)	0.5	1.0	0.5

	Other	Benzotriazole			0.03
	component	Polydimethylsiloxane			0.1

Total

100

100

100

100

100

EO/PO (PAG compound, molar ratio)	61:39	61:39	61:39	62:38	0:100
EO/PO (PAG compound, molar ratio)	1.6	1.6	1.6	1.6	0.0
Total content of component (B)	1.5	1.5	1.5	—	—
and component (C) (mass %)
[(C)/(B)]	0.5	2.0	0.5	—	—
[(B)/{(B) + (C)}]	0.7	0.3	0.7	—	—
40° C. kinamatic viscosity of lubricating	104.6	104.3	101.5	100.7	103.2
oil composition (mm2/s)
Diatomaceous earth passability	93%	93%	95%	96%	4%
Evaporability test (evaporation ratio)	0.4%	3.2%	0.5%	92.9%	—
Panel coking test (mg)	14.9	7.9	18.3	5.0 or less	—

				Comparative	Comparative	Comparative
				Example 3	Example 4	Example 5
	Lubricating oil	PAG	PAG 1		39	39
	composition	compound	PAG 2	55	60	60
	(mass %)	(A)
		PAG	PAG 3	45
		compound	PAG 4
		(A)′

	Naphthylamine-based antioxidant (B)		1.0
	Diphenylamine-based antioxidant (C)			1.0

	Other	Benzotriazole
	component	Polydimethylsiloxane

Total

100

100

100

	EO/PO (PAG compound, molar ratio)	38:62	61:39	61:39
	EO/PO (PAG compound, molar ratio)	0.6	1.6	1.6
	Total content of component (B)	—	1.0	1.00
	and component (C) (mass %)
	[(C)/(B)]	—	0.0	—
	[(B)/{(B) + (C)}]	—	1.0	0.0
	40° C. kinamatic viscosity of lubricating	101.3	102.8	102.2
	oil composition (mm2/s)
	Diatomaceous earth passability	60%	96%	—
	Evaporability test (evaporation ratio)	—	7.4%	65.9%
	Panel coking test (mg)	—	50.2	5.6

The following is found from the results shown in Table 1.

It is understood that each of the lubricating oil compositions of Examples 1 to 3 is excellent in passability through a water-containing stratum, and is also excellent in evaporation-suppressing property and coking resistance.

In contrast, it is found that the lubricating oil composition of Comparative Example 1 that is free of the naphthylamine-based antioxidant (B) and the diphenylamine-based antioxidant (C) is poor in evaporation-suppressing property, though the composition is excellent in passability through a water-containing stratum and coking resistance.

In addition, it is found that when the ratio [(EO)/(PO)](in other words, the ratio [(EO)/(RO)]) of the PAG compound is less than 1.0 like the lubricating oil compositions of Comparative Examples 2 and 3, the passability through a water-containing stratum is poor.

Further, it is found that the lubricating oil composition of Comparative Example 4 that is free of the diphenylamine-based antioxidant (C) is poor in coking resistance, though the composition is excellent in passability through a water-containing stratum and evaporation-suppressing property.

In addition, it is found that the lubricating oil composition of Comparative Example 5 that is free of the naphthylamine-based antioxidant (B) is poor in evaporation-suppressing property.