EP4692286A1

EP4692286A1 - Grease composition

Info

Publication number: EP4692286A1
Application number: EP24779884.6A
Authority: EP
Inventors: So NAKAJIMA
Original assignee: Idemitsu Kosan Co Ltd
Current assignee: Idemitsu Kosan Co Ltd
Priority date: 2023-03-31
Filing date: 2024-03-21
Publication date: 2026-02-11
Also published as: JPWO2024203765A1; CN120897983A; WO2024203765A1

Abstract

Provided is a grease composition, which has an appropriate oil separation and is excellent in water resistance while using hydrophilic nanofibers. The grease composition includes: a base oil (A); hydrophilic nanofibers (B) each having a thickness (d) of from 1 nm to 500 nm; and particles (C). The hydrophilic nanofibers (B) are one or more kinds selected from cellulose nanofibers (B1) and modified cellulose nanofibers (B2). The particles (C) each have a structure in which a hydrophobic group is present on an outermost surface of a hydrophilic parent structure, and in the grease composition, the particles (C) have an average particle diameter of from 1 nm to 500 nm.

Description

Technical Field

The present invention relates to a grease composition.

Background Art

Grease is more easily sealed than a lubricating oil is, and hence enables the downsizing and weight reduction of a machine to which the grease is applied. Accordingly, the grease has heretofore been widely used for the lubrication of the various sliding portions of, for example, an automobile, electrical equipment, industrial machinery, and engineering machinery.
In recent years, to provide a grease composition having a low environmental load, a grease composition using a thickener having biodegradability has been proposed. In, for example, PTL 1, there is a proposal of a grease composition using cellulose nanofibers (hereinafter also referred to as "CNFs") as a thickener.

Citation List

Patent Literature

PTL 1: JP 2017-210612 A

Summary of Invention

Technical Problem

However, a grease composition using hydrophilic nanofibers such as CNFs as a thickener has a problem in that the composition is liable to undergo oil separation, and is insufficient in water resistance.
Accordingly, there has been desired a grease composition, which has an appropriate oil separation and is excellent in water resistance while using the hydrophilic nanofibers.
In view of the foregoing, an object of the present invention is to provide a grease composition, which has an appropriate oil separation and is excellent in water resistance while using hydrophilic nanofibers.

Solution to Problem

The inventor of the present invention has found that a grease composition including a base oil, specific hydrophilic nanofibers, and specific particles can solve the above-mentioned problem. Thus, the inventor has completed the present invention.
That is, the present invention provides the following item [1].

[1] A grease composition, including:
- a base oil (A);
- hydrophilic nanofibers (B) each having a thickness (d) of from 1 nm to 500 nm; and particles (C),
- wherein the hydrophilic nanofibers (B) are one or more kinds selected from cellulose nanofibers (B1) and modified cellulose nanofibers (B2), and
- wherein the particles (C) each have a structure in which a hydrophobic group is present on an outermost surface of a hydrophilic parent structure, and in the grease composition, the particles (C) have an average particle diameter of from 1 nm to 500 nm.

Advantageous Effects of Invention

The present invention can provide the grease composition, which has an appropriate oil separation and is excellent in water resistance while using hydrophilic nanofibers.

Description of Embodiments

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 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.

[Grease Composition]

A grease composition according to an embodiment of the present invention is a grease composition including: a base oil (A); hydrophilic nanofibers (B) each having a thickness (d) of from 1 nm to 500 nm; and particles (C).
The hydrophilic nanofibers (B) are one or more kinds selected from cellulose nanofibers (B1) and modified cellulose nanofibers (B2).
The particles (C) each have a structure in which a hydrophobic group is present on an outermost surface of a hydrophilic parent structure, and in the grease composition, the particles (C) have an average particle diameter of from 1 nm to 500 nm.
The inventor of the present invention has made extensive investigations with a view to solving the above-mentioned problem.
As the base material of a grease composition which has biodegradability and is excellent in lubricity and heat resistance, it is useful to use cellulose nanofibers as a thickener .
Herein, the cellulose nanofibers have the following problem: the nanofibers are liable to absorb moisture because of their high hydrophilicity; thus, the nanofibers are flowed by water, and hence hardly stay in a sliding portion. In addition, the nanofibers have a problem in that the nanofibers are liable to undergo oil separation.
The inventor of the present invention has made extensive investigations with a view to solving such problems. As a result, the inventor has found that a grease composition blended with the following particles (C) can solve those problems: the particles each have a structure in which the hydrophobic group is present on the outermost surface of the hydrophilic parent structure; and the particles each have a specific particle diameter.
Herein, in the base oil (A), the hydrophilic nanofibers (B) easily form a higher-order structure. In addition, the hydrophilic nanofibers (B) are uniformly dispersed in the base oil (A) with ease.
In addition, the grease composition of this embodiment includes the particles (C) as an essential component. The particles (C) are the following particles: the particles (C) each have a structure in which the hydrophobic group is present on the outermost surface of the hydrophilic parent structure; and in the grease composition, the particles (C) have an average particle diameter of from 1 nm to 500 nm.
The particles (C) impart an appropriate oil separation to the grease composition, and impart water resistance thereto. Although a detailed mechanism therefor is unclear, the mechanism is assumed to be, for example, as described below.
It is assumed that the hydrophilic nanofibers (B) and the particles (C) form some higher-order structure, and hence the grease composition has an appropriate oil separation and is excellent in water resistance. In addition, as a result of the formation of the higher-order structure, even when the content of the hydrophilic nanofibers (B) is small, and the content of the particles (C) is small, the composition may easily have a moderate worked penetration.
In the following description, the "base oil (A)," the "hydrophilic nanofibers (B)," and the "particles (C)" are also referred to as "component (A)," "component (B)," and "component (C)," respectively.
In the grease composition of this embodiment, the total content of the component (A), the component (B), and the component (C) 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 (100 mass%) of the grease composition. In addition, the total content is typically 100 mass% or less, preferably less than 100 mass%, more preferably 99 mass% or less, still more preferably 98 mass% or less.
The grease composition of this embodiment may include any other component except the component (A), the component (B), and the component (C) to the extent that the effect of the present invention is not impaired.
As described later, an "antioxidant (D)," a "rust inhibitor (E)," and an "additive (F)" that may be incorporated into the grease composition of this embodiment are also referred to as "component (D)," "component (E)," and "component (F)," respectively.
In the grease composition of this embodiment, the total content of the component (A), the component (B), the component (C), the component (D), the component (E), and the component (F) 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 (100 mass%) of the grease composition. In addition, the total content is typically 100 mass% or less, preferably less than 100 mass%, more preferably 99 mass% or less, still more preferably 98 mass% or less.
The grease composition of this embodiment may include any other component except the component (A), the component (B), the component (C), the component (D), the component (E), and the component (F) to the extent that the effect of the present invention is not impaired.

The grease composition of this embodiment includes the base oil (A).
The base oil (A) is, for example, one or more kinds selected from the group consisting of: a mineral oil; a synthetic oil; and a vegetable oil.
Examples of the mineral oil include: a normal-pressure residual oil obtained by distilling a crude oil, such as a paraffin base crude oil, an intermediate base crude oil, or a naphthene base crude oil, under normal pressure; a distillate oil obtained by distilling such normal-pressure residual oil under reduced pressure; a mineral oil obtained by subjecting the distillate oil to one or more of treatments, such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, contact dewaxing, and hydrorefining; and a wax isomerized mineral oil.
The mineral oils may be used alone or in combination thereof.
Examples of the synthetic oil include a hydrocarbon-based oil, an aromatic oil, an ester-based oil, an ether-based oil, and a fatty acid ester.
The synthetic oils may be used alone or in combination thereof.
Examples of the hydrocarbon-based oil include n-paraffin, isoparaffin, polybutene, polyisobutylene, poly-α-olefins (PAOs), such as a 1-decene oligomer and a co-oligomer of 1-decene and ethylene, and hydrogenated products thereof.
Examples of the aromatic oil include: alkylbenzenes, such as a monoalkylbenzene and a dialkylbenzene; and alkylnaphthalenes, such as a monoalkylnaphthalene, a dialkylnaphthalene, and a polyalkylnaphthalene.
Examples of the ester-based oil include: diester-based oils, such as dibutyl sebacate, di-2-ethylhexyl sebacate, dioctyl adipate, diisodecyl adipate, ditridecyl adipate, ditridecyl glutarate, and methyl acetyl ricinoleate; aromatic ester-based oils, such as trioctyl trimellitate, tridecyl trimellitate, and tetraoctyl pyromellitate; polyol ester-based oils, such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol-2-ethylhexanoate, and pentaerythritol pelargonate; and complex ester-based oils such as an oligoester of a polyhydric alcohol and a mixed fatty acid containing a dibasic acid and a monobasic acid.
Examples of the ether-based oil include: polyglycols, such as polyethylene glycol, polypropylene glycol, a polyethylene glycol monoether, and a polypropylene glycol monoether; and phenyl ether-based oils, such as a monoalkyl triphenyl ether, an alkyl diphenyl ether, a dialkyl diphenyl ether, pentaphenyl ether, tetraphenyl ether, a monoalkyl tetraphenyl ether, and a dialkyl tetraphenyl ether.
A fatty acid having 8 to 22 carbon atoms is preferred as a fatty acid for forming the fatty acid ester, and specific examples thereof include caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, erucic acid, palmitoleic acid, oleic acid, linoleic acid, linolenic acid, isostearic acid, arachic acid, ricinoleic acid, and 12-hydroxystearic acid.
Specific examples of the fatty acid ester include a glycerin fatty acid ester, a polyglycerin fatty acid ester, and a propylene glycol fatty acid ester.
Examples of the glycerin fatty acid ester include glycerin monooleate, glycerin monostearate, glycerin monocaprylate, glycerin dioleate, glycerin distearate, and glycerin dicaprylate.
Examples of the polyglycerin fatty acid ester include diglycerin monooleate, diglycerin monoisostearate, diglycerin dioleate, diglycerin trioleate, diglycerin monostearate, diglycerin distearate, diglycerin tristearate, diglycerin triisostearate, diglycerin monocaprylate, diglycerin dicaprylate, diglycerin tricaprylate, triglycerin monooleate, triglycerin dioleate, triglycerin trioleate, triglycerin tetraoleate, triglycerin monostearate, triglycerin distearate, triglycerin tristearate, triglycerin tetrastearate, triglycerin monocaprylate, triglycerin dicaprylate, triglycerin tricaprylate, triglycerin tetracaprylate, diglycerin monooleic acid monostearic acid ester, diglycerin monooleic acid distearic acid ester, diglycerin monocaprylic acid monostearic acid ester, triglycerin monooleic acid monostearic acid ester, triglycerin dioleic acid distearic acid ester, triglycerin dioleic acid monostearic acid ester, triglycerin monooleic acid monostearic acid monocaprylic acid ester, diglycerin monolaurate, diglycerin dilaurate, triglycerin monolaurate, triglycerin trilaurate, triglycerin trilaurate, diglycerin monomyristate, diglycerin dimyristate, triglycerin monomyristate, triglycerin dimyristate, triglycerin trimyristate, diglycerin monolinoleate, diglycerin dilinoleate, triglycerin monolinoleate, triglycerin dilinoleate, triglycerin trilinoleate, decaglycerin monooleate, decaglycerin monostearate, and decaglycerin monocaprylic acid monooleic acid ester.
Examples of the propylene glycol fatty acid ester include propylene glycol monooleate, propylene glycol monostearate, propylene glycol monocaprylate, and propylene glycol monolaurate.

<<Vegetable Oil (A1)>>

The base oil (A) preferably includes a vegetable oil (A1).
When the base oil (A) includes the vegetable oil (A1), an environmental load is suppressed, and hence the safety of the grease composition can be improved.
Examples of the vegetable oil (A1) include: a raw oil obtained by squeezing and extracting a natural vegetable oil raw material; a refined oil obtained by subjecting the raw oil to various kinds of refining treatment, such as filtration for removing a floating impurity in the raw oil, degumming for removing a phospholipid or the like, deacidification for removing a free fatty acid, decolorization for removing coloring matter, and dewaxing for removing a wax content; and a modified oil obtained by subjecting the refined oil to treatment, such as hardening, fractionation, ester exchange, or hydrogenation.
Specific examples of the vegetable oil (A1) include: oils derived from vegetables, such as rapeseed oil, peanut oil, corn oil, cottonseed oil, canola oil, soybean oil, camellia oil, olive oil, peanut oil, sunflower oil (preferably high-oleic type), Carthamus tinctorius oil (preferably high-oleic type), safflower oil (preferably high-oleic type), palm oil, palm kernel oil, coconut oil, rice oil, sesame oil, perilla oil, linseed oil, and grape seed oil; and vegetable-derived base oils such as an estolide ester.
The vegetable oil (A1) may include only one kind of vegetable oil, or may be a mixed vegetable oil including two or more kinds of vegetable oils.
A fatty acid for forming the vegetable oil (A1) is, for example, one or more kinds selected from oleic acid, linoleic acid, linolenic acid, palmitic acid, stearic acid, palmitoleic acid, arachidic acid, eicosenoic acid, behenic acid, erucic acid, lignoceric acid, lauric acid, and myristic acid. A particularly typical example thereof is oleic acid.
The term "fatty acid for forming the vegetable oil (A1)" means a fatty acid in a compound for forming the vegetable oil in which the fatty acid and glycerin are ester-bonded to each other.
From the viewpoint of further facilitating an improvement in oxidation stability of the grease composition, the content of oleic acid among the fatty acids for forming the vegetable oil (A1) is preferably 50 mass% or more with respect to the total amount of the constituent fatty acids in the vegetable oil (A1). From the same viewpoint, the content of oleic acid is more preferably 55 mass% or more, still more preferably 60 mass% or more with respect to the total amount of the constituent fatty acids in the vegetable oil (A1). In addition, the content of oleic acid is typically less than 85 mass% with respect to the total amount of the constituent fatty acids in the vegetable oil (A1).
A method of measuring the fatty acid composition of the vegetable oil (A1) is, for example, a method including: extracting a lipid from the vegetable oil (A1) with an organic solvent; evaporating the organic solvent after the extraction; then preparing a fatty acid methyl ester from the resultant lipid; and subjecting the fatty acid methyl ester to gas chromatography mass spectrometry (GC-MS analysis).
From the viewpoint of further facilitating an improvement in oxidation stability of the grease composition, the content of a saturated fatty acid among the fatty acids for forming the vegetable oil (A1) is preferably 3 mass% or more, more preferably 4 mass% or more, still more preferably 5 mass% or more with respect to the total amount of the constituent fatty acids in the vegetable oil (A1).
The total content of oleic acid, linoleic acid, and linolenic acid among the fatty acids for forming the vegetable oil (A1) is preferably 50 mass% or more, more preferably 70 mass% or more, still more preferably 80 mass% or more with respect to the total amount of the constituent fatty acids in the vegetable oil (A1).
The content of erucic acid among the fatty acids for forming the vegetable oil (A1) is preferably from 0 mass% to 45 mass%, more preferably from 1 mass% to 35 mass%, still more preferably from 2 mass% to 20 mass% with respect to the total amount of the constituent fatty acids in the vegetable oil (A1).
The content of the vegetable oil (A1) is appropriately adjusted in accordance with the composition of the vegetable oil (A1). For example, the content of the vegetable oil (A1) is 10 mass% or more, 20 mass% or more, 30 mass% or more, 40 mass% or more, 50 mass% or more, 60 mass% or more, 70 mass% or more, 80 mass% or more, 90 mass% or more, or 100 mass% with respect to the total amount of the base oil (A).
From the viewpoint of facilitating the exhibition of the effect of the present invention, the 40°C kinematic viscosity of the base oil (A) to be used in this embodiment is preferably 10 mm²/s or more, more preferably 20 mm²/s or more, still more preferably 30 mm²/s or more, still further more preferably 35 mm²/s or more.
In addition, from the viewpoint of facilitating the exhibition of the effect of the present invention, the 40°C kinematic viscosity of the base oil (A) of this embodiment is preferably 120 mm²/s or less, more preferably 100 mm²/s or less, still more preferably 90 mm²/s or less, still further more preferably 80 mm²/s or less.
The upper limit values and lower limit values of those numerical ranges may be freely combined. Specifically, the 40°C kinematic viscosity is preferably from 10 mm²/s to 120 mm²/s, more preferably from 20 mm²/s to 100 mm²/s, still more preferably from 30 mm²/s to 90 mm²/s, still further more preferably from 35 mm²/s to 80 mm²/s.
From the viewpoint of facilitating the exhibition of the effect of the present invention, the 100°C kinematic viscosity of the base oil (A) to be used in this embodiment is preferably 2.0 mm²/s or more, more preferably 3.0 mm²/s or more, still more preferably 4.0 mm²/s or more.
In addition, from the viewpoint of facilitating the exhibition of the effect of the present invention, the 100°C kinematic viscosity of the base oil (A) of this embodiment is preferably 20 mm²/s or less, more preferably 18 mm²/s or less, still more preferably 16 mm²/s or less.
The upper limit values and lower limit values of those numerical ranges may be freely combined. Specifically, the 100°C kinematic viscosity is preferably from 2.0 mm²/s to 20 mm²/s, more preferably from 3.0 mm²/s to 18 mm²/s, still more preferably from 4.0 mm²/s to 16 mm²/s.
A mixed base oil, which is obtained by combining a high-viscosity base oil and a low-viscosity base oil so that its kinematic viscosity may be adjusted within the above-mentioned ranges, may be used as the base oil (A) to be used in this embodiment.
From the viewpoint of facilitating the exhibition of the effect of the present invention, the viscosity index of the base oil (A) to be used in this embodiment is preferably 90 or more, more preferably 110 or more, still more preferably 130 or more.
In this description, the kinematic viscosity and viscosity index of the base oil (A) each mean a value measured or calculated in conformity with JIS K2283:2000.
In the grease composition of this embodiment, the content of the base oil (A) is preferably 50 mass% or more, more preferably 60 mass% or more, still more preferably 70 mass% or more, still further more preferably 80 mass% or more with respect to the total amount (100 mass%) of the grease composition. In addition, the content of the base oil (A) is preferably 100 mass% or less, more preferably 98 mass% or less, still more preferably 97 mass% or less, still further more preferably 95 mass% or less with respect to the total amount (100 mass%) of the grease composition.

The hydrophilic nanofibers mean fibrous matter having a thickness of 500 nm or less, which is composed of a formation material containing a compound having hydrophilicity, and is distinguished from particulate matter.
The hydrophilic nanofibers (B) have a low environmental load, and are hence excellent in safety to a human body. Accordingly, the incorporation of the hydrophilic nanofibers (B) suppresses the environmental load, and hence can improve the safety of the grease composition.

(Criteria for Judgment of "Hydrophilicity")

Whether or not nanofibers are "hydrophilic" is judged as described below.
The nanofibers (fibrous matter) of interest are molded into sheet-like matter, and a water droplet is dropped onto the surface of the sheet-like matter. At that time, (1) when a contact angle with the water is 90° or less, or (2) when the dropped water droplet is quickly absorbed by the sheet-like matter, the nanofibers are judged to be "hydrophilic".

("Thicknesses" of Hydrophilic Nanofibers)

Further, the definition of the "thickness" of each of the hydrophilic nanofibers is the same as a definition related to the thickness of general fibrous matter.
Specifically, in a cut surface at the time of cutting in a direction perpendicular to a tangential direction at an arbitrary point on the side surface of each of the hydrophilic nanofibers, when the cut surface is a circle or an ellipse, its diameter or long diameter is the "thickness" of the hydrophilic nanofiber. When the cut surface is a polygon, the diameter of the circumscribed circle of the polygon is the "thickness" of the hydrophilic nanofiber.
When a flaky, powdery, or particulate hydrophilic compound having a size of several micrometers or more is blended as a thickener into the base oil, the hydrophilic compound are liable to aggregate in the base oil to form a so-called "lump". As a result, the aggregate of the hydrophilic compound is deposited on the surface of the grease composition to be obtained, and hence the dispersed state thereof is liable to be nonuniform. In this case, an increase in worked penetration of the grease composition to be obtained requires the addition of a large amount of the hydrophilic compound. However, the grease composition becomes poor in wear resistance because the composition includes particles that are larger than an oil film thickness.
Meanwhile, in the grease composition of the present invention, the hydrophilic nanofibers each having a thickness (d) of from 1 nm to 500 nm are blended into the base oil. Accordingly, in the base oil, a higher-order structure is formed by the hydrophilic nanofibers while the hydrophilic nanofibers are uniformly dispersed without aggregation of the hydrophilic nanofibers. As a result, there can be obtained a grease composition, which has a moderate worked penetration despite the fact that the content of the hydrophilic nanofibers is small.

(Thicknesses (d) and Aspect Ratios of Hydrophilic Nanofibers)

In the present invention, the "thickness (d) of each of the hydrophilic nanofibers" represents the thickness of each of the hydrophilic nanofibers dispersed in the base oil, and is distinguished from the "thickness (d') of each of hydrophilic nanofibers" serving as a raw material before the blending into the base oil to be described later.
However, there is substantially no difference between the "thickness (d) of each of the hydrophilic nanofibers" dispersed in the base oil and the "thickness (d') of each of the hydrophilic nanofibers" serving as a raw material before the blending into the base oil. Accordingly, the "thickness (d) of each of the hydrophilic nanofibers" dispersed in the base oil and the "thickness (d') of each of the hydrophilic nanofibers" serving as a raw material before the blending into the base oil may be regarded as being substantially identical to each other.
The thickness (d) of each of the hydrophilic nanofibers dispersed in the base oil is from 1 nm to 500 nm. However, from the viewpoint of forming the higher-order structure by the hydrophilic nanofibers in the base oil and the viewpoint of more uniformly dispersing the hydrophilic nanofibers therein, the thickness is preferably from 3 nm to 300 nm, more preferably from 5 nm to 200 nm, still more preferably from 10 nm to 100 nm, still further more preferably from 15 nm to 70 nm, yet still further more preferably from 20 nm to 50 nm.
With regard to the hydrophilic nanofibers in the grease composition of the present invention, at least the dispersion of the hydrophilic nanofibers whose thicknesses (d) fall within the above-mentioned ranges only needs to be recognized, and the hydrophilic nanofibers whose thicknesses (d) deviate from the above-mentioned ranges may be dispersed.
However, in the grease composition of one aspect of the present invention, from the viewpoint of forming the higher-order structure by the hydrophilic nanofibers in the base oil and the viewpoint of more uniformly dispersing the hydrophilic nanofibers therein, the average of the thicknesses (d) of 10 hydrophilic nanofibers randomly selected from the hydrophilic nanofibers dispersed in the base oil is from 1 nm to 500 nm, preferably from 3 nm to 300 nm, more preferably from 5 nm to 200 nm, still more preferably from 10 nm to 100 nm, still further more preferably from 15 nm to 70 nm, yet still further more preferably from 20 nm to 50 nm.
In addition, from the above-mentioned viewpoints, 1 or more (more preferably 5 or more, still more preferably 7 or more) hydrophilic nanofibers whose thicknesses (d) fall within the above-mentioned ranges are preferably present in the 10 randomly selected hydrophilic nanofibers among the hydrophilic nanofibers in the grease composition of the present invention. It is more preferred that each of the thicknesses (d) of the 10 selected hydrophilic nanofibers fall within the above-mentioned ranges.
In the grease composition of one aspect of the present invention, the aspect ratio of each of the hydrophilic nanofibers is preferably 5 or more, more preferably 10 or more, still more preferably 15 or more, still further more preferably 30 or more, yet still further more preferably 50 or more.
The term "aspect ratio" as used herein refers to the ratio (length/thickness) of the length of each of the hydrophilic nanofibers to be observed to the thickness thereof, and the "length" of the hydrophilic nanofiber refers to a distance between the two points of the hydrophilic nanofiber that are most distant from each other.
In addition, in the case where part of the hydrophilic nanofibers to be observed are brought into contact with the other hydrophilic nanofibers to make it difficult to identify the "length", the following only needs to be satisfied: when the length of only a portion whose thickness is measurable among the hydrophilic nanofibers to be observed is measured, the aspect ratio of the portion falls within the above-mentioned ranges.
Further, the average of the aspect ratios (hereinafter also referred to as "average aspect ratio") of 10 hydrophilic nanofibers randomly selected from the hydrophilic nanofibers in the grease composition of the present invention is preferably 5 or more, more preferably 10 or more, still more preferably 15 or more, still further more preferably 30 or more, yet still further more preferably 50 or more.

(Thicknesses (d') and Aspect Ratios of Hydrophilic Nanofibers)

The thickness (d') of each of the hydrophilic nanofibers serving as a raw material before mixing with the base oil is from 1 nm to 500 nm, preferably from 3 nm to 300 nm, more preferably from 5 nm to 200 nm, still more preferably from 10 nm to 100 nm, still further more preferably from 15 nm to 70 nm, yet still further more preferably from 20 nm to 50 nm.
In addition, the average aspect ratio of the hydrophilic nanofibers serving as a raw material before the mixing with the base oil is preferably 5 or more, more preferably 10 or more, still more preferably 15 or more, still further more preferably 30 or more, yet still further more preferably 50 or more.
In this description, the "thickness (d)" of each of the hydrophilic nanofibers dispersed in the base oil and the "thickness (d')" of each of the hydrophilic nanofibers serving as a raw material before the blending into the base oil, and the aspect ratios of these hydrophilic nanofibers are values measured with an electron microscope or the like.

(Material for forming Hydrophilic Nanofibers)

The hydrophilic nanofibers to be used in one aspect of the present invention each only need to include a formation material containing a compound having hydrophilicity. Examples of the compound having hydrophilicity include a compound having a functional group having a hydrogen-bonding property, such as a hydroxy group or an amino group, and a metal oxide.
However, from the viewpoint of providing a grease composition, which has a low environmental load and is excellent in safety to a human body, and the viewpoint of making the affinity of the nanofibers for the base oil satisfactory, each of the hydrophilic nanofibers to be used in one aspect of the present invention preferably contains a polysaccharide, more preferably contains one or more kinds of polysaccharides selected from cellulose, carboxymethylcellulose, chitin, and chitosan, still more preferably contains cellulose, and is still further more preferably a cellulose nanofiber.
Lignocellulose may be used as a raw material for the cellulose nanofibers. The lignocellulose is a composite hydrocarbon polymer for forming the cell wall of a plant, and it has been known that the lignocellulose mainly includes cellulose and a hemicellulose serving as polysaccharides, and a lignin serving as an aromatic polymer. The cellulose for forming the cellulose nanofibers may be one or more kinds selected from lignocellulose and acetylated lignocellulose. In addition, the cellulose nanofibers may each contain one or more kinds selected from a hemicellulose and a lignin. Further, the cellulose for forming the cellulose nanofibers may be chemically bonded to one or more kinds selected from a hemicellulose and a lignin.
In addition, the hydrophilic nanofibers to be used in one aspect of the present invention may be used after their surfaces have been subjected to modification treatment.
More specifically, there may be used hydrophilic nanofibers whose surfaces have been subjected to one or more kinds of modification treatment selected from: esterification such as acetylation; phosphorylation; urethanization; carbamylation; etherification; carboxymethylation; oxidation with 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO); and periodic acid oxidation.
The content of the polysaccharide in the hydrophilic nanofibers to be used in one aspect of the present invention is preferably from 60 mass% to 100 mass%, more preferably from 70 mass% to 100 mass%, still more preferably from 80 mass% to 100 mass%, still further more preferably from 90 mass% to 100 mass% with respect to the total amount (100 mass%) of the hydrophilic nanofibers.
The polymerization degree of the polysaccharide is preferably from 50 to 3,000, more preferably from 100 to 1,500, still more preferably from 150 to 1,000, still further more preferably from 200 to 800.
In the present invention, the polymerization degree of the polysaccharide polymer means a value measured by a viscosity method.

(Content of Hydrophilic Nanofibers)

In the grease composition of one aspect of the present invention, the content of the hydrophilic nanofibers (B) is preferably from 0.1 mass% to 15 mass%, more preferably from 0.5 mass% to 13 mass%, still more preferably from 0.7 mass% to 10 mass%, still further more preferably from 1.0 mass% to 8.0 mass%, yet still further more preferably from 1.5 mass% to 6.0 mass% with respect to the total amount (100 mass%) of the grease composition.
When the content of the hydrophilic nanofibers (B) is 0.1 mass% or more, a grease composition having a moderate worked penetration is easily prepared. In addition, when the content of the hydrophilic nanofibers (B) is 20 mass% or less, a grease composition is easily prepared.

The grease composition of this embodiment includes the particles (C).
The particles (C) each have a structure in which the hydrophobic group is present on the outermost surface of the hydrophilic parent structure, and in the grease composition, the particles (C) have an average particle diameter of from 1 nm to 500 nm.
When the grease composition of this embodiment includes the particles (C), there can be provided a grease composition, which has an appropriate oil separation and is excellent in water resistance while using the hydrophilic nanofibers.
The average particle diameter (average particle diameter of the primary particles) of the particles (C) is preferably from 3 nm to 400 nm, more preferably from 5 nm to 300 nm.
The average particle diameter of the particles (C) may be measured from, for example, an image of the primary particles obtained by observing the grease composition with an electron microscope.
The hydrophilic parent structure of each of the particles (C) is not particularly limited as long as the particles (C) can be controlled so as to have an average particle diameter in the above-mentioned ranges. A material for forming the structure is also not particularly limited as long as the material is a hydrophilic material.
Herein, from the viewpoint of satisfying the condition that "the average particle diameter is from 1 nm to 500 nm," examples of the hydrophilic material for forming the hydrophilic parent structure of each of the particles (C) include: mineral-based materials such as bentonite; and metal oxide-based materials, such as silica and alumina.
Among them, silica is preferred, and fumed silica is more preferred from the viewpoint of improving the effect of the present invention.
In addition, the hydrophobic group of each of the particles (C) is not particularly limited as long as the group is a hydrophobic group that can be bonded to the outermost surface of the hydrophilic parent structure.
For example, when the material for forming the hydrophilic parent structure is silica, the silica preferably has one or more kinds of hydrophobic groups selected from the group consisting of: a dimethylsilyl group; a trimethylsilyl group; and a dimethylpolysiloxane group. Those hydrophobic groups may each be formed on the surface of the silica by treating the surface of the silica with one or more kinds of surface treatment agents selected from the group consisting of: dimethyldicyclosilane; hexamethyldisilazane; and a silicone oil.
A more preferred aspect of the particles (C) is as follows: fumed silica has one or more kinds of hydrophobic groups selected from the group consisting of: a dimethylsilyl group; a trimethylsilyl group; and a dimethylpolysiloxane group (preferably a dimethylsilyl group). Those substituents may each be formed on the surface of the fumed silica by treating the surface of the fumed silica with one or more kinds of surface treatment agents selected from the group consisting of: dimethyldicyclosilane; hexamethyldisilazane; and a silicone oil (preferably with dimethyldicyclosilane).
Examples of the particles (C) that may be suitably used include: Aerosil (trademark) series (Aerosil (trademark) R 972, R 974, R 9200, R 976, R 976 S, NX 130, RY 200 S, R 202, RX 200, R 8200, RY 200, RY 200 L, RX 300, R 812, R 812 S, and RY 300) manufactured by Evonik Industries AG; and HDK (trademark) series (HDK (trademark) H15, H15P, H17, H18, H20, H30, and H2000) manufactured by Wacker Asahikasei Silicone Co., Ltd.
The particles (C) may be used alone or in combination thereof.
The content of the particles (C) is preferably 6.0 mass% or more, more preferably 6.5 mass% or more, still more preferably 7.0 mass% or more, still further more preferably 10.0 mass% or more with respect to the total amount (100 mass%) of the grease composition, and is preferably 20.0 mass% or less, more preferably 18.0 mass% or less, still more preferably 16.0 mass% or less.

The grease composition of this embodiment may include the antioxidant (D).
When the base oil (A) in the grease composition of this embodiment includes the vegetable oil (A1), the incorporation of the antioxidant (D) into the grease composition can improve the oxidation stability of the grease composition.
Examples of the antioxidant (D) include a phenol-based antioxidant (D1), a naphthylamine-based antioxidant (D2), and a diphenylamine-based antioxidant (D3).
Among them, the phenol-based antioxidant (D1) or the naphthylamine-based antioxidant (D2) is preferred from the viewpoint of improving an oxidation-suppressing effect.

<<Phenol-based Antioxidant (D1)>>

A phosphorus-free phenol-based antioxidant generally used as an antioxidant for a lubricating oil composition may be used as the phenol-based antioxidant (D1).
The phenol-based antioxidants (D1) may be used alone or in combination thereof.
From the viewpoint of further facilitating an improvement in effect of the present invention, the phenol-based antioxidant (D1) preferably includes a compound (D1-1) represented by the following general formula (d1-1).
From the same viewpoint, the content of the compound (D1-1) in the phenol-based antioxidant (D1) 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%, even more preferably from 95 mass% to 100 mass% with respect to the total amount of the phenol-based antioxidant (D1).
In the general formula (d1-1), R^d1 represents an alkylene group having 1 to 5 carbon atoms.
From the viewpoint of further facilitating an improvement in effect of the present invention, the number of the carbon atoms of the alkylene group that may be selected as R^d1 is preferably from 1 to 4, more preferably from 1 to 3, still more preferably 1 or 2.
Specific examples of the alkylene group that may be selected as R^d1 include: linear alkylene groups, such as a methylene group, an ethylene group, a n-propylene group, a n-butylene group, and a n-pentylene group; and branched alkylene groups, such as an isopropylene group, an isobutylene group, a sec-butylene group, a tert-butylene group, an isopentylene group, and a neopentylene group.
In the general formula (d1-1), R^d2 represents an alkyl group having 1 to 25 carbon atoms.
From the viewpoint of further facilitating an improvement in effect of the present invention, the number of the carbon atoms of the alkyl group that may be selected as R^d2 is preferably 2 or more, more preferably 4 or more, still more preferably 6 or more. In addition, the number is preferably 22 or less, more preferably 21 or less, still more preferably 20 or less. The upper limit values and lower limit values of those numerical ranges may be freely combined. Specifically, the number is preferably from 2 to 20, more preferably from 4 to 15, still more preferably from 6 to 10.
Specific examples of the alkyl group that may be selected as R^d2 include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, an icosyl group, a henicosyl group, a docosyl group, a tricosyl group, a tetracosyl group, and a pentacosyl group. Those groups may be linear or branched.
The compounds (D1-1) may be used alone or in combination thereof.

<<Naphthylamine-based Antioxidant (D2)>>

A naphthylamine-based antioxidant generally used as an antioxidant for a lubricating oil composition may be used as the naphthylamine-based antioxidant (D2).
The naphthylamine-based antioxidants (D2) may be used alone or in combination thereof.
The naphthylamine-based antioxidant (D2) may be unsubstituted, or may have a substituent such as an alkyl group.
From the viewpoint of further facilitating an improvement in effect of the present invention, the naphthylamine-based antioxidant (D2) preferably includes a compound (D2-1) represented by the following general formula (d2-1).
From the same viewpoint, the content of the compound (D2-1) in the naphthylamine-based antioxidant (D2) 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%, even more preferably from 95 mass% to 100 mass% with respect to the total amount of the naphthylamine-based antioxidant (D2).
In the general formula (d2-1), R^b1 represents an alkyl group having 1 to 30 carbon atoms.
When the number of the carbon atoms of the alkyl group is from 1 to 30, the effect of the present invention is easily improved.
From the viewpoint of further facilitating improvements in effects of the present invention, the numbers of the carbon atoms of the alkyl group that may be selected as R^b1 are each independently preferably from 1 to 20, more preferably from 4 to 16, still more preferably from 4 to 14.
Specific examples of the alkyl group that may be selected as R^b1 include those given as the examples of the alkyl group that may be selected as each of R^a1 and R^a2. The alkyl group may be linear or branched.
In the general formula (d2-1), nb1 represents an integer of from 0 to 5.
From the viewpoint of further facilitating an improvement in effect of the present invention, nb1 preferably represents 0 or 1.
The compounds (D2-1) may be used alone or in combination thereof.

<<Diphenylamine-based Antioxidant (D3)>>

A diphenylamine-based antioxidant generally used as an antioxidant for a lubricating oil composition may be used as the diphenylamine-based antioxidant (D3).
The diphenylamine-based antioxidants (D3) may be used alone or in combination thereof.
From the viewpoint of further facilitating an improvement in effect of the present invention, the diphenylamine-based antioxidant (D3) preferably includes a compound (D3-1) represented by the following general formula (d3-1).
From the same viewpoint, the content of the compound (D3-1) in the diphenylamine-based antioxidant (D3) 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%, even more preferably from 95 mass% to 100 mass% with respect to the total amount of the diphenylamine-based antioxidant (D3).
In the general formula (d3-1), R^a1 and R^a2 each independently represent an alkyl group having 1 to 30 carbon atoms.
When the number of the carbon atoms of the alkyl group is from 1 to 30, the effect of the present invention is easily improved.
From the viewpoint of further facilitating an improvement in effect of the present invention, the numbers of the carbon atoms of the alkyl groups that may be selected as R^a1 and R^a2 are each independently preferably from 1 to 20, more preferably from 4 to 16, still more preferably from 4 to 14.
Specific examples of the alkyl group that may be selected as each of R^a1 and R^a2 include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, an icosyl group, a henicosyl group, a docosyl group, a tricosyl group, a tetracosyl group, a pentacosyl group, a hexacosyl group, a heptacosyl group, an octacosyl group, a nonacosyl group, and a triacontyl group. Those groups may be linear or branched.
In the general formula (d3-1), na1 and na2 each independently represent an integer of from 1 to 5.
From the viewpoint of further facilitating an improvement in effect of the present invention, na1 and na2 each independently represent preferably from 1 to 3, more preferably 1 or 2, still more preferably 1.
The compounds (D3-1) may be used alone or in combination thereof.
The content of the antioxidant (D) is preferably 0.8 mass% or more, more preferably 1.0 mass% or more, still more preferably 1.5 mass% or more, still further more preferably 3.0 mass% or more, yet still further more preferably 5.0 mass% or more with respect to the total amount (100 mass%) of the grease composition, and is preferably 10 mass% or less, more preferably 9.0 mass% or less, still more preferably 8.0 mass% or less.

The grease composition of this embodiment may include the rust inhibitor (E).
The grease composition of this embodiment includes the hydrophilic nanofibers (B). However, the hydrophilic nanofibers (B) are liable to absorb moisture because of their high hydrophilicity, and hence rust may occur. In view of the foregoing, the incorporation of the rust inhibitor (E) into the grease composition of this embodiment can improve the rust-inhibiting property of the grease composition.
Examples of the rust inhibitor (E) include fatty acid soap, a carboxylic acid-based rust inhibitor, a carboxylic acid salt-based rust inhibitor, a sulfonic acid metal salt, an organic phosphorous acid ester, an organic phosphoric acid ester, an organic phosphoric acid ester amine salt, an organic phosphoric acid metal salt, an alkenyl succinic acid ester, an alkenyl succinic acid polyhydric alcohol ester, an amine-based rust inhibitor, a fatty acid amine, thiadiazole and a derivative thereof, and benzotriazole and a derivative thereof.
The rust inhibitors (E) may be used alone or in combination thereof.
Among them, fatty acid soap, an organic phosphoric acid ester amine salt, and a sulfonic acid metal salt are preferred.
The fatty acid soap is not particularly limited, and examples thereof include a fatty acid zinc salt, a fatty acid aluminum salt, a fatty acid calcium salt, and a fatty acid magnesium salt.
Among them, a fatty acid zinc salt is preferred.
The number of the carbon atoms of the fatty acid zinc salt is preferably from 4 to 30, more preferably from 6 to 24, still more preferably from 8 to 20.
Examples of the fatty acid zinc salt include zinc stearate and zinc neodecanoate.
The organic phosphoric acid ester amine salt is not particularly limited, and is, for example, an organic phosphoric acid ester amine salt having a hydrocarbon group having 1 to 30 carbon atoms. The number of the carbon atoms of the organic phosphoric acid ester amine salt is preferably from 4 to 24, more preferably from 8 to 18.
The sulfonic acid metal salt is a metal salt of each of various sulfonic acids. Examples of the various sulfonic acids for forming the sulfonic acid metal salt include an aromatic petroleum sulfonic acid, an alkyl sulfonic acid, an aryl sulfonic acid, and an alkylaryl sulfonic acid. More specifically, preferred examples thereof include dodecylbenzenesulfonic acid, dilaurylcetylbenzenesulfonic acid, benzenesulfonic acid substituted with paraffin wax, benzenesulfonic acid substituted with polyolefin, benzenesulfonic acid substituted with polyisobutylene, naphthalenesulfonic acid, and dinonylnaphthalenesulfonic acid.
A metal for forming the sulfonic acid metal salt is preferably, for example, sodium, magnesium, calcium, zinc, or barium.
The base number of such salt is not particularly limited, and the salt may be neutral or overbased.
The fatty acid zinc salt and the organic phosphoric acid ester amine salt are preferably incorporated in combination as the rust inhibitors (E). Examples of the fatty acid zinc salt to be used as a mixture with the organic phosphoric acid ester amine salt include zinc stearate and zinc neodecanoate.
When the fatty acid zinc salt and the organic phosphoric acid ester amine salt are used in combination, from the viewpoint of a rust-inhibiting property, the total content of the fatty acid zinc salt and the organic phosphoric acid ester amine salt is preferably from 0.1 mass% to 10.0 mass%, more preferably from 0.3 mass% to 8.0 mass%, still more preferably from 0.4 mass% to 5.0 mass%, still further more preferably from 0.5 mass% to 3.0 mass%, yet still further more preferably from 0.8 mass% to 1.5 mass%.
When the fatty acid zinc salt and the organic phosphoric acid ester amine salt are used in combination, the ratio of the fatty acid zinc salt to the organic phosphoric acid ester amine salt is preferably from 1/9 to 9/1, more preferably from 3/7 to 8/2, still more preferably from 5/5 to 7/3.
The content of the rust inhibitor (E) is preferably from 0.1 mass% to 10.0 mass%, more preferably from 0.3 mass% to 8.0 mass%, still more preferably from 0.4 mass% to 5.0 mass%, still further more preferably from 0.5 mass% to 3.0 mass%, yet still further more preferably from 0.8 mass% to 2.0 mass% with respect to the total amount (100 mass%) of the grease composition.

The grease composition of this embodiment may further include the additive (F) to be blended into a general grease composition to the extent that the effect of the present invention is not impaired.
Examples of the additive (F) include a dispersant, a lubricity improver, a thickener, a detergent dispersant, a corrosion inhibitor, an extreme pressure agent, and a metal deactivator.
Those various additives may be used alone or in combination thereof.

(Dispersant)

The dispersant only needs to be a solvent having satisfactory compatibility with both of water and an oil, and examples thereof include one or more kinds selected from: aprotic polar solvents, such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols, such as propanol, ethylene glycol, propylene glycol, and hexylene glycol; and surfactants, such as a polyglycerin fatty acid ester, a sucrose fatty acid ester, citric acid monoglyceride, diacetyl tartaric acid monoglyceride, a polyoxyethylene sorbitan acid ester, and a sorbitan acid ester.
Specific examples of the dispersant include sorbitan trioleate, a succinic acid half ester, urea, and various surfactants.
When the grease composition of one aspect of the present invention includes the dispersant, the content of the dispersant is preferably from 0.01 mass% to 20 mass%, more preferably from 0.1 mass% to 10 mass%, still more preferably from 0.2 mass% to 5 mass% with respect to the total amount (100 mass%) of the grease composition.

(Lubricity Improver)

Examples of the lubricity improver include sulfur compounds (e.g., a sulfurized oil and fat, a sulfurized olefin, a polysulfide, a sulfurized mineral oil, thiophosphoric acids such as triphenyl phosphorothioate, thiocarbamic acids, thioterpenes, and dialkyl thiodipropionates), and phosphoric acid esters and phosphorous acid esters (e.g., tricresyl phosphate and triphenyl phosphite).
When the grease composition of this embodiment includes the lubricity improver, the content of the lubricity improver is preferably from 0.01 mass% to 20 mass%, more preferably from 0.1 mass% to 10 mass%, still more preferably from 0.2 mass% to 5 mass% with respect to the total amount (100 mass%) of the grease composition.

(Thickener)

The thickener increases the viscosity of the base oil as required, and is blended for adjusting the kinematic viscosity of the base oil containing the thickener to an appropriate value.
Examples of the thickener include a polymethacrylate (PMA), an olefin copolymer (OCP), a polyalkylstyrene (PAS), and a styrene-diene copolymer (SCP).
When the grease composition of this embodiment includes the thickener, the content of the thickener is preferably from 0.01 mass% to 20 mass%, more preferably from 0.1 mass% to 10 mass%, still more preferably from 0.2 mass% to 5 mass% with respect to the total amount (100 mass%) of the grease composition.

(Detergent Dispersant, Corrosion Inhibitor, Extreme Pressure Agent, and Metal Deactivator)

Examples of the detergent dispersant include a succinimide and a boron-based succinimide.
Examples of the corrosion inhibitor include a benzotriazole-based compound and a thiazole-based compound.
Examples of the extreme pressure agent include a phosphorus-based compound, zinc dithiophosphate, and an organic molybdenum.
An example of the metal deactivator is benzotriazole.
When the grease composition of this embodiment includes those additives, the content of each of these additives is preferably from 0.01 mass% to 20 mass%, more preferably from 0.1 mass% to 10 mass%, still more preferably from 0.2 mass% to 5 mass% with respect to the total amount (100 mass%) of the grease composition.

[Characteristic of Grease Composition]

In the grease composition of this embodiment, the hydrophilic nanofibers (B) are spuriously hydrophobized because the hydrophilic groups of the hydrophilic nanofibers (B) are protected by the particles (C). Accordingly, the grease composition of this embodiment has an appropriate oil separation and is excellent in water resistance.
In addition, in the grease composition of this embodiment, the higher-order structure is easily formed by the hydrophilic nanofibers (B), and the hydrophilic nanofibers (B) are uniformly dispersed in the base oil (A). In addition, the particles (C) are uniformly dispersed in the base oil (A). Accordingly, the grease composition of this embodiment easily has a moderate worked penetration even when the contents of the hydrophilic nanofibers (B) and the particles (C) are small.

[Physical Properties of Grease Composition]

From the viewpoints of pumpability and the suppression of oil leakage, the worked penetration of the grease composition of this embodiment at 25°C is preferably from 220 to 430, more preferably from 240 to 400, still more preferably from 250 to 345, still further more preferably from 255 to 300, yet still further more preferably from 265 to 295.
In this description, the worked penetration of the grease composition means a value measured at 25°C in conformity with JIS K2220:2013 (Article 7).

The grease composition of this embodiment may be evaluated for its oil separation by a method described in Examples to be described later.
The oil separation of the grease composition of this embodiment is preferably 10 mass% or less, more preferably 8.0 mass% or less, still more preferably 7.0 mass% or less from the viewpoint of suppressing oil leakage at the time of its use, and the oil separation is preferably 0.5 mass% or more, more preferably 0.8 mass% or more, still more preferably 1.0 mass% or more.
In this description, the oil separation of the grease composition means a value measured in conformity with the oil separation test method of JIS K2220:2013 (Article 11) under the conditions of a temperature of 100°C and a time period of 24 hours.

The grease composition of this embodiment may be evaluated for its water resistance through the measurement of its rinsing water resistance by a method described in Examples to be described later.
From the viewpoint of water resistance, the rinsing water resistance of the grease composition of this embodiment is preferably 10 mass% or less, more preferably 9.0 mass% or less, still more preferably 8.0 mass% or less.
In this description, the rinsing water resistance of the grease composition means a value measured in conformity with the rinsing durability test of JIS K2220:2013 (Article 16) under such a condition that water at 79°C is used.

The grease composition of this embodiment may be evaluated for its oxidation stability through the measurement of its oxidation stability degree by a method described in Examples to be described later.
From the viewpoint of oxidation stability, the oxidation stability degree of the grease composition of this embodiment is preferably 80 or less, more preferably 75 or less, still more preferably 70 or less.
In this description, the oxidation stability degree of the grease composition means a value measured in conformity with the oxidation stability degree test of JIS K2220:2013 (Article 12) under the conditions of a temperature of 99°C, a time period of 100 hours, and an oxygen pressure of 750 kPa.

The grease composition of this embodiment may be evaluated for its rust-inhibiting property through the performance of a bearing rust inhibition test by a method described in Examples to be described later.
From the viewpoint of the rust-inhibiting property, it is preferred that when the grease composition of this embodiment is evaluated by a method described in Examples to be described later, no reddish brown or black color change (rust) is observed on the surface of a bearing.
In this description, the rust-inhibiting property of the grease composition means a value evaluated in conformity with the bearing rust inhibition test of ASTM D 1743 under the conditions of a temperature of 52°C and a time period of 48 hours.

[Method of producing Grease Composition]

A method of producing the grease composition of this embodiment preferably includes the following steps (1) to (3):

·Step (1): a step of mixing an aqueous dispersion, which is obtained by blending hydrophilic nanofibers each having a thickness (d') of from 1 nm to 500 nm, preferably from 3 nm to 300 nm, more preferably from 5 nm to 200 nm, still more preferably from 10 nm to 100 nm, still further more preferably from 15 nm to 70 nm, yet still further more preferably from 20 nm to 50 nm into water, a base oil, and a dispersant to prepare a mixed liquid;
·Step (2): a step of removing the water from the mixed liquid to prepare grease; and
·Step (3): a step of blending the grease with the particles (C).

The step (2) may be a step of removing the water and the dispersant from the mixed liquid.
In the grease composition obtained through such steps, the aggregation of the hydrophilic nanofibers in the base oil is suppressed, and hence the hydrophilic nanofibers each having a thickness (d) of from 1 nm to 500 nm, preferably from 3 nm to 300 nm, more preferably from 5 nm to 200 nm, still more preferably from 10 nm to 100 nm, still further more preferably from 15 nm to 70 nm, yet still further more preferably from 20 nm to 50 nm can be dispersed under a state in which the nanofibers maintain their fiber shapes. It is assumed that as a result of the foregoing, the hydrophilic nanofibers and the particles (C) form some higher-order structure in the base oil, and hence there is prepared a grease composition, which has a moderate oil separation and is excellent in water resistance.
The steps (1) to (3) are described below.

The step (1) is a step of mixing an aqueous dispersion, which is obtained by blending hydrophilic nanofibers each having a thickness (d') of from 1 nm to 500 nm, preferably from 3 nm to 300 nm, more preferably from 5 nm to 200 nm, still more preferably from 10 nm to 100 nm, still further more preferably from 15 nm to 70 nm, yet still further more preferably from 20 nm to 50 nm into water, a base oil, and a dispersant to prepare a mixed liquid.
Details about the hydrophilic nanofibers and the base oil to be used in the step (1) are as described above.
As described above, the term "thickness (d')" as used herein refers to the thickness of each of the hydrophilic nanofibers serving as a raw material before the blending into the base oil or the water, and suitable ranges of the "thickness (d')" are the same as those described above.
The solid content concentration of the aqueous dispersion obtained by blending the hydrophilic nanofibers into the water is typically from 0.1 mass% to 70 mass%, preferably from 0.1 mass% to 65 mass%, more preferably from 0.1 mass% to 60 mass%, still more preferably from 0.5 mass% to 55 mass%, still further more preferably from 1.0 mass% to 50 mass% with respect to the total amount (100 mass%) of the aqueous dispersion.
The aqueous dispersion may be prepared by: blending the hydrophilic nanofibers, and as required, a surfactant or the like into the water; and sufficiently stirring the mixture manually or with a stirring machine.
The following may be performed: powderized hydrophilic nanofibers are used as the hydrophilic nanofibers; and water is added thereto to provide an aqueous dispersion.
The blending amount of the dispersant in the mixed liquid to be prepared in the step (1) is preferably from 0.1 mass% to 50 mass%, more preferably from 0.5 mass% to 40 mass%, still more preferably from 1.0 mass% to 30 mass%, still further more preferably from 1.0 mass% to 20 mass%, yet still further more preferably from 1.0 mass% to 10 mass% with respect to the total amount (100 mass%) of the mixed liquid.
The blending amount of the water in the mixed liquid to be prepared in the step (1) is preferably from 1 mass% to 60 mass%, more preferably from 3 mass% to 50 mass%, still more preferably from 5 mass% to 40 mass% with respect to the total amount (100 mass%) of the mixed liquid.
The blending amount ratio (water/dispersant) of the water to the dispersant in the mixed liquid to be prepared in the step (1) is preferably from 0.01 to 600, more preferably from 0.05 to 400, still more preferably from 0.1 to 300, still further more preferably from 0.2 to 200 in terms of mass ratio.
The above-mentioned various additives to be blended into a general grease composition may each be added to the mixed liquid together with the aqueous dispersion obtained by blending the hydrophilic nanofibers into the water, the base oil, and the dispersant. The mixed liquid may be prepared by: mixing those components; and sufficiently stirring the mixture manually or with a stirring machine.

The step (2) is a step of removing at least the water from the mixed liquid prepared in the step (1).
In this step, the dispersant may be removed from the mixed liquid together with the water.
A method of removing the water and the dispersant is preferably a method including heating the mixed liquid to evaporate and remove the water and the dispersant.
Conditions at the time of the evaporation and removal of the water are preferably as follows: the mixed liquid is heated under an environment at a pressure of from 0.001 MPa to 0.1 MPa in the temperature range of from 0°C to 100°C.
In addition, conditions at the time of the evaporation and removal of the dispersant are preferably as follows: the mixed liquid is heated under an environment at a pressure of from 0.001 MPa to 0.1 MPa in the temperature range of from [boiling point (°C) of dispersant]-120°C to [boiling point (°C) of dispersant]-0°C.
The evaporation and removal of the water and the dispersant may be performed by normal-pressure distillation.
The grease is prepared by the step (2).

The step (3) is a step of blending the grease prepared in the step (2) with the particles (C).
Specifically, the grease composition of this embodiment is prepared by, for example, mixing the grease prepared in the step (2) and the particles (C), and subjecting the mixture to treatment such as uniformization with a homogenizer, a roll mill, or the like.
In addition, the antioxidant (D), the rust inhibitor (E), and the additive (F) may be blended as required at the timing of the mixing of the particles (C).

[Applications of Grease Composition]

The grease composition of this embodiment has an appropriate oil separation and is excellent in water resistance while using the hydrophilic nanofibers. Accordingly, the composition can exhibit excellent lubrication performance even under a moisture-absorbing environment.
A mechanism part to which the grease composition of this embodiment is applicable is, for example, a bearing or a gear. More specific examples thereof include: various bearings, such as a slide bearing and a ball bearing; a gear; an internal combustion engine; a brake; a part for a torque transmission device; a fluid coupling; a part for a compression device; a chain; a part for a hydraulic device; a part for a vacuum pump device; a watch part; a part for a hard disk; a part for a refrigerating machine; a part for a cutting machine; a part for a rolling mill; a part for a squeezing and drawing bench; a part for a rolling machine; a part for a forging machine; a part for a heat treatment device; a part for a heat exchanger; a part for a washing machine; a part for a shock absorber; and a part for a sealing device.
The grease composition of this embodiment is suitable for an application where a sliding portion, such as a general bearing or a bearing for a construction machine, is lubricated.
According to one aspect of the present invention, there are provided the following items [1] to [13].

[1] A grease composition, including:
- a base oil (A);
- hydrophilic nanofibers (B) each having a thickness (d) of from 1 nm to 500 nm; and
- particles (C),
- wherein the hydrophilic nanofibers (B) are one or more kinds selected from cellulose nanofibers (B1) and modified cellulose nanofibers (B2), and
- wherein the particles (C) each have a structure in which a hydrophobic group is present on an outermost surface of a hydrophilic parent structure, and in the grease composition, the particles (C) have an average particle diameter of from 1 nm to 500 nm.
[2] The grease composition according to the above-mentioned item [1], wherein the base oil (A) includes a vegetable oil (A1).
[3] The grease composition according to the above-mentioned item [1] or [2], wherein a content of the hydrophilic nanofibers (B) is from 0.1 mass% to 20 mass% with respect to a total amount of the grease composition.
[4] The grease composition according to any one of the above-mentioned items [1] to [3], wherein the hydrophilic parent structure includes silica particles.
[5] The grease composition according to any one of the above-mentioned items [1] to [4], wherein a content of the particles (C) is 6.0 mass% or more with respect to a total amount of the grease composition.
[6] The grease composition according to any one of the above-mentioned items [1] to [5], further including an antioxidant (D).
[7] The grease composition according to the above-mentioned item [6], wherein the antioxidant (D) includes one or more kinds selected from a phenol-based antioxidant and a naphthylamine-based antioxidant.
[8] The grease composition according to the above-mentioned item [6] or [7], wherein a content of the antioxidant (D) is 0.8 mass% or more with respect to a total amount of the grease composition.
[9] The grease composition according to any one of the above-mentioned items [1] to [8], further including a rust inhibitor (E).
[10] The grease composition according to the above-mentioned item [9], wherein the rust inhibitor (E) includes a fatty acid zinc salt and an organic phosphoric acid ester amine salt.
[11] The grease composition according to the above-mentioned item [9] or [10], wherein a content of the rust inhibitor (E) is from 0.5 mass% to 3.0 mass% with respect to a total amount of the grease composition.
[12] The grease composition according to any one of the above-mentioned items [1] to [11], wherein the grease composition has a worked penetration at 25°C of from 220 to 430.
[13] A lubrication method, including lubricating a lubrication site with the grease composition of any one of the above-mentioned items [1] to [12].

Examples

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

[Various Physical Property Values]

Methods of measuring various physical property values were as described below.

(1) 40°C Kinematic Viscosity and Viscosity Index of Base Oil (A)

The 40°C kinematic viscosity and viscosity index of the base oil (A) were measured and calculated in conformity with JIS K2283:2000.

(2) Thicknesses and Aspect Ratios of Hydrophilic Nanofibers serving as Thickener (B)

The thickness and length of each of 10 randomly selected hydrophilic nanofibers were measured with a transmission electron microscope (TEM), and a value calculated from the expression "length"/"thickness" was adopted as the "aspect ratio" of each of the hydrophilic nanofibers of interest.

(3) Worked Penetration (One-half Scale) of Grease Composition

The worked penetration (one-half scale) of the grease composition was measured at 25°C in conformity with JIS K2220:2013 (Article 7).

[Raw Material]

The base oil (A), a hydrophilic nanofiber-dispersed liquid, the particles (C), and a dispersant used as raw materials for preparing a grease composition in each of Examples 1-1 to 1-4 and Comparative Examples 1-1 to 1-6 were as described below.

·Base oil (A1): a vegetable oil (rapeseed oil, product name: "FUMI SAWAYAKA OIL", manufactured by Showa Sangyo Co., Ltd., 40°C kinematic viscosity: 37.71, viscosity index: 208)
·Base oil (A2): a poly-α-olefin (PAO) (40°C kinematic viscosity: 57.14, viscosity index: 146)

·Hydrophilic nanofiber-dispersed liquid (product name: nanoforest-S, manufactured by Chuetsu Pulp & Paper Co., Ltd., aqueous dispersion containing cellulose nanofibers (CNFs) each having a polymerization degree of 600 (thickness (d'): from 20 nm to 50 nm (average: 35 nm), aspect ratio: 100 or more (average: 100 or more)))

·Particles (C1): hydrophobic silica gel (product obtained by treating the surface of fumed silica with dimethyldichlorosilane, product name: Aerosil (trademark) R972, manufactured by Evonik Industries AG, average primary particle diameter: about 16 nm)
·Particles (C2): organic bentonite (product name: Baragel 3000, manufactured by Elementis PLC)
·Particles (C3): organic bentonite (product name: Bentone (trademark) 27, manufactured by Elementis PLC)
·Particles (C4): organic modified mineral (product name: GARAMITE (trademark)-7303, manufactured by BYK-Chemie GmbH)
·Comparative component (C'1): polybutene
·Comparative component (C'2): polymethacrylate (PMA)

·Antioxidant (D1): a phenol-based antioxidant (n-octyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate)
·Antioxidant (D2): a phenol-based antioxidant (n-octadecyl 3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate)
·Antioxidant (D3): naphthylamine
·Antioxidant (D4): dinonyldiphenylamine

·Rust inhibitor (E1): a mixture of zinc stearate and an organic phosphoric acid ester amine salt (mass ratio of zinc stearate to the organic phosphoric acid ester amine salt=7/3, number of the carbon atoms of the organic phosphoric acid ester amine salt: from 10 to 12)
·Rust inhibitor (E2): a mixture of zinc neodecanoate and an organic phosphoric acid ester amine salt (product name: G-1340, manufactured by King Industries, Inc., number of the carbon atoms of the organic phosphoric acid ester amine salt: from 10 to 12)

·Rust inhibitor (E3):	zinc stearate
·Rust inhibitor (E4):	an organic phosphoric acid ester amine salt
·Rust inhibitor (E5):	a mixture of calcium sulfonate and sodium sulfonate
·Rust inhibitor (E6):	barium sulfonate (overbased)
·Rust inhibitor (E7):	sodium sulfonate (neutral)
·Rust inhibitor (E8):	sodium sulfonate (overbased)

·Dispersant: sorbitan trioleate

(Example 1-1)

200 Grams (amount of the CNFs among them: 20.0 g) of the hydrophilic nanofiber-dispersed liquid serving as the hydrophilic nanofibers (B), 438 g of the base oil (A1), and 2.0 g of the dispersant serving as the additive (F) were mixed, and the mixture was sufficiently stirred at 25°C to prepare a mixed liquid.
Next, the mixed liquid was heated to 90°C under normal pressure (atmospheric pressure) so that water was evaporated and removed from the mixed liquid.
Next, the mixed liquid was cooled to room temperature (25°C), and then 40.0 g of the particles (C1) were added thereto, followed by sufficient stirring. After that, the resultant was subjected to homogenization treatment with a triple roll mill to prepare a grease composition of Example 1-1.

(Examples 1-2 to 1-4 and Comparative Examples 1-1 to 1-6)

Grease compositions of Examples 1-2 to 1-4 and Comparative Examples 1-1 to 1-6 were prepared in the same manner as in the grease composition of Example 1-1 except that the components and their contents were changed as shown in Table 1 and Table 2.
Next, the grease compositions of Examples 1-1 to 1-4 and Comparative Examples 1-1 to 1-6 were each evaluated for its oil separation and water resistance.

[Evaluation of Oil Separation]

The mass ratio of the oil separated from each of the grease compositions was measured in conformity with the oil separation test of JIS K2220:2013 (Article 11) under the conditions of a temperature of 100°C and a time period of 24 hours. When the oil separation of the composition was 10 mass% or less, the oil separation was judged to be appropriate.

[Evaluation of Water Resistance]

The mass (rinsing water resistance) of each of the grease compositions rinsed off with water with respect to the mass (100 mass%) of the grease composition before its test was measured with water at 79°C in conformity with the rinsing durability test of JIS K2220:2013 (Article 16). When the rinsing water resistance of the composition was 10 mass% or less, the water resistance thereof was judged to be satisfactory.
The compositions, physical property values, and evaluation results of the grease compositions of Examples 1-1 to 1-4 and Comparative Examples 1-1 to 1-6 are shown in Table 1 and Table 2.

[Table 1]

Table 1

			Example
			1-1	1-2	1-3	1-4
Base oil (A)	Base oil (A1)	mass%	87.60	85.60	83.60	-
Base oil (A)	Base oil (A2)	mass%	-	-	-	87.60
Hydrophilic nanofibers (B)	Hydrophilic nanofiber-dispersed liquid	mass%	4.00	4.00	4.00	4.00
Particles (C)	Particles (C1)	mass%	8.00	10.00	12.00	8.00
	Particles (C2)	mass%	-	-	-	-
	Particles (C3)	mass%	-	-	-	-
	Particles (C4)	mass%	-	-	-	-
	Comparative component (C'1)	mass%	-	-	-	-
	Comparative component (C'2)	mass%	-	-	-	-
Additive (F)	Dispersant	mass%	0.40	0.40	0.40	0.40
Total		mass%	100.00	100.00	100.00	100.00
Physical property value	Worked penetration	-	344	314	283	353
Evaluation result	Oil separation	mass%	6.5	3.9	1.9	7.1
Evaluation result	Rinsing water resistance	mass%	7.9	2.5	1.9	8.6

[Table 2]

Table 2

			Comparative Example
			1-1	1-2	1-3	1-4	1-5	1-6
Base oil (A)	Base oil (A1)	mass%	89.00	87.60	87.60	87.60	87.60	87.60
Base oil (A)	Base oil (A2)	mass%	-	-	-	-	-	-
Hydrophilic nanofibers (B)	Hydrophilic nanofiber-dispersed liquid	mass%	10.00	4.00	4.00	4.00	4.00	4.00
Particles (C)	Particles (C1)	mass%	-	-	-	-	-	-
	Particles (C2)	mass%	-	8.00	-	-	-	-
	Particles (C3)	mass%	-	-	8.00	-	-	-
	Particles (C4)	mass%	-	-	-	8.00	-	-
	Comparative component (C'1)	mass%	-	-	-	-	8.00	-
	Comparative component (C'2)	mass%	-	-	-	-	-	8.00
Additive (F)	Dispersant	mass%	1.00	0.40	0.40	0.40	0.40	0.40
Total		mass%	100.00	100.00	100.00	100.00	100.00	100.00
Physical property value	Worked penetration	-	270	336	341	324	342	361
Evaluation result	Oil separation	mass%	5.4	11.4	More than 15	12.9	More than 15	More than 15
Evaluation result	Rinsing water resistance	mass%	More than 90	37.8	9.7	5.2	45.2	57.9

As shown in Table 1, it was found that each of the grease compositions of Examples 1-1 to 1-4 had an appropriate oil separation and was excellent in water resistance.

(Example 2-1)

Next, a grease composition of Example 2-1 was prepared in the same manner as in Example 1-1 except that in Example 1-1, 25 g of the antioxidant (D1) was also added to the mixed liquid simultaneously with 40.0 g of the particles (C1).

(Examples 2-2 to 2-13)

Grease compositions of Examples 2-2 to 2-13 were prepared in the same manner as in the grease composition of Example 2-1 except that the components and their contents were changed as shown in Table 3 and Table 4.
Next, the grease compositions of Examples 2-1 to 2-13 were each evaluated for its oil separation, water resistance, and oxidation stability.

[Evaluation of Oil Separation]

The oil separation of each of the grease compositions was measured in the same manner as in the above-mentioned section "Evaluation of Oil Separation." When the oil separation was 10 mass% or less, the oil separation was judged to be appropriate.

[Evaluation of Water Resistance]

The rinsing water resistance of each of the grease compositions was measured in the same manner as in the above-mentioned section "Evaluation of Water Resistance." When the rinsing water resistance was 10 mass% or less, the water resistance thereof was judged to be satisfactory.

[Evaluation of Oxidation Stability]

A pressure drop was measured in conformity with the oxidation stability degree test of JIS K2220:2013 (Article 12) under the conditions of a temperature of 99°C, a time period of 100 hours, and an oxygen pressure of 750 kPa. When the oxidation stability degree of the composition was 80 or less, the oxidation stability thereof was judged to be satisfactory.
The compositions, physical property values, and evaluation results of the grease compositions of Examples 2-1 to 2-13 are shown in Table 3 and Table 4.

[Table 3]

Table 3

			Example
			2-1	2-2	2-3	2-4	2-5	2-6	2-7
Base oil (A)	Base oil (A1)	mass%	80.60	78.60	77.60	76.60	81.60	78.60	82.60
Hydrophilic nanofibers (B)	Hydrophilic nanofiber-dispersed liquid	mass%	4.00	4.00	4.00	4.00	4.00	4.00	4.00
Particles (C)	Particles (C1)	mass%	10.00	12.00	13.00	14.00	12.00	12.00	12.00
Antioxidant (D)	Antioxidant (D1)	mass%	5.00	5.00	5.00	5.00	2.00	5.00	-
	Antioxidant (D2)	mass%	-	-	-	-	-	-	1.00
	Antioxidant (D3)	mass%	-	-	-	-	-	-	-
	Antioxidant (D4)	mass%	-	-	-	-	-	-	-
Additive (F)	Dispersant	mass%	0.40	0.40	0.40	0.40	0.40	0.40	0.40
Total		mass%	100.00	100.00	100.00	100.00	100.00	100.00	100.00
Physical property value	Worked penetration	-	295	283	273	255	279	283	271
Evaluation result	Oil separation	mass%	2.6	1.8	1.6	1.1	1.8	1.8	2.2
	Rinsing water resistance	mass%	4.4	1.9	2.8	1.1	1.6	1.9	1.6
	Oxidation stability degree	kPa	80	75	70	80	65	75	100

[Table 4]

Table 4

			Example
			2-8	2-9	2-10	2-11	2-12	2-13
Base oil (A)	Base oil (A1)	mass%	80.60	78.60	83.10	82.60	78.60	78.60
Hydrophilic nanofibers (B)	Hydrophilic nanofiber-dispersed liquid	mass%	4.00	4.00	4.00	4.00	4.00	4.00
Particles (C)	Particles (C1)	mass%	12.00	12.00	12.00	12.00	12.00	12.00
Antioxidant (D)	Antioxidant (D1)	mass%	-	-	-	-	-	-
	Antioxidant (D2)	mass%	3.00	5.00	-	-	-	-
	Antioxidant (D3)	mass%	-	-	0.50	1.00	5.00	-
	Antioxidant (D4)	mass%	-	-	-	-	-	5.00
Additive (F)	Dispersant	mass%	0.40	0.40	0.40	0.40	0.40	0.40
Total		mass%	100.00	100.00	100.00	100.00	100.00	100.00
Physical property value	Worked penetration	-	288	277	269	272	274	302
Evaluation result	Oil separation	mass%	2.1	2.9	1.5	2.3	2.4	1.8
	Rinsing water resistance	mass%	2.7	2.4	1.8	2.8	2.6	2.1
	Oxidation stability degree	kPa	60	85	120	80	85	80

As shown in Table 3 and Table 4, it was found that each of the grease compositions of Examples 2-1 to 2-13 had an appropriate oil separation and was excellent in water resistance. In addition, the following result was obtained: each of the grease compositions of Examples 2-1 to 2-6, 2-8, and 2-11 was also excellent in oxidation stability.

(Example 3-1)

Next, a grease composition of Example 3-1 was prepared in the same manner as in Example 1-1 except that in Example 1-1, 25 g of the antioxidant (D1) and 5.0 g of the rust inhibitor (E1) were also added to the mixed liquid simultaneously with 20.0 g of the particles (C1).

(Examples 3-2 to 3-8)

Grease compositions of Examples 3-2 to 3-8 were prepared in the same manner as in the grease composition of Example 3-1 except that the components and their contents were changed as shown in Table 5.
Next, the grease compositions of Examples 3-1 to 3-8 were each evaluated for its oil separation, water resistance, and rust-inhibiting property.

[Evaluation of Oil Separation]

[Evaluation of Water Resistance]

[Evaluation of Rust-inhibiting Property]

Each of the grease compositions was loaded into a high-humidity thermostat, and whether or not rust occurred was visually observed in conformity with the bearing rust inhibition test of ASTM D 1743 under the conditions of a temperature of 52°C and a time period of 48 hours. When the composition was evaluated as "A" in the following evaluation criteria, its rust-inhibiting property was judged to be satisfactory.

-Evaluation Criteria-

A: No reddish brown or black color change (rust) was observed on the surface of a bearing.
B: A reddish brown or black color change (rust) was observed on the surface of a bearing.

The compositions, physical property values, and evaluation results of the grease compositions of Examples 3-1 to 3-8 are shown in Table 5.

[Table 5]

Table 5

			Example
			3-1	3-2	3-3	3-4	3-5	3-6	3-7	3-8
Base oil (A)	Base oil (A1)	mass%	77.60	77.60	77.60	77.60	77.60	77.60	77.60	77.60
Hydrophilic nanofibers (B)	Hydrophilic nanofiber-dispersed liquid	mass%	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00
Particles (C)	Particles (C1)	mass%	12.00	12.00	12.00	12.00	12.00	12.00	12.00	12.00
Antioxidant (D)	Antioxidant (D1)	mass%	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00
Rust inhibitor (E)	Rust inhibitor (E1)	mass%	1.00	-	-	-	-	-	-	-
	Rust inhibitor (E2)	mass%	-	1.00	-	-	-	-	-	-
	Rust inhibitor (E3)	mass%	-	-	1.00	-	-	-	-	-
	Rust inhibitor (E4)	mass%	-	-	-	1.00	-	-	-	-
	Rust inhibitor (E5)	mass%	-	-	-	-	1.00	-	-	-
	Rust inhibitor (E6)	mass%	-	-	-	-	-	1.00	-	-
	Rust inhibitor (E7)	mass%	-	-	-	-	-	-	1.00	-
	Rust inhibitor (E8)	mass%	-	-	-	-	-	-	-	1.00
Additive (F)	Dispersant	mass%	0.40	0.40	0.40	0.40	0.40	0.40	0.40	0.40
Total		mass%	100.00	100.00	100.00	100.00	100.00	100.00	100.00	100.00
Physical property value	Worked penetration	-	294	288	297	281	300	302	287	284
Evaluation result	Oil separation	mass%	2.5	2.1	2.2	1.7	2.8	3.0	2.3	2.0
	Rinsing water resistance	mass%	2.1	1.7	1.6	2.2	2.4	2.6	2.0	1.7
	Bearing rust inhibition test	-	A	A	B	B	B	B	B	B

As shown in Table 5, it was found that each of the grease compositions of Examples 3-1 to 3-8 had an appropriate oil separation and was excellent in water resistance. In addition, the following result was obtained: each of the grease compositions of Examples 3-1 and 3-2 was also excellent in rust-inhibiting property.

Claims

A grease composition, comprising:
a base oil (A);

hydrophilic nanofibers (B) each having a thickness (d) of from 1 nm to 500 nm; and

particles (C),

wherein the hydrophilic nanofibers (B) are one or more kinds selected from cellulose nanofibers (B1) and modified cellulose nanofibers (B2), and

wherein the particles (C) each have a structure in which a hydrophobic group is present on an outermost surface of a hydrophilic parent structure, and in the grease composition, the particles (C) have an average particle diameter of from 1 nm to 500 nm.
The grease composition according to claim 1, wherein the base oil (A) comprises a vegetable oil (A1).
The grease composition according to claim 1 or 2, wherein a content of the hydrophilic nanofibers (B) is from 0.1 mass% to 20 mass% with respect to a total amount of the grease composition.
The grease composition according to any one of claims 1 to 3, wherein the hydrophilic parent structure comprises silica particles.
The grease composition according to any one of claims 1 to 4, wherein a content of the particles (C) is 6.0 mass% or more with respect to a total amount of the grease composition.
The grease composition according to any one of claims 1 to 5, further comprising an antioxidant (D).
The grease composition according to claim 6, wherein the antioxidant (D) comprises one or more kinds selected from a phenol-based antioxidant and a naphthylamine-based antioxidant.
The grease composition according to claim 6 or 7, wherein a content of the antioxidant (D) is 0.8 mass% or more with respect to a total amount of the grease composition.
The grease composition according to any one of claims 1 to 8, further comprising a rust inhibitor (E).
The grease composition according to claim 9, wherein the rust inhibitor (E) comprises a fatty acid zinc salt and an organic phosphoric acid ester amine salt.
The grease composition according to claim 9 or 10, wherein a content of the rust inhibitor (E) is from 0.5 mass% to 3.0 mass% with respect to a total amount of the grease composition.
The grease composition according to any one of claims 1 to 11, wherein the grease composition has a worked penetration at 25°C of from 220 to 430.
A lubrication method, comprising lubricating a lubrication site with the grease composition of any one of claims 1 to 12.